STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 - A Preliminary Report Geological Survey P. p. # 1028j D„ W. Rankin, editor Metz Reference Room Civil Engineering Department B106 C.E. Building University of Illinois Urbana, Illinois 61801 \ University of Illinois Library at Urbana-Champaign Oak Street Y =UGN j Studies Related to the i-.z., Charleston, South Carolina, Earthquake of 1886— RECEIVED APR ' 3 1978 It ML NEWMAN A Preliminary Report [WAR 27 Ideological SURVEY PROFESSIONAL PAPER 1028 - v. 1 XX,* /A f V ' r-^v ;Vv -s r_—_ 1 _^r ■ Iftin ? .iw v r \ » \ i \ * * .-. v*. -A ir >. a.' VAN v - v c v kTV *. A r ’JUU \! • i‘ 3AM ■ S3tk ^ o United States Department of the Interior National Center, Mail Stop 928 February 17, 1978 GEOLOGICAL SURVEY RESTON, VIRGINIA 22092 Professional Paper 1028, "Studies related to the Charleston, South Carolina, earthquake of 1886 -- A preliminary report" is now published. Please accept this copy with our compliments. This study was funded largely by the Nuclear Regulatory Commission, Office of Nuclear Regulatory Research under Agreement No. AT (A9~25)~1000. Other funds were provided by the U.S. Geological Survey, the National Science Foundation, and the State of South Carolina. As the title indicates, it is a preliminary report. Much of the work has now been superseded. Few of the conclusions are changed, but our body of knowledge is now considerably larger. Those of you that are able, might be interested in attending a symposium "Studies related to the Charleston, South Carolina, Earthquake of 1886" to be presented at the meeting of the Southeastern Section, Geological Society of America on Thursday, April 6, 1978 in Chattanooga, Tennessee. Much of the newer information will be presented at that meeting. Douglas W. Rankin Coordinator for Charleston Earthquake Studies STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886—A PRELIMINARY REPORT Studies Related to the Charleston, South Carolina, Earthquake of 1886—A Preliminary Report Edited by Douglas W. Rankin A. Studies Related to the Charleston, South Carolina, Earthquake of 1886—Introduction and Discussion, by Douglas W. Rankin B. Reinterpretation of the Intensity Data for the 1886 Charleston. South Carolina. Earthquake, by G. A. Bollinger C. The Seismicity of South Carolina Prior to 1886. by G. A. Bollinger and T. R. Visvanathan D. Recent Seismicity Near Charleston, South Carolina, and its Relationship to the August 31, 1886, Earthquake, by Arthur C. Tarr E. Lithostratigraphv of the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, South Carolina, by Gregory S. Gohn, Brenda B. Higgins, Charles C. Smith, and James P. Owens F. Biostratigraphy of the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, South Carolina, bv J. E. Hazel. L. M. Bybell. R. A. Christopher, N. O. Frederiksen, F. E. May, D M. McLean, R. Z. Poore, C. C. Smith, N. F. Sohl, P C. Valentine, and R. J. Witmer G. Geochemistry of Subsurface Basalt From the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, South Carolina—Magma Type and Tectonic Implications, by David Gottfried, C. S. Annell, and L. J. Schwarz. H. Heat Flow From a Corehole Near Charleston, South Carolina, bv J. H. Sass and John P Ziagos I. The Nature of the Geophvsical Basement Beneath the Coastal Plain of South Carolina and North¬ eastern Georgia, by Peter Popenoe and Isidore Zietz J. Magnetic Basement Near Charleston. South Carolina—A Preliminary Report, by Jeffrey D, Phillips K. Bouguer Gravity Map of the Summerville-Charleston, South Carolina, Epicentral Zone and Tec¬ tonic Implications, by Leland Timothv Long and J. W. Champion, Jr. L. Exploring the Charleston, South Carolina, Earthquake Area With Seismic Refraction—A Prelimi¬ nary Study, by Hans D. Ackermann M. A Preliminary Shallow Crustal Model Between Columbia and Charleston, South Carolina, Deter¬ mined From Quarry Blast Monitoring and Other Geophysical Data, by Pradeep Talwani N. Electric and Electromagnetic Soundings Near Charleston. South Carolina—A Preliminary Report, by David L. Campbell O. Correlation of Major Eastern Earthquake Centers With Mafic/’Ultramafic Basement Masses, bv M. F. Kane. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 78— 600007 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03047-1 CONTENTS Page Abstract_ 1 Introduction _ 2 Seismicity _ 3 Geologic setting: the Atlantic Coastal Plain_ 5 Geology beneath Coastal Plain rocks and tectonic setting_ 9 The source area: method of approach_ 10 Studies of the meizoseismal area_ 11 Earthquake origins_ 13 Conclusions _ 13 References cited _ 14 ILLUSTRATIONS Page Figure 1. Map showing comparison of areas of observed Modified Mercalli in¬ tensities for four major United States earthquakes_ 4 2. Map of the vicinity of Charleston, S.C., showing the approximate area of the meizoseismal area of the 1886 earthquake_ 6 3. Map of the southeastern United States showing location of Charles¬ ton, S.C., relative to regional features_ 7 m Studies Related to the Charleston, South Carolina, Earthquake of 1886—Introduction and Discussion By DOUGLAS W. RANKIN GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-A I I 1 s i 1 CONTENTS Page (A) Studies related to the Charleston, South Carolina, earthquake of 1886— introduction and discussion, by Douglas W. Rankin_ 1 (B) Reinterpretation of the intensity data for the 1886 Charleston, South Carolina, earthquake, by G. A. Bollinger_ 17 (C) The seismicity of South Carolina prior to 1886, by G. A. Bollinger and T. R. Visvanathan_ 33 (D) Recent seismicity near Charleston, South Carolina, and its relationship to the August 31, 1886, earthquake, by Arthur C. Tarr_ 43 (E) Lithostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, by Gregory S. Gohn, Brenda B. Higgins, Charles C. Smith, and James P. Owens _ 59 (F) Biostratigraphy of the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina, by J. E. Hazel, L. M Bybell, R. A. Christopher, N. 0. Frederiksen, F. E. May, D. M. McLean, R. Z. Poore, C. C. Smith, N. F. Sohl, P. C. Valentine, and R. J. Witmer_ 71 (G) Geochemistry of subsurface basalt from the deep corehole (Clubhouse Crossroads corehole 1) near Charleston, South Carolina—magma type and tectonic implications, by David Gottfried, C. S. Annell, and L. J. Schwarz 91 (H) Heat flow from a corehole near Charleston, South Carolina, by J. H. Sass and John P. Ziagos___ 115 (I) The nature of the geophysical basement beneath the Coastal Plain of South Carolina and northeastern Georgia, by Peter Popenoe and Isidore Zietz_ 119 (J) Magnetic basement near Charleston, South Carolina—a preliminary re¬ port, by Jeffrey' D. Phillips_ 139 (K) Bouguer gravity map of the Summerville-Charleston, South Carolina, epicentral zone and tectonic implications, by Leland Timothy Long and J. W. Champion, Jr_ 151 (L) Exploring the Charleston, South Carolina, earthquake area with seismic refraction—a preliminary study, by Hans D. Ackermann_ 167 (M) A preliminary shallow crustal model between Columbia and Charleston, South Carolina, determined from quarry blast monitoring and other geo¬ physical data, by Pradeep Talwani_ 177 (N) Electric and electromagnetic soundings near Charleston, South Carolina— a preliminary report, by David L. Campbell_ 189 (O) Correlation of major eastern earthquake centers with mafic/ultramafic basement masses, by M. F. Kane_ 199 ' . STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886—INTRODUCTION AND DISCUSSION By Douglas W. Rankin “One [Cordilleran earthquake], in 1812, destroyed a church in Los Angeles, California, killing a score or more people. Together with the Charleston earthquake [of 1886] this shock is entitled to a peculiar place in our history; these two shocks being the only earthquakes which have caused any loss of life in the country.” N. S. Shaler (1899) “The absence of large-magnitude earthquakes in eastern North Ameri¬ ca since the Charleston, S.C., earthquake of 1886 has resulted in com¬ placency, or perhaps unawareness on the part of the general populace of the existence of any earthquake threat to them.” Otto Nuttli (1973) ABSTRACT The seismic history of the southeastern United States is dominated by the 1886 earthquake near Charleston, S.C. An understanding of the specific source and the uniqueness of the neotectonic setting of this large earthquake is essen¬ tial in order to properly assess seismic hazards in the south¬ eastern United States. Such knowledge will also contribute to the fundamental understanding of intraplate earthquakes and will aid indirectly in deciphering the evolution of Atlan¬ tic-type continental margins. The 15 chapters in this volume report on the first stage of an ongoing multidisciplinary study of the Charleston earthquake of 1886. The Modified Mercalli intensity for the 1886 earthquake was X in the meizoseismal area, an elliptical area 35 by 50 km, the center of which was Middleton Place. Seismic ac¬ tivity is continuing today in the Middleton Place-Summerville area at a higher level than prior to 1886. The present seis¬ micity is originating at depths of 1 to 8 km, mostly in the crystalline basement beneath sedimentary rocks of the Coastal Plain. The crystalline basement beneath the Charleston-Summer- ville area is not simply a seaward extension of crystalline rocks of the Appalachian orogen that are exposed in the Piedmont to the northwest, but has a distinctive magnetic signature that does not reflect Appalachian orogenic trends. The area underlain by this distinctive geophysical basement, the Charleston block, may represent a broad zone of Tri- assic and (or) Jurassic crustal extension formed during the early stages of the opening of the Atlantic Ocean. The Charleston block is characterized in part by prominent, roughly circular magnetic and gravity highs that are thought to reflect mafic or ultramafic plutons. A continuously cored borehole put down over the shallow¬ est (about 1.5 km deep) of these magnetic anomalies on the edge of the meizoseismal area bottomed at 792 m in amygda- loidal basalt. Although the K-Ar ages of about 100 m.y. for the basalt are consistent with the Late Cretaceous (Ceno¬ manian) age of the overlying Cape Fear Formation, this must be a minimum age as a result of chemical alteration. The interpreted magmatic composition of the basalt most closely resembles the high-Ti quartz-normative tholeiites of Late Triassic and Early Jurassic age from eastern North America; age of the basalt is probably similar. Various geo¬ physical surveys suggest that Coastal Plain sedimentary rocks do not simply dip homoclinally to the southeast on a gently dipping basement surface but are disturbed by struc¬ tures not yet clearly deciphered. The present stress regime of the Charleston-Summerville area appears to be one of NE.-SW. compression rather than of extension as it presumably was in the Mesozoic. The present stress regime seems similar to that of much of the eastern United States. Comparison of several seismic source areas in eastern North America shows that epicenters are typically near the periphery of positive gravity features interpreted to represent mafic or ultramafic bodies. Earth¬ quakes may be caused by the concentration of regional stress around the peripheries of these inhomogeneities in an other¬ wise more homogeneous plate. Whether the inhomogeneities are more or less rigid than the surrounding material is un¬ certain. 1 2 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 INTRODUCTION One of the largest historic earthquakes in eastern North America, and by far the largest earthquake in southeastern United States took place about 9:50 p.m. on August 31, 1886, near Charleston, S.C. The major shock lasted less than 1 minute, but resulted in about 60 deaths and extensive damage to the city of Charleston. Because the event took place before seismological instrumentation, estimates of its loca¬ tion and size must come from observations of the damage and effects caused by the earthquake. Most of what we know of the event and the resulting dam¬ age comes from a comprehensive report by C. E. i Dutton published in 1889. A review of Dutton’s ! (1889) intensity data by Bollinger (this volume) confirms a Modified Mercalli intensity of X for the meizoseismal area (area of maximum damage) and j of intensity IX for the city of Charleston. Dutton’s report shows the location of craterlets formed of sand, but does not report any surface faulting. No fault, in fact, is known to be exposed at the surface within 100 km of the meizoseismal area. The cause of the Charleston earthquake has not been adequately explained in the 90 years since the event. An understanding of this large earthquake is essen¬ tial in order to properly assess seismic hazards in the j southeastern United States. The specific source of the earthquake and its tectonic setting must be es¬ tablished, both to permit evaluation of expectable j future seismicity in the Charleston region and to ! determine whether that region differs in any tec- ! tonically significant fashion from other parts of the Eastern United States. With a few notable exceptions, the earth’s seis¬ micity is largely associated with plate boundaries or active volcanic areas within the plates such as Hawaii (see Tarr, 1974). Plate boundaries, in fact, were originally defined on the basis of seismicity. Oceanic crust is created at divergent plate boun¬ daries and consumed at convergent plate boundaries. Where plate motion is parallel to the plate boundary, crust is neither created nor consumed. Continents are envisaged as being carried passively within mov¬ ing oceanic crust. Because of its lower density, con¬ tinental crust is probably not consumed (is not sub¬ ducted) at convergent margins but becomes pro¬ gressively deformed and crumpled, causing orogenic belts. A divergent plate boundary may originate within a continental mass through rifting and the subse¬ quent formation of an oceanic ridge within the grow¬ ing rift. Such a history is hypothesized • for the growffh of the Atlantic Ocean during the Mesozoic. Rifting within the large continental mass that in¬ cluded North America and Africa began in the Tri- assic as evidenced by the cratonal Triassic basins preserved in eastern North America. By the Cre¬ taceous, a significant ocean basin had formed be¬ tween the United States and Africa (Pitman and Talwani, 1972). As spreading from the Mid-Atlantic Ridge continued, the eastern margin of the North American continent was carried passively farther from the active spreading center. This continental margin is now commonly referred to as the type ex¬ ample of the seismically quiet Atlantic-type conti¬ nental margin (Dewey and Bird, 1970). The meizoseismal area of the Charleston earth¬ quake, although close to the Atlantic coastline, is well within the North American continental mass if one accepts the East Coast magnetic anomaly more than 200 km to the southeast as the continental mar¬ gin (Taylor and others, 1968). The active plate boundary, the Mid-Atlantic Ridge, is about 3,000 km east-southeast of Charleston. An in-depth study of the Charleston area offers an opportunity to advance the fundamental understanding of Atlantic-type continental margins. Earthquakes along boundaries of plates, such as the San Francisco 1906 and Alaska 1964 earthquakes, are readily understood in terms of relative plate motions and plate tectonics. No similar understanding yet exists for deformation and earthquakes such as Charleston which occur within platfes. The Charleston earthquake shares its intraplate setting with the other largest historical earthquakes in eastern and central United States. Those took place in 1755 near Cape Ann, Mass., and in the win¬ ter of 1811-12 in the Mississippi Valley near New Madrid, Mo., as a series of three widely felt shocks. All of these large intraplate earthquakes occurred before instrumentation. An adequate explanation of any of them should aid in explaining the others (see Kane, this volume). The epicenter of the Cape Ann earthquake ap¬ pears to have been offshore. It is, thus, not well lo¬ cated and not easy to study. The available data for the Mississippi Valley series, however, have been extensively analyzed by several seismologists (see particularly Nuttli, 1973 and 1976; and Evernden, 1975 and 1976) and the epicentral area is under in¬ vestigation today. The Mississippi Valley series included three major shocks: one on Dec. 16, 1811, one on Jan. 23, 1812, and one on Feb. 7, 1812. The February shock is con¬ sidered the largest. Gupta and Nuttli (1976) recent¬ ly revised upward the maximum intensity for each INTRODUCTION AND DISCUSSION 3 of these events, and Nuttli is quoted by Mosaic maga¬ zine (1976) as rating the surface-wave magnitude (Af s ) of these events in chronologic order as 8.0, 7.7, and 8.2, respectively. Nuttli (oral commun. 1977) states that this revision assumes that the surface- wave behavior of the Mississippi Valley earthquakes is similar to that of interplate earthquakes, such as those which have occurred in California. He would not necessarily change his published (Nuttli, 1976) estimates of the body-wave magnitude (m b ) for these events of 7.2, 7.1, and 7.4, respectively. Bollinger (this volume) arrived at a body-wave magnitude (ra b ) estimate for the Charleston earth¬ quake of 6.8 using the attenuation of intensity as a function of distance from the epicenter and Nuttli’s (1976) intensity-particle velocity data for the cen¬ tral United States. As noted by Bollinger (this vol¬ ume), the number of significant earthquakes in the central United States for which both intensity and particle velocity data are available is quite small be¬ cause of the short period of instrumented record relative to the low rate of earthquake occurrence. Using the more abundant western United States in¬ tensity-particle velocity data, he estimates the m b for the Charleston earthquake at 7.1. Seismologists disagree as to what is the most ap¬ propriate measure of earthquake size, particularly when comparing earthquakes in different geologic terranes, for example, in the eastern United States and the western United States (Nuttli, 1976; Evern- den, 1976; and Bollinger, this volume). In a general way, however, the Mississippi Valley earthquakes of 1811-12 have been equated with the San Francisco earthquake of 1906 (these are probably the largest historic earthquakes in the conterminous United States), and the Charleston earthquake has been equated with the San Fernando earthquake of 1971 which had an instrumentally determined magnitude M l = 6.4 (Allen and others, 1973) and a m b = 6.0 esti¬ mated from intensity data (Nuttli, 1976). The hazards which must be considered in any dis¬ cussion of South Carolina seismicity include not only ground breakage and ground motion in the epicen- tral area, but significant ground motion at consid¬ erable distance from the epicenter as is common with all larger earthquakes. Because of the low attenua¬ tion of seismic energy in the East, the area of equiv¬ alent damage for earthquakes of the same magnitude is far larger for eastern and central United States earthquakes than for those taking place on the west coast (fig. 1). The Charleston earthquake, for ex¬ ample, produced intensity V effects in Chicago (Bol¬ linger, this volume), 1,200 km from the earthquake epicenter. Important recent work related to the seismicity of the Charleston earthquake includes the monitoring of microearthquakes in the summer of 1971 in the Summerville area by Bollinger, a review of seismic activity in South Carolina by Bollinger (1972), and the establishment of a 5-station reconnaissance seis- mographic network in March 1973 by the Seismo- logical Investigations Group of NOAA under the auspices of the AEC. The Geological Survey assumed responsibility for the operation of this network and began some geophysical surveys of the area in 1973. In the spring of 1974, these efforts were expanded into a large multidisciplinary study of the Charles¬ ton earthquake by the Geological Survey funded largely by the NRC, Office of Nuclear Regulatory Research under Agreement No. AT(49-25)-1000. The studies include the operation of an enlarged seismographic network, a wide variety of geophysi¬ cal studies, a geological program of mapping surface and near surface features, a deep-drilling program, a geochemical and paleontologic study of appropri¬ ate samples, and detailed and regional synthesis of the results. This research is continuing today. This volume is a preliminary report of these and some related studies. Most of the papers reflect data gathered before June 1976. Work in progress and future work undoubtedly will modify some of the conclusions, but should not change the data presented. In this introductory chapter, I have tried to pro¬ vide a framework into which the individual studies fit and to summarize the more significant findings of the studies. Some promising directions of continued research are suggested by what these studies have shown. I am indebted to Carl Wentworth as well as the members of the various Charleston projects for numerous lengthy discussions concerning the cause of the seismicity of the Charleston area. SEISMICITY A commonly held view is that the 1886 event took place in an area that had been essentially aseismic for nqprly two centuries. An archival study by Bol¬ linger and Visvanathan (this volume) reports 18 probable earthquakes in South Carolina between 1698 and 1886. Of these, 13 appear to have been in the Charleston area. The maximum intensity of the pre-1886 events appears to be V or possibly VI. Bollinger and Visvanathan (this volume) conclude that although South Carolina was not aseismic in the I 4 STUDIES RELATED TO CHARLESTON', SOUTH CAROLINA, EARTHQUAKE OF 1886 c u O c; o o -*-> 73 <13 ' x: “ 55 £ *3 O CJ fc* -*-> ^ c .73 o 3) o ” x o =5 A >» -j « ,, .ti — — r* — CO O 2 -3 X. O 73 ~r -n — * 73 *o •" c 3 ^ c w c rz c D Tf c C 3 O 3 *D 2 C5 a • • rz >> 5 o> f CO a > s o - Ci fi. 3 ’D - s ~z H —> —> '5 .5 73 c O tf ZJ J o *3 VI M o t: a. « « u i; 43 73 sr 3 Ci £ - £ . <3 fc- «zi g 73 C« ® -r «8 £ - ^ ^ D W ^ X -C C 5) *> * -C #, 73 . %—( r i "3 C t-i 73 ° '£ • oo o> - J g C e s «^ S = £ U in w z of 3> 3 o 1 i) -u-> c —- 3 2 iT 3 <—< c £ — O . 08 w c. M o 3 C BIS > u c; o pS •*“ 73 w — 3 ’31 ear g ZJ cc C VI w « - u ^ w 3 ~ : ^ 1 ~ zjS g? "* O ■-■ CJ C *0 Sf £3 *5 5 £ 15 ^5 x c j a £ C x £ * _ £ ~ 0) _3 > V — j > 3 z g n -3 a 73 > ti C3 •-*- 2 O & 0 08 73 73 3 2 3 > 08 — 73 1> 3 S £ 7J D 0> t£ t: d: d w ^ w £- . ~ 73 C J? *"* O 2 •—» 2 5 _ — 2 tj 2 c ^ ® © •— I . a S ^15 > § .5 ~ >» w u 3 £ .t? 5 a. 5 £ 3 3 C o - .- -x INTRODUCTION AND DISCUSSION 5 50-year period prior to 1886, the seismic activity does not appear to have been anomalously high rela¬ tive to the surrounding States either in number of events or in energy levels. It should be pointed out, however, that their catalog lists nine events in South Carolina during this 50-year period, eight of which are approximately located in the vicinity of a spe¬ cific town. Four of these were in the Charleston area. The 1886 earthquake was followed by a series of aftershocks which may still be underway today. Of 435 or more earthquakes reported to have taken place in South Carolina between 1754 and 1975, more than 300 were aftershocks in the first 35 years fol¬ lowing 1886 (Tarr, this volume). The 1886 earthquake and its aftershocks dominate the seismic record of the southeast. The seismotec- tonic map of Hadley and Devine (1974) based upon the record from 1800 to 1972 depicts the Charleston area as having the highest concentration of epicen¬ ters and the largest single event in the southeastern United States. Several seismic frequency contours close around Charleston, yet these authors show the area to have no known geologic control for the seis¬ micity. On the other hand, on the basis of a 225-year record (1754-1970) of felt earthquakes in the south¬ ern Appalachians, Bollinger (1972) suggests a dif¬ fuse zone of seismicity trending NW. across South Carolina roughly perpendicular to the structural grain of the Appalachians. Whether the Charleston seismicity is part of a broad NW.-trending zone (Bollinger, 1972) or whether the seismicity origi¬ nates in an isolated source area (Hadley and Devine, 1974) is a question that has not been resolved. Most of the historical seismicity in the East, however, is associated spatially with the Appalachian orogen. As we shall see, Charleston is outside the Appalachian orogenic belt and is the locus of the only significant seismic energy release in the Coastal Plain province. The meizoseismal area of the 1886 earthquake has been variously drawn by several investigators, but has been most recently reinterpreted by Bollinger (this volume) as an elliptical area roughly 35 by 50 km trending northeast between Charleston and Jed- burg and including Summerville (fig. 2). This inter¬ pretation contrasts considerably with the dual epi- centrums of Dutton (1889). Middleton Place is ap¬ proximately in the center of Bollinger’s ellipse, about midway between the two epicenters shown by Dutton. The South Carolina seismographic network has recorded about 30 events in the South Carolina area between its inception in March 1973 and December 1975 (Carver and others, 1977). It should be noted that the 8-month period from March through Decem¬ ber 1973 was a reconnaissance study with 5 stations operating in the Charleston area. No stations op¬ erated during the period from January to May 1974, and 10 stations operated over a larger area in South Carolina between May 1974 and December 1975. Thirteen of the 30 events recorded are in the Charleston-Summerville area, mostly between Sum¬ merville and Middleton Place. Tarr (this volume) now refers to this area specifically as the Middleton Place-Summerville seismic zone, defined by a cluster of epicenters trending roughly NW. The hvpocenters are at approximate depths of 1 to 8 km. Tarr notes that the coordinates of the epicenters are well deter¬ mined, but that because none of the seismograph sta¬ tions of the 10-station network were close to the seis¬ mic activity, the depths of the hypocenters are not well determined. The largest event thus far recorded by the South Carolina seismographic network is a magnitude (m bLg ) 3.8 event on Nov. 22, 1974 very close to Middleton Place at a depth of 4.1 km (Tarr, this volume). Analysis of first-motion of this event indicates NE.-SW. compression acting on planes striking N. 42°W. and dipping either 12° NE. or 78° SW. Tarr (this volume, fig. 7) favors the plane dipping steeply to the SW. because the hypocenters of other events in the Middleton Place-Summerville seismic zone are closer to this plane. The historic record suggests that the Charleston- Summerville area had a continuum of low-level seis¬ micity prior to 1886 and that a low-level of strain energy release continues in the same area today. More specifically, Middleton Place is roughly in the center of the meizoseismal area of the 1886 event and at the SE. end of a zone, perhaps 15 km long, of continuing microearthquake activity. These recent events, in the depth range of 1 to 8 km, are thought to be either the continuation of the aftershock series or events located along closely related structural features as the result of the modification of the stress environment by the 1886 earthquake. For lack of contrary evidence here or elsewhere, we assume that the 1886 earthquake originated at similar depths. GEOLOGIC SETTING: THE ATLANTIC COASTAL PLAIN The Middleton Place-Summerville seismic zone is near the shoreline of the emerged part of the At¬ lantic Coastal Plain, which consists of an essentially eastward-thickening wedge of very gently seaward- 6 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 80°30' 15' 80° 33° 45' 32*30' MP + Middleton Place ®< XC t Clubhouse crossroads core hole 1 ®ccc; Clubhouse crossroads corehole 2 ®ccc 3 Clubhouse crossroads corehole 3 — ZOO - —200 - * ■ 1 TO O' — " EXPLANATION 0 sgs Seismograph station operating as of April 1977. Letters refer to specific stations (A C. Tart, written commun.. 1977) - 200 ——- Magnetic contour—Interval. 100 gammas Patterned between -200 and +400 as shown The lowest dosed contours are hachured. Data from U S Geological Survey (1975) - w - Bouguer gravity anomaly contour—Interval. 5 milligals. The lowest closed contours are hachured Data from Long and Champion (this volume) X—X--X Approximate meizoseismal area of the 1886 earthquake—From Bollinger (this volume). Dotted where particularly difficult to reconcile Dutton's base map of 1889 with modem maps + C*fB Charleston Air Force Base - 100 — - 200 — Earthquake epicenters, 1973-75 Moncks Comer Jedburg Walter boro^Y (0 MILES 5 to KILOMETERS Figure 2. —Map of the vicinity of Charleston, S.C., showing: the approximate location of the metzoseismal area of the 1886 earthquake in reference to the locations of coreholes, seismograph stations, magnetic contours, Bouguer gravity anom¬ alies, earthquake epicenters (1973-75), and geographic features. INTRODUCTION AND DISCUSSION 7 dipping unconsolidated and semi con soli dated sedi¬ mentary rocks. The wedge thickens from a feather edge against the crystalline rocks of the Appalachi¬ an orogenic belt along the Fall Line to thicknesses of more than 1 km near the coast in southeastern South Carolina (fig. 3). The rocks range in age from 36° 92° 32° 28° 0 100 200 MILES 1- T - L — 1- H 0 100 200 300 KILOMETERS iRYLANJ OHIO INDIANA ILLINOIS L'WEST VIRGINIA MISSOURI KENTUCKY NORTH CAROUNA ARKANSAS f \ / Segment of x >-Orangeburg scarp / ALABAMA Atlanta MISSISSIPPI DUNBARTON/C BASIN A\ (BURIED) C \ Savannah' Charleston > GEORGIA SOUTHEAST GEORGIA EMBAYMENT GULF COASTAL PLAIN LOUISIANA Jacksonville EXPLANATION EXPOSED ROCKS OF THE APPALACHIAN OROGEIC BELT GIVEN FLORIDA Edge of Coastal Plain sedimentary rocks Upper Triassifc and (or) Lower Jurassic rocks Piedmont Figure 3. —Map of the southeastern United States showing location of Charleston, S.C., relative to regional features. The Fall Line represents the downstream extent of crystal¬ line rock outcrops of the Piedmont and is roughly the same as the mapped edge of Coastal Plain rocks. Contour interval for submarine topography is 1,000 m. The east coast magnetic anomaly shown is from Schouten and Klitgord (1977) and Taylor and others (1968). The Blake Spur fracture zone is from Schouten and Klitgord (1977). 8 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Mesozoic to Holocene; the oldest rocks exposed at the surface decrease in age toward the coast. Most of the following description of the physi¬ ography of the Coastal Plain comes from Colquhoun and Johnson (1968). The Coastal Plain in South Carolina consists of three physiographic belts rough- i ly parallel to the Atlantic coast. These are referred to from northwest to southeast as the Upper, Middle, j and Lower Coastal Plains. The Upper Coastal Plain is a surface of fluvial or more rarely eolian erosion, which slopes irregularly from a maximum elevation of 150 to 180 m along the Fall Line to about 75 m on its southeastern side. It is separated from the Middle Coastal Plain by the Orangeburg scarp which has a relief of about 30 m across a distance of a few kilo¬ meters (fig. 3). The scarp is the locus of upper Miocene and Pliocene ( ?) shoreline deposits and coin¬ cides roughly with earlier Eocene shorelines as well (Colquhoun and Johnson, 1968). The Middle Coastal , Plain consists of a surface on which fluvial erosion has proceeded to the point where the primary depo- sitional topography, although present, is generally not obvious. The Charleston-Summerville area is within the Lower Coastal Plain. The surface of the Lower j Coastal Plain consists mainly of primary deposition- | al topography formed during the Pleistocene and Holocene. Larger landforms, such as barrier island chains and marsh surfaces, can be seen, particularly close to the present shoreline. As Colquhoun and Johnson (1968) point out, individual storm-beach ; ridges can be seen on aerial photographs and topo¬ graphic maps. Six barrier-beach systems (old shore¬ lines) have been recognized on the Lower Coastal Plain ; their landward surfaces rise to approximately 33, 21, 12, 8, 5, and 3 m. They have been named the Wicomico, Penholoway, Talbot, Pamlico, Princess Anne, and “Silver Bluff,” respectively. The oldest rocks known to date for which there is paleontologic control in the Coastal Plain of South Carolina are of Late Cretaceous (Cenomanian) age (Hazel and others, this volume), although at least one tectonic basin containing red terrigenous sedi¬ mentary rocks (Dunbarton basin, see below) is buried beneath the Upper Cretaceous rocks. Most of our detailed knowledge of the subsurface stratigra¬ phy in the Charleston-Summerville area comes from the continuously cored drill hole (Clubhouse Cross¬ roads corehole 1, hereafter called CCC 1) put down in 1974 and 1975 about 40 km west-northwest of Charleston as part of the Charleston studies (fig. 2). Results from this corehole not only provide detailed information on the stratigraphy of the Charleston- Summerville area but modify considerably the pre¬ viously held interpretations of the geologic history of the Coastal Plain, (see papers in this volume by Gohn and others, Hazel and others, and Gottfried and others). CCC 1 penetrated 750 m of Tertiary and Upper Cretaceous clastic and calcareous sedimentary rocks with good core recovery. The hole bottofned in 42 m of amygdaloidal basalt with excellent core recovery; the total depth was 792 m (Gohn and others, this volume). A general change in paleoenvironments is indicated from continental and marginal marine in the lower part of the Upper Cretaceous section to mostly marine in the upper Upper Cretaceous and younger section (Gohn and others, this volume, and Hazel and others, this volume). The change is not a simple transgression, however, but involves several transgressive-regressive cycles and four large time gaps. The most recent hiatus is within the Cooper Formation (the uppermost of the Tertiary units penetrated by CCC 1) composed of monotonous, bio- turbated marine deposits (Gohn and others, this vol¬ ume). The Cooper was deposited in an outer sublit¬ toral (outer shelf) or deeper environment, and the hiatus which spans the Eocene-Oligocene boundary represents approximately the early Oligocene (Hazel and others, this volume). Work in progress suggests that this hiatus is useful in mapping nearsurface ge¬ ology (L. M. Force, G. S. Gohn, B. B. Higgins, and Laurel Bybell, written and oral commun., 1976). One of the most significant results from the deep¬ drilling program has been the recovery and analysis of samples of lava flows of amygdaloidal basalt from the bottom of the corehole (CCC 1). Two flows have been identified. The interpretation by Gottfried and others (this volume) of the geochemistry of the basalt considerably constrains the various models proposed for the Mesozoic tectonic setting of Charleston. Analyses show that the basalt has under¬ gone slight to extreme oxidation, hydration, and hy¬ drothermal alteration. The effects of alteration are greatest near the margins of the flows, and the least altered rocks can be identified. Gottfried and others (this volume) report that the light rare earth ele¬ ment (REE) enriched pattern and low K Rb indi¬ cate an origin for the basalt from an undepleted source area in the upper mantle. The abundances of the REE, Ti, Zr and Nb, and the pattern of light REE enrichment are most similar to those obtained by other workers from the high-Ti quartz-normative tholeiites of Mesozoic age from eastern North America (ENA). INTRODUCTION AND DISCUSSION 9 These continental basalts were erupted during rifting and crustal extension in the early stages of continental breakup when North America separated from Africa. In eastern North America these basalts are of Late Triassic or Early Jurassic age (Johnson and McLaughlin, 1957, and Cornet and others, 1973). This age is consistent with the evidence from mag¬ netic anomaly patterns and deep-sea drilling for the initiation of the opening of the North Atlantic Ocean 180 m.y. ago (Pitman and Talwani, 1972; Vogt, 1973). K-Ar ages of 94.8±4.2 m.y. and 109±4 m.y. were obtained for the CCC 1 basalt by Richard Mar¬ vin, U.S. Geological Survey. Although the K-Ar ages are consistent with a Late Cretaceous (Cenomanian) age of the overlying Cape Fear Formation (Hazel and others, this volume), such a young age for the corehole basalt poses problems. By the beginning of Late Cretaceous time, the North Atlantic Ocean was already a significant feature and Charleston would have been on the order of 1,600 km from the Mid- Atlantic spreading center (see Pitman and Talwani, 1972). Gottfried and others (this volume) note that a magma-generating regime in the Charleston area 100 m.y. ago would almost certainly have been dif¬ ferent from that of 180 m.y. ago which produced spacially related ENA tholeiitic basalts. Because of the close geochemical sirhilarities of the corehole basalt to the ENA tholeiitic basalts, they conclude that these basalts are related in time as well as space. If that is true, a buried Triassic and (or) Jurassic basin may be present in the Charleston-Summerville area. Because of the documented chemical alteration of the corehole basalt, the K-Ar ages must be considered minimum ages and permissive of an original Tri¬ assic or Jurassic age of the basalt (Gottfried and others, this volume). The alteration and minimum ages may reflect postmagmatic processes associated with Cretaceous (?) tectonic activity. Sass and Ziagos (this volume) report on tem¬ perature measurements made in CCC 1 at 3-m inter¬ vals from the surface to the lowest depth of the hole. They obtain an average heat flow value of 1.3 ±0.12 HFU (1 Heat-Flow-Unit (HFU)=10 -6 cal cm -2 sec -1 ). They note that this value is within the range of other values measured in eastern ■ United States and that no thermal anomaly is associated with the Charleston-Summerville area. In the Upper Coastal Plain in the area of the Savannah River, a buried graben containing terres¬ trial redbeds has been documented in the subsurface by extensive drilling and geophysical surveys, and has been named the Dunbarton basin by Marine and ■ Siple (1974) (fig. 3). The basin is about 10 km wide and 50 km long and is wholly overlain by the Tusca¬ loosa Formation of Late Cretaceous age. As much as 903 m of maroon mudstone, sandstone of fluvial origin and fanglomerate fill the basin; no basalt flows were penetrated in the drilling. No fossils have been recovered from the basin drill holes, but by comparison with graben containing dated redbeds that are exposed elsewhere in the east, Marine and Siple (1974) suggest that the redbeds of the Dun¬ barton basin are of Triassic age. GEOLOGY BENEATH COASTAL PLAIN ROCKS AND TECTONIC SETTING The basement surface upon which the Coastal Plain sediments were deposited now dips gently sea¬ ward, on the average, but it is deformed by several transverse structures and contains at least one Meso¬ zoic graben (the Dunbarton basin). The most im¬ portant of these transverse structures in the south- | east are the Cape Fear arch near the North Carolina- South Carolina border and the Peninsular arch (also called the Ocala arch or uplift) of Florida (fig. 3). These two arches are separated by the Southeast Georgia embayment. As summarized by Owens (1970), the Cape Fear arch divides the Atlantic Coastal Plain into two large poorly-defined sedi¬ mentary’ basins. Whereas glauconite-rich clastic rocks are dominant in the emerged Coastal Plain north of the arch, carbonate rocks are increasingly important southward and culminate in the Florida carbonate platform. The carbonate section is rela¬ tively thin over the Peninsula arch. A further differ¬ ence in the basins is that Lower Cretaceous rocks are abundant in the Coastal Plain north of the Cape Fear arch, but, as noted in the previous section, have not been identified in South Carolina. Over the Cape Fear arch, rocks as old as Cre¬ taceous have been planed off by marine erosion which took place as early as late Miocene and which continued into the Pliocene and Pleistocene (Colqu- houn and Johnson, 1968). Recent work by Winker and Howard (1977) has shown that the conventional wisdom of correlating barrier-beach systems (old shorelines) by elevation above present sea level (for example, Colquhoun and Johnson, 1968) is not valid. The old shorelines have been deformed into the Plio¬ cene and Pleistocene by persistent Cenozoic struc¬ tural features such as the Cape Fear arch and the Peninsular arch. A study of the deformation of these old shorelines offers one of the most promising ap- 10 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 proaches to the elucidation of Quaternary deforma¬ tion in the Charleston-Summerville area. The basement rocks in which the present seismici¬ ty' takes place and in which the 1886 earthquake is presumed to have originated are not directly accessi¬ ble for observation because of the Coastal Plain sedi¬ mentary cover. These basement rocks traditionally have been considered to be an extension of the meta- morphic rocks of the Piedmont province exposed northwest of the feather edge of the Coastal Plain section (Rodgers, 1970). Aeromagnetic data suggest that this is not true. The basement beneath the Coastal Plain in the Charleston-Summerville area differs from the metamorphic terrane of the Pied¬ mont in having a relatively smooth and continuous low-amplitude magnetic field which Popenoe and Zietz (this volume) suggest represents undeformed tuffaceous clastic rocks intermixed with basaltic and rhyolitic flows and ash-fall deposits. High-amplitude, steep-gradient, generally circular magnetic and gravity positive anomalies (fig. 2) within and ad¬ jacent to this basement block are interpreted to re¬ flect mafic or ultramafic plutons in the basement (see papers by Popenoe and Zietz, Phillips, Long and Champion, and Kane, this volume). For convenience, this distinctive basement terrane is hereafter called the Charleston block. All of the area shown on figure 2 is thought to be within the Charleston block. The extent and boundaries of the Charleston block are not well known, but it does appear to underlie a sizeable area of the emerged and submerged Coastal Plain. The Orangeburg scarp (fig. 3) appears to coincide with the northwestern boundary of the Charleston block and may be structurally controlled (Popenoe and Zietz, this volume; Higgins and oth¬ ers, 1976). A possible delineation of the other bound¬ aries of the Charleston block is presented in the paper by Popenoe and Zietz (this volume). Why did the 1886 earthquake occur in the Charles¬ ton-Summerville area rather than elsewhere in the Charleston block ? Is it reasonable, in fact, to restrict the probability of a recurrence of an 1886 earth¬ quake to the Charleston block at all? Clearly, we need to know more about the Charleston block and about the nature and location of the boundaries of this block. In southwesternmost Georgia and northern Flori¬ da the mainly carbonate rocks of the Coastal Plain are underlain by nonfolded and nonmetamorphosed clastic rocks containing fossils ranging in age from Early Ordovician to probably Middle Devonian (Rodgers, 1970, and references therein). These fos- siliferous Paleozoic rocks are clearly not part of the Appalachian orogenic belt. They have not been de¬ formed by Appalachian otogenic events and they contain a pelecypod fauna that is closest to that of central Bohemia and Poland but that also has simi¬ larities to that of Nova Scotia, North Africa, and South America (Pojeta and others, 1976). Florida may thus represent a fragment of Africa (Rodgers, 1970) that was attached to North America during the closing of the late Precambrian and early Paleo¬ zoic Iapetus Ocean (Odom and Brown, 1976) and was then left behind during the opening of the pres¬ ent Atlantic Ocean basin (Rodgers, 1970). The Charleston block separates Florida from the Ap¬ palachian orogenic belt. As suggested by Popenoe and Zietz (this volume) and Long and Lowell (1973), the Charleston block basement may have formed as a zone of Mesozoic crutsal extension simi¬ lar to the exposed Triassic and (or) Jurassic basins but much larger. This zone of extension is quite broad, perhaps as wide as 100 to 200 km, and is pre¬ sumably related to the initial stages of the Mesozoic opening of the present Atlantic. The extension was apparently accompanied by the extrusion of conti¬ nental tholeiitic lava such as that penetrated in CCC 1, and probably by the intrusion of large mafic plutons. Charleston probably shared this general Mesozoic extensional setting with New Madrid. The zone of extension at New Mardrid can reasonably be called an aulocogen or failed-arm trough (Burke and Dewey, 1973), the location of which may have been controlled by an even earlier failed-arm trough (Ervin and McGinnis, 1975). The zone of extension, if that is what it is, occupied by the Charleston block, is more analogous to, but much larger than, the ex¬ posed Triassic and (of) Jurassic basins in eastern North America. THE SOURCE AREA: METHOD OF APPROACH An adequate explanation of the Charleston earth¬ quake of 1886 must include detailed studies of the source area. Earthquakes in the area are presum¬ ably caused by the sudden release of gradually ac¬ cumulated strain by faulting as no active volcanoes are present in the Coastal Plain. A first step in the study of the source area is, therefore, to identify and analyse faults. The history of fault movement through time, as well as the recency of movement must be determined, and this can be done only through study of the Coastal Plain section which records younger geologic events. From an under¬ standing of the patterns of behavior of the fault through time as well as of changes in these patterns INTRODUCTION AND DISCUSSION 11 it may be possible to place limits on the likely future behavior of the faults. It may be possible to estab¬ lish a crude recurrence interval of movement for a given fault. It may be possible to establish the proba¬ bility of the length of a fault that would break in a single event and to predict the geometry of that movement. The modern regional stress field should be estab¬ lished from an analysis of the most recently formed structures and from in situ stress measurements. In unraveling these complex relations, a thorough un¬ derstanding of the geologic history is necessary; the structure, structural history, and stress field must make sense in terms of the geologic history of the area. A next step in understanding the source area is to characterize the modern seismicity including earth¬ quake distribution and focal mechanisms. Only then, but not necessarily then, can one hope to relate the seismicity to a given fault or fault system. A reevaluation of the historical record shows that Middleton Place is roughly in the center of the meizoseismal area of the 1886 earthquake (Bol¬ linger, this volume). Seismicity continues in the same area today (the Middleton Place-Summerville seismic zone), and is either a continuation of the aftershock series or strain-energy release along structural features closely related to the origins of the 1886 event (Tarr, this volume). Most, if not all, of this recent seismicity originates in the basement beneath the Coastal Plain sedimentary rocks. Observations of the basement may be made only by drilling and by various geophysical measure¬ ments. Drilling is extremely expensive but the infor¬ mation obtained is indispensible for calibrating the geophysical measurements as well as for the obvious direct geologic returns discussed earlier. Interpreta¬ tion of the basement geology involves the construc¬ tion of geologically reasonable models from the syn¬ thesis of the geophysical measurements, from drill¬ hole data (including extrapolation from the petrolo¬ gy of the cores) and from an understanding of the geology of the eastern seaboard. Structures identified in the basement must be traced upward into the youngest rocks possible in order to determine as much as possible about the history and recurrence of movement and the ge¬ ometry of movement. The interpretation of the deep¬ er Coastal Plain geology involves the same process of constructing geologically reasonable models from drill-hole data and geophysics as in the interpreta¬ tion of the basement geology. In addition, informa¬ tion on the biostratigraphy and lithostratigraphy is provided from the drill holes. For the shallow sub¬ surface, a wealth of information can be obtained from shallow drilling. Given a mappable geophysical horizon and the control from shallow drilling, a de¬ finitive map of the shallow structure should be ob¬ tainable. These studies are underway. Geologic map¬ ping of surface exposures will provide data on the Quaternary history and information to guide shal¬ low drilling and shallow geophysical exploration and interpretation. Study of the raised and deformed barrier-beach systems are underway, and should contribute to the understanding of the Quaternary structural history. Along with this, a review of available geodetic leveling may provide information on any modern crustal movement. The seismicity monitored by the South Carolina Seismographic Network must be related to basement structures as constrained by structures in Coastal Plain rocks. In the long run, this may be the most difficult as well as the most relevant task. Finally, the stress axes measured in drill holes by hydrofrac¬ turing must be related to stress axes inferred from focal mechanisms. STUDIES OF THE MEIZOSEISMAL AREA Clubhouse Crossroads corehole 1 w’as drilled over the center of the largest of the positive magnetic anomalies in the meizoseismal area (fig. 2). This magnetic anomaly coincides with a positive Bouguer gravity anomaly described by Long and Champion (this volume). Some of the numerous findings re¬ sulting from this drilling have already been cited and reported in this volume in articles by Gohn and others, Hazel and others, and Gottfried' and others. Unfortunately, the corehole drilling had to be aban¬ doned in the basalt and did not reach the body caus¬ ing the anomaly and (or) crystalline basement. Ad¬ ditional deep drilling is in progress, in part to sample the source of that geophysical anomaly. 1 Figure 2 1 Additional information has been received since this report was written. Clubhouse Crossroads corehole 3 (CCC 3), the deepest of the drill holes, reached a total depth of 1,152 m (3,780 ft) before a broken drill rod on May 19, 1977, forced abandonment of the drilling operation (see fig. 2). The drL’l penetrated basalt at 774 m (2,540 ft), passed through a thick section of it that contained one or two thin sedimentary rock inter¬ layers, and entered red sandstone and shale at 1,031 m (3,384 ft) (Gohn, oral commun., 1977). No fossils have yet been recovered from * these sedimentary rocks, but they do resemble rocks from exposed Triassic and (or) Jurassic basins. Thus, at CCC 3, 257 m of basalt are underlain by at least 121 m of red, probably terrestrial, clastic rocks. The vertical electrical sounding (VES) data which suggest that the basalt near CCC 1 and CCC 3 is less than 75 m thick (Campbell, this volume) is in obvious conflict with the drill-hole findings. The cause of this discrepancy is under investigation. Ackermann’s (this volume) cal¬ culations of basement depths were made assuming a thin basalt layer. These depth estimates must now also be increased, at least in the vicinity of CCC 3. 12 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 shows in a simplified way the geographic relation¬ ship between many of the features discussed in the meizoseismal area in this and other articles. Phillips (this volume) presents the results of automated depth analysis performed on the aero- magnetic data. His work suggests that tops of most of the mafic plutons presumed to cause the pro¬ nounced magnetic anomalies are at a depth of 2.5 to 3.5 km and probably extend to depths of 4.5 km. His analysis also suggests that the crystalline base¬ ment is at a depth of 0.6 to 1.5 km. Hence, the mafic plutons are within the crystalline basement. The mafic body causing the large anomaly at CCC 1 appears to extend up to the top of the crystalline basement and is probably the only mafic pluton in the area which does so. The east-west magnetic low under Middleton Place is modeled as a nonmagnetic zone within the crystalline basement. Phillips sug¬ gests that this could be an altered zone along a fault, but also discusses the possibility that the magnetic pattern could be caused by reversed polarity of basalt lavas. The presence of reversely polarized basalts would also modify some other interpreta¬ tions, and these complications are recognized by Phillips. Ackermann (this volume) reports on the results of seismic refraction studies in the meizoseismal area, designed for full reverse coverage from a base¬ ment horizon 600 to 1,000 m deep, and partial re¬ verse coverage from shallower horizons. Of three refracting horizons discovered, the shallowest cor¬ responds to a thin well-indurated calcarenite at the base of the Santee Limestone. This horizon, which is about 100 m deep in CCC 1, may prove useful for mapping structures in the shallow subsurface. The intermediate refracting horizon corresponds to the top of the basalt at a depth of 750 m in the core¬ hole. Ackermann determined this horizon had seis¬ mic velocities which range from 5.8 km/s at the corehole to 4.3 km s at Middleton Place. He sug¬ gests that the velocity decrease could be caused either by increased fracture porosity or by a termi¬ nation of the basalt layer and notes that the lowest velocity coincides with the epicentral area of the Nov. 22, 1974, earthquake. The surface of the in¬ termediate refracting layer shows a broad trough¬ like depression trending N. or NNW. (Ackermann, this volume, fig. 4). The trough could be more than 50 m deep and is several kilometers west of Middle- ton Place. Arrivals from the basalt layer, the intermediate refracting layer (assumed to be basalt), are shingled suggesting that this relatively high-veloc¬ ity layer is interlayered in a lower velocity sequence. One interpretation of this is that the basalt is under¬ lain by sedimentary rock. Ackermann calculates the depth to the lowest refracting horizon, which he interprets as high-velocity crystalline basement (6.3 to 6.5 km s) using as a model a zone of constant velocity (4.2 km s) between the basalt and the base¬ ment. Under this assumption, the basalt and base¬ ment horizons appear to diverge toward the south¬ east, with crystalline basement at -a depth of about 900 m at CCC 1 but at a depth of 2,000 m some 25 km to the ESE. (Ackermann, this volume, fig. 5). The basement surface, so calculated, appears to slope more steeply in the southeast part of the sur¬ veyed area and Ackermann suggests that this may be a flexure or a fault. Talwani (this volume) reports on the results of monitoring quarry blasts by portable seismographs in the area between Columbia and Georgetown, S.C. By combining data from monitoring blasts in the Berkeley quarry (about 35 km north of Summer¬ ville), data from seismic-refraction lines of Acker¬ mann (this volume and unpub. data, 1977), regional Bouguer gravity, and densities of samples from CCC 1, Talwani (this volume) has calculated three possible crustal models between the Berkeley quarry and the vicinity of Middleton Place. Two of the seismic-refraction lines used by Talwani in con¬ straining his model were run by Ackermann after he had submitted his manuscript for this volume. Campbell (this volume) reports on the results of 18 audio-frequency magnetotelluric (AMT) sound¬ ings and 9 Schlumberger d.c. resistivity soundings (vertical electrical soundings = VES) in the meizo¬ seismal area. The most significant results to date are from the latter. In analysing the data, the re¬ sistivity of the electrical basement is assumed to be high with respect to that of the overlying sedimen¬ tary rocks; a value of 200 ohm-m is arbitrarily chosen. None of the VES showed a high-resistivity layer near the 750 m depth analogous to the basalt in CCC 1. Campbell argues that the basalt is, there¬ fore, less than 75 m thick and is underlain by low- resistivity material. These data suggests that near the corehole the electrical basement is at 1,300 m and that the basalt is underlain by about half a kilo¬ meter of sedimentary rock. Three VES soundings in the triangle between CCC 1, Summerville, and Middleton Place show an electrical basement at 900 m, significantly shallower than the 1,100- to 1,300-m-deep electrical basement elsewhere. These are soundings VES 2, 4, and 7 shown as Campbell’s figure 2. Campbell suggests INTRODUCTION AND DISCUSSION 13 that this shallower highly resistive horizon (the deepest observed at those locations) may reflect thickened basalt rather than crystalline basement. If true, the basalt is not only thicker, but also deeper (down-faulted or down-bent) on the southeast side of a line trending northeast between CCC 1 and Summerville. Long and Champion (this volume) speculate a similar down-dropped basin (Triassic?) on the basis of the gravity data. EARTHQUAKE ORIGINS The present seismicity which takes place along a NW.-SE. zone between Summerville and Middleton Place, is either part of the aftershock series of the 1886 earthquake or strain release along structures related to that earthquake. This seismicity is orig¬ inating in the basement at depths significantly deeper than the basalt in CCC 1. The largest event so far recorded (Nov. 22, 1974) was at the SE. end of this zone under Middleton Place, also essentially the center of the meizoseismal area of the 1886 event. The focal mechanism for this event suggests compression along a NE.-SW. axis. One of the nodal planes determined for this event is consistent with nearly vertical (perhaps reverse) faulting on a plane striking N. 42° W. (Tarr, this volume). The suggested NE.-SW. compression is consistent with the pattern of compressive stress found by Sbar and Sykes (1973) for eastern North America. On the basis of this, the stress environ¬ ment of Charleston appears to have changed from the Mesozoic stress environment w r hich involved ex¬ tension on a NE.-SW. axis. Sbar and Sykes (1973) further speculate that the Charleston-Summerville seismicity may be localized at the continental mar¬ gin along the landward projection of the Blake Spur fracture zone (fig. 3). This hypothesis should cer¬ tainly be pursued. As yet, no geologic feature has been recognized in the exposed crystalline rocks of the Piedmont that can be related to the fracture zone such as the train of alkalic plutons of the White Mountain Plutonic Series in New 7 England which has been suggested as the continental pro¬ jection of the Kelvin Sea Mount chain (Diment and others, 1972, and Sbar and Sykes, 1973). Kane (this volume) notes a correlation between Bouguer gravity highs and seven w 7 ell-defined east¬ ern North American earthquake regions including Charleston, New Madrid, and Cape Ann. The gravity highs are interpreted as being caused by mafic or ultramafic plutons. He notes that the major seismic activity in each region does not coincide with the position of the gravity anomalies but is peripheral to them. He suggests that the plutons may act to concentrate regional stress around their peripheries —perhaps through plastic deformation of serpentin- ized rocks, a concept derived from the hole-in-plate problem of mechanics. A somew 7 hat similar hy¬ pothesis of stress amplification has been proposed by Long (1976) for the observed relationship be¬ tween mafic plutons (as deduced from the gravity field) and seismicity. His concept differs from Kane’s in that the inhomogeneities in the plate (the mafic plutons) are interpreted to be more rigid than the enclosing plate. CONCLUSIONS The more important concepts that are emerging from the work underway are as follows: 1. Seismic activity 7 is continuing today in the center of the meizoseismal area of the 1886 earth¬ quake at a higher level than prior to 1886. 2. The present seismic activity probably originates in the crystalline basement beneath the Coastal Plain sedimentary rocks. 3. The crystalline basement beneath the Charles¬ ton-Summerville area is not simply a seaw 7 ard • extension of crystalline rocks exposed in the Piedmont. 4. The Charleston block may represent a broad zone of Triassic and Jurassic crustal extension formed during the early stages of the opening of the Atlantic Ocean. 5. The present stress regime appears to be one of NE.-SW. compression rather than extension, and is similar to the stress regime of a large part of the eastern United States. 6. The structure in the Charleston-Summerville area is not a simple homocline of Coastal Plain sedimentary rocks dipping tow 7 ard the sea on a gently dipping basement surface. 7. Various geophysical surveys yield interpreta¬ tions that are not yet consistent. 8. Geologic mapping is incomplete but should yield valuable results. 9. Studies of one large eastern earthquake area may yield results that are useful to studies of others; for example, the association of seis¬ micity with the margins of positive gravity anomalies. The cause of the Charleston-Summerville seismic¬ ity 7 is still not determined. We certainly know 7 a great deal more about the area than w 7 e did a few 7 years ago and can begin to draw 7 some constraints around some of the possibilities. The papers in this volume 14 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 report on the first stage of an ongoing study. If the seismicity is to be understood, the answer will come from the multidisciplinary approach we are trying to pursue. Such detailed geologic and geophysical studies seeking to understand the geologic history and current tectonic regime of a seismic source area are very few and for intraplate sources areas, repre¬ sent a new dimension in seismic analysis. REFERENCES CITED Allen. C. R., Hanks, T. C., and Whitcomb, J. H., 1973, San j Fernando Earthquake: seismological studies and their ! tectonic implications, in U.S. Natl. Oceanic and Atmos- ! pheric Admin., San Fernando, California, earthquake of ] February 9, 1971: Washington, D.C., U.S. Govt. Printing Office, v. 3, p. 13-21. Bollinger, G. A., 1972, Historical and recent seismic activity in South Carolina: Seismol. Soc. America Bull., v. 62, no. 3, p. 851-864. Burke, Kevin, and Dewey, J. F., 1973, Plume-generated triple junctions: key indicators in applying plate tec¬ tonics to old rocks: Jour. Geology, v. 81, no. 4, p. 406- j 433. Carver, David, Turner, L. M., and Tarr, A. C., 1977, South I Carolina seismological data report May 1974—June 1975: | LLS. Geol. Survey open-file rept. 77—429, 66 p. Colquhoun, D. J., and Johnson, H. S., Jr., 1968, Tertiary sea-level fluctuation in South Carolina: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 5, no. 1, p. 105-126. Cornet, Bruce, Traverse, Alfred, and McDonald, N. G., 1973, Fossil spores, pollen, and fishes from Connecticut indi¬ cate Early Jurassic age for part of the Newark Group: Science, v. 186, p. 1243-1247. Dewey, J. F., and Bird, J. M., 1970, Mountain belts and the new global tectonics: Jour. Geophys. Research, v. 75, j no. 14, p. 2625-2647. Diment, W. H., Urban, T. C., and Revetta, F. A., 1972, Some geophysical anomalies in the eastern United States, in Robertson, E. C., ed.. The nature of the solid earth: | New York, McGraw-Hill Book Co., p. 544-572. Dutton, C. E., 1889, The Charleston earthquake of August j 31, 1886: U.S. Geol. Survey, Ninth Ann. Rept. 1887-88, j p. 203-528. Ervin, C. P., and McGinnis, L. D., 1975, Reelfoot Rift; reac¬ tivated precursor to the Mississippi Embayment: Geol. Soc. America Bull., v. 86, no. 9, p. 1287-1295. Evemden, J. F., 1975, Seismic intensities, “size” of earth¬ quakes and related parameters: Seismol. Soc. America Bull., v. 65, no. 5, p. 1287-1313. - 1976, A reply to “Comments on ‘Seismic intensities, “size” of earthquakes and related parameters’ by Otto ! W. Nuttli”: Seismol. Soc. America Bull., v. 66, p. 339- 340. Gupta, I. N., and Nuttli, O. W., 1976, Spatial attenuation of intensities for central U. S. earthquakes: Seismol. Soc. America Bull., v. 66, no. 3, p. 743-751. Hadley, J. B., and Devine, J. F., 1974, Seismotectonic map of j the eastern United States: U.S. Geol. Survey Misc. , Field Studies Map MF-620. Higgins, B. B., Owens, J. P., Popenoe, Peter, and Gohn, G. S., 1976, Structures in the Coastal Plain of South Carolina: Geol. Soc. America Abstracts with Programs, v. 8, no. 2, p. 195-196. Johnson, M. E., and McLaughlin, D. B., 1957, Trtassic forma¬ tions in the Delaware Valley: Geol. Soc. America Guide¬ book for field trips, Atlantic City 1957, Field Trip No. 2, p. 31-56. Long, L. T., 1976, Speculations concerning southeastern earthquakes, mafic intrusions, gravity anomalies, and stress amplification: Earthquake Notes, v. 47, p. 29-35. Long, L. T., and Lowell, R. P„ 1973, Thermal model for some continental margin sedimentary basins and up¬ lift zones: Geology, v. 1, no. 2, p. 87-88. Marine, I. W., and Siple, G. E., 1974, Buried Triassic basin in the central Savannah River area, South Carolina and Georgia: Geol. Soc. America Bull., v. 85, no. 2, p. 311— 320. Mosaic, 1976, Quakes in search of a theory: Mosaic, v. 7, no. 4, p. 2-11. Nuttli, O. W., 1973, The Mississippi Valley earthquakes of 1811 and 1812; intensities, ground motion and magni¬ tudes: Seismol. Soc. America Bull., v. 63, no. 1, p. 227- 248. - 1976, Comments on “Seismic intensities, ‘size’ of earthquakes and related parameters” by Jack F. Evernden: Seismol. Soc. America Bull., v. 66, no. 1, p. 331-340. Odom, A. L., and Brown, J. F., 1976, Was Florida a part of North America in the lower Paleozoic?: Geol. Soc. America Abs. with Programs, v. 8, no. 2, p. 237-238. Owens, J. P., 1970, Post-Triassic tectonic movements in the central and southern Appalachians as recorded by sedi¬ ments of the Atlantic Coastal Plain, in Fisher, G. W., and others, eds., Studies of Appalachian geology, central and southern: New York, Intersci. Publishers, p. 417- 427. Pitman, W. C., Ill, and Talwani, Manik, 1972, Sea-floor spreading in the North Atlantic: Geol. Soc. America Bull., v. 83, no. 3, p. 619-646. Pojeta, John, Jr., Kriz, Jin, and Berdan, J. M., 1976, Silu- rian-Devonian pelocypods and Paleozoic stratigraphy of subsurface rocks in Florida and Georgia and related Silurian pelecypods from Bolivia and Turkey: U.S. Geol. Survey Prof. Paper 879, 32 p. Rodgers, John, 1970, The tectonics of the Appalachians: New York, Intersci. Publishers, 271 p. Sbar, M. L., and Sykes, L. R., 1973, Contemporary compres¬ sive stress and seismicity in eastern North America: An example of intraplate tectonics: Geol. Soc. America Bull., v. 84, no. 6, p. 1861-1881. Schouten, Hans, and Klitgord, K. D., 1977, Mesozoic mag¬ netic anomalies, western North Atlantic: U.S. Geol. Survey Misc. Field Studies Map MF-915. Scott, N. H., 1973, Felt area and intensity of San Fernando earthquake, in U.S. Natl. Oceanic and Atmospheric Admin., San Fernando, California, earthquake of Feb¬ ruary 9, 1971: Washington, D.C., U.S. Govt. Printing Office, v. 3, p. 23-48. Shaler. N. S., 1899, Aspects of the Earth: New York, Charles Scribner’s Sons, 344 p. Tarr, A. C., comp., 1974, World seismicity map: Reston, Va., U. S. Geol. Survey. INTRODUCTION AND DISCUSSION 15 Taylor, P. T., Zietz, Isidore, and Dennis, L. S., 1968, Geo¬ logic implications of aeromagnetic data for the eastern continental margin of the United States: Geophysics, v. 33, no. 5, p. 755-780. U.S. Geological Survey, 1975, Aeromagnetic map of Charles¬ ton and vicinity, South Carolina: U.S. Geol. Survey open-file map 75-590. Vogt, P. R., 1973, Early events in the opening of the North Atlantic, in Tarling, D. H., and Runcorn, S. K., eds., Implications of continental drift to the earth sciences, NATO Advance Study Institute, Newcastle-upon-Tyne, England (1972), v. 2, p. 693-712. Winker, C. D., and Howard, J. D., 1977, Correlation of tectonically deformed shorelines on the southern At¬ lantic Coastal Plain: Geology, v. 5, p. 123—127. Reinterpretation of the Intensity Data for the 1886 Charleston, South Carolina, Earthquake By G. A. BOLLINGER STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA. EARTHQUAKE OF 1.886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-B ■ • 3 < tV'fPl CONTENTS Page Abstract- 17 Introduction _ 17 Intensity effects in the epicentral region_ 18 Intensity effects throughout the country_ 22 Attenuation of intensity with epicentral distance_ 27 Magnitude estimate _ 29 Conclusions _ 31 References cited_ 31 ILLUSTRATIONS Page Figure 1. Epicentral area maps for the 1886 Charleston, S.C., earthquake _ 20 2. Isoseismal map showing the State of South Carolina for the 1886 Charleston earthquake- 22 3-5. Maps of the Eastern United States showing: 3. Distribution of intensity observations for the 1886 Charleston earthquake_ 23 4. Isoseismal map contoured to show the more localized variations in the reported intensities for the 1886 Charleston earthquake_ 24 5. Isoseismal map contoured to show the broad regional patterns of the reported intensities for the 1886 Charleston earthquake_ 25 6. Histogram showing distribution of intensity as a function of epicentral distance for the 1886 Charles¬ ton earthquake_ 28 7, 8. Graphs showing attenuation of intensity with epicentral distance for various fractiles of intensity at given distance intervals for the 1886 Charleston earthquake _28, 29 9. Histogram showing distribution of epicentral distances for given intensity levels of the 1886 Charles¬ ton earthquake_ 30 10. Graph showing body wave magnitude estimates for the 1886 Charleston earthquake based on Nuttli’s technique _ 31 TABLES Page Table 1. Variation of intensity effects along the South Carolina Railroad_ 21 2. Number of intensity observations as a function of epicentral distance intervals for the 1886 Charles¬ ton, S.C., earthquake _ 27 III . STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT REINTERPRETATION OF THE INTENSITY DATA FOR THE 1886 CHARLESTON, SOUTH CAROLINA, EARTHQUAKE By G. A. Bollinger 1 ABSTRACT In 1889, C. E. Dutton published all his basic intensity data for the 1886 Charleston, S.C., shock but did not list what intensity values he assigned to each report, nor did he show the distribution of the locations of these data re¬ ports on his isoseismal map. The writer and two other seis¬ mologists have each independently evaluated Dutton’s 1,300 in„ensit 3 reports (at least two of the three interpreters agreed on intensity values for 90 percent of the reports), and the consensus values were plotted and contoured. One map was prepared on which contours emphasized the broad regional pattern of effects (with results similar to Dutton’s); another map was contoured to depict the more localized variations of intensity. As expected, the latter map shows considerable detail in the epicentral region as well as in the far-field. In particular, intensity VI (Modified Mercalli (MM)) effects are noted as far away as central Alabama and the Illinois-Kentuckv-Tennessee border area. Dutton’s ; “low intensity zone” in West Virginia appears on both isoseismal maps. A maximum MM intensity of X for the epicentral region and IX for Charleston appears to be appropriate. Epicentral effects included at least 80 km of railroad track seriously damaged and more than 1,300 km 2 of extensive cratering and fissuring. In Charleston, the railroad-track damage and cratering were virtually absent, whereas many, but not most, buildings on both good and poor ground were de¬ stroyed. The epicentral distances to some 800 intensity-observa¬ tion localities were measured, and the resulting data set was analyzed by least-square regression procedures. The attenua¬ tion equation derived is similar to others published for dif¬ ferent parts of the eastern half of the United States. The technique of using intensity-distance pairs rather than isoseismal maps has the advantages, however, of com¬ pletely bypassing the subjective contouring step in the data handling and of being able to specify the particular fractile of the intensity data to be considered. When one uses intensities in the VI to X range, and their associated epicentral distances for this earthquake, body- wave magnitude estimates of 6.8 (Central United States in¬ tensity-velocity data published by Nuttli in 1976) and 7.1 1 Virginia Polytechnic Institute and State University, Blacksburg, Va. (Western United States intensity-velocity data published by Trifunac and Brady in 1975) are obtained. INTRODUCTION The problems associated with the description of seismic ground motion in a minor seismicity area i such as the Southeastern United States are well known. In that region, the largest events took place ; before instruments were available to record them, so | that only qualitative descriptions of their effects exist. During the past few decades, when instru¬ ments began to be used, no event having m b > 5 has taken place. Thus we have quantitative data only for small events, and we need to analyze the qualitative data, which are all that is available for larger events. The purpose of this study is to review thoroughly the data that do exist and to derive as much infor¬ mation as possible concerning regional seismic | ground motions. Fortunately, the largest earthquake known to have occurred in the region, the 1886 Charleston, S.C., earthquake, was well studied by ; Dutton (1889) and his coworkers. An excellent suite of intensity information is thus available for that im¬ portant earthquake. Secondly, the Worldwide Stand¬ ard Seismograph Network (WWSSN) stations in the Eastern United States provide data on the radia¬ tion from the regional earthquakes that have oc¬ curred since installation of the stations. Finally, intensity-particle-velocity relationships as well as attenuation values for various seismic phases have | been proposed that can be utilized in an attempt to synthesize the above data types. The initial part of this paper is concerned with a reevaluation of the intensity data for the 1886 Charleston earthquake, and the second part, with a consideration of the attenuation of intensity as dis¬ tance from the epicenter increases. (The distance 17 18 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 from the epicenter is hereafter called epicentral dis- | tance.) The concluding section presents a magnitude estimate for the 1886 shock. This research was conducted while the author was on study-research leave with the U.S. Geological Sur¬ vey (U.S.G.S.) in Golden, Colo. Thanks are extended to the members of the Survey, particularly Robin McGuire and David Perkins, for their many helpful discussions. Robin McGuire did the regression analy¬ sis presented in this paper, and Carl Stover pro¬ vided a plot program for the intensity data. Thanks are also due to Rutlage Brazee (National Oceano¬ graphic and Atmospheric Administration, N.O.A.A.) and Ruth Simon (U.S.G.S.) for interpreting the sizable amount of intensity data involved in this study. This research was sponsored in part by the Na¬ tional Science Foundation under grant No. DES 75- 14691. INTENSITY EFFECTS IN THE EPICENTRAL REGION Dutton assigned an intensity X as the maximum epicentral intensity for the 1886 shock. He used the Rossi-Forel scale; conversion to the Modified Mer- calli (MM) scale results in a X-XII value. However, the revised edition (through 1970) of the “Earth¬ quake History of the United States’’ (U.S. Environ¬ mental Data Service, 1973) downgraded Dutton’s value to a IX-X (MM). Because of this revision, it is appropriate to compare the scale differences be¬ tween these two intensity levels (IX and X) with the meizoseismal effects as presented by Dutton. Ground effects, such as cracks and fissures, and damage to structures increase from the intensity IX to the intensity X level, whereas damage to rails is first listed in the MM scale at the X level. Taken literally, rail damage is indicative of at least inten- sity-X-level shaking. Richter (1958, p. 138) also listed “Rails bent slightly” for the first time at in¬ tensity X. However, he instructed (p. 136) that, ! “Each effect is named at that level of intensity at which it first appears frequently and characteris- J tically. Each effect may be found less strongly, or in 1 fewer instances, at the next lower grade of intensity; more strongly or more often at the next higher grade.” Thus, widespread damage to rails is a firm indicator of intensity-X shaking. In discussing building damage, it is convenient to use Richter’s (1958, p. 136-137) masonry A, B, C, D classification: Masonry A. Good workmanship, mortar, and design; re¬ inforced, especially laterally, and bound together by using steel, concrete, etc.; designed to resist lateral forces. Masonry B. Good workmanship and mortar; reinforced, but not designed in detail to resist lateral forces. Masonry C. Ordinary workmanship and mortar; no ex¬ treme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces. Masonry D. Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally. j At the IX level, masonry D structures are destroyed, masonry' C structures are heavily damaged, some¬ times completely collapsed, and masonry B struc¬ tures are seriously damaged. Frame structures, if not bolted, are shifted off their foundations and have their frames racked at IX-level shaking, whereas at intensity X most such structures are destroyed. Nearly complete destruction of buildings up to and including those in the masonry B class is a charac¬ teristic of the intensity-X level. Only in Charleston do we have a valid sample of the range of structural damage caused by the 1886 earthquake. It was the only nearby large city, and it contained structural classes up to the range be¬ tween masonry C and masonry B. Many of the im¬ portant public buildings, as well as mansions and churches, had thick w r alls of rough handmade bricks joined with an especially strong oyster-shell-lime mortar. The workmanship was described as excel¬ lent, but nowhere in Dutton’s (1889) account is reference made to special reinforcement or design to resist lateral forces. Structures outside the Charleston area (as in Summerville, see p. 21) w r ere built on piers, some 1-2 m (3-6 ft) high, thereby making the structures inverted pendulums. Dutton’s report for Charleston indicates that although the damage was indeed extensive (see below), most masonry buildings and frame structures were not destroyed. This fact plus Dutton’s report on the absence of rail damage and extensive ground effects in the Charleston area indicates an intensity level of IX. The following quotations from Dutton’s report (1889, p. 248-249, 253) contain detailed descriptions of the structural damage in Charleston caused by the earthquake of 1886: There was not a building in the city which had wholly escaped injury, and very few had escaped serious injury. The extent of the damage varied greatly, ranging from total demolition down to the loss of chimney tops and the dislodgment of more or less plastering. The number of buildings which were completely demolished and leveled to the ground was not great. But there were several hundred which lost a large portion of their walls. There were very many also which remained standing, but so badly shattered REINTERPRETATION OF THE INTENSITY DATA 19 that public safety required that they should be pulled down altogether. There was not, so far as at present known, a brick or stone building which was not more or less cracked, and in most of them the cracks were a permanent disfigure¬ ment and a source of danger or inconvenience. A majority of them however were susceptible of repair by means of long bolts and tie-rods. But though the buildings might be made habitable and safe against any stresses that houses are liable to except fire and earthquake, the cracked walls, warped floors, distorted foundations, and patched plaster and stucco must remain as long as the buildings stand per¬ manent eye-sores and sources of inconveniences. As soon as measures were taken to repair damages the amount of in¬ jury disclosed was greater than had at first appeared. In¬ numerable cracks which had before been unnoticed made their appearance. The bricks had “worked” in the embedding mortar and the mortar was disintegrated. The foundations were found to be badly shaken and their solidity was great¬ ly impaired. Many buildings had suffered horizontal dis¬ placement; vertical supports were out of plumb; floors out of level; joints parted in the wood work; beams and joists badly wrenched and in some cases dislodged from their sockets. The wooden buildings in the northern part of the city usually exhibited externally few signs of the shaking they received except the loss of chimney tops. Some of them had been horizontally moved upon their brick foundations, but none were overthrown. Within these houses the injuries were of the same general nature as within those of brick, though upon the whole not quite so severe. The amount of injury varied much in different sections of the city from causes which seem to be attributable to the varying nature of the ground. The peninsula included be¬ tween the Cooper and Ashley Rivers, upon which Charleston is built, was originally an irregular tract of comparatively high and dry land, invaded at many points of its boundary by inlets of low swampy ground or salt marsh. These in¬ lets, as the city grew, were gradually filled up so as to be on about the same level as the higher ground. * * * As a general rule, though not without a considerable number of exceptions, the destruction was greater upon made ground than upon the original higher land. [p. 248-249] * * * In truth, there was no street in Charleston which did not receive injuries more or less similar to those just described. To mention them in detail would be wearisome and to no purpose. The general nature of the destruction may be summed up in comparatively few words. The destruction was not of that sweeping and unmitigated order which has be¬ fallen other cities, and in which every structure built of ma¬ terial other than wood has been either leveled completely to the earth in a chaos of broken rubble, beams, tiles, and planking, or left in a condition practically no better. On the I contrary, a great majority of houses were left in a condi¬ tion shattered indeed, but still .susceptible of being repaired. Undoubtedly there were very many which, if they alone had suffered, would never have been repaired at all, but would have been torn down and new structures built in their places; for no man likes to occupy a place of business which suf¬ fers by contrast with those of his equals. But when a com¬ mon calamity falls upon all, and by its very magnitude and ' universality renders it difficult to procure the means of re- i construction, and where thousands suffer much alike, his action will be different. Thus a very large number of build¬ ings were repaired which, if the injuries to them had been exceptional misfortunes instead of part of a common dis¬ aster, would have been replaced by new structures. Instances of total demolition were not common. This is probably due, in some measure, to the stronger and more enduring character of the buildings in comparison with the rubble and adobe work of those cities and villages which are famous chiefly for the calamities which have be¬ fallen them. Still the fact remains that the violence of the quaking at Charleston, as indicated by the havoc wrought, was decidely less than that which has brought ruin to other localities. The number of houses which escaped very serious injuries to their walls was rather large; but few are known to have escaped minor damages, such as small cracks, the loss of plastering, and broken chimney tops. [p. 253] Damage to the three railroad tracks that extend north, northwest, and southwest from Charleston be¬ gan about 6 km (3.7 mi) northwest of the city and was extensive (fig. 1A). More than 80 km (62 mi) of these tracks was affected. The effects listed were; lateral and vertical displacement, formation of S- shaped curves, and the longitudinal movement of hundreds of meters of track. A detailed listing of the effects along the South Carolina Railroad tracks, w r hich run northwest from Charleston directly through the epicentral region, is given in table 1. Ground cracks from wrhich mud or sand are ejected and in w r hich earthquake fountains or sand craters are formed begin on a small scale at intensity VIII, become notable at IX, and are large and spec¬ tacular phenomena at X (Richter, 1958, p. 139). The formation of sand craterlets and the ejection of sand were certainly widespread in the epicentral area of the 1886 earthquake. Many acres of ground were overflowed with sand, and craterlets as much as 6.4 m (21 ft) across were formed. Dutton (1889, p. 281) wrote: “Indeed, the Assuring of the ground within certain limits may be stated to have been universal, w r hile the extravasation of water w 7 as confined to cer¬ tain belts. The area within which these fissures may be said to have been a conspicuous and almost uni¬ versal phenomenon may be roughly estimated at nearly 600 square miles [1,550 sq. km].” By com¬ parison, the elliptical intensity-X contour suggested by the present study encloses an area of approxi¬ mately 1,300 km-. The distribution of craterlets taken from Dutton (1889, pi. 28) is also showm in figure 1A. In a few localities, the water from the craters probably spouted to heights of 4.5-6 m (15-20 ft), as indi¬ cated by sand and mud on the limbs and foliage of trees overhanging the craters. Other ground effects indicating the intensity-X level are fissures as much as a meter wide running parallel to canal and streambanks, and changes of 20 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 EXPLANATION *•"**■* Railroad track damaged ■ Building destroyed # MarkecPhonzontal displacement MP -f- Middleton Place O Craterlet area O Chimney destroyed 80 ° 30 ° Figure 1.—Epicentral area maps for the 1886 Charleston, S.C., earthquake. A, This study. Dashed contour encloses intensity-X effects. B, Dutton’s map and C, Sloan’s map (modified from Dutton, 1889, pis. 26 and 27, respectively) show contours enclosing the highest intensity zone, although neither Dutton nor Sloan labeled his contours. Base map modified from Dutton (1889). Rivers flowing past the Charleston peninsula are the Ashley River flowing from the northwest and the Cooper River flowing from the north. the water level in wells (Wood and Neuman, 1931). bank of the Ashley River about a meter wide and Dutton (1889, p. 298) reported that a series of wide some tens of meters long across the field of view of cracks opened parallel to the Ashley River, (see cap- the photograph. tion, fig. 1) and that the sliding of the bank river- In a belt of craterlets (trend N. 80° E., length ward uprooted several large trees, which fell over ~5 km) about 10 km (6.2 mi) southeast of Summer- into the water. His plate 23 shows a crack along the ville, Sloan reported (Dutton, 1889, p. 297) that REINTERPRETATION OF THE INTENSITY DATA 21 Table 1. —Variation of intensity effects along the South Carolina Railroad [Based on Dutton, 1889, p. 282-287. Refer to fig. 1 for locations mentionedj Distance from Charleston (km) (mi) <5.8_ <3.66 _ Occasional cracks in ground; no marked disturbance of track or roadbed. 5.8_ 3.66_ Rails notably bent and joints between rail opened. 5.8-8_ 3.66-5_ Ground cracks and small craterlets. 8 _ 5 _ Fishplates torn from fast¬ enings by shearing of the bolts; joints between rails opened to 17.5 cm (7 in.). 9.6 _ 6 _ Joints opened, roadbed per¬ manently depressed 15 cm (6 in.). 14.4_ 9 _ Lateral displacements of the track more frequent and greater in amount; serious flexure in the track that caused a train to derail; more and larger crater- lets. 16 _ 10_ Craterlets seemed to be greater in size (as much as 6.4 m (21 ft) across) and number; many acres overflowed with sand. 16-17.6_ 10-11 _ Maximum distortions and dislocations of the track; often displaced laterally and sometimes alternately depressed and elevated; occasional severe lateral flexures of double curva¬ ture and gTeat amount; many hundreds of meters of track shoved bodily to the southeast; track parted longitudinally, leaving gaps of 17.5 cm (7 in.) between rail ends; 46 cm (18 in.) depression or sink in roadbed over a 18-m (60-ft) length. 17.6—24_ 11-15 _ Many lateral deflections of the rails. I Table 1. —Variation of intensity effects along the South Carolina Railroad —Continued Distance from Charleston ( km ) ( mi ) Effects 33.9_ 21_ Tracks distorted laterally and vertically for a con¬ siderable distance. 34.9_ 21.66_ At Summerville—many flex¬ ures, one of which was a sharp S-shape; broken culvert under tracks in a sharp double curvature. 35.4-44.3 22-27.5 _ Disturbance to track and roadbed diminishes rapid¬ ly- 44.3_ 27.5_ At Jedburg—a severe buck¬ ling of the track. wells had been cracked in vertical planes from top to bottom, and that the wells had been almost uni¬ versally disturbed, many overflowing and subse¬ quently subsiding, others filling with sand or becom¬ ing muddy. In Summerville, whose population at that time was about 2,000, the structures were supported on wood posts or brick piers 1-2 m high and, though especial¬ ly susceptible to horizontal motions, the great ma¬ jority did not fall. Rather, the posts and piers were driven into the soil so that many houses settled in an inclined position or were displaced as much as 5 cm. Chimneys, which were constructed to be inde¬ pendent of the houses, generally had the part above the roofline dislodged and thrown to the ground. Be¬ low the roofs, many chimneys were crushed at their bases, both bricks and mortar being disintegrated and shattered, allowing the whole column to sink down through the floors. This absence of overturn¬ ing in piered structures plus the nature of the dam¬ age to chimneys was interpreted by Dutton as evi¬ 24—25.6 _ 15—16 _ Epicentral area—a few wooden sheds with brick chimneys completely col¬ lapsed; railroad alinement distorted by flexures; ele¬ vations and depressions, some of considerable amount, also produced. 29-30.6 _ 18.5-19_Flexures in track, one in an • 8.8-m (29-ft) section of single rails had an S-shape and more than 30 cm (12 in.) of distortion. 32 - ==20 _ “. . . a still more complex flexure was found. Beneath it was a culvert which had been strained to the north¬ west and broken” (p. 286); a long stretch of the road¬ bed and track distorted by many sinuous flexures of small amplitude. dence for predominantly vertical ground motions. The preceding discussion indicates an intensity-X level of shaking in the epicentral area. Figure 1 A depicts the approximate extent of this region along with the locations of rail damage, craterlet areas, building damage, and areas of marked horizontal displacements. Dutton and his coworkers did not map the regions of pronounced vertical-motion ef¬ fects, but they did emphasize the importance of these effects in the epicentral region. Also shown in figure 1 (B and C) is the extent of the highest intensity zone, as given by Dutton and by Sloan. Because of the sparsely settled and swampy nature of the region, the meizoseismal area cannot be defined accurately. 22 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 INTENSITY EFFECTS THROUGHOUT THE COUNTRY Dutton (1889) published all his intensity reports, some 1,337, but he did not list the intensity values that he assigned to each report, nor did he show the location of the data points on his isoseismal map. By using the basic data at hand, a reevaluation was at¬ tempted to present another interpretation of the data (in the MM scale) and to determine whether additional information could be extracted concern¬ ing this important earthquake. The writer and two other seismologists (Rutlage Brazee, N.O.A.A., and Ruth Simon, U.S.G.S.) each independently evaluated Dutton’s intensity data listing according to the MM scale. For the resulting 1,047 usable reports, ranging from MM level I to X, at least two of the three inter¬ preters agreed on intensity values for 90 percent of the reports. As would be expected, most of the dis¬ agreement was found at the lower intensity levels (II-V). A full listing of the three independent in¬ tensity assignments for each location was made by Bollinger and Stover (1976). The consensus values, or the average intensity values, in the 10 percent of the reports where all three interpreters disagreed were plotted at two dif¬ ferent map scales and contoured (figs. 2-5). When multiple reports were involved, for example, those from cities, the highest of the intensity values ob¬ tained was assigned as the value for that location. The greatest number of reports (178) for an indi¬ vidual State was from South Carolina. Figure 2 pre¬ sents the writer’s interpretation of these data. Even Figure 2.—Isoseismal map showing the State of South Carolina for the 1886 Charleston earthquake. Intensity ob¬ servations are indicated by Arabic numerals, and the contoured levels are shown by Roman numerals. REINTERPRETATION OF THE INTENSITY DATA 23 Figure 3.—Eastern United States showing the distribution of intensity observations for the 1886 Charleston earth¬ quake. Solid circles indicate felt reports; small crosses indicate not-felt reports. 24 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Ficure 4.— Isoseisma! map of the Eastern United States contoured to show the more localized variations in the re¬ ported intensities for the 1886 Charleston earthquake. Contoured intensity levels are shown by Arabic numerals. REINTERPRETATION OF THE INTENSITY DATA 25 Figure 5. —Isoseismal map of the Eastern United States contoured to show the broad regional patterns of the reported intensities for the 1886 Charleston earthquake. Contoured intensity levels are shown in Roman numerals. 26 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 in contouring the mode of the intensity values, as i was done here, intensity effects vary considerably | with epicentral distance within the State. In particu¬ lar, two intensity-VI zones are shown that trend j northeastward across the State and separate areas of intensity-VIII effects. Although some of this vari- 1 ation may be due to incomplete reporting and (or) population density, it seems more likely that the local effects of surficial geology, soils, and water- [ table level are being seen. Interpreted literally, a very complex behavior of intensity is seen in the epi¬ central region. The intensity data base and interpretive, isoseis- mal lines throughout the Eastern United States are shown in figures 3-5. In figure 4, the data are con¬ toured to emphasize local variations, whereas figure 5 depicts the broad regional pattern of effects. Rich¬ ter (1958, p. 142-145), in discussing the problem of . how to allow for or represent the effect of ground in drawing isoseismal lines, suggested that two isoseis- mal maps might be prepared. One map would show the actual observed intensities; the other map would show intensities inferred for typical or average ground. The procedure followed here was to contour the mode of the intensity values (figs. 2 and 4) so as to portray the observed intensities in a manner that emphasizes local variations. Those isoseismal lines were then subjectively smoothed to produce a second isoseismal map showing the regional pattern of ef¬ fects (fig. 5). The two maps that result from this procedure seem to the writer to represent reasonable extremes in the interpretation of intensity data. The subjectivity always involved in the contouring of intensity data is well known to workers concerned with such efforts. The purpose of the dual presenta¬ tion here is to emphasize this subjectivity and to point out that, depending on the application, one form may be more useful than the other. Both local and regional contouring interpretations are to be found in the literature for U.S. earthquakes. Figures 4 and 5 show T that a rather complex iso¬ seismal pattern, including Dutton’s low-intensity zone (epicentral distance = a= 550 km (341 mi)) in West Virginia, was present outside South Carolina. Intensity-VIII effects were observed at distances of 250 km (150 mi) and intensity-VI effects were ob¬ served 1,000 km (620 mi) from Charleston. Indi¬ vidual reports, given below, are all paraphrased from Dutton (1889). They note what took place in areas affected by intensity VI (MM) or higher at epi¬ central distances greater than about 600 km (372 mi). Some of these reports were ignored in the con¬ touring showm in figure 4. Intensity VI-VIII in Virginia (±==600 km (372 mi)): Richmond (VIII)—Western part of the city: bricks shaken from houses, plaster and chimneys thrown down, entire population in streets, peo¬ ple thrown from their feet; in other parts of the city, earthquake not generally felt on ground floors, but upper floors considerably shaken. Charlottesville (VII)—Report that several chim¬ neys w r ere overthrowm. Ashcake (VI)—Piano and beds moved 15 cm (6 in.) ; everything loose moved. Danville (VI)—Bricks fell from chimneys, walls cracked, loose objects thrown down, a chande¬ lier swung for 8 minutes after shocks. Lynchburg (VI)—Bricks thrown from chimneys, walls cracked in several houses. Intensity VII in eastern Kentucky and western West Virginia (±==650 km (1+01+ mi)): Ashland, Ky. (VIII)—Town fearfully shaken, sev¬ eral houses thrown down, three or four persons injured. Charleston, W T . Va.—“A number of chimneys top¬ pled over” (p. 522). Mouth of Pigeon, W. Va.—Chimneys toppled off to level of roofs, lamps broken, a house swayed violently. Intensity VI in central Alabama (±sz700 km (1+31+ mi)): Clanton (VII)—Water level rose in wells, some went dry and others flowed freely ; plastering ruined. Cullman—House wall cracked, lamp on table thrown over. Gadsden—People ran from houses. Tuscaloosa—Walls cracked, chimneys rocked, blinds shaken off, screaming women and children left houses. Intensity VII in central Ohio (±==800 km (1+96 mi) ): Lancaster—Several chimneys toppled over, decora¬ tions shaken down, hundreds rushed to the streets. Logan—Bricks knocked from chimney tops, houses shaken and rocked. Intensity VI in southeastern Indiana and northern Kentucky (±==800 km (1+96 mi)): Rising Sun, Ind.—Plaster dislodged, ornaments thrown down, glass broken. Stanford, Ky.—Some plaster thrown down, hanging lamps sv/ung 15 cm (6 in.). REINTERPRETATION OF THE INTENSITY DATA 27 Intensity VI in southern Illinois, eastern Tennessee, and Kentucky (&==950 km (590 mi) ): Cairo, Ill.—Broken windows, “houses settled con¬ siderably” (p. 430) in one section, ceiling cracked in post office. Murpnysboro, Ill.—Brick walls shook, firebell rang for a minute, suspended objects swung. Milan, Tenn.—Cracked plaster, people sitting in chairs knocked over. Clinton, Ky.—Some bricks fell from chimneys. Intensity VI in central and western Indiana. ( A =1,000 km (620 mi)) : Indianapolis—Earthquake not felt on ground floors; part of a cornice displaced on one hotel, people prevented from writing at desks, clock in court house tower stopped, a lamp thrown from a mantle. Terre Haute—Plaster dislodged, sleepers awakened; in Opera House, earthquake felt by a few on the ground floor, but swaying caused a panic in the upper galleries. Madison—Several walls cracked, chandeliers swung. Intensity VI in northern Illinois and Indiana (&==il,200 km (7J+b mi)): Chicago, Ill.—Plaster shaken from walls and ceil¬ ings in one building above the fourth floor; barometer at Signal Office “stood 0.01 inches higher than before the shock for eight minutes” (p. 432) ; earthquake not felt in some parts of City Hall, especially noticeable in upper stories of tall buildings, not felt on streets and lower floors. Valparaiso, Ind.—Plaster thrown down in hotel, chandeliers swung, windows cracked, pictures thrown from walls. The preceding reports indicate that structural damage extended to epicentral distances of several hundred kilometers and that apparent long-period effects were present at distances exceeding 1,000 km (620 mi). Persons also frequently reported nausea at these greater distances. Dutton apparently contoured his isoseismal map in a generalized manner, which is an entirely valid procedure. The rationale in that approach is to de¬ pict not the more local variations, as was presented in the above discussion, but rather the regional pat¬ tern of effects from the event. Figure 5 is the writ¬ er’s attempt at that type of interpretation, and the resulting map is very similar to Dutton’s. ATTENUATION OF INTENSITY WITH EPICENTRAE DISTANCE The decrease of intensity with epicentral distance is influenced by such a multiplicity of factors that it is particularly difficult to measure. The initial task in any attenuation study is to specify the distance (or distance range) associated with a given inten¬ sity level. Common selections are: minimum, maxi¬ mum, or average isoseismal contour distances or the i radius of an equivalent area circle. In all these ap¬ proaches, the original individual intensities are not considered; rather, isoseismal maps are used. Per¬ haps a better, but more laborious, procedure has been suggested by Perkins (oral commun., 1975), wherein the intensity distribution of observations is plotted for specific distance intervals. In this man¬ ner, all the basic data are presented to the reader without interpretation by contouring. He is then in a position to know exactly how the data base is handled and thereby to judge more effectively the results that follow. Once the intensity-distance data are cast in this format, they are then also available for use in different applications. The epicentral distances to some 800 different locations affected by the 1886 shock were measured and are listed in table 2. For these measurements, the center of the intensity X (fig. 1) area was as¬ sumed to be the epicenter. Figure 6 presents the resulting intensity distributions as functions of epi¬ central distance. The complexity present in the iso¬ seismal maps (figs. 4 and 5) is now transformed to specific distances, and the difficulty of assigning a single distance or distance interval to a given inten¬ sity level is clearly shown. The approach followed here was to perform a regression analysis on the intensity-distance data set, using an equation of the form, Table 2.— Number of intensity observations as a function of epicentral distance intervals for the 1886 Charleston, S. C., earthquake Epicentral distance (km) IX VIII VII VI v IV II-III Number of obser¬ vations 50- 99 3 4 3 3 3 16 100- 199 2 18 18 17 18 1 74 200- 299 _ 9 22 25 30 5 91 300- 399 _ 3 16 12 31 8 70 400- 499 _ 2 3 10 26 19 12 72 500- 599 1 3 11 13 19 7 54 600- 699 _ 1 3 3 14 33 11 65 700- 799 _ 3 4 22 16 22 67 800- 899 1 2 29 20 20 72 900- 999 _ _ _ _ 3 18 17 30 68 1,000- 1,249 _ 4 24 19 48 95 1,250- 1,499 6 6 20 32 1,500- 1,749 _ _ _ _ _ 1 3 4 Totals 5 38 72 94 234 164 173 780 28 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 (16) (74) IX VIII VII VI V IV ll-lll EPICENTRAL DISTANCE, IN KM Figure 6. —Distribution of intensity (Modified Mercalli, MM) as a function of epicentral distance (km) for the 1886 Charleston earthquake. Intensity distribution is shown for specific distance intervals. 7 = 7 0 + a + bA + c log- A, where a, b, c are constants, A is the epicentral dis¬ tance in kilometers, 7 0 is the epicentral intensity, and 7 is the intensity at distance A. This equation form was selected because it has been found useful by other investigators (for example, Gupta and Nuttli, 1976). The resulting fit for the median, or 50-percent fractile, was, 7 = 7 0 + 2.87 - 0.00052a - 2.88 log A. The standard deviation, between the observed and predicted intensities, is 1.2 intensity units for these data. For the 75-percent fractile, the a con¬ stant is 3.68; for the 90-percent fractile, the a con¬ stant is 4.39. The b term is very small and could perhaps be deleted, as it results in only half an in¬ tensity unit at 1,000 km. The minimum epicentral distance at which the equation is valid is probably 10-20 km. The intensity-distance pairs extend to ■within only 50 km of the center of the epicentral region, but that region (fig. 1) has a diameter of approximately 20 km. The curves for the 50-, 75-, and 90-percent frac- tiles are shown in figures 7 and 8 along with other published intensity attenuation curves for the Cen¬ tral and Eastern United States. Isoseismal maps Figure 7.—Attenuation of intensity (MM) with epicentral distance (km) for various fractiles of intensity at given distance intervals for the 1886 Charleston earthquake (heavy solid curves). Attenuation functions by Howell and Schultz (1975), Gupta and Nuttli (1976), and Cornell and Merz (1974) are shown by light dashed curves. IA VIII VII VI > K c n z LU t- ? V IV ll-lll Number of observations (91) (70) (72) (54) (65) I l I l (67) (72) I l Number of observations 40 20 0 -I-1 (68) (95) (32) (4) o> U~3 REINTERPRETATION OF THE INTENSITY DATA 29 Figure 8.—Attenuation of intensity (MM) with epicentral distance (km) for various fractiles of intensity at given distance intervals for the Charleston earthquake (solid curves). Evernden’s attenuation curves (1975) (Rossi- Forel intensity scale; L=10 km, C=25 km, k=l and 114) are shown by dashed curves for /c=X. were utilized to develop these latter curves, and the general agreement between the entire suite of curves is remarkable. A direct comparison between curves, which may not be valid because of different data sets and different regions, would suggest that the Howell and Schultz (1975) curve is at about the 85-percent fractile, the Gupta and Nuttli (1976) curve is at the 80-percent fractile, and the Cornell and Merz (1974) curve is at the 70-percent fractile. At the intensity- VI level and higher, note that there is less than one intensity-unit difference among the Central United States, Central and Eastern United States, and Northeastern United States curves and the 75- and 90-percent fractile curves of this study. Evernden’s (1975) curves (fig. 8) forhisk = l and k = l 14 , factors lie between the 50- and 90-percent fractile curves of this study. Evernden used k fac¬ tors to describe the different patterns of intensity decay with distance in the United States. A value of k = lwas found for the Gulf and Atlantic Coastal Plains and the Mississippi Embayment and a k = l for the remainder of the Eastern United States. Evernden prefers to work with the Rossi-Forel (R- F) intensity scale. The difference between the R-F and MM scales is generally about half an intensity unit, and conversion to R-F values would essentially result in translating the fractile curves of this study upward by that amount. This would put the 75- percent fractile curve in near superposition with Evernden’s k = l curve. Such a result is perhaps not surprising because approximately two-thirds of the felt area from the 1886 shock is in Evernden’s k = l region, and isoseismal lines are often drawn to en¬ close most of the values at a given intensity' level. Although differences in intensity attenuation may exist between various parts of the Eastern United States, it would appear from this study that the dispersion of the data ( D O LU to z 20 r n.c. — r - - Iji a 0□CL L. — ±L S.C. ~ - n □ 1 n m i_i_ i . ;_i_l-1 1830 40 50 60 70 80 1890 YEAR Figure 7. —Equivalent number of magnitude 3.0 earthquakes (N ) versus time (1836 to August 31, 1886) for South Carolina and nearest States. Two-year increments were plotted (wide bars), except for the year 1886 (narrow bar) ; data tabulated in table 6. Conversion of intensity (I,, maximum epicentral intensity) to magnitude (M) ac¬ cording to M=l-i-(2/3) I 0 (Gutenberg and Richter, 1956). Conversion of M to N* according to N„=10 u ~ u ~ 10) (Allen and others, 1965). REFERENCES CITED Allen, C. R., St. Amand, P., Richter, C. F., and Nordquist, J. M., 1965, Relationship between seismicity and geologic structure in the southern California region: Seismol. Soc. America Bull., v. 55, no. 4, p. 753-797. Bollinger, G. A., 1975, A catalog of Southern United States earthquakes—1754 through 1974: Virginia Polytech. Inst, and State Univ. Research Div. Bull. 101, 68 p. Coffman, J. L., and von Hake, C. A., eds., 1973, Earthquake history of the United States: U.S. Dept. Commerce, Pub. 41-1, (Revised ed., through 1970), 208 p. Dutton, C. E., 1889, The Charleston earthquake of August 31. 1886: U.S. Geol. Survey Ann. Rept. 9, 1887-88, p. 203-528. Gibbes, L. R., 1859, Notice of the phenomena attending the shock of the earthquake of Dec. 19, 1857 [Charleston, S.C.]: Elliott Soc. Nat. History Charleston Proc., v. 1, p. 288-289 (paper read Sept. 1, 1858). Gutenberg, B., and Richter, C. F., 1956, Earthquake magni¬ tude, intensity, energy, and acceleration: Seismol. Soc. America Bull., v. 46, no. 2, p. 105-145. MacCarthy, G. R., 1957, An annotated list of North Caro¬ lina earthquakes: Elisha Mitchell Sci. Soc. Jour., v. 73, no. 1, p. 84-100. -1961, North Carolina earthquakes, 1958 and 1959, with additions and corrections to previous lists: Elisha Mit¬ chell Sci. Soc. Jour., v. 77, no. 1, p. 62-64. McCrady, Edward, 1897, The history of South Carolina under the proprietary government, 1670-1719: New York, Macmillan Co., 762 p. (see especially p. 307-308). South Carolina State Board of Agriculture, 1883, South Carolina. Resources and Population. Institutions and Industries: Charleston, S.C., Walker, Evans & Cogswell, 724 p. (see especially p. 381-83, 529-534). [Reproduced in 1972 by The Reprint Co., Spartanburg, S.C.] Taber, Stephen, 1914, Seismic activity in the Atlantic Coastal Plain near Charleston, South Carolina: Seismol. Soc. Bull., v. 4, no. 3, p. 108-160. Wallace, D. D., 1934, The history of South Carolina: New York, Am. Hist. Soc., 3 v. (see especially v. 3, p. 333- 334). -1961, South Carolina, a short history, 1520-1948: Columbia, S.C., Univ. South Carolina Press, 753 p. (see especially p. 56). Woollard, G. P., 1968, A catalogue of the earthquakes in the United States prior to 1925; based on unpublished data compiled by Harry Fielding Reid and published sources prior to 1930: Hawaii Inst. Geophysics Data Rept. 10 (HIG-68-9). [approx. 156 p.]. Recent Seismicity Near Charleston, South Carolina, and its Relationship to the August 31, 1886, Earthquake By ARTHUR C. TARR STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-D - m. I'vi . ‘ ■ . CONTENTS Page Abstract_ 43 Introduction _ 43 South Carolina-Georgia seismic zone_ 44 Spatial distribution of earthquakes_ 44 Temporal distribution of earthquakes_ 4B Recent seismicity near Charleston_ 50 Prior instrumental studies_ 50 Reconnaissance study March 1973-December 1973 _ 50 South Carolina seismographie network_ 52 Seismicity May 1974-December 1975 _ 52 Interpretation of results_ 55 References cited_ 56 ILLUSTRATIONS Pape Figure 1-3. Maps showing: 1. Seismicity of the southeastern United States, 1961-75 _ 45 2. Seismicity in South Carolina and adjoining States, 1754- 1975 _______ 46 3. Seismicity in the Charleston, S.C., area, 1754-1972 _ 47 4. Graph showing the cumulative number of earthquakes in South Carolina versus maximum Modified Mercalli intensity, 1754- 1975 ______. 49 5. Map showing the Charleston area seismicity and seismograph sta¬ tions, March 1973-December 1975 _ 51 6. Map showing the South Carolina seismographie network, May 1974-December 1975 _ 53 7. Profile of the Middleton Place-Summerville seismic zone_ 54 8. Isoseismal map and focal mechanism of the November 22, 1974, earthquake _ 55 TABLES Page Table 1 . South Carolina earthquakes, 1754-1975 _ 48 2. A representative South Carolina crustal model _ 52 3. Hypocenter summary__ 52 hi STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT RECENT SEISMICITY NEAR CHARLESTON, SOUTH CAROLINA, AND ITS RELATIONSHIP TO THE AUGUST 31, 1886. EARTHQUAKE By Arthur C. Tarr ABSTRACT The hypoeenters of recent earthquakes located in the epi- central area of the intensity-X August 31, 1886, Charleston, S.C., earthquake are inferred in this study to be associated with the probable source volume of the main shock. The hypoeenters were determined from data recorded by a 5- station temporary seismic network which operated near Summerville for 8 months in 1973 and by a 10-station per¬ manent seismic network which began operating in May 1974. The temporary network revealed an ill-defined cluster of activity northwest of Charleston and south of Summerville. Earthquakes recorded from May 1974 through December 1975 by the larger, permanent network defined a linear trend of hypoeenters which extended northwest from Middle- ton Place to Summerville and to depths as great as 8 km. A well-constrained focal mechanism was determined for the largest earthquake in the zone. The strike (N. 42° W.) and dip (78°) of one nodal plane are similar to the strike and dip of the seismicity. The historical catalog of Charleston-Summerville earth¬ quakes shows that the area has experienced declining earth¬ quake activity since the 1886 main shock; however, the seismic activity has not yet reached pre-1886 levels. The persistence of seismic activity during nine decades and the observation that the nearly vertical zone of recent seismicity is located near the center of the zone of highest epicentral intensities of the 1886 shock, suggest that the Middleton Place- Summerville zone is closely associated with the rupture sur¬ face of the 1886 shock. The results of this study do not support a hypothetical connection, along a continuous north- west-trending seismic zone, of the Middleton Place-Summer- ville seismic activity with activity offshore to the southeast or with a persistent cluster of earthquakes near Bowman to the northwest. INTRODUCTION The most destructive earthquake in the history of . the southeastern United States took place at 9:51 p.m. (local time) on August 31, 1886. The shaking destroyed much of Charleston, S. C., killed approxi¬ mately 60 persons, and caused injury to many others (Dutton, 1889). Intensity-IX (Modified Mercalli scale) effects were observed in Charleston, and a maximum intensity of X was reported in the epi¬ central area, inferred to be near the town of Sum¬ merville, 25 km northwest of Charleston (Dutton. 1889; Bollinger, this volume). The magnitude (m bLg ) has been estimated at 6.8-7.1 (Bollinger, this vol¬ ume) . The main shock w r as preceded by several fore¬ shocks (Dutton, 1889; Taber, 1914) and followed by an extensive aftershock series (Dutton, 1889; Taber, 1914; Bollinger, 1975). The largest after¬ shock occurred about 8 minutes after the main shock and was of sufficiently high intensity that reference is made (U.S. Environmental Data Service, 1973, p. 25) to the “earthquakes of August 31, 1886.” The catalog of earthquakes of South Carolina (Bollinger, 1975) shows that the Charleston-Summerville area was quite active for at least three decades after the main shock. Most of the details contained in Dutton’s (1889) report of the Charleston earthquake do not need to be reviewed. For the purposes of this study, how¬ ever, several facts in the Dutton (1889) report are significant: 1. Personal observations and distribution of dam¬ age indicate that the epicenter was nearer to Summerville than to Charleston. This fact is significant for interpretation of the recent seismicity instrumentally located near Sum¬ merville and Middleton Place. 2. The extensive survey of earthquake effects showed an elongate, NNE-trending zone of highest intensities which, to Dutton, indicated the presence of two epicenters. Later, Taber (1914) reinterpreted the elongate zone as in¬ dicating the presence of a buried NNE-trend- ing fault passing under Woodstock, a railroad stop 10 km southeast of Summerville. 43 44 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 3. Dutton’s (1889) survey of intensities (reevalu¬ ated by Bollinger, this volume) throughout the Eastern United States showed that the area in which the earthquake was felt was very large and that major geological features (for ex¬ ample, the Appalachian Mountains) strongly affected the intensity pattern. The 1886 shock is very important for earthquake hazards studies (see, for example, Algermissen and Perkins, 1976) because it took place within an in¬ traplate region characterized by infrequent earth¬ quakes of relatively small and moderate magnitudes. In terms of damage and the size of the area in which it was felt, the shock ranks with the 1663 St. Lawrence Valley, 1755 Cape Ann, and 1811-1812 Mississippi Valley shocks as the most significant in the Eastern and Central United States. The char¬ acterization of recent earthquake activity near Charleston and the relationship of the activity to the South Carolina-Georgia seismic zone are the prin¬ cipal topics of this chapter. Incorporated herein are the results of seismological studies conducted in the Charleston area by the U.S. Geological Survey dur¬ ing the period 1973 through 1975. Support for this study was provided by the Atomic Energy Commission and the Nuclear Regu¬ latory Commission, Office of Nuclear Research, un¬ der Agreement No. AT (49-25)-1000. The efforts of Kenneth W. King in the design, in¬ stallation, and operation of the South Carolina seis¬ mograph network and of David L. Carver in seis¬ mogram analysis and data processing activities were particularly important contributions to this study. The critical review of the manuscript in its early stages by Margaret Hopper, Charley Langer, and Frank McKeown and the continually fruitful col¬ laboration and support of G. A. Bollinger and Pra- deep Talwani are gratefully acknowledged. SOUTH CAROLINA-GEORGIA SEISMIC. ZONE SPATIAL DISTRIBUTION OF EARTHQUAKES The South Carolina-Georgia seismic zone is a broad band of seismicity that extends southeast¬ ward from the southern Appalachian seismic zone across most of South Carolina and northeastern Georgia (fig. 1). The zone is mapped on the basis of information from Bollinger’s (1975) earthquake catalog, which is a listing of hypocenter coordinates, intensities, and (or) magnitudes (if determined) of earthquakes that took place in the time period 1754-1974. The catalog has been supplemented by new information on several pre-1886 shocks (Bol¬ linger and Visvanathan, this volume) and by in- strumentally recorded information on earthquakes which took place in 1975 (Carver and others, 1977). The Bollinger (1975) catalog lists both macro- seismic epicenters and instrumental hypocenters for the southeastern United States. Macroseismic epi¬ centers are qualitative estimates of earthquake lo¬ cations inferred from one or more subjective in¬ tensity observations. Instrumental hypocenters are quantitatively determined from precisely timed ar¬ rivals of P- and S-waves of earthquakes recorded by at least four seismograph stations. A rectangle approximately 500 km long and 350 km wide defines the South Carolina-Georgia seismic zone in this study. Its long axis (A-A') is oriented northwesb-southeast, perpendicular to the southern Appalachian seismic zone (fig. 1), and the rectan¬ gle is similar in shape and orientation to Bollinger’s (1975) definition of the zone. The largest South Carolina and northeast Georgia earthquakes, in addition to several clusters of sig¬ nificant seismic activity, are enclosed by the rectan¬ gle. These clusters are located in the Charleston- Summerville area, in the Orangeburg-Bowman area, and in the vicinity of Clark Hill Reservoir (figs. 1 and 2). Previously, several workers (Woollard, 1969; Bol¬ linger, 1973; Sbar and Sykes, 1973) noted that the South Carolina-Georgia seismic zone appears to trend northwest, perpendicular to the trend of the southern Appalachians in eastern Tennessee and western North Carolina (fig. 1). The axis (A-A') of the seismic zone, inferred from an alinement of well-determined epicenters of earthquakes which took place in the period 1961-1975, is a northwest¬ trending line that passes near Greenville, Columbia, Orangeburg, and Charleston (fig. 1). The axis A-A' is the approximate center line of the rectangle de¬ fining the seismic zone (fig. 1) ; extension of the axis to the southeast and northwest connects several .epicenters on the Continental Shelf and the epi¬ center of the eastern Tennessee earthquake of No¬ vember 30, 1973 (Bollinger and others, 1976). Figure 3 is a map showing epicenters of earth¬ quakes that took place in the Charleston area prior to the initiation of U.S. Geological Survey seismo¬ logical studies there in 1973. The distribution of epi¬ centers of the 1886 earthquake, of its aftershocks, and of subsequent activity is mapped in figure 3 and suggests the presence of a source area north¬ west of Charleston and near Summerville. However, RECENT SEISMICITY AND ITS RELATIONSHIP TO EARTHQUAKE 45 ~ NOV. 30, 1973 ^4 % > 0 MARYVILLE W ^TENNESSEE ' r • - NORTH CAROUNA iQi ITH CAROLIN, Summerville, |NOlV. 22. 1974^ iiddlIeton PLACE 1 GEORGIA ALABAMA* 1 , o 50 100 1 50 200 KILOMETERS EXPLANATION Modified Mercalli (MM) intensity Macroseismic Instrumental FLORIDA Figure 1. — Seismicity of the southeastern United States, 1961-75. Figure shows the relationship of the South Carolina-Georgia seismic zone to other seismicity in the southeastern United States. The axis A-A' is the approximate center line of the rectangle defining the South Carolina-Georgia seismic zone. Numbers beside the epicenter symbols show the number of events recorded. The macroseismic epicenters were determined from accounts of damage and felt reports, whereas instrumental epicenters were determined from seismo- graphic data. a more precise location of the source area is virtual¬ ly impossible because of the inherent inaccuracies in the macroseismic observations used to locate the historical epicenters. TEMPORAL DISTRIBUTION OF EARTHQUAKES The temporal aspects of earthquake statistics are conventionally represented by the Gutenberg and Richter (1944) relationship log N = a + b M (1) where N is the number of earthquakes recorded during time interval T, M is a magnitude (such as M l , mi, M s , or m bLg ), and a and b are constants. The number N may be either incremental number NAM) where N i is the number of earthquakes having mag¬ nitude in the interval M- - ^M .oo \ \ \ •A oO o o o Columbia^' AUG. 2. 1974 V o V SOUTH GEORGIA o') o CAROLINA £ 'Orangeburg O Bowman q 33 B - 320-^ .14. Summerville Mt ~ _ /nu SU ~tW3 -e- o . Maaieton ^ O NOV. 22. 1974 !v:-.>s 0 0 Charleston' ~~ i - ’ "\ 4 J ^ 78 ° 100 KILOMETERS , "0 X\° l - ^ h! EXPLANATION MM Intensity l 0 FELT l-ll III IV V VI VII VIII IX X 1 c » Macroseismic ° ° ooooOOOl 5> uT Instrumental Magnitude* 139-2V, 3 3Pi 4V, 5 5*3 6’5 7 7V, •M = 1-Vj/ 0 Figure 2. —Seismicity in South Carolina and adjoining States, 1754-1975. Earthquakes are indicated by circles of varying sizes, which represent the maximum Modified Mercalli intensities shown in the explanation. Numbers beside the epi¬ center symbols show the number of events recorded. Earthquakes are from the catalog of Bollinger (1975), supple¬ mented by earthquakes reported by Carver and others (1977) and Bollinger and Visvanathan (this volume). statistical treatment. Questions exist regarding (1) the accuracy of the values of maximum intensity /„ for the historical shocks (Were maximum inten¬ sity effects always reported to newspapers and other data sources if population densities were low in the epicentral area?), (2) the completeness of the cata¬ log (Could significant numbers of seismic events, es¬ pecially of lower l 0 events, have been missed because of low population densities or inadequate report¬ ing?), and (3) the size of the data sample (Is it sufficiently free from fluctuation to be characterized by equations (1) or (3)?). In order to determine the constants in a fre¬ quency-maximum intensity relationship of the form RECENT SEISMICITY AND ITS RELATIONSHIP TO EARTHQUAKE 47 81 ° 00 ' 80 ° 00 ' 79 ° 00 ' Figure 3.—Seismicity in the Charleston, S. C., area, 1754-1972. Data are from the same catalogs as in figure 2. The earthquakes shown are principally, but not exclusively, macroseismic epicenters. Numbers beside the epicenter symbols show the number of events recorded. of equation (3), several assumptions have been made: (1) the information in table 1 is complete in listing earthquakes of the highest intensities down to those of an intensity level where the cumulative number departs from the straight-line relationship by 10 percent or more; (2) searches of data sources, such as those by Bollinger (1975) and Bollinger and Visvanathan (this volume), have been exhaustive; (3) all large and moderately large events were re¬ ported to newspapers and other data sources; and (4) population densities in South Carolina were suf- ficently high that reports were made on all events having intensities stronger or equal to the intensity where the cumulative number departs from the straight-line relationship by 10 percent or more. The Bollinger (1975) catalog is used and is sup¬ plemented by information on pre-1886 events (Bol¬ linger and Visvanathan, this volume) and on several events in the instrumental catalog of Carver and others (1977) which were felt. The catalog entries have been separated according to incremental maxi¬ mum intensity class and decade of occurrence for 48 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Table 1. —South Carolina earthquakes, 175\-1975 [C.-S.. Charleston-Summerville area; S.C.. South Carolina exclusive of Charleston-Summerville area Leaders (_) indicate no earthquake reported, Data from Bollinger (19T5), Carver and others (1977), and Bollinger and Visvanathan (this volume).] Maximum Modified Mercalli intensity I- II- III- IV- V- VI- VII- VIII- IX- Felt I II II III III IV IV V V VI VI VII VII VIII VIII IX IX X X Total Grand Total 1750-60: C.-S. 2 c c 1760-70: C.-S. Q C 1770-80: C.-S. 1 1780-90: C.-S. s.c. 1790-1800: C.-S. 1 SC 2 1800-10: C.-S. s r 1810-20: C.-S. 1 S.C. 1 1820-30: C.-S. S.C. 1 1830-40: C.-S. s c 1840-50: C.-S. 1 SC 1 1850-60: C.-S. 1860-70: C.-S. 3 S.C. 1 1870-80: C.-S. 1 S.C. 1 1880-90: C.-S. 155+ s.c. 1890-1900: C.-S. 72_ s.c. 1900-10: C.-S. 51 S.C. 1910-20: C.-S. 16 S.C. 1920-30: C.-S. 1 S.C. 2 1930-40: C.-S. 10 S.C. 3 1940-50: C.-S. 12 S.C. 1 1950-60: C.-S. 4 1_ 0.0 1.2 2.5 1.4 2 _ 1.2 2.3 5.8 3.3 3 _ 3.5 28.5 6.2 3.5 Half space . 32.0 --- 8.2 4.6 Program HYP071 (Lee and Lahr, 1975) was used for hypocenter computations. A representative crustal model assumed three layers on a half space (table 2). A summary of hypocenter locations, depths, and times is given in table 3. The reconnaissance survey demonstrated, as had Bollinger’s (1972) survey, that low-level seismic ac¬ tivity was indeed taking place near Summerville and furthermore, that the activity was located near the inferred location of the 1886 earthquake and aftershocks. However, it was not possible to dis¬ cern whether the cluster represented a small source area or was only a segment of a longer zone of ac¬ tivity that extended beyond the network perimeter. This difficulty is due to the tendency for more smaller earthquakes to be detected and located near the center of a network (where the detection thresh¬ old is lowest) than in the area outside the network (where the detection threshold increases). SOUTH CAROLINA SEISMOGRAPHIC NETWORK The design of the South Carolina network and the objectives of the seismological program have been discussed by Tarr and King (1974). One of the principal objectives of the program was to monitor the South Carolina-Georgia seismic zone and pro¬ vide, for the first time, an instrumental data base for earthquakes in the zone. Ten stations were in¬ stalled in the Coastal Plain astride the hypothetical northwest-trending axis of the seismic zone. Sta¬ tions were spaced 50-100 km apart at the north¬ western end of the network to insure wide areal coverage and 30-60 km apart at the southeastern end where a lower detection threshold was desirable. A nominal three-station detection threshold of M l = 2.5 or better was estimated for the area bounded roughly by the cities of Columbia, Sumter, Georgetown, Charleston, Beaufort, and Aiken. The 10-station network became operational in May 1974. All data channels were radio-telemetered to a central site at Columbia where all the signals were recorded on 16-mm film and magnetic tape, and where three channels were recorded on visual drum-type recorders for monitoring purposes. The network configuration was altered slightly in 1975 to locate more accurately aftershocks of the August 2, 1974, earthquake at Clark Hill Reservoir; how¬ ever, the network that was operative during the oc¬ currence of most of the earthquakes discussed in the next section is shown in figure 6. SEISMICITY MAY 1974-DEC EMBER 1975 Between May 1974 and December 1975, earth¬ quakes in the South Carolina-Georgia seismic zone were recorded and located by the use of data from Table 3 .—Hypocenter summary [Leaders (_), insufficient data for computation] Date Origin time ( Universal Coordinated Time) RMS 1 (sec) Latitude ( degrees) ERY - (km) Longitude ( degrees ) ERX 1 (km) Depth (km) ERZ * (km) Duration 5 magnitude Number of observa¬ tions 1973 March 25 04 29 31.6 0.11 32.953N. 80.080W. 0.9 4 April 18 10 06 10.7 .56 33.044N. 80.190W. _ _ 2.5 _ _ _ 4 23 21 32 38.2 .38 33.012N. 80.280W. 3.3 4 June 9 19 24 52.7 .29 32.942N. 0.7 80.153W. 1.5 3.5 395.6 9 12 20 45 25.0 .29 33.018N. 1.2 SO. 168 W. 1.8 3.7 496.8 __ 6 August 25 09 17 30.1 .27 32.944N. .5 80.186W. .9 D. i 4.3 1 November 13 15 10 03.0 .05 32.945N. 80.205W. 1.3 4 December 19 10 16 18.3 _ 33.008N. _ _ 80.275W, _ _ 8.3 „ _ _ _ _ 4 1574 May 28 05 01 36.1 0.22 33.388N. 1.0 80.697W. 0.9 3.6 268.7 1.6 12 September 2 08 54 47.0 .33 33.053N. 79.742W. _ _ 2.6 _ __ 4 November 22 05 25 55.8 .15 32.902N. .9 80.147W. 2.0 4.1 261.0 3 ’. 8 6 22 06 22 43.9 .35 32.874N. 1.7 80.145W. 4.4 2.2 541.8 2.6 7 1975 April 28 05 46 52.3 0.04 32.986N. 0.2 80.215W. 0.3 3.7 63.9 3.0 T : RMS ; Root-mean-square of travel time residuals : ERY: Standard error in latitude : ERX: Standard error in longitude 4 ERZ : Standard error in depth 'Duration magnitude is an estimation of Richter magnitude M found by M — 0.87-f-2.00 log (r) —0.0035 A where r is signal duration in seconds and 1 is epicentral distance in kilometers (see Lee, Bennett, and Meagher, 1972) RECENT SEISMICITY AND ITS RELATIONSHIP TO EARTHQUAKE 53 36* 84 c 83* 82* 79* TENNESSEE ne- NORTH CAROLINA Figure 6 .—South Carolina seismogTaphic network, May 1974-December 1975. Seismograph station abbreviations, coordinates, and instrumentation are discussed by Carver and others (1977). the 10-station network. Although the region of uni¬ form detection threshold is quite large, most of the earthquakes detected and located were concen¬ trated near Summerville (table 3). The most im¬ portant of the shocks outside the Charleston-Sum- merville area was the August 2, 1974, Clark Hill Reservoir earthquake (fig. 1). This event and its aftershock sequence have been described by Talwani and others (1975) and need not be discussed fur- ' ther. Another significant event took place on May 28, 1974, and was located near Bowman (fig. 1). Epicenters of earthquakes taking place in the Charleston-Summerville area tend to cluster about 20-25 km northwest of Charleston and about 0-10 km south of Summerville, near Middleton Place and near the cluster of small earthquakes recorded by the five-station network in the reconnaissance study of 1973 (fig. 5). Figure 5 shows that the seismic activity of the 3-year period is confined principally to a zone be¬ tween Middleton Place and Summerville. The zone of epicenters seems to trend northwest, and the depth section shows that the activity is in the depth interval 1-8 km (fig. 7). Although the epicenter co- 54 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 B u 1 1 1 1 7 1 |- / / I 2.5 km/s • 2 — 4 ■ • • • / / / / / / • 5.8 km/s 4 — - $ W / 1 . • S [— / / / / / 6.2 km/s 8 | in - / • J!!1!1 8 10 DISTANCE, IN KILOMETERS 12 14 16 18 EXPLANATION <1,0 or? 1.0-1.9 2.0-2.9 Duration magnitude 3.0-3.9 Figure 7. —Profile of the Middleton Place-Summerville seismic zone. Hypocenters are projected onto a ver¬ tical plane passing through B-B', which is shown in figure 5. The plane B-B' is perpendicular to the strike of the two nodal planes (dashed lines) determined for the November 22, 1974, earthquake (fig. 8). The upper layers of the crustal model, used in the hypocenter computations, are shown by horizontal inter¬ faces and compressional (P) velocities. ordinates are well determined, the vertical extent of the zone cannot be known with comparable certain¬ ty because of large errors in the estimates of depth (table 3). The absence of seismograph stations close to the seismic activity is the principal reason for the lack of depth control. The largest earthquake of the period had a magnitude (m bLff ) of 3.8 and took place on November 22, 1974; its focus was very near Mid¬ dleton Place at a depth of about 4.1 km. Despite the relatively small magnitude of this earthquake, it was widely felt in South Carolina (fig. 8A). A focal-mechanism solution was determined from 13 short-period P-w r ave first-motion observations of the November 22, 1974, event (fig. 85).'Eight of the first-motions of the aftershock recorded by 10 network stations are consistent with the first mo¬ tions of the main shock. One of the permissible fo¬ cal-mechanism solutions, in which all observations are consistent, is shown in figure 85. The B- (null-) axis in this solution is horizontal and parallel to the strike of the two planes. The strike (N. 42° W.) of the preferred (A) nodal plane is well-constrained by observations from stations both within and out¬ side the seismic network. The dip (78°) is controlled by the position of the auxiliary (C) nodal plane. The profile B-B' in figure 7 is oriented perpendic¬ ularly to the B-axis and, hence, the two nodal planes are viewed end-on in the figure. Uncertainties in hypocenter coordinates and permissible variations of the crustal velocity model allow' for slightly dif¬ ferent nodal plane orientations than the ones shown | in figure 85. RECENT SEISMICITY AND ITS RELATIONSHIP TO EARTHQUAKE 55 Figure 8. —Isoseismal map (A) and focal mechanism ( B ) of the November 22, 1974, earthquake. Figure (modified from fig. 11 of Stover and others, 1976) depicts interpretations of Modified Mercalli intensities from questionnaire canvass of postmasters in the area. Maximum intensity was VI. INTERPRETATION OF RESULTS The seismological studies conducted from 1973 through 1975 have provided new information about the South Carolina-Georgia seismic zone near Charleston. Our data suggest that the small but persistent zone of seismic activity between Middle- ton Place and Summerville may be associated with the source volume of the 1886 Charleston earth¬ quake. In addition, the Middleton Place-Summer- ville zone appears to coincide with anomalous seis¬ mic velocities of shallow-layered structures under Middleton Place (Ackermann, this volume) and large local gravity (Long and Champion, this vol¬ ume), electrical and electromagnetic (Campbell, this 56 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 volume), and aeromagnetic (Popenoe and Zietz, this volume; Phillips, this volume) anomaly pat¬ terns. The seismological data presented in this chapter suggest that current seismic activity in the Middle- ton Place-Summerville area is taking place along a nearly vertical plane striking northwest and passing near Middleton Place (fig. 5). The largest recent event took place (November 22, 1974) at the south¬ eastern end of the zone. The Middleton Place-Sum¬ merville seismic activity takes place within the zone of highest intensities of the 1886 shock as contoured by Sloan in the Dutton (1889) report and by Bol¬ linger (this volume). Furthermore, the epicenter of the November 22, 1974, shock was located midway between the two epicenters in Dutton’s (1889) con¬ touring of the maximum intensity zone. The pre¬ ferred nodal plane determined from the focal mech¬ anisms of the November 22, 1974, event is nearly vertical and suggests reverse faulting on a plane striking N. 42° W.; the trace of this plane at the surface is along the course of the Ashley River (fig. 5). In prior studies, Taber (1914) and Bollinger (1972) suggested that the earthquakes felt in the Charleston-Summerville area since August 31, 1886, are related to, if they are not actually aftershocks of, the main shock. Certainly the frequency of oc¬ currence statistics of felt earthquakes in the Charleston-Summerville area (table 1) shows a de¬ cline of activity typical of aftershock sequences (Richter, 1958) and, even in the last two decades, the frequency has not decreased to pre-1886 levels. Therefore, it may be that the current Middleton Place-Summerville activity is taking place within the rupture zone of the main shock. The current seismic data cannot discriminate between the al¬ ternative hypotheses that the earthquake activity represents either true seismic afterslip in the rup¬ ture zone or a response to the regional stress field in locally weak geologic structures near the 1886 rupture zone. Determination of the dimensions of the Middle- ton Place-Summerville zone must await detection and location of further seismic activity and will re¬ quire more precisely determined hypocenters than were possible for this study. Five new local seismic stations currently provide the seismographic net¬ work with this capability. In addition, several new coastal stations northeast and southwest of Charles¬ ton will expand the monitoring capability neces¬ sary to detect and locate the possible extension of the seismic zone onto the Continental Shelf. Be¬ cause of the current configuration of the network, the detection and location capability fall off rapidly to the southeast. The seismic data from this study do not indicate that a seismic relationship exists be¬ tween the Middleton Place-Summerville and Bow¬ man source areas. » The identification of the subsurface structure re¬ sponsible for the current seismicity in the Middleton Place-Summerville zone and by inference, for the 1886 Charleston earthquake, cannot be made at this time. Only one focal mechanism has been de¬ termined thus far, and many more will be required to determine the nature of faulting throughout the entire zone and to establish the relationship of the relatively deeper seismicity to shallow structures under Middleton Place. REFERENCES CITED Algermissen, S. T., and Perkins, D. M., 1976, A probabilistic estimate of maximum acceleration in rock in the contig¬ uous United States: U.S. Geol. Survey open-file rept. 76-416, 45 p., 2 pis.. 1 table. Bollinger, G. A., 1972, Historical and recent seismic activity in South Carolina: Seismol. Soc. America Bull., v. 62, p. p. 851-864. - 1973, Seismicity of the southeastern United States: Seismol. Soc. America Bull., v. 63, p. 1785-1808. - 1975, A catalog of southeastern United States earth¬ quakes 1754 through 1974: Virginia Polytech. Inst, and State Univ., Research Div. Bull. 101, 68 p. Bollinger, G. A., Langer, C. J., and Harding, S. T., 1976, The eastern Tennessee earthquake sequence of October through December, 1973: Seismol. Soc. America Bull., v. 66, no. 2, p. 525-547. Carver, David, Turner. L. M., and Tarr, A. C., 1977, South Carolina seismological data report May 1974-June 1975: U.S. Geol. Survey open-file rept. 77-429, 66 p. Dutton, C. E., 1889, The Charleston earthquake of August 31, 1886: U.S. Geol. Survey 9th Ann. Rept., 1887-88, p. 203-528. Gutenberg, Beno, and Richter, C. F., 1942. Earthquake mag¬ nitude, intensity, energy, and acceleration: Seismol. Soc. America Bull., v. 32, p. 163-191. - 1944, Frequency of earthquakes in California: Seismol. Soc. America Bull., v. 34, p. 185-188. Karnik, Vit, 1969, Seismicity of the European area, Part I: Dordrecht, Holland, D. Reidel Pub. Co., 364 p. Lee, W. H. K., Bennett, R. E., and Meagher, K. L., 1972, A method of estimating magnitude of local earthquakes from signal duration: U.S. Geol. Survey open-file rept., 28 p., 5 figs., 1 table. Lee, W. H. K., and Lahr. J. C., 1975, HYP071 (revised), A computer program for determining hypoeenter, magni¬ tude, and first motion pattern of local earthquakes: U.S. Geol. Survey open-file rept. 75-311, 59 p., 5 figs., 48 tables. McKee, J. H., 1973, A geophysical study of microearthquake activity near Bowman, South Carolina: Atlanta, Ga., Georgia Inst, of Tech., unpub. Masters thesis, 75 p. RECENT SEISMICITY AND ITS RELATIONSHIP TO EARTHQUAKE 57 Page, Robert, 1968, Aftershocks and microaftershocks of the great Alaska earthquake of 1964: Seismol. Soc. America Bull., v. 58, p. 1131-1168. Richter, C. F., 1958, Elementary seismology: San Francisco, Calif., W. H. Freeman, 768 p. Sbar, M. L., and Sykes, L. R., 1973, Contemporary compres¬ sive stress and seismicity in eastern North America; an example of intra-plate tectonics: Geol. Soc. America Bull., v. 84, no. 6, p. 1861-1881. Stover, C. W., Simon, R. B., and Person, W. J., 1976, Earth¬ quakes in the United States, October-December 1974: U.S. Geol. Survey Circ. 723-D, p. D1-D27. Taber, Stephen, 1914, Seismic activity in the Atlantic Coastal Plain near Charleston, South Carolina: Seismol. Soc. America Bull., v. 4, p. 108-160. Talwani, Pradeep, Secor, D. T., and Scheffler, P., 1975, Pre¬ liminary results of aftershock studies following the 2 August 1974 South Carolina earthquake: Earthquake Notes, v. 46, no. 4, p. 21-28. Tarr, A. C., and King, K. W., 1974, South Carolina seismic program: U.S. Geol. Survey open-file rept. 74—58, 15 p., 4 figs., 2 tables. U.S. Environmental Data Service, 1973, Earthquake history of the United States: U.S. Environmental Data Service Pub. 41—1, rev. ed. (through 1970), 208 p. Utsu, T., 1961, A statistical study on the occurrence of after¬ shocks: Geophys. Magazine, v. 30, p. 521-605. Woollard, G. P., 1969, Tectonic activity in North America as indicated by earthquakes, in Hart, P. J., ed.. The earth’s Mon. 13, p. 125-133. crust and upper mantle: Am. Geophys. Union Geophys. ' Lithostratigraphy of the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, South Carolina,' By GREGORY S. GOHN. BRENDA B. HIGGINS, CHARLES C. SMITH, and JAMES P. OWENS STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-E CONTENTS Page Abstract _ 59 Introduction _ 59 Core stratigraphy _ 61 Basalt _ 61 Upper Cretaceous Series_ 61 Tertiary System _ 62 Texture and mineralogy _ 63 Calcium carbonate content and textural analysis_ 63 Sand-fraction mineralogy _ 63 Clay-fraction mineralogy_ 66 Formation descriptions _ 66 Depositional history _ 69 References cited_ 70 ILLUSTRATIONS Pape Figure 1. Map showing location of Clubhouse Crossroads corehole 1 _ 60 2. Stratigraphic column for the Clubhouse Crossroads core_ 62 3. Graphs showing distribution of the acid-soluble fraction and the textural composition of the core sediments_ 64 4. Graphs showing mineralogy of the clay-sized fraction and the light-mineral split of the sand-sized fraction _ 65 5. Graph showing stratigraphic distribution of heavy minerals in the sand-sized fraction of the core sediments _ 66 6 . Diagram showing distribution of granitic, low-rank metamorphic, and high-rank metamorphic minerals in heavy-mineral suites of 57 core samples _ 67 7. Diagram showing generalized paleoenvironments represented by Cretaceous and Tertiary sediments of the Clubhouse Crossroads core_ 69 ■ STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886- A PRELIMINARY REPORT LITHOSTRATIGRAPHY OF THE DEEP COREHOLE (CLUBHOUSE CROSSROADS COREHOLE 1) NEAR CHARLESTON, SOUTH CAROLLNA By Gregory S. Gohn, Brenda B. Higgins, Charles C. Smith, and James P. Owens ABSTRACT The continuously cored Clubhouse Crossroads test hole j is in the Cottageville 15-minute quadrangle, 40 km (25 mi) j west-northwest of Charleston, S.C. Total depth of the hole j is 792 m (2,599 ft). Seven hundred and fifty meters (2,462 ft) of Cenozoic and Upper Cretaceous sediments was pene- j trated, and 70 percent of the core was recovered. Below that, 42 m (137 ft) of amygdaloidal basalt was penetrated, and 100 percent of this part of the core was recovered. The formations recognized in the core, their ages, and ap¬ proximate thicknesses are: amygdaloidal basalt (K/Ar minimum ages: 94.8±4.2 m.y. and 109±4 m.y.), greater than 42 m (137 ft); Cape Fear Formation, Upper Creta¬ ceous, 59 m (194 ft) ; Middendorf Formation, Upper Cre¬ taceous, 124 m (408 ft); Black Creek Formation, Upper Cretaceous, 159 m (520 ft); Peedee Formation, Upper Cre¬ taceous, 164 m (540 ft); Beaufort(?) Formation, Paleocene, 52 m (170 ft); Black Mingo Formation, Paleocene and Eocene, 67 m (220 ft) ; Santee Limestone, Eocene, 56 m (183 ft); Cooper Formation, Eocene and Oligocene, 64 m (211 ft); unconsolidated sediments, Pleistocene!?), 5 m (16 ft). The Cape Fear Formation contains feldspathic sand and interbedded clay similar to those in the overlying Midden¬ dorf Formation, but it also contains marginal marine de¬ posits of thinly interbedded fine sand and dark clay. The Middendorf Formation contains fining-upward cycles of conglomeratic to fine-grained quartzose, feldspathic sand, and mottled clay deposited in continental environments. The Black Creek Formation is a heterogeneous sequence of sandy mud, well-sorted sand, shelly sand, and dark clay de¬ posited in marine and marginal marine environments. The Peedee and Beaufort(?) Formations are homogeneous marine sequences of dark-gray silty clay and muddy sand. The Black Mingo Formation contains silty clay, muddy sand, thinly interbedded sand and clay, and shelly limestone deposited in marine and marginal marine environments. The Santee Limestone and Cooper Formation are dominant¬ ly impure glauconitic marine limestone containing varying amounts of silt- and sand-sized quartz. INTRODUCTION In January, February, and March 1975, a 792-m (2,599-ft) continuously cored test hole, the Club¬ house Crossroads corehole 1, was drilled near Charleston, S.C. (fig. 1). The hole was drilled in sup¬ port of the Charleston Project, a multidisciplinary investigation by the U.S. Geological Survey, to de¬ termine the cause of seismicity in the Coastal Plain near Charleston, S.C. Prior to the analysis of the core, the lithologic character and three-dimensional stratigraphy of the Coastal Plain sediments in this area were poorly understood. Analysis of the core has established the stratigraphic column necessary for the construction of a regional stratigraphic framework. The corehole is at lat 32°53.25' N., long 80°21.4T W., near the center of the Cottageville 15-minute quadrangle, 3.5 km (2.2 mi) southwest of Clubhouse Crossroads and approximately 40 km (25 mi) west- northwest of Charleston. The Clubhouse Crossroads hole was drilled to a total depth of 792 m (2,599 ft) below a surface elevation of 6 m (20 ft). Of that total, 244 m (800 ft) of Cenozoic and 506 m (1,662 ft) of Cretaceous sediments were penetrated, and 70 percent of the core was recovered. The basal 42 m (137 ft) is composed of weathered and fresh basalt; 100 percent of this part of the core was recovered. Mechanical problems prevented further drilling. The diameter of most of the upper 225 m (738 ft) of the core is 15 cm (6 in.) ; that of the lower 567 m (1,861 ft) is 7 cm (2.75 in.). The drilling was done by a U.S. Army Corps of Engineers team using hydraulic rotary drilling equipment. Eight geophysical logs were made by the Schlumberger Corp. (Rhodehamel, 1975). Descriptive logs of the core were made at the drill site before more detailed laboratory investigations. The preliminary bio strati graphic and lithostrati- graphic analyses of the core sediments and fossils have been completed, and the basic stratigraphic 59 60 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 81° 80° Figure 1 . — Location of Clubhouse Crossroads corehole 1, Charleston, S.C. LITHOSTRATIGRAPHY OF DEEP COREHOLE 61 framework and sediment composition are described herein. The purpose of this report is to describe the mineralogy, texture, sedimentary structures, and general appearance of the core sediments and the nomenclature and contact relationships of the for¬ mations that have been identified. The biostrati- graphic framework for the core sediments has been described by Hazel and others (this volume). This work was supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Research, AgreementNo. AT (49-25)-1000. Melodie Hess, Ray Schneider, and Steve Perlman performed the many laboratory analyses of the core sediments. The potas¬ sium-argon radiometric age analyses were done by R. F. Marvin, U.S. Geological Survey. We wish to express particular appreciation to the Westvaco Corporation for allowing us to locate the drilling site on their land and for their interest and cooperation throughout this project. CORE STRATIGRAPHY BASALT The Clubhouse Crossroads corehole bottomed in 42 m (137 ft) of basalt of Cretaceous (?) age. K/Ar age determinations on two basalt samples gave ages of 109±4 m.y. (latest Early Cretaceous) and 94.8±4.2 m.y. (earliest Late Cretaceous). These ages are considered to be minimum ages, however, be¬ cause of observed geochemical alteration of the basalt (Gottfried and others, this volume). Both ages are consistent with the Woodbinian (Cenomanian) age, as determined by microfossils from sediments immediately overlying the basalt (see Hazel and others, this volume). The distribution of weathered, amvgdaloidal, and massive basalts within the core indicates at least two distinct flow units. Seven meters (23 ft) of the lower flow were recovered. This flow displays distinct ver¬ tical changes in lithology. The lower 1.8 m (6 ft) contains a network of fine fractures along which con¬ siderable alteration of the basalt has occurred. Above the fractured basalt the remaining 5.2 m (17 ft) shows an upward increase in the size and abundance of amygdules. The upper flow (34.7 m, 114 ft) con¬ tains a similar sequence of lithologies with 3 m (10 ft) of fractured basalt below 22.9 m (75 ft) of fresh, dark-gray amygdaloidal basalt. Amygdules increase in size and abundance upward in the fresh basalt. The dark-gray basalt grades upward into dark-red weathered basalt (4.2 m, 14 ft), which grades into red-, yellow-, and white-mottled clay (4.6 m, 15 ft) displaying a relict amygdaloidal texture. UPPER CRETACEOUS SERIES Upper Cretaceous sediments (fig. 2) are present from the 750-m (2,462-ft) level to the 244-m (800- ft) level in the core, a thickness of 507 m (1,662 ft). The section is provisionally divided into four forma¬ tions that have been recognized previously as out¬ crop and subsurface units in the Carolina Coastal Plain (Swift and Heron, 1969). From the base to the top, the formations are the Cape Fear (59 m, 194 ft), Middendorf (124 m, 408 ft), Black Creek (159 m, 520 ft), and Peedee (164 m, 540 ft). Unlike the Tertiary formations, contacts between the four Cre¬ taceous units are difficult to identify precisely be¬ cause of poor core recovery in some intervals and similar sediment types in more than one interval. The base of the Cape Fear Formation (750 m, 2,462 ft) is drawn between an upper conglomeratic muddy sand and the top of the basalt. The Cape Fear-Middendorf boundary (691 m, 2,268 ft) is placed at the base of the lowest 0.3 m (1 ft) or thicker feldspathic sand in an interbedded sand-clay sequence. Recognition of the Middendorf-Black Creek boundary is hampered by a lack of core recovery for several intervals. Core' recovery between 575 m (1,787 ft) and 567 m (1,860 ft) is poor, and the small amount of sediment that was recovered con¬ sists of friable sand and dark clay lithologically similar to overlying thinly interbedded sand and clay. The section between 567 m (1,860 ft) and 750 m (2,462 ft) consists mostly of interbedded 1- to 5- m-thick (3—15—ft) yellowish-grav-, greenish-gray-, and red-mottled clay and coarse feldspathic sand. On the basis of the most pronounced lithologic change, the Middendorf-Black Creek contact is placed at 567 m (1,860 ft). Rock resistivity also de¬ creases significantly at this point (fig. 2). Despite the fact that the Middendorf-Black Creek contact is not sharply defined, the paleontologic data (Hazel and others, this volume) suggest that an un¬ conformity of considerable temporal magnitude is present. Fossils at about 560 m (1,837 ft) indicate a late Austinian age; at 586 m (1,923 ft), early Eagle- fordian (late Cenomanian) fossils are present. The 26-m (85-ft) interval between these two points is barren of diagnostic fossils. The data suggest that either the low 7 er Austinian and middle and upper Eaglefordian (that is, approximately Coniacian and Turonian) deposits are absent, or they are repre¬ sented by only 26 m (85 ft) of section. Data from 62 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 CLUBHOUSE CROSSROADS CORE DORCHESTER COUNTY, SOUTH CAROLINA z > o z K > < I- D < Q o > X Q_ 3 h- < o Z u rsi UJ X C/3 (J (/) < < DEPTH IN IN FEET METERS 0—0 _ _ _ _ ____ _ _ _ _ _ _ 0-50 0-50 0-50 0-50 0-50 0-50 0-50 0-50 0-50 0-50 0-50 0-50 0-100 source for the andalusite and associated minerals found in the younger Cretaceous sediments. CLAY-FRACTION MINERALOGY Variations in the mineralogy of the clay-sized fraction primarily involve small changes in the rela¬ tive percentages of kaolinite, illite, and mixed-layer illite-smectite (fig. 4). In Cape Fear, Middendorf, and basal Black Creek sediments below 541 m (1,775 ft), kaolinite and mixed-layer clay are most abun¬ dant. In Black Creek, Peedee, and basal Beaufort (?) sediments between 541 m (1,775 ft) and 238 m (780 ft), kaolinite, illite, and illite-smectite are present in nearly equal proportions. Siliciclastic and carbonate sediments above 238 m (780 ft) contain little kaolin- I ite and abundant illite and mixed-layer clay. In addition to the three clay minerals, cristobalite and clinoptilolite are abundant within parts of the Tertiary section (fig. 4). The abundance of these two minerals in the Tertiary sediments and their ab¬ sence in the Cretaceous sediments represent a fun¬ damental change in the origin of some of the sedi¬ ments deposited in the South Carolina Coastal Plain. Specifically, many authors have used clinoptilolite and cristobalite as indicators of diagenetically al¬ tered volcanic and volcaniclastic materials in the Atlantic and Gulf Coastal Plains (Heron, 1969: Reynolds, 1970; and others). Figure 5.—Stratigraphic distribution of heavy minerals in the sand-sized fraction of the core sediments. An interesting and significant heavy-mineral suite, perhaps related to the tectonic history of the Charleston area, is present in the upper half of the Black Creek Formation at 455 m (1,492 ft). This suite contains 95 percent andalusite. The remaining 5 percent consists of biotite, minor cordierite, and trace amounts of zircon, tourmaline, garnet, am- phibole, kyanite, epidote, chloritoid, and sillimanite. The dominant association of andalusite, biotite, and cordierite suggests that the source of the assemblage was the contact aureole of a magma body intruded into pelitic rocks. The unusually high concentration of one mineral, in this case andalusite, suggests that this heavy-mineral suite was deposited very close to its source area. Some credence is given to this idea by the presence of Cretaceous!?) basalt in the Clubhouse Crossroads core and the presence of diabase in the nearby Sum¬ merville test well (Cooke, 1936). The unroofing of Cretaceous or older Mesozoic shallow intrusive rocks and their contact zones could have provided a local FORMATION DESCRIPTIONS Cape Fear Formation The Cape Fear Formation overlies the basalt and contains unconsolidated interbedded clay and sand. The lower 13 m (42 ft) is composed primarily of red or brown clay and less common feldspathic sand and has a basal unit of muddy conglomeratic sand. The clay is typically silty, noncalcareous, and unfossilif- erous, and contains a trace of mica and pyrite. The clay has a knobby appearance and lacks primary sedimentary structures. Rare muddy feldspathic sand is interbedded with the nodular clay. The sand is massive, noncalcareous, and typically contains 10 to 20 percent feldspar. The middle part (24 m, 80 ft) of the formation consists of thinly interbedded gray-olive silty clay and light-greenish-gray fine-grained sand. The sand contains abundant mica, and sparse shell fragments are found in both the sand and clay. The upper 22 m (72 ft) of the Cape Fear Formation is composed of knobby clay similar to that in the lower part of the formation. The upper clay is reddish-brown to yel¬ lowish-gray, noncalcareous, and micaceous. LITHOSTRATIGRAPHY OF DEEP COREHOLE 67 2450 ZIRCON TOURMALINE MONAZITE ® RUTILE 2031 APATITE /2v- ^ — (^1872 1923 ( 5 ) 2428 J- ®1934 2428. 865 .899 /\ 178 125 • _ KYANITE STAUROLITE GARNET SILLIMANITE AMPHIBOLE ANDALUSITE 1492 ’525 1003 # 237 1150 698 * *620 1751 1607 1634 1257 ® 302 EXPLANATION Sample below 540 m (1770 ft) Depth given in feet (1 ft equals 0.3048 m) Sample above 534 m (1751 ft) Depth given in feet Granitic minerals MIL) Metamorphic minerals of lower pressure/temperature M(H) Metamorphic minerals of higher pressure/temperature ' ~7 \ V M(H) - EPIDOTE CHLORITOID - MIL) Figure 6 .—Distribution of granitic, low-rank metamorphic, and high-rank metamorphic minerals in heavy-mineral suites of 57 core samples. Samples from above and below 540 m (1,770 ft) fall into two separate distinct fields. Middendorf Formation The Middendorf Formation is a thick sequence of feldspathic sand, clayey silt, and sandy and silty clay. The clay is typically pale red or reddish brown, or it may be red and gray green mottled. The fine-grained deposits are noncalcareous, micaceous, and lack pri¬ mary sedimentary structures. The sand is poorly sorted, feldspathic, and noncalcareous. Typical col¬ ors for the sand are mottled combinations of red, reddish brown, and gray green. The sand is fine to coarse grained, and thin quartz-pebble-rich conglom¬ eratic sand is common. Horizontal and inclined bed¬ ding are common in the sand. The clay, silt, sand, and conglomeratic sand are arranged in 1- to 5-m- thick (3—15—ft) fining-upward cycles. The cycles tend to be better defined and more complete in the upper half of the formation than in the lower half. Black Creek Formation The Black Creek Formation is the most hetero¬ geneous of the Upper Cretaceous formations in the ; core. Abundant fossiliferous silty clay, muddy sand, and clean sand alternate in 15- to 46-m-thick (50- 150 ft) sequences with thinly interbedded sand and clay and less common shelly limestone. The silty clay and muddy sand are typically medi¬ um-gray or gray-green, calcareous, fossiliferous sediments that resemble similar sediment types in the overlying Peedee Formation. Quartz sand and I silt constitute as much as 50 to 60 percent of the coarser grained beds, and calcium carbonate content reaches a maximum of 40 percent. Macrofossil shells and shell fragments and microfossil tests vary from sparse to very abundant. Unusually heavy concentra¬ tions of shells are preserved in impure, shelly lime- ! stone. Minor constituents of the mud and muddy sand are glauconite, phosphate, mica, and pvrite. Physical sedimentary structures are rare in the muddy sediments, probably because of extensive | bioturbation. Light-colored, feldspathic, quartz silt and fine sand are interbedded with dark-gray clay near the base 68 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 of the Black Creek. Bedding in this basal unit is planar, wavy, or discontinuous on a 1-10-mm scale. The clay contains as much as 20 percent black car¬ bonaceous debris. Well-sorted, calcareous quartz sand is found at the top of the Black Creek. Core recovery is poor in this interval, but relatively clean, poorly consolidated, very fine to fine-grained quartz sand was collected throughout the interval. Peedee Formation The Peedee Formation is a thick sequence of cal- ' careous muddy sand and calcareous mud. The lower half of the formation is dominantly medium-gray to olive-gray fossiliferous muddy sand. The sand typi¬ cally contains 50 to 60 percent fine- to medium¬ grained quartz sand, 30 to 40 percent clay and silt, and 10 to 20 percent whole macrofossils, shell frag¬ ments, and microfossils. Trace amounts to a few per¬ cent of sand-sized glauconite, phosphate, and mica are also present. The acid-soluble fraction consists almost entirely of fossils, although calcite-cemented nodules and layers do exist. The upper half of the formation is composed of medium-gray to olive-gray, silty or sandy calcareous clay. These beds are domi¬ nantly silty clay but may contain as much as 20 to 25 percent quartz sand and abundant macrofossils and microfossils. Glauconite, pvrite, phosphate, and mica are accessory constituents. Calcium carbonate content varies from 10 to 40 percent and represents both fossils and calcite cement. Physical sedimentary structures are rare in the Peedee and are restricted to thin intervals of wavy or disrupted laminations or thin beds. Biogenic sedi¬ mentary structures, including distinct burrows and bioturbated fabric, typify the poorly sorted Peedee sediments. Beaufortf ?) Formation The Beaufort (?) Formation is primarily com¬ posed of medium-gray-green silty clay above a basal unit of nodular muddy sand. The dominant sediment type is a yellowish-gray to greenish-gray, moderate¬ ly calcareous silty clay or locally sandy clay. Calcium carbonate averages 10 to 20 percent and is contained almost exclusively in microfossil tests. Accessory constituents include sand-sized glauconite, carbon¬ ized woody material, and pyrite. Physical sedimen¬ tary structures include planar, wavy, and disrupted laminae and thin bedding, and micrograded silt and fine sand. The bedding is typically disrupted or ob¬ literated by burrows. The basal 11 m (37 ft) of the formation consists of grav-green to light-gray glauconitic muddy sand containing an abundant microfauna. If physical sedi¬ mentary structures were originally present in this interval, they have been obliterated by intense bio- turbation. Calcite-cemented nodules averaging 60 to 80 percent calcium carbonate are common through¬ out; sediment between the nodules averages 15 to 25 percent calcium carbonate. Black Mingo Formation The Black Mingo is the most heterogeneous of the Tertiary formations recovered in the core. Much of the middle and lower part of the formation is gray- green bioturbated silty clay and muddy sand similar to those of the underlying Beaufort (?) and Peedee Formations. The uppermost 7 m (22 ft) of the for¬ mation is light-colored, fine-grained, impure lime¬ stone similar to the limestone in the upper part of the overlying Santee Limestone. Other sediment types in the Black Mingo are thin¬ ly interbedded sand and clay and less common quartzose shelly limestone. The interbedded sand and clay form a regularly alternating sequence of olive-gray silty clay that often contains abundant wood fragments, and light-colored, fine-grained, slightly calcareous quartz sand or shelly limestone. Stratification is typically in the form of flaser and lenticular beds that are disrupted by vertical and horizontal burrows. The shelly limestone is 0.3-1 m (1-3 ft) thick, and is typically composed of oyster and other pelecypod valves and fine- to medium¬ grained quartz sand. Santee Limestone The Santee Limestone is herein divided into three informal lithologic units: a lower bryozoan-pelecy- pod sand, 11m (35 ft) thick; a middle shelly lime¬ stone, 7 m (23 ft) thick; and an upper fine-grained limestone, 38 m (125 ft) thick. The basal part of the Santee consists of porous bryozoan-pelecypod sand. Calcium carbonate averages 60 to 80 percent; the remainder of the unit is composed of quartz and sand-sized glauconite and phosphate. Sedimentary structures are restricted to inclined bedding, prob¬ ably representing crossbedding, and to local concen¬ trations of the noncarbonate fraction that may represent burrows. The middle part of the formation is composed of vellowish-gray glauconitic, quartzose fossiliferous limestone. These rocks average 60 to 70 percent cal¬ cium carbonate. Whole shells and valves of pelecy- LITHOSTRATIGRAPHY OF DEEP COREHOLE 69 pods plus foraminifers and ostracode shells dom¬ inate the fauna. The upper unit resembles similar sediments in the overlying Cooper Formation but is a pale yellowish gray rather than the more greenish gray of the Cooper sediments. This unit is composed primarily of microfossil tests and fine quartz sand, with minor glauconite and clay minerals. Calcium carbonate averages 40 to 60 percent. Hard cemented nodules and occasional burrows are present in the fine¬ grained limestone. Cooper Formation The Cooper Formation is a monotonous sequence of impure limestone. The amount of calcium car¬ bonate ranges from 60 to 75 percent. Minor litho¬ logic components include quartz sand (5-25 percent), glauconite (1-10 percent), phosphatic sand and peb¬ bles (1-5 percent), bone material (1-5 percent), pelecvpod shell hash (1-5 percent), mica (1 per¬ cent), and clay minerals (10-30 percent). Subtle differences in the abundance of these components result in the variability observed in the Cooper sedi¬ ments. Colors of the sediment vary from moderate or pale greenish or yellowish gray to olive brown and pale olive. Grain-size analysis indicates that most of the Cooper is made up of sand-sized foram- iniferal tests (40-70 percent). Physical sedimentary structures are rare in the Cooper and are restricted to thin wavy laminae. The Cooper sediments are either thoroughly bioturbated or contain distinct burrows. DEPOSITIONAL HISTORY From the preliminary study of the core sediments, generalized depositiona! paleoenvironments may be assigned to the formations in the core (fig. 7). In figure 7, the terms “continental,” “marginal ma¬ rine,” “inner shelf,” and “middle to outer shelf” are each used to represent a spectrum of specific related paleoenvironments. The term “continental” poten¬ tially represents both fluvial sediments and residual materials. Marginal marine sediments accumulated in estuarine, lagoonal, tidal-flat, or barrier-system environments. Inner- and middle- to outer-shelf sediments were deposited on the marine shelf at various distances from the shoreline and under var¬ ious energy regimes. Unfossiliferous reddish clay and coarse-grained feldspathic sand in the Cape Fear and Middendorf Formations are assigned to the continental environ¬ ment. Rare fossiliferous clay in the Middendorf and FORMATION o (J f 5 1 i COOPER SANTEE LIMESTONE BLACK MINGO BEAUFORT!?) PEEDEE BLACK CREEK s MIDDENDORF - CAPE FEAR Figure 7.— Generalized paleoenvironments repre¬ sented by Cretaceous and Tertiary sediments of the Clubhouse Crossroads core. fossiliferous thinly interbedded sand and dark clay in the Cape Fear indicate a marginal marine setting for those specific beds. The heterogeneous Black Creek Formation represents a cyclic alternation of poorly sorted marine deposits and marginal marine beds including shelly or well-sorted sand and dark clay. The more homogeneous Peedee Formation con¬ tains sandy and silty marine clay that may also be present in cyclic units. The large-scale sequence of paleoenvironments found in the Upper Cretaceous sediments of the Clubhouse Crossroads core is superficially similar to that described by Swift, Heron, and Dill (Swift and Heron, 1969; Swift, Heron, and Dill, 1969) for sedi¬ ments in the Cretaceous outcrop belt of North and South Carolina. These authors described a vertical sequence composed of thin basal estuarine? sedi¬ ments (Cape Fear), then fluvial sediments (Midden¬ dorf), estuarine sediments (Black Creek), and neritic sediments (Peedee). In general, a large-scale transgressive sequence (Middendorf, Black Creek, Peedee) is indicated. The temporal distribution of sediments in the Clubhouse Crossroads core does not readily support the model of Swift, Heron, and Dill (1969). In the Clubhouse Crossroads core, most or all of the fluvial facies (Middendorf) is restricted to the Woodbinian 70 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 and lower Eaglefordian and is separated by a con- | siderable period of time from the upper Austinian and younger estuarine and neritic facies (Black > Creek and Peedee). Time transgression of facies is implicit in a transgressive sedimentary sequence. However, large time gaps should not exist in a ver¬ tical sedimentary sequence produced by transgres¬ sing lithosomes of a single horizontal facies sequence. The repetition of coarse- and fine-grained units ! in the Black Creek and Peedee Formations on a scale of 15-46 m (50-150 ft) is similar to that shown by cyclic units in the Cretaceous section of New Jersey described by Owens and Sohl (1969). Such transgressive-regressive cycles are on a smaller scale than the single transgression described by ' Swift, Heron, and Dill (1969) and would require more frequent shifts of the shoreline and of paleo- environments. These two concepts may possibly be reconciled, at least for the Black Creek and Peedee, if the smaller cycles are considered second-order J features superimposed on a generally transgressive upper Austinian to Navarroan sequence. In such a setting, environmental shifts within each of the smaller cycles may have been only of limited mag¬ nitude. Environmental shifts within Black Creek cycles would have involved marginal marine and marine lithotopes, whereas Peedee cycles would have involved only marine deposits of the outer, middle, and inner shelf. The fine-grained microfossiliferous Beaufort(?) Formation represents an open marine environment, whereas cyclic deposits in the Black Mingo Forma¬ tion include marine and marginal marine beds and represent a large-scale Tertiary regressive sequence. Fine-grained limestones of the Santee Limestone and Cooper Formation are monotonous bioturbated ma¬ rine deposits. Skeletal limestone in the Santee does not appear to be a part of a peritidal facies mosaic and is also assigned a marine (subtidal) origin. Pooser (1965), Colquhoun and Johnson (1968), Inden and Zupan (1976), and others have assigned Tertiary sediments in South Carolina to a wide range of marine and nonmarine environments. Their studies are based primarily upon surface and sub¬ surface data from the updip outcrop belt. Predict¬ ably, the more downdip location of the Clubhouse Crossroads core is reflected in the dominantly ma¬ rine nature of the Tertiary sediments in the core. REFERENCES CITED Brown, P. M., 1958, Well logs from the Coastal Plain of North Carolina: North Carolina Div. Mineral Resources Bull. 72, 68 p. , - 1959, Geology and ground-water resources in the Greenville area, North Carolina: North Carolina Div. Mineral Resources Bull. 73, 87 p. Colquhoun, D. J., and Johnson, H. S., Jr., 1968, Tertiary sea- level fluctuations in South Carolina, in Tanner, W. F., ed., Tertiary sea-level fluctuations: Paleogeography, Paleoclimatology, Paleoecology, v. 5, no. 1, p. 105-126. Cooke, C. W., 1936, Geology of the Coastal Plain of South Carolina: U.S. Geol. Survey Bull. 867, 196 p. Heron, S. D., Jr., 1969, Mineralogy of the Black Mingo mud- rocks: South Carolina Div. Geology, Geol. Notes, v. 13, no. 1, p. 27-41. Inden, R. F., and Zupan, Alan-Jon W., 1976, Facies and facies equivalents of the Santee Limestone (Lower Ter¬ tiary) in South Carolina: Geol. Soc. America Abs. with Programs, v. 8, no. 2, p. 204-205. Malde, H. E., 1959, Geology of the Charleston phosphate area, South Carolina: U.S. Geol. Survey Bull. 1079, 105 p. Owens, J. P., and Sohl, N. F., 1969, Shelf and deltaic paleo- environments in the Cretaceous-Tertiary formations of the New Jersey Coastal Plain, in Subitzky, Seymour, ed., Geology of selected areas in New Jersey and east¬ ern Pennsylvania and guidebook of excursions: New Brunswick, N.J., Rutgers Univ. Press, p. 235-278. Pooser, W. K., 1965, Biostratigraphy of Cenozoic ostracoda from South Carolina: Kansas Univ. Paleont. Contr., Arthropoda, art. 8, p. 1-80. Reynolds, W. R., 1970 Mineralogy and stratigraphy of Lower Tertiary clays and claystones of Alabama, in Symposium on environmental aspects of clay minerals: Jour. Sed. Petrology, v. 40, no. 3, p. 829-838. Rhodehamel, E. C., 1975, Geophysical logs from a geologic test hole near Charleston, South Carolina: U.S. Geol. Survey open-file report 75-247, 1 p. Siple, G. E., 1975, Ground-water resources of Orangeburg County, South Carolina: South Carolina Div. Geology Bull. 36, 59 p. Swift, D. J. P., and Heron, S. D., Jr., 1969, Stratigraphy of the Carolina Cretaceous: Southeastern Geology, v. 10, no. 4, p. 201-245. Swift, D. J. P„ Heron, S. D., Jr., and Dill, C. E., Jr„ 1969, The Carolina Cretaceous—Petrographic reconnaissance of a graded shelf: Jour. Sed. Petrology, v. 39, no. 1, p. 18-33. Biostratigraphy of the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, South Carolina, By J. E. HAZEL. L. M. BYBELL, R. A. CHRISTOPHER. N. O. FREDERIKSEN. F. E. MAY, D. M. McLEAN, R. Z. POORE. C. C. SMITH, N. F. SOHL. P. C. VALENTINE, and R. J. WITMER STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1S86-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-F CONTENTS Page Abstract _ 71 Introduction_ 71 Biostratigraphy _ 73 Stage placement _ 73 Correlation _ 75 Paleoenvironment_ 75 Trangressions and regressions_ 75 Rates of sedimentation_ 78 Page Fauna and flora_ 80 Ostracodes _' 80 Cretaceous larger invertebrates_ 81 Tertiary calcareous nannofossils_ 82 Foraminifers_ 83 Spores and pollen_ 85 Dinofiagellates _ 86 References cited_ 88 ILLUSTRATIONS Page Figure 1 . Generalized geologic map of South Carolina showing location of the Clubhouse Crossroads corehole 1 _ 72 2. Stratigraphic column and spontaneous potential and resistivity logs for the Clubhouse Crossroads core _ 73 3. Generalized correlation chart for the Upper Cretaceous and lower Tertiary of the Atlantic and Gulf Coastal Plains_ 76 4. Diagram showing generalized transgressive and regressive cycles in the Gulf and Atlantic Coastal Plains_ 78 TABLES Page Table 1 . The thickness, duration, and calculated sedimentation rate for each provincial stage represented in the Clubhouse Crossroads core_ 79 2. The thickness, duration, and calculated sedimentation rate for each European stage represented in the Clubhouse Crossroads core _ 79 ill . ' STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA. EARTHQUAKE OF 1886— A PRELIMINARY REPORT BIOSTRATIGRAPHY OF THE DEEP COREHOLE (CLUBHOUSE CROSSROADS COREHOLE 1) NEAR CHARLESTON, SOUTH CAROLINA By J. E. Hazel, L. M. Bybell, R. A. Christopher, N. O. Fredericksen, F. E. May, D. M. McLean, R. Z. Poore, C. C. Smith, N. F. Sohl, P. C. Valentine, and R. J. Witmer abstract The biostratigraphy is summarized as follows: Microfossils (calcareous nannoplankton, dinoflagellates, foraminifers, ostracodes, and sporomorphs) and mollusks have been used to date the sedimentary' part of a 792-m (2,599—ft) core from a test hole (Clubhouse Crossroads corehole 1) drilled 40 km (25 mi) west-northwest of Charles¬ ton, S.C. The sedimentary section is 750 m (2,462 ft) thick and is of Late Cretaceous and (except for a few meters of probable Pleistocene) early Tertiary age. The drillhole bot¬ tomed in amygdaloidal basalt of Cretaceous (?) age. The Cretaceous section is composed almost entirely of clastic sediments. The oldest Cretaceous sedimentary unit is the Cape Fear Formation which contains Cenomanian (Woodbinian) fossils. The Middendorf Formation overlies the Cape Fear. Rare fossils in the Middendorf indicate a Cenomanian (lower Eaglefordian) placement. No fossils suggestive of a Turonian or Coniacian Age (middle Eagle¬ fordian to early Austinian) ware found. The overlying Black Creek Formation is of probable late Santonian and early Campanian Age (late Austinian and early Tayloran). The youngest Cretaceous unit is the Peedee Formation. The Peedee contains assemblages indicative of a late Cam¬ panian to middle Maestrichtian Age (late Tayloran and Navarroan). The Cretaceous-Tertiary boundary is at 244 m (800 ft). The oldest Tertiary unit is the Beaufort (?) Formation, the clays and sands of which contain early' Paleocene as¬ semblages (Danian; early and middle Midwayan). Overly¬ ing the Beaufort (?) are clayey sands and sandy clays as¬ signed to the Black Mingo Formation. This unit is of Paleo¬ cene and early Eocene age (Thanetian and Ypresian; late Midwayan and Sabinian). The overlying Santee Limestone is of middle and late Eocene age (late Lutetian and Bar- tonian; late Claibornian and Jacksonian). The youngest Tertiary formation present is the Cooper Formation. The lower part of the Cooper is late Eocene (Bartonian; Jack¬ sonian). The upper part, however, is of late Oligocene age (Chattian; Chickasawhayan). Both the Cooper and the Santee are dominantly carbonate. Formation Thickness (m) Age 5 Pleistocene (?). Cooper... 64 Chattian and Bartonian. Santee __ 66 Bartonian and Lutetian. Black Mingo _ 67 Ypresian and Thanetian. 62 Peedee _ 164 Maestrichtian and Campanian. Black Creek ... 169 Campanian and Santonian (?). 124 Cape Fear _ 69 Cenomanian. Palynomorphs, foraminifers, and ostracodes were studied from both the Cretaceous and Tertiary parts of the core. Mollusks have been examined from only the Cretaceous part, and calcareous nannofossils from only the Tertiary. INTRODUCTION The sedimentary rocks of a 792-m (2,599-ft) core from the Clubhouse Crossroads corehole 1 (CCC 1) in Dorchester County, S.C., (fig. 1) have been ex¬ amined for micro- and macrofossils. Most of the core is 15 cm (6 in.) in diameter for approximately the upper 225 m (738 ft) and 7 cm (2.75 in.) in diameter for the remaining 567 m (1,861 ft). The following groups have been studied in some detail: calcareous nannofossils (by Bybell), dinoflagellates (by May, McLean, and Witmer), mollusks (by Sohl), ostracodes (by Hazel and Valentine), planktic foraminifers (by Poore and Smith), and sporo¬ morphs (by Christopher and Frederiksen). Other calcareous forms are rare or absent, and no siliceous microfossils were observed. In this paper, the lithostratigraphic units deline- ! ated in the core are dated paleontologically, assigned to commonly used provincial and international j stages, and correlated with other lithostratigraphic 1 units in the Atlantic and Gulf Coastal Province. In- 71 72 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 80° - L —r — 1 7 ---j- | Lower Upper Cretaceous 0 50 100 KILOMETERS Figure 1 . —Generalized geologic map of South Carolina showing location of the Clubhouse Crossroads corehole 1. The exact site is in the Cottageville, 1943, 15' quadrangle, at lat 32°53.25' N., long 80°21.4' W., 3.5 km (2.2 mi) southwest of Clubhouse Crossroads and 40 km (25 mi) west-northwest of Charleston, S.C. Geology modified from Cooke (1936). terpretations as to environment of deposition are also presented. Most of the studies are still incom¬ plete at this writing, and investigations are continu¬ ing. We are confident of the age assignments of most of the beds seen in the core, but some assignments are tentative, and examination of additional samples in some intervals will be needed to document more precisely both zone and stage boundaries. At pres¬ ent, paleoenvironmental conclusions are very gen¬ eralized and quite tentative. This paper is divided into two parts—in the first part, our interpretations are presented on biostratig¬ raphy and paleoenvironment of the core; in the sec¬ ond part are short discussions on the occurrence, value, and degree of study of the six biologic groups used. Without the excellent technical support of many persons, this report could not have been prepared in the time allotted. Ellen E. Compton, Diane V. Mc- Neave, W. A. Bryant, and Patricia B. Swain of the U.S. Geological Survey deserve special mention for their efforts. This study was funded in part by the U.S. Nuclear Regulatory Commission, Office of Nuclear Regula¬ tory Research, under agreement number AT (49- 25)-1000. BIOSTRATIGRAPHY OF THE DEEP COREHOLE 73 BIOSTRATIGRAPHY STAGE PLACEMENT CLUBHOUSE CROSSROADS CORE DORCHESTER COUNTY, SOUTH CAROLINA Figure 2 is a generalized lithologic log of the core showing the spontaneous potential and resistivity curves and the formations recognized. The lithologies present are discussed in some detail by Gohn and others in this volume. Except for the upper 5 m (16 ft) of unconsolidated Pleistocene(?) sediments, the core is entirely of Paleogene and Late Cretaceous age. Below the Pleistocene (?) to a depth of 137 m (449 ft), the deposits are dominantly carbonate. The lowermost sedimentary rocks, present in the core above 13 m (43 ft) of mottled red clay which perhaps represents weathering of the underlying basalt, extend from 750 m (2,462 ft) to about 691 m (2,268 ft) and are assigned to the Cape Fear For¬ mation. Rare late Cenomanian planktic foraminifers are present in the Cape Fear, and pollen in the unit suggests a Woodbinian assignment. The Woodbinian is placed in the middle Cenomanian by Pessagno (1969). Beds assigned to the Middendorf Formation are present between about 691 m (2,268 ft) and 567 m (1,860 ft). These beds are largely unfossiliferous; however, planktic foraminifers of the Rotalipora cushmani~R. greenhornensis Subzone of late Ceno¬ manian Age (early Eaglefordian) are present at 586 m (1,923 ft). This indicates that the unfossil¬ iferous interval between 586 and 560 m (1,923-1,837 ft), including whatever disconformities that may be present, represents the Turonian and Coniacian ! Stages, or in provincial terms, the middle and upper Eaglefordian and most, if not all, of the lower Aus- tinian. According to the time scale of van Hinte (1976), this interval would approximate 11 million years. An erosional disconformity probably is pres¬ ent in this 26-m (85-ft) interval, although the pale¬ ontological data do not indicate definitely whether this presumed disconformity is between the Mid¬ dendorf and Black Creek Formations or within the Middendorf. Heavy-mineral data (Gohn and others, this volume) suggest that it is at the base of the Black Creek Formation. The sands and clays between 567 and 408 m (1,860-1,340 ft) are assigned to the Black Creek Formation. The fauna and flora of the Black Creek in the core indicate that the unit is of questionable Santonian to Campanian Age and is referable to the upper part of the Austinian and lower part of the Tayloran Provincial Stages. No diagnostic fos¬ sils have been found in the lower 7 m (23 ft) of the Black Creek. The interval from about 560 to 533 m O t DEPTH EXPLANATION ▲ Glauconite • Phosphate -- Fine-grained - - ■ limestone Coarse-grained limestone v Sandy 1 ' ; ' limestone S 1 — Silty clay i - — Sandy clay LiJrj Clayey sand ■ ;Sand ——- Unconformity Figure 2. —Stratigraphic column for the Clubhouse Cross¬ roads core. Spontaneous potential (S.P.) and resistivity logs are also shown. 74 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 (1,837-1,749 ft) contains mollusks, ostracodes, and i pollen of early late Austinian Age, which may be Santonian (Pessagno, 1969) or early Campanian (Young, 1963) in age. The Austinian-Tayloran boundary in the core is between 522 and 473 m (1,713-1,552 ft). Foraminifer data suggest that this provincial stage boundary should be placed in | the lower part of this interval, between 522 and 487 m (1,713-1,598 ft). Ostracode data, on the other hand, suggest that the stage boundary is at about 480 m (1,575 ft). For the purposes of this paper, the last depth is tentatively accepted, with the real- I ization that the Austinian-Tayloran boundary may ultimately be revised downward in the core (but al¬ most certainly not upward). The uppermost Cretaceous lithostratigraphic unit recognized in the core is the Peedee Formation; it is represented by sandy clays and clayey sands within the interval between 408 and 244 m (1,340- 800 ft). The lower beds of the Peedee from between 408 and 335 m (1,340-1,100 ft) are Tayloran on the basis of both benthic macrofaunas and benthic and planktic microfaunas. The mollusks, forami- nifers, ostracodes, dinoflagellates, and sporomorphs of the 335- to 244-m (1,100- to 800-ft) interval in- j dicate that these beds are Navarroan. The Cam- panian-Maestrichtian boundary is at about 335 m (1,100 ft). According to Pessagno (1969), the Campanian-Maestrichtian and provincial Tayloran- Navarroan Stage boundaries are coincident. This assignment is followed herein, although this prac¬ tice is not accepted by many workers, and more re¬ search on the problem is called for. The more argil¬ laceous upper part of the formation from between 296 and 281 m (971-922 ft) to 244 m (800 ft) is of middle Maestrichtian age as determined by plank¬ tic foraminifers, and is assignable to the Globotrun- cana gansseri Subzone of Pessagno (1969). Overlying the Peedee is a silty- to sandy-claystone unit not known from outcrops in the area of the Clubhouse Crossroads core. Beds of the same age do crop out to the northeast on the Black River near Georgetown, S.C. The unit is at least biostratig- raphically equivalent in part to and is here question¬ ably referred to the Beaufort Formation of North Carolina (see Gohn and others, this volume). The stratigraphic interval between 244 and 192 m (800- 630 ft) is of Danian Age. The upper part, from about 213 m (700 ft) to 192 m (630 ft), is in the P 2 planktic foraminifer zone, and the lower part is in the P 1 zone. The Black Mingo Formation overlies the Beau¬ fort (?) Formation and is present from 192 to 125 m (630-410 ft). The unit consists mostly of clayey sands and sandy clays; sandy limestone beds are in the middle and at the top of the formation. The base of the Sabinian Stage is at 180 m (590 ft) ; thus, the lower 12 m (40 ft) of the Black Mingo, as iden¬ tified in the core, is of Midwayan Age. Above this, from 180 m to about 132 m (590-433 ft), the Black Mingo is late Paleocene in age (Thanetian). The upper beds of the Black Mingo, at least from 132 to 125 m (433-410 ft), are lower Eocene (Ypresian and upper Sabinian). The upper bed of the Black Mingo can be placed in the NP 12 nannoplankton zone, which is the middle zone of the Ypresian (Berggren, 1972). An unconformity between the Black Mingo and the Santee Limestone seems to represent the upper Ypresian and the lower Lute¬ tian. The Santee Limestone is present from 125 to 69 m (410-227 ft) in the core. The lower part of the Santee, from 125 to 102 m (410-336 ft), is of late Lutetian Age (NP 16-17) and is correlative with upper Claibornian formations in the Gulf Coast (Bybell, 1975). Nannofossils suggest that the mid¬ dle Eocene-late Eocene boundary (Lutetian-Bar- tonian) is between 104 and 101 m (341-331 ft) in the core. This age assignment is consistent with os¬ tracode data that suggest that the Claibornian- -Jacksonian boundary is between 110 and 95 m (361- 312 ft). The boundary is placed at 102 m (336 ft) in the core, at a point where the resistivity increases markedly. The interval between 104 and 82 m (341- 269 ft) can be placed in the upper Eocene nannofos- sil zones NP 13 or NP 19, which are approximately equivalent to P 15 and P 16 in the planktic foram¬ inifer zonation of Blow (1969) (see Berggren, 1972, p. 203). The contact between the lower part of the Cooper Formation and the underlying Santee Limestone is quite sharp. The upper beds of the Santee are ex¬ tensively bored, and the borings are filled by glauconitic calcareous sediment of the lower part of the Cooper. Phosphate pebbles are concentrated near the boundary; some pebbles are as much as 3 cm (1.2 in) in diameter. The beds above and below this contact are upper Eocene (Bartonian and Jack¬ sonian), however, and both are placed in nannofos- sil zone NP 20. The Cooper Formation, as that term has been used in South Carolina (for example, Pooser, 1965; Sanders, 1974), is present from 69 to 5 m (227-16 ft) and contains beds of late Eocene and Oligocene age. At least in the area of the Clubhouse Crossroads core, the contact between the Eocene and Oligocene BIOSTRATIGRAPHY OF THE DEEP COREHOLE 75 parts of the Cooper is an unconformity representing the lower Oligocene (Lattorfian and Rupelian Stages), a duration of approximately 5 million to 6 million years (Berggren, 1972). This indicates that the lower part of the Cooper is part of a separate depositional cycle, even though the lithologies of the upper and lower parts perhaps cannot be consistently differentiated in the field without fossil control. The Cooper Formation present between 69 and 55 m (227-180 ft) is of late Eocene age on the basis of its planktic flora and fauna. Ostracodes suggest assignment to the Jacksonian Provincial Stage. The upper 50 m (164 ft) of the Cooper from a depth of 55 to 5 m (180-16 ft) is of Oligocene age. Ostra¬ codes, particularly, suggest the presence of at least upper Vicksburgian sediments, and the Vicksbur- gian-Chickasawhayan boundary is placed at about 35 m (115 ft) depth. However, calcareous nanno- plankton zones NP 21, 22, and 23 (see Martini, 1971) are seemingly absent; all of the Cooper above 55 m (180 ft) is referable to NP 24, therefore, suggesting that the entire upper part of the Cooper is Chickasawhayan and upper Chattian. However, foraminifers questionably indicating planktic fo- raminifer zone P 20 are present at 52 m (171 ft), and P 20 is considered indicative of the lower Chat¬ tian and equates with the upper part of nannoplank- ton zone NP 23 according to Berggren (1972). Thus, the nannoplankton, ostracode, and planktic foram- inifer data are not compatible, although the apparent age disagreement is comparatively slight. The prob¬ lem probably is in the correlation of the ostracode range zones with the nannoplankton (NP) and planktic foraminifer (P) zones. Data from all three groups do indicate that most, if not all, of the lower Oligocene is missing. For the purposes of this paper, the upper part of the Cooper Formation (above 55 m; 180 ft) is con¬ sidered to be late Vicksburgian and Chickasawhayan in age, and foraminifer zone P 20 is placed in the lower Chattian (and upper Vicksburgian), which was suggested as a possibility by Berggren (1972, fig. 3). Thus, the entire Oligocene part of the Cooper is considered to be Chattian. CORRELATION Figure 3 is a generalized chart showing our con¬ clusions as to correlation of the units of the Club¬ house Crossroads core with outcropping lithostrati- graphic units in the Atlantic and Gulf Coastal Plains. The figure is self-explanatory, but some comments are appropriate. Note that several of the lithostratigraphic units of the Carolinas approach stage magnitude in temporal extent. Note also the significant segments of time that are not represented by sediments in the core or in the North Carolina outcrop. PALEOENVIRONMENT Incomplete studies of the fauna and flora of the Clubhouse Crossroads core lead to the following con¬ clusions. The Cape Fear Formation represents inner sublittoral to brackish-water conditions of deposi¬ tion (also see Swift and Heron, 1969). The Midden- dorf Formation was deposited under fluvial to marginal marine conditions. The lower part (San- tonian?) of the Black Creek contains a nearshore possibly brackish-water assemblage, whereas the upper part was deposited at inner to middle sub¬ littoral depths. The Peedee Formation was generally deposited in middle to outer sublittoral environ¬ ments. The lower part of the Beaufort ( ?) Formation was deposited at middle to outer sublittoral depths; the upper part was deposited in the inner to middle sub¬ littoral zone. The lower part of the Black Mingo was deposited in the middle to inner sublittoral zone with considerable oscillation in the depth of deposi¬ tion, but the upper part of the formation seems to represent outer sublittoral deposition. The lower part of the Santee Limestone probably represents middle to outer sublittoral depths. The upper part of the Santee and both the lower and upper parts of the Cooper Formation were deposited at outer sub¬ littoral (outer shelf) or even greater depths. TRANSGRESSIONS AND REGRESSIONS Figure 4 is a generalized comparison of transgres¬ sive and regressive cycles in the northeast Texas and Mississippi embayments with those in the core and with those in other areas in the Atlantic Coastal Plain. The Clubhouse Crossroads core contains more major hiatuses, w ? hich represent regressions or pe¬ riods of nondeposition. Otherwise, the curves for the Gulf Coast are generally similar to the curve for the core, except (1) in late Eaglefordian time, when there w'as regression in the eastern Gulf Coastal re¬ gion and Atlantic Coastal region and transgression in the western Gulf Coastal region; and (2) in late Oligocene time, when apparently there w^as major transgression in the southern Atlantic Coastal re¬ gion but minor regression in the Gulf Coastal re¬ gion. The Jacksonian and Chickasaw r havan transgres¬ sions of the core did not take place in the Salisbury embayment of the Virginia-Maryland-Delaware re- 76 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 SERIES European Stage Provincial Stage UJ UJ Chattian Chickasawhayan g O Rupelian Vicksburgian lattorfian Barton ian Jacksonian EOCENE Lutetian Claiborman Ypresian Sabmian Thanetian 1 - UJ O o UJ < Danian Midwayan k 1 Maesirichtian ii Navarroan Campanian Tayioran GO h o L < UJ cc Santonian Austiman o CO a_ Comacian ID Turoman Eaglefordian r Cenomanian Woodbmian New Jersey North Carolina Clubhouse Crossroads core Trent Marl (part) Shark River Formation Castle Hayne Limestone Manasquan Formation Vincentown Formation Homerstown Sand Red Bank Sand Navesmk Formation Beaufort Formation Mount laurel Sand Wenonah Fm. Peedee Formation 1 Marshalltown Fm Englishtown Fm Woodbury Clay Merchantville Formation Black Creek Formation TT Magothy Fm, Raritan Formation Middendort Fm - 7 - Cape Fear Fm - 7—1 - 1 Upper part of Cooper Formation Lower part of Cooper Formation Santee Limestone Black Mingo Formation Beaufort (?) Formation Peedee Formation Black Creek Formation Middendort Fm Cape Fear Fm. T he Sabmetown, Rockdale and Segum Formations of ’he Wilcox Group of Plummer 119331 are herein adopted for U S Geological Survey useage 'The Bergstrom and Sprinkle Formations of Young 1965) are herein adopted for US Geological Survey useage Figure 3. —Generalized correlation chart for. the Upper Cretaceous and lower Tertiary of the Atlantic and Gulf Coastal Plains. The interpreted stratigraphic position of the formations of the Clubhouse Crossroads core is shown. The Cape Fear Formation at Clubhouse Crossroads is un¬ derlain by basalt which has yielded K-Ar whole-rock ages of 94.8 m.y. and 109 m.y. Gottfried and others (this volume) present arguments that these are probably mini¬ mum ages and that the basalt is most likely of Late Triassic or Early Jurassic age. BIOSTRATIGRAPHY OF THE DEEP COREHOLE 77 Clubhouse Crossroads core Chattahoochee River area Western Alabama Texas Upper part of Cooper Formation Paynes Hammock Sand Chickasawhav Formation Byram Formation Catahoula Sandstone (part) i—,— Marianna Limestone Fled Bluff Clay CJ o CD Lower part of Cooper Formation Santee Limestone Black Mingo Formation Beaufort (?) Formation Ocala Limestone Whitsett Fm Yazoo Clay Manning Fm. Wellborn Sandstone Moodvs Branch Formation -?- Moodys Branch Formation Lisbon Formation Gosport Sand Lisbon Formation Caddell Fm Moodys Branch Formation Yegua Fm Cook Mountain Fm Sparta Sand Tallahatta Fm Tallahatta Fm. Wecnes Fm Hatchetigbee Formation Tuscahoma Fm Nanafalia Fm "i r -J—?—c- Clayton Formation Hatchetigbee Formation K Queen City Sand Reklaw Fm Camzo Sand Sabmetowri Fm.’ Tuscahoma Fm Rockdale Fm 1 Nanafalia Fm Seguin Fm Naheola Fm Porters Creek Clay Wills Point Fm Clayton Fm Kincaid Fm o o Peedee Formation Black Creek Formation Middendorf Fm Cape Fear Fm. m Providence Sand Prairie Bluff Chalk Ripley Fm Ripley Fm Cussetta Sand Member Demopolis Chalk Corsicana Marl Nacatoch Sand Nevtandville Marl Bergstrom Fm? Pecan Han Chall Blufftown Formation Mooreville Chalk Eutaw Fm Eutaw Fm Tuscaloosa Formation Gordo Fm -?- Coker Fm Wolfe City Sand Sprinkle Fm 2 Burditt Marl Dessau Fm Lower Austin Group i : i ; i: South Bosque Formation -Lake,Waco Fm.. Lake Waco Fm Pepper Shale Member of Woodbine Formation 3 O cj < Figure 3.—Continued. 78 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 —*■ Regression Transgression ■*— Figure 4.—Generalized transgressive and regressive cycles in the Gulf and Atlantic Coastal Plains. gion or in the Raritan embayment of New Jersey. Published (Brown and others, 1972, plates 24, 27) and unpublished data indicate, however, that sedi¬ ments deposited during the Chickasawhayan trans¬ gression are present as far north as the Richmond, Va., area. RATES OF SEDIMENTATION By use of the time scales recently published by Berggren (1972), Obradovich and Cobban (1975), and van Hinte (1976), the duration in millions of years (m.y.) for the provincial and European stages recognized in the study of the Clubhouse Crossroads core has been calculated. From these data, the rate of sedimentation per million years for each stage also can be calculated. However, because differences in compaction rates of the lithologies present have not been taken into consideration, the values can only be considered rough approximations. Table 1 lists the thickness, duration, and calculated sedimen¬ tation rate for each provincial stage, and table 2 gives the same data for each European stage. Miss¬ ing section is taken into consideration. For example, the fossils suggest that only the upper Claibomian (upper Lutetian) is present; thus the sedimentation rate given, 7.7 m/m.y. (25 ft/m.y.), is based on 3.0 m.y. rather than on the 6.0-m.y. estimated duration of the Claibornian Stage (Berggren, 1972). Where significant parts of stages are represented by dis- conformities, the stages are divided into parts BIOSTRATIGRAPHY OF THE DEEP COREHOLE 79 —► Regression Transgression ■*— Figure 4.—Continued. Table 1 . —The thickness, duration, and calculated sedimen- j tation rate for each provincial stage represented in the Clubhouse Crossroads core Table 2. —The thickness, duration, and calculated sedimen¬ tation rate for each European stage represented in the Clubhouse Crossroads core Stage Time represented (m.y.) Thickness (m) Rate (m/m.y.) Chickasawhayan _ 6.0 30 5.0 Upper Vicksburgian _ 2.0 20 10.0 Lower Vicksburgian _. 0 0 Jacksonian ... 5.5 47 8.5 Upper Claibornian _ 3.0 23 7.7 Lower Claibornian _ 0 0 Sabinian _ 7.0 55 7.9 Midwayan _ 6.0 64 10.7 Navarroan __ 3.0 91 30.3 Tayloran _ — 6.0 145 24.2 Upper Austinian . 7.0 87 12.4 Lower Austinian _ _ 0 0 Middle and Upper Eaglefordian 0 0 Lower Eaglefordian _ 2.7 124 45.9 Woodbinian _ _ 2.7 59 21.9 Stage Time represented (m.y.) Thickness (m) Rate (m/m.y.) Chattian __ 8.0 60 6.3 Rupelian _ 0 0 Lattorfian _ _ 0 0 Bartonian _ 6.5 47 8.5 Upper Lutetian _ 3.0 23 7.7 Lower Lutetian _ 0 0 Ypresian _ 2.6 12 4.8 Thanetian _ 6.6 55 8.5 Danian_ _ 4.0 52 13.0 Upper Maestrichtian __ 0 0 Lower and Middle Maestrichtian 3.0 91 30.3 Campanian __ 8.0 198 24.8 Santonian (?) __ 5.0 34 6.8 Coniacian ___ 0 Turonian _ _ 0 0 Middle and Upper Cenomanian 6.4 183 33.9 80 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 (lower, middle, or upper) to reflect this in the tables. The biostratigraphic assignment of the lithostrat- igraphic units of the core suggests that a rapid rate of clastic sedimentation during the Woodbinian and early Eaglefordian (middle and late Cenomanian) was followed by a regression resulting in the ab¬ sence of middle and upper Eaglefordian and lower Austinian deposits (Turonian and Coniacian). After this event, the rate of clastic deposition increased throughout the remainder of the Cretaceous. The end of this period of marine deposition is marked by the unconformity at the Cretaceous-Tertiary boundary. Rates of sedimentation were generally lower in the Tertiary. The sedimentation rate decreased dur¬ ing the Paleocene and early Eocene, culminating in an unconformity seemingly equivalent to the upper part of the Sabinian and the lower part of the Clai- bornian. The Paleocene and lower Eocene sediments, like the Cretaceous, are dominantly clastic. The re¬ mainder of the Tertiary was characterized by two episodes of carbonate deposition apparently sepa¬ rated by a regressive phase as evidenced by an un¬ conformity representing the lower Oligocene. FAUNA AND FLORA As stated in the “Introduction,” six fossil groups were studied—calcareous nannofossils, dinoflagel- lates, mollusks, ostracodes, planktic foraminifers, and sporomorphs. However, not all groups were found or studied throughout the core. To date, mol¬ lusks have been studied only in the Cretaceous, and calcareous nannofossils only in the Tertiary. OSTRACODES Ostracodes are absent below 538 m (1,765 ft) ex¬ cept for a small assemblage in the Cape Fear Formation at 721 m (2,365 ft). The presence of F ossocytheridea lenoirensis Swain and Brown, 1964, at 721 m (2,365 ft) suggests placement in the brack¬ ish water facies of Unit F of Brown and others (1972). At 538 m (1,765 ft) Asciocythere macropunctata (Swain, 1952) occurs with “Cytheveis” canteriolata Crane, 1965, and other species. This sample is prob¬ ably correlative in part with Unit C of Brown and others (1972). Assemblages of probable late Aus¬ tinian Age are present from a depth of about 538 m (1,765 ft) to about 481 m (1,578 ft). An assemblage at 481 m (1,578 ft) includes Haplocytheridea? nanifaba Crane, 1965, Cytheveis dallasensis Alex¬ ander, 1929, B r achycythere pyriforma Hazel and Paulson, 1964, and “Phacorhabdotus” pokornyi Hazel and Paulson, 1964 (late form). This assem¬ blage suggests correlation with the Burditt Marl or lowermost beds of the Sprinkle Formation of Texas. Assemblages suggesting uppermost Austinian to, perhaps, lower Tayloran are present from between 481 and 473 m (1,578-1,552 ft) up to 408 m (1,340 ft). The early to middle Tayloran species “ Cytheveis” plummevi Alexander, 1929, is present at 408 m (1,340 ft) and at 398 m (1,306 ft). Haplocytheridea insolita (Alexander and Alexander, 1933), “Veenia” gapensis (Alexander, 1929), and Brachycy there povosa Crane, 1965, also occur in the deeper sample. At 385 m (1,263 ft), the Tayloran markers Haplo¬ cytheridea insolita and “Veenia” gapensis again occur. From between 385 and 366 m (1,263-1,201 ft) to just below 322 m (1,056 ft), the ostracode as¬ semblage consists of species that occur in both the Navarroan and the Tayloran. The Peedee Formation above 322 m (1,056 ft) contains a typical Navar¬ roan assemblage including such species as Haplo¬ cytheridea renfroensis Crane, 1965, “Cytheveis” pidgeoni Berry, 1925, Brachycythere ovata Berry, 1925, “ Cytheveis ” huntensis (Alexander, 1929), and Haplocytheridea everetti (Berry, 1925). A diverse assemblage indicative of an early Mid- wavan Age is present in-the lower part of the Beau- fort(?) Formation. Occurring in the lower part of the Beaufort (?), among others, are Phractocyther- idea ruginosa Alexander, 1934, Loxoconcha atlan- tica (Alexander, 1934), Hermanites gibsoni Hazel, 1968, H. midivayensis (Alexander, 1934), Phacor¬ habdotus sculptilis (Alexander, 1934), Acanthocy- thereis icashingtonensis Hazel, 1968, Brachycythere plena Alexander, 1934, and Opimocythere browni Hazel, 1968. Ostracode assemblages of late Mid- wayan Age are not well known in the Gulf and At¬ lantic Coastal Plains. Therefore, although ostracodes occur in the lower part of the Black Mingo Forma¬ tion and upper part of the Beaufort (?) Formation in the core, they have not been as biostratigraphi- cally definitive as the other fossil groups studied in this interval. Ostracode assemblages suggesting a Sabinian Age are present in the core from about 150 m (492 ft) to at least 125 m (410 ft). Ouachitaia broussardi (Howe and Garrett, 1934), Haplocytheridea leei (Howe and Garrett, 1934), Acanthocythereis hil- gardi (Howe and Garrett, 1934), Buntonia alaba- mensis (Howe and Garrett, 1934), Phractocyther- idea moodyi (Howe and Garrett, 1934), “Cytheveis” dictyolobus of Pooser (1965), Hermanites bassleri BIOSTRATIGRAPHY OF THE DEEP COREHOLE 81 (Ulrich, 1901), and Opimocythere cf. 0. nanafaliana (Howe and Garrett, 1934), occur in this interval. The Claibornian indicators Opimocythere martini (Murray and Hussey, 1942) and Actinocythereis gosportensis (Blake, 1950) occur at about 110 m (361 ft). Ostracodes are fairly common in that part of the Cooper assigned to the Jacksonian, 69-55 m (227-180 ft). Cytheretta jacksonensis (Meyer, 1887), Acanthocythereis spinomuralis Howe and Howe, 1973, Echinocythereis jacksonensis (Howe and Chambers, 1935), Acanthocythereis fiorienensis (Howe and Chambers, 1935), and several others are typical Jacksonian species that are present in this interval. Several Vicksburgian guide species occur in the core between 55 and 35 m (180-115 ft). These in¬ clude Actinocythereis dacyi (How’e and Law% 1936), A. thomsoni (Howe and Law, 1936), Buntonia sul¬ cata Butler, 1963, and Buntonia huneri Howe and Law, 1936. The Chickasawhavan upper part of the Cooper Formation contains a diverse ostracode as¬ semblage of about 40 species. Actinocythereis way- nensis Butler, 1963, and Leguminocythcreis scarabaeus of Brown (1958) and Pooser (1965), are two of the diagnostic species that occur in this interval. In the uppermost part of the Cooper, above about the 10-m (33-ft) depth in the core, species common in lower Miocene sediments, for example Echinocythereis clarkana (Ulrich and Bassler, 1904), make their first appearance. CRETACEOUS LARGER INVERTEBRATES Macrofossils are common at many Upper Creta¬ ceous levels in the core between 537 and 245 m (1,763-804 ft). Although the shelled larger inver¬ tebrate groups of worms, sponges, bryozoans, scap- hopods, and cephalopods are all represented, the assemblages are dominated by bivalves and gastro¬ pods. No single sample contains more than 20 species of mollusks, but this is only because of the limited 1 nature of the core samples. Samples equivalent in volume to outcrop collections would undoubtedly yield greater diversity, including many of the larger forms not recoverable in cores. Below 537 m (1,763 ft) only one sample, at 721 m (2,365 ft), contained fossil mollusks; these are poorly preserved and nondiagnostic. The species Ostrea cretacea Morton, 1834, was found from 537 to 535 m (1,763-1,754 ft). This species is common to the upper part of the Eutaw Formation of the Chattahoochee River region of Alabama and Georgia, and to the Tombigbee Sand Member of the Eutaw of central and western Ala¬ bama ; it has been reported from the uppermost part of the Magothy Formation at Cliffwood, N.J. In addition, it has been encountered in wells in North and South Carolina at a similar stratigraphic level. In outcrop, the Tombigbee Sand Member contains ammonites of the upper Austinian which Young (1963) has assigned to the early Campanian. Up to 444 m (1,456 ft) the association of abundant Lucina glebula Conrad, 1875, Veniella mullenensis Stephenson, 1923, Camptonectes per- lamellosa Whitfield, 1885, and others is similar to assemblages that occur through the middle and lower parts of the Blufftown Formation of the Chat¬ tahoochee River region and in the Woodbury Clay of New Jersey. The occurrence of a large Trigonarca 1 and Aphrodina of the A. regia Conrad, 1875, type and several other mollusks in the assemblages at 411 and 410 m (1,347 and 1,345 ft) suggests a correla¬ tion with the type section of the Snow Hill Member of the Black Creek Formation of North Carolina and the basal part of the Cusseta Sand Member of the Ripley Formation of Georgia and eastern Ala¬ bama. The low r est depth in the core at w T hich Exogyra ponderosa Roemer, 1849, is found is 354 m (1,162 ft). This species ranges through the upper Austinian into the Tayloran and forms the basis for a strati- graphically broad but well-recognized zone through¬ out the Gulf and Atlantic Coastal Plains. Navarroan molluscan assemblages are present from a depth of at least 332 m (1,090 ft) to 245 m (804 ft). Specimens of Exogyra assignable to E. costata Say, 1820, occur sporadically throughout this interval; the varietal form spinifera is common. In outcrop, the Exogyra costata zone has been con¬ sidered coordinate in range with the Navarroan. ■ Flemingostrea subspatulata (Forbes, 1845), occurs commonly in samples at depths between 325 and 314 m (1,066-1,030 ft). This species ranges through the lower and middle Navarroan, but the specimens from the core material represent the form common to the lower part of the range zone that on the out¬ crop is associated with Exogyra cancellata Stephen¬ son, 1914. The Exogyra cancellata zone is considered lower Navarroan. Thus, the interval from about 335 to about 314 m (1,100-1,030 ft) appears to be as¬ signable to the E. cancellata zone. This early form of Flemingostrea subspatidata occurs in the upper¬ most part of the Wenonah Formation of New Jersey, the basal part of the Peedee Formation in North Carolina, and in the upper part of the Cusseta Sand Member of the Ripley Formation of Georgia and eastern Alabama. 82 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 TERTIARY CALCAREOUS NANNOFOSSILS Calcareous nannofossils are present throughout the Paleogene part of the core except for a 6-m (20- ft) interval at the Paleocene-Eocene boundary. Coccolith abundance and diversity are less than nor¬ mally observed in Gulf Coast Paleogene material, but coccoliths are present in sufficient numbers to be one of the more biostratigraphically useful groups in the Clubhouse Crossroads core. Martini’s (1971) standard calcareous nannoplank- ton zonation contains 25 NP zones in the Paleocene through Oligocene. Because many calcareous nanno- fossil marker species are absent from the core, and because several unconformities are present, only eight of these zones can be confidently recognized. The Beaufort (?) Formation from 244 to 192 m (800-630 ft) is entirely within the lower Paleocene NP 3 zone (Danian). Some of the lowest nannofos- sil diversities are in the Paleocene part of the core; however, the first appearances of Chiasmolithus danicus (Brotzen, 1959) and Coccolithus pelagicus (Wallich, 1877) within this interval indicate zone NP 3. Other typical Paleocene species are Cocco- lithus cribellum (Bramlette and Sullivan, 1961), Cruciplacolithus tenuis (Stradner, 1961), Neococ- colithes protenus (Bramlette and Sullivan, 1961), and Zygolithus sigmoides (Bramlette and Sullivan, 1961). The Black Mingo Formation from 192 to 125 m (630-410 ft) is divided among three zones—NP 4, NP 5-9, and NP 12. The first late Paleocene (Thanetian) species appear within this formation. Chiasmolithus bidens (Bramlette and Sullivan, 1961) and Toweius craticulus Hay and Mohler, 1967, occur in the assemblage and indicate the presence of zone NP 4. From 177 to 138 m (581—454 ft), the Black Mingo Formation covers zones NP 5 through NP 9 (upper Paleocene). Because of low species diversity, indi¬ vidual zones within this interval could not be recog¬ nized. Ellipsolithus distichus (Bramlette and Sullivan, 1961) and Fasciculithus typmpaniformis Hay and Mohler, 1967, have their first occurrences within the NP 5 zone, and Discoaster multiradiatus Bramlette and Riedel, 1954, has its extinction at the top of zone NP 9. Between 138 and 136 m (451 and 445 ft), the Black Mingo Formation is barren of calcareous nan¬ nofossils, and this interval may include an uncon¬ formity encompassing zones NP 10-11. From 131 to 125 m (431 to 410 ft), the Black Mingo Forma¬ tion is within the lower Eocene (Ypresian) NP 12 zone. Here the first Discoaster barbadiensis (Tan Sin Hok,- 1927) Sphenolithus moriformis Bron- nimann and Stradner, 1960), and several discolith species, among them Pontosphaera duocava (Bram¬ lette and Sullivan, 1961), P. pulchra (Deflandre, 1954), P. ocellata (Bramlette and Sullivan, 1961), and P. vesca (Sullivan, 1965), are found. Some late Paleocene species have been reworked into these sediments. The contact of the Black Mingo Formation with the overlying Santee Limestone represents an un¬ conformity covering zones NP 13-16. The lower part of the Santee Limestone from 125 to 104 m (410-341 ft) is in zones NP 16-17 (middle Eocene, Lutetian). Typical middle Eocene species first oc¬ curring within this interval include Blackites spi- nosvs (Deflandre and Fert, 1954), Cyclococcolithus formosus Kamptner, 1963, Cyclococcolithus retic- ulatus Gartner and Smith, 1967, H elicopoyitosphaera compacta (Bramlette and Wilcoxon, 1967), Ponto¬ sphaera pulcheroides (Sullivan, 1956), Reticulo- fenestra umbilica (Levin, 1965), and Zygrhablithus bijugatus (Deflandre, 1954). The calcareous nannofossil species diversity in¬ creases from 16 species in the Paleocene part of the core to 54 species in the upper Eocene. Part of this increase is due to a normally increasing- diversity of calcareous nannofossils in the Paleocene and Eocene, but in the Clubhouse Crossroads core, it is also due to either more favorable living conditions in the water column or better preservation of upper Eocene sediments. Several species, normally com¬ mon throughout the middle and upper Eocene, are present only in upper Eocene sediments in the core. The Santee Limestone interval between 101 and 83 m (331-271 ft) represents zones NP 18-19 (up¬ per Eocene, Bartonian). Helicopontosphaera bram- letti Muller, 1970, H. euphratis (Haq, 1966), and Sphenolithus obtusits Bukry, 1971, first appear here, and many other typical middle and late Eocene species are also found. Pentaliths are especially abundant within this interval. The uppermost part of the Santee Limestone and lowermost part of the Cooper Formation, 81-55 m (266-180 ft) can be placed in the upper Eocene NP 20 zone. Sphenolithus pseudoradians Bramlette and Wilcoxon, 1967, and Helicopontosphaera intermedia (Martini, 1965) first occur in this zone, whereas species with their last occurrences here include Dis¬ coaster barbadiensis (Tan Sin Hok, 1927), D. saipanensis Bramlette and Riedel, 1954, and Mic- rantholithus procerus Bukry and Bramlette, 1969. An unconformity occurs within the Cooper For¬ mation at 55 m (180 ft) and represents zones NP BIOSTRATIGRAPHY OF THE DEEP COREHOLE 83 21-23. From 55 to 5 m (180-16 ft) the Cooper For¬ mation is in zone NP 24 (upper Oligocene, Chat- j tian), which is marked by the first occurrence of Helicopontosphaera recta (Haq, 1966) and Poiv- tosphaera clathrata (Roth and Hay, 1967). Helio¬ cop ontosphaera compacta Bramlette and Wilcoxon, 1967, and Sphenolithus predistentus Bramlette and Wilcoxon, 1967, which become extinct within zone NP 24, are found throughout this interval. Consid¬ erable reworking of late Eocene species into the lower half of this zone has taken place. FORAMINIFERS Late Cretaceous planktic foraminifers are com¬ mon to abundant throughout much of the Cretaceous strata penetrated in the Clubhouse Crossroads core; they are particularly abundant in the Campanian and Maestrichtian parts of the Peedee and Black Creek Formations. Samples from these units have yielded superbly preserved faunas, many having a species diversity nearly identical to the rich faunas documented from the Upper Cretaceous of the west¬ ern Gulf Coastal Plain area (Pessagno, 1967; Smith and Pessagno, 1973). Within the Campanian-Mae- strichtian interval, however, is strong faunal and floral evidence, supported by lithologic data, of rapid and in some instances rather extensive changes in paleoenvironmental depositional settings. Within these more shallow neritic intervals, diversity of the j planktic foraminifers is quite low (although pres¬ ervation remains excellent) and is accompanied by the appearance of or increase in abundance of typi¬ cal shallow-water benthic foraminifers. The more shallow neritic parts of the Campanian and Mae¬ strichtian sections, as well as the majority of pre- Campanian samples that were studied, are characterized by low species diversity and the ab¬ sence of many key species normally present in more open marine paleoenvironments. This has resulted in questionable biostratigraphic assignments for several samples. In instances such as these, the zonal assignment of the sample is questioned or, where practicable, referred to an interval of two or more biostratigraphic zones. The biostratigraphic zonal assignment and chronostratigraphic correlation of these samples closely follows that utilized by Pes¬ sagno (1967, 1969) in his studies of the western Gulf Coast. Both the Middendorf and the Cape Fear Forma¬ tions, from the total depth of available samples, 741- 579 m (2,430-1,900 ft), contain a restricted shallow- water foraminiferal fauna, many samples being barren of planktic foraminifers. However, this interval contains rare individuals of Heterohelix moremani (Cushman), Hedbergella brittoneyisis Loeblich and Tappan, Globigerinelloides cf. G. caseyi (Bolli, Loeblich, and Tappan), and very abundant and excellently preserved Guembelitria harrisi Tap- pan, which are referable to the Rotalipora cushmani- greenhornensis Subzone of late Cenomanian Age. This subzone is also present within the upper parts of the Woodbine Formation and about the lower half of the Eagle Ford Group of Texas (Pessagno, 1967; 1969). In the interval from 579 to 522 m (1,900—1,713 ft), all samples that were studied were barren or contained no biostratigraphically important planktic foraminiferal faunas. In the lower part of the Black Creek Formation, a sample at 522 m (1,713 ft) con¬ tained Marginotruncana angusticarenata (Gan- dolfi), as well as several species of the genus Archaeoglobigerina. The concurrent range of these species is indicative of the Marginotruncana co'n- cavata Subzone of Santonian Age. The Black Creek section between 518 and about 494 m (1,700-1,620 ft) contains a sparse and poorly ! preserved foraminiferal fauna lacking the key [ species that would permit a precise biostratigraphic zonal assignment. Tentatively these faunas can be referred to the lower and middle Campanian Globo¬ truncana fomicata and Archaeoglobigerina blowi Subzones. According to Pessagno (1969), these subzones are representative of the upper part of the Austinian and the lower part of the Tayloran Pro¬ vincial Stages. A sample at 487 m (1,599 ft), in the middle part of the Black Creek Formation, con¬ tains a moderately diverse planktic foraminiferal fauna that can be assigned to the Archaeoglobig erina blowi Subzone of middle Campanian Age. Samples from the upper part of the Black Creek Formation and the lower part of the Peedee Forma¬ tion, from about 480 to 375 m (1,575-1,230 ft), are assignable to the Campanian Globotruncana elevata Subzone. Planktic foraminifers within this interval are very abundant, moderately diverse, and gen¬ erally well preserved. Characteristic of this interval is the overlap between the ranges of Globig erinel¬ loides multispina (Lalicker) and Globotruncana I linneiana (d’Orbigny) and those of Ventilabrella glabrata (Cushman), Rug o globig erina trading- housensis Pessagno, and Globotmincana elevata (Brotzen). Units normally placed in the middle and upper parts of the Tayloran Provincial Stage con¬ tain an assemblage characteristic of this subzone (see Pessagno, 1967, 1969; Olsson, 1975; Petters, i 1976). Lower and middle parts of the Peedee strata 84 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 within the interval between about 375 and 312 m (1,230-1,025 ft) are predominantly shallow ma¬ rine; they contain nondiagnostic planktic foram- inifers but probably are equivalent to either the upper Campanian Globotruncana elevata Subzone or to the lower Maestrichtian Rugotruncana subcir- cumnodifer Subzone. Samples from the Peedee within the interval be¬ tween about 312 and. 290 m (1,025-950 ft) are as¬ signable to the lower Maestrichtian Rugotruncana subcircumnodifer Subzone. Although the planktic foraminiferal faunas within this interval are less diverse than those from overlying strata, the pres¬ ence of Globigerinelloides yaucoensis (Pessagno), Archaeoglobigerina blowi Pessagno, Globotruncana bulloides (Vogler), and G. fornicata Plummer, is indicative of an early Maestrichtian Age and in¬ dicates placement of the interval in the lower part of the Navarroan Provincial Stage. The upper part of the Peedee Formation, within the interval between 290 and 244 m (950-800 ft), is assignable to the middle Maestrichtian Globotmn- cana gansseri Subzone. Samples within this section have yielded an excellently preserved and diverse planktic formaminiferal assemblage including abun¬ dant individuals of Guembelitria cretacea Cushman, Heterohelix glabrans (Cushman), Plano glob ulina carseyae (Plummer), Pseudotextularia deformis (Kikoine), Globotruncana aegyptiaca Nakkady, G. duwi Nakkady, G. gansseri Bolli, G. trinidadensis Gandolfi, Rugoglobigerina hexacamerata Brbnni- mann, R. reicheli Bronnimann, and Globotruncanella monmouthensis (Olsson). None of these species have been documented from strata older than mid¬ dle Maestrichtian in age (Smith and Pessagno, 1973). Planktic foraminiferal faunas assignable to the uppermost Maestrichtian Abathomplalus may- aroensis Subzone were not found during this inves¬ tigation. Because an obvious erosional unconformity exists between the Maestrichtian Peedee Formation and the overlying Danian Beaufort(?) Formation, and because both lithologic units are richly micro- fossiliferous, it seems reasonable to presume that strata assignable to the A. mayaroensis Subzone were removed (rather than never deposited) during latest Maestrichtian and (or) Danian time. Examination of the rather poorly preserved planktic foraminifers from the Cenozoic parts of the Clubhouse Crossroads core has resulted in the following biostratigraphic zonal assignments and age determinations. The lower part of the Beau¬ fort (?) strata from its unconformable contact with the underlying Peedee Formation at 244 m (800 ft) to approximately 215 m (705 ft) contains Globo- conusa daubjergensis (Bronnimann) and other species diagnostic of the G. daubjergensis Zone (zone P 1) of early Paleocene (early Danian) age. Beaufort(?) strata in the interval from between about 215 and 201 m (705 and 659 ft) are ques¬ tionably assigned to the Morozovella uncinata—M. angidata Zone (P 2) of late early Paleocene (late Danian) age. The uppermost 8-15 m (25-50 ft) of the Beau¬ fort (?) Formation and the basal few 7 feet of the Black Mingo Formation contain Morozovella angu- lata (Bolli), Subbotina triloculinoides (Plummer), S. pseudobulloides Plummer, Planorotalites com- pressa (Plummer), and P. ehrenbergi (Bolli). This assemblage is typical of the early part of the late Paleocene (early Thanetian) and is assignable to the Morozovella pusilla pusilla-M. angidata Zone (P 3). A sample at 170 m (557 ft) from the middle of the Black Mingo Formation contains a fauna that includes Morozovella angidata (Bolli), M. pusilla (Bolli), Planorotalites imitata (Subbotina), P. ehrenbergi (Bolli), P. cf. P. pseudomenardii (Bolli), P. compressa (Plummer), and Truncorota- loides esnaensis (Le Roy). This assemblage is refer¬ able to the Planorotaloides pseudomenardii Zone (zone P 4) of late (although not latest) Paleocene age. The upper part of the Black Mingo Formation contains a distinctive early Eocene planktic forami¬ niferal assemblage, although because of the lack of diagnostic species, the Paleocene-Eocene boundary (between zones P 6a and P 6b, Berggren, 1972) can be defined no more precisely than somewhere within the interval between 169 to 133 m (555-435 ft). Most of the Santee Limestone strata within the interval between about 120 and 77 m (393-254 ft), contain poorly preserved and generally nondiag¬ nostic planktic foraminiferal assemblages assignable to zones P 11 through P 14 of middle Eocene (Clai- bornian) age. The lower part of the Cooper Formation contains a rare and generally poorly preserved fauna, al¬ though species such as Cassigerinella eocaena Cor- aey, Chiloguembelina cubensis (Palmer), C. martini (Pijpers), Pseudohastigerina barbadoensis Blow, Globigerina ouachitaensis Howe and Wallace, G. angiporoides Hornibrook, G. praebulloides Blow, Hantkenina primitiva Cushman and Jarvis, Globi- gerinatheka mexicana (Cushman), and Globorotalia cerroazidensis (Cole) are present and indicate a late Eocene age assignment (zones P 15-17 of Blow, 1969). BIOSTRATIGRAPHY OF THE DEEP COREHOLE 85 A sample from the Cooper Formation at 57 m (188 ft) is questionably referred to the Globigerina tapuriensis Zone (P 18) of early Oligocene age; however, a late Eocene age assignment is also quite possible. If the sample at 57 m (188 ft) is Oligocene, then the Eocene-Oligocene boundary is between 59 m (193 ft) and 57 m (188 ft). If the sample at 57 m (188 ft) is of late Eocene age, then the Eocene- Oligocene boundary is somewhat higher, between 57 and 55 m (188-179 ft). From about 55 to 25 m (179-83 ft), the presence of forms related to Globi¬ gerina angulisuturalis Bolli suggests assignment to the upper part of the Globigerina ampliapertura Zone (zone P 20 of Blow). Foraminifers recovered between about 25 and 10 m (83-32 ft) are referable to zone P 21. Samples from the upper part of the Cooper For¬ mation, from a depth of 10-5 m (32-16 ft), contain i Cassigerinella chipolensis (Cushman and Ponton), j Globigerina angulisuturalis Bolli, G. anguliofficin- alis Blow, G. angustiumbilicata Bolli, G. ciperoensis Bolli, and Globoquadrina cf. G. globularis Ber¬ mudez, among others, which suggest correlation with the Globigerina angulisuturalis—Globorotalia opima Zone or the Globigerina angulisuturalis Zone (zones P 21-22 of Blow, 1969) of late Oligocene age. SPORES AND POLLEN Very little has been published on spores and pol¬ len from the Tertiary of the Atlantic Coast; there¬ fore, study of the Clubhouse Crossroads core has provided an opportunity to gather the first detailed information on the stratigraphic distribution of spores and pollen in the Paleogene rocks of the southern Atlantic Coastal Plain. No spore-pollen zonation of the Paleogene rocks of the Gulf Coast has been proposed in the literature, but the ranges of many Gulf Coast species are fairly well known from the Midwayan to the lower Vicksburgian and can be compared with the ranges of the species de¬ termined in the South Carolina material. All samples below 733 m (2,404 ft) were barren | of palynomorphs. Spore-pollen assemblages from the Cretaceous System of the core were correlated with the informal palynological zones established by Brenner (1963), Doyle (1969), S^.rkin (1974), and Wolfe (1976) for the Middle Atlantic States. From 733 to 714 m (2,404-2,342 ft), the assem¬ blage is considered equivalent to Doyle’s (1969) Zone IV, on the basis of samples from the Wood- bridge Clay Member of the Raritan Formation of New Jersey. At 714 m (2,342 ft), this assemblage includes only the Normapolles genera Atlantopollis and Complexiopollis, along with Tricolpites cras- simurus (Groot and Penny, 1960) Singh, 1971, “Re- titricolpites” georgensis Brenner, 1963, and “R” geranioides (Couper, 1958) Brenner, 1963. All samples examined between 714 and 580 m (2,342-1,902 ft) in the core were barren of paly¬ nomorphs. Between the 580- and 550-m (1,902- and 1,804-ft) depths in the core, units equivalent to the South Amboy Fire Clay Member of the Raritan For¬ mation were identified on the basis of the presence of Porocolpopollenites spp., Labrapollis sp., Tria- triopollenites spp., Complexiopollis spp., Santalacites spp., and Pseudoplicapollis spp. A comparison of samples from the Clubhouse Crossroads core with outcropping units from New Jersey indicates that the biostratigraphic equiva¬ lents of the Magothy Formation extend from 550 to 520 m (1,804-1,706 ft) in the core. Within this in¬ terval occur the Normapolles Praecursipollis sp., Santalacites spp., Complexiopollis funiculus Tschudy, 1973, C. abditus Tschudy, 1973, Pseudo¬ plicapollis spp., Plicapollis rusticus Tschudy, 1975, and Trudopollis spp., along with an abundance of oblate triangular foveoreticulate tricolporates. In the Middle Atlantic States, Wolfe’s (1976) Zone CA-2 encompasses the entire Merchantville Formation of the Raritan embayment and the lower part of the Merchantville Formation of the Salis¬ bury embayment. In the core this zone is recognized between 520 and 490 m (1,706-1,607 ft) ; it is char¬ acterized by an assemblage that includes Holkopol- lenites sp. A (CP3D-1), Plicapollis rusticus Tschudy, 1975 (NE-1), Propylipollis sp. B (PR-1), Complexiopollis abditus Tschudy, 1973 (NB-1), Santalacites sp. (NB-2), and Osculapollis cf. O. per- spectus Tschudy, 1975 (NO-4). (Note: the alphanu¬ meric code following each binomen refers to Wolfe’s, 1976, species designation.) Zone CA-3 of Wolfe (1976), established for the Woodbury Clay of the Raritan embayment and the upper part of the Merchantville Formation of the Salisbury embayment, extends from 490 to 430 m (1,607-1,410 ft) in the core. Within this interval the following species occur: Brevicolporites sp. A (CP3F-1), Tricolporites sp. C (C3C-2), and Hol- kopollenites sp. B (CP3D-2). In the core, the biostratigraphic equivalent of the Englishtown Formation of the Middle Atlantic States (Wolfe’s, 1976, Zone CA^f) was difficult to delineate. However, the assemblage that character¬ izes this unit in both the Salisbury and Raritan em- bayments and in what is interpreted to be English¬ town equivalent in the core includes Santalacites sp. 86 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 (NB-2), ''.Plicapollis sp. A (ND-1), PlicapoUis rusticus Tschudy, 1975 (NE-1), and Holkopol- lenites sp. B (CP3D-2). The unit extends from the 430- to 420-m (1,410- to 1,378-ft) depths in the core. Wolfe’s (1976) Subzone CA-5, characteristic of the Marshalltown Formation of the Raritan and Salisbury embayments, is between 420 and 360 m (1,378-1,181 ft) in the-core. The concurrent ranges of Complexiopollis abditus Tschudy, 1973 (NB-1), Pseudoculapollis admirabilis Tschudy, 1975 (NR- 1) , IPlicapollis sp. B (ND-2), Holkopollenites cf. i H. chemardensis Fairchild in Stover, Elsik, and Fairchild, 1966 (CP3D-3), and Plicapollis retusus Tschudy, 1975 (NE-3), among others, were used I to identify this biostratigraphic unit. Beds correlative with the Mt. Laurel Sand and Wenonah Formation of the Middle Atlantic States (Subzone CA-5 B of Wolfe, 1976) exist between the 360- and 290-m (1,181- and 951-ft) depths in the 1 core. The occurrence of Pseudoplicapollis endocuspis Tschudy, 1975 (NC-2), Osculapollis aequalis Tschu- dy, 1975 (NO-1), Triatriopollenites sp. (NP-2), I Labrapollis sp. (NV-1), and Pseudovacuopollis in- volutus Tschudy, 1975 (NT-1), help to correlate this interval. Biostratigraphic equivalents of the Navesink For¬ mation and Red Bank Sand of the Raritan embay- j ment and of the Monmouth Group of the Salisbury j embayment (Wolfe’s, 1976, Zone CA-6 MA-1) were identified between the 290- and 244-m (951- and : 800-ft) depths in the core. The lower boundary of this interval is marked by the last occurrence of Pseiidoplicapollis endocuspis Tschudy, 1975 (NC- | 2) , and ? Plicapollis sp. C (ND-3) and by the first appearance of Plicatopollis sp. (NN-2) and Momipi- tes sp. (NK-3). The Cretaceous-Tertiary boundary in the Club¬ house Crossroads core is marked by the first appear- 1 ances of Momipites coryloides Wodehouse, 1933, j Momipites cf. M. inaegualis Anderson, 1960, and Favitricolporites baculoferus (Pflug in Thomson and Pflug, 1953) Srivastava, 1972. Whether these three species also have first occurrences at this horizon in the Gulf Coast is not knowm. The middle Midwayan is characterized by the first occurrences of Trudopollis plena Tschudy, 1975, and probably also Triporopollenites n. sp. A (thin- walled) of Tschudy, 1973. The last occurrence of Pseudoplicapollis serena Tschudy, 1975, is within the upper Midwayan in both the Gulf Coast arid the core and is one of the most important spore-pollen extinc¬ tion events of the Paleogene. The first occurrence of Aesculiidites circumstriatus (Fairchild in Stover and others, 1966) Elsik, 1938, is at about the Mid- w T ayan-Sabinian boundary in both regions. In the middle Sabinian of the Gulf Coast and in the core, the best datum is provided by the last occurrences of Momipites dilatus (Fairchild in Stover and others, 1966) Nichols, 1973, and Momipites spp. of the Tenuipolus Group. The upper Sabinian (lower Eo¬ cene) both of the Gulf Coast and of the core from 137 to about 125 m (450-410 ft) is distinguished by the concurrence of Thomsonipollis magnifica (Pflug in Thomson and Pflug, 1953) Krutzsch, 1960, and N ax pollenites spp. and especially by the rather high relative abundance of Platycarya spp. and Platy- caryapollenites spp. In the Gulf Coastal Plain and in the Clubhouse Crossroads core, the Jacksonian and the uppermost Claibornian contain similar spore-pollen assem¬ blages. Important marker species present for this part of the Paleogene are Quercoidites microhenricii (Potonie, 1931) Potonie, 1960, and Pollenites ven- tosus Potonie, 1931. Nuxpollenites spp. is regularly present in the Claibornian but is extremely rare in the Jacksonian. The Chickasawhayan and late Vicks- burgian, 55- to 5-m (180- to 16-ft), spore-pollen assemblages of the Clubhouse Crossroads core are dominated by Cupressacites spp., Quercus spp., and a new r species of Momipites that is most similar morphologically and stratigraphically to Triatri¬ opollenites coryphaeus s. str. (Potonie, 1931) Thom¬ son and Pflug, 1953. Many species found in the lower Vicksburgian of the Gulf Coast are lacking from the upper Vicksburgian and Chickasawhayan of South Carolina, including Momipites coryloides Wode¬ house, 1933, and M. microfoveolatus (Stanley, 1965) Nichols, 1973. DINOFLAGELLATES Phytoplankton, primarily dinoflagellates, are pres¬ ent in many of the stratigraphic intervals of the Clubhouse Crossroads core. Dinoflagellates are rare in the deepest interval sampled, 734—714 m (2,404- 2,342 ft). No phytoplankton were observed in sam¬ ples from an interval between 704 and 580 m (2,309- 1,902 ft), and diversity is very low- between 559 and 543 m (1,835-1,781 ft). Dinoflagellates are present in moderate abundance and diversity from between 543 and 518 m (1,781-1,700 ft) to 244 m (800 ft). From 244 to 198 m (800-649 ft), dinoflagellates are abundant, but diversity is generally low. In the in¬ terval from 190 to 124 m (623—407 ft), dinoflagel¬ lates are virtually absent. Betw r een 120 and 5 m BIOSTRATIGRAPHY OF THE DEEP COREHOLE 87 (394-16 ft) dinoflagellates are generally abundant and diversity is variable. Only three species of dinoflagellates were observed in the core below 559 m (1,835 ft). Odontochitina costata Alberti, 1961, Florentinia lasciniata Davey and Verdier, 1975, and Oligosphaeridium pidcherri- mum (Deflandre and Cookson) Davey and Williams, 1966, occur between 733 and 714 m (2,404-2,342 ft). The concurrent ranges of these species suggest a Cenomanian Age for the interval. Because of low species diversity, the sampled in¬ terval between 559 and 543 m (1,835-1,781 ft) could not be dated precisely. The concurrence of the ranges of Deflandrea granulifera Manum, 1963, and Palaeo- hystrichophora infusorioides Deflandre, 1935, sug¬ gests an age not older than Santonian. On the basis of the concurrent ranges of Horo- loginella apiculata Cookson and Eisenack, 1962, De¬ flandrea sverdrupiana Manum, 1963, and Palaeosto- macystis laevigata Drugg, 1967, it is suggested that the 518- to 383-m (1,700- to 1,257-ft) interval is of Campanian Age. Other species present in this inter¬ val are Dinogymnium undulosum Cookson and Eisenack, 1970, Dinogymnium euclaensis Cookson and Eisenack, 1970, Dinogymnium digitus (Deflan¬ dre) Evitt and others, 1967, Dinogymnium denticu- ! latum (Alberti) Evitt and others, 1967, and Deflan¬ drea echinoidea Cookson and Eisenack, 1960. The assemblages in the 366- to 305-m (1200- to 1000-ft interval of the core are distinctly different from those of the Maestrichtian units above. The most characteristic feature is the presence of species j that appear to be restricted to the Marshalltown and W^enonah Formations of New Jersey. Included in the flora are Deflandrea victoriensis Cookson and Ma¬ num, 1964, Deflandrea cf. D. armata Cookson and Eisenack, 1970, Palaeocystodinium sp., Dinogymni¬ um sp., and Amphidinium mitratum Vozzhennikova, 1967. The apparent correlation of this interval with the Marshalltown and W 7 enonah Formations of New Jersey is also supported by the occurrence of the fol¬ lowing species: Odontochitina costata Alberti, 1961, Deflandrea n. sp., and Trigonopyxidia ginella (Cook¬ son and Eisenack) Downie and Sarjeant, 1964, Palaeohystrichophora infusorioides Deflandre, 1935, and Phoberocysta ceratioides Deflandre, 1937. The age of this interval is late Campanian. The interval from 296 to 270 m (970-885 ft) con¬ tains an assemblage that, in terms of the outcrop sec¬ tion, is a mixture of species restricted to either the Mount Laurel Sand and older units of the northern Atlantic Plain or to the Navesink and Red Bank 1 interval. These forms are Deflandrea pannucea- \ Stanley, 1965, Gonyaidacysta sp., Achomosphaera ramulifera (Deflandre) Evitt, 1963, Ophiobolus lapi- daris W T etzel, 1933, Samlandia angustivela (Deflan¬ dre and Cookson) Eisenack, 1963, Palaeohystricho¬ phora infusorioides Deflandre, 1935, Systematophora n. sp., and Phoberocysta ceratioides Deflandre, 1937. This assemblage is what one might expect in rocks equivalent to the disconformitv between the Mount Laurel Sand and the Navesink Formation of north¬ ern New Jersey. The suggested age is late Campani¬ an or early Maestrichtian. Between 267 and 244 m (875—800 ft), dinoflagel- late-acritarch assemblages are similar to those of the Navesink Formation-Red Bank Sand sequence of northern New Jersey. Species restricted to both this interval in the core and the Navesink-Red Bank interval are Palaeocystodinium australinum (Cook¬ son) Lentin and WTlliams, 1976, Gonyaulacysta sp., Deflandrea speciosa Alberti, 1961, Drugg, 1967, and Dinogymnium westralium (Cookson and Eisenack) Evitt and others, 1967. Between 267 and 259 m (875 and 850 ft), the assemblage also contains three un¬ described species that are known to occur elsewhere only in the Navesink Formation of New Jersey. These are new species of Dinogymnium, Hystricho- kolpoma, and Trithyrodinium. The above occur¬ rences suggest that the 267- to 244-m (875- to 800-ft) interval is Navesink equivalent (lower Maestrichtian). The presence of at least three species of the Cre¬ taceous genus Dinogymnium at 245 m (804 ft) and 244 m (800 ft) suggests that the Cretaceous-Terti¬ ary boundary is at about 244 m (800 ft). Between 244 and 198 m (800-649 ft) an assem¬ blage is present that suggests an early Paleocene age and correlation, at least in part, with the Bright- seat Formation of Maryland. Present in this inter¬ val are Deflandrea n. sp., which is known only from the Brightseat Formation; Deflandrea magnifica Stanley, 1965; Deflandrea obscura Drugg, 1967; De¬ flandrea dilwynensis Cookson and Eisenack, 1965; Spiniferites buccina (Davey and Williams) Sar¬ jeant, 1970; Spiniferites septatus (Cookson and Eisenack) McLean, 1971; and Spinidinium densi- pinatum Stanley, 1965. 'In the interval from 190 to 124 m (623-407 ft), dinoflagellates are too rarely represented to be of biostratigraphic value. The interval from 120 to 55 m (394-179 ft) is tentatively considered to be of mid¬ dle and late Eocene age. The difficulty of separating the middle from the late Eocene in this core by means of dinoflagellates was also experienced by Gradstein and Williams (1976) on the Labrador 88 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Shelf. Species that characterize the middle and late Eocene assemblages in both areas include Cyclo- nop helium ordinatum Williams and Downie, 1966, Wetzeliella articulata Eisenack, 1938, W. coleo- thrypta Williams and Downie, 1966, W. tenuivirgula Williams and Downie, 1966, and Diphyes colligerum (Deflandre and Cookson) Cookson, 1965. The first appearance of Pentadinium laticinctum Gerlach, 1961, and Achilleodinium biformoides (Eisenack) Eaton, 1976, at 69 m (225 ft) suggests a late Eocene age because P. laticinctum Gerlach, 1961, was found in the upper Eocene of wells on the Grand Banks, Newfoundland (Gradstein and Williams, 1976). j Also, Achilleodinium biformoides (Eisenack) Eat¬ on, 1976, has been found only in upper Eocene sedi¬ ments from other areas. Some of the other more ! common species recovered from this interval include Dedandrea heterophlycta Deflandre and Cookson, 1955, Hystrichokolpoma rigaudae Deflandre and Cookson, 1955, Homotryblium (= Cordosphaeri- i dium) floripes Eisenack, 1963, Pentadinium lati¬ cinctum subsp. granulation Gocht, 1969, Spiniferites pseudofurcatus (Klumpp) Sarjeant, 1970, Tectado- dinium spp., Systematophora placacantha (Deflandre and Cookson) Davey and others, 1969, Gonyaula- cysta giuseppei (Morgenroth) Sarjeant, 1969, and a new species of Leptodinium. The assemblage of the interval 53-12 m (175- 41 ft) contains Chiropteridium lobospinosum Gocht, 1960, and suggests an Oligocene age. Other species present in this interval include Hystrichokolpoma rigaudae Deflandre and Cookson, 1955, Pentadinium laticinctum subsp. granulatum Gocht, 1969, Wetz¬ eliella articulata Eisenack, 1938, Homotryblium ( = Cordosphaeridium) floripes Eisenack, 1963, Cor- dosphaeridium funiculatum Morgenroth, 1966, : Gonyaulacysta cantharellum (Brosius) Gocht, 1969, Lingulodinium machaerophorum (Deflandre and ! Cookson) Wall, 1967, Deflandrea heterophlycta De¬ flandre and Cookson, 1955, Thalassiphora pelagica Eisenack and Gocht, 1960, Gonyaulacysta giuseppei (Morgenroth) Sarjeant, 1969, Chiropeter idium aspinatum (Gerlach) Brosius, 1963, and Spiniferites spp. The interval from 11 to 5 m (37-16 ft) in the upper part of the Cooper Formation contains, among 1 other forms, Tuberculodinium vancampoae (Rossignol) Wall, 1967, which in other areas is used as a marker for the Miocene. REFERENCES CITED Berggren, W. A., 1972, A Cenozoic time-scale—some implica¬ tions for regional geology and paleobiogeography: Lethaia, v. 5, no. 2, p. 195-215. Blow, W. H., 1969, Late middle Eocene to Recent planktonic foraminiferal biostratigraphy, in Bronnimann, P., and Renz, H. H., eds.. International Conference on Planktonic Microfossils, 1st, Geneva, 1967, Proceedings: Leiden, Netherlands, E. J. Brill, v. 1, p. 199-422, 54 pis. Brenner, G. J., 1963, The spores and pollen of the Potomac Group of Maryland: Maryland Dept. Geology, Mines and Water Resources Bull. 27, 215 p. Brown, P. M., 1958, Well logs from the Coastal Plain of North Carolina: North Carolina Div. Mineral Resources Bull. 72, 68 p. Brown, P. M., Miller, J. A., and Swain, F. M., 1972, Struc¬ tural and stratigraphic framework, and spatial distribu¬ tion of permeability of the Atlantic Coastal Plain, North Carolina to New York: U.S. Geol. Survey Prof. Paper 796, 79 p. Bybell, L. M., 1975, Middle Eocene calcareous nannofossils at Little Stave Creek, Alabama: Tulane Studies Geology and Paleontology, v. 11, no. 4, p. 178-252. Cooke, C. W., 1936, Geology of the Coastal Plain of South Carolina: U.S. Geol. Survey Bull. 867, 196 p. Doyle, J. A., 1969, Angiosperm pollen evolution and bio¬ stratigraphy of the basal Cretaceous formations of Maryland, Delaware, and New Jersey [abs.]: Geol. Soc. America Abs. with Programs, [v. 1], pt. 7, p. 51. Gradstein, F. M., and Williams, G. L., 1976, Biostratigraphy of the Labrador Shelf, Part I: Canada Geol. Survey, open file 349, 39 p. Mann, C. J., and Thomas, W. A., 1968, The ancient Missis¬ sippi River: Gulf Coast Assoc. Geol. Socs. Trans., v. 18, p. 187-204. Martini, Erlend, 1971, Standard Tertiary and Quaternary calcareous nannoplankton zonation, in Farinacci, Anna, and Matteucci, R., eds.. Proceedings of the II Planktonic Conference, Roma, 1970: Rome, Edizioni Tecnoscienza, v. 2, p. 739-785. Obradovich, J. D., and Cobban, W. A., 1975, A time-scale for the late Cretaceous of the Western Interior of North America: Geol. Assoc. Canada Spec. Paper 13, p. 31-54. Olsson, R. K., 1975, Upper Cretaceous and lower Tertiary stratigraphy of New Jersey coastal plain: Petroleum Exploration Soc. New York. Second Ann. Field Trip Guidebook, May 3, 1975, 49 p. Owens, J. P., and Sohl, N. F., 1969, Shelf and deltaic paleo- environments in the Cretaceous-Tertiary formations of the New Jersey Coastal Plain, in Subitzky, Seymour, ed., Geology of selected areas in New Jersey and eastern Pennsylvania and guidebook of excursions: New Bruns¬ wick, N. J., Rutgers Univ. Press, p. 235-278. Pessagno, E. A., Jr., 1967, Upper Cretaceous planktonic Foraminifera from the western Gulf Coastal Plain: Paleontographica Americana, v. 5, no. 37, p. 245—445, pi. 48-101, fig. 1-63. - 1969, Upper Cretaceous stratigraphy of the western Gulf Coast area of Mexico, Texas, and Arkansas: Geol. Soc. America Mem. Ill, 139 p., 60 pis. Petters, S. W., 1976, Upper Cretaceous subsurface stratig¬ raphy of Atlantic Coastal Plain of New Jersey: Am. Assoc. Petroleum Geologists Bull., v. 60, no. 1, p. 87- 107, fig. 1-7. BIOSTRATIGRAPHY OF THE DEEP COREHOLE 89 Plummer, F. B., 1933, The geology of Texas, part 3, Cenozoic systems in Texas: Texas Univ. Bull. 3232, p. 519-818. Pooser, W. K., 1965, Biostratigraphy of Cenozoic Ostracoda from South Carolina: Kansas Univ. Paleont. Contr. [38], Arthropoda, art. 8, 80 p. Sanders, A. E., 1974, A paleontological survey of the Cooper Marl and Santee Limestone near Harleyville, South Carolina—preliminary report: Columbia, South Carolina Div. Geology Geol. Notes, v. 18, no. 1, p. 4-12. Sirkin. L. A., 1974, Palynology and stratigraphy of Creta¬ ceous strata in Long Island, New York, and Block Island, Rhode Island: U.S. Geol. Survey Jour. Research, v. 2, no. 4, p. 431—440. Smith, C. C., and Pessagno, E. A., Jr., 1973, Planktonic foraminifera and stratigraphy of the Corsicana Forma¬ tion (Maestrichtian) north-central Texas: Cushman Found. Foram. Research Spec. Pub. 12, p. 1-68. pi. 1-27, fig. 1-24. Swift, D. J. P., and Heron, S. D., Jr., 1969, Stratigraphy of the Carolina Cretaceous: Southeastern Geology, v. 10, no. 4, p. 201-245. Tschudy, R. H., 1973, Stratigraphic distribution of signifi¬ cant Eocene palynomorphs of the Mississippi embay- ment: U.S. Geol. Survey Prof. Paper 743-B, p. B1-B24. van Hinte, J. E., 1976, A Cretaceous time scale: Am. Assoc. Petroleum Geologists Bull., v. 60, no. 4. p. 498-516. Wolfe, J. A., 1976, Stratigraphic distribution of some pollen types from the Campanian and lower Maestrichtian rocks (Upper Cretaceous) of the Middle Atlantic States: U.S. Geol. Survey Prof. Paper 977, 18 p. Young, Keith, 1963, Upper Cretaceous ammonites from the Gulf Coast of the United States: Texas Univ. Pub. 6304, 373 p. - 1965, A revision of Taylor nomenclature, Upper Cretaceous, central Texas: Texas Univ. Bur. Econ. Geology Geol. Circ. 65—3, 10 p. Geochemistry of Subsurface Basalt From the Deep Corehole (Clubhouse Crossroads Corehole 1) Near Charleston, South Carolina*— Magma Type and Tectonic Implications By DAVID GOTTFRIED, C. S. ANNELL. and L. J. SCHWARZ STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-G Twt_o7 ' 1 ! CONTENTS Page Abstract_ 91 Introduction _ 91 Description of basalt _ 92 Analytical methods _ 92 Analytical results _ 93 Major elements_ 93 Normative composition_ 95 Trace elements_ 95 Large cations _ 96 Rare-earth elements _ 97 High-valence cations_ 97 Ferromagnesian elements _ 98 Alteration effects_ 99 Age _ 100 Comparison with basalts of other provinces_ 102 Major elements _ 102 Trace elements _ 103 Tectonic setting_ 105 Summary and conclusions_ 109 References cited_ 110 ILLUSTRATIONS Page Figure 1 . Normative mineralogy of Clubhouse Crossroads corehole 1 basalts plotted on diopside-hypersthene-olivine-nepheline-quartz dia¬ gram _ 95 2-5. Graphs showing— 2. Average abundances of rare-earth elements (REE) in ba¬ salts from the corehole normalized to chondrites_ 98 3. Comparison of alteration effects for selected elements in altered marginal samples relative to less altered in¬ terior samples of corehole basalt flows_ 100 4. Variations of large cations and smaller high-valence cations with depth in corehole basalts__ 101 5. Average abundances of REE of tholeiitic diabases and ba¬ salts normalized t6 chondrites _ 104 6. Samples of corehole basalts and some tholeiitic basalts and dia¬ bases from continental provinces plotted on discrimination diagrams of Pearce and Cann (1973) _ 106 7. Graph showing comparison of REE patterns of corehole basalts with chilled margins of diabase from three continental prov¬ inces _ 108 ill IV CONTENTS TABLES Papre Table 1 . Major-oxide and normative mineral compositions, in weight percent, of basalt from Clubhouse Crossroads corehole 1 near Charleston, S.C _ 94 2. Trace-element abundances, in parts per million, in basalt from Club¬ house Crossroads corehole 1 near Charleston, S.C_ 96 3. K-Ar ages and analytical data T>f basalts from Clubhouse Crossroads corehole 1, 32°53.2' N., 80°21.5' W., Dorchester County, S.C_ 101 4. Geochemical comparison of basalt from the Clubhouse Crossroads corehole 1 (CCC 1) near Charleston, S.C., with basaltic rocks from other provinces_ 102 5. Rank ordering of chemical similarities between basalts from the Club¬ house Crossroads corehole 1 near Charleston, S.C., and selected comparison basaltic rocks _ 103 STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT GEOCHEMISTRY OF SUBSURFACE BASALT FROM THE DEEP COREHOLE (CLUBHOUSE CROSSROADS COREHOLE 1) NEAR CHARLESTON, SOUTH CAROLINA— MAGMA TYPE AND TECTONIC IMPLICATIONS By David Gottfried, C. S. Annell, and L. J. Schwarz ABSTRACT Geophysical studies indicate that mafic volcanic and as¬ sociated plutonic rocks may be important components of the ! basement beneath the Coastal Plain of the southeastern United States. A drill hole, 40 km northwest of Charleston, S.C., over a magnetic and gravity high penetrated 750 m of Coastal Plain sediments and bottomed in 42 m of basalt. Petrographic and major element data on 11 samples of basalt selected from the drill core, representing at least 2 lava flows, indicate that the basalts have undergone slight to extreme oxidation, hydration, and hydrothermal altera¬ tion. Effects of alteration are greatest in the marginal zones and least in the interior parts of the flows. The upper parts of the flows have abundant amygdules which contain laumon- tite, calcite, and chlorite. Petrochemical data on the least altered samples indicate that the basalts are of the quartz- normative tholeiitic magma type and closely resemble the Mesozoic high-Ti quartz-normative chilled diabase of eastern North America. Eight samples were analyzed for 27 trace elements, in¬ cluding rare-earth elements (REE), Rb, Ba, Sr, Th, Zr, Hf, Nb, Ta, Ni, Co, Cr, and Cu, by neutron activation, emission spectrography, and spectrophotometry. Concentrations of K, Rb, Ba, and Sr are highly variable in the marginal zones and reflect the mobility of these elements during postmag- matic processes. K/Rb ratios of the least altered samples are in the range 300-400. The abundances of REE, P, Ti, Zr, Nb, and Th show little (<20 percent) or no variation regardless of the degree of alteration and indicate that the two flows were originally of the same chemical composition. The contents of minor and trace elements of the corehole basalts are compared with those of rocks of tholeiitic com¬ position which occur on Atlantic-type passive continental margins (eastern North America, Tasmania, Antarctica, South Africa: all of Mesozoic age) and with basalts from island arc and oceanic-ridge settings. The low abundances of REE, Ti, Zr, and Nb in the corehole basalt and in quartz- normative tholeiites from eastern North America and Tas¬ mania are more similar to those of island arc and oceanic- ridge basalt^ than to those of “average” continental basalts. However, the pattern of enrichment in light REE and the low ratio, K/Rb, for the corehole basalt indicate that it originated from an undepleted source area in the upper mantle. The abundances of REE, Ti, Zr, and Nb and the light-REE enrichment patterns are strikingly similar to those of the high-Ti quartz-normative tholeiitic diabases of eastern North America. K-Ar analyses of a relatively fresh i sample and an altered sample yield ages of 94.8 m.y. and 109 m.y., respectively. These are considered minimum ages and may be significantly younger than the time of volcanism. The characteristic geochemical features of the corehole ba¬ salts suggest that they have a temporal as well as spatial relationship with the Late Triassic and Early Jurassic tho¬ leiitic province of eastern North America which formed | during an extensional tectonic regime. The subsurface ba- ! salts may be associated with structural features produced I by tensional faulting and suggest the possible presence of a buried Triassic basin beneath the Charleston area. INTRODUCTION Recent geophysical surveys of the Charleston area show pronounced positive magnetic and gravity anomalies which are interpreted as mafic or ultra- mafic plutons associated with mafic volcanic rocks (Popenoe and Zietz, this volume; Long and Cham¬ pion, this volume; Kane, this volume; Phillips, this volume). Contrasting models based largely on the magnetic and gravity patterns have been proposed for the tectonic setting of the crust beneath the Coastal Plain: Zietz and others (1976) proposed an ocean-floor or island arc tectonic setting, and Pope¬ noe and Zietz (this volume) propose a zone of conti¬ nental extension. A deep hole w T as drilled over a magnetic and gravity high about 40 km west-north- west of Charleston as part of the program to investi¬ gate the seismicity of the Charleston-Summerville area (see fig. 3 of Rankin, this volume). The drill hole, called Clubhouse Crossroads corehole 1 (CCC 1), penetrated 750 m of Coastal Plain sediments and bottomed in 42 m of basaltic lavas which may be genetically related to the bodies causing the mag¬ netic and gravity highs. The basalt is overlain by fossiliferous sediments of early Late Cretaceous (Cenomanian) age (Hazel and others, this volume; Gohn and others, this volume). A geochemical study of the basaltic rocks recovered from the corehole should provide direct evidence for constraining mod- 91 92 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 els of the regional tectonic setting that have been inferred from the geophysical and geological data. The purpose of the present study is to present new data on major elements and trace elements for the corehole basalts and to discuss the implications of the data with regard to tectonic setting. The first stage of the study is to determine the composition of the parental magma of the basalts. This, by neces¬ sity, includes an assessment of the degfee of altera¬ tion of the basalts. The second stage is to infer the tectonic setting of the basalts from a comparison of geochemical features of the corehole basalts with those of basalts of known tectonic setting. The third stage is to apply this information to an interpreta¬ tion of the tectonic setting of the Charleston area at j the time of extrusion of the basalts; therefore, knowledge of the age of the basalts is essential. In reading the detailed discussion that follows, the j reader should keep in mind that one of our major conclusions is that no single group or pair of geo- chemically associated elements can be used alone for i distinguishing magma type and tectonic setting. We have benefited greatly from many helpful dis¬ cussions with D. W. Rankin, Peter Popenoe, J. P. j Owens, B. B. Higgins, R. L. Smith, and S. L. Russell, j J. D. Fletcher provided preliminary semiquantitative spectrographic analyses which later expedited quan¬ titative spectrographic determinations. M. E. Mrose and E. J. Dwornik provided important mineralogical data by X-ray diffraction and scanning electron microscopy. We are grateful to Michael Fleischer, G. T. Faust, L. P. Greenland, and J. G. Arth for critical reviews of this paper and for their sugges¬ tions for its improvement. Part of the work was sup¬ ported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research, Agreement No. AT (49-25)-1000. DESCRIPTION OF BASALT The basalt can be divided into two flows on the basis of criteria described by Nichols (1936) for distinguishing successive flows. The following cri¬ teria have been used in the present study: distribu¬ tion of vesicles (now amygdules), grain size, and j petrography. The upper flow is 35 m thick. The top I of the flow (5 m) is highly oxidized, is reddish brown, and contains abundant amygdules. This zone grades downward into relatively fresh dark gray basalt containing fewer amygdules. The basal zone of the flow (3 m) is essentially aphyric and green¬ ish ; fractures are greenish black. Only 7 m of the lower flow were penetrated before drilling was ter¬ minated. The top of the lower flow is amygdaloidal, but less oxidized than the top of the upper flow, and grades downward into less altered gray fractured basalt. Limited mineralogical studies have been carried out to date because the basalts are altered and the matrix is fine grained. Petrographic data obtained on least altered samples from the upper flow show that the basalt is composed mainly of clinopvroxene, plagioclase (An 60 )> and Fe-Ti oxides. Preliminary studies by scanning electron microscope and X-ray diffraction techniques have thus far identified the following secondary minerals: laumontite, calcite, chlorite (all in large amygdules), and stilbite. ANALYTICAL METHODS Major-element oxide, water, and CCb contents were determined by the rapid rock-analysis methods described by Shapiro (1975). Abundances of 10 major constituents were determined from a single solution obtained by a nitric acid dissolution of a sample fused with lithium metaborate-lithium tetra¬ borate. CaO, MgO, Na 2 0, and K : 0 contents were determined by atomic absorption spectrometry; SiCk, ALO ; , FejO , TiCb, P : 0-„ and MnO contents were determined spectrophotometrically. Separate sample portions were used to determine FeO, H : 0 ( + and - ), and CCk contents. Minor- and trace-element abundances were deter¬ mined by means of chemical, emission spectro¬ graphic, and instrumental neutron-activation analy¬ ses. Niobium content was determined by a spectro- photometric method (Greenland and Campbell, 1974). After decomposition by hydrofluoric acid and evaporation to volatilize silica, the samples were fused with pyrosulfate and dissolved in hydrochloric acid-tartaric acid. After separation by a thiocyanate extraction with amyl alcohol and back-extraction with dilute hydrofluoric acid, the niobium was re¬ acted with 4-(2-pyridylazo)-resorcinol. Analytical error, calculated on the basis of replicate analyses of eight U.S.G.S. (U.S. Geological Survey) standard rocks, ranges from 2.9 to 6.4 percent in the concen¬ tration range from 10 to 27 ppm (parts per million). Three modifications of d.c. (direct-current) arc emission spectroscopy were used to determine the concentrations of 17 minor and trace elements. 1. A 15-ampere arc in air was used to determine Ba, Co, Cr, Cu, Ga, Mn, Ni, Sc, Sn, Sr, V, Y, and Zr concentrations; this method has a co- GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 93 efficient of variation of approximately 15 per¬ cent (Bastron and others, 1960). 2. A 25-ampere arc in an argon atmosphere was used to determine Pb and Zn concentrations by fractional volatilization of a sample buffered by a Na 2 C0 3 admixture. This method has a coefficient of variation of approximately 10 percent at the concentrations reported by An- nell (1967). 3. A 15-ampere arc in air and a sample buffered with KoC0 3 were used to determine Rb and Li concentrations. A coefficient of variation of 10 percent is realized by this technique (Annell, 1964). Synthetic standards and U.S.G.S. standards, BCR-1, W-l, and AGV-1 (Flanagan, 1973), were used to establish analytical curves for the element concentrations determined spectrographically. Instrumental neutron activation was used to de¬ termine La, Ce, Sm, Eu, Tb, Yb, Lu, Hf, Ta, and Th concentrations. Three 0.15-g replicate samples packed in polyethylene vials were irradiated for 2 hours at a flux of 5x 10 J3 neutrons cm -2 sec -1 at the National Bureau of Standards reactor, Gaithers¬ burg, Md. A standard was synthesized from an analyzed obsidian doped with solutions of selected trace elements, dried, reground, and calibrated rela¬ tive to seven U.S.G.S. standard rocks: BCR-1, G-2, AGV-1,.GSP-1, PCC-1, DTS-1, and W-l (Flana¬ gan, 1973). The samples and standards were counted on a Ge(Li) detector 1 week and 6 weeks after ir¬ radiation. The tantalum content was determined by counting on a low’-energy photon detector 5 months following irradiation. The spectral data were proc¬ essed on an IBM 370 1 computer by means of the program SPECTRA (Baedecker, 1976). ANALYTICAL RESULTS MAJOR ELEMENTS Major-element analyses were made on 11 corehole samples that represent the different zones of the two flow units as characterized by texture and grain size and that show to varying degrees the effect of posteruptive processes. The major-oxide composi¬ tions and the normative mineral compositions of the basalts are presented in table 1, where the data are arranged in order of increasing depth of the samples in the corehole. The contact between the upper and lower flow units is between samples B-5A and B-6. Chemical evidence of variable alteration in the suite 1 Any trade names in this publication are used for descriptive purposes only and do not constitute endorsement by the U.S. Geological Survey. of samples is clearly indicated by the relatively high and variable contents of H 2 0 and C0 2 . As expected from megascopic observation, sample B-l, which is reddish brown, amygdaloidal, and extensively zeo- litized, and which is taken from the top of the upper flow, has the highest H 2 0 content and Fe 2 0 3 /FeO ratio; it also has the lowest MgO content. Some im¬ portant petrogenetic elements have been remobilized, and possibly others have been introduced along the contact between the upper and lower flow units. The lowest K 2 0 and Na-,0 contents (0.02 and 0.48 per¬ cent, respectively) are in the top of the lower flow (sample B-6), which resembles the top of the upper flow (B-l) in texture and zeolite content and has the third highest content of H 2 0, second highest con¬ tent of Fe 2 0 ; „ and next lowest MgO content. This sample also has an exceptionally high Si0 2 content and is strongly depleted in A1 2 0 3 . The highest con¬ tents of K 2 0 (1.3-1.4 percent) and Na-0 (3.4 per¬ cent) are found in samples (B-5, B-5A) of massive aphanitic rock in which amygdules are virtually ab¬ sent. The basal zone showing potassium enrichment extends approximately 1 m above the contact be¬ tween the flows. The presence of an analogous zone of potassium enrichment extending downward for approximately 3 m from the contact is indicated by the relatively high potassium content in B-7. The approximate symmetry of alteration indicates that ip addition to weathering, oxidation, and hydration processes, which have affected the tops of the two flows to a greater degree than the interior parts, hy¬ drothermal activity has affected the contact zone between the two flows. An important feature of the chemistry of the sam¬ ples is the uniformity of concentrations of two minor constituents, phosphorus and titanium. P 2 0 : concen¬ trations are virtually the same (range, 0.12 to 0.15 percent) within the limits of error of the analytical method; titanium is nearly as uniform in its dis¬ tribution except for the two most altered samples, B-l and B-6, which show a depletion of about 15 percent in TiO ; content (range, 0.82-0.87 percent) relative to that of the other samples (range, 0.95- 1.1 percent). Although the introduction of water has strongly influenced the variations in the oxida¬ tion state of the rocks, the total iron concentrations are relatively unaffected. The distribution pattern for the values of total iron is approximately parallel to the pattern for the values of titanium. Prior studies (Cann, 1970; Pearce and Cann, 1971, 1973; Pearce and others, 1975; Floyd and Winchester, 1975; Winchester and Floyd, 1976) have shown that Ti and P concentrations have remained stable in 94 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Table 1 . —Major oxide and normative mineral compositions, in iveight percent, of basalt from Clubhouse Crossroads corehole 1, near Charleston, S.C. [Analyses by F. W. Brown. S. D. Botts. and Leonard Shapiro, using methods described by Shapiro (1975)] Sample No. _ - B-1 B-2 B-3 B-3A B-4 B-4 A B-5 B-6 B-7 B-8 Depth below surface (meters) _ - 758 764 771 774 779 782 784.8 785 785.4 789 791 Major-oxide composition SiCL _ 51.3 52.6 54.4 52.8 53.5 50.5 52.1 64.2 50.7 51.1 A.I 2 O - - 14.4 13.3 13.4 14.0 13.5 14.2 13.4 14.4 6.8 12.8 13.6 F e.O _ _ 8.2 2.9 2.7 3.6 2.4 3.0 3.1 3.2 4.1 3.9 3.2 FeO __ - .68 7.5 8.4 8.0 8.7 8.5 8.2 8.1 5.5 7.4 8.0 MgO . . - 9 9 5.6 D. < 5.7 5.9 6.0 6.0 6.2 4.5 6.4 5.9 CaO _ _ 6.7 7.7 9.2 9.0 9.0 9.0 7.8 7.5 7.0 6.6 8.7 Na O _ _ 2.7 3.1 2.4 2.3 2.4 2-2 3.4 3.4 .48 3.3 2.7 K,0 _ _ .70 .73 .50 .53 .62 .48 1.3 1.4 .02 .98 .48 H;0-t- .. _ 8.6 4.4 1.7 1.4 2.0 2.0 2.6 2.1 4.3 3.6 3.4 H.O— _ _ 4.0 1.2 .66 1.1 .61 .93 1.2 1.5 1.2 1.6 .88 TiCL_ _ .75 .89 .96 .98 .96 1.1 .99 1.0 .77 .93 .91 P_0-, _ _ .13 .12 .14 .12 .14 .13 .14 .13 .12 .13 .13 MnO _ _ __ - .12 .19 .21 .24 99 .26 .24 .26 .18 .22 .21 CO- _ .08 .03 .04 .05 .02 .07 .01 .03 .62 .07 .01 Total _ _ 99 99 99 101 99 101 99 101 100 99 99 Major-oxide composition recalculated volatile-free SiO : _ - 57.5 55.0 54.7 55.0 54.6 54.4 53.1 53.3 68.5 54.3 53.8 ALO, _ - 16.7 14.3 13.9 14.2 14.0 14.4 14.1 14.7 7.3 13.7 14.3 FejO ■ _ - 9.5 3.1 2.8 3.6 2.5 3.1 3.3 3.3 4.4 4.2 3.4 FeO _ — .79 8.0 8.7 8.1 9.0 8.6 8.6 8.3 5.9 7.9 8.4 MgO _ — 2.6 6.0 5.9 5.8 6.1 6.1 6.3 6.4 4.8 6.9 6.2 CaO _ 7.8 8.3 9.6 9.1 9.3 9.2 8.2 i . 1 7.5 7.1 9.2 Na O _ _ 3.1 3.3 2.5 2.3 2.5 2.2 3.6 3.5 .51 3.5 2.8 K:0 _ _ .81 .78 .52 .54 .64 .49 1.4 1.4 .02 1.1 .51 TiO’ _ _ .87 .95 1.0 .99 .99 1.1 1.0 1.0 .82 1.0 .96 P,0, _ .15 .13 .15 .12 .14 .13 .15 .13 .13 .14 .14 MnO _ .14 .20 .22 .24 .23 .26 .25 .27 .19 .24 .22 Total _ __ _ 100 100 100 100 100 100 100 100 100 100 100 Normative mineral composition 1 Q - 12.49 3.86 7.08 8.80 6.68 8.16 41.51 1.14 4.14 OR _ 4.84 4.63 3.08 3.17 3.80 2.88 8.09 8.48 .13 6.22 2.99 AB _ 26.75 28.15 21.13 19.71 21.03 18.93 30.31 29.47 4.32 29.97 24.12 AN_ 29.40 21.70 25.30 26.64 25.10 27.91 18.40 20.38 17.39 18.48 24.88 WO _ 3.45 7.80 8.96 7.23 8.52 6.67 9.03 6.88 6.22 6.55 8.40 EN_ 6.42 14.97 14.77 14.37 15.22 15.19 12.19 13.97 11.93 17.11 15.51 FS _ 12.24 14.24 14.80 15.01 14.79 14.85 11.79 13.08 12.92 15.46 15.17 FO _ 2.49 1.30 _ ___ _ FA _ _ 2.65 1.34 __ _ _ _ _ MT_ 2.28 2.62 2.72 2.75 2.72 2.75 2.80 0 72 2.36 2.83 2.78 IL _ 1.67 1.81 1.90 1.38 1.89 2.12 1.98 1.95 1.56 1.90 1.82 AP _ .31 .28 .34 .28 .34 .31 .34 .31 .28 .31 .28 CC _ .21 .07 .10 .12 .05 .16 .02 .07 1.50 .17 .02 Total __ 100 100 100 100 100 100 100 100 100 100 100 1 Based on analyses recalculated to 100 percent water-free oxides; Fe-jOa/FeO ~ Fe;0-. ratio assumed to be 0.15. mafic rocks that have undergone weathering, deu- teric alteration, and greenschist facies metamor¬ phism; these studies have used concentrations of Ti and P in various combinations with concentrations of other stable trace elements, such as Zr, Y, and Nb, to characterize magma types and tectonic environments. These stable elements are indicators of the stage of fractional crystallization during the early and middle stages of differentiation of tholeiitic magmas (Anderson and Greenland, 1969). Their contrasting patterns of variation, such as progressive enrich¬ ment in tholeiitic differentiates and depletion as con¬ tents of K,0 and SiOj increase in calc-alkaline lavas, are also useful as indicators of petrologic province and hence geologic setting (Anderson and Gottfried, 1971; Miyashiro, 1974; Miyashiro and Shido, 1975; Martin and Piwinskii, 1972). The results of the present study provide further confirmation that P and Ti are relatively insensitive to processes of chemical alteration. Thus, we inter¬ pret the uniform distribution of these elements plus that of total iron (1) as a primary feature of the flows, (2) as an indication that the random varia¬ tions of the other petrogenetic elements within and between the flow's are a reflection of secondary alter- GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 95 ation processes, and (3) as evidence that all the rocks were originally of the same chemical composition. , NORMATIVE COMPOSITION Plots of the normative compositions of the core¬ hole samples in the normative diopside-hypersthene- olivine-nepheline-quartz diagram are shown in figure 1. Because hydration and attendant oxidation affect the position of the points in the diagram, the norma¬ tive calculations were made on water-free oxides, and the Fe 2 03 /(Fe 0 + Fe 2 0 3 ) ratio was assumed to be 0.15. On the basis of their normative mineralogy, two of the basalt samples are olivine-normative tho- leiites, but the other nine are quartz-normative tho- leiites according to the classification scheme of Yoder and Tilley (1962). The olivine-normative tho- leiites (samples B-5, B-5A) and the strongly quartz-normative tholeiite (sample B-6), which to¬ gether represent the opposite extremes in composi¬ tion of the compositions plotted on the diagram, re¬ flect the introduction of potassium and silica, re¬ spectively, during alteration of the marginal zones of the flows. The next most altered samples (B-7, B-l) plot on the diagram in positions intermediate between the extremely altered and the least altered samples. Inasmuch as the effects of secondary proc¬ esses appear to be gradational, any distinction based on the degree of alteration is arbitrary’. Analyses representing the interior parts of the flows, (samples B-2, B-3, B-4, B-8) cluster closely together in the quartz-tholeiite field. Petrographic and chemical evi¬ dence indicate that these are the least altered sam¬ ples. This suggests that their composition best approximates the composition of the liquids from which they were formed. The average.major-element composition of these samples is also assumed to be close to the original composition of the two lava flows. This will be discussed further when compari¬ sons are made between basalts from CCCl and mafic rocks representative of a variety’ of magma types and tectonic settings. TRACE ELEMENTS The results of analyses for 27 trace elements in individual samples are given in table 2. The data are subdivided according to the scheme recommended by Taylor (1965) and Tayior and White (1966) where¬ in elements are grouped mainly on the basis of their geochemical associations and within each group are listed according to their ionic radius and charge. As : noted above, the nature of the variations of the major elements indicates a complicated history of Di Figure 1 .— Normative mineralogy of Clubhouse Crossroads corehole 1 basalts from table 1 (prefix B is omitted here) plotted on a diopside (Di)-hypersthene (Hy)-olivine (Ol)-nepheline (Ne) -quartz (Qz) diagram. 96 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Table 2.— Trace-element abundances, in parts per million , in basalt from Clubhouse Crossroads corehole 1 near Charleston, S.C. Sample No.- B-l B-2 B-3 B-3A B-4 B-4A B-5 B-5A B-6 B-7 B-8 Large cations Rb* - 2 26 14 16 19 10 43 52 0.7 34 ’ 19 Ba 2 *- 155 230 120 140 135 140 360 330 20 235 110 K‘ - 6,700 6,500 4,300 4,500 5,300 4,100 11,600 11,600 200 9,ltJ0 4,200 Sr* . 140 190 200 160 200 220 160 160 40 250 160 Ca 2 * - 56,000 59,000 69,000 65,000 66,000 66,000 59,000 55,000 54,000 51,000 66,000 Pb 2 * - 5.4 5.0 5.0 3.9 4.6 4.6 4.5 4.6 4.7 3.9 4.2 K/Rb - 3,400 250 310 280 280 410 270 230 285 270 220 Ba/Rb- 78 8.8 8.6 8.8 7.1 14 8.4 6.3 29 6.9 5.8 K/Ba- 43 28 36 32 39 29 32 35 10 39 38 High-valence cations Th 1 *_ 1.7 1.9 2.0 2.2 2.1 1.7 2.0 2.0 Zr‘* _ 65 74 82 77 70 71 71 73 Hf'*_ 1.6 1.8 2.1 2.1 2.0 1.7 1.7 2.0 Nb 3 *_ 7.0 7.0 7.4 6.8 7.4 7.5 7.2 7.7 Ta 3 * ... .33 .28 .32 .26 * .33 .18 .32 .30 Zr Hf- 41 41 39 37 35 42 42 37 NbxlOO Ti .13 .13 .13 .12 .13 .16 .13 .14 Nb 21 25 23 26 22 42 22 26 Ferromagnesian elements Co 2 - _ 35 49 47 40 46 52 42 44 42 45 43 Cu 2 *_ _ 8 28 34 25 20 28 26 24 38 19 16 Li*_ 16 6 7 5 14 10 16 16 36 25 11 Ni 2 * _ 24 19 20 19 17 16 16 14 15 11 14 Zn 2 ‘ _ _ 55 80 90 84 82 96 80 32 95 78 88 Cr* _ 39 38 36 35 34 28 32 31 30 37 32 Ga 3 *_ 26 14 15 15 15 18 14 12 16 16 15 Sc 3 * _ _ 40 54 50 52 56 52 54 52 48 48 46 V 3 *_ 210 310 310 310 320 400 400 330 310 280 250 Ni/Co _ .69 .39 .43 .48 .37 .31 .38 .32 .36 .24 .33 Rare earth elements La _ 10 10 11 11 11 10 11 9 Ce _ . -- 17 18 19 19 19 17 18 18 Nd _ <28 <36 <36 10 10 <36 <37 <37 Sm _ 2.9 2.7 3.1 3.0 3.0 2.6 3.5 3.0 Eu _ .99 .90 1.02 .95 .96 .75 .99 .99 Tb _ .62 .61 .73 .68 .70 .59 .66 .68 Yb _ _ 1.9 1.8 2.1 2.3 2.4 1.8 2.1 2.1 Lu _ .38 .42 .47 .44 .44 .38 .47 .44 Y _ 28 34 33 34 32 34 31 28 alteration for the lava flows; therefore it is im- 1 5 to B-7) that straddles the contact betw*een the portant to evaluate the effects of alteration on the suites of trace elements before they can be used for characterization of magmatic and tectonic affinities. LARGE CATIONS The elements in the group of large cations that have the largest ionic radii, Rb, Ba, and K, cannot conveniently fit into the crystal structures of the major minerals (pyroxenes, plagioclase, olivine) of basaltic rocks. These elements are probably highly concentrated in interstitial material, and thus, they may be more susceptible to leaching and remobiliza¬ tion by geologic processes during and after crystalli¬ zation. The concentrations of these elements vary the most in the altered zone (extending from sample B- upper and low flows. The patterns of abundance variations for Rb and Ba are similar to those for K. In samples from the altered zone in which the K content is relatively high, the contents of Rb (34- 52 ppm) and Ba (235-360 ppm) are relatively high; in sample B-6, in which the K content is low, Rb (0.7 ppm) and Ba (20 ppm) contents are also low. An extreme depletion of Rb relative to Ba and K is found in the strongly hydrated and oxidized sample (B-l) from the top of the upper flow, although K, and to a lesser extent Ba, are somewhat enriched in this sample with respect to the least altered samples of the suite. Sample B-2, which in figure 1 is shown to be similar in normative composition to the least altered basalts, has distinctly higher Rb (26 ppm) GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 97 and Ba (230 ppm) contents than B-l (2 ppm Rb and 155 ppm Ba), though both have essentially the same K contents. Rubidium appears to be the most sensitive indicator of alteration and is particularly useful for subdividing the heterogeneously altered flows into alteration domains. The wide variations for Rb, between adjacent samples (B-l and B-2) from the top of the upper flow T (thirteenfold) and between the top of the lower flow and adjacent sam¬ ples (approximately fiftyfold), outhne the extent of the alteration zones developed at the margins of the flow’s. In the least altered zone, the Rb variation is less than twofold (10-19 ppm). Except for the anomalously high value for the K/Rb ratio (3,400) in sample B-l, the K/Rb ratios range from 220 to 410. Because of the lack of any appreciable differ¬ ence between the K/Rb ratios of the relatively un¬ altered samples and those of the strongly altered samples, it might be argued that the K 7 Rb ratio of the least altered samples does not reflect the original K Rb ratio of the rocks. However, the original K Rb ratio may have been modified to a limited extent be¬ cause of the close geochemical coherence of these two elements in hydrothermal as well as magmatic processes. Inasmuch as the magnitude of the change is unknowm, the K/Rb ratio would have to be used with caution in distinguishing between magma types. The range of variation of Sr concentration is less than that of Rb, Ba, and K concentrations. The max¬ imum change (fivefold) is in the lower alteration zone where Sr is strongly depleted in B-6 (40 ppm) relative to adjacent samples B-5A and B-7 (160 and 250 ppm, respectively). In this zone, the variation of | Sr concentration is similar to that of the alkali metals. However, in the least altered zone and in the upper alteration zone (upper flow 7 ) the concentration pattern for Sr is nearly the same as that observed for Ca. Possible explanations for the diverse be¬ havior of Sr are (1) Sr is in part located in sites similar to those occupied by the alkali metals; (2) Sr is incorporated to some extent in plagioclase, py¬ roxene, and zeolites; and (3) the nature and (or) degree of the alteration may vary in different parts of the basalt flow’s. The uniformity in Pb contents (3.9 to 5.4 ppm) in the suite of samples is somew’hat surprising. Lead is generally considered a K-related element in igneous rocks, and we w’ould have expected somew’hat analo¬ gous variations. RARE-EARTH ELEMENTS The rare-earth elements (REE) are probably the most important single group of geochemically co¬ herent trace elements that have been used in studies on modes of origin, petrologic evolution, and tec¬ tonic setting of basaltic rocks (Frey and others, 1968; Kay and others, 1970; Schilling and Win¬ chester, 1967; Jakes and Gill, 1970). Moreover, prior studies have showrn that the processes of weathering, low’-grade metamorphism (greenschist facies), and spilitization do not change or alter the primary REE pattern (Frey and others, 1968; Kay and Senechal, 1976; Herrmann and others, 1974). It is, therefore, important to extend such studies to include the core¬ hole basalts. The abundances of eight REE and Y w’ere determined (by neutron-activation analysis and emission spectrography, respectively) in eight samples. The data given in table 2 indicate that the abundances are virtually the same in each of the samples, regardless of the degree of alteration. The REE data also support our previous interpretation based on the uniformity of Ti and P concentrations, that all samples were of the same composition prior to alteration. The normalized REE pattern of the basalts is showrn in figure 2, in w’hich the ratios of the average concentration for each REE in the eight samples to the average concentration of the same REE in chondrites are plotted on a logarithmic ordi¬ nate against a linear scale of the REE atomic num¬ ber along the abscissa. For normalization purposes, the average concentrations of the individual REE in 20 ordinary chondritic meteorites (table 8 in Has- kin and others, 1966) w 7 ere used. The REE pattern of the corehole basalts show’s enrichment of the light REE (La-Sm) relative to the heavy REE (Gd-Lu). A comparison of this REE pattern w’ith those of basalts of known tectonic setting and magma type is made in a later section. HIGH-VALENCE CATIONS The contents of Th, Zr, Hf, Nb, and Ta are re¬ markably uniform in this suite of samples, except for Ta depletion in highly altered sample B-6. Their uniformity leaves little doubt about the immobility of these elements during alteration. These elements generally show threefold to fivefold increases in abundances in the course of differentiation of conti¬ nental tholeiitic magmas (Gottfried and others, 1968; Eales and Robey, 1976). The constancy of these elements in the two flow’s again suggests the absence of any differentiation trends in the two flow’s. The stability of the high-valence cations is an important feature because in this study w T e use Zr ROCK/CHONDRIIE 98 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Figure 2. —Average abundances of rare-earth elements (REE) in basalts from the Clubhouse Crossroads corehole 1 near Charleston, S.C., normalized to average REE abundances in 20 chondrites (Haskin and others, 1966). Each vertical bar indicates the range of normalized values of eight samples. in conjunction with other immobile minor and trace elements, in particular Ti, Y, and Nb, in our attempt to identify the tectonic setting of the volcanic rocks according to the classification scheme of Pearce and Cann (1973). Discrimination diagrams, such as Ti- Zr, Ti-Zr-Y, and Ti-Zr-Sr, have been widely utilized by many workers to investigate past environments of a wide variety of mafic rocks (Pearce, 1975; Seidel, 1974; Bickle and Nisbet, 1972; Pearce and Cann, 1971; Smewing, Simonian, and Gass, 1975; Kay and Senechal, 1976; Perfit, 1977). FERRO MAGNESIAN ELEMENTS Features of petrologic interest in the group of ferromagnesian elements are the low abundance lev¬ els of Ni, Co, Cr, and especially Cu as compared to the average abundances given by Prinz (1967) of these elements in quartz-normative tholeiites. Al¬ though Cu is a chalcophile element, it is included here for convenience. Variations in the abundances of these elements between the least altered and strongly altered samples of the corehole basalts are generally less than twofold to threefold. The effect of alteration is readily apparent for Cu which is depleted (8 ppm) in the strongly oxidized sample (B—1) from the top of the upper flow and somewhat enriched (38 ppm) in the intensely altered sample (B—6) from the top of the lower flow relative to the least altered samples (16-34 ppm). Mobility of Cu during weathering and alteration of tholeiitic dia¬ base from Pennsylvania has been noted by Smith and others (1975). Copper is generally enriched in the residual liquids during tholeiitic differentiation until a copper-rich sulfide appears. This is clearly shown in the well- studied Palisades sill (Walker, 1969) and in differ¬ entiated diabase intrusions of the Tasmanian tholeiitic province (Greenland and Lovering, 1966). In the latter province, crystallization of chalcopyrite during the middle stage of differentiation resulted in strong depletion of Cu in the later differentiates (McDougall and Lovering, 1963; and McDougall, 1964). The low copper contents of the corehole basalt might reflect prior crystallization of sulfides. Sample B-l is also depleted in cobalt, zinc, scandium, and vanadium. Chromium and scandium contents are uniform and do not appear to have been affected by alteration. Bloxam and Lewis (1972) previously noted that chromium is fairly stable during altera¬ tion. The Ni Co ratios of the corehole basalts (0.24- 0.48) are significantly lower than the Ni Co ratios commonly found in nonorogenic tholeiitic basalts (2-3). Variation of this ratio in several tholeiitic diabase-granophyre suites has been reviewed by Fleischer (1968) who showed that the Ni Co ratio decreases systematically during differentiation (—3 GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 97 and Ba (230 ppm) contents than B-l (2 ppm Rb and 155 ppm Ba), though both have essentially the same K contents. Rubidium appears to be the most sensitive indicator of alteration and is particularly useful for subdividing the heterogeneously altered flows into alteration domains. The wide variations for Rb, between adjacent samples (B-l and B-2) from the top of the upper flow (thirteenfold) and between the top of the lower flow and adjacent sam¬ ples (approximately fiftyfold), outhne the extent of the alteration zones developed at the margins of the flows. In the least altered zone, the Rb variation is less than twofold (10-19 ppm). Except for the anomalously high value for the K 7 Rb ratio (3,400) in sample B-l, the K/Rb ratios range from 220 to 410. Because of the lack of any appreciable differ¬ ence between the K 'Rb ratios of the relatively un¬ altered samples and those of the strongly altered samples, it might be argued that the K 'Rb ratio of the least altered samples does not reflect the original K 'Rb ratio of the rocks. However, the original K 'Rb ratio may have been modified to a limited extent be¬ cause of the close geochemical coherence of these two elements in hydrothermal as well as magmatic processes. Inasmuch as the magnitude of the change is unknown, the K/Rb ratio would have to be used with caution in distinguishing between magma types. The range of variation of Sr concentration is less than that of Rb, Ba, and K concentrations. The max¬ imum change (fivefold) is in the lower alteration zone where Sr is strongly depleted in B-6 (40 ppm) relative to adjacent samples B-5A and B-7 (160 and 250 ppm, respectively). In this zone, the variation of Sr concentration is similar to that of the alkali metals. However, in the least altered zone and in the upper alteration zone (upper flow) the concentration pattern for Sr is nearly the same as that observed for Ca. Possible explanations for the diverse be¬ havior of Sr are (1) Sr is in part located in sites similar to those occupied by the alkali metals; (2) Sr is incorporated to some extent in plagioclase, py¬ roxene, and zeolites; and (3) the nature and (or) degree of the alteration may vary in different parts of the basalt flows. The uniformity in Pb contents (3.9 to 5.4 ppm) in the suite of samples is somewhat surprising. Lead is generally considered a K-related element in igneous rocks, and we would have expected somewhat analo¬ gous variations. RARE-EARTH ELEMENTS The rare-earth elements (R.EE) are probably the most important single group of geochemically co¬ herent trace elements that have been used in studies on modes of origin, petrologic evolution, and tec¬ tonic setting of basaltic rocks (Frey and others, 1968; Kay and others, 1970; Schilling and Win¬ chester, 1967 ; Jakes and Gill, 1970). Moreover, prior studies have shown that the processes of weathering, i low-grade metamorphism (greenschist facies), and spilitization do not change or alter the primary R.EE ! pattern (Frey and others, 1968; Kay and Senechal, 1976; Herrmann and others, 1974). It is, therefore, important to extend such studies to include the core¬ hole basalts. The abundances of eight R.EE and Y were determined (by neutron-activation analysis and emission spectrography, respectively) in eight samples. The data given in table 2 indicate that the abundances are virtually the same in each of the samples, regardless of the degree of alteration. The REE data also support our previous interpretation based on the uniformity of Ti and P concentrations, that all samples were of the same composition prior to alteration. The normalized R.EE pattern of the basalts is shown in figure 2, in which the ratios of the average concentration for each REE in the eight samples to the average concentration of the same 1 REE in chondrites are plotted on a logarithmic ordi- j nate against a linear scale of the REE atomic num- | ber along the abscissa. For normalization purposes, the average concentrations of the individual REE in 20 ordinary chondritic meteorites (table 8 in Has- kin and others, 1966) were used. The REE pattern of the corehole basalts shows enrichment of the light REE (La-Sm) relative to the heavy REE (Gd-Lu). A comparison of this REE pattern with those of ; basalts of known tectonic setting and magma type is made in a later section. high-valence cations The contents of Th, Zr, Hf, Nb, and Ta are re¬ markably uniform in this suite of samples, except for Ta depletion in highly altered sample B-6. Their uniformity leaves little doubt about the immobility of these elements during alteration. These elements | generally show threefold to fivefold increases in abundances in the course of differentiation of conti¬ nental tholeiitic magmas (Gottfried and others, 1968; Eales and Robey, 1976). The constancy of these elements in the two flows again suggests the absence of any differentiation trends in the two flows. The stability of the high-valence cations is an j important feature because in this study we use Zr nOCK/CHONDR!TE Figure 2. —Average abundances of rare-earth elements (REE) in basalts from the Clubhouse Crossroads corehole 1 near Charleston, S.C., normalized to average REE abundances in 20 chondrites (Haskin and others, 1966). Each vertical i bar indicates the range of normalized values of eight samples. in conjunction with other immobile minor and trace elements, in particular Ti, Y, and Nb, in our attempt to identify the tectonic setting of the volcanic rocks according to the classification scheme of Pearce and Cann (1973). Discrimination diagrams, such as Ti- Zr, Ti-Zr-Y, and Ti-Zr-Sr, have been widely utilized by many workers to investigate past environments of a wide variety of mafic rocks (Pearce, 1975; Seidel, 1974; Bickle and Nisbet, 1972; Pearce and Cann, 1971; Smewing, Simonian, and Gass, 1975; Kay and Senechal, 1976: Perfit, 1977). FERROMAGNESIAN ELEMENTS Features of petrologic interest in the group of ferromagnesian elements are the low abundance lev¬ els of Ni, Co, Cr, and especially Cu as compared to the average abundances given by Prinz (1967) of these elements in quartz-normative tholeiites. Al¬ though Cu is a chalcophile element, it is included here for convenience. Variations in the abundances of these elements between the least altered and strongly altered samples of the corehole basalts are generally less than twofold to threefold. The effect of alteration is readily apparent for Cu which is depleted (8 ppm) in the strongly oxidized sample (B—1) from the top of the upper flow and somewhat enriched (38 ppm) in the intensely altered sample (B-6) from the top of the lower flow relative to the least altered samples (16-34 ppm). Mobility of Cu during weathering and alteration of tholeiitic dia- j base from Pennsylvania has been noted by Smith and others (1975). Copper is generally enriched in the residual liquids j during tholeiitic differentiation until a copper-rich sulfide appears. This is clearly shown in the well- studied Palisades sill (Walker, 1969) and in differ- . entiated diabase intrusions of the Tasmanian I tholeiitic province (Greenland and Lovering, 1966). In the latter province, crystallization of chalcopyrite during the middle stage of differentiation resulted in j j strong depletion of Cu in the later differentiates (McDougall and Lovering, 1963; and McDougall, 1964). The low copper contents of the corehole basalt . might reflect prior crystallization of sulfides. Sample ’ B-l is also depleted in cobalt, zinc, scandium, and ; vanadium. Chromium and scandium contents are uniform and do not appear to have been affected by [ alteration. Bloxam and Lewis (1972) previously noted that chromium is fairly stable during altera¬ tion. The Ni Co ratios of the corehole basalts (0.24- ; 0.48) are significantly lower than the Ni/Co ratios 1 commonly found in nonorogenic tholeiitic basalts (2-3). Variation of this ratio in several tholeiitic ^ diabase-granophyre suites has been reviewed by i Fleischer (1968) who showed that the Ni Co ratio decreases systematically during differentiation (~3 GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 99 to <1) and is useful as an index of fractionation. Thus the low Ni/Co ratios and also the low Cr con¬ tents (28—39 ppm) could be accounted for if the corehole basalts represent residual liquids derived from a more primitive magma that has undergone extensive fractionation prior to eruption. However, low Ni and Co contents and low Ni/Co ratios are considered characteristic features of some calc-al¬ kali c basalts and andesites of island arcs and active continental margins. The petrogenetic significance of these elements has been extensively discussed by Taylor (1969), Taylor and others (1969, 1971), Hedge (1971), and Marsh (1976). Taylor and others (1969, 1971) suggested that neither fractional crys¬ tallization of basalt nor partial melting of undiffer¬ entiated mantle can account for the relative abun¬ dances of these elements in orogenic magmas; they postulated a two-stage model to explain the low Ni, Co, and Cr contents and low Ni 'Co ratios. Therefore, caution is required in using contents of these ele¬ ments by themselves to identify magma type and tectonic setting because similar abundance relations may occur in mafic volcanic rocks of contrasting magma types (calc-alkalic and tholeiitic) and, hence, tectonic settings (orogenic and non-orogenic). ALTERATION EFFECTS In this study, our efforts were directed mainly at “seeing through” the effects of alteration by an em¬ pirical approach. Detailed studies of the zeolites and other secondary minerals dispersed through parts of the flow are clearly needed before we can achieve an adequate understanding of the mechanism or the nature of the alteration processes. Petrographic, mineralogic, and chemical data indicate that altera¬ tion is greatest in the marginal zones and least in the interior parts of the flows. A gross correlation exists among the degrees of alteration indicated by (1) trace-element composition, (2) megascopic ex¬ amination, for example, of color variation (Gohn and others, this volume), and (3) H : 0 and Fe-Ck con¬ tents. By analogy with alteration studies of sea-floor basalts, in which altered rims are compared with less altered interior parts of pillow fragments (Hart, 1969, 1971; Hart and others, 1974; Philpotts and others, 1969), we have compared chemical variations between margin and interior samples. Various aspects of the effects of alteration of the major and minor elements have been discussed in preceding sections. Some of the results are sum¬ marized in figure 3, where selected major-, minor-, and trace-element concentrations in six samples from altered marginal zones are compared with the aver- ] age concentration of the least altered samples from the interiors of the flow's. The elements selected are those that have been used for identification of mag¬ ma type or tectonic setting. They are plotted in figure 3 in order of decreasing ionic radii. In a general wav, this also is their order of relative susceptibility of alteration; the larger ions are most easily altered (Rb>K=^Ba> Sr>Ca), and the REE and highly charged cations (Zr, Nb, Ti, and P) are the least easily altered. The large alkali trace elements are the most sensitive indicators of alteration, and they en¬ hance the recognition of the existence and extent of subtle alteration effects in the flows. Figures 3 and 4 show that the magnitude and di¬ rection of the chemical changes resulting from al¬ teration in B-l and B-2 (top of upper flow) are dif¬ ferent from those of the chemical changes in B—6 (top of lower flow). The two contrasting alteration trends may be the effects of different degrees or types of alteration. The trends for B-l and B-2 sug¬ gest a stage of alteration during which the alkali metals were added to these samples. How'ever, con¬ tinued alteration, or some later process, resulted in preferential loss of Rb relative to K and hence the anomalously high K/Rb ratio (3.400). Except for Rb loss in B-l. this alteration trend for the large¬ sized ions is similar to those observed in slightly weathered margins of submarine basalt fragments (Hart, 1971). The alteration trend for B-6 suggests that the top of the lower flow may have undergone alteration of a different type. Alkali metals in this sample are depleted by more than an order of mag¬ nitude relative to the least altered samples. The frac¬ tionation of Ta from Nb, as indicated by the high Nb/Ta ratio (42), is also an important feature of this sample. Samples immediately above (B-o, B- 5A) and below (B-7) have alteration patterns showing relatively strong enrichment of the alkali elements and seem qualitatively to have a comple¬ mentary relationship to that of sample B-6. These abrupt chemical changes along the contact zone may reflect the effects of a hydrothermal event which took place after extrusion of the upper flow. Weathering (hydration and oxidation), hydro- thermal activity, and possibly low-grade metamor¬ phism (zeolite facies) have contributed in varying degrees to the complex chemical and mineralogical changes that much of the lava flow's have undergone. However it is extremely difficult, if at all possible, in some samples from the lava flow’s, to determine the sequence in w'hich some of these processes oc¬ curred or to assign them correctly. 100 - o 0 . 5 < 0.8 0 . 6 ' 04 - 0.2 0.1 0.08 O 0.06 < > < 0.04 0.02 0.01 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1S86 A ® — A Rb & o A •* O •o. Z — e - -I # O AA S • U £ AA 0 ▲ & EXPLANATION Altered marginal samples • 6-1 O $ 6-2 6-5 and 5A ZL 6-7 A 6-6 -Average concentration of least altered samples 16-3, 6-3A 6-<. 6-AA.and 6-8) 6a Pb Sr Ca ELEMENTS REE Nb f ! ! :! T t r Figure 3.— Comparison of alteration effects on concentrations of selected elements in altered marginal samples relative to the average concentration of the less altered interior samples of the basalt flows penetrated in the Clubhouse Cross¬ roads corehole 1 near Charleston, S.C. AGE Knowledge of the time of volcamsm and associ¬ ated tectonic activity is essential to understanding I the pre-Late Cretaceous structural evolution of the j Charleston area. Potassium-argon (K-Ar) whole- rock ages were determined on a fine-grained sam¬ ple (3—3) from the least altered zone and an al- j tered aphyric sample (B-5A) from the base of the upper flow. The K-Ar ages and analytical data are given in table 3. The similarity in age between the least altered sample (94.8 m.y.) and a more altered K-rich sample (109 m.y.) indicates that these are minimum ages, probably dating some posteruptive processes that have affected the volcanic rocks. These apparent ages are not in stratigraphic con¬ flict with the Cenomanian Age (Hazel and others, this volume) of the overlying sedimentary rocks. Currently assigned estimates of the time interval of the Cenomanian Stage are 90 m.y. to 94 m.y. (Obradovich and Cobban, 1975). Prior to the de¬ tailed analysis of the geochemistry of the basalts and because of the similarity in ages of the basalts and the overlying rocks there was little reason to question the K-Ar ages. However, the possibility cannot be precluded that this similarity is fortui¬ tous and that the basalts are significantly older than the K-Ar ages indicate. Several investigators have evaluated the effects of alteration on K-Ar ages of basaltic rocks whose ages were well established by geologic evidence or by in- GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 101 Figure 4.—Variations of large cations and smaller high-valence cations in samples from different depths in the two basalt flows penetrated in the Clubhouse Crossroads corehole 1 near Charleston, S.C. Table S. — K-Ar ages and analytical data of basalts from Clubhouse Crossroads corehole 1, !2‘ 55.2' N., 80‘ 21.5' TT., Dorchester County, S.C. [Analysts: R. F. Marvin, H R Mebnert. and Violet Merritt, U.S. Geological Survey] Sample Depth below K*0 1 Ar 4< ' 1 Ar 40 Calculated age (m.y.) * 8 * No. surface (meters) (wt. percent) (1C»* IC moies/g 1 (wt. percent) ^20 B-3 ... 770 0.63,0.62 0.896S 83 94.8r-L2 .626 avg. B-5A. 785 1.37,1.43 2.309 85 109*4 1.40 avg. : Radiogenic argon. 8 Constants : K 40 X< =r 0.585 X 10“ 10 /yr. X* = 4.72 x- 10 /yr. K 4C = 1.19 X lCr* atomic abundance. dependent radiometric methods. Kaneoka (1972) found that rocks containing more than 1 percent H 2 0 (+) generally show much younger ages than fresh rocks as a result of radiogenic 4(1 Ar loss due to hydration. Mankinen and Dalrvmple (1972) showed that significant amounts of radiogenic 40 Ar have been lost from basaltic rocks in w’hich K is concentrated in interstitial glass and which show no evidence of chemical alteration. They suggested that only holocrvstalline rocks, in w’hich K is bound in primary minerals, are suitable for age work. Low t and variable K-Ar ages of Mesozoic tholeiitic basalts from Antarctica were ascribed to significant loss of radiogenic 4 Ar w r hich varies inversely w r ith amount of devitrified matrix in the samples (Fleck and others, 1977). Fleck and others (1977) found no chemical or petrographic evidence for loss or gain of K in the samples and postulated that argon loss occurred only during the devitrification process. Armstrong and Besancon (1970) discussed in some ! detail the problems encountered in interpreting the I geologic significance of K-Ar ages for the Upper ! Triassic tholeiitic hvpabyssal and extrusive rocks of eastern North America. They believed that, regard¬ less of the close grouping of most of the ages be- 1 tween 180 m.y. and 200 m.y., the ages do not record the time of igneous activity but are probably the re¬ sult of zeolite facies regional metamorphism which i has affected all of the sedimentary and volcanic rocks. Taking into consideration the alteration his¬ tory of the corehole basalts and the chemical and (or) petrographic criteria suggested by various geo- chronologists for selection of samples for age de¬ termination, we think that all of the corehole basalts and, for that matter, most of the older basalts are unsuitable for yielding reliable K-Ar ages. The age of the lavas can only be considered as pre-Late Cretaceous until additional detailed geochronologi- cal studies, such as, 4C Ar/ 3 Ar experiments, are car¬ ried out. 102 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 COMPARISON WITH BASALTS OF OTHER PROVINCES A comparison of the major-element and selected trace-element abundances in the corehole basalts, with mean compositions of several types of basalts representative of diverse magma types and tectonic settings is shown in table 4. The composition given for the corehole basalts is the average of five sam¬ ples (B-3, B-3A, B-4, B-4A, and B-8), which on the basis of petrographic and chemical criteria are the least altered. Data for basalts chosen for com¬ parative study are taken from compilations by other authors. (See references in table 4.) Because of their proximity' to the study area, compositions of the main tholeiitic magma types represented by eastern North America (ENA) chilled diabases were selected to represent continental tholeiites. Data from the well-studied continental tholeiitic province from Tasmania are also given for com¬ parison. MAJOR ELEMENTS Misleading comparisons can be made using the major-element chemistry of the corehole basalts es¬ pecially with regard to the alkali elements. However, some important petrochemical features appear to have been retained that are important for characteri¬ zation of magma type. For example, except for the Table 4. —Geochemical comparison of basalt from the Clubhouse Crossroads corehole 1 (CCC 1) near Charleston, S.C., with basaltic rocks from other provinces CCC 1 basalt Eastern North America tholeiitic diabases Island i arc series Oceanic- ridge tholeiitic basalt Mesozoic tholeiitic diabase Higb-A) Calc-Alk. basalt Tholeiitic basalt High-Ti Qt2--Nonn. Higb-Fe Low-Ti Qti.-Norm. Qtz.-Norm, Olivine- Norm. Major oxides (weight percent) SiO; _ Al-Oo .. F 02 0 3 ..... FeO __ MgO _ CaO _ Na,0 ..._ K,0 TiO,.. - 54.0 _ 14.0 _ 3.08 _ 8.56 - . 6.01 _ 8.99 _ 2.75 .97 52.1 14.2 1 11.6 7.41 10.66 2.12 .66 1.12 52.7 14.2 1 13.9 5.53 9.86 2.51 .64 1.14 51.7 15.0 1 11.8 7.44 10.8 2.23 .48 .76 47.9 15.3 12.1 10.5 10.7 2.0 .29 .59 53.4 16.4 .52 8.32 6.72 11.49 1.60 .91 .59 50.59 16.29 3.66 5.08 8.96 9.50 2.89 1.07 1.05 51.57 15.91 2.74 7.04 6.73 11.74 2.41 .44 .80 49.9 17.3 2.01 6.9 7.3 11.9 2.8 .16 1.5 Trace elements (parts per million) Rb ___ _ 16 21 22 15 8 33 10 5 1 Ba __ _ 130 160 115 100 115 75 10 Sr __ _ 190 186 180 186 115 130 375 200 135 Zr __ .. 77 92 90 60 44 55 100 52 95 Hf ___ _ 2.0 2.5 1.5 1.1 .8 2.6 1.0 2.9 Nh . . . . _ 7.2 9.5 8.0 5 3 5 2.5 2 5 Th ... 2.0 2.4 .4 3.2 1.1 .5 .18 Ni __ 18 II 34 48 308 67 25 12 97 Co __ .. 46 49 52 53 65 42 40 34 32 K/Rb _ _ 310 280 240 285 300 200 340 900 1,060 NbxlOO/Ti ... _ .12 .13 .12 .11 .14 .13 .05 .03 .06 Zr/Hf _ _ 39 37 40 40 69 38 52 33 Ni/Co . .39 1.7 .65 .91 4.7 1.3 .63 .4 3.0 Sources of data Major elements _ CCC 1 basalt . 1 , average of least altered samples (B—3. B—3A, B—4, B-4 A, End B— 8 ). Eastern North America tholeiitic diabases .Weigand and Ragland, 1970. Tasmania Mesozoic tholeiitic diabase ... _ Edwards, 1942. Island arc series _... Ocean-ridge tholeiitic basalt _ _ Engel and others, 1965 Trace elements _ CCC 1 basalt . 2 , average of leEst Eltered sEmples < B-3, B—3A. B—4, B-4 A, End B— 8 ). Eastern North America tholeiitic diabases -Weigand and Ragland, 1970; Ragland and others. 1968; Smith and others. 1975; David Gottfried, unpub. data. Tasmania Mesozoic tholeiitic diabase _ _ Heier and others, 1965; Compston and others, 1968; Gott- fried 1 and others. 1968. Island arc series_... and White . 1970. 1972a, b; Pearce and Cann. 1973; Taylor, 1969; Gill. 1970. Oceanic-ndge tholeiitic basalt _ _Engel and others, 1965; Tatsumoto and others. 1965; Hart, 1971 Pearce and Cann. 19 73; Dsvid Gottfried, unpub. data * Total Fe as FctOc. GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 103 most altered samples, the magnitude of the effects of alteration for total Fe, Mg, and A1 is small com¬ pared with the variations of these elements between ' the corehole basalts and some other groups of basalt. The contents of Fe, Mg, and Al, thus, are still use¬ ful for comparative purposes. To explore the relationship between magma type and tectonic setting a rank ordering of chemical similarities between the corehole basalts and the 1 comparison basalts was obtained, following the pro¬ cedure of R.agland and others (1968). The follow¬ ing statistics were calculated: 1. The sum, for all oxides of the quantity | % x - % Ch | %Ch 2. The sum, for all oxides of the quantity (%x-%Ch) s fcCh where %x is the oxide abundance in the comparison basalt and e ycC h is the oxide abundance in the core¬ hole basalt The two comparison numbers obtained permit a rank ordering of the comparison basalts compared to the corehole basalts; the smaller values indicate closer similarity between the comparison basalts and the corehole basalts. Results of the com¬ parison are given in table 5 in which the comparison basalts are listed according to this rank-ordering, j This rank-ordering shows that the corehole basalts are most similar to the Mesozoic continental high- | Fe-Ti quartz-normative tholeiitic diabases from eastern North America. Table 5. —Rank ordering of chemical similarities between basalts from the Clubhouse Crossroads corehole 1 near Charleston , S.C., and selected comparison basaltic rocks Comparison basalts 1 z- Value i — 7,-Cb VrCb Rank : V' (9ex- A. <7c Value -7rChK Cb Rank ENA 5 High-Fe, quartz-norma¬ tive tholeiitic diabase_ 0.76 1 0.58 1 ENA 5 High-Ti, quartz-norma¬ tive tholeiitic diabase _ 1.15 2 . .9? 2 ENA 5 Low-Ti, quartz-norma¬ tive tholeiitic diabase _ 1.15 2 1.19 3 Island arc, tholeiitic basalt_ 1.26 3 1.68 4 Island arc, high-Al. calc-alk. basalt __ 2.13 4 3.41 6 Tasmania Mesozoic tholeiitic diabase_ 2.26 5 2.59 5 ENA 5 olivine-normative tholeiitic diabase_ 2.32 6 5.14 8 Oceanic-ridge tholeiitic basalts. 2.32 6 3.46 7 ' Data from table 4, Fe as FeO. and analyses recalculated water-free. 2 ENA = Eastern North America. TRACE ELEMENTS For the most part, variations in trace elements in these basaltic provinces can be related to (1) the composition and mineralogy of the source region and (2) the degree of partial melting and subse¬ quent fractional crystallization during ascent of magma in the mantle and in the crust. Other fac¬ tors that may play a role are “wall rock reaction,” which is discussed in detail by Green and Ringwood (1967), and contamination resulting from interac¬ tion of basaltic magma with rocks of the conti¬ nental crust (Faure and others, 1974). Although these processes affect the major-element chemistry to some extent, the trace-element concentrations are affected much more and, more importantly, can pro¬ vide clues to the history and origin of basalts that are not readily observed from the relations among the major elements. Comparison of the trace-element data in table 4 indicates that the corehole basalts show the great¬ est similarity’ to the ENA quartz-normative dia¬ bases, in particular to the high-Fe and high-Ti types. Wiegand and R.agland (1970) and R.agland, Brun- felt, and Weigand (1971) regarded the ENA high- Fe type as a subgroup of the ENA high-Ti group with which it shares similarities (REE, Ti, Rb, Sr, Zr, Co) but from which it is different in having a lower Ni content and hence a lower Ni/Co ratio. The corehole basalts are closer in composition to the high Fe-tvpe than to the high-Ti type. A close simi¬ larity exists between the corehole basalts and the high-Fe type in abundances of stable minor and trace elements (Ti, Zr, Hf, Nb) and of mobile ele¬ ments (K, Rb), as well as in the unusually low Ni/ Co ratios. This similarity’ can be extended to in¬ clude Cu which, as noted previously (table 2), is unusually low in the corehole basalts (—25 ppm). In high-Fe types, Cu decreases as the Ni/Co ratio de¬ creases to values as low as 37—44 ppm (Weigand and Ragland, 1970). A comparison of the REE pattern of the corehole basalts (from fig. 2) with average REE patterns for the ENA high-Ti quartz-normative diabase (an average which includes the high-Fe type), ENA olivine-normative diabase, and oceanic-ridge tho¬ leiitic basalts is shown in figure 5. The abundances and REE pattern for the corehole basalts and for the ENA high-Ti type are virtually the same. The REE pattern for the ENA olivine-normative type is similar to that for the high-Ti type but is dis¬ tinctly lower in absolute abundance. Light-REE en¬ riched patterns intermediate between those of the 104 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 EXPLANATION A OCEANIC-RIDGE THOLEIIT1C BASALT Figure 5. —Averag-e abundances of rare-earth elements (REE) in tholeiitic diabases and basalts normalized to average REE abundances in 20 chondrites (Haskins and others, 1966). Data sources: oceanic-ridge tholeiitic basalt (Schilling, 1971) ; eastern North America olivine-normative diabase and eastern North America high-Ti quartz-normative diabase (Ragland, Brunfelt, and Weigand, 1971) ; and basalts from Clubhouse Crossroads corehole 1 near Charleston, S.C., table 2 (this paper). high-Ti and olivine-normative types characterize the low-Ti type (fig. 4 in Ragland, Brunfelt, and Weigand, 1971; Ragland, Weigand, and Brunfelt, 1971). The light-REE depleted patterns of oceanic- ridge tholeiitic basalt (fig. 5) are in sharp contrast to the light-REE enriched patterns of the corehole basalts and ENA high-Ti and olivine-normative tholeiitic types. Other trace-element contents and interelement ratios that clearly distinguish the core¬ hole basalts and the ENA tholeiitic diabases from oceanic-ridge tholeiitic basalts are higher Rb, Ba, and Th contents, higher Nb/Ti ratios, and lower K/Rb ratios. However, the abundance levels for Nb, Zr, and heavy REE (Sm-Lu) are either similar or even lower in the ENA low-Ti type, ENA olivine- normative type, and Tasmanian tholeiitic diabase than in the average oceanic- ridge tholeiitic basalt. Depletion of trace elements of large ionic radii such as Cs, Rb, Ba, Th, and U and the high K/Rb, K/Ba, and K/Cs ratios have led to the interpretation that the oceanic-ridge tholeiitic basalt was derived from a source in the mantle that had previously under- : gone one or more periods of partial melting (Tatsu- ! moto and others, 1965; Gast, 1968; Hart, 1969, 1971; Hart and others, 1970). These geochemical i features once considered unique to the worldwide : oceanic-ridge system have also been recognized in volcanic suites, designated by Jakes and Gill (1970) 1 as the island arc tholeiitic series. The REE patterns of these rocks are indistinguishable from those of the oceanic-ridge tholeiitic basalt (Jakes and Gill, 1970), as are several other geochemical features | shown in table 4. Thus far, it can clearly be shown that the core¬ hole basalt is neither an ocean-floor basalt that i originated from the mid-Atlantic ridge system nor a basaltic rock with affinities to the island arc tho- ‘ leiite series. The corehole basalt, and for that mat- ! ter, the ENA quartz-normative types, have many features in common with basalt, or basaltic andesite, of the calc-alkalic series of island arcs. These include , the low Ti, Ni, Co, and Cr contents, which, as pre¬ viously mentioned, are considered distinctive of the calc-alkalic basalts (table 4) and andesites. They GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 105 also have similar K/Rb ratios (Jakes and White, 1970) and light-REE enrichment patterns (Jakes and Gill, 1970). However, important differences that can be observed are the generally higher K ? 0, Al-Oa, and Sr abundances in the calc-alkalic basaltic rocks, than in the ENA high-Ti and high-Fe quartz-norma¬ tive diabases. These differences may be related to differences in water pressure and oxygen fugacity of the magmas from which they formed. The higher water content in calc-alkalic magmas favors crys¬ tallisation of large amounts of plagioclase (Yoder, 1969) which results in the higher A1 and Sr con¬ tents. The presence of low Ni, Co, Cr, and Cu in the continental ENA high-Ti quartz-normative diabase cannot be explained by the two-stage model of Taylor (1969) which involves partial melting of oceanic crust carried down the Benioff Zone in an island arc environment. The geochemical data indi¬ cate that the ENA and corehole basalt magmas were derived from a mantle undepleted with regard to the lithophilic elements. We suggest that prior to extrusion, olivine and copper-bearing minerals may have crystallized and depleted the Ni, Co, Cr, and Cu of the magma. Studies of immiscible sulfide melts and polvmineralic sulfide minerals from Hawaiian tholeiitic basalts (Skinner and Peck, 1969; Desborough and others, 1968) indicate that crystallization of chalcopyrite and Cu- and Ni-rich pvrrhotite solid solutions has taken place. These studies, and those referred to earlier, show that these features are the result of late-stage fractiona¬ tion processes of basaltic magma. The (Nbxl00)/Ti ratio (0.05) in high-Al basalts (from data in Pearce and Cann, 1973) is lower than that in the corehole basalts (0.12) and continental tholeiites (0.11 to 0.14). This ratio may serve as a useful discriminant because it remains relatively constant during the early and middle stages of dif¬ ferentiation of tholeiitic magmas (Gottfried and others, 1968). Variation of Zr/Hf in the compari¬ son suites, with the exception of suites of the Tas¬ manian province, has not yet been adequately docu¬ mented and at this stage provides no more informa¬ tion than Zr alone. The preceding discussions of the implications of the geochemical data with regard to magma type can be summarized as follows: (1) the corehole basalts can be classified as quartz-normative tho¬ leiites and were derived from an undepleted source area, presumably the upper mantle; (2) striking similarities between major-element, trace-element, and REE compositions of the corehole basalts and those from the Mesozoic ENA tholeiitic province in¬ dicate that the corehole basalts are continental tho¬ leiitic basalts. TECTONIC SETTING In recent years, extensive geochemical studies have shown that young mafic volcanic rocks in a given geographic or geologic setting have consistent or characteristic geochemical features that characterize their tectonic setting. These studies have led'to the recognition of several trace elements that were shown to be relatively insensitive to alteration processes and that were characteristic of magma type and tectonic setting. Subsequently, the abund¬ ance patterns of these trace elements were used to construct binary or ternary discrimination dia- grams which allowed comparisons to be made be¬ tween trace-element abundances in an unknown rock and those in volcanic rocks of known tectonic set¬ ting. The volcanic rocks are generally classified according to a scheme that combines tectonic set¬ ting and chemical characteristics (Pearce and Cann, 1973). Tectonic setting is based on the rela¬ tive motions of lithospheric plates. Thus mafic vol¬ canic rocks are designated as (1) ocean-floor basalts (diverging plate margins), (2) low-K tholeiites and calc-alkalic basalt of the island arc series (converg¬ ing plate margins), and (3) oceanic island and con¬ tinental basalts (intraplate oceanic crust and intra¬ plate continental crust). Elements generally used are Ti, Zr, Y, Nb, P and, also, K and Sr in situa¬ tions where the rocks are considered unaltered (Pearce and Cann, 1973; Floyd and Winchester, 1975; Winchester and Floyd, 1976; Pearce and others, 1975). The most widely applied discrimina¬ tion diagrams are the Ti-Zr, Ti-Zr-Y, and Ti-Zr-Sr diagrams of Pearce and Cann (1973). The results of plotting the abundances of these elements in the corehole basalts and in some com¬ parison continental basalt samples on these three : diagrams are presented in figure 6. The comparison samples include basalts or chilled diabases from the | eastern North America, Tasmania, Antarctica, and ' Karroo tholeiitic provinces. The Ti and Zr values for the corehole basalts and ENA high-Ti types plot in fields B and C on the Ti-Zr diagram (fig. 6A) ! which would indicate the samples are calc-alkalic i basalts of the island-arc series. The other con¬ tinental tholeiitic diabases show affinities to island arc tholeiitic basalt or ocean-floor basalt. Figure 6 B (Ti-Zr-Y) shows that the corehole basalts are ; in the field of ocean-floor basalts; the diabase from Ti. IN PPM 106 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 14.000 12.000 10.000 8000 6000 4000 2000 0 0 20 40 60 80 100 120 140 160 180 Zr, IN PPM EXPLANATION A FIELDS A—8 Low-potassium tholeiitic basalt of island arc series &-C Calc-alkalic basalt of island arc series B—D Ocean-floor basalt BASALTS AND DIABASES OF EASTERN NORTH AMERICA Corehole basalts, Charleston, S. C. © B-i, 2. 5, 6, 7 (most altered) • &-3, 4, 8 (least altered) Eastern North America ® High-Ti quartz-normative diabase ■+ Low-Ti quartz-normative diabase ® Olivine-normative diabase DIABASES OF OTHER CONTINENTS ■ Tasmanian diabase X Ferrar diabase, Antarctica A Karroo diabase. South Africa Figure 6. —Samples of corehole basalts and some tholeiitic basalts and diabases from continental provinces plotted on discrimination diagrams of Pearce and Cann (1973). A, Samples plotted on the Ti-Zr discrimination diagram. Data sources: basalts from Clubhouse Crossroads core- hole 1 near Charleston, S.C., tables 1 and 2 (this paper); eastern North America (ENA), table 3 of Ragland, Brunfelt, and Weigand (1971) ; Great Lake sheet, Tas¬ mania, McDougall (1964), Gottfried ond others (1968); the Palisades sill is in the field of calc-alkalic basalts. In figure 6C (Ti-Zr-Sr), the samples cluster at or near the junctions of the fields: ocean-floor basalt, island arc tholeiitic basalt, and calc-alkalic basalts of the island-arc series. It is thus clear that these diagrams are inappropriate for identification of tectonic setting and magma type of these types of continental tholeiites. Ferrar diabase, Gunn (1966) ; Karroo basalts of south¬ ern province. South Africa, table 3 of Cox and others (1967). B, Plot of corehole basalts and chilled diabase of Palisades sill on Zr-Ti-Y discrimination diagram. Note that the corehole basalts and chilled margin of the Palisades sill (Walker, 1969) plot in fields B and C, respectively. C, Samples of corehole basalts and diabases from eastern North America plotted on Zr-Ti-Sr dia¬ gram. Each of the continental tholeiitic magma types that we used for comparison occurs on Atlantic-type passive continental margins and represents the early cycles of magmatism that preceded or coin¬ cided with rifting, continental drift, and sea-floor spreading. A comparison between the chondrite- normalized REE abundance pattern for the core¬ hole basalts and the chondrite-normalized REE GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 107 EXPLANATION FIELDS A-B Low potassium tholeiitic basalt of island arc series B Ocean-floor basalt B-C Calc-alkalic basalt of island arc series D Intraplate basalt: oceanic island or continental basalt BASALTS AND DIABASES OF EASTERN NORTH AMERICA ' Corehole basalts, Charleston, S. C. ® B-1, 2, 5, 6, 7 (most altered) • B-3, 4, 8 (least altered) C Palisades sill, New Jersey, chilled margin Ti/100 /' r X -k / r B. Yx3 A B C © EXPLANATION FIELDS Low-potassium tholeiitic basalt of island arc series Calc-alkalic basalt of island arc series Ocean-floor basalt BASALTS AND DIABASES OF EASTERN NORTH AMERICA Corehole basalts, Charleston, S. C. Ti/100 \ Sr/2 abundances for the Karroo, Ferrar, and Red Hill diabases (Philpotts and Schnetzler, 1968) is shown in figure 7. Plots of the REE data for each of these suites are on a single variation curve and show that the suites are indistinguishable from each other on the basis of their REE variation patterns. Other important geochemical features shared by the corehole basalts, rocks of the ENA tholeiitic ROCK/CHONDRITE 108 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 EXPLANATION ELEMENTS Figure 7. —Comparison of chondrite-normalized rare-earth element (REE) abundance patterns in basalts from Clubhouse Crossroads corehole 1 near Charleston, S.C. (see fig, 2, this paper), with average REE abundances of chilled margins of Karroo diabase, South Africa; Ferrar diabase, Antarctica; and Red Hill diabase, Tasmania (Philpotts and Schnetz- ler, 1968). province, and the Tasmanian-Antarctic and Karroo tholeiitic rocks are their unusually low abundances of Ti, Nb, and Zr. Schilling (1971) suggested that the relatively low abundances of REE in the chilled diabases of these provinces might be related to dif¬ ferences between extrusive and intrusive crystalliza¬ tion regimes. This difference in regime appears not 1 to be the reason for different REE patterns because the REE patterns and absolute abundances are es¬ sentially the same in the Palisades diabase (Phil- J potts and Schnetzler, 1968) and in the Watchung Basalt flow (Donnelly and others, 1973). Further¬ more, Smith, Rose, and Lanning (1975) found that flows, sills, and dikes of the high-Ti quartz-norma- j tive magma type in Pennsylvania are nearly identi- ! cal in chemical composition. For comparisons with the corehole basalts, we selected samples or suites ; of continental rocks of similar composition to mini¬ mize effects of fractional crystallization or other dif¬ ferentiation processes on the abundance variations. The petrogenetic implications of the similar or lower abundance levels of lithophilic elements of small ionic radii (Ti, Zr, Hf, Nb, Ta) in a Tasmanian diabase-granophyre suite as compared to those in oceanic-ridge tholeiitic basalt were discussed previ¬ ously (Gottfried and others, 1968). Ragland, Brun- felt, and Weigand (1971) showed that the olivine- normative tholeiitic dikes from North Carolina have half the absolute REE concentrations of oceanic- ridge tholeiitic basalt. Smith and others (1975) pointed out similarities between the low-Ti quartz- normative diabase and island arc tholeiitic basalt. The rocks selected by Pearce and Cann (1973) for establishing the field of continental basalts on their discrimination diagrams are from the African Rift Valley, Karroo, and Deccan provinces. Their mean contents in parts per million for Ti (15,150) ; Zr (215), and Nb (20) are at least twice those of the abundances found in the corehole basalts, in the chilled margins of the sills and dikes of the ENA province, and in the Tasmanian province. In the Red Sea region, which is considered the modem analog of early Mesozoic rifting and continental drift, magma associated with rifting and crustal extension was dominantly of the alkalic basalt type, and magma in the median trough of the Red Sea was tholeiitic (Gass, 1970; Mohr, 1972). The REE patterns of the tholeiites from the Red Sea trough are similar to those of basalts from the oceanic- ridge system (Schilling, 1969; 1973). Both the GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 109 Karroo and Deccan provinces contain a wide range of tholeiitic basalts as well as intermediate, felsic, | and under saturated alkalic rocks (Ghose, 1976; Cox, 1970). Clearly, the basalts (containing 328 ppm Zr and 27 ppm Nb) of the Karroo province selected by Pearce and Cann (1973) are more strongly frac¬ tionated than the Karroo samples we selected for comparison. A similar situation may exist with re¬ gard to REE abundances in the Deccan basalts. The REE pattern for the Deccan basalt given by Frey and others (1968) was considered “typical” of con¬ tinental basalts but shows light REE absolute abundances two to three times those shown by the comparison suites shown in figure 7 and is more , similar to that of alkalic basalts. The REE pat- j tern of the average Deccan plateau basalt given by Nakamura and Masuda (1971) is believed to be more representative of the province and shows REE abundances similar to those of our comparison quartz-normative tholeiites. The examples noted above indicate that part of the data used for char¬ acterizing continental basalt provinces seems to have given misleading impressions about the represen¬ tative geochemical features for some continental tholeiitic provinces. The preceding discussion noted that most of the suites of samples selected for construction of the discrimination diagrams discussed previously rep¬ resented magmas of alkalic affinity or highly evolved tholeiitic magmas. By way of contrast, the tholeiitic magmas of Mesozoic age in eastern North America represent perhaps the most primitive end of the spectrum of tholeiitic magma types erupted into con¬ tinental crust. This is particularly true for the oli¬ vine-normative type (R.agland and others, 1972; J Ragland, Weigand, and Brunfelt, 1971; Gottfried and Greenland, 1972). By virtue of the striking similarities of geochemical features of the corehole basalts with those of the quartz-normative tholeiitic magmas of this province, it is reasonable to assume | a similar mode of origin and tectonic environment j at the time of extrusion. The similarity of K-Ar ages of the corehole basalt and the age of -the overlying sediments presents a knotty problem. Although we considered the K-Ar. age (~100 m.y.) as a mini-' mum age because of posteruption alteration effects, it could be argued that alteration occurred shortly after extrusion and that the basalt is of Cretaceous age. However, on the basis of their characteristic ' geochemical features, we suggest that the corehole | basalts are related to the tholeiitic province of east¬ ern North America in time as well as space. The age of the quartz-normative tholeiites in the northern ! part of the province is Late Triassic or Early Juras¬ sic on the basis of geologic and paleontologic evi¬ dence (Johnson and McLaughlin, 1957; Cornet and others, 1973). In general, it is believed that the magmas representing these tholeiitic flows, sills, and dikes were intruded at the onset of rifting and sepa¬ ration of North America from North Africa (Faust, 1975). Evidence from deep drilling and magnetic anomaly patterns indicate that continental drift be¬ gan 180 m.y. ago (Pitman and Talwani, 1972; Vogt, ; 1973). Magmatism, rifting, and an extensional tec¬ tonic regime were associated with upwelling of the mantle and a steep geothermal gradient. A rather high degree of partial melting of the upper mantle in this area of high heat flow could account for the “primitive” nature of the ENA tholeiitic magmas and their intrinsic geochemical features. If the core¬ hole basalt is approximately 100 m.y. old. then vol- canism took place after the continental margin | moved progressively further from the spreading i axis. It is reasonable to assume that the petro¬ chemical features of later formed magma types would be different because of differences in the thermal regimes that prevailed at the time of their formation. SUMMARY AND CONCLUSIONS The results of our geochemical study of 11 core¬ hole samples can be summarized as follows: 1. Major- and minor-element and petrographic data indicate that the basalts have undergone slight to extreme oxidation, hydration, and hydrothermal alteration. Effects of these proc¬ esses are strongest on the marginal zones of the flows and are reflected mineralogically by the presence of zeolites, calcite, and chlorite, and chemically by high abundances of H t O, Fe 2 0 3 , and C0 2 and by remobilization of K, Ba, Rb, and Sr. Rubidium is the most sensitive in¬ dicator of alteration and is useful for assess¬ ing the degree and extent of alteration in the flows. The minor elements P and Ti and the trace elements Zr, Nb, Hf, Th, and REE are essentially uniform in the two flows and indi¬ cate the absence of any in situ differentiation trends. Normative compositions of the least altered samples indicate that the basalts are of the quartz-normative tholeiitic magma type. 2. The major-element and trace-element composi¬ tion of the corehole basalts is compared with available data on rocks of theoleiitic composi¬ tion from Atlantic-type, passive continental 110 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 margins (eastern North America, Tasmania, Antarctica, South Africa) and on basalts from island arc (calc-alkalic basalt, low-potassium tholeiitic basalt) and mid-oceanic ridge set¬ tings. The light REE enrichment pattern and low K/Rb ratio (—300) of the corehole basalts contrast markedly with corresponding data on island arc tholeiitic basalt and oceanic-ridge basalt which characteristically are depleted in light REE and have high K 'Rb ratios ( — 950- • 1050). These data indicate that the corehole tholeiitic magma w r as derived from an unde¬ pleted source area in the upper mantle. Some geochemical features believed to be character¬ istic of orogenic high-Al basalts and andesites are shared by both the corehole basalts and the high-Fe quartz-normative tholeiitic dia¬ base of the eastern North America province. ! These include low Ti, Cr, Cu, Ni, and Co con¬ tents and low Ni/Co ratios. We attribute some of these features to low-pressure fractional crystallization processes involving separation of olivine and Cu- and Ni-bearing sulfides dur¬ ing magmatic ascent. On the basis of the REE pattern, as well as absolute abundances, the corehole basalts are virtually indistinguishable from the quartz-normative tholeiitic rocks from eastern North America, Karroo (South j Africa), Ferrar (Antarctica), and Red Hill (Tasmania). 3. The low abundances of ions of relatively small ionic radii, (Ti, Zr, Hf, Nb, and Ta) in the corehole basalts and in our comparison suites of continental tholeiites are more similar to those found in island arc and oceanic-ridge basalts than to “average” abundances in con¬ tinental basalts. As a result of these primitive features, Mesozoic continental tholeiitic dia¬ bases from eastern North America, Tasmania, and Antarctica would be erroneously classi- ! fied as oceanic-ridge and (or) island arc tho¬ leiites on the basis of their Ti-Zr-Nb abund¬ ance relations. In the present study, no single | group or pair of geochemically associated ele- | ments could be used alone for distinguishing magma type and tectonic setting of the core- ■ hole basalts from basalts of all of the con- | trasting tectonic environments considered This emphasizes the importance of using trace elements of widely different chemical proper- j ties and sizes for discrimination purposes. 4. K-Ar analyses of two whole-rock samples yield j ages of 94.8 m.y. for the least altered sample, 1 and 109 m.y. for an altered K-rich sample. These dates are considered minimum ages which may be significantly younger than the time of volcanism. Geochemical data on the corehole basalts consti¬ tute a substantial body of information which has placed certain constraints on proposed models of the regional tectonic setting. The geochemical fea¬ tures of the basalts suggest to us that they are re- lated in space and time to the tholeiitic province of • eastern North America and have features in com¬ mon with quartz-normative tholeiitic suites found on other rifted continental margins. The tectonic regime associated with magmatic activity during early rifting is dominated by extensional tec¬ tonics. Flows, dikes, and sills of the eastern North America province are closely associated with Tri- assic-Early Jurassic rift valleys and deep-seated grabens. 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R., 1977, Petrology and geochemistry of mafic rocks from the Cayman Trench—Evidence for spread¬ ing: Geology, v. 5, no. 2, p. 105-110. Philpotts, J. A., and Schnetzler, C. C., 1968, Genesis of con¬ tinental diabases and oceanic tholeiites considered in light of rare-earth and barium abundances and partition coefficients, in Ahrens, L. H., ed., Origin and distribution of the elements: New York, Pergamon Bress, p. 939-947. Philpotts, J. A., Schnetzler, C. C., and Hart, S. R., 1969, Sub¬ marine basalts—Some K, Rb, Sr, Ba. rare-earth, H,0, and CO- data bearing on their alteration, modification by plagioclase, and possible source materials: Earth and Planetary Sci. Letters, v. 7, no. 3, p. 293-299. Pitman, W C., Ill, and Talwani, Manik, 1972, Sea-floor spreading in the North Atlantic: Geol. Soc. America Bull., v. 83, no. 3, p. 619—646. GEOCHEMISTRY OF SUBSURFACE BASALT FROM DEEP COREHOLE 113 Prinz, Martin, 1967., Geochemistry of basaltic rocks—Trace elements, in Hess, H. H., and Poldervaart, Arie, eds., Basalts; The Poldervaart treatise on rocks of basaltic composition, v. 1: New York, Interscience Publishers, p. 271-323. Ragland, P. C., Brunfelt, A. 0., and Weigand, P. W., 1971, Rare-earth abundances in Mesozoic dolerite dikes from eastern United States, in Brunfelt, A. 0., and Stemnes, Eiliv, eds., Activation analysis in geochemistry and cosmochemistry: Oslo, Universitetsfarlaget, p. 227-235. R.agland, P. C., Fullagar, P. D., Wiegand, P. W.. and Brun¬ felt. A. 0., 1972, 'Primitive’ nature of Mesozoic dolerites from the southeastern U.S. [abs.]: Intemat. Geol. Cong., 24th, Montreal, 1972, Abstracts [Volume], p. 53. Ragland. P. C., Rogers. J. J. W., and Justus, P. S., 1966. Origin and differentiation of Triassic dolerite magmas, North Carolina, U.S.A.: Contr. Mineralogy and Pe¬ trology, v. 20, no. 1, p. 57-80. Ragland, P. C., Weigand, P. W., and Brunfelt, A. 0., 1971, Rare-earth element abundance patterns in Mesozoic dolerites from eastern United States [abs.]: Geol. Soc. America, Abs. with Programs, v. 3. no. 5, p. 342-343. Schilling, J.-G., 1969, Red Sea floor origin; rare-earth evi¬ dence: Science, v. 165, no. 3900, p. 1357-1360. - 1971, Sea-floor evolution; rare-earth evidence: Royal Soc. London Philos. Trans., ser. A, v. 268, no. 1192, p. 663-706. - 1973, Afar mantle plume; rare earth evidence: Nature: Physical Sci., v. 242. no. 114. p. 2-5. Schilling, J.-G., and Winchester, J. W., 1967. Rare-earth fractionation and magmatic processes, in Runcorn, S. K., ed., Mantles of the earth and terrestrial planets; New York. Interscience Publishers, p. 267-283. Seidel, Eberhard, 1974, Zr contents of glaucophane-bearing meta-basalts of Western Crete, Greece: Contr. Mineral¬ ogy and Petrology, v. 44, no. 3, p. 231-236. Shapiro, Leonard, 1975. Rapid analysis of silicate, carbonate, and phosphate rocks—revised edition: U.S. Geol. Survey Bull. 1401, 76 p. Skinner, B. J. and Peck, D. L., 1969, An immiscible sulfide melt from Hawaii in Wilson, H. D. B., and Bateman, A. M., eds., Magmatic ore deposits—a symposium: Econ. Geol. Mon. 4, p. 310-322. Smewing, J. D., Simonian. K. O., and Gass, I. G., 1975, Meta¬ basalts from the Troodos Massif, Cyprus; Genetic im¬ plication deduced from petrography and trace element geochemistry: Contr. Mineralogy and Petrology, v. 51, no. 1, p. 49-64. Smith, R. C. II, Rose. A. W., and Lanning, R. M., 1975, Geology and geochemistry' of Triassic diabase in Pennsyl¬ vania: Geol. Soc. America Bull., v. 86, no. 7, p. 943-955. Tatsumoto, M., Hedge, C. E., and Engel, A. E. J., 1965, Potassium, rubidium, strontium, thorium, uranium, and the ratio of strontium-87 to strontium-86 in oceanic tholeiitic basalt: Science, v. 150, no. 3696, p. 886—888. Taylor, S. R., 1965, Geochemical analysis by spark source mass spectrography': Geochim. et Cosmochim. Acta, v. 29, no. 12, p. 1243-1261. - 1969, Trace element chemistry of andesites and as¬ sociated calc-alkaline rocks, in McBimey, A. R.., ed., Proceedings of the Andesite Conference, Eugene and Bend, Oreg., July 1-6, 1966: Oregon Dept. Geology and Mineral Industries Bull. 65, p. 43—63. Taydor, S. R., Kaye, Maureen, White. A. J. R., Duncan, A. R., and Ewart, A., 1969, Genetic significance of Co, Cr, Ni, Sc and V content of andesites: Geochim. et Cos¬ mochim. Acta, v. 33, no. 2, p. 275-286. Taylor, S. R., and White, A. J. R.. 1966, Trace element abundances in andesites: Bull. Volcanol., v. 29, p. 177- 194. Taylor, S. R., White. A. J. R.. Ewart. A., and Duncan, A. R... 1971, Nickel in high-alumina basalts; A reply': Geochim. et Cosmochim. Acta, v. 35, no. 5, p. 525-528. Vogt. P. R., 1973, Early events in the opening of the North Atlantic, in Tarling, D. H., and R.uncom, S. K., eds.. Implications of continental drift to the earth sciences New York, Academic Press, v. 2, p. 693-712. Walker, K. R.. 1969, The Palisades sill, New Jersey; a rein¬ vestigation: Geol. Soc. America Spec. Paper 111, 178 p. Weigand, P. W„ and P-agiand. P. C., 1970, Geochemistry of Mesozoic dolerite dikes from eastern North America: Contr. Mineralogy and Petrology', v. 29, no. 3, p. 195-214. Winchester, J. A.., and Floyd, P. A., 1976, Geochemical magma type discrimination; Application to altered and meta¬ morphosed basic igneous rocks: Earth and Planetary Sci. Letters, v. 28. no. 3, p. 459-469. Toder, H. S., Jr., 1969, Calcalkalic andesites—experimental data bearing on the origin of their assumed characte¬ ristics, in McBirney, A. R., ed., Proceedings of the Ande¬ site Conference, Eugene and Bend, Oreg.. July 1-6, 1968: Oregon Dept. Geology' and Mineral Industries Bull. 65, p. 77-89. Yoder, H. S., Jr., and Tilley, C. E., 1962, Origin of basalt magmas; an experimental study of natural and syn¬ thetic rock systems: Jour. Petrology', v. 3, no. 3, p. 342-529. Zietz, Isidore, Popenoe, Peter, and Higgins, B. B., 1976, Regional structure of the southeastern United States as interpreted from new aeromagnetic maps of part of the Coastal Plain of North Carolina, South Carolina, Georgia, and Alabama [abs.]: Geol. Soc. America, Abs. with Programs, v. 8, no. 2, p. 307. F ' F I 1 r ■ . r F I F » IF IF D Heat Flow From a Corehole Near Charleston, South Carolina, By J. H. SASS and JOHN P. ZIAGOS STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-H CONTENTS Page Abstract _ 115 Introduction _ 115 Heat-flow data _ 115 Regional significance_ 116 References cited _ 116 ILLUSTRATIONS Page Figure 1. Graph showing temperature versus depth in Clubhouse Cross¬ roads corehole 1 _ 116 TABLE Page Table 1. Temperature gradient, harmonic mean thermal conductivity, and heat flow for nearly linear segments of the temperature profile 116 f ■ STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT HEAT FLOW FROM A COREHOLE NEAR CHARLESTON, SOUTH CAROLINA By J. H Sass and John P. Ziagos ABSTRACT Temperature measurements were made at 3-m intervals from the surface to total depth (790 m) in Clubhouse Cross¬ roads corehole 1 located near the epicenter of the 1886 Charleston earthquake. The temperature profile is irregular, reflecting the observed variability of thermal conductivity with depth. With the exception of one 150-m-thick interval within the Middendorf Formation (Upper Cretaceous), temperature gradient varies inversely with thermal con¬ ductivity over individual stratigraphic units, resulting in internally consistent component values of heat flow averag¬ ing 1.3=0.12 hfu (1 heat flow unit, hfu^lO -0 cal cm' ; s _1 =41.8 roW m' s ). This value is within the range of other values measured in the region; thus, no thermal anomaly is associated with the observed seismicity’ in the area. INTRODUCTION As an adjunct to the geological, geophysical, and seisinological investigations related to the 1886 Charleston earthquake, thermal studies were made in the Clubhouse Crossroads corehole 1 (CCC 1), located about 40 km west-northwest of Charleston (fig. 2, Rankin, this volume). The thermal studies were undertaken primarily to obtain a well-docu¬ mented value of regional heat flux but incidentally to discover if any thermal anomalies were associ¬ ated with the observed seismicity (Bollinger and Visvanathan, this volume; Tarr, this volume) of the area. In general, thermal anomalies would be ex¬ pected only in regions of strong and recent tectonic strain, such as plate margins (Lachenbruch and Sass, 1973) or where igneous activity has occurred during the last few million years over areas having horizontal dimensions on the order of a * crustal thickness (Lachenbruch and others, 1976; Lachen¬ bruch and Sass, 1977). Thermal anomalies might also be expected in regions of large-scale convective water movement (see, for example, Lachenbruch and Sass, 1977). This report summarizes the ther¬ mal studies in CCC 1 and their relation to regional heat flux and the tectonic setting. The thermal meas¬ urements are discussed in detail by Ziagos, Sass, and Munroe (1976). HEAT-FLOW DATA Temperatures were measured at 3-m intervals | over the entire length of the hole (fig. 1). Irregu¬ larities in the temperature profile, which indicate | abrupt changes in thermal conductivity, reflect the I stratification of the sedimentary section. Ninety measurements of thermal conductivity on water-saturated sections of core showed a stratifica¬ tion that corresponded closely to the variations in thermal gradient with depth. The lowest thermal conductivities (2-4 cal cm -1 s _1 °C _1 were measured in the Cooper Formation in the uppermost 60 m of the hole and in shale and clay-rich units. Basalt ! had a small range of conductivities (4.2-4.6 cal cm -1 s _1 °C -:1 ), and gravelly and sandy sections and well-indurated sandy and silty limestone and sand¬ stone had the highest conductivities (5-8 cal cm -1 ! s- 1 °c- 1 ). With the exception of the depth interval 555- 698 m (mostly within the Middendorf Formation (Upper Cretaceous) ; Gohn and others, this volume), component heat flows range from 1.1 to 1.5 hfu (1 hfu, heat flow unit, =10 -6 cal cm~ 2 s _1 = 41.8 mW m~ 2 ) (table 1). The mean of these seven values, weighted according to the thickness of the inter¬ vals, is 1.30 = 0.12 hfu. The value of 0.71 hfu with¬ in the Middendorf Formation is significantly lower than the mean for the other intervals. This forma¬ tion contains layers of coarse sand and pebbles, and we attribute the low heat flow mainly to the con¬ vective transfer of heat across the formation by moving ground water. Ziagos, Sass, and Munroe (1976) used a number of methods for reducing the data, and, in all in- 115 116 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Figure 1.—Temperature versus depth in Clubhouse Crossroads eorehole 1. Temperatures were measured at 3-m inter¬ vals. Lithologic column based on figure 2 of Gohn and others, this volume. Table 1 .—Temperature gradient, harmonic mean thermal conductivity, and heat flow for nearly linear segments of the temperature profile (fig. 1) in Clubhouse Crossroads eorehole 1 Depth interval (m) Temperature gradient. CC km->) Number of samples Thermal conductivity (10* 3 cal cm -1 s-i ’C* 1 ) Heat flow (10—^ cal cm" 2 s _1 ) 274-396 .. ..21.0 11 5.992:0.29 1.26 405-442 .. .23.0 5 6.38— S3 1.47 442-469 .. _40.1 5 3.39* 16 1.36 46&-515 -- 6 5.28=: 56 1.34 509-555 .30.1 4 3.76=: .82 1.13 565—696 .. ..18.4 15 3.88=: .32 0.71 713-738 .. .-42.2 5 3.43= 11 1.45 764-789 .. .30.4 16 4.23= .13 1.28 stances, the heat flow calculated was within the limits of 1.3 ± 0.2 hfu. REGIONAL SIGNIFICANCE The heat flow of 1.3 hfu is within the range of values commonly found in the Coastal Plains physio¬ graphic province and adjoining parts of the Ap¬ palachian province of Fenneman (1.946) (see fig. 1 of Ziagos and others. 1976, fig. 2 of Sass and others, 1976, or fig. 1 of Lachenbruch and Sass, 1977, for the most recent maps). From a thermal standpoint, the Charleston area seems to be indis¬ tinguishable from the rest of the United States east of the Great Plains. It thus appears that steady- state frictional heating within the upper crust does not contribute significantly to the observed heat flux in this region of relatively high seismicity. In contrast, in the San Andreas fault zone north of the Transverse Ranges in California, Lachenbruch and Sass (1973) estimated that as much as 0.8 hfu might be produced by shear-strain heating asso¬ ciated with relative movement between the Pacific and American plates (Atwater, 1970) over a band about 100 km wide, roughly coincident with the fault zone. REFERENCES CITED Atwater, Tanya, 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geol. Soc. America Bull., v. 81, no. 12, p. 3513-3535. Fenneman, N. M.. 1946, Physical divisions of the United States: Map prepared in cooperation with the Physio¬ graphic Committee, U.S. Geological Survey, U.S. De- HEAT FLOW FROM A COREHOLE 117 partment of the Interior, Washington, D. C. scale 1: 7,500,000. Lachenbruch, A. H., and Sass, J. H., 1973, Thermo-mechani¬ cal aspects of the San Andreas Fault system, in Pro¬ ceedings of the conference on tectonic problems of the San Andreas fault system: Standford Univ. Pubs. Geol. Sci., v. 13, p. 192-205. Lachenbruch, A. H., and Sass, J. H., 1977, Heat flow in the United States and the thermal regime of the crust: Am. Geophys. Union Geophys. Mon. 20, in press. Lachenbruch, A. H.. Sass, J. H., Munroe, R. J., and Moses, T. H„ Jr., 1976, Geothermal setting and simple heat conduction models for the Long Valley caldera: Jour. Geophys. Research, v. 81, no. 5, p. 769-784. Sass, J. H., Diment, W 7 . H., Lachenbruch, A. H., Marshall, E. V., Munroe, R. J., Moses, T. H., Jr., and Urban, T. C., 1976, A new heat-flow contour map of the conterminous United States: U.S. Geol. Survey open-file rept. 76-756, 24 p. Ziagos, J. P., Sass, J. H., and Munroe, R. J., 1976, Heat flow near Charleston, South Carolina: U.S. Geol. Survey open-file rept. 76-148, 21 p. The Nature of the Geophysical Basement Beneath the Coastal Plain of South Carolina and Northeastern Georgia By PETER POPENOE and ISIDORE ZIETZ STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA. EARTHQUAKE OF 1886-A PRELIMINARY REPORT SURVEY PROFESSIONAL PAPER 1028-1 GEOLOGICAL CONTENTS Pagt Abstract_ 119 Introduction _ 119 Regional setting_ 120 Aeromagnetic and gravity surveys_ 125 Interpretation of the aeromagnetic and gravity fields_ 128 Conclusions _ 135 References cited _ 136 ILLUSTRATIONS Figure 1-7 Maps: 1. Index map showing the location of the area discussed in this report and boundaries of the individual aeromag¬ netic surveys _ 120 2. Structure-contour map of the surface of the geophysical basement in parts of North Carolina, South Carolina, Georgia, Florida, and Alabama_ 124 3. Generalized aeromagnetic map of southeastern South Carolina and eastern Georgia_ 126 4. Simple Bouguer anomaly map of the South Carolina and Georgia Coastal Plain in the area of aeromagnetic coverage _ 127 5. Interpretive map showing the major geophysical and geo¬ logic basement units underlying the Coastal Plain of South Carolina and eastern Georgia_______ 129 6. Location map showing major geophysical basement units and localities discussed in text_ 130 7. Interpretive map showing the larger diabase dikes of as¬ sumed Triassic or Jurassic age present beneath the Coastal Plain of South Carolina and eastern Georgia . 131 TABLE Pape Table 1 . Data on wells penetrating basement rocks in South Carolina and northeastern Georgia _ 121 ill STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886- A PRELIMINARY REPORT THE NATURE OF THE GEOPHYSICAL BASEMENT BENEATH THE COASTAL PLAIN OF SOUTH CAROLINA AND NORTHEASTERN GEORGIA By Peter Popenoe and Isidore Zietz ABSTRACT Geophysical data delineate two distinctive crustal prov¬ inces beneath the Coastal Plain of Georgia and South Caro¬ lina. The province adjacent to and east of the Fall Line is a continuation of the Piedmont, composed chiefly of schist and gneiss units, which geologically and geophysically re¬ flect the fabric of the Appalachian orogen. In structure and composition, the basement rocks are similar to those of the Carolina slate belt and the Charlotte belt immediately west of the Fall Line. At least two small Triassic basins are present within the province and are clearly delineated by the magnetic data. The second basement provice that underlies southeastern South Carolina and east-central Georgia has no counterpart in the exposed southern Appalachians. This basement is com¬ posed of undeformed tuffaceous clastic rocks intermixed with basaltic and rhyolitic flows and ash-fall deposits, which are associated with a relatively smooth, low-amplitude magnetic and gravity field. Emplaced within and directly adjacent to this sequence of rocks are a number of mafic plutons. which produce high-amplitude, steep-gradient, circular aeromag- netic and gravity positives. These positives occur within broad areas of higher magnetic level, which may reflect extensive basaltic flows, but also appear to be related to deeper mafic crustal sources. The association of the mafic plutons with basaltic flows at the basement surface suggests that the two are genetically related. The boundary between the region of northeast-trending aeromagnetic anomalies and the smooth magnetic field con¬ sists of a number of long, straight segments. This boundary probably reflects a series of major faults, which juxtapose a metamorphic and nonmetamorphic terrane, The association of mafic plutons with the boundary suggests a deep crustal break. The smooth gravity and magnetic fields associated with basement in southeastern South Carolina and east-central Georgia are similar to those associated with Triassic basins, suggesting that a large area of the basement is underlain- by a deep structural, basin filled with Triassic (?) clastic and volcanic material, and intruded by a number of Triassic (?) or later mafic plutons. INTRODUCTION Although the area of the 1886 earthquake and that of present earthquake activity near Charleston, S. C., geographically belongs to the Coastal Plain physiographic province, analyses of the depth of ; focus of present seismicity (Tarr, this volume; Bollinger, 1972) and the depth estimation of the | 1886 earthquake (Dutton, 1889) indicate that ! these earthquakes occurred within the basement ! underlying the Coastal Plain sedimentary section. | This “basement” is traditionally believed to be the 1 buried eastward extension of the Appalachian Pied¬ mont province which, where exposed, consists of highly to moderately deformed metamorphic rocks and intrusive igneous rocks of Precambrian and ! Paleozoic age, overprinted in places by a system of downfaulted basins containing clastic rocks and a system of northwest-trending mafic dikes, both of Triassic (?) age. Because the crystalline rocks in eastern South Carolina and eastern Georgia are covered by up to 2 km of Coastal Plain sedimentary rocks and cannot be mapped directly, geophysical methods must be employed to aid in understanding the composition and structure of these rocks, the so- called geophysical basement. The geophysical basement is defined as the top of a moderate-to high-velocity layer identified by seis¬ mic refraction surve} r s. Velocities of this layer range generally from 4.3 to 6.8 km/s, and are character¬ istic of well-indurated sedimentary rocks, volcanic flows or sills, or crystalline basement. The surface of this layer is the lower limit of the poorly-indur¬ ated Coastal Plain sedimentary rocks, with veloci¬ ties typically less than 3.0 km/s. Within the last several years, detailed gravity and magnetic surveys covering the Coastal Plain area of southeastern South Carolina and eastern Georgia have been obtained. These data document that two very different basement provinces are pres¬ ent beneath the Coastal Plain of South Carolina and Georgia, one with a geophysical expression that is | typical of rocks of the Appalachian orogen and one 119 120 87“ STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 8E ° S5 ' 84“ 83“ 87“ 81' 80“ 79“ 78“ v ° 7gC ■L — - T \ / \ \ \ i c Atlanta Figure 1.—Index map showing the location of the area discussed in this report and boundaries of the individual aeromag- netic surveys. that appears to have no counterpart in the exposed Appalachian system. Geophysical differences be¬ tween these two provinces and probable sources are discussed. REGIONAL SETTING The area of aeromagnetic data coverage (fig. 1) lies in parts of two physiographic provinces: the Appalachian Piedmont province underlain chiefly by highly deformed and metamorphosed sedimen¬ tary and igneous rocks of Precambrian and Paleo¬ zoic age, and the Atlantic Coastal Plain physio¬ graphic province underlain chiefly by consolidated and unconsolidated younger sedimentary rocks of Cretaceous to Holocene age. The line dividing the two provinces is known as the Fall Line. At this line, the ancient crystalline rock surface, which slopes gently toward the coast, becomes covered by the younger sedimentary rocks of the Atlantic Coastal Plain. For a general discussion and summary of the geology of the exposed Piedmont within the area covered by the aeromagnetic survey, see Overstreet and Bell (1965a, b) ; Bell and others (1974) ; Georgia Geological Survey (1976a, b) ; Crickmay j (1952) ; Hurst (1970) ; and Snoke, Secor, and Metzgar (1977). The rocks of the Piedmont are gen¬ erally discussed in terms of geologic “belts” that have distinctive lthologies or metamorphic grades, j In many places, these belts are distinct tectonic ] units bounded by faults. The northwest edge of the Survey area is within the Carolina slate belt of South Carolina and the similar Bel Air belt of Georgia, which consist of low-grade, mildly de¬ formed metasedimentary and metavolcanic rocks. 1 Locally, adjacent to the Fall Line is the Kiokee belt, which consists of various granitoid gneisses, bio- tite muscovite gneiss, and hornblende gneiss. The major difference between the two belts has been in¬ terpreted to be not their original composition, but metamorphic grade and type of deformation. The Carolina slate belt-Bel Air belt rocks are low-grade greenschist facies/rocks, which have been only mild¬ ly deformed and folded along northeast-trending, subhorizontal axes, whereas the Kiokee belt rocks are highly deformed and injected albite-epidote- amphibolite to amphibolite facies rocks. The relationship of the two metamorphic and tec¬ tonic styles exhibited by these two belts is poorly GEOPHYSICAL EASEMENT BENEATH COASTAL PLAIN 121 understood. In places, the contact between the belts is clearly along major fault systems. The rocks of both the Carolina slate belt-Bel Air belt and the Kiokee belt are cut by granodioritic to quartz-monzonitic plutons whose crosscutting re¬ lationships and lack of strong foliation clearly show that these plutons were emplaced after the main metamorphic event of the Appalachian orogen. The two main bodies of these porphyritic granites ex¬ posed in the survey area are the Liberty Hill and Pageland plutons of South Carolina (Bell and Popenoe, 1976), which intrude Carolina slate belt rocks. On the basis of their unmetamorphosed char¬ acter and because they are intruded by mafic dikes believed to be of Triassic age, these intrusive bodies have been considered late Paleozoic or Carbonifer¬ ous in age. Rubidium-strontium whole rock isochron ages of the granites range from 249 to 332 m.y. (Fullagar, 1971; Butler and Fullager, 1975). Although no large structural basins containing Triassic age rocks are exposed in the area covered by the survey, the area is just south of the Wades- boro-Deep River Triassic basin of North Carolina, and structural basins of similar-appearing clastic rocks of presumed Triassic age are known to exist beneath the Coastal Plain of both South Carolina and Georgia (Marine and Siple, 1974;. The Triassic basins of the east coast characteristically are north¬ east-trending, deep trough-shaped grabens, filled with coarse- to fine-grained continental clastic rocks, representing both high and lower energy deposits. The structures probably formed during the initial stages of continental rifting that accompanied the ' formation of the present Atlantic Ocean. Basaltic volcanism and intrusion of mafic rock accompanied or closely followed this rifting. Massive diabase stocks, dikes, and sills were emplaced both within the Triassic and in the surrounding older rocks. The dikes were emplaced in a remarkably systematic regional pattern, trending consistently northwest in the southern Appalachians, north-northeast in the central Appalachians, and northeast in the northern Appalachians (King, 1971). The structure and composition of the basement rocks underlying the Coastal Plain of South Caro¬ lina and Georgia are not well known. Some drill holes have penetrated basement in widely scattered areas of the Coastal Plain. Published descriptions of basement samples within the survey area are i listed in table 1. Table 1 . —Data on wells penetrating basement rocks in South Carolina and northeastern Georgia Well No. Name and location Type of basement rock Altitude of bifiement (meters) Sourcf 1 .... Town of Hartsville water well, Darlington County, S.C. Pre-Cretaceous schist (granite). -80 Maher, 1971 (granite) Woollard, Bonini, and Mever, 1957 (pre- Cretaceous schist). 2_ Town of Dillon water well, Dillon County, S.C. Rhyolite breccia __ -147 Maher, 1971. 3_ Town of Florence water well, Florence County, S.C. Triassic olivine diabase_ -173 Maher, 1971. 4_ Town of Marion water well, Marion County, S.C. Pre-Cretaceous schist_ -193 Maher, 1971. 5_ Palmetto Drilling Allsbrook No. 1, 1.6 km N of Alls¬ brook, Horry Countv, S.C. “Pre-Cretaceous” ....__ -318 Maher, 1971. 6_ Pioneer Oil, Smart No. 1, 19 km SW of Conway, Horry' County, S.C. “Pre-Cretaceous” _ -417 Maher, 1971. 7 .... Myrtle Beach, Horry County, S.C. Chlorite schist_ —433 Zupan and Abbot, 1976. 8_ Calabash, Brunswick County, N.C. Chlorite schist_ -395 Zupan and Abbot, 1976. 9 .... Town of Sumter water well, Sumter County, S.C. Pre-Cretaceous granite_ -189 Maher, 1971. 10 .... Oil test between Perry and Wagner, Aiken County, s.cr Pre-Cretaceous granite_ -59 Maher, 1971. 11_ Town of Aiken water well, 226, 1.6 km S of center of Aiken, Aiken County, S.C. Pre-Cretaceous granite_ -12 Maher, 1971; Daniels, 1974. 12_ Survey Drilling oil test, 8 km SW of Aiken County, S.C. Pre-Cretaceous granite_ -41 Maher, 1971. 122 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Table 1 . —Data on wells penetrating basement rocks in South Carolina and northeastern Georgia — Continued Well No. Name and location Type of basement rock Altitude of basement (meters) Source 13 .... Aiken County. S.C.__ Chlorite schist __...._ -108 Daniels, 1974. 14_ Aiken Countv, S.C. ...__ Hornblende chlorite_ -210 Daniels, 1974. 15_ Aiken County, S.C. _... Epidote chlorite schist__ -196 Daniels, 1974. 16_ Aiken County, S.C. ___ Hornblende chlorite schist „ -198 Daniels, 1974. 17_ Aiken County, S.C._... Chlorite schist___ -178 Daniels, 1974. 18_ Aiken Countv, S.C. __ Quartz-feldspar gneiss_ -187 Daniels, 1974. 19_ Aiken Countv, S.C. ___ Hornblende chlonde schist __ -192 Daniels, 1974. 20_ Aiken County, S.C. ___ Mica quartzite and chlorite- biotite schist. -205 Daniels, 1974. 21_ Barnwell County, S.C._ Triassici?) fanglomerate Daniels, 1974. 22_ Barnwell County’, S.C._ Triassic(?) siltstoDe . — -309 Daniels. 1974. 23_ Sumerville _..._ Diabase ____ -740 Cooke, 1936. 24_ USGS Clubhouse Cross¬ roads corehoie 1, Dor¬ chester County, S.C. Amygdaioidal basalt _ -744 Gottfried, this volume. 25 _ Seabrook Island, Charleston County’, S.C. Fine-grained quartzitic sandstone (basement?). -814 Charles Speier, drillers log 26 .... Richmond County, Ga. - Talcose schist_ _ _ -58 Daniels, 1974. 27_ Allens Station, 14.5 km S of Augusta, Richmond County, Ga. Talcose schist _ __ -6 Milton and Hurst, 1965; Daniels, 1974. 28_ Burke Countv, Ga. _ _ _ Chlorite-sencite schist _ _ -144 Daniels, 1974. 29 .... Middle Georgia Oil and Gas, 19 km NW of Sanders- ville, Washington County, Ga. Crystalline rock ..._ +27 Milton and Hurst, 1965. 30 _ Town of Sandersville, Washington County, Ga. Quartzite, biotite gneiss and schist- Maher, 1971. 31 _ Layne-Atlantic NSC water well, 3.2 km SW of Ten- nile, W T ashington County, Ga. Granite _____ Milton and Hurst, 1965. 32 _ A. F. Lucas and Georgia Petroleum oil well, 5.6 km SW of Louisville, Jefferson County, Ga. Crvstalline rock ___ Milton and Hurst, 1965. 33 .... Grace McCain No. 1, 0.8 km S of Minter, Laurens County, Ga. Diabase _____... -691 Milton and Hurst, 1965. 34 _ Barnwell No. 1, Jim Gillis, 4.8 km S of Soperton, Treutlen County, Ga. “Paleo” (metaquartzite) _ -879 Milton and Hurst. 1965. 35 .... McCain and Nicholson H. Gillis No. 1, 11 km E of Soperton. Treutlen County, Ga. “Basement” (biotite gneiss). -865 Milton and Hurst, 1965. 36 _ Prvor No. 1 , 6.5 km NE of Newington, Screven County, Ga. “Granite” (no bedrock sam¬ ple recovered) . -777 Milton and Hurst, 1965; Pickering (oral com- mun.) 1976. 37_ J. E. Weatherford-Lonnie Wilkes No. 1, .8 km S of Eiggston. Montgomery County, Ga. Diabase _ -943 Milton and Hurst, 1965. 38 _ Tropic Oil Co., Gibson No. 1, 11 km SE of Vidalia, Toombs County, Ga. Conglomeratic arkose _ -1062 Milton and Hurst, 1965. 39 _ S. J. Felsenthal No. 1, 11 km W of Baxley, Appling County, Ga. “Quartzite” (sandstone prob¬ ably indurated by intrusive diabase) . — 1180 (appr.) Milton and Hurst, 1965. 40 ...1 J. E. Weatherford. S. J. Felsenthal, W. E. Bradley, No. 1,1.6 km W of Baxley, Appling County, Ga. Amygdaioidal basalt _ -1182 Milton and Hurst. 1965. 41 _ Jelks-Rogers No. 1, LaRue and others. S of Retreat, Liberty County, Ga. Quartz rhyolite porphyry _ -1289 Milton and Hurst, 1965. 42 _ Union Bag & Paper Co. No. 1. 11 km N of Gardi, W T ayne County, Ga. Volcanic ash _ -1372 Milton and Hurst. 1965. GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 123 Table 1.— Data on wells penetrating basement rocks in South Caroline and northeastern Georgia —Continued Well No. Name and location Type of basement rock AJtitude of baaement (meters) Source 43_ Brunswick Peninsular No. 1, 2.9 km E of McKinnon, Wavne Countv, Ga. Tuffaceous arkose _ -1386 Miltor. and Hurst, 1965. 44_ California Co., Brunswick Peninsula Corp. No. 1, Wavne Countv, Ga. Gray and pink arkosic quartz¬ ite and diabase. -1375 Applin and Applm, 1964. 45 .... Humble Oil Co., W. C. McDonald No. 1, Ga. Mili¬ tary District 1499, SW of Brunswick, Glynn County, Ga. Granite_ -1436 Maher, 1971. The basement descriptions (Milton and Hurst, 1965; Maher, 1971; Siple, 1967; Cooke, 1936; Daniels, 1974) show that a heterogeneous basement underlies the Coastal Plain. Many of the basement rocks appear similar in composition to rocks of the Piedmont, particularly Dear the Fall Line; however, in other areas, only fine-grained igneous rocks, such as basalt and diabase, have been found. A few holes have bottomed in clastic rocks resembling those in exposed Triassic basins. Of particular interest to this study are a series of fine-grained igneous rocks and clastic pocks (Milton and Hurst, 1965) known to underlie a large area of the Coastal Plain of Georgia. These rocks show little or no deformation, and include volcanic flows of both basaltic and rhyolitic composition, ash- fall deposits, and tuffaceous arkose (Milton and Hurst, 1965). The surface of the basement is an erosional sur¬ face, which slopes gently eastward beneath the Coastal Plain sediments. During the Early Creta¬ ceous, this surface apparently occupied a regional topographic high, wfiiich was not fully inundated until late in the Cretaceous Period. Figure 2 is a regional structure-contour map of the surface of the geophysical basement which is in part this erosion surface. The map is based chiefly on published depths of drill holes that have reached basement or near basement, and the seismic refraction studies of Ackerman (this volume) ; Bonini (1957) ; Bonini and Woollard (1960) ; Meyer (1956) ; Pooley (1960) ; Hersey and others (1959) ; and unpublished seismic reflection data of the U.S. Geological Survey. Only broad, regional structures are shown on the map because of the wide data spacing. The most prominent of these structures in northeastern South Carolina is the Cape Fear arch, w’hich trends north¬ west and brings the basement to within 400 m of the surface near the coast at the North Carolina- South Carolina State boundary. A thick section of I Lower Cretaceous rocks are present north of the arch, but these rocks are absent over the arch and to the south. This, plus a vertical displacement of strand lines at the surface attest to a long tectonic history. The second most prominent feature is the southeast Georgia embayment, which is recessed into the coast roughly between the Georgia-South Carolina and the Georgia-Florida State boundaries. The embayment is believed to be primarily a tec¬ tonically passive feature (Maher, 1971), and the uplift probably occurred on the Peninsular arch of Florida and Cape Fear arch, rather than dowmwarp in the embayment. In addition to the broader regional structures, a basement feature known as the Yamacraw Ridge lies parallel to the coast in eastern Georgia. The Yamacraw Ridge, a ridge of about 350 m relief, w r as defined on the basement surface by seismic re¬ fraction studies (Meyer, 1956; Pooley, 1960). Drill¬ ing in Georgia in the area of the ridge indicates that it apparently is not reflected in the overlying beds (Maher, 1971). Basement contours and the apparent lack of deformation of the overlying sediments sug¬ gest that the ridge may be topography on the base¬ ment erosional surface. In South Carolina, the Beaufort or Burton High are along the strike of the Yamacraw Ridge. These features and a trough to the west named “Ridge- land trough” were defined by structure contours on different elevations of Tertiary horizons (Siple, 1969: Heron and Johnson, 1966). The expression of these features within the Tertiary rocks suggests a basement structural control, but our structure-con¬ tour map indicates that these are only minor fea¬ tures on the basement in the area in w r hich they w’ere defined. We have used the name Ridgeland trough for the basement depression west of the Yamacraw Ridge in Georgia. The refraction work of Ackerman (this volume) and the electrical work of Campbell (this volume) 124 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 1 1 1 1 ! n V / s 1 —-T --1 1 _ r _ % j v y —-—— T 1 ! - 1 — r' • 1 | ^ Figure 2. —Structure-contour map of the surface of the geophysical basement in parts of North Carolina, South Carolina, Georgia, Florida, and Alabama. Elevations are in meters below sea level. Basement is defined as the top of the crystalline rocks, top of the Triassic rocks or Jurassic volcanic rocks, or top of the high-velocity refractor in South Carolina and Georgia. GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 125 have defined a trough-shaped depression on the surface of the amygdaloidal basalt horizon (base¬ ment?) in the seismically active area west of Charleston, S. C. The significance of this feature is still under study. A discussion of the geology of the Coastal Plain rocks is given by Gohn and others (this volume) and R.ankin (this volume), and will not be repeated here. The only important physiographic feature relating to this study is the Orangeburg or Citronelle escarp¬ ment (see fig. 1). This escarpment produces a 15 to 60 m change in the general topographic level along a 965-km line, extending from Florida to Washing¬ ton, D. C. (Doering, 1960), and its maximum de¬ velopment is in South Carolina. At its northern end, it follows the Fall Line, but in South Carolina, it divides the upper from the middle Coastal Plain with its toe at about 60 m and its crest at about 110 m elevation. The Orangeburg scarp in South Caro¬ lina is believed to be composite in origin (Colqu- houn, 1965), formed by a late Miocene and perhaps Oligocene sea level transgression. AEROMAGNETIC AND GRAVITY SURVEYS Figure S presents a composite aeromagnetic map of southeastern South Carolina and eastern Georgia, compiled from the six smaller surveys shown in figure 1. These surveys were conducted by the U.S. Geological Survey in cooperation with the U.S. Nu¬ clear Regulatory Commission, the Coastal Plains Regional Commission of the U.S. Department of Commerce, and the former U.S. Atomic Energy Commission. The map has a contour interval of 100 gammas and is a simplification of the more detailed individual maps, which have contour intervals of 10 to 50 gammas. Data in all areas were obtained at a nominal flight elevation of 152.4 m above mean terrain, and have been reduced to remove the inter¬ national geomagnetic reference field (Fabiano and Peddie, 1969), which permits a comparison of mag¬ netic levels over widely separated areas. Flight line spacing was 1.6 km for all data but area 1, where it was 0.8 kim The patterns shown on the magnetic map reflect structure and lithology in crystalline and -meta- morphic rocks. The crystalline and metamorphic rocks are the basement rocks or in thin intrusive rocks or volcanic flows within the sedimentary sec¬ tion. The magnetic contribution of sedimentary rocks, such as those of the Coastal Plain, is negli¬ gible. The only significant effect of the sedimentary rocks is to increase the distance between the anom¬ aly-producing rocks of the basement and the air¬ borne magnetometer. The effect of this increased distance causes both a smoothing and merging of anomalies from deeper sources and a lowering in their amplitude and gradient. The magnetic properties of crystalline and meta¬ morphic rocks are generally related to magnetite content, but similar rocks may vary widely in magnetization because of such factors as magnetic size domain and remanent and induced components. Generally, mafic rocks are more magnetic than felsic varieties, but this is not always true. Daniels (1974) measured the magnetic susceptibility of exposed rocks in the central Savannah R.iver area and found that only a small fraction of Carolina slate belt rocks are significantly magnetic (10 -1 to 10 -1 cgs cm 3 ), whereas a large fraction of felsic gneisses from a small area of the Kiokee belt fall in this magnetic range. Amphibolitic schists (chlorite- epidote-hornblende schist) from basement cores in the Savannah River Plant area are also fairly high in susceptibility (==2.1 X10 -3 cgs'em 3 ). The differ¬ ence in this measured susceptibility appears to be related to metamorphic grade rather than to compo¬ sition. The concept that metamorphic grade affects magnetic susceptibility was outlined by R.eed, Owens, and Stockard (1968), who suggested that medium-grade metamorphic rocks (epidote-amphi- bolite facies) are enhanced in magnetite relative to those of either higher or lower grade. Figure 4 is a simple Bouguer gravity map of the area of the magnetic survey. The map is based on the simple Bouguer anomaly map of South Caro¬ lina and Georgia by Long,- Bridges, and Dorman (1972) and Long, Talwani, and Bridges (1976), as well as on unpublished data from H. L. Envoy. Contour intervals are 5 and 10 mGals based on a station spacing of 4 to 6 km. The patterns on gravity maps reflect density* i variations associated w*ith lithologic changes in the Earth’s crust. These changes are of both regional j and local extent and derive from both near-surface j and deep sources. In figure 4, most of the anomalies ! are believed to be produced by intra-basement sources because of the limited thickness and lateral homogeneity of the Coastal Plain rocks in this area. An established practice in interpreting gravity and magnetic maps is to observe the anomalv- lithology relations in areas of exposed or known geology and to use these observations as principal guidelines in interpreting anomalies in covered ; areas. This is the method that we have used in the following interpretation. Insight into the anomaly source is provided by shape, amplitude, and the cor- 126 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Figure 3. —Generalized aeromagnetic map of southeastern South Carolina and eastern Georgia. See figure 1 for location of area and sources of data. Contour interval is 100 gammas, and values are relative to a datum of 53,715.02 gam¬ mas. The data have been reduced to remove the international geomagnetic reference field (Fabiano and Peddie. 1969). relation or lack of correlation of the gravity and magnetic fields. As examples, granitic plutons of the Piedmont generally produce gravity lows, and mafic plutons produce gravity highs. Usually felsic plutons produce aeromagnetic lows, but many produce aero¬ magnetic highs. Magnetic aureoles in the country GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 127 35 c 8< e _8T 7 B? 8 '!'' -V bo 7? 7r Figure 4. —Simple Bouguer anomaly map of the South Carolina and Georgia Coastal Plain in the area of aeromagnetic coverage. Contour intervals are 5 and 10 milligals; hachures indicate areas of lower gravity. See text for sources of data. rock and magnetic border phases or com positional | layering of the younger plutons are reflected in the magnetics. Linear aeromagnetic high anomalies, par- ticularly those associated with corresponding grav- j ity high anomalies, are often caused by bedded sources. Other linear high magnetic anomalies may reflect cataclastic zones or compositional layering in plutonic rocks. The least ambiguous of all geophysi¬ cal fields is the association of steep gradient, high amplitude, circular or elongate gravity and aero¬ magnetic highs with mafic or ultramafic plutons. Aeromagnetic and gravity maps also provide in¬ formation not only on lithology, but also on the structural deformation of the rocks of an area. For example, when rocks are subjected to a horizontal compressive stress exceeding their elastic limit, they fold, elongate, buckle, and shear, and at suffi¬ ciently high temperatures become injected along axes perpendicular to the direction of applied force. Even large bodies of solid granite, if subjected to sufficient stress and heat, flow through plastic de¬ formation to reflect the direction of applied stress. 128 STUDIES RELATED TO CHARLESTON. SOUTH CAROLINA, EARTHQUAKE OF 1886 The stress direction is imprinted in their geologic fabric, which, in turn, is reflected in the magnetic grain. Such a grain is strongly developed in the geologic and aeromagnetic patterns of rocks of the Appalach¬ ian system. The rocks involved in the Appalachian orogen have a common geologic fabric and a cor¬ responding geomagnetic pattern of anomalies that are elongated in the northeast direction. Regardless of the causative rocks or structures, this pattern of anomalies may be observed on aeromagnetic maps from diverse parts of the Appalachian system, in¬ cluding rocks beneath the Atlantic Coastal Plain. The pattern reflects lithologic changes associated with bedding repeated through folding, tilting, or faulting, and shear structures along the geologic grain. Later intrusive rocks can generally be differ¬ entiated from rocks involved in the orogen because they crosscut this geologic grain or do not show the above fabric. INTERPRETATION OF THE .AEROMAGNETIC AND GRAVITY FIELDS We have divided the magnetic map shown on fig¬ ure 3 into a number of areas of similar geophysical signature. Figure 5 shows these subdivisions, in ad¬ dition to drill hole locations keyed to numbers on table 1. To aid in locating the units with respect to cul¬ tural or political boundaries, we have also plotted these major units on a location map (fig. 6). Magnetic zones In, Is, and 2a all lie west of the Fall Line (fig. 1) where sources are generally ex¬ posed. Areas In and Is are underlain by bedded, tuffaceous metavolcanic rocks of the Carolina slate belt and area 2a by the higher grade gneissic gran¬ ite, granitic gneiss, hornblende gneiss, and quartz- microcline gneiss of the Kiokee belt. The most preeminent trend in the magnetics over both belts is northeast, reflecting the geologic grain of the Ap¬ palachian orogen. In the Carolina slate belt, the magnetic anomalies are sharp, and wavelengths are about i km, and amplitudes generally less than 200 gammas. Many are continuous for distances up to 50 km, suggesting a continuity of the causative units. Most of the field, however, is smooth. The anomaly pattern is known to reflect dipping bedding within the volcanic and depositional units of the belt. The tectonic style of the belt is one of broad regional folding along sub¬ horizontal axes. The beds are tilted gently to the east in area In (Bell and Popenoe, 1976). and gent¬ ly folded in area Is. The Kiokee belt in area 2a is characterized by a more complex flat to moderately convoluted mag¬ netic field of slightly higher amplitude and general¬ ly more discontinuous sharp linear northeast-trend¬ ing anomalies. These linear anomalies occur over both granitic gneiss and gneissic granite units. The linear zone dividing magnetic zone 1 from magnetic zone 2 is labeled Fj and reflects a major fault system that can be traced from North Caro¬ lina into Georgia by its magnetic patteni (Hatcher and others, 1977; Daniels, 1974; Bell and others, 1974; Bell and Popenoe, 1976). The fault system not only separates areas of differing geophysical char¬ acter, but is marked by short wavelength, linear magnetic highs characteristic of cataclastic zones of major faults of the Piedmont. Similar aeromag¬ netic highs have been noted associated with the Alexander City, Brevard, Towaliga, Bartletts Ferry, and Goat Rock fault systems (Neathery and others, 1976). The general gravity field of the Carolina slate belt is flat, and has an average Bouguer value of +5 mGals (fig. 4). This is high, relative to the more felsic rocks of the Kiokee belt. Magnetic highs and deep gravity lows of 10 to 15 mGals amplitude are associated with granitic to quartz-monzonitic plutons underlying areas la , (Pageland pluton) and lb (Liberty Hill pluton) (fig. 6). These plutons exhibit a strong discontinuous magnetic aureole. Both plutons exhibit some internal compositional differences evident in the aeromag¬ netic signature. Their crosscutting relationships identify that the plutons were emplaced after the last regional metamorphic event (Bell and Popenoe, 1976). This is evident from their magnetic signa- ; ture. A second prominent magnetic trend is evident in area In and in extensive areas over the Coastal Plain. These are northwest-trending linear aero¬ magnetic highs which correlate with diabase dikes of assumed Triassic or Jurassic age (King, 1961, 1971; Bell and Popenoe, 1976). Figure 7 shows our interpretation of the distribution of these dikes in the survey area. Where deeply buried, only the largest are traceable, and no doubt other dikes exist that are not shown on figure 7. The dikes can be recognized most easily where they cut low-suscep¬ tibility material, or in areas of aeromagnetic lows, but some are evident in areas of aeromagnetic highs. In addition to the northwest-trending set of dikes, a north-trending dike set is evident over the Coastal Plain near long 79°30' W. Dikes of this trend and j longitudinal position are exposed in the Deep River GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 129 EXPLANATION 3b Geophysical ares or feature discussed in text Composition of basement determined from drill hole descriptions. Numbers are Mafic intrusive pluton keyed to table 1. - «b Shale, sandstone, quartzite, or arkose 36 ? Pre-Cretaceous rocks - Geologic boundary or contact 40 0 Diabase or basalt ■ 45 ® Granite -Fault interpreted from geophysical data 42 A Granophyre, polite, or rhyolitic ash 35. Gneiss or schisl Figure 5.— Interpretive map showing the major geophysical and geologic basement units underlying the Coastal Plain of South Carolina and eastern Georgia. Triassic basin of North Carolina, where they were ! trending dikes as continuous at the surface, new mapped discontinuous!}' by Reinemund (1955). Al- magnetic data (U.S. Geol. Survey, 1974) indicate though Reinemund did not recognize the north- that this dike set is continuous at depth in that area. 130 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 •istocr i 200 KILOMETERS " < _• * r < • SOCM > "WOT. - ► r V*' ' •* ■ T^NNE^j^r- : -'V ► s ' 4* pow.s'X ^ , ^ -^ - ^'**%*, '* c ' i *** ** V, •“««'. ^ 5f {-«•»»« v ■-> r 'v .- . ».O w * I 35*1-_• > ! *?'/:?■ y' - it - ^ -»Vfc ■■-„.—•.-V - • .! * r i j ■ Vw ■* r ■ ■ > « s_ / v -,v- * * ‘ieh - --kT 4 * ', ^ ., —-V/O -g*#* V^,to> to*- V- - S- h.-—'“•/ -v s - .s^.v: ->% .■- A • V - ^_ i*LA • V. I- - ° ..to*; “S_ —j+t-AtST* ■ ** "**' 0 *‘ toto .jJ- '**' : /£•*. / V ‘ -* ..• ,y v !T “*v ^ • *-►-» , c.ee^»r. t ‘. . ‘ w to .* - • ^ V' ' t 7. --Q ^ y-^ ' \i ■ ■■■ **6»*rroN k -o*'G*». v ) ,,- jl > ^ >L. '• »V » w *•<*■*'»,, - , ww»»ei« --»S»»e® Y Ta ^ ' ,r '~ "*'-- ^-^1, .•S*^ >* f •■ \ ;ON» . L t.V -_0£ORGiA ALABAMA ; ^ OOOL c&udutrr [ • \ , > W7'm»n > -in..' I "OUSTOK s “lilt* ' f _' i*5sof x L ;.' ~' • - - v y -- - - 1 " S '- x . • A«n.*^M*S5E.E o' r ‘ 1 r a r- -ot^ — « ** ' k * O *» N -•O-SOft ^ X*;;: .-" .^-'Florida x V v—n- s '." “ L'=i— -S- ' ' - - - - _ A v c c :\ nass-v. - •** b/ ^ k ’' eW jaQVBO^'WU.E EXPLANATION 3b * Geophysical area or feature discussed in text ( Mafic intrusive pluton - Geologic boundary or contact — Fault interpreted from geophysical data to County or location mentioned in text '-J- —— Boundary of survey area Figure 6.—Location map showing major geophysical basement units and localities discussed in text. If the two widely separated areas are connected by continuous dikes, which they appear to be, the length of this swarm is thus greater than 275 km. Anomalies characteristic of the Kiokee belt and Carolina slate belt are extensive in a broad area under the Coastal Plain, and drill data confirm that similar sources cause the anomalies. We have labeled this area 2b on figure 5. In addition, several sub- areas labeled 2c and 2d have been delineated. The magnetic signature of area 2c was correlated by Daniels (1974) with exposures of low-grade rocks near Augusta, Ga. (Bel Air belt of Crickmay, 1952), and we have extended Daniels’ correlation to the south. The highly convoluted and distinctive mag¬ netic unit 2d appears to correlate with low-grade rocks exposed in river valleys north of Columbia, S. C. (lower metasiltstone unit of Daniels). W r e have extended this unit to the northeast of Columbia on the magnetic signature. The central area of zone 2b is characterized by a large, closed gravity low with a minimum value of -45 mGals and an average Bouguer value on the GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 131 SOUTH CAROLINA GEORGIA 50 106 160 KILOMETERS Figure 7. —Interpretive map showing the larger diabase dikes of assumed Triassic or Jurassic age present beneath the Coastal Plain of South Carolina and eastern Georgia. order of -25 mGals. Basement samples (holes 9- I magnetic and gravity highs and lows predominate, 12, fig. 5) and magnetic and gravity data indicate 1 indicating probable bedded sources. Areas under- that a large proportion of this area is underlain by ■ lain by this foliated granite have a northeast-trend- low-density gneissic granite. The proportion of ing linear magnetic grain similar to areas underlain granite to the rock it intrudes seems to be directly i by bedded sources. This grain probably reflects related to the Bouguer value. The granite appears compositional layering. to be the predominant basement lithology south and 1 We could find no aeromagnetic anomalies char- east of Columbia, S. C., but is probably subordinate ; acteristic of the large bodies of nonfoliated por- to metavolcanic rocks near the North Carolina State phyritic granites such as the Pageland and Liberty 7 boundary, where longer, more continuous linear Hill plutons in the area of 2b beneath the Coastal 132 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Plain. Gravity anomalies that suggest circular plu- tons are present south of drill hole 10 and at drill hole 9; however, the magnetic highs characteristic of the exposed plutons are not associated with these gravity lows. The linear aeromagnetic highs over the gravity minima indicate that the source of these gravity lows is probably foliated granite. A number of long, moderate amplitude aeromag¬ netic highs crossing areas 2a, 2b, and 2c are not as¬ sociated with gravity highs. Their anastomosing pat¬ tern suggests that they may reflect cataclastic zones associated with Piedmont faults. We have labeled one particularly prominent break, which is probab¬ ly, a fauit, F 2 . Daniels (1974) showed that amphibo¬ litic rocks (hornblende-chlorite schist) are prob¬ ably the cause of the long, linear highs north of area 2e. Area 2h has two pronounced 10+ mGal gravity highs associated with aeromagnetic highs and area 2f lies on a 15 mGal high. If the source of these anomalies derive from the contrast of amphibolitic gneiss (2.8-2.9 gs/cm 3 ) with more felsic rocks (2.7-2.8 gs/cm 3 ), the source would have a minimum depth extent of 1.2 and 1.8 km, respectively. If they derive from the contrast of diabase or gabbro (2.9- 3.0 gs/cm 3 ) with felsic volcanic rocks, their mini¬ mum depth extent would be 0.8 and 1.2 km. In both areas we favor intrusive gabbros as the source of the gravity highs. The Dunbarton Triassic (?) basin, described by Marine and Siple (1974), underlies area 2e. This basin is the cause of a deep, smooth, northeast¬ trending aeromagnetic low due to a considerable depth of nonmagnetic rock. Marine and Siple (1974) have shown from drill data and geophysical analysis that the basin is 50 km long, 10 km wide, and con¬ tains more than 900 m of fanglomerate, mudstone, and sandstone. No volcanic flows or sills are present in the sequence. Areas 2f and 2g are underlain by smooth, north¬ east-trending aeromagnetic lows, although the lows are not well developed, as in area 2e. Area 2f is also underlain at basement depth by a Triassic (?) structural basin, as a deep well at Florence, S. C., penetrated 221.6 m of Triassic-appearing sediment below basement depth before bottoming in “trap rock” (Darton, 1896, p. 218), and seismic refrac¬ tion velocities (Bonini and Woollard, 1960, station 43) are compatible with Triassic sediments at the basement (3.9 km/s). The identification of “trap rock” suggests that some of the linear aeromagnetic highs surrounding area 2f could reflect diabase sills or flows of Triassic age, as suggested by Bonini and Woollard (1960) from seismic velocity data; however, the 15 mGal gravity anomaly is more com¬ patible with a body of greater depth extent than a sill (previously discussed) and suggests that either an intrusive body or a dense lithologic unit under¬ lies the basin. The seismic refraction velocity in area 2g (Bonini and Woollard, 1960, station 44) of 6.8 km indicates that this magnetic low probably does not reflect a sediment-filled Triassic basin unless it is overlain by a diabase sill or flow. The geophysical fields discussed thus far are all typical of those produced by rocks of the Appalach¬ ian orogen. The anomalies of the older crystalline rocks have a northeast grain; the Triassic (?) basins cause smoth, northeast-trending aeromag¬ netic lows of limited width and length. The geo¬ physical fields of the southern part of the area dis¬ cussed in the subsequent section are quite different in character, suggesting profound differences in the crustal composition, structure, and (or) meta- j morphic history of these areas. The straight boundaries in segments between B,-B : , B 2 -B 3 , B 3 -B 4 and the profound change in basement character across the boundaries strongly suggest a series of faults juxtaposing high density magnetic rocks on the northwest and lower density nonmagnetic to mildly magnetic rocks on the south¬ east The abruptness of the gravity and magnetic breaks also suggests faulting. The postulated fault between B ; and B 3 is continuous with B 3 -A 4 , which together closely follow the base of the Orangeburg or Citronelie escarpment at the surface of the Coastal Plain for a distance of more than 150 km. The coincidence almost certainly reflects a structural control for the scarp, a conclusion reached on geo¬ logic evidence by Doering (1960), who believed the scarp marked a hinge line, but refuted by Colqu- houn (1965) on the basis of heavy-mineral assem¬ blages in the upper beds of the Coastal Plain. South of line Ai-A 2 -A 3 -A 4 -A 3 (fig. 5) and north of Bi-Bj-Bj-Bj (fig. 5), the crust is highly mafic, causing high-amplitude gravity and magnetic anom¬ alies, except for area 3c, the large gravity low cen¬ tered around Georgetown, S.C. The magnetic field south of B a -B;-B 3 -B 4 does not have the grain of the Appalachian orogen and is composed of a smooth southward decreasing field of low amplitude, ran¬ dom to east-trending anomalies, and several areas of higher magnetic level that enclose sharp, steep- gradient, high-amplitude circular anomalies. The Bouguer gravity field of this southern area is smooth and can be viewed as a broad regional low, averaging GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 133 - 10 mGals, on which are superimposed sharp, steep- gradient gravity positives. Part of the smoothing of the regional field south of line B : -B., can be accounted for by an increased section of Coastal Plain rocks in the southeast Georgia embayment. Figure 2 indicates that this thickening is not rapid, however, but gradual, and magnetic anomalies from northwest-trending dike sources are not significantly diminished. Therefore, the low-amplitude field south of the line indicates weakly magnetic rocks beneath the Coastal Plain sediments, caused by a major change in basement composition, metamorphic grade, tectonic style, or a combination of all three. The smooth field contains many low-amplitude, moderately sharp-gradient anomalies that can be shown to originate from sources at or near the depth of known or seismic basement; however, only linear anomalies reflecting dikes and a few circular anomalies, perhaps reflect¬ ing small plutons, have amplitudes of over 100 gammas. This suggests that many of these anoma¬ lies, particularly areas of low-amplitude button-like anomaly fields seem in the detailed 10-gamma con¬ tour aeromagnetic data, may originate from thin sources. Several possible lithologies could reasonably ac¬ count for the smooth magnetic and gravity field south of line B : -B 4 . From observations of the geo¬ physical fields of exposed Piedmont rocks, these would include high-grade schist or gneiss, granitic rocks, or a deep basin of flat-1 ying or unmetamor¬ phosed sedimentary rocks. Our interpretation is that most of the smooth field reflects a thick section of unmetamorphosed fiat-lying clastic and volcanic rocks within a structural basin for the following reasons: 1. The straight boundaries B^-Bo, B : -B 3 , and B s -B< suggest faults. 2. The general geophysical character of the entire southern province resembles that seen over some Triassic basins wiiere a flat magnetic field is associ¬ ated with stratified continental deposits, and anoma¬ lies are associated with exposures of mafic intrusive or extrusive rocks (see Kane and others, 1972) and, in some cases, secondary horst structures juxtapos¬ ing older rocks with the younger rocks. 3. No high-grade schist or gneiss is knowm from drill holes in the smooth field, with the exception of drill holes 34 and 35, w T hich are discussed later. In¬ stead, all drill holes within the area have bottomed in unmetamorphosed flat-lying volcanic flows, volcan- iclastic rocks, and tuffaceous clastic rocks (wells 23-25, 37-55, table 1). These lithologies within the survey area include quartz-rhyolite porphyry, amyg- daloidal basalt and diabase, volcanic ash, and arkosic to fine-grained sandstone. No felsic intrusive rocks, such as granites, are known from basement samples south of the boundary B ; -B 4 and north of Glynn, Pierce, and Coffee Counties, Ga. (fig. 6). In South Carolina, granitic basement has been suggested as the cause of the smooth magnetic field (Taylor and others, 1968, p. 776; Daniels, 1974), on the basis of reported granite at the basement recov¬ ered from a well that was drilled in Screven County, Ga. (well 36, table 1) ; however, new information reveals that no basement sample was recovered from the Screven Count}’ well (Sam Pickering, Jr., oral commun., 1976) and the identification of basement material as granite w’as based on the resistance to the drill. Seismic and magnetic evidence suggests that the “basement” in Screven County, Ga., is com¬ posed of stratified rocks. Two sub-horizontal seismic refractors were recorded by Pooley (1960) (refrac¬ tion line 4) only 3 km from the Screven Count}’ well, one at 0.7 km (the surface contoured in fig. 2) and a second at 2.1 km. No gravity or magnetic break is evident between the well location and seismic refrac¬ tion line location. If the resistive rock penetrated in the Screven County well was a volcanic flow, it would explain the high basement velocity of 5.3 km/s recorded by Pooley and the deeper subbasement refractor. 4. The shoreward extension of the Brunswick, Ga., branch of the East Coast magnetic high, discussed by Taylor, Zietz, and Dennis (1968), Emory and others (1970), and R.abinowitz (1974) touches the southern corner of our study area in area 3b. Along the shelf edge, the magnetic high, continuous with the Brunswick anomaly, has been interpreted as re¬ lated to Mesozoic continental extension. 5. Seismic, electrical, and magnetic analysis show that subhorizontal layers are present at several depths beneath the basalt (geophysical basement) in the Summerville, S.C., area (Ackermann, this vol¬ ume; Campbell, this volume; Phillips, this volume). Subhorizontal refractors w’ere also recorded below the geophysical basement in the area of smooth mag¬ netic field in Jasper County, S.C., and Effingham I County, Ga. (Pooley, 1960, refraction lines 10, 2). j The most probable explanation for the high velocity, ; high-resistivity layers beneath the geophysical base¬ ment depth (fig. 2) is deeper volcanic flows or sills, or the true crystalline basement. We favor a Triassic(?) age rather than a Paleo¬ zoic age for the unmetamorphosed sedimentary and 134 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 volcanic rocks within the structural basin for the following reasons: 1. The abruptness of the boundary dividing the | smooth field from the northeast-trending Piedmont : anomalies suggests structure rather than a meta- morphic gradient. 2. All of the rock penetrated in the area of the smooth field suggest Triassic lithologies, except for the rhyolites and ignimbrites. The rhyolitic litholo¬ gies do not preclude a Triassic age, as rhyolites and ignimbrites of Triassic age are extensive in the southern Florida volcanic province (Barnett, 1975, p. 127). 3. The area lies on the predictable continuation to the east-northeast of the deep Triassic graben sys¬ tem that underlies the region of the Apalachicola- southwest Georgia embayment in southwestern and south-central Georgia (Barnett, 1975; Marine and Siple, 1974) (fig. 2). In Florida and western Geor¬ gia, the graben system is more than 200 km wide (Barnett, 1975; Neatherv and Thomas, 1975; Ma¬ rine and Siple, 1974) and contains a number of sec¬ ondary horst blocks of older Paleozoic sedimentary rocks and Precambrian (?) or lower Paleozoic!?) crystalline rocks. The various basins of the graben system are filled with coarse red feldspathic and arkosic elastics, and tabular, sometimes amygdular, diabases believed to be intrusive sills (Barnett, 1975). The northern boundary of this graben system is structural, as a well in Pulaski County, Ga. (R. 0. Leighton-Dana No. 1, Milton and Hurst, 1965), pene¬ trated 1,176 m of coarse- to fine-grained sandstones and diabase layers of Triassic! ?) age below the Upper Cretaceous Coastal Plain sedimentary rocks only 30 km southeast of wells in Houston County, Ga., that bottomed at basement depth in Piedmont rocks (Tricon Minerals-Gilbert No. 1, Milton and Hurst, 1965). There is a distinct gravity break be¬ tween the Houston County, Ga., and the Pulaski County, Ga., wells (see Long and others, 1972). This break is similar to and along the strike of our mag¬ netic break and corresponding gravity break Bj-B; (fig. 5), suggesting a similar cause. 4. Northwest-trending anomalies probably reflect¬ ing mafic dikes cut the smooth field in South Caro¬ lina (fig. 7). These dikes are along the same trend and are assumed to be continuous with dike swarms of Triassic(?) or Jurassic(?) age that cut the ex¬ posed Piedmont rocks. Depth analyses on these dikes indicate that they have tops at basement depth (basalt) (0.7 to 1 km) in South Carolina. This would put a minimum probable age of Jurassic (?) on the basement material in the area of the smooth field in South Carolina. 5. There are no obvious structural breaks within the smooth field. Instead, magnetic data suggest that flow units are extensive in the basement and form zones of more or less localized, discontinuous bodies, which cause low-amplitude magnetic anom¬ alies. The flows appear to be mixed with and sepa¬ rated by intervening zones or layers of clastic ma¬ terials, causing areas of smoother aeromagnetic and gravity low's. Basement compressional velocities in the area of smooth magnetic field (Pooley, 1960, Bonini and Woollard, 1960) of 4.5-5.6 km/s are not compatible with velocities of Triassic sedimentary rocks (2.3— 4.5 km/s) but could be explained by the extensive volcanic flows or sills known to be present at the basement. Wells 34 and 35 are within the area of smooth magnetic field. These wells were reported to have bottomed in metamorphosed rocks (Milton and Hurst, 1965, p. 17). If the identification of basement in this area is correct, these rocks may occupy a small secondary horst structure within the pro¬ posed structural basin. Clearly, metamorphism is nil to low' grade in all other wells that have pentrated basement within the smooth magnetic field. The shape and amplitudes of the aeromagnetic and gravity highs in areas 4a, 4b, 4c, 4d, 4e, 4f, 4g, and 4h (figs, 4 and 5) clearly indicate that these areas are underlain by mafic intrusive masses. Mafic intrusive masses of this abundance, shape, and size are not known elsewhere in the south Appalachian Piedmont. A similar, but less pronounced, occur¬ rence of mafic plutons along the coast of Maine and Massachusetts w r as interpreted by Kane and others (1972) as marking a major crustal break resulting from adjustments between distinctive crustal blocks offshore and onshore. The association of the aero¬ magnetic and gravity' highs in areas 4a, 4b, and 4c with the boundary between Piedmont-type meta- morphic rocks and undeformed rocks, and the ar¬ rangement of the plutons within the ridgelike zone along an eastwest and northeast trend, suggest that the zone also marks a major crustal break. Anomalies 4e, 4f, and 4g have the classic gravity and magnetic forms of unmetamorphosed, stocklike mafic plutons, and may be feeders for basalt flows known to underlie the area (wells 23 and 24, table 1). Anomalies 4e and 4f are deep or may be pro¬ duced by plutons with outward-dipping contacts, whereas 4g is typical of an anomaly from a steep¬ sided, cylindrical, compositionally zoned pluton. GEOPHYSICAL BASEMENT BENEATH COASTAL PLAIN 135 Depth analyses on anomalies 4e, 4f, and 4h by the technique of Vaquier and others (1951) and by Phillips (this volume) show that the tops of the main plutonic masses lie at about 1.5-km depth or below basement (basalt) depth. If the three small circular anomalies associated with the top of anom¬ aly 4f are treated as separate sources, these anom¬ alies originate at about 0.8 km below ground level, or the known depth to the basalt flows drilled in this area (drill hole 2-4, table 1). The small anomaly centered between drill hole 23 and 24 similarly origi¬ nates from the basalt depth. Depth analyses by the same technique on anomaly 4g indicates a depth to the top of this pluton of about 1 km, or near the depth to the high velocity refractor (0.925 km) in this area (Bonini and Woollard, 1960; Pooley, 1960). Areas 4a, 4b, 4c, and 4d outline zones composed of a complex of mafic plutons which appear to in¬ trude a mafic crust, perhaps similar to the Bays of Maine igneous complex (Chapman, 1968; Kane and others, 1972). Individual plutons in the zone are not isolated, as are anomalies 4e-4h, but discrete plu¬ tonic masses can be identified from both the gravity and the magnetic data. On figure 5 we have pat¬ terned the most prominent of these plutons wfithin the zones. The plutons appear to be arranged reticu- larlv along apparent north-northeast and east trends. The zones surrounding individual plutons in areas 4a, 4b, and 4c, and zone 5 near plutons 4e, 4f, and 4h have an unusually high magnetic level. Our modeling and that of Phillips (this volume) in area 5 suggest that in this area much of the magnetic buildup (broad wavelength anomalies) originates | from a deep source; however, there is a strong pos¬ sibility that at least some of the high magnetic level originates from the cumulative effect of many shal¬ lower, thin magnetic sources. Depth analyses show’ that many individual anomalies wuthin this zone of high magnetic level originate near basement (basalt) depth. Our interpretation in area 5 and area 4c is that these areas of high level may, in part, outline areas of extensive mafic flow 7 s, al¬ though we w T ere unable to friodel the broader sources as flow’s. There is a deep gravity low underlying George¬ town, S. C., and a corresponding high magnetic level in area 3c of the magnetic map. This gravity low 7 has been interpreted by Talw r ani and others (1975) to be caused by a deep structural basin of low-density rock capped by a mafic sill or flow. The sill would explain high seismic velocities (6.7 km/s) (Bonini, 1957) and high magnetic level associated w r ith the gravity 7 low. A second interpretation, w’hich would also fit the data, is that the gravity anomaly reflects a low-density basement rock, such as a granite body, coincidentally overlain by diabase or basalt (Talwani and others, 1975). On the basis of the amplitude of the gravity low (35 mGals), the magnetic expression, which suggests a prismatic source, and the drill hole 7, which bottomed in chlorite schist, we favor the second interpretation, whereas Talw’ani favored the first. CONCLUSIONS The major point in our analysis is that the base- 1 ment beneath southeastern South Carolina and east- central Georgia is not simply an extension of the Appalachian Piedmont, as generally believed, but i differs strikingly from the Piedmont. The area near the Charleston earthquake zone is particular- i ly anomalous, as it is underlain by a number of large mafic plutons and an extensive volcanic field. These plutons intrude nonmagnetic and undeformed layered rocks that could be as old as Paleozoic, al- ! though some evidence in the Charleston, S. C., area suggests that here they may be Triassic(?) and per¬ haps as young as Cretaceous. The presence of mafic plutons of this size and abundance suggest that they mark deep structural breaks interpreted to be of Mesozoic age. The boundary between the deformed rocks of the Piedmont and the undeformed rocks beneath the Coastal Plain is abrupt, marked along most of its length by a ridgelike zone of mafic plutons. The straight segments along much of the zone suggests a series of faults, rather than a metamorphic gradient. If the smooth magnetic field in South Carolina and south-central Georgia does reflect a complex structural basin of Triassic(?) age rocks, this basin ! is an order of magnitude larger than any exposed Triassic(?) basin in the Eastern United States, and is a major geologic feature requiring explanation. The apparent overlap of-Florida has long been noted I in most continental reconstructions (Bullard and others, 1965; Dietz and Holden, 1970). The pro- ; posed graben system could furnish a zone along which the overlap could be accommodated. Also, it is possible that Florida, during the initial stages of continental separation, occupied a separate crustal fragment attached to the trailing edge of the Afri¬ can-South American blocks and later became stranded with the North American plate. There are 136 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 any number of interesting speculations; in fact, the extension of the graben system into South Carolina and eastern Georgia at this time is a speculation based on geophysics yet to be proved by drilling. REFERENCES CITED Applin, E. R., and Applm, P. L., 1964, Logs of selected wells in the Coastal Plains of Georgia: Georgia Geol. Survey Bull. 74, 229 p. Barnett, R. S., 1975, Basement structure of Florida and its tectonic implications: Gulf Coast Assoc. Geol. Socs. Trans., v. 25, p. 122-142. Bell, Henry, III, Butler, R. J.. Howell, D. E., and Wheeler, W. H., 1974, Geology of the Piedmont and Coastal Plain near Pageland, South Carolina and Wadesboro, North Carolina: South Carolina Devel. Board, Div. Geology’ [Carolina Geol. Soc. Field Trip Guidebook], 23 p. 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In¬ terpretation of basement rocks beneath the Atlantic Coastal Plain from reconnaissance aeromagnetic data (abs.) : Geol. Soc. America Spec. Paper 115, p. 182- 183. Reinemund, J. A., 1955. Geology of the Deep River coal field. North Carolina: U.S. Geol. Survey Prof Paper 246, 159 p. Siple, G. E., 1967, Geology and ground water of the Savannah River Plant and vicinity, South Carolina: U.S. Geol. Survey Water-Supply Paper 1841, 113 p. - 1969, Salt-water encroachment of Tertiary limestones along coastal South Carolina: South Carolina Div. Geo¬ logy Geol. Notes, v. 13, no. 2, p. 51-65. Snoke, A. W., Secor, D. T., Jr., and Merzgar, C. R., 1977, Batesburg-Edgefield catalastic zone—a fundamental tec¬ tonic boundary in the South Carolina Piedmont: Geol. Soc. America Abs. with Programs, v. 9, no. 2, p. 185-186. Talwani, Pradeep, Ressetar, Robert, McAleer, Jacqueline, Holmes, Tom, Grothaus, Train, Findlay, Marsh, Cable, Mark, and Amick, David, 1975, Gravity and magnetic profile across the Georgetown gravity low: South Caro¬ lina Div. Geology Geol. Notes, v. 19, no. 2, p. 24-32. Taylor, P. T., Zietz, Isidore, and Dennis, L. S., 1968, Geologic implications of aeromagnetic data for the eastern con¬ tinental margin of the United States: Geophysics, v. 33, no. 5, p. 755—780. U. S. Geological Survey, 1970, Aeromagnetic map of the Camden-Rershaw area, north-central South Carolina: U.S. Geol. Survey open-file map, scale 1:62,500. - 1974, Aeromagnetic map of parts of the Greensboro and Raleigh l c by 2° quadrangles, North Carolina: U.S. Geol. Survey open-file map 74-29, scale 1:250.000. - 1975, Aeromagnetic map of Charleston and vicinity. South Carolina: U.S. Geol. Survey open-file map 75—590. scale 1:250,000. - 1976a, Aeromagnetic map of parts of the Brunswick and Savannah l e, x2 c quadrangles, Georgia and South Carolina: U.S. Geol. Survey open-file map 76-155, scale 1:250.000. -- 1976b, Aeromagnetic map of parts of Georgia, South Carolina, and North Carolina: U.S Geol. Survey opeD- file map 76-181, scale 1:250,000. Vacquier, V., Steenland, N. C., Henderson, R. G., and Zietz, Isidore, 1951, Interpretation of aeromagnetic maps: Geol. Soc. America Mem 47, 151 p. (Reprinted 1963.) Woollard, G. P.; Bonini, W. E., and Meyer, R. P., 1957, A seismic refraction study of the subsurface geology of the Atlantic Coastal Plain and Continental Shelf be¬ tween Virginia and Florida: Madison, Wisconsin Univ. Dept. Geol. Geophys. Sec., 128 p. Zupan, Alan-Jon, and Abbott, W. H„ 1976, Appendix B— Comparative geology of onshore and offshore South Carolina, in Preliminary summary of the 1976 Atlantic margin coring project of the U.S. Geological Survey: U.S. Geol. Survey open-file rept. 76-844 p. 206-214. " ■ ' Magnetic Basement Near Charleston, South Carolina—A Preliminary Report By JEFFREY D. PHILLIPS STUDIES RELATED TO THE CHARLESTON. SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028 -J t CONTENTS Page Abstract_ 139 Introduction _i_...*_ 139 Aeromagnetic data _ 139 Depth analysis_ 142 Model studies _ 142 Discussion of results _ 145 Depth contours_ 146 Basalt horizon_..._ 146 Crystalline basement _-— 146 Mafic bodies _ 149 References cited_ 149 ILLUSTRATIONS Page Figure 1. Map showing location of the study area _ 140 2. Aeromagnetic map of the study area _ 141 3. Magnetic profile 66 and interpreted basement cross section_ 143 4. Magnetic profile 76 and interpreted basement cross section__ 145 5. Contour map of magnetic basement (deep solution) _ 147 6. Contour map of magnetic basement (shallow solution) ..._ 148 . STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA. EARTHQUAKE OF ]886— A PRELIMINARY REPORT MAGNETIC BASEMENT NEAR CHARLESTON, SOUTH CAROLINA— A PRELIMINARY REPORT By Jeffrey D. Phillips ABSTRACT Automated depth analysis has been performed on aero- magnetic profiles collected in an area of current and his¬ torical seismicity near Charleston, S. C. Model studies, based on the computer-generated depth estimates, have been used to construct basement cross sections along two north-south lines. These cross sections reveal that extensive surfaces of magnetization contrast are present at two different depths, with the deeper surface having the stronger contrast. The shallower surface corresponds to the seismicallv determined crystalline basement. The deeper surface is interpreted to be the top of a mafic intrusive complex. Linear zones of anomalously low magnetization within the crystalline rocks appear to be responsible for linear lows in the magnetic anomaly. Reversely magnetized volcanic flows are an alter¬ native interpretation. Contour maps of the computer-gen¬ erated depth estimates are used to study depths and trends of subsurface structures in the epicentral area. INTRODUCTION The aeromagnetic map of Charleston and vicinity, S.C. (U.S. Geol. Survey, 1975) is characterized by localized strong magnetic highs superimposed upon broad, but sharply defined regional highs. The over¬ all pattern suggests that interesting structures and lithologies exist within the crystalline rocks under¬ lying the South Carolina Coastal Plain. The defini¬ tion of these structural and lithologic features is of particular interest because of the historical seismic¬ ity of the Charleston area (Bollinger, this volume; Bollinger and Visvanathan, this volume; Tarr, this volume). The magnetization contrasts responsible for mag¬ netic anomalies usually are found at the boundaries of relatively homogeneous zones of magnetization. Such magnetization boundaries may be found at the crystalline basement surface (the contact between buried crystalline rock and the overlying sedimen¬ tary’ rock), at lithologic boundaries within the crys¬ talline rock, and at the surfaces of dikes, sills, and volcanic flows within the sedimentary column. In the Charleston region, there is evidence for all three forms of magnetization boundaries. To locate magnetization boundaries, an automated depth-analysis technique has been used on selected aeromagnetic profiles taken from the Charleston sur¬ vey (U.S. Geol. Survey, 1975). Two complementary* sets of depths have been calculated for each profile. Model studies, based on both sets of solutions, have been used to obtain basement cross sections along j two north-south lines. These cross sections indicate ; the presence of at least two extensive surfaces of magnetization contrast, the deeper surface having the stronger contrast. The shallower of these two surfaces is interpreted to be crystalline basement on the basis of seismic- refraction studies (Ackermann, this volume). The deeper surface is interpreted to be the top of a mafic intrusive complex. In addition to defining .these magnetic-basement surfaces, the depth esti¬ mates and model studies suggest that linear zones of anomalously low magnetization are present within the crystalline basement rocks. Contour maps of the computer-generated depth estimates are used to examine these linear features, as well as other apparent structures, in a 40 x 55 km area west of Charleston. This study was funded by the U.S. Nuclear Regu- ; latory Commission, Office of Nuclear Regulatory | Research, Agreement AT (49-25)-1000. AEROMAGNETIC DATA A total-field aeromagnetic survey was flown over the South Carolina Coastal Plain between lat 139 140 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 | I | ! I | | 0 5 10 15 20 25 30 KILOMETERS Figure 1.— Map showing location of the study area. Magnetic profiles 66 and 76 are indicated. The line with tick marks is the zero contour of the aeromagnetic map (U.S. Geological Survey, 1975). 33°30' N. and 32°30' N. in 1973. The flight lines were north-south at 1 mile (1.6 km) spacing. The flight elevation was 500 ft (152 m). The resulting total field anomaly map was released as an open- file report (U.S. Geol. Survey, 1975). The regional magnetic pattern consists of broad regional highs separated by broad belts of low mag¬ netic intensity. Superimposed upon the regional highs are stronger, more localized highs. In figure 1 the regional magnetic highs and lows are outlined, and the location of the detailed study area is indi¬ cated. The part of the aeromagnetic map covering this detailed study area is reproduced in figure 2. Indicated on this figure are the locations of Middle¬ ton Place, the Charleston Air Force Base, and the USGS corehole site (Clubhouse Crossroads corehole 1). Strong magnetic highs are found over the core¬ hole site, north and southeast of Middleton Place, and along the coast in the southeast quadrant and the southwest comer of the mapped area. Middle- ton Place itself sits on an east-west linear magnetic low, bounded on three sides by strong closed mag¬ netic highs and on the south by an east-west linear magnetic high. Both this linear high and the strong triple-peaked magnetic high over the USGS core¬ hole site correspond to similar highs in the gravity anomaly (Long and Champion, this volume) In ad¬ dition, the regional magnetic highs correspond, for MAGNETIC EASEMENT 141 1 1 LINE 76 LINE 66 0 5 10 15 KILOMETERS CONTOUR INTERVALS 50 GAMMAS Figure 2.—Aeromagnetic map of the study area (modified from U.S. Geological Survey, 1975). The southern ends of magnetic profiles 66 and 76 are indicated. 142 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 the most part, to similar regional gravity highs, i This correspondence suggests that both the strong magnetic highs and the regional highs are pro- i duced by mafic intrusive bodies at depth. DEPTH ANALYSIS In order to obtain magnetic-basement depth esti- 1 mates, digital aeromagnetic data, as recorded along ‘ the flight lines, were first interpolated to a rectang¬ ular grid. The resuiting interpolated profiles were oriented north-south, spaced at 1-mile (1.6 km) in- ! tervals, and sampled at quarter-mile (0.4 km) in¬ tervals. Selected profiles were analyzed for source depth using the automated high-resolution technique of Phillips (1975). In this technique a statistical source model is used to estimate depth from the autocorrelation function of the magnetic profile. The autocorrelation function is calculated within short windows about each observation point, and compared with a theoretical autocorrelation func¬ tion that would be produced by a two-dimensional basement having uncorrelated magnetization. This means that the magnetization contrast at the base- j ment surface is assumed to be statistically inde- I pendent from point to point. Where the fit between \ the observed and theoretical autocorrelation func¬ tions is good, a depth can be estimated. Where the fit is poor, it means the data violate the assumptions of the model, and no reliable depth estimate is pos- j sible. In practice then, the computer only locates j the segments of the basement where the assump- I tions of two dimensionality and uncorrelated mag- netization appear to be satisfied. When two slight¬ ly different statistical models (assuming zero-mean and nonzero-mean source magnetization) are used, two complementary solutions can be obtained—one emphasizing shallow sources and one emphasizing deep sources. The two solutions are combined in cross section form prior to structural or geologic interpretation. Because the depth-analysis technique uses a slid¬ ing data window to search for a continuous base¬ ment surface, its response to a sudden vertical change in the basement is often delayed, resulting in hyperbolic tails’ on the depth estimates. These computer artifacts are generally ignored in the sub¬ sequent interpretation. Horizontal shifts of the depth estimates due to phase effects of the magnetic anomaly are reduced in the technique, through use of the analytic signal (Nabigihian, 1972), which has its power centered directly over the sources. How¬ ever, some horizontal shifts will always remain as a result of the technique’s slow response to sud¬ den vertical changes. One possible source of error in the estimated depths is the assumption that two-dimensional trends in the magnetic anomaly are perpendicular to the flight lines. Where this assumption is not satisfied, the depths will be somewhat overestimated. Fortunately, the major two-dimensional features of the aeromagnetic map do trend at right angles to the flight lines. Although the required assumptions of two-dimen¬ sionality and uncorrelated source magnetization may seem unreasonable, this technique has been suc¬ cessfully tested, both on synthetic data (generated assuming uniformly magnetized bodies) and on real data collected in areas of known source depth. For this reason, we can have some confidence in the estimated depths. MODEL STUDIES Two-dimensional modeling techniques were used to obtain cross sections along profiles 66 and 76 (fig. 1). Where possible, computer-generated depths were used to define the models. In areas where no com¬ puter-generated depths were available, simple models were used to match the general character of the anomaly and thus complete the cross section. Uniform-induced magnetization in a small number of bodies was assumed in all models. It should be emphasized that model studies of this sort yield nonunique solutions, and commonly alternative solu¬ tions are reasonable. However, in areas where com¬ puter-generated depths are available, the present solutions fit both the amplitude and the correlation statistics of the observed magnetic profiles, thereby restricting the alternatives. Line 66 is near the eastern edge of the detailed study area, and extends beyond it to the north (see fig. 1). The magnetic profile shown at the top of fig¬ ure 3 is characterized by broad magnetic highs in the north and south separated by a broad low. The northern high and the low are outside the detailed study area. The southern high contains three strong peaks, two of which flank the Middleton Place mag¬ netic low (lat 32°54' N.). An interpreted basement cross section is shown at the bottom of the figure. The heavy lines on the cross section indicate depths obtained from the computer, and the lighter con¬ necting lines define the model used to fit the mag¬ netic anomaly. In the northern part of the cross section, a well- defined southward-dipping magnetic basement is seen at depths of 600-1,500 m below the ground MAGNETIC BASEMENT 143 S 600- 400- PROFILE 66 r600 Middleton Place 400 - 600 Flight 5000 J S -600 "-5000 N VERTICAL EXAGGATtON *5 0 5 10 15 20 MILES -i ! —;-r- 0 5 10 15 20 KILOMETERS Figure S.—Magnetic profile 66 and interpreted basement cross section. The observed magnetic profile is given by the solid curve at the top of the figure. The computed magnetic profile is given by the dashed curve. Heavy lines on the cross section represent computer-generated depths. Lighter lines have been added to form the model. Magnetizations ( J ) are given in gauss. Depths are relative to ground level. surface. Because these depths correspond to the seismically determined crystalline basement depths south of Middleton Place (Ackermann, this vol- j ume), this surface has been interpreted to be crys- j talline basement. The underlying material has been assigned a weak magnetization of 5x10“* gauss, which corresponds to a susceptibility of 10* 3 , a rea¬ sonable value for granitic or metamorphic rocks. Because of the low magnetization contrast and low relief of this basement surface, it contributes very little to the calculated magnetic anomaly. Con¬ sequently it could just as easily be modeled as an unconformity in a thick nonmagnetic sedimentary sequence, in either interpretation, the surface is de¬ fined magnetically by the dikes or volcanic flow’s that contribute to the short wavelength part of the mag¬ netic anomaly. No attempt has been made to include these features in the model. The computer-generated depths provide no in¬ formation on intrabasement sources in the northern part of this line, but deep sources have been added to the model, based on results from nearby lines. These sources consist of a strongly magnetized body at 3.6 km depth in the north and a weakly mag¬ netized or nonmagnetic body at similar depth un¬ der the broad magnetic low. The northern source has been interpreted to be a mafic intrusive body on the basis of the regional gravity map (Long and Champion, this volume), w’hich show’s a relative high over this feature. The modeled nonmagnetic body under the broad magnetic low’ does not make any significant contribution to the calculated mag¬ netic anomaly, and it could easily be replaced by a thickened section of the overlying material. The three magnetic highs and the regional high in the southern part of the line appear to be pro- 144 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 duced at depths of between 2.4 km and 3.6 km by a strongly magnetized source, which is again inter¬ preted to be a mafic intrusive body on the basis of a corresponding regional gravity high. The Middleton Place magnetic low (lat 32°54' N.) corresponds to a topographic high in the estimated magnetic basement. Depth analysis of adjacent magnetic profiles reveals that this apparent topo¬ graphic high is a linear feature, extending to the west under the magnetic low. On the cross section the topographic high.is interpreted to be the upper 1 boundary of a nonmagnetic zone within the crys- j talline rock. The width of this hypothetical zone i may be exaggerated by the depth-estimation tech¬ nique. This nonmagnetic zone is shown extending down into the top of the mafic intrusive body, and in fact, this relief at the top of the mafic body can account for nearly all of the observed decrease in magnetic intensity. Although this suggests that the source of the magnetic low is at great depth, the correlation between the low and the shallower depth estimates is an argument for a source nearer the j crystalline basement surface. The nonmagnetic zone is included in the model mainly on the basis of this correlation. This modeled nonmagnetic zone could represent i a zone of alteration around a fault. Three types of evidence support this interpretation: (1) The linearity" of the feature, (2) the close association with Middleton Place, an area of active seismicity (Tarr, this volume), and (3) the negative sign of the anomaly, which is what one would expect from a mechanically or hydrothermally altered zone j within granitic rock. Although none of the other geophysical studies indicate a w’est-trending fault j in the Middleton Place area, there is a pronounced j east-west linear high in the gravity anomaly im- ■ mediately to the south, along lat 32°52' N. (Long and Champion, this volume). An alternative explanation for the Middleton Place magnetic low- requires reversely magnetized material at or above the crystalline basement. The presence of such material is a distinct possibility, as basalt flows were encountered in the USGS core- I hole at a depth of 750 m (Gohn and others, this vol¬ ume), and similar flows could be present at greater depths. In order to produce a magnetic low, basalt flow's that have reversed remanent magnetization would be required. Topographic relief on the tops or bottoms of the flows would result in a magnetic anomaly. The presence of reversely magnetized flows of limited extent would help explain some of the differences between the magnetic and gravity anomalies. In the gravity anomaly linear lows are absent, and the gravity high is not broken up into separate peaks north and west of Middleton Place. Reversely magnetized basalt flow's could be the source of the linear magnetic low’s that break up this feature on the aeromagnetic map. The computer-generated surface at the southern end of the section (south of lat 32°45' N.) has been modeled as a low-angle fault. Although this surface is almost certainly a boundary of the mafic intru¬ sive' body at depth, displacement of the crystalline basement along this surface is hypothetical. At its northern end this apparent surface extends up into the sedimentary column and may level off at the top of the basalt horizon seen in the USGS corehole to the northwest. Computer artifacts (hyperbolic tails) may be responsible for the apparent continu¬ ity of this surface. Line 76 passes through the western end of the Middleton Place magnetic low (figs. 1 and 2). The magnetic profile, shown in figure 4, is similar to profile 66. The interpreted basement cross section differs from the previous one in several ways. Deep sources have been detected in the northern part of the section by using the technique of Phillips (1975). As before, these are modeled as a mafic in¬ trusive body in the north and as a nonmagnetic body under the broad magnetic low. The magnetiza¬ tion of the northern mafic body has decreased to 5xl0~ 3 gauss, whereas the magnetization of the southern mafic body has increased to 9xl0* 3 gauss. The latter value is unusually large, even for mafic rock. Because of the oblique trends of the anomalies relative to the flight line (fig. 2), the depth of the southern mafic body is likely to be overestimated. A shallower body would not require as strong a magnetization to fit the observed anomaly. Another difference is the growffh of the nonmag¬ netic zone overlying the southern mafic body. There are now two peaks in the upper boundary of this zone, which correlate with relative lows in the mag¬ netic anomaly at lat 32°5l' N. and 32°54' N. On the aeromagnetic map (fig. 2) the tw T o magnetic lows appear to be separate features, but contour maps of the computer-generated depth estimates, to be covered later, suggest the lows can be connected along a north-northw’est-trending line. Consequent¬ ly, the section of figure 4 may be presenting a nearly longitudinal view’ of an altered zone about a north-northwest-trending fault. An alternative interpretation, presented earlier, would explain the magnetic lows as the result of reversely magnetized DEPTH. IN METERS GAMMAS MAGNETIC BASEMENT 145 PROFILE 76 N r 600 1-5000 5000 k i o 5 10 15) 20 MILES |-—--- 1 - ! 0 6 10 15 20 KILOMETERS Figure 4.—Magnetic profile 76 and interpreted basement cross section. Symbols are as in fig-ure 3. basalt flows. In this interpretation no nonmagnetic intrabasement zones would be required. In a final difference, crystalline basement is shown deepening to the south above the southern mafic body. This brings the model into agreement with the gravity interpretation of Long and Champion (this volume) which places a fault with a similar sense of motion in this location. The shallow (~1 km) depth estimates at lat 32 c 43' N. would have to result from intrasediment volcanics in this inter¬ pretation. An alternative model more in agreement with the seismic refraction data (Ackermann, this volume) would have crystalline basement remain¬ ing flat and shallow at about 1,200 m depth, pass¬ ing through the shallow depth estimates. This modi¬ fication would not significantly affect the calculated anomaly. DISCUSSION OF RESULTS Both the depth estimates and the model studies suggest that several different crystalline materials I are present beneath the 'Coastal Plain sediments. The shallowest of these is the basalt unit encount¬ ered in the corehole. Below is the weakly magnetic granitic or metamorphic material w’hich forms the crystalline basement. Because of the weak magneti¬ zation, depths to this basement can only be esti¬ mated where dikes or volcanic flows are present. South of Middleton Place this surface is known to be crystalline basement from the seismic refraction results (Ackermann, this volume). If further geo¬ physical studies should prove that a shallow crystal¬ line basement is absent in the northern part of. the section, then the shallow magnetic sources there could be reinterpreted as dikes or volcanic flow’s at a surface of unconformity within a thick sedi¬ mentary column. The model studies have showm that most of the power in the magnetic anomalies is produced by strongly magnetized sources at depth. On the basis of the strong magnetization and the associated 146 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 gravity highs (Long and Champion, this volume), these sources have been interpreted to be mafic in¬ trusive bodies. Although some mafic material may extend up to the crystalline basement in parts of ! the study area, the mafic bodies seen in the cross sections generally appear to be covered by a kilo¬ meter or more of weakly magnetic crystalline ma¬ terial. Mafic intrusive bodies are absent beneath the broad magnetic and gravity low to the north of Summerville. Recent earthquake hvpocenters under Middleton Place cluster at depths of 1-8 km (Tarr, this vol¬ ume) . According to our models, the top of the mafic intrusive complex in this area is located at depths of 2.5-S.5 km; thus, the shallower seismic events could be occurring within the crystalline rock near the surface of the mafic intrusive bodies, as pro¬ posed by Kane (this volume). On both cross sec¬ tions crustal magnetization appears to be homo¬ geneous below about 4.5 km, so nothing can be said about structures associated with the deeper seismic events. DEPTH CONTOURS In order to examine more fully the relation be¬ tween the computer-generated depth estimates and the aeromagnetic anomalies, contour maps of the depth estimates have been prepared. There are dif¬ ficulties in interpreting such maps. The contours do not represent geologic surfaces such as the crys¬ talline basement or the tops of the mafic bodies. In¬ stead, they indicate spots or patches on these sur¬ faces where the correlation statistics of the mag¬ netic profiles could be used to estimate depth. These patches are often surrounded by deeper contours, which represent the fictitious surfaces of the hyper¬ bolic tails. It follows that topographic highs in the contoured depth estimates are more reliable indi¬ cators of geologic surfaces than are gradients. How¬ ever, our model studies have shown that some gradients can have a geologic interpretation. The depth-estimation technique introduces distortion by exaggerating the widths of vertical bodies, and by overestimating depths when anomaly trends are not perpendicular to the flight lines. In addition, in many areas depth estimates are unavailable be¬ cause of unacceptable behavior of the correlation statistics. Despite these difficulties depth estimates often show remarkable consistency from line to line, and, when contoured, show good correlation with features of the aeromagnetic map. In figures 5 and 6 the “deep” and “shallow” com¬ puter-generated depth solutions have been con¬ toured. These figures only contain information of the type shown by the heavy lines in figures 3 and 4. Blank areas indicate regions where no reliable depth estimates could be obtained. The shallower features of these contour maps will be compared with the seismic interpretation of Ackermann (this volume). The deeper features will be analyzed using the gravity data of Long and Champion (this volume). BASALT HORIZON The seismic interpretation places the basalt hori¬ zon above 900 m depth east of Middleton Place and west of long 80°15' W. Magnetic-basement depth estimates above 900 m are found on the “deep” solution (fig. 5) at the center right and on the “shallow” solution (fig. 6), both northeast of the corehole and within an east-west belt around lat 32°45' N. These depth estimates, ■which are shal¬ lower than 600 m in places, probably represent the basalt horizon. The seismic interpretation shows a north-south trough in the basalt horizon west of Middleton Place. Maximum depth to the horizon within the trough is 1 km. Closed highs in the mag¬ netic basement contours at depths above 1.2 km just west and south of Middleton Place (figs. 5 and ■ 6) may indicate the depressed region of the basalt horizon, or they may indicate crystalline basement. CRYSTALLINE BASEMENT The seismic interpretation has crystalline base- ■ ment dipping southeast, increasing in depth from 900 m near the corehole to 1,300 m under Middle- ton Place, and flexing to 2,000 m further southeast. Magnetic-basement depth estimates in the range 1.2-1.5 km are found throughout the detailed study area, not just to the southeast of Middleton Place. This depth range includes the apparent east-west linear topographic high passing through Middleton Place, and the apparent north-northwest linear topographic high half-way between Middleton Place and the corehole (fig. 6). If the tops of these linear topographic highs are located at the crystalline basement, then the basement appears to be at a fairly constant depth of 1.2 km. There is no evi¬ dence of a southeast dip or flexure. The linear topographic highs in the basement have been modeled as nonmagnetic zones within the crystalline rocks. These topographic highs are as¬ sociated with relative lows in the magnetic anomaly (fig. 2) and can be interpreted either as altered zones about faults within the crystalline rocks or as MAGNETIC BASEMENT 147 0 5 10 KILOMETERS CONTOUR INTERVAL 305 METERS BETWEEN 0-1524 METERS 610 METERS BETWEEN 1524-4572 METERS Figure 5.—Contour map of depths to magnetic basement corresponding to the computer-generated “deep” solution within the study area. Contours are dashed where inferred. Blank areas repre¬ sent regions where no depth estimates were generated. 148 STUDIES BELATED TO CHARLESTON, SOUTH CAROLINA. EARTHQUAKE OF 1886 ! 1 ' 1 1 I i 0 5 10 KILOMETERS CONTOUR INTERVAL 305 METERS BETWEEN 0-152* METERS 610 METERS BETWEEN 1574-4572 METERS Figure 6.—Contour map of depths to magnetic basement corresponding to the computer-generated “shallow” solution. Contours are dashed where inferred. Blank areas represent regions where no depth estimates were generated. MAGNETIC BASEMENT 149 reversely magnetized volcanic flows on top of the crystalline basement. A third interpretation may be advanced for the north-northwest-trending linear topographic high. The feature is located along the eastern boundary of the strong magnetic high over the USGS corehole site. If the top of the mafic body responsible for this magnetic high is at the crystalline basement as the gravity interpretation of Long and Champion (this volume) suggests, then the linear topographic high in the magnetic-basement depth estimates may represent a vertical contact between crystalline basement material to the east and mafic material to the west. Similar contacts may exist on other sides of the mafic body as well, as indicated by the 1.5-km depth estimates northwest and southeast of the corehole site in figure 6. MAFIC BODIES The estimated depths to magnetic basement con¬ toured in figure 5 reflect w’hat are interpreted to be mafic intrusive bodies at depths of 2—4.5 km. These sources are responsible for the strong magnetic highs and the regional magnetic high in the aero- magnetic map (fig. 2). The mafic bodies are shal¬ lowest in the northern part of the study area, w’here the magnetic anomalies are the strongest. They deepen to the south, most rapidly along the south- ward-dipping surface appearing in the southeast quadrant of figure 5. Mafic bodies are absent un¬ der the broad magnetic low’ immediately north of the study area (fig. 1). In discussing the mafic bodies, we w’ill ignore contours above a depth.of 2 km in figure 5, such as those near Middleton Place and those near the center of the figure. These contours represent the shallow magnetic sources discussed earlier. Shallow mafic sources, indicated by the magnetic highs north of the Middleton Place magnetic low in figure 2, are found in figure 5 to have estimated depths of between 2.0 and 2.7 km. Similar depths are indicated under the magnetic high to the south¬ east of Middleton Place (figs. 2, 5). Apparent small-scale relief on the magnetic base¬ ment, such as the V-shaped trough northwest of Middleton Place in figure 5, is most likely a com¬ puter artifact resulting from nonperpendicularity of the magnetic contours to the flight lines. It is in¬ teresting, however, that one leg of the trough trends northw’est along the trace of a nodal plane of the 22 November 1974 earthquake (Tarr, this volume), and the other leg trends northeast along a lineation defined by the gravity anomaly (Long and Champion, this volume). Both legs of the trough correspond to saddles in the magnetic anomaly (fig. 2 ). The positive magnetic anomaly over the USGS corehole site (fig. 2) is the strongest magnetic high in the area, and it is coincident with the strongest positive gravity* anomaly (Long and Champion, this volume). The strength of the magnetic anomaly and the results of the gravity modeling suggest that the 3.2-4.5 km depths indicated under this feature in figure 5 are unlikely to represent the top of the source body. It is more likely that the top is at the crystalline basement and that the contours of fig¬ ure 5 represent either the bottom of the source body or a mineralization boundary within the source ' body. This is probably the shallowest mafic body in the study area. None of the other mafic sources appear to extend up to the crystalline basement. The large elongate magnetic high in the south¬ east quadrant of the study area (fig. 2) corresponds to a gravity low (Long and Champion, this volume). This suggests that the material responsible for this magnetic high differs from the mafic material to the north. In our model studies, the magnetic source i has been placed at depths of 2.7-3 km. A final strong magnetic high is found in the south¬ west corner of the study area (fig. 2). A correspond¬ ing gravity high (Long and Champion, this volume) indicates a mafic source body. According to figure 5 the body is at a depth of 1.5 km. REFERENCES CITED Nabigihian, M. N., 1972, The analytic signal of two-di¬ mensional magnetic bodies with polygonal cross-section: Its properties and use for automated anomaly inter¬ pretation: Geophysics, v. 37, no. 4, p. 507-517. Phillips, J. D., 1975, Statistical analysis of magnetic pro¬ files and geomagnetic reversal sequences: Stanford, Calif., Stanford Univ., Ph.D. dissert, p. 81-134. U. S. Geological Survey, 1975, Aeromagnetic map of Charles- - ton and vicinity*, South Carolina: U.S. Geol. Survey open-file rept. 75-590. ' . . ' . Bouguer Gravity Map of the Summerville-Charleston, South Carolina, Epicentral Zone and Tectonic Implications By LELAND TIMOTHY LONG and J. \V. CHAMPION, Jr. STUDIES RELATED TO THE CHARLESTON. SOUTH CAROLINA. EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1 028-K CONTENTS Page Abstract_ 151 Introduction _ 151 Regional gravity _ 153 Analysis of line data_ 153 Residual gravity anomalies_ 160 Three-dimensional models _ 161 Possible earthquake mechanisms_ 163 Earthquake locations and Sloan’s intensity map_ 163 Conclusion _ 166 References cited _ 166 ILLUSTRATIONS Page Figure 1. Simple Bouguer anomaly map of Georgia and South Carolina_ 152 2. Index map of southern South Carolina showing the location of the area of study_ 154 3. Simple Bouguer gravity map of the Summerville-Charleston, S.C., epicentral zone___ 155 4. Index map showing the locations of the detailed gravity lines, the profile, and the earthquake epicenters_ 156 5—10. Profiles of: 5. Detailed gravity line G-G' showing the horizontal cylin¬ der, sphere, and simple fault model interpretations for three anomalies_ 157 6. Line L-L' showing a model for the central positive anomaly _ 158 7. Line A-A' compared with the theoretical anomaly from a fault model _ 158 8. Line F-F' compared with the theoretical anomaly from a horizontal cylinder_ 158 9. Eastern part of line D-D' compared with the theoretical anomaly for a two-dimensional rectangular struc¬ ture _ 159 10. Line 1-1' compared with a simple fault model_ 159 11. Residua] gravity map of the Summerville-Charleston, S.C., epi¬ central zone _ 160 12. Map showing the elevation contours of the interpreted subbase¬ ment surface_.... 161 13. Diagrams showing three-dimensional modeling of the gravity anomalies by use of polygons of anomalous density in stacked vertical sheets at intermediate depths and at maximum depths 162 14. Map showing superposition of isoseismal contours of Earl Sloan on Bouguer gravity anomalies_ 164 ill . . STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT BOUGUER GRAVITY MAP OF THE SUMMERMLLE-CHARLESTON. SOUTH CAROLINA. EPICENTRAL ZONE AND TECTONIC IMPLICATIONS Bv Leland Timothy Long 1 and J. W. Champion, Jr . 2 ABSTRACT A new Bouguer anomaly map of the Summerville—Charles- j ton, S.C., epicentral zone is interpreted to reveal a mafic in- 1 trusive body and associated flows and a northeast-trending Triassic(?) basin. Two shallow structures interpreted from j the gravity data and associated with the Triassic (?) basin j may be significant in determining the mechanism for the 1886 Charleston earthquake. The first is a border fault on j the northwest side of the basin striking N. 45 e E., and the second is a linear positive anomaly striking east. The first structure suggests the more conventional earthquake mech- j anism of reactivation of a basement fault. The second struc¬ ture suggests a newly proposed mechanism of stress ampli- j fication in the anomalously rigid structure responsible for I the linear positive anomaly. Intensity data from the August j 81, 1886, Charleston earthquake and epicenters of recent events favor stress amplification as the more likely explana- j tion for earthquake activity 1 5 in the Summerville—Charleston epicentral zone. INTRODUCTION The lack of a confirmed tectonic mechanism for the Charleston, S.C., earthquake of August 31, 1886, | and for its foreshocks and aftershocks has been a . major obstacle in the development of a realistic evaluation of seismic hazard in the Eastern United States. Although more than 400 events (Taber, j 1914; Bollinger, 1972) have been felt in the region since 1886, no fault to which the activity 7 can be definitely attributed has been observed at the sur- 1 face. The probable reason that no tectonic mechan¬ ism has yet been agreed upon is that the basement structures responsible are not only unknown but also hidden by more than 0.79 km of post-Paleozoic sedimentary’ and extrusive rocks. The Summerville-Charleston epicentral zone does not lie near an active plate boundary. Hence, the 3 School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, Ga. 5 Chevron Oil Co., Houston, Tex. association of the Summerville—Charleston events with plate tectonics would have to be indirect. Intra¬ plate movements near the epicentral zone, if present, are obscured by a lack of appropriate data except, perhaps, recent releveling data. Within the context of the theories of plate tectonics, the Charleston earthquake is one manifestation of intraplate tec¬ tonics that as vet has not been satisfactorily explained. Taber (1914), in one of the earliest attempted ex¬ planations of the seismic activity near Charleston, hypothesized a fault in "the crystalline basement. On the basis of intensity data only, he suggested that the unobserved fault trended in a general northeast- southwest direction and was near Woodstock, S.C., 8 km southeast of Summerville. A deep well near Summerville bottomed in basaltic rocks (diabase) (Cooke, 1936). The 0.26 km of sedimentary 7 rocks directly above the diabase was interpreted to be Tri- assic (Cooke, 1936). Mansfield (1936) placed this section in the Cretaceous. Woollard, Bonini, and Mey r er (1957) and Pooley, Mey 7 er, and Woollard (1960), assuming that Mansfield was correct, sug¬ gested that the tectonics of the Charleston epicentral area might be related to a topographic feature of the basement surface, the Yamacraw Ridge, and the as¬ sociated basement valley to the north, rather than to a Triassic basin perhaps related to the Florence Triassic basin. Bollinger (1973) conjectured that a general relation existed among seismicity, regional uplift, and old Appalachian structures. Oliver and Isacks (1972) and Fletcher, Sbar, and Sy 7 kes (1974) suggested that the Charleston activity 7 and other earthquakes of the South Carolina belt are related to the landward extension of a major transform fault 151 BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 153 and structures associated with the early opening of , the Atlantic Ocean. The first objective of this paper is to present an analysis and structural interpretation of gravity data covering the Summerville-Charleston epicen- tral zone. The gravity data consist of approximately 1,000 regional observations at a mean separation of 1.0 km and an additional 1,000 observations at a mean separation of 0.3 km along selected lines. The gravity analysis presented herein is largely a con¬ densation of the master’s thesis of the second author (Champion, 1975). A listing of the gravity data and details concerning its reduction are given in Champ¬ ion (1975). Standard reduction techniques were used, and the theoretical gravity was computed using the 1931 international gravity formula. The second objective of this paper is to discuss the interpretation of the structure in terms of possible mechanisms for the Charleston earthquake of August 31, 1886. In particular, we propose that stress amplification within an anomalously rigid crustal structure could have been responsible for the 1886 Charleston earthquake. The hypothesis of stress amplification should be given serious consideration as new data become available. The gravity data were obtained by the second author during the summer of 1974. His fieldwork, the data reduction, and the analysis were supported by the U.S. Geological Survey (grant 14-08-0001- G-127). Studies on the stress amplification mecha¬ nism were supported by the National Science Foun¬ dation under grant DES75-15756. REGIONAL GRAVITY The regional Bouguer gravity' pattern of the South Carolina and Georgia Coastal Plain is characterized by numerous sharp positive anomalies and smoother, less pronounced negative anomalies (fig. 1). The sharp positive anomalies range in magnitude from + 15.0 to +70.0 mGal. Those that were studied in detail have the size and character of mafic volcanic plugs and associated basaltic flows and dikes (Long, 1974). The negative anomalies could be explained by shallow basins, perhaps of Triassic age, or by blocks of less dense or thicker continental crust. In general, the gravity data imply a highly inhomogeneous up¬ per crust beneath the Coastal Plain. The area of detailed gravity coverage (fig. 2) in¬ cludes one of the sharp positive anomalies and a major part of the suspected epicentral zone of the 1886 earthquake, as well as the epicenters of more recent seismic events. The borders of the area inves¬ tigated are defined by the coordinates 32°37'30" N., 80° W., and 33°07'30" N., 80°22'30" W. The gravity map (fig. 3) is contoured at 1.0 mGal from individu¬ al gravity observations and has an estimated pre¬ cision of 0.2 mGal. The gravity' isogals in figure 3 indicate that the Summerville-Charleston epicentral zone is near the contact between a sharp positive anomaly, here in¬ terpreted as a volcanic plug, and a negative anomaly, here interpreted as a Triassic basin. In the western part of the study area (fig. 3), a positive anomaly having a peak value of 15.0 mGal exhibits a steep gravity gradient of 2.0-3.0 mGal /km on both its northern and its southern sides. To the east of this feature, the gravity gradient becomes less steep, and the isogals spread to form a noselike feature. To the south and to the northeast, negative anomalies ap¬ pear to form an arc around the noselike feature. The northwestern part of the study area is characterized by a reasonably constant negative anomaly of ap¬ proximately - 3.0 mGal. Three prominent zones of alinement in the contour lines and anomalies can be observed. The trend of the strongest alinement of contour lines is approxi¬ mately east-west and is formed by the steep gradient south of the central positive anomaly. Another aline¬ ment of contour lines is defined by the trend of the isogals north of the central positive anomaly and bears approximately N. 50° W. These two contour alinements appear to be a consequence of the shape of the central positive anomaly. A third alinement is defined by the western termination of the northeast¬ ern and southern negative anomalies. The north¬ western edges of these negative regions define an alinement of isogals trending N. 40°-50° E. (labeled NE linear anomaly on fig. 3) that intersects the cen¬ tral positive anomaly in the region where the isogals begin spreading to form the noselike positive anomaly. ANALYSIS OF LINE DATA Approximately one-half of the new data were ob¬ tained along lines at an average separation of 0.3 km. However, many of these lines (see fig. 4) were restricted to major rights- of-way or were obtained prior to knowledge of the strike of the crustal struc¬ tures. The lines that proved to be normal to the strike of the crustal structures could be used for interpre¬ tation of depths and structures using simple two- dimensional models. Lines G—G', the north part of A-A', and a profile L-L’, which all cross the positive anomalies in the north part of the map, give infor¬ mation on the depths to these structures. Lines A-A' 154 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Bowman O Figure 2. —Index map of southern South Carolina showing the location of the area of study. BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 155 Figitre 3. —Simple Bouguer gravity map of the Summerville-Charleston, S.C., epicentral zone (taken from Champion, 1975). Epicenters of recent earthquakes are from Tarr (this volume). ~/o 156 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Figure 4.— Index map showing the locations of the detailed gravity lines and the profile. Epicenters of recent earthquakes (Tarr, this volume) are plotted on the gravity map of figure 3. n: cq BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 157 and F-F' are used to model the negative anomaly in the northeast. Line D-D' provides information on the depth and size of the eastward protruding posi¬ tive anomaly. Lines 1-1', G-G' and F-F' give infor¬ mation on a possible northwest-trending structure, j Simple two- or three-dimensional models (Nettleton, ■ 1976) are used in the analysis. Corrections for end effects in two-dimensional models would not signifi¬ cantly affect the results. Similarly, more detailed models that remove the deviations of the observed gravity from the modeled gravity would not change any of the conclusions. Most of the irregularities that have magnitudes less than 1.0 mGal are inversely j proportional to the elevation. However, the implied Bouguer reduction densities are often unreasonably low, or even negative. The explanation probably re¬ lates to a fundamental difference between near-sur- face materials in the dry elevated areas and those in the river bottoms. - The northern part of line G-G' crosses two posi¬ tive anomalies. The anomaly at 4.0 km on line G-G’ (see fig, 5) can be modeled with a horizontal cylinder that has a 1.3-km depth to its center and a radius of 0.45 km at a density contrast of 0.3 g/cm 3 . The anomaly at 9.0 km can be modeled by a sphere that has a depth to its center of 1.7 km and a radius of 0.9 km at a density contrast of 0.3 g/cxrr. Both of these anomalies are consistent with a depth of 0.8 km to the tops of these structures. Profile L-L’, which was interpolated from the con¬ tours because of the lack of appropriate rights-of- way for a detailed profile, crosses the largest posi¬ tive anomaly (fig. 6). Examination of this anomaly indicated that the steepest gradient lies on the north side and has a half-width of about 2.0 km. The cen¬ tral peak was modeled as a 4.0-km-wide two-dimen¬ sional structure that has its top at a depth of 1.5 km and its bottom at 4.0 km (fig. 6). The interpretation of a structure at depths less than 1.5 km along line L-L’ is difficult because of the smoothing that is in¬ herent in the interpolation from contours. The sides of the anomaly can be modeled with a thinner or less dense structure that also has its top at 1.5 km and its bottom at 4.0 km. The depth to the top of the largest positive anomaly is consistent with depths to basement structures toward the northeast. The negative gravity values to the south are modeled by a less dense basin. Line A-A' intersects the prominent negative anomaly in the northeastern part of the study area. The structure under line A-A' (fig. 7) can be mod¬ eled as a nearly vertical fault that offsets a block 0.3 g/cm 3 denser than the overlying material. The fault is interpreted as striking N. 40° E. (labeled NE linear anomaly in fig. 3) and intersects line A-A' at 6.0 km (fig. 4). The throw of the fault is 0.65 km, and the upper boundary of the upthrown block on the northwest is at a depth of 0.8 km. The positive residual at 5.0 km is derived in part from the posi- 12 - re 0 NORTHEAST Cylinder Sphere Simple fault model Z,1.0 T,0.2 Ap,0.3 //Vi v / / / / / / ' Z i A- -T -rV'Vn/-r V VtH G' | • • • • .■ "southwest z R A p T EXPLANATION • Observed gravity points - Theoreticai gravity anomaly Depth to center, in km Radius, in km Density contrast, in g/cm 3 Thickness, in km 12 16 20 DISTANCE, IN KILOMETERS 1 J 1 Figure 5.—Detailed gravity line G-G' showing the horizontal cylinder (at 4.0 km) and sphere (at 9.0 km) interpretations for the two northern positive anomalies and the simple fault model interpretation for the anomaly at 16.0 km. The location of G-G' is shown in figure 4. DEPTH. IN km BOUGUER ANOMALY, IN mGal 158 16 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA. EARTHQUAKE OF 1886 DISTANCE, IN KILOMETERS Ap=0.15 Ap=0.3l Ap=a i 5 _| Ap= -0.1 6 - EXPLANATION • • ♦ • Gravity anomaly interpolated from observed contours ——* * Theoretical gravity anomaly Ip Density contrast in g/cm 3 Figure 5.—Profile L-L' 3howtng a model for the central posi¬ tive anomaly. This profile was interpreted from the con¬ tour lines. tive density anomalies on the upthrown block re¬ lated to the two positive anomalies of line G-G', pre¬ viously noted. It may also be derived in part from the inverse relation of gravity to elevation caused by applying a Bouguer reduction density of 2.67 to Coastal Plain consolidated sedimentary rocks. Line B-B' gives similar results (Champion, 1975). Line F~F’ (see tig. 8) was used to estimate the extent of the feature responsible for the negative anomaly—a possible basin—corresponding to the downthrown side of the fault interpreted under lines A-A' (fig. 7) and B-B'. A horizontal cylinder was used for the model and indicated a depth to its cen¬ ter of 3.7 km and a radius of 1.8 km at a density con¬ trast of -0.2 g/cm 1 . Maximum depths to the tops of two-dimensional structures can be computed using 0.65 times the ratio of the maximum anomaly to the maximum gradient. The positive anomalies on the north end of line F-F’ are an extension of the posi- DISTANCE, IN KILOMETERS Figure 8. —Line F-F’ compared with the theoretical anomaly from a horizontal cylinder that has a depth of 3.7 km to its center and a radius of 1.8 km at a density contrast of —0.2 g/cnr. tive anomaly at 4.0 km in line G-G’. Its northern gradient and its maximum anomaly (respectively, 1.1 mGal/km and 3 mGal) indicate a maximum depth of 1.8 km, which is consistent with, or slightly deeper than, the structures along line G-G'. How¬ ever, the northern and southern edges of the basin have respective gradients of 2.0 and 1.2 mGal km for a maximum anomaly of -8.0 mGal. The respec¬ tive maximum depths to the tops of the northern and southern edge of the basin would be 2.6 and 4.3 km. BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 159 Furthermore, the scatter in the gravity data on the southern edge of the basin can be almost completely removed by using a correction factor proportional to the elevation. Consequently, the negative anomaly is interpreted as a basin in the depth range of 0.8 (from line G-G') to perhaps 5.5 km. The westward extension of the basin is interpreted as being trun¬ cated by a fault. The northern edge could also be interpreted to be a fault. The southern edge is deeper and probably represents a density' contrast within or below the basin. The maximum depth of the basin cannot be estimated without better control on the density contrast. The southeastern part of line D-D' crosses the eastward protruding positive anomaly. This struc¬ ture is modeled (fig. 9) as a rectangular bar whose width is 8.0 km, depth to top is 1.5 km, and depth to bottom is 4.0 km on the basis of a density contrast | of 0.2 g/ cm 3 . The actual depth to the bottom may be greater since density contrasts become more difficult to discriminate at greater depths. The residuals are 1 I ! i 1 1——J 1 1 1_l_—t—1—1 30 32 34 36 38 40 42 44 DISTANCE. IN KILOMETERS E ° 1 ! ! ! 1 . -XT. — X i_ Ap =0.2 EXPLANATION • • • • Observed gravity points —— Theoretical gravity anomaly A p Density contrast in g/cm 3 Figure 9.—Eastern part of line D-D' compared with the theoretical anomaly for a two-dimensional rectangular structure. DISTANCE, IN KILOMETERS to ir UJ UJ 5 2 Z O l- < > LU _ J UJ 30 20 10 EXPLANATION .Observed gravity points —-Theoretical gravity anomaly -Gravity corrected for near- surface anomalies Figure 10.—Line 1 - 1 ' compared with a simple fault model that has a depth of 0.8 km to its center and a throw of 0.2 km at a density contrast of 0.3 g/cm 1 within the precision of the data. The maximum gradient of 2.4 mGal, km and maximum anomaly of 12 mGal imply a maximum depth of 3.25 km to the top of the structure. On the basis of this computa¬ tion and the depth computations for line F-F’, the top of the eastward-protruding positive anomaly may be deeper than the tops of the other structures to the northwest. The difference in depths to the tops of the structures is consistent with the fault inter¬ pretation for line A-A’. In an analysis of earthquakes and structures near Bowman, S.C., McKee (1974) noted an alinement of epicenters N. 40° W. parallel to Bollinger’s (1973) transverse South Carolina-Georgia seismic belt. Tarr (this volume) also noted the N. 40° W. aline¬ ment of epicenters. Lines G-G', and F-F’ are normal to this trend (fig. 4) and have been examined for evidence of a faultlike structure. Line /-/' and part of line G-G' are consistent with the model for a thin faulted slab with the downthrown side assumed to be infinitely deep (see Nettleton, 1976, p. 195). The depth to its center is 0.8 km, and its thickness is 0.2 km at a density contrast of 0.3 g/cm 3 . The thin faulted slab model is mathematically identical to a 160 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 semi-infinite horizontal sheet and does not necessari¬ ly require interpretation as a fault. Line /-/' indi¬ cates a possible thin slab truncated at 5.5 km from the south end (fig. 10; see fig. 4 also). Line G-G' shows a possible thin slab truncated at 16.0 km from the northeast end (fig. 5). The scatter in the data about the theoretical anomaly for the thin faulted slab in line /-/' correlates inversely with the eleva¬ tion (see fig. 10). The scatter is related to the near¬ surface density‘variations which correlate with the topography, and, hence, the scatter can be reduced prior to computation of the model parameters. How- I ever, the proportionality constant is often too large to be explained completely by an improper Bouguer j reduction density and implies that near-surface structures are controlled in part by their elevation, j Line F-F’ has a negative deviation from the theo- , retical anomaly (fig. 8) at 15.0-16.0 km and again s at 12 km from the north end. However, the interpre¬ tation that these deviations result from a thin : faulted slab is inappropriate because they can be virtually eliminated by a correction factor propor- i tional to the elevation and, hence, are related to near¬ surface structures. The two thin faulted slab models line up along a strike of N. 70° W. This strike is not consistent with the N. 40° W. alinement of regional epicenters noted by Bollinger (1973), McKee (1974), and Tarr (this volume). The gravity data do not indicate the existence of a continuous fault that has vertical displacements greater than 200 m t oriented N. 40° W. The N. 70° W. alinement is con¬ sistent with, but displaced from, some recent shallow epicenters plotted in figure 4 (Tarr, this volume). ! However, the interpretation that the alinement of the thin faulted models is a single fault is not unique. An alternate interpretation that these features are the edges of basalt flows is preferred by the authors since this would be consistent with the interpretation that the positive anomalies are volcanic plugs and associated flows; also the regional data and lack of a fau’tlike structure along line F-F' do not support a continuous linear structure. RESIDUAL GRAVITY ANOMALIES In order to facilitate further analysis, gravity values were computed at a regular grid interval of 1.0 km using a distance-weighted mean-value inter¬ polation algorithm. The area of the interpolated gravity data encompassed the area defined by the simple Bouguer gravity map. A regional field was obtained by convolving the gravity data with a smoothing operator which has an effective half-width of 2.5 km. The residual gravity map (fig. 11) is the arznr arts' miTW Figure 11.— Residual gravity map of the Summerville- Charleston, S.C., epicentral zone. Contour interval, 0.5 mGal. Hachures are used to indicate the negative side of the zero and all negative contour lines. difference between the simple Bouguer gravity map and the regional field already described. Positive residual anomalies are shown in the central part of figure 11 and are located in the same positions as the positive anomalies in the original map (fig. 3), im¬ plying that parts of the source of these anomalies are less than 2.5 km deep (the radius of the smoothing operator). The westernmost large positive anomaly is resolved by the residual anomalies into two peaks. These two peaks are in a line with the two positive anomalies to the northeast. The line strikes N. 45° E. A long, narrow negative residual anomaly that also strikes N. 45° E. occurs approximately 4.0 km south¬ east of the line of positive anomalies. This negative residual anomaly connects the northwest edge of the basin (where a fault, shaded on fig. 3, was inter¬ preted along line A-A') and the northwest edge of a negative simple Bouguer anomaly (fig. 3) in the southwest part of the area shown on the map. This long, narrow negative residual anomaly is inter¬ preted as evidence of the southwest continuation of the edge fault for the basin interpretation of the BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 161 negative Bouguer anomaly near 80° W., 33° N. The negative residual anomaly occurs on the downthrown block of a fault. The positive anomaly protruding to the east (fig. 11) is smoother than the peaks to the northwest and supports the hypothesis of a deeper source for this anomaly. The northeastern and southwestern negative Bou¬ guer anomalies may be part of one continuous basin (see fig. 1). The total depth of the basin is unknown but could be as deep as 5.5 km on the basis of the [ horizontal cylinder model applied to line F-F’ (fig. 83. Mann and Zablocki (1961) noted very similar i anomalies associated with the Jonesboro fault, a ; border fault on the southeast edge of the Deep River | Triassic basin in North Carolina. However, they in¬ terpreted the density contrast to be -0.1 g/cm 3 . If a density contrast of 0.1 g/cm 3 applies to the basin near Charleston, its depth may exceed the 5.5-km ( estimate from the cylinder model applied to line F-F'. The displacement of shallow or more recent flows near the fault could be significantly less or even nonexistent, since some of the volcanic activity and the faulting could have been contemporaneous. : THREE-DIMENSIONAL MODELS Although gravity data generally do not allow a unique structural interpretation, the addition of con¬ straints on the acceptable structures will often allow a direct inversion that provides insight into the dis¬ tribution of the anomalous masses. One simple con¬ straint is to assume that the anomalies are derived from a variation in the thickness of less dense rocks near the surface. In this model, the sedimentary rocks or less dense volcanic rocks were assumed to provide a density contrast of 0.3 g/cm 3 above more dense basic volcanic rocks or intrusive rocks. A lower density contrast or greater depth would lead to numerical instability for the shallow anomalies in the northwest part of the study area. Coastal Plain ! sediments were neglected in this model. An iterative process was used to effect a perfect fit of the theo¬ retical anomaly to the data interpolated at a grid interval of 1.0 km. In this type of reduction, shallow anomalies are made more sharp, while the deeper sources remain shiooth. The effect is similar to a downward continuation of the gravity field. The elevation contours of the interpreted sub¬ basement (fig. 12) are similar to those of the gravity anomalies, except that they show steeper gradients and tend to emphasize 3ome of the structural con¬ tacts. In this model, the sharp peaks in the northwest part of the study area are at depths equivalent to 0.7-2.1 km. The eastward-protruding positive anom- Figure 12.—Elevation contours of a surface derived by modeling Bouguer gravity anomalies assuming a density contrast of 0.3 g, cm 1 . The contours are in kilometers above an arbitrary surface at a depth of 3.5 km. Hachures in¬ dicate areas below 3.5 km. Epicenters of recent earthquakes (solid circles) are from Tarr (this volume). CCC 1, Club¬ house Crossroads corehole 1. aly now appears as a ridge and has an average gradi¬ ent significantly less than those on the edges of the large positive anomaly to the northwest. The smooth gradient provides additional support for the hy¬ pothesis of a possible deeper structure as the cause of this anomaly. Figure 13 shows two results of three-dimensional . modeling of the gravity anomalies using the method of Talwani and Ewing (1960). Regional gravity data to the west of the area were included in this modeling to minimize the edge effects related to the positive anomaly. The models were obtained by as¬ suming a structure compatible with the line data and perturbing the model until the computed gravity anomaly varied no more than ±2 mGal from the ob¬ served data. Regional data, interpolated onto a grid 162 STUDIES RELATED TO CHARLESTON. SOUTH CAROLINA, EARTHQUAKE OF 1886 A A +0.23 g/cm3 +0.13 g/cm3 -0.27 g/cm3 EXPLANATION — 1 ■■ Map area B + 0.33 g/cm3 -0.27 g/cm3 Figure 13. —Diagrams showing three-dimensional modeling of the gravity anomalies following the method of Talwani and Ewing (1960). This method uses polygons of anomalous density in stacked vertical sheets at intermediate- depths (A) and at maximum depths ( B ). The dashed lines on the top blocks outline the area of the gravity map in figure 3. The vertical dimension is exaggerated by a factor of five, and the sheets are separated for clarity. The maximum error for either model is 10 percent. with a 5.0-km separation, were used to the west of the study area for the comparison. In the study area, the data interpolated to a 1.0-km grid were used for the comparison, and the maximum error allowed was 1.0 mGal. Resolution of models or structures at depths less than 2.5 km was not practical for data interpolated to a 1.0-km grid interval. Figure 13.4 shows a model based on the depths and structures interpreted from the line data. The top sheet contains most of the sources for the positive anomalies. These extend over parts of the second and third sheet where no anomalous masses are shown. Hence, some of these may be interpreted as near¬ surface flows or sills. The second sheet contains the BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 163 sources for the negative anomalies and the eastward extension of the positive anomalies. The third sheet contains only a source for part of the eastward pro¬ truding anomaly. Figure 135 shows the result of extending the mod¬ eled structures as deep as possible. However, in both models, shallow structures (2.5—4.5 km) are re¬ quired to maintain a satisfactory fit to the observed data; only the core of the large positive anomalies can be extended to significantly greater depths. Both models suggest that a vertical offset of the structure responsible for the eastw'ard-protruding positive anomaly is compatible with the gravity data. POSSIBLE EARTHQUAKE MECHANISMS Two large-scale features of the interpreted struc¬ tures are considered significant to the mechanism of the 1886 Charleston earthquake. The first is a fault in the basement interpreted as the northwest edge of a basin. The fault strikes generally N. 45 s E., but is displaced to the northwest by more than 6.0 km from the hypothesized Woodstock fault described by Taber (1914). The offset on the fault is estimated to be at least 0.6 km on the basis of the interpretation of the gravity data for the zones that cross this fea¬ ture. The 0.6-km vertical displacement is reasonable for basins associated with rift zones and is compara¬ ble to the displacement of the Deep River basin in North Carolina (Reinemund, 1955). The second sig¬ nificant large-scale feature is the linear positive anomaly extending east from the positive high. This is interpreted to be a ridge or barlike structure of high-density material in the depth range of 1.0-6.0 km or greater in the crust. High density rocks typi¬ cally have high seismic velocities. Higher velocities would imply that this protrusion has a higher modu¬ lus of rigidity than would be expected for the sur¬ rounding basin materials. These two large-scale features of the interpreted crustal structure pose two independent explanations for the occurrence of earthquakes near Charleston. The first explanation, and perhaps the more conven- i tional, is that the earthquakes are the result of the ; reactivation of the northeast-striking basement fault. This mechanism is essentially that proposecTby Taber (1914), except that the fault interpreted from gravity data is farther to the northwest than the proposed Woodstock fault. In this first explanation, earthquake epicenters and intensities should be closely associated with the fault trace. The second explanation is that the earthquakes are the result of fracture of the structure responsible for the eastward protruding positive anomaly. The contrast between the rigidity of this structure and the rigidity of the surrounding sedimentary or igne¬ ous rocks would allow' the concentration of stress through the mechanism of “stress amplification.” In stress amplification, the geometry of the structure is such that regional shear stress is concentrated in an anomalously rigid crustal unit, and the stress within the anomalously rigid unit is amplified at geometri¬ cally appropriate locations. Consequently, stress am¬ plification could lead to .stress levels significantly higher than expected for an homogeneous medium. During a steady change in regional ambient stress, fracture will occur first at geometrically appropriate locations in the more rigid structures. The existence of contemporary stress changes related to a flexure of the crust is implied by differential vertical move¬ ment (Meade, 1971; Brown and Oliver, 1976) meas¬ ured along the South Carolina coast. However, ap¬ propriate data prior to the 1886 Charleston earth¬ quake are not available. Nevertheless, bending of the east-west axis of the structure could account for the concentration of stress necessary for the occurrence of earthquakes in or near the structure. Earthquakes in this model would be expected to occur at points of weakness or stress concentration, such as where the structure thins or joins a larger structure. For the mass of rigid material identified near Charleston, one possible point of weakness or stress concentration would be near lat 32° 52' 20” N. between long 80° 7' 30” W. and 80° 12' W. Near this point, the structure in the inversion model (fig. 12) shows a slight thinning and is near its junction with the larger structure. The east-west strike of the structure would imply a higher susceptibility to stress amplification from regional shear stress ori¬ ented normal to the strike. These stresses would be conducive to a generally north-south strike to a plane of rupture caused by stress amplification. EARTHQUAKE LOCATIONS AND SLOAN’S INTENSITY M AP Sufficiently precise epicenters and intensity maps might now allow identification of one of the two above-described mechanisms as the more likely mechanism of the Charleston earthquake. Unfor¬ tunately, the reported epicenters for the historic ac¬ tivity near Charleston are based largely on intensity reports or seismograms written at distant stations. The precision of such data is insufficient for a reso¬ lution of mechanism of the Charleston earthquakes. The November 22, 1974, earthquake, however, was recorded by the U.S. Geological Survey’s recently in¬ stalled seismic net as well as by more distant sta- 164 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 33°07'30" 30°22'30" 30°15' 30°07'3Cr 80°Q0' 79°52'3Cr 33°00' |- 32°52'3CT !- 32°15 EXPLANATION ~>-/£l 4E Dunon’s epicenters xxx Fsosetsmal contours of Eari Sioan; | ^ range from most intense to least j* L^. intense • Recent earthquake epicenter @ Epicenter of November 22, 1974. j- earthquake hbbbb interpreted fault m basement. dotted where quesnonable qCCC 1 Qubhouse Crossroads corehole 1 32°37'30' 0 2 4 S 3 10 KILOMETERS Figure 14.—Superposition of isoseismal contours of Earl Sloan (Dutton, 1889) on Bouguer gravity anomalies of figure 3. The solid line indicates the near-surface position of the interpreted basement fault. Epicenters of recent earth¬ quakes are from Tarr (this volume). tions, and consequently its hypocenter should be more precise (see fig. 14). The hypocenter (Tarr, this vol¬ ume) falls 7.0 km southeast of the proposed base¬ ment fault, but less than 2.0 km north of the center of the eastward-protruding ridge. The hypocenter of an aftershock of the November 22, 1974, earthquake was near the center of the protruding ridge. These hypocenters are compatible with the hypothesized hypocenters for the stress amplification mechanism. Their depths of 4.1 km and 2.2 km, respectively, are BOUGUER GRAVITY MAP OF THE EPICENTRAL ZONE AND TECTONIC IMPLICATIONS 165 close to or within the structures modeled. Unfor¬ tunately, both the precision of the hvpocenter depth computations and the maximum depth of the struc¬ ture are unknown and difficult to compute with exist¬ ing data. Many of the other events occur where the gravity model indicates dense, shallow, and hence more rigid structures, such as near-surface flows. If these events are shallow they could be attributed to stress amplification in the shallow structures. This could explain part of the apparent scatter in the epicenters. Unfortunately, the gravity data are not sufficient to resolve the details of these shallow struc¬ tures. The isoseismal contours according to Earl Sloan (Dutton, 1889) are shown in figure 14 for compari¬ son to the epicenters, gravity contours, and crustal structure. Unfortunately, Sloan’s map was not com¬ pletely true to scale, and the contours have been ad¬ justed to fit the known locations of still-existing towns or stable physiographic features shown on his map. The intensity data are distributed so as to form two apparent “maximum intensity areas” at the ends of a linear zone of maximum intensity striking N. 20° E. This distribution prompted Taber (1914) to propose the Woodstock fault. The isoseis¬ mal contours do not correspond to an intensity dis¬ tribution that would be expected from reactivation of the basement fault interpreted from the gravity data. While the intensity contours may be partially controlled by the near-surface soils and the distribu¬ tion of intensity observations, the predominance of the higher intensities to the southeast of the fault would be difficult to explain completely by variations in near-surface soils. The distribution of the inten¬ sities given by Sloan does, however, correspond re¬ markably well to the intensity that one would expect from an earthquake resulting from fracture in the eastward protruding structure because of stress am¬ plification. Consequently, the epicentral zone of the 1886 earthquake may be presumed to be midway be¬ tween the dual epicenters of Dutton (1889) and sub¬ sequently near the epicenter of the November 22, 1974, earthquake. However, this immediate epicen¬ tral zone was sparsely populated, and remains so today. Although the completeness of the data and the effects of soil response to intensities in the im¬ mediate epicentral zone are questionable, the distri¬ butions of intensities near the two macroseismic epi¬ centers were well documented. Remarkably, the crustal structure interpreted from the gravity data is consistent with the intensity pattern. In the stress amplification mechanism, the seimic waves would originate largely in the more rigid, higher velocity structure and would propagate into the lower veloc¬ ity adjacent basins. Because of a possible combina- | tion of focusing of the seismic w 7 aves and a change [ in acoustical impedance near the edge of the struc¬ ture, the seismic waves could undergo dynamic amplification near the observed dual epicenters. In general, Sloan’s intensities are lower where the ! gravity data imply denser and, consequently, higher velocity crustal structures. As a tribute to Sloan’s evaluation of the intensities about 5.0 km north- northwest of Dutton’s northern epicenter (see fig. 14), we note that even the apparent reduction in the intensities—which was discounted by Dutton (1889) —is supported by the crustal structures interpreted from the gravity data and the supposition that the intensities over the more rigid crustal rock were sub¬ dued. The intensity data for the historic and recent 1 Summerville-Charleston earthquakes show that audible sounds are associated with these earthquakes j (Louderback, 1941). Louderback (1941) also sug¬ gested that these unusual sounds imply the fractur¬ ing of fresh rock under high stress rather than j movement along established faults. These sounds may imply anomalously high corner frequencies and, j hence, the existence of a greater proportion of | energy 7 at frequencies higher than generally observed I for earthquakes of equivalent magnitudes occurring in. the other seismic zones. By consideration of the spectral theory of earthquakes (Randall, 1973), these conditions imply relatively high stress drops for the Summerville-Charleston earthquakes. Fur¬ thermore, the width of the eastward protruding ridge would allow an effective fault radius of about 6 km if the entire structure ruptures according to the stress amplification mechanism. A magnitude ( m b ) range of 6.8-7.1 (Bollinger, this volume) is implied by intensity data. Since m b is equivalent to 1Mi. in the 6.0-7.0 magnitude range, these magnitudes and a fault radius of 6 km would theoretically allow 7 : stress drops of 40-200 bars (Randall, 1973). Ac- ' cording to Gibowicz (1973), a magnitude-7.0 event ; would normally have a stress drop of .about 22 bars and (after Randall, 1973) a fault radius of 25 km. A 50-km-long fault is not reasonable for the Charles¬ ton event. If correct, the maximum intensity and a reasonable fault radius imply an abnormally high stress drop for the 1886 Charleston earthquake. These observations are consistent with the stress amplification mechanism for the Summerville- ! Charleston earthquakes. 166 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 CONCLUSION Of the two mechanisms—reactivation of a base¬ ment fault and stress amplification—suggested by the structures interpreted from the gravity data, the stress amplification mechanism better satisfies the historic intensity data and recent-event epicen¬ ters. Stress amplification should be considered care¬ fully as a probable mechanism for the great Charleston earthquake of 1886 as more data become available. REFERENCES CITED Bollinger, G. A., 1972, Historical and recent seismic activity in South Carolina: Seismol. Soc. America Bull., v. 62, no. 3, p. 351-364 - 1973, Seismicity and crustal uplift in the southeastern United States: Am. Jour. Sci., in The Byron N. Cooper Volume 273-A. p. 396—108. Brown, L. D., and Oliver, J. E., 1976, Vertical crustal move¬ ments from leveling data and their relation to geologic structure in the Eastern United States: Rev. Geo¬ physics and Space Physics, v. 14, no. 1, p. 13-35. Champion, J. W., Jr., 1975, A detailed gravity study of the Charleston, South Carolina, epicentral zone: Atlanta, Georgia Inst, of Tech., Master's thesis, 97 p. Cooke, C. W., 1936, Geology of the Coastal Plain of South Carolina. U.S. Geol. Survey Bull. 867, 196 p. Dutton, C. E., 1889, The Charleston earthquake of August 31, 1886: U.S. Geol. Survey Ann. Rept. 9, 1887-88, p. 203-528. Fletcher, J. P., Sbar. M. L., and Sykes, L. R., 1974, Seismic zones and travel time anomalies in eastern North Amer¬ ica related to fracture zones active in the early opening of the Atlantic [abs.]: in EOS (Am. Geophys. Union Trans.), v. 55, no. 4, p. 447. Gibowicz, S. J., 1973, Stress drop and aftershocks: Seismol. Soc. America Bull., v. 63, no. 4, p. 1433-1446. Long, L. T., 1974, Bouguer gravity anomalies of Georgia, in Symposium on the petroleum geology of the Georgia Coastal Plain: Georgia Geol. Survey Bull. 87, p. 141-166. Long, L. T., Bridges, 3. R., and Dorman. L. M., 1972, Simple Bouguer gravity map of Georgia: Georgia Geol. Survey. Long, L. T., Talwani, P., and Bridges, S. R., 1975, Simple Bouguer gravity map of South Carolina: South Carolina Div. Geology. Map. Ser. 21, 27 p. Louderback, G. D., 1941, The personal record of Ada M. Trotter of certain aftershocks of the Charleston earth¬ quake of 1886: Seismol. Soc. America Bull., v. 31, no. 4, p. 199-206. Mann, V. I., and Zablocki, F. S., 1961, Gravity features of the Deep River-Wadesboro Triassic basin of North Carolina: Southeastern Geology, v. 2, no. 4, p. 191-215. Mansfield, W. C., 1936, Some deep wells near the Atlantic Coast in Virginia and the Carolinas: U.S. Geol. Survey Prof. Paper 186-1, p. 159-161. McKee, J. H., 1974, A geophysical study of microearthquake activity near Bowman, South Carolina: Atlanta, Georgia Inst, of Tech., Master's thesis, 65 p. Meade, B. K., 1971, Report of the sub-commission on recent crustal movements in North American: Internat. Assoc. Geodesy, 15th General Assembly, Moscow, USSR. Nettleton, L. L., 1976, Gravity and Magnetics in oil pros¬ pecting: New York, McGraw-Hill Inc., 464 p. Oliver, Jack, and Isacks, Bryan, 1972, Seismicity and tectonics of the eastern United States [abs.] Earthquake Notes, v. 43, no. 1, p. 30. Pooley, R. N., Meyer, R. P., Woollard, G. P., 1960, Yamacraw Ridge, pre-Cretaceous structure beneath South Carolina- Georgia Coastal Plain [abs.]: Am. Assoc. Petroleum Geologists Bull., v. 44, no. 7, p. 1254—1255. Randall, M. J., 1973, The spectral theory of seismic sources: Seismol. Soc. America Bull., v. 63, no. 3, p. 1133—1144. Rememund, J. A.. 1955, Geology of the Deep River coal field, North Carolina: U.S. Geol. Survey Prof. Paper 246, 159 p. Taber, Stephen, 1914, Seismic activity in the Atlantic Coastal Plain near Charleston, S.C.: Seismol. Soc. America Bull., v. 4, no. 3, p. 108-160. Talwani, Mamk, and Ewing, W. M., 1960, Rapid computa¬ tion of gravitational attraction of three-dimensional bodies of arbitrary shape: Geophysics, v. 25, no. 1, p. 203-225. Woollard, G. P., Bonmi, W. E., and Meyer, R. P., 1957, A seismic refraction study of the subsurface geology of the Atlantic Coastal Plain and continental sheif between Virginia and Florida: "Wisconsin Univ., Dept. Geology Geophysics Sec., 128 p. n I | I Exploring the Charleston, South Carolina, I Earthquake Area with Seismic Refraction— A Preliminary Study f Bv HANS D. ACKERMANN I STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT | --- GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-L I I n . i ' CONTENTS Page Abstract_ 167 Introduction _ 167 The survey _ 1 __ 167 Results _ 169 Character of refracted arrivals_ 169 Variations in velocity_ 169 Structural interpretations_ 169 Discussion _.___ 173 References cited_ 175 ILLUSTRATIONS Page Figure 1. Location map showing area of study _ 168 2. Map showing lateral variations in compressional velocity of in¬ termediate horizon _ 170 3-5. Contour maps showing: 3. Depths to the marker horizon within the Santee Lime¬ stone _ 171 4. Depths to the intermediate marker horizon_ 172 5. Depths to crystalline basement horizon _ 174 STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886— A PRELIMINARY REPORT EXPLORING THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE AREA WITH SEISMIC ■REFRACTION-—A PRELIMINARY STUDY By Hams D. Ackermann ABSTRACT Seismic refraction soundings northwest of Charleston, S.C., show: 1. A marked decrease in the velocity of an approximately 800-m-deep basalt layer (of Cretaceous or older age). 2. A possible small downwarp in this basalt layer and the shallower sedimentary section. 3. A flexure or fault in the 1.000- to 2,000-m-deep crys¬ talline basement. The spatial near coincidence of these three features with the high-intensitv zone of the 1886 earthquake and recent 1 earthquake epicenters may strengthen the argument that j the Charleston seismic trend is an area of high stress along j a preexisting fault zone. INTRODUCTION During the spring of 1975, 12 seismic-refraction spreads, each approximately 2,600 m long, were re¬ corded near Charleston, S. C. The purpose of this work was to investigate deformations that may be related to the historical earthquakes of the area. The problem is twofold—(1) to find deformations, and (2) to determine whether they are recent. One possible approach to this problem is to iden¬ tify deformations that involve recent geologic fea¬ tures. On the Atlantic Coastal Plain, this approach would mean locating basement faults that extend into the shallow sedimentary section. The seismic- reflection method is well suited for this purpose. This approach presents two problems, however. First of all,.an earthquake-generating fault need not dis- j rupt shallow rocks. Second, although seismic re¬ flection is very sensitive for delineating vertical displacements, it cannot detect strike-slip displace¬ ments. Nevertheless, a high-resolution seismic-re¬ flection survey was seriously considered in the early stages of this project. We felt that in order to in¬ vestigate recent earthquakes, the survey should focus to depths between at least 200 and 1,000 m, the latter having been the assumed depth to the crystalline basement. In addition, resolution must be sufficient to detect vertical displacements as small as 15 m, and the survey should be sufficiently extensive to cross the Charleston structure if one exists. The cost of contracting a survey to meet these minimum requirements was prohibitive. A seismic-refraction survey, on the other hand, also permits the study of recent tectonism, but from a different point of view. In refraction, the path traveled by the recorded signal is largely horizontal instead of vertical. Hence, although the refraction method is considerably less accurate than the re¬ flection method for delineating vertical displace¬ ments, the long horizontal travel path permits the accurate calculation of lateral velocity variations in the layers recorded. One important cause for lateral velocity changes in a layer is inhomogeneity result¬ ing from changes in fracture porosity (Wyllie and others, 1956; Ackermann and others, 1975). One may certainly expect large variations in fracture porosity in a zone of recent earthquakes because fractures caused by the large stresses have not had the opportunity to close. Hence, we chose to use seismic refraction to search for lateral velocity vari¬ ations in the deep rocks of the Charleston area. Furthermore, refraction is a powerful reconnais¬ sance tool, and the results are extremely valuable for possible later high-resolution reflection studies. This work was supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Regula¬ tory Research, Agreement no. AT (49-25)—1000. THE SURVEY Locations of the 12 seismic spreads are shown in figure 1. Also shown are the centers of highest in- 167 168 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 9CP5' 30°00' 33°00' 32 ° 15 ’ 10 15 KILOMETERS Figure 1 . —Location map showing area of study near Charleston, S. C. EXPLORING EARTHQUAKE AREA WITH SEISMIC REFRACTION 169 tensity (Dutton, 1889) for the 1886 MM intensity X earthquake; the location of the 792-m-deep Club¬ house Crossroads corehole 1 (CCC 1) ; and the epi¬ center, near the historic Middleton Place planta¬ tion, of the Nov. 22, 1974, magnitude-3.8 earthquake (Tarr, this volume). The plan was to tie the CCC 1 to the refraction profiles and to extend coverage towards the most southern of Dutton’s centers and also to the Middleton Place epicenter. The seismic spreads were recorded with sufficient shot points to obtain full reverse coverage from a basement horizon 600—1,000 m deep, and partial reverse cov¬ erage from shallower horizons within the sedimen¬ tary sequence. RESULTS The refraction interpretations revealed three dis¬ tinct seismic marker horizons that were continuous throughout the area surveyed. The shallowest and intermediate refracting horizons were penetrated by CCC 1 and are evident on both the geologic and sonic well logs. The deepest horizon is below the total depth of the hole (792 m). The shallowest refractor correlates with a thin, well-indurated crossbedded calcarenite at the base of the Santee Limestone (Eocene), which is about 100 m deep at the well site. The velocity of this calcarenite member of the Santee is 2.5-2.7 km 's. Its higher velocity is a result of cementation by car¬ bonate rocks dissolved from the overlying section. The intermediate horizon corresponds to the top of the Cretaceous ( ?) basalt lava flows, intersected by CCC 1 at a depth of 750 m. (See Gottfried and other geophysical data to obtain a preliminary crust- basalt.) Its velocity near the well 3ite is 5.8 km/s. Drilling was terminated after 42 m of basalt had been cored. The deepest horizon recorded has a velocity of about 6.3-6.5 km's. This undoubtedly represents the crystalline basement. We note, once again, that shot locations were planned to provide complete reverse coverage for a single high-velocity basement horizon 600-1,000 m deep. The recordings revealed two high-velocity 7 layers; one, the basalt, which was within the ex¬ pected interval, and the other, the deeper crystalline basement. Full coverage was not obtained from the crystalline basement. Consequently, velocity and depth control are incomplete for this layer. The sonic well log showed several layers between the shallow Santee Limestone (Eocene) and the Cretaceous (?) basalt that have velocities slightly more than 2.7 km s. However, the position of these layers in the sedimentary section did not permit them to appear as conspicuous events on a refrac¬ tion record. Thus, for all practical purposes, the refraction data revealed only the three above-men¬ tioned layers. CHARACTER OF REFRACTED ARRIVALS The arrivals recorded from the marker horizon in the Santee attenuate rapidly and cannot be iden¬ tified on the recordings beyond 700 to 1,100 m from any shot point. Apparently, the high-velocity-sand- i stone part of the Santee is too thin to transmit a seismic wave efficiently. Similarly, arrivals from the basalt layer also at¬ tenuate rapidly and are generally shingled. A shingle is a form of multiple arrival (Spencer, 1965; Cassinis and Borgonovi, 1966) associated with a high-velocity layer imbedded in a lower velocity medium. Therefore, we infer that the basalt is again underlain by lower velocity rocks, possibly a pre-Upper Cretaceous sedimentary sequence VARIATIONS IN VELOCITY Interpretations for the intermediate horizon, which correlates with the basalt at CCC 1, are that its velocity ranges from 4.3 to 5.8 km s. Significant velocity variations for the shallow marker horizon in the Santee Limestone, on the other hand, were not identified. Furthermore, data from the deep crystalline basement rocks were insufficient to cal¬ culate velocity changes for this layer. Figure 2 shows the lateral variations in compres- sional velocity of the intermediate (basalt) horizon. We see a general eastward decrease in velocity. The focus of the November 22, 1974, magmtude-3.8 earthquake (Tarr, this volume) was at a depth of 4.1 km, directly under the area of lowest velocity. STRUCTURAL INTERPRETATIONS Figures 3 and 4 are contour maps of interpreted depths to the marker horizon within the Santee Limestone and to the intermediate (basalt) marker horizon, respectively. Depths were calculated using a vertical velocity function obtained from the sonic log of CCC 1. By so doing, the countoured depths for these two horizons agree with their actual measured depths at the well site. We point out, however, that because of the shingling of the basalt arrivals, depth calculations for this horizon are based, in part, on subjectively shifting recorded data. Thus, to some extent, the interpreted depths and structure for the basalt horizon are uncertain. 170 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 30°15' 30°00' 1 ! 33°00' . I 32°*5' + X ® EXPLANATION Seismic spread Epicenter of November 1974 earthquake Center of highest intensity of 1886 earthquake Clubhouse Crossroads corehole 1 Boundary between zones of different compressionai velocity of the intermediate (basalt) horizon. Given in km/s 10 15 KILOMETERS FiGtTRE 2.—Lateral variations in compressionai velocity of the intermediate (basalt) horizon. EXPLORING EARTHQUAKE AREA WITH SEISMIC REFRACTION 171 8C°15’ 80°00' 33°00 Vi \ 32°i5' + X ® - 100 - explanation Seismic spread Epicenter of November 1974 earthquake Center of highest intensity of 1886 earthquake Clubhouse Crossroads corehole 1 Contour showing interpreted depth to marker horizon within Santee Limestone. Given in meters below ground surface, contour interval. 10 m 5 10 15 KILOMETERS Figure 3. Contour map of interpreted depths to the marker horizon within the Santee Limestone. Ground-surface ele¬ vation approximately 7.5 m. 172 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 80°15’ ao°ocr 33°00' ■ 32°15' X ® EXPLANATION Seismic spread Epicenter of November 1974 earthquake Center of highest intensity of 1886 earthquake Clubhouse Crossroads corehole 1 - 750 - Contour showing interpreted depth to intermediate (basalt) marker horizon. Given in meters below ground surface: contour interval, 50 m 0 10 15 KILOMETERS Ftgure 4. —Contour map of interpreted depths to the intermediate (basalt) marker horizon. EXPLORING EARTHQUAKE AREA WITH SEISMIC REFRACTION 173 Figure 3 shows that the Santee is nearly hori¬ zontal throughout the area surveyed. The data sug¬ gest, however, that the Santee contains an approxi¬ mately 20-m-deep north-trending troughlike de¬ pression near the eastern edge of surveyed area. The most striking feature of the basalt-horizon contours (fig. 4) is also a north-trending broad troughiike depression which is offset slightly west- , ward from the axis of the overlying Santee trough. The size of this depression may exceed 50 m, though it is difficult to determine because it modulates the regional dip which is to the southeast. The axis of this trough also parallels the trend of the basalt- velocity variations noted above, though offset several ! kilometers westward from the velocity minimum. Figure 5 is a contour map of interpreted depths to the deep crystalline basement. Because data from this horizon are incomplete, structural details could not be determined. In particular, the downward con¬ tinuation of the troughlike feature from the Santee and basalt horizons could not be calculated. Crystal¬ line basement depths were calculated, assuming a constant 4.2-km s velocity between basement and the overlying basalt. If a sedimentary section under¬ lies the basalt, its average velocity may be less than 4.2 km s. If so, the calculated crystalline basement depths are too large. In any case, the basalt and basement horizons diverge southeastward. Near CCC 1, the distance between the two may be less than 200 m. In the southeastern part of the area sur¬ veyed, it may be 1,000 m. The more tightly spaced depth contours in the southeastern part of the area indicate either a flexure or a fault in the crystalline basement. Unfortunately, data for this horizon are insufficient to make a definitive judgment. DISCUSSION The seismic-refraction data allow one to map three horizons, the upper two of which were iden¬ tified in CCC 1. The intermediate horizon correlates with a Cretaceous ( ?) basalt at the depth of 750 m in the corehole. The velocity of this horizon is defi¬ nitely variable, decreasing from a high value of about 5.8 km/s at the well to about 4.5 km. s along the eastern edge of the area surveyed, which in¬ cludes the epicenter of the November 22, 1974, earthquake. Furthermore, a north-trending trough¬ like feature, which parallels the velocity trend, has been tentatively identified in the intermediate hori¬ zon. The axis of this trough is offset a few kilo¬ meters west of the velocity minimum. The data also | suggest a similar but smaller depression in the shallowest horizon, which is within the 100-m-deep Santee Limestone. Shingling in the refracted arrivals for the inter¬ mediate (basalt) horizon indicates that this horizon is underlain by lower velocity materials, possibly a pre-Upper Cretaceous sedimentary section, which thickens southeastward. The deepest of the three mappable horizons is the true crystalline basement. Because data from this horizon are incomplete, details of its structure and velocity cannot be calculated. However, the data do indicate either a flexure or a fault of the crystal¬ line basement in the southeastern part of the area surveyed. Neither the intermediate (basalt) nor the shallow Santee horizons show evidence of this struc¬ ture. Furthermore, the velocity of the intermediate (basalt) layer attains a minimum value in the im¬ mediate area of this basement flexure or fault. In addition, the troughlike depression in the basalt and one of the two centers of highest intensity (Dutton, 1889) are just a few kilometers westv/ard. Thus we see the near coincidence of four events or features: (1) a line of earthquake activity de¬ fined by the Dutton high-intensity points and the recent epicenter at Middleton Place; (2) a compres- sional velocity minimum of a Cretaceous (?) basalt layer; (3) possible downwarping of this basalt layer and a shallower Eocene horizon: and (4) a flexure or fault in the crystalline basement. The spa¬ tial proximity of these four suggests a genetic re¬ lationship. Sbar and Sykes (1973) delineated a region of high horizontal compressive stress in eastern North America. They proposed that the Charleston seismic trend is due to these high stresses acting along a zone of weakness, such as an unhealed fault or the continental extension of a major oceanic fracture zone. Our interpretation of the Charleston seismic data then suggests the possibility that the flexure or fault found in the crystalline basement may ac¬ tually be a manifestation of such a zone of crustal weakness. Additional data are necessary to define this structure clearly and to determine its trend. The small downwarp tentatively identified in the two shallower horizons may then be the product of more recent earthquake activity. We further note the decrease in compressional velocity of the in¬ termediate (basalt) layer and suggest that this may be due to increased fracture porosity also resulting from earthquake activity over an extended period of time. Another possible explanation for the low- 174 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 80°15' 80 ° 00 ' 33°00' 32°15' + X ® EXPLANATION Seismic spread Epicenter of November 1974 earthquake Center of highest intensity of 1886 earthquake Clubhouse Crossroads corehoie 1 \\ / " 7 000 — Contour showing interpreted depth to crystalline basement horizon. Given in meters below ground surface: contour interval. 100 m 10 15 KILOMETERS Figure 5.—Contour map of interpreted depths to the crystalline basement horizon. EXPLORING EARTHQUAKE AREA WITH SEISMIC REFRACTION 175 ered compressional velocity is that it simply repre¬ sents the termination of the basalt layer and that the arrivals recorded there are from the rocks that elsewhere underlie the basalt. The velocity of 4.5 km s, for example, is an acceptable value (Stewart and others, 1973) for Triassic sedimentary rocks. REFERENCES CITED Ackermann, H. D., Godson, R. H„ and Watkins. J. S., 1975, A seismic refraction technique used for subsurface in¬ vestigations at Meteor Crater, Arizona: Jour. Geophvs. Research, v. 80, no. 5, p. 765-775. Cassinis, R. and Morgonovi, L., 1966, Significance and impli¬ cations of shingling in refraction records: Geophys. Prospecting, v. 14, no. 4, p. 547-565. Dutton, C. E., 1889, The Charleston earthquake of August 31, 1886: U.S. Geol. Survey, Ann. Rept. 9, 1887-1888, p. 203-528. Sbar, M. L., and Sykes, L. R., 1973, Contemporary- com¬ pressive stress and seismicity in eastern North America; an example of intraplate tectonics: Geol. Soc. America Bull., v. 84. no. 6, p. 1861-1881. Spencer, T. W., 1965, Refraction along a layer: Geophysics, v. 30, no. 3, p 369-388. Stewart, D. M., Ballard, J. A., and Black, W. W., 1973, A seismic estimate of depth of Triassic Durham basin. North Carolina: Southeastern Geology, v. 15, no. 2, p. 93-103. Wyllie. M. R. J., Gregory, A. R., and Gardner, L. W., 1956, Elastic wave velocities in heterogeneous and porous media: Geophysics, v. 21, no. 1, p. 41-70. ■ . A Preliminary Shallow Crustal Model Between Columbia and Charleston, South Carolina, Determined from Quarry Blast Monitoring and Other Geophysical Data By PRADEEP TALWANI STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-M . t i :<2 .'toi ijsrl £ ns aid Twloj CONTENTS Page Abstract_1_ 177 Introduction_ 177 Data collection _ 177 Results _ 178 Discussion _ 179 References cited _ 185 ILLUSTRATIONS Pa*e Figure 1 . Location map around Charleston, S.C., showing study area, quarries monitored, refraction lines, per¬ manent seismograph stations, refraction shot points, and interpreted Triassic basins _ 178 2. Traveltime curves for quarry blasts in the Coastal Plain of South Carolina _ 180 3. Velocity models southeast of Columbia _ 185 4. Simple Bouguer anomaly map of area around Charleston, S.C., showing location of refraction data -- 186 5. Observed gravity profile and three interpretative shallow crustal models_ 187 in STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886- A PRELIMINARY REPORT A PRELIMINARY SHALLOW CRUSTAL MODEL BETWEEN COLUMBIA AND CHARLESTON, SOUTH CAROLNA. DETERMINED FROM QUARRY BLAST MONITORING AND OTHER GEOPHYSICAL DATA By Pradeep Talwani 1 ABSTRACT To obtain the velocity model under the Coastal Plain of South Carolina, blasts at five quarries were monitored. The data, incorporated with other refraction and gravity data, suggest that the crustal model is extremely complicated. A Triassic(?) graben is inferred near Summerville, and its border faults may be associated with the seismicity observed in the Summerville-Charieston area. INTRODUCTION The 10-station South Carolina seismographic net¬ work went into operation in May 1974 (Tarr, this volume). To best use the data accumulated by this network for the location of hypocenters, we must first determine the subcrustal velocity structure. Data pertinent to the velocity structure of the Coast¬ al Plain of South Carolina were obtained at five quarries. These data have been incorporated with other geophyhical data to obtain a preliminary crust¬ al model between Columbia and Summerville. This study was supported by U.S. Geological Sur¬ vey Contract No. 14-08-0001-14553. I am grateful to my students, Donald Stevenson, David Amick, and Robert Van Nieuwenhuise, for their help in carrying out the fieldwork, and to the various quarry superintendents for their cooperation. I thank Hans Ackermann for allowing me to use some of his unpublished seismic refraction data. I also benefited from the discussions with A. C. Tarr of the U.S. Geological Survey and Prof. Donald T. Secor of the University of South Carolina. I also thank Dr. John Sumner of Lehigh University, who reviewed the manuscript and offered valuable comments. ' D*pt. of GooL. Univorvity of South Carolina. Columbia, S.C. 29208. DATA COLLECTION Locations .—Most of the blast data were collected in the summers of 1975 and 1976. Of the five quar¬ ries monitored, two are in the crystalline rocks near the Fall Line, and the others are in the Coastal Plain (fig. 1). The Columbia quarry (COQ) and the Cayce quarry (CAQ) produce granite and are in the northwest quadrant of the study area. I Berkeley quarry (BEQ) produces fine-grained clas- : tic limestone of the Santee Limestone, and the Georgetown (GTQ) and Bass (BAQ) quarries pro¬ duce indurated recrystallized limestone of the lower part of the Santee Limestone. BEQ, GTQ, and BAQ provided data from the southeast quadrant of the study area. Locations of various shots at BEQ and GTQ were determined from quarry maps (1 inch = 200 feet). The location of each shot was deter¬ mined to ±0.01' (about 20 m) by tieing the shot point to a Coast and Geodetic Survey triangulation station. The blasts were monitored at remote station sites lying along the various refraction lines (fig. 1). The locations of stations within 6 km of the blast site were determined from the quarry map or from aerial photographs of the area surrounding the quar¬ ry (1 inch = 800 feet). Thus, the accuracy of locating stations close to the blast site was ±0.02' (±40 m). The locations of more distant stations were deter- 1 mined from 7.5' topographic quadrangle maps or county maps, and the accuracy was ± 100 m. All remote stations were within 45 km of the blast site, as beyond that distance the blast could not be detected on portable seismographs. Traveltime .—The origin times of all quarry blasts , were obtained by recording the shot at the quarry 178 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 Figure 1 . —Location map around Charleston, S.C., showing study area, quarries monitored, refraction lines, permanent seismograph stations of the South Carolina seismographic network, refraction shot points of Wooilard and others (1957), and interpreted Triassic (Tr). The locations of the center of seismic activity, Middleton Place, and of the wells to the basalt at the Clubhouse Crossroads core hole 1 (CCC 1) are also shown. The town of Orange¬ burg is 6 km west of OSC. Abbreviations: COQ, Columbia quarry; CAQ, Cayce quarry; BEQ, Berkeley quarry; GTQ, Georgetown quarry; BAQ, Bass quarry. Seismograph station abbreviations, coordinates, and instrumenta¬ tion are discussed by Carver, Turner, and Tarr (1977). on a portable seismograph equipped with a paper speed of 300 mm, min. Origin times at quarry sites and the P- and 5-wave arrival times on seismographs at remote stations were read to an accuracy of at least r0.02 s by using a low-power microscope. A WWVB time signal was recorded on all seismograms to obtain a uniform absolute time. Traveltimes from various blasts to stations of the seismographic net¬ work were also incorporated wherever they were available. RESULTS Velocity values from most traveltime data pre¬ sented below were obtained by drawing an eyeball fit curve through the data. Local geology and the quali¬ ty of the first arrivals were incorporated where a single line did not pass through all the data points. The velocity was also calculated by a method of least squares. The standard deviation and the coefficient of determination, r a , were also determined by linear re¬ gression. The velocity values obtained by the two methods agree well, and the values obtained by eye¬ ball fit were used in computing depths. The least square values are given in figure captions 2 C-F. The two quarries, located on either side of the Congaree River to the south of Columbia (COQ and CAQ), are about 1 km apart. The traveltime data for the Columbia blasts recorded in a southeast direction were plotted, and those for COQ and CAQ were grouped together (fig. 2 A). The data suggest a P- wave velocity ( v P ) of 6.0 km/s. This velocity is rea¬ sonable for crystalline basement, which outcrops near Columbia. Figure 2 B shows two possible interpretations of traveltime data for blasts at BEQ and recorded with¬ in 3 km in a northwest direction. In the first inter¬ pretation (a), the traveltimes for the near stations (within 1 km) and for the distant stations (near 5 A PRELIMINARY SHALLOW CRUSTAL MODEL 179 km and 8 km) lie on a 3.65 km s line. Both near sta¬ tions lie in the Berkeley quarry, and 3.65 km s probably represents the velocity’ of Santee Lime¬ stone. From borehole data north of Santee River (Alan-Jon Zupan, oral commun., 1977), Santee Limestone is known to be less than 100 m thick. Traveltime data from BEQ, in a northwest direction beyond 7 km, lie on 5.70 km s curve (fig. 2C). A simple two-layer example of a 3.65-km s overlying a 5.70-km s layer would imply that the'3.65-km s lay¬ er is 1.8 km thick. North of BEQ, Santee Limestone is known to overlie Black Mingo and Peedee Forma¬ tions, which are a few hundred meters thick. Thus, a 3.65-km s layer, if it exists, has to be younger than the Black Mingo Formation. In interpretation a (fig. 2 B), data from a station 1.74 km distant were not incorporated. These data indicate a 2.2-km s layer (interpretation b ). In this latter interpretation, data from the two nearer stations have been neglected on the assumption that they represent the thin Santee Limestone, which has a higher P -wave velocity’ be¬ cause of its greater induration. The sedimentary’ rocks, which have a P -wave velocity of 2.2 km s, in turn overlie a 0.56-km-deep layer with a 5.70-km/s P-wave velocity. This depth, 0.56 km, appears to be more reasonable in view of a 5.5 km,s layer 0.5 km deep at Woollard’s (1957) station 53 (fig. 1). Figure 2C shows the preferred traveltime data for blasts at BEQ recorded in a northwest direction out to a distance of about 31 km. This profile lies almost completely on the mapped Santee Limestone (see, for example, the Coastal Plain geology’ taken from Cooke (1936) and incorporated on the gravity map on South Carolina (Talwani and others, 1975). If interpretation b from figure 2 B is accepted, the 5.70- km s layer is offset by 0.7 km, 5 km from BEQ. The postulation of a fault is based on data from a single station. If data from that station are neglected, the depth to the 5.70-km/s layer is 0.90 km. The P-wave velocity of 5.70 km/s suggests a basalt flow, and the implications thereof will be discussed later. In recording blasts at BEQ to the south (fig. 2D) there are no data within 3 km. Assuming that the velocity of the near-surface material is 2.2 km/s, the calculated depth to the 5.2-km s layer is 0.8 km. If we assume that the true P-wave velocity’ of this layer (basalt flow) is 5.70 km s, then an apparent velocity of 5.2 km s suggests a southerly dip of 2.3°. Figure 2 E shows the reversed profile between GTQ to the north of Santee River and BAQ to the south. Sedimentary rocks that have a P-wave veloci¬ ty of 2.05 km s overlie a 6.0 km s layer (crystalline basement), which dips from a depth of 0.48 km be¬ low BEQ to 0.63 km below GTQ. In figure 2 F, two interpretations are presented for travel-time data for blasts at GTQ and BAQ re¬ corded to the southwest. The data from the two quar¬ ries are grouped together in figure 2P. Two inter¬ pretations were made owing to the uncertainty re¬ sulting from a lack of data between 11 and 24 km. In interpretation a, a layer with an apparent velocity’ of 5.70 km/s. underlies a 2.0 km/s. sedimentary lay¬ er. The underlying layer is 0.57 km deep and dips 1° j to the southwest. Alternatively, if we assume that a 6.0-km/s. layer underlies the sedimentary’ rocks (in¬ terpretation b), it is 0.57 km deep, and somewhere between 11 and 24 km from BAQ. it is downthrown by 0.64 km. DISCUSSION The quarry blast data presented above are sparse i and somewhat inconclusive. However, by incorporat¬ ing gravity, magnetic, and other seismic refraction data, some constraints can be applied and a prelimi¬ nary’ interpretation made. Figure 3A shows seismic refraction data of Wool- lard and others (1957). Only profiles at locations 48, 49, and 53 were reversed. A low velocity of 4.82 km/s. at location 51 together with an aero magnetic low has been inferred by Daniels (1974) to indicate a possible Triassic basin. This basin lies to the north¬ east of the Dunbarton basin, where red beds of as¬ sumed Triassic age are known to occur in the subsur¬ face (fig. 1) (Marine and Siple, 1974). Data from Figures 2 A and 2C were combined with those from figure 3A to obtain a schematic model along a line southeast from Columbia (fig. 3B). Seismic refraction data obtained at BEQ (and re¬ corded to the south) were insufficient to obtain a velocity model in the Summerville area. However, other refraction data are available in the area (Ack- ermann, this volume, and written commun.). These were combined with drill-hole data at Clubhouse Crossroads and the gravity map of the area to ob¬ tain a velocity model. Figure 4 shows a part of the Bouguer anomaly map of South Carolina (Talwani and others, 1975). The location of the seismic refraction profiles from quarries, and those of Ackermann (unpub. data, and this volume) are also shown. BEQ and the Club¬ house Crossroads corehole 1 both lie on broad gravi¬ ty highs, which are separated by an east-west gravi¬ ty low. This gravity low coincides with a broad low seen on the aeromagnetic map of the area (Phillips, this volume). Unpublished refraction data from Figure 2.—Traveltime curves for quarry blasts in the Coastal Plain of South Carolina. A, Shots at Columbia (COQ and CAQ) recorded in a southeast direction. Interstate (I), Knightsville (K) profiles (Acker- mann, written commun.) and those at Middleton Place (MP), Clubhouse Crossroads (CC), County Line East (CLE) and Bees Ferry (BF) (Acker- mann, this volume) were extrapolated to a north- j south profile from BEQ (fig. 4). The direction of extrapolation was along strike of the structures sug- j gested by the gravity contours. This profile passes | through BEQ in the north, and through the eastern ; end of a gravity high to the south of Summerville. Ackermann (this volume) noted that some seismic i refractions were shingled. These were interpreted as being caused by a thin basalt flow, and the P -wave velocity was found to vary between 4.5 and 5.8 km s. Figure 5 shows the observed gravity profile and three proposed shallow crustal models. In making the gravity models, the density values used were those obtained at the Clubhouse Crossroads corehole I: 2.1 g/cm 3 and 2.9 g/cm 3 for Tertiary and Upper Cretaceous sedimentary rocks and the basalt, respec¬ tively (Brenda Higgins, written commun.). Below the sedimentary rocks that have v P = 2.0 -2.2 km/'s., seismic data at BF and I indicated an absence of the 5.3 km 's, horizon, which had been ob¬ served at BEQ and CC. At CLE and K, a velocity of 5.0—5.4 km/'s was obtained, and the refractions were associated with shingling. At Middleton Place (MP), the velocity decreased from 5.5 to 4.5 km/s. east¬ ward. This is interpreted as the edge of the basalt flow. A 6.2 km/s. horizon (crystalline basement) was observed at BF, CLE, and MP. The low-velocity, low-density sediments of the Coastal Plain are 600-950 m thick. If the standard Bouguer density of 2.67 g/cm 3 is used in reduction of gravity data, the contribution of these sediments is -20 mGal for a -0.6-g/cm 3 density contrast and an 800-m thickness. Since the Coastal Plain is only a few tens of meters above sea level, a normally com¬ pensated crust would have a slightly negative Bou¬ guer anomaly associated with it. However, the Sim¬ ple Bouguer gravity values in the area are positive, ranging from 0 to 10 mGal and indicating a thinned continental crust in this part of the Coastal Plain. To model the near-surface geology (to a depth of 2.5-3 km), a datum density value of 2.5 g/cm 3 rather than the standard 2.67 g/cm 3 was used. This has the effect of removing the regional gravity gradient due to a deep basement structure. The gravity models were constrained by the depths obtained from refraction data (short thick lines, fig. 5). In the first model, a 500-m-thick basalt flow is assumed below CC, a sedimentary basin below I, and a basalt flow below BEQ. The basalt flow below CC is at least 100 million years old, and possibly as old as Triassic (Gottfried and others, this volume), which suggests that the sedimentary horizon {v P = 4.4 km/s) is older—possibly of Triassic age. A density of 2.4 g/cm 3 was used to model this horizon. TRAVELTIME. IN SECONDS A PRELIMINARY SHALLOW CRUSTAL MODEL 181 FIGURE 2.—Continued. B, Shots at BEQ recorded in a northwest direction within 8 km of the site—two possible interpreta¬ tions (see text for discussion). TRAVELTIME, IN SECONDS TRAVELTIME, IN SECONDS 182 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 = u DISTANCE, IN KILOMETERS S. D Figure 2.—Continued. C, Shots at BEQ recorded in a northwest direction for distances greater than 7 km from the site. Curve is projected to zero time at distances shorter than 7 km and shows that if interpretation b (fig. 2 B) is accepted, faulting is suggested close to BEQ. Velocity obtained by eyeball fit (5.70 km/s) agrees well with that obtained by the method of least squares (5.68 km/s, standard deviation 0.13, and r~0.994). D, Shots at BEQ recorded to the south. Dashed line represents the assumed velocity of near-surface material. Velocity by eyeball fit (5.2 km/s) agrees with that obtained by method of least squares (5.07 km/s, standard deviation 0.06, and r=0.9985). This value was used by Marine (1974) to model Bouguer anomaly due to a basalt flow (~500 m postulated Triassic rocks at Dunbarton basin. A thick) below CC is insufficient to match the observed graben is required to model the gravity low. The ; gravity anomaly. KILOMETERS TRAVEL1IME, IN SECONDS A PRELIMINARY SHALLOW CRUSTAL MODEL 183 6.0 km/s 1 '-;- C VERTICAL EXAGGATION *2.5 Figure 2.—Continued E, Reversed refraction profile between blasts at GTQ and BAQ, as well as an interpretative model. Velocities by eyeball fit (6.5 km s and 5.5 km s) agTee well with those obtained by method of least squares (6.29) km/s. standard deviation 0.26, ^=0.992; and 5.49 km s, standard deviation 0.07, r-=0.999, respectively). The near-surface velocity of 2.05 km s obtained at GTQ was also used at BAQ. 184 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1386 F Figube 2. Continued. F, Shota at GTQ and BAQ recorded in a southwest direction. Two interpretations are given. Veloc- ty values obtained by eyeball fit (5.70 and 6.0 km/s agree with those obtained by the method of least squares (5.65 km/s, standard deviation 0.09, r^O.9988; and 6.12 km/s, standard deviation 0.12, ^=0.9988, respectively). In the second model, the basalt flow below CC is replaced by a broad volcanic plus having' an umbrel¬ lalike flow at the top. The shingling of refractions and v P < 5.8 km/s observed at CLE, MP and K is interpreted to be due to thin basalt flows around the plug, whose stem, lying below CC, is associated with no shingling and a velocity of 5.8 km/s. BF and I lie outside the plug where the post-Cretaceous sedi¬ mentary rocks ( v P = 2.0 — 2.2 km/s and underlain by Triassic sedimentary rocks (v P = ±A km/s). The crystalline basement (v P = 6.2 km/s) dips gently to the south and extends from BEQ to the south of BF, being interrupted by a volcanic plug at CC. The Bouguer anomaly associated with this model fits the observed data over the gravity highs on the ends of the profile but does not match the gravity low. To match the gravty low, a buried graben is required below I (model 3). Thus, model 3 represents the preferred interpre¬ tation of the observed gravity data using constraints supplied by seismic refraction data and drilling. Some of the features of this model are: a. A broad volcanic plug was punched into a broad Triassic basin. b. The seismic velocity is 5.8 km/s in the stem of this plug, while on the flanks (associated with basalt flows) it decreases to 5.0 km/s and causes shingling of refractions. A PRELIMINARY SHALLOW CRUSTAL MODEL 185 COLUMBIA 47 ORANGEBURG ESCARPMENT 50 _ 4S 5.76 n - 179 _ SEA LEVELn 100 - to cr ^ 200 - £ 300 - Q. 400 - 500 - A 49 'T.79 51 6.06' '?-_1.76 6.12 — _ S2_ 1 ~ 73 -^— ' 1 .97 4.32 T, .^27 6 . 12 ' S3 .2 38 5.52 NW SE. a 10 20 30 « 50 KILOMETERS Figure 3.—Velocity models southeast of Columbia. A, Model between Columbia and location 53 of Woollard and others (1957) The numbers 47-53 are the refraction locations (see fig. 1), and the others are the observed seismic velocities Ocm/s). B, Interpreted model along a line southeast from Columbia through location 52 to the Berkeley quarry. Seis¬ mic velocity values in km/s are incorporated. c. The broad lows on the aeromagnetic and gravity maps are due to a broad Triassic basin below Summerville. d. Middleton Place lies on top of the southern flank of the graben, and the observed seismicity there may be associated with the border faults of this gTaben. e. The gravity high at BEQ is associated with a shallow crystalline basement having a thin basalt cap. (This is suggested by an absence of the basalt on the southwest profile from GTQ figr- 4). These results are preliminary, and this summer (1977) H. D. Ackermann and I will collect more data to test the model. REFERENCES CITED Carver, David, Turner, L. M., and Tarr, A. C., 1977, South Carolina seismological data report, May 1974-June 1975: U.S. Geo. Survey open-file report 77—129, 66 p. Cooke, C. W’., 1936, Geology of the Coastal Plain of South Carolina U.S. Geol. Survey Bull. 867, 196 p. Daniels, D L., 1974, Geologic interpretation of geophysical maps, central Savannah River area. South Carolina and Georgia U.S. Geol. Survey Geophys. Inv. Map GP-893 [19751. 186 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 80° 8T 34 ° £ 79° Columbia 'Rll ■ 11 * Wm $ EXPLANATION A Quarry (see fig. 1 for names) Ackermanrrs seismic refraction profile Woollard's refraction profile Seismic refraction profile from quarry - 10 — Bouguer anomaly (mgal) from Talwani and others (1975) + Middleton P'ace ® Clubhouse Crossroads corehote 1 52 ^ Town Figure 4.—Simple Bouguer anomaly map (modified from Talwani and others. 1975) of the area around Charleston, S.C.. showing the location of refraction data from Ackermann (this volume and written commun.). Location of a north-south profile from BEQ is also shown. This profile was extrapolated from unpublished refraction data from Interstate (I) and Knightsville (K) profiles (Ackermann, written commun.). as well as profiles at Middleton Place (MP), Clubhouse Crossroads (CC), County Line East (CLE), and Bees Ferry- (BF) (Ackermann, this volume). The direction of extrap¬ olation (dashed lines) was along strike of the structures suggested by the gravity contours. DEPTH, IN KILOMETERS mGal A PRELIMINARY SHALLOW CRUSTAL MODEL 187 S. Summerville N. 2 . 0 - 2.2 ( 2 . 1 ) 2 . 0 - 2.2 ( 2 . 1 ) 2 . 0 - 2.2 ( 2 . 1 ) Figure 5.—Observed gravity profile and three interpretative shallow crustal models. The P -wave velocities in km/S of the Tertiary and Upper Cretaceous sedimentary rocks, Triassic(?) sedimentary rocks, basalt, and crystalline basement are, respectively 2.0-2.2, 4.4, 5.7-5.8, and 6.2. The corresponding density values (in parentheses) are 2.1, 2.4, 2.9 and 2.. g/cm\ Computed gravity due to model 1 (circles), model 2 { triangles), and model 3 (squares) is also shown along with the observed gravity profile. Locations such as BF and, CLE, extrapolated to the north-south profile, are shown in figure 4. The short thick lines at velocity boundaries show depths obtained from, refraction data. Marine, I. W., 1974, Geohydrolog;.’ of buried Triassic basin at Savannah River plant, South Carolina: Am. Assoc. Petroleum Geologists Bull., v. 58, no. 9, p. 1825-1837. Marine, I. W\, and Siple, G. E., 1974, Buried Triassic basin in the central Savannah River area, South Carolina and Georgia Geol. Soc. America Bull., v. 85, no. 2. p. 311- 320. Talwani, P., Long, L. T., and Bridges, S. R., 1975, Simple Bouguer anomaly map of South Carolina South Caro¬ lina Div. Geology Map Ser. MS-21. Woollard, G. P., Bonini, W, E.. and Meyer, R. P., 1957, A seismic refraction study of the subsurface geology of the Atlantic Coastal Plain and Continental Shelf between Virginia and Florida: Wisconsin Univ., Dept. Geology Geophys. Sec., 128 p. . Electric and Electromagnetic Soundings Near Charleston, South Carolina-— A Preliminary Report Bv DAVID L. CAMPBELL STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-N rn\_or> ■ ■ CONTENTS Page Abstract_ 189 Background _ 189 Audio-frequency magnetotelluric soundings _ 190 Vertical electric soundings _ 193 Cross sections_ 195 Interpretation and some speculations _ 197 References cited_ 198 ILLUSTRATIONS Page Figure 1. Sketch of electric microlaterolog of Clubhouse Crossroads core¬ hole 1 .......__ 190 2. Map showing locations of vertical electric soundings (VES) and audio-frequency magnetotelluric (AMT) resistivity sound¬ ings _ 191 3. Hand-contoured maps of AMT apparent resistivities at six fre¬ quencies for north-south oriented E (electrical)-field_ 192 4. Hand-contoured map of AMT apparent resistivities at six fre¬ quencies for east-west oriented E-field _ 194 5. Interpreted VES projected onto section A-A’ _ 196 6. Interpreted VES projected onto section B-B' _ 196 7. Interpreted VES projected onto section C-C' _ 197 TABLE Page Table 1. Interpreted VES solutions _ 196 STUDIES RELATED TO THE CHARLESTON. SOUTH CAROLINA. EARTHQUAKE OF 1886— A PRELIMINARY REPORT ELECTRIC AND ELECTROMAGNETIC SOUNDINGS NEAR CHARLESTON, SOUTH CAROLINA—A PRELIMINARY REPORT By David L. Campbell .ABSTRACT In an attempt to outline structural features which may bear on earthquake activity near Charleston, S. C., the U.S. Geological Survey has completed 9 Schiumberger d.c. resistivity soundings and 18 audio-frequency magnetotelluric (AMT) soundings in the region. Typical soundings show up to 60 m of surface sediments of variable resistivity, un¬ derlain by 100-250 m of 15-25 ohm-m material and 500- 1,000 m of 4-10 ohm-m material. The resistivity soundings failed to detect a basalt (Creta¬ ceous or older) encountered at 750 m depth in U.S. Geologi¬ cal Survey Clubhouse Crossroads corehole 1. Drilling had been stopped after penetrating 42 m into this basalt We now estimate this flow to be less than 75 m thick; it is under¬ lam by low-resistivity material. Interpretation of the sound¬ ings indicates that the depth to high-resistivity electric base¬ ment near the corehole is approximately 1,300 m. The AMT data outline a higher resistivity zone approxi¬ mately 11 km wide, trending northeast-southwest, roughly corresponding to the higher isoseismal region of the 1886 earthquake. This zone seems to be bordered on the north¬ west by a lineament interpreted by Long and Champion in 1975, on the basis of gravity, to represent a steeply dipping fault with the southeast side downthrown. Three d.c. sound¬ ings over this zone, however, show shallower electric base¬ ment (around 900 m) than those outside it. If this base¬ ment represents a thickened version of the Cretaceous(?) basalt encountered in the corehoie, some 150 m of vertical displacement would be indicated along this fault since Cre¬ taceous time. BACKGROUND On August 31, 1886, Charleston, S.C., was shaken by a large earthquake which was felt throughout the eastern United States. Dutton (1889) studied this earthquake, finding maximum isoseismals along an elongated northeast-trending region between Charleston and the town of Summerville, which is about 35 km inland. The seismicity of South Caro¬ lina has been studied by Bollinger (1972), who finds historical earthquakes occurring in a north¬ west-trending band across South Carolina through Charleston and Summerville, roughly perpendicular to the coast and the Appalachian Mountains. Tarr (this volume) reports that a magnitude 3.8 earth¬ quake occurred November 22, 1974, and 15 km west of Charleston, at a depth of 4.1 km. The focal mechanism of this earthquake was well determined and involved either a reverse fault or a thrust that strikes N. 42° W. Thus the scene is set: earthquakes seem to occur at shallow depths in a northwest¬ trending belt which passes under Charleston, but a northeast-trending feature, perhaps only in the shal¬ low subsurface between Charleston and Summer¬ ville, transmitted and focused the shaking of the 1886 quake. In the winter of 1975, the U.S. Geological Survey drilled a deep corehoie near Clubhouse Crossroads, 24 km southwest of Summerville. This corehoie (Clubhouse Crossroads corehoie 1) ■was located on an aeromagnetic and gravity’ high; geophysical analysis predicted a mafic basement at about 1,300 m depth. Instead, at least two successive basalt flows were encountered, beginning at 750 m depth. The core had penetrated 42 m into these basalts be¬ fore the core barrel became wedged in the hole and the hole was abandoned. Gottfried and others (this volume) reports that K-Ar ages of 94.8 ±4.2 m.y. and 109it4 m.y. for the basalt must be considered minimum ages because geochemical studies indicate that all samples are altered somewhat. The K-Ar ages are consistent, however, with a Late Creta¬ ceous age for the overlying Cape Fear Formation (Hazel and others, this volume). Figure 1 shows an electric log (microlaterolog) of Clubhouse Crossroads corehoie 1, with a tentative stratigraphic description by Gohn and others (this 189 190 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 STRATIGRAPHY MICROLATEROIOG 1 10 100 1000 ohm-m i 100 200 - 300 t/> ac iu 5 400 Z X t- CL UJ Q -i 500 - 600 700 TD 792 800 Figure 1. —Sketch of electric microlaterolog of Clubhouse Crossroads corehole 1 near Clubhouse Crossroads, S. C. Also shown is a tentative stratigraphic identification by Gohn and others (this volume). TD, total depth. volume). In general, the logged resistivities do not j seem particularly indicative of lithologic or strati¬ graphic boundaries. An exception is the thin 30 ohm-m zone between 116 and 126 m in depth, which corresponds to the tight calcareous sands at the base i of the Santee Limestone. Below this zone, resistivi¬ ties vary between 2 and 20 ohm-m throughout the sedimentary section. The interface between fresh¬ water and saltwater should be no deeper than 220 m at this location, but no specific resistivity drop on the log is identified as due to this cause. The micro- laterolog shows a resistivity of 600 ohm-m for the basalt at the bottom of the corehole. In June 1975, the U.S. Geological Survey com¬ pleted further geophysical work in the 'Charleston area, including 6 refraction seismic spreads, 18 audio-frequency magnetotelluric (AMT) resistivity soundings, and 9 Schlumberger d.c. vertical electric soundings (VES). The results of the refraction seis¬ mic work are reported by Ackerman (this volume), and this paper discusses the resistivity results. The locations of the VES and AMT soundings are shown on figure 2. This study has been funded by the U.S. Nuclear Regulatory Commission, Office of Nuclear Regula¬ tory Research, Agreement No. AT (49-25)-1000. Charles Tippens and Harold Kaufmann made the AMT soundings. The material in this report was presented orally March 26, 1976, at the Combined Meeting of the Northeastern-Southeastern Sections of the Geologi¬ cal Society of America, at Arlington, Va. AUDIO-FREQUENCY MAGNETOTELLURIC SOUNDINGS A general description of audio-frequency magne¬ totelluric (AMT) techniques and theory may be found in Chapter IV of Keller and Frischknecht (1966). Each AMT measurement described here yielded 11 apparent resistivity values for a given site, each value corresponding to one of 11 different frequencies in the band from 7.5 Hz to 18.6 Hz. Schematically, one may regard each apparent re¬ sistivity value as a weighted average of true re¬ sistivities in the earth below that site, with succes¬ sively lower frequencies weighting successively deeper resistivities more heavily. In the Charleston area, near-surface resistivities were too low for very deep penetration of audio-frequency electromagnetic waves, so that negligible weights resulted at even the lowest AMT frequency for depths greater than about 700 m. The equipment used in the Charleston area re¬ ceived electromagnetic waves broadcast by the light¬ ning strokes in thunderstorms. Ideally, the equip¬ ment should be oriented to measure the maximum ELECTRIC AND ELECTROMAGNETIC SOUNDINGS 191 30 ° 15 ' 80 ° 00 ' 33 ° 00 ’ 3r«- Ficure 2.—Map showing locations of vertical electric soundings (VES) (heavy lines) and audio-frequency magneto- telluric (AMT) resistivity soundings (heavy L’s) described in this report. Also shown are the locations of Club¬ house Crossroads corehole 1 (CCC 1), the historical plantation Middleton Place (MP), and section lines A-A , B-B', and C-C (figs. 5, 6, and 7). The broken line indicates a trend which Long and Champion (this volume) picked on the basis of gravity to represent a basement fault with the southeast side dropped 0.65 km. In the pres¬ ent study we prefer that Long and Champion’s line be shifted somewhat to the position indicated by heavy rail¬ road bars. horizontal electnc field at each frequency; in prac¬ tice, however, the direction to the particular storm (s) in progress is unknown, and therefore the direction of maximum field is unknown. Therefore, two electric fields are measured, in north-south and east-west directions, and later combined in a way Marnetot«i)uric ttehmou* actually involvr* nmultintoui me*»ur*m*nt t lonUl mifntl •! d i t - .» th# «l«ctne ft#ld In prtetic*. ho*mr the •lactncal held it of chief concern to ut. at it it found to vary in mafnttude much morr than the magnetic held. which varies with the interpreter. According to Stodt (oral common., 1975), this azimuthal uncer¬ tainty can give rise to a scatter of as much as an order of magnitude in the derived apparent resis¬ tivity values. Other problems with AMT data in¬ volve changes in relative calibrations from fre¬ quency to frequency due to drift in the electronic gear, contamination by cultural noise, and near field effects due to very local storms (in Charleston 192 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 ■MTS' an o' 30*15' 30*00' ^*[79 L 77 33*90'i 285 Hz (5/2 = 51 METERS) Summerville .ijo A * V % = 1200 256 75 14 4.5 200 d= VES 3 2.6 10.1 64 310 190 CO p= 80 146 63 7 25 8 200 d= VES 4 2.3 4.9 27 59 194 1186 co »— 120 25 60 16 50 4 200 d= VES 5 1.5 8 26 110 150 900 co 1200 300 15 40 7 200 d= VES 6 1.5 14 64 96 1350 30 0 — 45 91 21 40 10.6 4.2 200 d= VES 7 1.65 5 14.5 47 409 1113 co P—- 320 64 302 20.6 52.1 4.8 200 d= VES 8 1.7 4.4 11.3 182 320 965 00 9 = 1000 74.5 38.4 124 23 7.6 200 d= VES 9 2.3 25 68.4 139.2 889 1475 30 9 — 700 72 15 6 200 d= 10.6 70.5 226 1300 co NORTHWEST A VES 8 Oh 75 —'-38 124 1200 i— 7.6 1400!— 1600 1 — 200 ohrrwn VES 3 VES 4 200 SOUTHEAST A' VES 5, 7“ - 3 < u_ — 20(fj VERTICAL EXAGGERATION < 25 Figure 5.—Interpreted VES projected onto section A-A‘. ) Thin lines indicate divisions between different electrical \ layers. Numbers are resistivities in ohm-meters. Dotted i horizons indicate features inferred from other evidence, \ but not seen in the VES data. NORTHWEST SOUTHEAST VES 1 VES 7 VES 2 VES 6 VERTICAL EXAGGERATION x 25 shown on tig. 2). A second fault in the subsurface is shown parallel to the Long-Champion trend and 11 km southeast of it in order to separate the different resistivity structures of VES 4 and 5. The position Figure 6.—Interpreted VES projected onto section B-B'. Thin lines indicate divisions between different electrical layers. Numbers are resistivities in ohm-meters. Dotted horizons indicate features inferred from other evidence, but not seen in the VES data. ELECTRIC AND ELECTROMAGNETIC SOUNDINGS 197 of this fault has been chosen to coincide with some linear segments on the aeromagnetic contour map. Both faults are conjectural and interpreted as faults mainly to separate soundings of differing layer thicknesses and resistivities. Though the conjectured faults also separate regions of shallow and deep electric basement, any two adjacent basement depths may be connected with slopes not exceeding 5°. A •rather gentle anticline would therefore fit the base¬ ment depth data as well as the faulted structure shown on sections A-A' and B-B'. Section C-C' (fig. 7) trends northeast between the two conjectured faults. We see that the shallow electric basement of VES 2 and 4 has been lost to the southwest under VES 9. i INTERPRETATION AND SOME SPECULATIONS The original gravity interpretation of Long and Champion (this volume) involved downdrop of the block to the southeast of the Long-Champion line. The VES interpretations shown on sections A-A' and B-B’, however, show shallower basements southeast of this line. I would reconcile these two interpretations by postulating that the “basement” seen on VES 2, 4, and 7 is, in fact, a thickened in¬ terval of the Cretaceous (?) basalt encountered in Clubhouse Crossroads corehole 1. If the Long- Champion fault were already active in the Late Cretaceous, some surface relief may have been ex¬ pressed along it at that time. The basalt floods which inundated the area at that time would have filled the valley southeast of the fault resulting in greater basalt thicknesses there. Computer models indicate that a 100-m thickness of 200 ohm-m basalt, even when underlain by one-half kilometer of low- resistivity sediments, would be sufficient to give an “electric basement” signature equal within experi¬ mental error to those obtained from VES 2, 4, and 7. The hypothesis of a thickened basalt interval un¬ der VES 2, 4, and 7 may explain the apparent change in basement depth seen along section C-C'. The deep basement seen at VES 9 would represent crystalline basement some one-naif kilometer below the basalt flow. The flow itself presumably is too thin to be seen at the VES 9 location. The basalt is present at 750 m Clubhouse Crossroads corehole 1 to the west of the fault, but (according to the thick¬ ened basalt hypothesis) at around 900 m under VES 2 , 4, and 7 east of it. Thus, some 150 m of post- Cretaceous vertical offset could be indicated across the Long-Champion line in the vicinity of these particular soundings. Long and Champion’s (this SOUTHWEST c Or 200 - VES 9 72 15 VES 2 VES 4 75 14 16 “5CT -60 NORTHEAST C 400 — r 600 — V S 800- o. LU a 1000 — U 1200 — 1400 >— L 1600- 4.5 200 200 ohm-m 200 VERTICAL EXAGGERATION x 25 Figure 7 .—Interpreted VES projected onto section C-C'. Thin lines indicate divisions between different electrical layers. Numbers are resistivities in ohm-meters. Dotted horizons indicate features inferred from other evidence, but not seen in the VES data. volume) gravity interpretation involved a 600-m vertical offset in the basement across this line at a location 12 km further northeast, indicating either that displacement increases northeasterly, or that substantial vertical displacement was already there before the basalt flow came. I tend to favor the first interpretation, on the grounds that the interpreted basement offset in the southeast is small, with no significant difference between VES 1 and VES 9. The structure east of the Long-Champion line may be a Cretaceous (?) analog of the Atlantic coast Triassic and Jurassic basins farther to the north. The Long-Champion fault would be the west¬ ern border fault of this basin with the postulated thickened basalt and west-dipping flow within its sedimentary pile. (Some west dip seems likely be¬ tween VES 2 and 7 as shown on section B-B’.) The lower resistivity seen above basement within the 198 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1S86 structure (4 ohm-m as opposed to 10 ohm-m outside : it) could be indicative of ground water pooled in such a basin. Though the question is not closed, indications are that present-day earthquake activity has little to do with the postulated Long-Champion fault. The one well-determined earthquake focal mechanism (Tarr, this volume) and general seismicity trends (Bol- | linger, 1972) are at nearly right angles to it. There is no longer any surface expression of the Long- Champion fault. Its only significance for present- day earthquake planners may be the transmission and concentration of shaking along its associated shallow basin in response to large earthquakes which take place deeper in the section. A second U.S. Geological Survey corehole, 2.6 km southwest of the center of VES 1, encountered basalt between 774 m and 1,031 m depth, and passed into well-indurated sediments below 1,031 m depth. The total basalt thickness at this location is 257 m compared with 75-m maximum thickness I have in¬ terpreted from data at the VES 1 site. REFERENCES CITED Bollinger, G. A., 1972, Historical and recent seismic activity ; in South Carolina: Seismol. Soc. America Bull., v. 62, j no. 3, p. 851-864. Campbell, D. C., 1977, A model for estimating electric m?.- croanisotropy coefficient tf fractured aquifers: Geo¬ physics, v. 42, no. 1, p. 114-117. Dutton, C. E., 1889, The Charleston earthquake of August 31, 1886: U.S. Geol. Survey Ann. Rept. 9, 1887-88, p. 203-528. Goldstein, M. A., and Strangway, D. W., 1975, Audio-fre¬ quency magnetotellurics with a grounded electric dipole source: Geophysics, v. 40, no. 4, p. 669-70. Keller, George, and Frischknecht, Frank, 1966, Electrical methods in geophysical prospecting: New York, Per- gamon Press, 519 p. Zohdy, Adel A. R., 1974a, A computer’ program for the automatic interpretation of Schlumberger sounding curves over horizontally stratified media: U.S. Natl. Tech. Inf. Service PB-232703/AS, 25 p. -- 1974b, Use of Dar Zarrouk curves in the interpreta¬ tion of vertical electrical sounding data: U.S. Geol. Sur¬ vey Bull. 1313-D, 41 p. - 1975, Automatic interpretation of Schlumberger sounding curves, using modified Dar Zarrouk functions: U.S. Geol. Survey Bull. 1313-E, 39 p. Zohdy, Adel A. R., Anderson, L. A., and Muffler, L. J. P., 1973, Resistivity, self-potential and induced polarization surveys of a vapor-dominated geothermal system: Geo¬ physics, v. 38, no. 6, p. 1130-1144. Zohdy, Adel A. R., Eaton. G. P., and Mabey, D. R.. 1974, Application of surface geophysics to ground-water in¬ vestigations, Chapter Dl, of Book 2 of Techniques of water-resources investigations of the U.S. Geol. Survey: Washington, D. C., U.S. Govt. Printing Office, p. 10-22. Correlation of Major Eastern Earthquake Centers With Mafic/Ultramafic Basement Masses By M. F. KANE STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-0 ' CONTENTS Page Abstract_ 199 Introduction _._ 199 Comparison of gravity anomalies and earthquake areas_ 199 A possible source mechanism _ 201 References cited_ 203 ILLUSTRATIONS Page Figure 1. Sketch maps showing gravity and seismicity data for seven major earthquake regions in eastern North America _ 200 2. Sketch maps showing gravity and contemporary epicenter data for the New Madrid, Mo., and Charles¬ ton, S. C., earthquake areas_ 202 III STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886- A PRELIMINARY REPORT CORRELATION OF MAJOR EASTERN EARTHQUAKE CENTERS WITH MAFIC/ULTRAMAFIC BASEMENT MASSES By M. F. Kane ABSTRACT Extensive gravity highs and associated magnetic anomalies are present in or near seven major eastern North American earthquake areas. The seven include the five largest of these centers. The immediate localities of the gravity anom¬ alies are, however, relatively free of seismicity, particularly of the largest seismic events. The anomalies are presumably caused by extensive mafic or ultramafic masses embedded in the crystalline basement. Laboratory experiments show that serpentinized gabbro and aunite fail under stress in a creep mode rather than in a stick-slip mode. A possible explana¬ tion of the correlation between the earthquake patterns and the anomalies is that the mafic ultramafic masses are ser¬ pentinized and can sustain only low-stress fields, thereby acting to concentrate regional stress outside their boundaries. The proposed model is analogous to the hole-in-plate prob¬ lem of mechanics whereby stresses around a hole in a stressed plate may reach values several times the average. INTRODUCTION Earthquakes of the Eastern United States are markedly lower in frequency and magnitude than those of the western regions, particularly when com¬ pared with those occuring along the San Andreas fault of California. Because of the low damping of earthquake energy in the Eastern United States, however, relatively high intensities are anticipated when compared with the intensities resulting from corresponding magnitudes of the western earth¬ quakes (Nuttli, 1973). A second aspect of the east¬ ern earthquake region that contrasts with that of western regions is the sparsity of readily identifi¬ able major faults. To some extent, this lack may be attributed to a thick cover of incompetent sedimen¬ tary strata, but it seems surprising that ongoing studies have not uncovered direct evidence of major fault systems in the major eastern earthquake regions. As part of the earthquake investigation program of the U.S. Geological Survey, aeromagnetic and gravity studies of the New Madrid, Mo., and Charleston, S.C., earthquake areas began in 1972. Coverage of much of these regions was completed by 1975, although surveys in the New Madrid area are still underway. The initial efforts were directed towards discernment of linear magnetic or gravity features that could be attributed to major faults in the crystalline, presumably magnetic, basement rocks, but evidence of such features was not de¬ tected, at least not in the sense of readily apparent lineaments or discontinuities. Major magnetic and gravity highs were recognized in the near-epicentral regions of both the New Madrid and Charleston areas, but coincidence seemed to be the most plaus¬ ible explanation. Positive magnetic and gravity anomalies have now been identified, however, for the seven major Eastern United States earthquake areas as defined by Hadley and Devine (1974), so that implications other than coincidence must be considered. COMPARISON OF GRAVITY ANOMALIES AND EARTHQUAKE AREAS Figure 1 illustrates the comparison of earthquake epicenter areas with gravity anomalies for seven well-identified eastern North American earthquake regions. The dashed line shown on each map of the figure is the maximum frequency contour line show¬ ing the total number of earthquakes per 10 4 knv from 1800 to 1972 that have had an intensity of Modified Mercali III or larger (Hadley and Devine. 1974). As explained by Hadley and Devine, the contours are “only * * * a guide for estimating regional seismicity.’’ Also shown on figure 1 is the 199 200 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 EXPLANATION Bouguer gravity contour in milligals relative to sea level dashed in areas of sparse data. Hachures indicate areas of relatively lower gravity —— — 64— —- Maximum frequency contour show¬ ing total number of seismic events per lO'km 2 from 1800 to 1972 a Approximate location of largest seismic event for each region. Two events of equal intensity are shown for Cape Ann, Mass., area (C) 0 50 100 KILOMETERS I_I_L_l_l_1_1 . Figure 1. —Sketch maps showing gravity and seismicity data for seven major earthquake regions in eastern North America. Seismicity data were modified from Hadley and Devine (1974). Gravity data in .4 and B are from American Geophysical Union (1964); gravity data in C and D from Kane and others (1972); gravity data in E from Heiskanen and Uotila (1956) ; gravity data in F from Revetta and Diment (1971) ; and gravity data in G from Thompson and Garland (1957). CORRELATION OF EARTHQUAKE CENTERS WITH MAFIC BASEMENT MASSES 201 earthquake of maximum intensity within each re¬ gion. The fact that these largest earthquakes are all within the maximum contour lines gives assur¬ ance that the contour lines also locate the areas of maximum energy release. An examination of the small-scale maps of figure 1 shows that positive gravity anomalies of 10 milli- gals or greater and horizontal extents of more than 30 km are present in each of the earthquake regions. The New Madrid, Mo., area (fig. 1.4) is notable for two large circular anomalies northwest and south of the zone of maximum epicenter frequency. The largest seismic event is also located between the gravity highs. In the Charleston, S.C., area (fig. i 15), the largest event and the center of maximum epicenter frequency are both just east of a gravity high that has an easterly elongation. In the Cape Ann, Mass. (fig. 1C), Anna, Ohio (fig. IE), and Attica N. Y. (fig. IF), areas the zones enclosed by the contour of maximum epicenter frequency are elongated, one end of the zone overlapping the grav¬ ity high in each area. In each of these last three areas, the event of maximum intensity is near but outside the locus of the gravity high. In the Cape Ann area (fig. 1C) two events of approximately equal intensity are indicated; the second event is north of the seismicity zone, well removed from any notable gravity high. The strongest known earth¬ quakes of this area, however, took place in the early and mid-18th century and are approximately located in the region east of the gravity high (Richard Holt written commun., 1976). In the Massena, N.Y. (fig. 15), and Baie St. Paul, Quebec (fig. 1G), areas, the gravity highs are quite broad and have local highs superimposed. The maximum frequency contour is within the broad high; the events of maximum in¬ tensity are near but outside the superimposed gravity highs. In general the gravity anomalies, and hence their sources, tend to be peripheral to the earthquake maximum frequency contour. As this contour en¬ closes, for the most part, the earthquake of maxi¬ mum intensity this relation also indicates that the sources of the gravity highs are outside the region of maximum strain energy release. Figure 2 illustrates a more precise comparison of earthquake incidence and gravity anomalies for the New Madrid, Mo., and Charleston, S.C., areas. The earthquake plot for the New Madrid area (fig. 2A) (Stauder and others, 1976) represents cumula¬ tive seismic events from June 29, 1974, to March 31, 1976. Events in the patterned zones are too close to be shown individually. In figure 2A, the earth¬ quake epicenters are shown, for the most part, be¬ tween the tw T o prominent gravity highs north and south of the earthquake zone. A suggestion of an arcuate zone is seen southeast of the northern gravity' high. Earthquakes are sparse or lacking in the immediate vicinity of the gravity highs. In the Charleston area (fig. 25), the earthquakes (A. C. Tarr, this volume; C. E. Dutton, 1889) are east of the gravity high, which in detail has the shape of a sharp nose (Long and Champion, this volume). In both areas, depths to the earthquakes generally are less than 15 km (A. C. Tarr, this volume, William Stauder, oral commun., 1976). A POSSIBLE SOURCE MECHANISM In reviewing possible causal relationships be¬ tween the gravity anomalies and the earthquakes, we have considered isostatic effects, intrusive ac¬ tivity, and anomalies in the distribution of regional stress. Isostatic effects would appear to be negligible as the loads represented by the gravity highs are small compared with surface loads imposed by topography. Intrusive activity might be a factor, but the anomaly in the Baie St. Paul area is asso¬ ciated with mafic masses of Precambrian age, seem¬ ing to rule out this possibility for at least one of the areas. Of the three factors, the most plausible one would seem to be a relationship between the distribution of the regional stress field and crustal lithology. Long (this volume), in reporting on the gravity high in the Charleston, S.C., earthquake area, suggested that stress amplification caused by lithologic contrast may be related to the occurrence of the earthquakes. In a study of the relations between rock type, stress, and mechanical failure, Byerlee and Brace (1968) concluded that serpentinized gabbro and dunite, limestone, and porous tuff failed by creep rather than by stick slip, a small-scale analog to earthquakelike failure. When one considers the gravity anomalies in the region of the earthquakes shown in figure 1, plausible sources of the anomalies are large masses of mafic and (or) ultramafic rock imbedded in a crust of generally more silicic rock. If these masses are serpentinized, they may, as sug¬ gested by Byerlee and Brace’s results, deform con¬ tinuously by creep rather than intermittently by stick slip as regional stress changes. The behavior of the stress in the host rock enclosing these masses might, therefore, be similar to that which takes place in a rigid plate near a hole or plastic plug. Timoshenko and Goodier (1951, p. 78-82) showed 202 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 9,0 90 o g 9 o 81 ° 80 ° 79 ° EXPLANATION Souguer gravity contour in rruliigals relative to sea level. Hachures indicate areas of relatively lower gravity Location of a well-determined recent seismic event 100 KILOMETERS Area where seismic events are too I closely grouped to be shown separately Isoseismal zone of the 1886 Charleston. S. C.. earthquake Figure 2.—Sketch maps showing gravity and contemporary epicenter data for the New Madrid, Mo., and Charleston, 3. C., earthquake areas. Sources of gravity data are given in figure 1. Epicenter data in the New Madrid, Mo., area are from Stauder and others (1976). Epicenter data in the Charleston, S. C., area are from Tarr (this vol¬ ume). Isoseismal boundary is from Dutton (1889). CORRELATION OF EARTHQUAKE CENTERS WITH MAFIC BASEMENT MASSES 203 that stresses are localized at the margin of a hole in a plate and attain values several times those of the applied stresses. The thrust of this model is that large rock masses that have distinctive defor¬ mation contrasts may distort regional stress fields in much the same way that distinctive magnetiza¬ tion and density contrasts distort the magnetic and gravity fields. The role of serpentine in the mode of deforma¬ tion of the San Andreas fault has been commented on by Allen ( 1968 ) . He noted the “great abundance” of serpentine in the part of the fault zone char¬ acterized by creep and suggested that the creep may be related to the presence of serpentine. Although th geometry of the model described above and that of the San Andreas fault zone are greatly different, the two situations may be linked by the unusual de¬ formation properties of serpentine. The stress concentration near holes in plates is dependent, among other things, on the direction and type of stresses, shapes of the holes, and on the rela¬ tive location of plate boundaries. The arcuate zone (fig. 2.4), for example, might be analogous to the high-stress zone that exists between a hole-in-a-plate and a nearby boundary. In the New Madrid, Mo., area, a boundary may be indicated by the southwest¬ trending zone for earthquakes that is southwest of the arcuate zone (fig. 2 A). As such the zone would represent a fault influenced by the location of ser- pentinized mafic-'ultramafic masses near either end. Similarly, the earthquakes near the eastern nose of the gravity anomaly in the Charleston region (fig. 2 B) might be analogous to high-stress zones asso¬ ciated with the ends of narrow cracks in plates when tension is applied normal to the crack. Undoubtedly, the model of the hole-in-a-plate, if valid, is greatly oversimplified, as the masses are more analogous to plastic plugs, and geologic bodies are three dimensional. Uncertainties are also pres¬ ent in other aspects of the data including the pre¬ cise cause of the gravity anomalies, the directions and types of stress, the shapes and orientations of the anomalous masses, and the dimensions and boundaries of the host rock in which the anamalous masses are embedded. The only densities, however, that could reasonably explain the high positive grav¬ ity amplitudes, are those associated with mafic or ultramafic rocks. At present, no direct evidence of serpentinization exists. Perhaps the major question that arises about a relationship between mafic basement masses and stress-field distribution is why other regions in eastern North America underlain by large positive | gravity anomalies do not have associated earthquake activity. Lack of serpentinization would be the most obvious answer. Other possible answers include the lack of a sufficiently large or changing regional stress field, or inappropriate geometric relations be¬ tween the causative masses and stfess-field direc¬ tions. i The present evidence indicates, for example, that most, if not all the masses so far considered are at depths where they would be enclosed in highly competent basement. Mafic masses in softer, less competent sedimentary strata that yield more easily would presumably not give rise to the same stress concentrations. Possibly, also, the continental stress field, probably imparted by plate-tectonic conditions, is strongly zoned in a regional sense. The southwest alinement of earthquake areas from the Gulf of St. Lawrence to the New Madrid region and the similar trend in the broad earthquake region of the Ap¬ palachians indicated by the seismotectonic map of Hadley and Devine (1974) may be expressions of regional zoning of the continental stress field. In summary, a correlation has been shoum to exist between major eastern North American earthquake areas and the presence of mafic-ultramafic masses as evidenced by gravity anomalies. It is not true, however, that all mafic-ultramafic masses are asso¬ ciated with earthquake areas. A model has been pro¬ posed whereby stress is concentrated near the mar¬ gin of these masses in much the same manner as stress concentrations take place near the margins of defects or holes in plates under stress. This model has major implications in the consideration of east- 1 ern North America seismicity, as it suggests that larger earthquakes are restricted to relatively local areas. The model may also explain why major . through-going faults of continental or subcontinent¬ al dimensions are not evident in eastern North America. Presumably the faults associated with the localized stress zones would be similarly localized and of relatively small dimensions, perhaps 10 km long or less. REFERENCES CITED Allen, C. R., 1968, The tectonic environments of seismically active and inactive areas along: the San Andreas fault system, in Dickinson, W. R., and Grantz, Arthur, eds.. Proceedings of Conference on greolopic problems of San Andreas fault system: Stanford Univ. Pub. Geol. Sci., v. 11, p. 70-80. American Geophysical Union. Special Committee for the Geophysical and Geologrical Study of the Continents, 196-4, Boufuer gravity anomaly map of the United 204 STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 States (exclusive of Alaska and Hawaii) : Washington, D.C., U.S. Geol. Survey, 2 sheets, scale 1:2,500,000. Byerlee, J. D., and Brace, W. F., 1968, Stick slip, stable sliding, and earthquakes—Effect of rock type, pressure, strain rate, and stiffness: Jour. Geophys. Research, v. 73, no. 18, p. 6031-6037. Dutton, C. E., 1889, The Charleston earthquake of August 31, 1886: U.S. Geol. Survey Ninth Ann. Rept., p. 203- 528. Hadley, J. B., and Devine, J. F., 1974, Seismotectonic map of the Eastern United States: U.S. Geol. Survey Misc. Field Studies Map MF-620. Heiskanen, W. A., and Uotila, U. A. K., 1956, Gravity survey of the State of Ohio: Ohio Div. Geol. Survey, Rept. Inv. 30, 34 p. Kane, M. F., Simmons, Gene, Diment, W. H., Fitzpatrick, M. M., Joyner, W. B., and Bromery, R. W., 1972, Bouguer gravity and generalized geologic map of New England and adjoining areas: U.S. Geol. Survey Geophys. Inv. Map GP-839. Nuttli, Otto, W., 1973, The Mississippi Valley earthquakes of 1811 and 1812: intensities, ground motion, and magni¬ tudes, Seismol. Soc. America Bull., v. 63, no. 1, p. 227- 248. Revetta, F. A., and Diment, W. 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