Ash-flow Eruptive Megabreccias of the Manhattan and Mount Jefferson Calderas, Nye County, Nevada By DANIEL R. SHAWE ard DAVID B. SNYDER U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1471 Megabreccias associated with the Manhattan and Mount Jefferson calderas possess characteristics that suggest origin by ash-flow eruption rather than by caldera-wall collapse UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in-Publication Data Shawe, Daniel R., 1925- Ash-flow eruptive megabreccias of the Manhattan and Mount Jefferson calderas, Nye County, Nevada. (U.S. Geological Survey professional paper ; 1471) Bibliography: p. Supt. of Docs. no.: I 19.16:1471 1. Breccia—Nevada—Nye County. 2. Volcanic ash, tuff, ete. -Nevada-N ye County. 8. Calderas—Nevada—Nye County. I. Snyder, David B. II. Title. III. Series. QE471.15.B79525 1988 552'.2 87-600088 Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 PLATE FIGURE CONTENTS Page Pue Oe 1 IntrOGUCtION ...s lae aa a kkk kkk n n n d k k k k kkk nn n d n d d r nnn nn n r re 1 Acknowledgments lll kkk keke kkk kkk kkk ke ekke eke e ees 1 Geologic setting of the Manhattan and Mount Jefferson calderas ................... 2 Regional S@ttIMG . .k kkk kkk kkk kk kkk kkk kkk kkk k kk keke es 2 Southern Toquima Range setting ........................l kkk kk rrr kk kee} 2 Distribution, form, and character of the megabreccia units ........................ 5 Description of the megabreccia units .......................... kkk kkk eee ece s 5 Megabreccia unit of Sloppy Gulch ... .ll kkk kkk 00 scc 6 Megabreccia unit of Mariposa Canyon .ll kkk kk} cs 10 Middle member of the Round Rock Formation ............................... 10 Rhyolite megabrecci@ kkk keke kkk k kkk keke kk keke es 12 Megabreccia unit of Silver Creek kkk kk 0s 13 Megabreccia of Jefferson CanyOM ...ll keel keke e ees 15 Basal contact of a megabreccia Outflow lll kkk kle} 17 nea oo aie 18 Deep-level, subvoleanic "explosion-breccia'' kkk csak} es 21 Physics of megabreccia eruption ......................... .ll kkk kkk keke ea ee ees 22 SUMMAry |... 22222 lll lala aa ee ekke k k k k k k k kk 6 n n n d d d d e e kkk n k d d d d d k nn n n r r ae 24 RefereNCe$ Cited .ll lel l aaa a a a a a eee e e e e e e e e e eee eee e eee e eee 27 ILLUSTRATIONS [Plates are in pocket] 1. Geologic and gravity map of the Manhattan caldera, Nye County, Nevada, including geologic cross sections A-A ' through G-G'. 2. Map showing aeromagnetic contours, and calderas and granite plutons of part of the southern Toquima Range, Nye County, Nevada. 3. Geologic map of part of the southwest margin of the Mount Jefferson caldera and adjacent terrane, Nye County, Nevada, including geologic cross sections H-H' and I-I'. 1. Generalized geologic map of the southern Toquima Range, showing locations of the Manhattan, Mount Jefferson, and Meadow Canyon calderas, and areas of plates ........................ .ll kkk kkk kkk kkk k kkk eek kee rre es 2-15. Photographs showing: 15 Middle member of Round Rock Formation, showing large clasts in dominant tuff matrix ............... Interlayered ash-flow tuff, mesobreccia, and megabreccia of megabreccia unit of Sloppy Gulch .... ...... Margin of immense clast of Permian Diablo Formation bounded by tuff matrix, megabreccia unit of Sloppy Gulch Brecciated limestone, forming part of an immense clast in the megabreccia unit of Sloppy Gulch .. ...... Fine-grained tuff rind at the margin of a large clast of tuff in the megabreccia unit of Sloppy Gulch ..... Deformed reddish-brown rhyolite of the middle member of the Round Rock Formation ................. Large knobby rhyolite clast in rhyolite megabreccia of the upper member of the Round Rock Formation .. Megabreccia unit of Silver Creek, clasts virtually entirely granite in a comminuted granite matrix ....... Megabreccia of Jefferson Canyon, containing large block of Cambrian quartzite ....................... Large clast of partly silicified Paleozoic limestone in megabreccia of Jefferson Canyon ................. Intensely brecciated quartzite clast in the megabreccia of Jefferson Canyon .......................... Close view of intensely brecciated quartzite clast in megabreccia of Jefferson Canyon .................. Devitrified glass rind on rhyolitic welded ash-flow tuff clast in megabreccia of Jefferson Canyon ........ Contact of tuff of the megabreccia of Jefferson Canyon on Paleozoic limestone ........................ 16. Sketch showing generation of a pyroclastic flow and megabreccia kee s kkk} cc ecs 17. Plot of hindered-settling velocities of megabreccia clasts ks. lla kk kerr rarer ns III Page ASH-FLOW ERUPTIVE MEGABRECCIAS OF THE MANHATTAN AND MOUNT JEFFERSON CALDERAS, NYE COUNTY, NEVADA By DANIEL R. SHAWE and DAVID B. SNYDER ABSTRACT The Manhattan and Mount Jefferson calderas in the southern Toquima Range formed about 25 and 27 Ma, respectively. The Man- hattan caldera formed with eruption of silicic ash-flow tuffs of the Round Rock Formation, interspersed in which are several megabrec- cia units, some containing immense clasts (hundreds of meters long). The presence of some clast lithologies that are nowhere recognized in caldera walls, the evidence that some clasts became brecciated in sub-caldera levels prior to emplacement of megabreccia, and the oc- currence of some megabreccia units as outflow suggest an origin by eruption rather than by collapse of caldera walls. Varied development of indurated ash rinds on clasts that occur side by side in the megabrec- cia suggests differences in temperature of clasts when they were in- corporated into the ash matrix of the megabreccia, again indicating derivation of some materials from deep and hot levels. The Mount Jefferson caldera formed with eruption of silicic ash-flows of the tuff of Mount Jefferson; a megabreccia unit also appears to have been erupted. This unit contains an ash-flow-tuff matrix locally welded to black vitrophyre, suggesting eruption of ash-flow megabreccia at very high temperatures. INTRODUCTION Volcanic megabreccia in the San Juan Mountains caldera field in southwestern Colorado (Lipman, 1976; Hon and others, 1983) has been attributed to collapse of oversteepened caldera walls into ash deposits within the caldera; however, some volcanic megabreccias observed in this study in the Great Basin provide evidence of an origin largely by the mechanism of ash- flow eruption and appear unrelated to collapse of caldera walls. Evidence includes extensive brecciation of clast materials prior to ash eruption, differential heating of clasts prior to ash eruption, derivation of clasts from sub-caldera levels rather than from caldera walls, distribution and form of intracaldera megabreccia units similar to those of associated ash-flow deposits, decrease in clast size outward from apparent vent zones, and deposition of tabular megabreccia units as outflow beyond the structural margins of calderas. Not all the described megabreccias exhibit these attributes, though their general similarity suggests a common origin. An important factor in interpretation of the erup- tive origin of a megabreccia is its content of indurated ("healed") brecciated clasts that appear identical to "explosion-breccia'"' formed in the root zones of Tertiary calderas and other igneous centers elsewhere, as discussed later in this report. Although the cited evidence suggests to us an eruptive origin of the megabreccias described here, the occurrence of collapse breccias in these and other calderas of the Great Basin also is possible. The features of ash-flow eruptive megabreccias described here were observed in and near the Manhat- tan caldera and along the southwest margin of the Mount Jefferson caldera, both in the southern Toquima Range, northern Nye County, Nev. These features were delineated during field studies by the senior author dur- ing the past 15 years (Shawe, 1981a,b). Recent geophys- ical studies by the junior author provide a more complete definition of the Manhattan caldera and its volcanic fill, and his calculations of physical conditions during ash-flow eruptions provide corroboration for the eruptive transport of large megabreccia clasts. ACKNOWLEDGMENTS Comments on the manuscript by P.D. Rowley, R.F. Hardyman, G.R. Osburn, and TH. Druitt clarified our presentation and tempered our interpretations. R.F. Hardyman, P.W. Lipman, and H.J. Moore directed the authors to useful references that describe various features and mechanisms of origin of '"explosion- breccias," pyroclastic flows, "co-ignimbrite lag-fall" deposits, "proximal-facies'' deposits, and related phenomena. Moore aided in the mathematical analysis of ash-flow eruption of megabreccia. David Jones made modal point-count analyses of thin sections of samples of the tuff of Round Mountain and of the Round Rock and the Diamond King Formations. R.E. Schoenfeld kindly photographed hand specimens. Margaret Clemensen typed and proofread the manuscript. 2 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA GEOLOGIC SETTING OF THE MANHATTAN AND MOUNT JEFFERSON CALDERAS REGIONAL SETTING The western part of the Great Basin, in which lie the Manhattan, Mount Jefferson, and numerous other calderas of Tertiary age, is a region of great geologic complexity. It spans the continental margins that ex- isted in the Paleozoic and Mesozoic Eras, and its geology preserves a fragmentary history of repeated crustal-plate interactions that occurred sporadically throughout that long interval of time. Stewart (1980) has summarized the Paleozoic, Mesozoic, and Tertiary geologic history of the region. During extensive silicic volcanism in the Tertiary, numerous calderas were formed, and some appear to be localized along major northwest- or northeast-trending strike-slip structural zones (for example, Snyder and Healey, 1983). In a general way, calderas and their associated voluminous ash-flow deposits in the western Great Basin lie in a broad northeast-trending zone of low gravity (Woollard and Joesting, 1964). The low gravity suggests that the crust in this part of the Great Basin may contain numerous large silicic batholiths at depth. Evidence is lacking, however, as to whether some of these batholiths are related to the Tertiary calderas and their silicic volcanic rocks, or whether they were emplaced at an earlier time. sOUTHERN TOQUIMA RANGE SETTING The southern Toquima Range typifies the block- faulted and uplifted mountains, bounded by young alluvium-filled valleys, of the Basin and Range province. The southern Toquima Range is made up of Paleozoic sedimentary and metamorphic rocks invaded by two large Cretaceous granite plutons, in turn intruded by or overlain by a variety of Tertiary hypabyssal and volcanic rocks (fig. 1) (Ferguson, 1921, 1924; Kleinhampl and Ziony, 1985; Shawe, 1977, 1981a,b). The Paleozoic rocks are marine sedimentary rocks: quartzite, silty argillite, argillite, and limestone of Cam- brian age; and argillite, limy argillite, limestone, dolomite, chert, and quartzite of Ordovician age. In places, Paleozoic argillite has been metamorphosed to phyllitic argillite; near granite contacts it consists of chloritoid and muscovite-biotite schist. Locally near granite contacts, limestone has been metamorphosed to tremolite and other cale-silicate minerals. Cretaceous granite crops out in two large plutons, one southeast of Round Mountain and one south of Manhattan (fig. 1). Southeast of Round Mountain, the pluton referred to here as the granite of Shoshone Mountain extends for more than 20 km southeastward to near Belmont. It is a compound body consisting of an oval mass, here called the Round Mountain lobe, about 13 km long and composed of coarse-grained granite separated from porphyritic coarse-grained granite on its south side by a narrow screen of Paleozoic schist. A second oval mass of similar size, the Belmont lobe, adjoins the Round Mountain lobe on its southeast side. The Belmont lobe consists of an outer broad an- nular zone of porphyritic coarse-grained granite that grades inward to a core of coarse-grained nonporphyritic granite. The pluton south of Manhattan, here called the granite of Pipe Spring, forms an irregular-oval body of coarse-grained granite about 15 km long. The granite plutons are intruded locally by dikes and other small masses of aplite and pegmatite. A swarm of rhyolitic and subordinate andesitic dikes and a small stock of granodiorite-all of Oligocene age-intrude the granite and adjacent Paleozoic rocks east and south of Round Mountain. The Manhattan and Mount Jefferson calderas formed during eruption of voluminous silicic ash-flow tuffs in late Oligocene time (fig. 1 and pl. 2). The 25-Ma Man- hattan caldera forms a crude oval structure about 11X17 km in size whose long axis trends west- northwest. The 27 Ma Mount Jefferson caldera is at least 15 km in diameter, and the nearby Meadow Can- yon caldera (fig. 1), not discussed further in this paper, is at least 10 km in diameter. The Manhattan caldera, whose structural margin is well defined by inward- dipping, high-angle faults, several shallow-level igneous plugs, and concentrations of megabreccia (pl. 1), is filled with more than 1,000 m of silicic ash-flow and ash-fall tuff layers. The Mount Jefferson caldera, incompletely defined, is bounded on its southwest side by a steep fault (pl. 3) and apparently is filled with several thou- sand meters of silicic ash-flow tuffs. The Manhattan and Mount Jefferson calderas are well defined by their geophysical properties. A substantial gravity low coinciding with the Manhattan caldera sug- gests 1-2 km of volcanic fill (Snyder, 1983) (pl. 1; cross section A-A '). Aeromagnetic data (pl. 2) indicate, by curvilinear trends of parallel magnetic contours, the oval caldera boundary along its northern, southeastern, and southwestern margins, and show the general form of a late andesitic stock that may represent a phase of resurgence, discussed in more detail later in this report. The form of the Mount Jefferson caldera is indicated by its magnetic properties (pl. 2). The Manhattan and Mount Jefferson calderas, since their formation, have been modified by Basin-range faults (Shawe, 1981a,b). The faults, however, have not significantly disrupted 117°00' ANHATTAN AND MOUNT JEFFERSON CALDERAS O L c >4~_°n - M . _ ve cvs aay ags t d a A bok gs GEOLOGIC SETTING, L t < 0 co c +0 + + 2C: 0 s a§08»%<%&§§ von vor bov 10 KILOMETERS 5 MILES EXPLANATION E < > 5 h a a el k L 8 s 3 O Cretaceous porphyritic granite Tertiary volcanic rocks Cretaceous granite ks Paleozoic sedimentary and metamorphic ro Contact Caldera rim-Dashed where projected; queried where uncertain Mount Jefferson, and v + + of* L 'D.E-R A a N E V A D A figure I~ IElManhattan io Reno Area of FIGURE 1.-Generalized geologic map of the southern Toquima Range, showing locations of Manhattan, Meadow Canyon calderas, and areas of plates 1-3. 4 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA or obscured the forms of the calderas. Plate 1 shows the insignificant effect of Basin-range faults on the Manhat- tan caldera. Silicic ash-flow tuffs whose eruption led to collapse of the Manhattan caldera are referred to the Round Rock Formation (Shawe, 1987). The Round Rock For- mation in the southern Toquima Range is virtually con- fined within the Manhattan caldera, leading to the interpretation that it came from the caldera. We do not know whether or not significant outflow was deposited around the caldera nor the extent to which erosion may have removed possible outflow. The Round Rock Formation has been dated (K-Ar method on biotite; fission-track method on zircon) as about 25 Ma (latest Oligocene) (Shawe and others, 1986). The formation consists of a lower tuff member which includes two megabreccia units, a middle member that is itself a megabreccia unit, and an upper tuff member which includes two megabreccia units. The lower and upper members are light-colored, poorly consolidated latitic, quartz latitic, and rhyolitic ash-flow and ash-fall tuff layers, apparently randomly associated, that occur in several cooling units. The lower member (unit Trl, pl. 1 and cross sections) is probably at least 1,000 m thick, and the upper member (unit Tru, pl. 1 and cross sections) is 200-3800 m thick. The poorly consolidated ash-flow and ash-fall tuffs of these members are characterized by abundant lithic and pumice fragments. Lithic frag- ments may constitute from 1 to almost 40 percent of the rocks. They are crystal-poor rocks that contain about 10-27 percent phenocrysts (mostly 1-2 mm) con- sisting of about 50 percent sodic plagioclase and lesser subequal amounts of quartz, sanidine, and mafic minerals. Mafic minerals are mostly biotite, though hornblende is also present in some rocks. Iron-titanium oxide minerals, apatite, and zircon are accessory minerals. The upper and lower members contain relatively thin units of strongly welded ash-flow tuff, of vitrophyric ash-flow tuff, of air-fall tuff, and of volcanic breccia other than the megabreccia units, all of whose compositions are generally similar to those of the main parts of the upper and lower members. The megabreccia units locally overlap rocks beyond the structural margin (ring-fracture zone) of the Manhat- tan caldera, and they appear to form intrusive masses (vent material) in the structural margin of the caldera. Near the southwest margin of the caldera, 3 km north- west of Manhattan, a small hill composed almost en- tirely of reddish-brown rhyolite is believed to be a plug and associated dikes (unit Trmr, pl. 1) that are related to the middle member of the Round Rock Formation. The reddish-brown rhyolite intrudes the middle member and is lithologically similar to much of the clast material in the megabreccia of the middle member. Small rhyolite and quartz latite plugs along the east margin of the Manhattan caldera (unit Tr, pl. 1) probably were emplaced in the ring-fracture zone of the caldera. The rhyolite is a light-brownish-gray, flow-layered, crystal-poor rock containing conspicuous biotite phenocrysts. The quartz latite is a light-gray, crystal- poor rock containing sparse biotite. The age of one of the rhyolite plugs is 24.8+0.9 Ma (K-Ar method on biotite; Shawe and others, 1987, p. 6). Overlying the Round Rock Formation in the southern Toquima Range is the Diamond King Formation (Shawe, 1987) of latest Oligocene age (about 25 Ma) (Shawe and others, 1986). The Diamond King Forma- tion (unit Tdk, pl. 1) is exposed extensively within the Manhattan caldera, where it is about 150-200 m thick. The Diamond King is a light-buff to light-pinkish- brown, generally welded, rhyolitic ash-flow tuff that contains about 25-50 percent crystals (mostly 1-3 mm) of dominantly quartz and lesser amounts of sanidine and sodic plagioclase. Traces to a few percent of biotite and iron-titanium oxides are present; zircon, apatite, and allanite are accessories. The Diamond King is char- acterized in hand specimens by conspicuous smoky quartz dipyramids and by much fewer lithic and pumice fragments than found in the underlying Round Rock Formation. The source of the Diamond King is unknown. The Dia- mond King possibly was derived from a caldera now buried beneath the alluvium of Big Smoky Valley west of Round Mountain. Overlying the Diamond King Formation are buff- colored lacustrine claystone and siltstone beds and fluvial sandstones and conglomerates of the Bald Moun- tain Formation (Shawe, 1987), also of latest Oligocene age (Shawe and others, 1986). These beds are made up mostly of volcanic detritus. The formation is about 200-250 m thick. The tuff of Peavine Creek (Shawe, 1987) is the youngest stratigraphic unit in the volcanic succession in the southern Toquima Range (together with the underlying Bald Mountain Formation, designated as unit Tvy on pl. 1). This informal unit consists of poorly consolidated, light-greenish-buff and buff, quartz-latitic and rhyolitic ash-flow and ash-fall tuff, and of inter- layered light-brown, highly welded ash-flow tuff. The tuff of Peavine Creek is about 24.6 Ma (Shawe and others, 1986). In the southern Toquima Range the unit is present only within the Manhattan caldera where it overlies the Bald Mountain Formation; its maximum remnant thickness is about 250 m. The tuff probably was erupted from a caldera mapped by G.F. Brem (Brem and Snyder, 1983; Brem, written commun., June 1983) in the southern Toiyabe Range near Peavine Creek, 20-30 km west-northwest of Manhattan. The poorly consolidated tuffs and the welded tuffs both con- tain low to moderate amounts of phenocrysts as varied amounts of quartz, sanidine, and sodic plagioclase. Biotite is present in small but locally conspicuous amounts; it appears to be most abundant in the lower part of the unit. Iron-titanium oxide minerals, apatite, and zircon are accessories. At Round Mountain, north of the Manhattan caldera, an ash-flow tuff unit has been dated as 26.1 Ma (K-Ar method on sanidine; Silberman and others, 1975) and more recently was dated as 27 Ma ( biotite and on sanidine; Shawe and others, 1987, p. 5). This unit, referred to here as the tuff of Round Moun- tain, is likely correlated with parts of the tuff of Mount Jefferson that have been dated as about 26 Ma (Marvin and others, 1973). The tuff at Round Mountain contains about 20-40 percent phenocrysts; these consist of relatively more plagioclase and biotite, and less sani- dine and quartz, than do phenocrysts of the Diamond King. The Crone Gulch Andesite was emplaced into volcanic rocks within the Manhattan caldera about 3 km north- east of Manhattan primarily in the form of an andesite stock about 3 km long and 1 km wide (Shawe, 1987). Numerous associated sills were intruded near the stock, mostly into the Bald Mountain lake beds, and abundant andesite dikes and plugs of similar composition were emplaced in volcanic rocks within the Manhattan caldera. The Crone Gulch Andesite (unit Tc, pl. 1) is an olive-brown porphyritic rock that contai cent labradorite and lesser amounts of augite, set in a groundmass of plagioclase laths, minor augite, and iron- titanium oxide minerals embedded in a devitrified glassy matrix. Apatite is a relatively abundant ac- cessory mineral. Some of the sills contain numerous vesicles filled with chalcedony or calcite. The rocks give suspect isotopic ages of about 22 Ma (Shawe and others, 1986); they are probably older, based on geologic evidence cited later in this report. Two large plugs and satellitic dikes and sills of flow- layered volcanic rock identified by Ferguson (1924, p. 53) as dacite (unit Td, pl. 1) form a low hill 5 km north-northwest of Manhattan. The rocks are about 24.5 Ma (Shawe and others, 1986). Petrographic studies suggest that these rocks are varied in composition. They are crystal-poor rocks whose phenocryst composi- tions indicate a range from biotite latite to biotite- quartz latite. About 85 percent of the rock is a devitrified glassy matrix. Black vitrophyric layers occur locally. Late in the history of the Manhattan caldera, prob- ably as a result of resurgence, an apparent gravity slide moved a large mass of the younger volcanic units (unit Tvy, pl. 1) westward into the western part of the caldera. THE MEGABRECCIA UNITS 5 DISTRIBUTION, FORM, AND CHARACTER OF THE MEGABRECCIA UNITS Five significant megabreccia units occur around the periphery of and within the Manhattan caldera (see in- set map, pl. 1). They are, in apparent sequence from oldest to youngest, the megabreccia unit of Sloppy Gulch (unit Tris) and the megabreccia unit of Mariposa Canyon (unit Trim), both part of the lower member of the Round Rock Formation; the middle member of the Round Rock Formation (unit Trm), a member composed wholly of megabreccia; a rhyolite megabreccia (unit Trur) within the upper member of the Round Rock; and the megabreccia unit of Silver Creek (unit Trus) near or at the top of the upper member of the Round Rock and beneath the Diamond King Formation (pl. 1). A sixth unit, the megabreccia of Jefferson Canyon (unit Tjc) near the southwest margin of the Mount Jefferson caldera (pl. 3), is also described here because certain well- developed features suggest an ash-flow eruptive origin for it. A few thin lenses of megabreccia that occur sporadically in the Round Rock and younger formations in the southern Toquima Range are not described here. The five Manhattan caldera megabreccia units occur either as vent facies near the structural wall of the caldera, as intracaldera facies, or as outflow facies. Some units consist of all three facies. The megabreccia of Jef- ferson Canyon appears to be outflow related to the Mount Jefferson caldera (pl. 3). Southwest of the mouth of Jefferson Canyon, the megabreccia of Jefferson Can- yon and the overlying tuff of Mount Jefferson appear to correlate with a lithologically similar megabreccia unit and the overlying tuff of Round Mountain. The similar megabreccia and the tuff of Round Mountain both thicken outward from Round Mountain, suggest- ing that a paleotopographic high underlies Round Mountain. DESCRIPTION OF THE MEGABRECCIA UNITS The megabreccia units described here have common characteristics that suggest their eruptive origin, although all of the criteria that we interpret to indicate an eruptive origin are not evident in each of the mega- breccia units. Caldera-wall collapse may have formed some megabreccia, but that cannot have been a major cause of megabreccia emplacement. Each unit has dif- ferences in the character of matrix material and clasts that point to variations in the sources of contained rocks and in mechanisms of eruptive formation. Distinctions among matrix materials are made on the basis of their composition, structure, and degree of welding. Distinc- tions among clasts are made on the basis of their size, 6 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA FigGurRE 2.-Middle member of the Round Rock Formation, showing large clasts in dominant tuff matrix. Faceted clast at lower right is part of a 50-m-long fragment of rhyolite (pick at arrow on left-facing faceted surface of clast gives scale). View to east in southwest part of Manhattan caldera. distribution, shape, composition, internal structure, and rinds. Commonly, but with notable exceptions, clast lithology is similar to that of nearby caldera walls; this relation suggests that clasts were derived either by vent plucking during eruption along the ring-fracture zone or by gravity collapse of the walls into the vent. Characteristics that we describe in later pages lead us to believe that vent plucking was the dominant process. Clasts of all sizes, in all of the megabreccia units, vary in shape from angular to rounded, and shapes having varied degrees of rounding may occur side by side. Large fragments generally are more rounded than small fragments, which grade in size down to particles that disappear in tuff matrix. Most fragments are more or less equidimensional, although some large clasts of the Paleozoic sedimentary rocks tend to be well layered and slablike. Large blocky clasts generally have rounded corners. Some large clasts show well-faceted, somewhat striated surfaces (fig. 2) suggestive of abrasion. MEGABRECCIA UNIT OF SLOPPY GULCH This unit, well exposed near Sloppy Gulch just north of Manhattan (pl. 1), was described by Ferguson (1924, p. 43-44) as a talus breccia and originally was named by him the Hedwig Breccia Member of the Esmeralda Formation. This name is now abandoned and these rocks are reassigned to the informal Sloppy Gulch unit of the lower member of the Round Rock Formation (Shawe, 1987). Ferguson described the unit as con- sisting of breccia of small angular fragments of the various Paleozoic rocks and granite, but he remarked that none of the materials were mixed and thus he in- ferred that they were from a local source, even though topographically higher areas of appropriate granite and Paleozoic rock sources generally are not evident. Ferguson stated, surprisingly, that the unit contains no volcanic material, even though it locally contains significant exposed tuff and some large-as long as THE MEGABRECCIA UNITS T 100 m-volcanic clasts embedded in tuff matrix. Ap- parently he considered that the tuff of the Round Rock Member simply inundated his Hedwig unit following talus deposition. The megabreccia unit of Sloppy Gulch is exposed in a broad arc along the south margin of the Manhattan caldera (pl. 1). Near Manhattan the Sloppy Gulch is believed to be dominantly a vent facies megabreccia because it occurs at the structural margin of the caldera, shows steep contacts with Paleozoic wall rocks, and occupies a position where later venting of the reddish- brown rhyolite took place. Curvilinear faults that bound the caldera near Manhattan are steep faults, dipping into the caldera; the sharply curving segment 2 km east of Manhattan (pl. 1) is well exposed in a steep ravine, where it strikes N. 40° E. and dips 80° NW. In this area the megabreccia is exposed through vertical relief of more than 200 m. Drill holes in a small patch of megabreccia inside the caldera margin about 3 km east of Manhattan bottom in the megabreccia at a depth of about 200 m. We believe that a steep gravity gradient at the margin of the caldera near Manhattan (pl. 1) in- dicates locally deep volcanic fill that may correlate with a vent zone near or along the caldera ring fracture. The steep gravity gradient, however, also could simply in- dicate a steeper walled caldera with somewhat less tuff fill. Because geologic evidence suggests a vent zone here, we prefer the vent interpretation. Layers of the megabreccia unit of Sloppy Gulch are intercalated in the lower member of the Round Rock Formation and occur at the top of the lower member, about 1 km north of Manhattan (pl. 1, cross section A-A"), where they are interpreted to be intracaldera facies. None of the Sloppy Gulch, however, is known to occur in or at the top of the lower member in the cen- tral part of the caldera where those rocks are exposed through a distance of several kilometers. Near East Manhattan Wash and the mouth of Bald Mountain Wash, 4-10 km east of Manhattan and just west of the Maris Mine, the megabreccia unit of Sloppy Gulch occurs widely as a relatively thin layer on an almost flat surface of Paleozoic rocks (pl. 1, cross sec- tions C-C" and D-D'), extending perhaps 1 km beyond the structural margin of the Manhattan caldera. Megabreccia clasts in this area are as long as 250 m (pl. 1). A small patch of the Sloppy Gulch lies about 4 km south of the south margin of the Manhattan caldera (see inset map, pl. 1); it is at an altitude of 7,400 ft, about the same as that of the caldera margin near Manhat- tan. This patch of megabreccia, characterized by clasts of Paleozoic rocks probably no more than a few meters in size in tuff matrix identical in appearance to that of the Sloppy Gulch north of Manhattan, is about 0.5 km south of the north contact of the granite of Pipe Spring. It and megabreccia east of Manhattan south and east of the structural margin of the caldera are considered to be outflow facies. Because the southeast margin of the Manhattan caldera cannot be correlated with sharp differences in the gravity field (pl. 1), the nature and configuration of the megabreccia unit of Sloppy Gulch here are some- what uncertain. The presence of breccia dikes in the Round Rock Formation (pl. 1), of large clasts within the megabreccia unit of Sloppy Gulch (pl. 1, cross section D-D"), and of small rhyolite plugs about 1 km north of the area of megabreccia (pl. 1) suggests that a vent zone for breccia and rhyolite eruption marks this segment of the caldera margin. Part of the megabreccia west of the Maris Mine thus is interpreted to be vent facies, and part is inferred to be outflow facies (pl. 1, cross section D-D"). The matrix of the megabreccia unit of Sloppy Gulch is identical in character to ash-flow tuff of the lower member of the Round Rock Formation. The matrix typically contains abundant small (1-2 cm) lithic fragments consisting of the common Paleozoic rock types and less abundant granite, aplite, and volcanic rocks. Structure in the tuff matrix generally is poorly defined; locally in the vent zone along the south margin of the Manhattan caldera, compaction foliation manifested by flattened pumice lapilli strikes about east-west and dips steeply north. Near the mouth of Bald Mountain Wash, gently east dipping (outflow?) layers of megabreccia (with Round Rock tuff matrix) that contain abundant Paleozoic clasts and some granite and aplite clasts are interlayered with thin con- formable layers that contain clasts generally less than 1 m in size-termed "mesobreccia'' by Lipman (1976, p. 1398)-in Round Rock tuff matrix and with layered tuff of the lower member of the Round Rock Formation (pl. 1, cross section D-D', and fig. 3). Fragments in megabreccia in places dominate the tuff matrix to the extent that at the surface no tuff is visible. In some areas where megabreccia fragments of disparate lithol- ogies are large (in the order of tens to hundreds of meters across), and tuff matrix is not evident at the sur- face, the senior author spent much time fruitlessly at- tempting to map rational structural relations before discovering-in prospect pits (fig. 4), mine workings, and road cuts-small amounts of interstitial tuff matrix that proved the true character of the material. Locally the tuff matrix may constitute half or more of the megabreccia, and megabreccia fragments can be seen to be embedded in tuff matrix. The largest blocks (as large as 200 X300 m) in the megabreccia unit of Sloppy Gulch are found in what we believe to be vent facies because of its position along the structural margin where gravity data suggest a 8 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA FIGURE 3.-Megabreccia unit of Sloppy Gulch. Megabreccia and mesobreccia interlayered with ash-flow tuff of the lower member of the Round Rock Formation near mouth of Bald Mountain Wash 1 km southeast (outside) of inferred structural margin of Manhattan caldera; view to northeast. Mesobreccia layer about 3 m thick overlies ash-flow tuff layer in cliff, and is overlain on higher slope by interbedded ash-flow and mesobreccia layers; megabreccia layers cap hills in background and underlie older ash-flow tuff of the lower member a few hundred meters to the left of the view. (See also pl. 1, cross section D-D'.) vent. One of these blocks, northwest of Manhattan, is Ordovician limestone and argillite (pl. 1); and another, near the mouth of Bald Mountain Wash (pl. 1, cross sec- tion D-D"), is Cretaceous granite. However, some large blocks are found in what we believe to be outflow facies near the structural margin of the caldera but close to inferred vents. One large block of Paleozoic limestone 120 X 250 m in size occurs near East Manhattan Wash (pl. 1), and a huge slab of Paleozoic limestone and argillite about 650 m long is found near the mouth of Bald Mountain Wash (pl. 1, cross section D-D'). Most clasts in the megabreccia unit of Sloppy Gulch are Paleozoic sedimentary rocks. Near Manhattan, Or- dovician argillite and limestone dominate. Quartzite, probably from Cambrian and Ordovician formations, is common; one large clast of Lower Cambrian brown siltstone was mapped (pl. 1). Knotted schist, including some of the Lower Ordovician(?) Mayflower Schist, is a common component of clasts. Cretaceous aplite and granite are less abundant but they are conspicuous where found. Olive-brown argillite, siltstone, sandstone, and conglomerate (containing pebbles as large as 4 cm) of the Permian Diablo Formation (Ferguson, 1924, p. 25-26; Ferguson and Cathcart, 1954) form an im- mense clast (more than 300 m long) 2 km northwest of Manhattan (pl. 1), and small fragments of clastic rocks of the Diablo Formation are scattered elsewhere in the megabreccia unit of Sloppy Gulch near Manhattan. Oc- casional clasts of rhyolitic tuffs, one as wide as 100 m, are found. The tuffs are unlike any mapped in the southern Toquima Range. Compositions of clasts in the megabreccia unit of Sloppy Gulch in the vicinity of East Manhattan Wash and Bald Mountain Wash are similar to those near Manhattan. However, no clasts of the THE MEGABRECCIA UNITS 9 FIGURE 4.-Megabreccia unit of Sloppy Gulch, showing a margin of immense, locally brecciated clast of Permian Diablo Formation (dark- colored indurated layered clastic sedimentary rock) bounded by tuff matrix (light-colored lithic tuff). Diablo were recognized in the eastern areas, and white vein quartz in small fragments was observed near East Manhattan Wash. All of the Cambrian and Ordovician rock types, and the Cretaceous aplite and granite, occur at the surface south or southeast of the Manhattan caldera. The near- est outcrop of the Diablo Formation, however, now lies more than 10 km to the southwest. There the Diablo in part is overlain by thrusted Cambrian and Ordovician rocks (Poole and Wardlaw, 1978) lithologically similar to the Cambrian and Ordovician rocks at Manhattan; the Diablo thus also may be present at depth near Man- hattan. If the Diablo once overlay the lower Paleozoic rocks near the Manhattan caldera prior to late Cenozoic erosion, there are no vestiges remaining there now. Almost all of the clasts of the megabreccia unit of Sloppy Gulch are themselves intensely brecciated. This fact of course led Ferguson (1924, p. 43-44) to describe the rocks as talus that contains fragments mostly only 1-2 in. across. The individual clasts consist of monolith- ologic breccias, even where groups of clasts are of a great variety of lithologies. Some of the megabreccia clasts have been so intense- ly brecciated that the materials within them appear to have been crushed, milled, and stirred. One such clast, an immense slab of Paleozoic limestone about 200 m long and enclosed in vent facies of the megabreccia unit of Sloppy Gulch about 1.5 km east of Manhattan (pl. 1), has been thoroughly "mixed" (fig. 5). Matrix of this clast appears to consist entirely of comminuted lime- stone. The clast was mineralized by minor though per- vasive thin quartz and calcite veins, locally abundant iron oxides, and minor patches of abundant manganese oxide minerals. Another large clast, about 100 m across, of fine-grained, orangish-buff, intensely brecciated tuff occurs in the megabreccia unit of Sloppy Gulch north- west of Manhattan. Within the clast, small fragments of this tuff are surrounded by a fine-grained matrix of fragmental material ranging to as small as submicro- scopic size. In addition, weblike seams of fine-grained material that merges with the matrix appear to ramify through the larger fragments. None of the wall rocks of the Manhattan caldera are brecciated in the manner of the clasts described here, nor is it likely that upper parts of the structural margin of the caldera were so brecciated and then removed entirely by post-caldera erosion. For example, just beyond the east structural margin of the caldera the Diamond King Formation overlies Cretace- ous granite, the megabreccia unit of Sloppy Gulch, and the megabreccia unit of Silver Creek, indicating uneven erosion (locally almost no erosion) of the caldera margin at the time of emplacement of the Diamond King short- ly after caldera collapse. Local preservation of the Dia- mond King in this area indicates that not much more erosion has occurred since the Diamond King was depos- ited, and therefore higher parts of the original caldera wall locally remain, which exhibit no brecciated rocks similar to those of the megabreccia clasts. Some clasts in the megabreccia unit of Sloppy Gulch, large and small, are silicified, iron mineralized, replaced by cale-silicate minerals, or otherwise mineralized. A number of the larger mineralized clasts contain prospect pits. Anomalous metal contents, as high as 2 ppm (parts per million) Ag, 30 ppm Mo, 300 ppm Cu, 700 ppm Zn, and 100 ppm Ni have been detected in geochemical samples collected from prospects in the larger clasts. Much mineralization probably occurred before emplace- ment of the megabreccia, as suggested by the fact that mineralized clasts are embedded in unmineralized tuff, although some of the mineralization was fault con- trolled and appears to have postdated megabreccia emplacement. Rinds of indurated tuff on clasts in the megabreccia unit of Sloppy Gulch are neither conspicuous nor com- mon. A few large fragments, however, show distinctive rinds of tuff a few centimeters thick. One large clast of tuff is surrounded by a finely layered rind of fine-grained tuff surrounded in turn by coarser grained tuff matrix of the megabreccia (fig. 6). Microscope study shows that the rind consists of tuff that contains a mixture of abun- dant noncompacted devitrified shards, phenocrysts, and rock particles occurring as thin layers that appear to be successively plated onto the surface of the clast. The surrounding tuff matrix is coarser grained and contains more crystal and lithic fragments and fewer identifiable shards. Most fragments seem to be surrounded by porous tuff identical to that of the common tuff matrix of the megabreccia. An example of a clast without a rind of tuff is the immense clastic rock fragment of the 10 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA FiGurE 5.-Brecciated limestone, forming part of an immense clast in the megabreccia unit of Sloppy Gulch. Breccia fragments appear to be intensely crushed, milled, and stirred. Diablo Formation northwest of Manhattan (fig. 4). The few clasts in the megabreccia unit of Sloppy Gulch seen to have rinds are all volcanic rocks. MEGABRECCIA UNIT OF MARIPOSA CANYON The megabreccia unit of Mariposa Canyon occurs in a small area along the north margin of the Manhattan caldera about 1 km north of Mariposa Canyon (pl. 1). Much of the unit contains large fragments of Paleozoic rocks; here the unit is interpreted to fill a vent (pl. 1, cross section A-A '). A steep gravity gradient at the margin of the caldera in this area and the presence of a gravity "low'" just south of exposed megabreccia (pl. 1) suggest thick caldera fill and a possible vent zone; alternatively a steeper caldera wall and thinner fill are possible. Thin layers of mesobreccia as intracaldera facies extend into the lower member of the Round Rock Formation, to which this unit is assigned, south- ward from the interpreted vent. The matrix of the megabreccia unit of Mariposa Canyon is similar to that of the megabreccia unit of Sloppy Gulch. Blocks as large as 100 m occur in the megabreccia unit of Mariposa Canyon. Most of the fragments in the megabreccia unit of Mariposa Canyon are schist that is lithologically similar to schist that forms the wall rock of the Round Mountain lobe of the granite of Shoshone Mountain. Other clast lithologies (granite, for example) are present in small amounts, but the megabreccia was not studied in enough detail to allow description of all the rock types that make up the clasts. Insufficient study of clasts in the megabreccia unit of Mariposa Can- yon prohibits a description of their internal structure and mineralization, and no information was recorded on possible clast rinds. MIDDLE MEMBER OF THE ROUND ROCK FORMATION The middle member of the Round Rock Formation consists mostly of fragments of reddish-brown rhyolite THE MEGABRECCIA UNITS FIGURE 6.-Sample of fine-grained rind broken from margin of a large (several meters) clast of tuff in the megabreccia unit of Sloppy Gulch. Rind consists of the finely layered tuff on left half of sample. Right half of specimen is typical tuff matrix of the megabreccia. and gray andesite. The rhyolite previously was con- sidered to compose a separate formation that original- ly was named (Ferguson, 1924, p. 50-51) the Maris Rhyolite, for outcrops in the vicinity of the Maris Mine east of the mouth of Bald Mountain Wash. Ferguson and Cathcart (1954) subsequently abandoned the name Maris Rhyolite in favor of the Oddie Rhyolite, and Shawe (1987) restricted the Oddie Rhyolite from the Manhattan area and reassigned the rhyolite to the mid- dle member of the Round Rock. The middle member is widely distributed as intracaldera facies throughout the Manhattan caldera (pl. 1), forming a more or less regular, thin layer averaging about 40 m thick (pl. 1, and cross sections). Near a rhyolite plug and dikes (unit Trmr), 3-4 km northwest of Manhattan, the member appears to be much thicker and is interpreted there to be in part vent facies (pl. 1, cross section The vent zone of the middle member probably is within the in- terpreted principal vent zone of the megabreccia unit of Sloppy Gulch near the plug and dikes. Small patches outside the caldera near East Manhattan Wash and the Maris Mine are considered to be outflow facies. The initial volume of the member was about 2 km. Tuff matrix is virtually absent from much of the middle member of the Round Rock Formation, partic- ularly in the northern third of the Manhattan caldera. Where matrix is absent, small to large fragments of gray andesite are contained in reddish-brown rhyolite, or con- versely, small to large fragments of reddish-brown rhyolite are contained in gray andesite. These relations will be described in more detail in the discussion of the 11 megabreccia clasts. In the southern two-thirds of the caldera, where tuff matrix is irregularly present, the matrix may constitute only thin weblike seams sepa- rating breccia fragments of great size range, or it may make up half or more of the megabreccia (fig. 2). The tuff matrix of the middle member of the Round Rock appears identical in character to tuff of the upper member. Distribution and size of clasts in the middle member of the Round Rock Formation have been difficult to evaluate. For example, zones where significant tuff matrix is present appear to have random distribution throughout the southern two-thirds of the Manhattan caldera, and only where appreciable ash-tuff matrix is present at the ground surface can sizes of individual clasts be discerned. Locally large clasts more than 10 m across lie in tuff, isolated by a few hundred meters from other clasts. Blocks of reddish-brown rhyolite and gray andesite 10 m across are common in the matrix; smaller blocks are much more abundant, however. One large faceted block (fig. 2) is about 50 m long. Where the tuff matrix appears to be absent and the only lithologies evi- dent are rhyolite and andesite, blocks of one lithology within the other commonly are 10 m across and rarely are much larger. No consistent pattern of size distribu- tion of megabreccia clasts in the middle member throughout the Manhattan caldera was recognized. Clast material of the middle member of the Round Rock Formation varies in composition between reddish- brown rhyolite and gray andesite. Compositions of the clasts range from about 56 to 76 percent SiO,, and other major oxides display more or less straight-line trends between the end members of the series (Shawe and Lepry, 1985). Rock types that are represented in- clude rhyolite, quartz latite, latite, and andesite, although reddish-brown rhyolite and gray andesite are the conspicuous types seen in outcrops. These rocks are extensively brecciated and large masses of one type commonly contain small to large fragments of the other type. At several localities where rhyolite and andesite occur together, andesite dominates in the upper part and rhyolite dominates in the lower part of the member. The reddish-brown rhyolite is a heterogeneous rock type that varies in character throughout. In places, it appears to be a strongly brecciated flow-layered rhyolite containing sparse small phenocrysts, in which are embedded somewhat rounded fragments of crystal-rich structureless rhyolite. In other places, the rhyolite may be entirely an autobrecciated(?) flow-layered rock. In yet other places, it consists of welded ash-flow tuff, with well-defined flattened pumice lapilli, that is partly to extensively brecciated. Disrupted welded ash-flow tuff locally contains rounded fragments of crystal-rich struc- tureless rhyolite (fig. 7). At one locality 3 km northwest of Manhattan, a large area of mostly unbrecciated FIGURE 7.-Deformed reddish-brown rhyolite of the middle member of the Round Rock Formation. Welded ash-flow tuff that contains conspicuous flattened pumice lapilli is cut by healed diagonal frac- tures that offset pumice fragments. Note rounded clast of crystal- rich structureless rhyolite at lower left, sheared tangentially along a diagonal fracture. Thin light-gray edge at top is part of a clast of gray andesite embedded in the reddish-brown rhyolitic welded tuff. rhyolite contains conspicuous flattened pumice lapilli striking about N. 70° E. and dipping 75° N. This large slab apparently is an undeformed but rotated remnant of ash-flow tuff deposited initially near an eruptive center now marked by the plug and dikes of reddish- brown rhyolite (Trmr). Small (boulder-size) clasts of undeformed welded ash-flow tuff are common in the middle member of the Round Rock near the mouth of Bald Mountain Wash. Andesite of the middle member is plagioclase-rich, generally brecciated rock that contains conspicuous phenocrysts of hornblende and biotite in varied amounts. Ferguson (1924, p. 52-53) described these rocks as hornblende and biotite andesite porphyry dikes that "cut the Round Rock member and apparently also the Maris rhyolite, although no clear proof of this could be found." Hornblende- and biotite-dominant types tend to be segregated within the megabreccia, and are not known to be extensively mixed in particular localities. The internal structure of the andesite is more simple than that of the rhyolite. Commonly it consists of a breccia of subrounded fragments of hard gray andesite 0.1-0.5 m across in a soft (pulverized?) andesite matrix. In places the clasts of gray andesite are themselves in- ternally brecciated. ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA At one locality at the margin of the Manhattan caldera about 0.5 km southwest of the plug of reddish- brown rhyolite (unit Trmr), conformable layers of flow- layered (non-brecciated) andesite strike about N. 65° W. and dip 75° SW. The flow-layered andesite appears to be interlayered with megabreccia and tuff. Possibly these layers formed on the flank of a volcano, once located near the site of the reddish-brown rhyolite plug but later destroyed. Subsequent eruption(s) from the volcano (to emplace the middle member of the Round Rock) and intrusion of the rhyolite plug may have caused steep tilting of the andesite layers. None of the rhyolite-to-andesite rock types that dominate the middle member of the Round Rock For- mation are known more than a few hundred meters beyond the margin of the Manhattan caldera. The source of the material appears to have been the erup- tive center now marked by the rhyolite plug and dikes just within the caldera northwest of Manhattan. In some areas of megabreccia near Manhattan, the middle member contains not only brecciated rhyolite and andesite clasts but also abundant brecciated clasts of a variety of Paleozoic lithologies derived from the megabreccia unit of Sloppy Gulch. These areas, because of dominance of Paleozoic fragments, were mapped as the megabreccia unit of Sloppy Gulch, and they were not subsequently remapped as the middle member following recognition of their true identity. Incorpora- tion of material from the older Sloppy Gulch into the middle member probably occurred close to an eruption vent when the middle member was erupted through the Sloppy Gulch. The middle member of the Round Rock Formation is complex in that the various types of clast materials each also constitute a matrix for fragments of the other types. Whether as clasts in tuff or as matrix to the other rock type, the distinctive lithology of the rhyolite and andesite is maintained. Little evidence of mineralization of clasts in the middle member has been observed. Clasts in the middle member of the Round Rock For- mation show rinds only where they are embedded in tuff matrix. Only the reddish-brown rhyolite possesses rinds. One large faceted rhyolite clast (shown in fig. 2) has a thin rind of hard tuff that is "plastered" onto the lightly striated surface of the clast. RHYOLITE MEGABRECCIA Thin layers of rhyolite megabreccia probably no more than 10 m thick are interlayered in the upper member of the Round Rock Formation within the Manhattan caldera near its northeast margin (pl. 1). All of the rhyolite megabreccia is considered to be intracaldera THE MEGABRECCIA UNITS 13 FIGURE 8. -Large (2X3 m) rhyolite clast in the rhyolite megabreccia of the upper member of the Round Rock Formation (pick at upper left gives scale). Note knobby rounded surface that may have formed by a process of spalling, abrasion, or both, in which erosion was greatest along fractures. facies. The matrix of the rhyolite megabreccia unit is the same as that in the enclosing upper member ash- flow tuff. Clasts in the rhyolite megabreccia are smaller on average than those in the other megabreccias. Max- imum size is about 5 m. No areal pattern of size distribu- tion was observed. The rhyolite megabreccia unit contains clasts that are generally more rounded than those of the other megabreccia units. Some large rounded fragments in the rhyolite megabreccia unit display a knobby surface (fig. 8), formed possibly by fracture-controlled spalling or abrasion or by a combina- tion of these mechanisms. Some clasts in the rhyolite megabreccia have irregularly curved surfaces that show a high polish. The clasts consist almost wholly of light- gray rhyolite that contains a moderate amount of phenocrysts of quartz, sodic plagioclase, sanidine, and biotite in a devitrified glass matrix. Quartz phenocrysts are particularly conspicuous in some clasts. Rhyolite of this character has been recognized only in the rhyolite megabreccia. Granite boulders occur locally as clasts in the rhyolite megabreccia. Clasts in the rhyolite megabreccia unit in the upper member of the Round Rock Formation are both brec- ciated and unbrecciated. Brecciated clasts consist of porphyritic rhyolite, with devitrified glassy ground- mass, broken into fragments a few centimeters and less in size, set in a matrix of pulverized rhyolite. In hand specimens the matrix is not conspicuous, but it is marked by a generally vuggy character. As seen in thin section the matrix is locally cemented by minute rami- fying seams of silica (microcrystalline quartz or chalcedony). No rinds have been observed on clasts in the rhyolite megabreccia unit. MEGABRECCIA UNIT OF SILVER CREEK The megabreccia unit of Silver Creek covers a large area along the northeast margin of the caldera (pl. 1). 14 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA Small areas of exposure occur for several kilometers northwest and south of the main patch of megabreccia. A cluster of small rhyolite plugs within the megabrec- cia mark a vent zone that is believed to have been also a principal conduit for earlier eruptions of the megabrec- cia (pl. 1, cross section F-F"). The megabreccia in this area is exposed through a vertical relief of at least 200 m. Again, a steep gravity gradient here at the caldera margin and proximity to a gravity low within the caldera (pl. 1) are interpreted to indicate a vent zone at the caldera margin. Within the caldera and no more than 1 km from its structural margin, the megabreccia forms a thin layer of intracaldera facies rock lying at the top of the upper member of the Round Rock For- mation and beneath the Diamond King Formation (pl. 1, cross sections E-E' and F-F'). The megabreccia unit of Silver Creek occurs also as outflow facies (pl. 1, cross sections E-E", F-F', and G-G') where it extends as far as about 4 km outside the caldera margin at about the same altitude as the margin. In one locali- ty outside the structural margin of the caldera (pl. 1, cross section G-G') the megabreccia lies on a rather steep (25°) slope that is suggestive of a caldera topographic wall. Much of the matrix of the megabreccia unit of Silver Creek consists of tuff identical in appearance to much of the tuff of the upper member of the Round Rock For- mation. In places, however, the matrix of the mega- breccia appears to be entirely comminuted granite (fig. 9). The matrix as seen under the microscope con- sists of pulverized crystals of the granite; the fragments range in size down to nearly submicroscopic. Some of this material, however, as viewed in thin section, con- tains small particles as large as 1 mm that may be devitrified glass, and finer grained (in part submicro- scopic) material that may also be in part devitrified glass. The megabreccia unit of Silver Creek contains most- ly granite clasts, some of which are as much as 10 m across. The larger clasts show a systematic distribu- tion such that fragments as much as 8-10 m across are concentrated near the cluster of rhyolite plugs near the south fork of Silver Creek, those of 4-5 m maximum size occur as much as 1 km outward from the cluster of plugs, and those of 1-2 m maximum size are found beyond (pl. 1). Most of the clasts are nonpor- phyritic coarse-grained granite; some are aplitic or por- phyritic granite. All are identical in appearance to comparable types in the Belmont lobe. In a few places small clasts of flow-layered rhyolite are evident, and in the south half of the outcrop area of the megabrec- cia unit of Silver Creek, brown to reddish-brown clasts of welded ash-flow tuff that contain prominent large flattened pumice lapilli are present. Fragments of other FIGURE 9.-Megabreccia unit of Silver Creek. Clasts (as much as 2 m across) are virtually all granite in a comminuted granite matrix (pocket knife at arrow gives scale). Note general rounding of fragments. lithologies such as the common Paleozoic rock types and brown andesite have been seen only rarely in the megabreccia. The brown ash-flow tuff fragments may have been derived from similar-appearing welded ash- flow tuff layers in the Round Rock Formation, which in this area would be buried deeply within the caldera; no such material is exposed outside the caldera in this vicinity. Virtually all of the clasts in the megabreccia unit of Silver Creek appear to be unbrecciated internally. Like clasts in the rhyolite megabreccia unit in the upper member of the Round Rock Formation, those in the megabreccia unit of Silver Creek are generally more rounded than those in the other megabreccia units. No rinds have been observed on clasts in the megabreccia unit of Silver Creek. THE MEGABRECCIA UNITS 15 MEGABRECCIA OF JEFFERSON CANYON The megabreccia of Jefferson Canyon is widely ex- posed through an area of several square kilometers southwest of the margin of the Mount Jefferson caldera (pl. 3). The megabreccia consists of a relatively thin layer of large areal extent lying upon a nearly flat sur- face of Paleozoic rocks (pl. 3, cross sections H-H' and I-I'). Much of the unit lies outside any known caldera structure and thus is considered to be outflow. Vent- facies megabreccia is nowhere exposed along the mapped margin of the Mount Jefferson caldera (pl. 3), although vent-facies materials are suggested to occur, partly faulted out, in the subsurface (pl. 3, cross sec- tion H-H"). The presence of immense (100-300 m) megabreccia blocks (pl. 3) may indicate proximity to a vent zone near the edge of Big Smoky Valley. The matrix of the megabreccia of Jefferson Canyon is similar in appearance to, though generally it is less welded than, the tuff of Mount Jefferson, to which we believe it to be related. In most places the matrix of the megabreccia of Jefferson Canyon is more welded than that of megabreccias of the Manhattan caldera, and flat- tened pumice lapilli are locally evident. Commonly, tuff is strongly layered adjacent to megabreccia blocks a meter or more in diameter, and layering manifested by flattened pumice lapilli is deflected conformably around blocks; these relations suggest compaction of hot ash around blocks during welding. The tuff matrix in places contains only sparse small lithic fragments that local- ly are more abundant near large megabreccia clasts; elsewhere the matrix is charged with small lithic frag- ments. The matrix shows a chemical kinship with the tuff of Mount Jefferson in that it has a similar content of SiO, (about 69-70 percent) and of other major com- ponents, although slightly less of Al,O, and Na,0 (data from Shawe and Lepry, 1985). At one place on the north side of Jefferson Canyon (shown near the middle of pl. 3), the matrix of the megabreccia appears to be entirely black crystal-rich (quartz-latitic) glass. Crystals are euhedral to frag- mental sodic plagioclase, quartz, biotite, augite, and sanidine, with accessory apatite, zircon, and iron- titanium oxides, set in a groundmass of foliated black glass. As seen in thin section, small lithic particles as much as 5 mm across are present; larger fragments are megascopically visible. The black glass matrix is inter- preted to be the vitrophyric basal part of a welded ash- flow tuff deposit which here forms the matrix of the megabreccia of Jefferson Canyon. Clasts as much as 50 m in dimension occur in the black vitrophyre unit. Fragments in the megabreccia of Jefferson Canyon are of many sizes; one block of Cambrian Gold Hill For- mation quartzite and Paleozoic limestone (pl. 3) is longer than 300 m. Some of the larger blocks occur within what appears to be a channel-fill of megabreccia outflow, ex- tending from an inferred vent near the margin of the Mount Jefferson caldera east of the area of plate 3. Geologic cross section I-I' (pl. 3), drawn approximate- ly along the longitudinal axis of the possible channel, suggests that transport of megabreccia was westward, if no significant warping of the surface took place follow- ing megabreccia emplacement. Fragment distribution in the megabreccia of Jefferson Canyon is irregular (fig. 10). At one locality, megabreccia consisting of large closely spaced blocks of Paleozoic rock in tuff matrix gives way laterally to ash-flow tuff in which megabrec- cia clasts, except for scattered particles as small as a few centimeters, are virtually absent. Clasts of the megabreccia of Jefferson Canyon are of a great variety of lithologic types. Most abundant are fragments of Paleozoic limestone (fig. 11), quartzite (in- cluding that of the Cambrian Gold Hill Formation), argillite, silicified limestone and argillite (in part jasperoid), and Cretaceous granite (some aplitic and some pegmatitic). Lesser amounts of knotted and phyllitic schist, rhyolite welded ash-flow tuff, and black vitrophyre, and minor medium-grained granodiorite and andesite have been seen locally. Most of these rock types are present just south of the Mount Jefferson caldera. A possible source for the rhyolite welded ash- flow tuff and black vitrophyre clasts, however, has not yet been identified, but it is probably the Mount Jef- ferson caldera. Black vitrophyre occurs dominantly as a vitrophyric basal part of the megabreccia, as previ- ously described. Near the west end of the "channel-fill" of the mega- breccia of Jefferson Canyon (pl. 3), a group of quartzite blocks of the Gold Hill Formation, some as much as 20 m across, are linearly disposed within the megabreccia. Such groupings of clasts of the same lithology are not as common as groupings of clasts of varied lithology. Clasts in the megabreccia of Jefferson Canyon are commonly either unbrecciated or only slightly or part- ly so, particularly at their margins (fig. 11), or they are intensely brecciated throughout (fig. 12). Intensely brec- ciated quartzite (fig. 13) consists of angular to sub- rounded fragments varying in size from a few tens of centimeters to tiny particles, all embedded in indurated comminuted quartzite. Brecciated quartzite almost identical to that illustrated in figure 13 has been de- scribed and illustrated by Bowes and Wright (1961, p. 300; Plate XVIIA) and ascribed by them (1961, p. 307-311) to formation as deep-level "explosion- breccia'' more or less in place in the Kentallen, Argyll igneous complex, Scotland. Brecciated granite clasts in the megabreccia of Mount Jefferson are similar in appearance to those of quartzite; 16 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA FiGurE 10.-Megabreccia of Jefferson Canyon. Large isolated block in foreground is Cambrian quartzite embedded in ash-flow tuff; low hills beyond contain numerous closely spaced large blocks of Paleozoic rock of various lithologies in a less conspicuous tuff matrix. View is westward along axis of a probable megabreccia channel fill. microscope study shows that they consist of sub- rounded fragments in a matrix of angular particles of pulverized granite. Considerable pore space in the fine- grained matrix commonly is cemented by iron oxide minerals (deposited as pyrite before near-surface weathering?). A large clast-several meters in size-of brecciated rhyolitic welded ash-flow tuff as seen in thin section con- sists of angular to subrounded fragments set in a shard- charged groundmass in which shards appear to be molded around or deflected against fragments. Sparse interstitial porosity in this rock has been filled with chalcedonic silica. Many clasts in the megabreccia of Jefferson Canyon have well-formed devitrified welded-tuff rinds or hard layered non-welded-tuff rinds. Clasts with diverse rinds or without rinds may be side by side within megabrec- cia matrix; some large clasts appear to have different types of rinds on different parts of their surfaces. One granite clast 4 m in diameter, so thoroughly brecciated and pulverized that it appears megascopically to be jasperoid, is surrounded by a rind a few centimeters thick of layered ash-flow tuff that contains numerous devitrified flattened pumice lapilli and glass shards oriented parallel with the margin of the clast. The layered rind grades outward through a zone about 10 ecm thick into more-or-less structureless ash-flow tuff matrix of the megabreccia. A second clast of similar size, but of light-gray rhyolitic welded ash-flow tuff, is surrounded by a devitrified glass rind about 3 em thick that merges outward through several centimeters into more-or-less structureless tuff (fig. 14). Microscope study of the contact of the rind and the clast shows small microbrecciated streaks of the clast penetrating the rind tangentially. Within the inner 1 cm of the devitrified glass rind are small wisps of flattened glass shards. Outward the rind merges into a zone 1-2 cm thick of strongly layered devitrified welded ash-flow tuff THE MEGABRECCIA UNITS 17 FIGURE 11.-Large clast of partly silicified Paleozoic limestone in the megabreccia of Jefferson Canyon. Thin-bedded limestone was isoclinally folded (in late Paleozoic or Mesozoic time) and later was silicified along bedding layers before incorporation in the megabreccia. FIGURE 12.-Intensely brecciated quartzite clast in the megabreccia of Jefferson Canyon. Tuff rind still clings to part of the upper left surface of the clast. marked by numerous flattened small pumice lapilli and oriented glass shards. This layer of the rind in turn merges outward through several centimeters into nearly structureless less welded ash-flow tuff. Another rhyolite welded tuff clast 3 m in diameter is surrounded by a layered rind 5-15 cm thick of gray welded tuff that has a glassy matrix marked by numerous small flattened pumice lapilli conformable with the surface of the clast. This fragment is embedded in black glass matrix of the megabreccia of Jefferson Canyon. BASAL CONTACT OF A MEGABRECCIA OUTFLOW A basal contact of a megabreccia outflow unit is well exposed only on the south side of Jefferson Canyon; here the megabreccia of Jefferson Canyon rests on Paleozoic limestone just north of an immense quartzite clast of the Gold Hill Formation (pl. 3). At the contact buff-colored ash-flow tuff matrix of the megabreccia contains numerous clasts of limestone and phyllitic 18 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA FiGurE 13.-Close view of intensely brecciated quartzite clast in the megabreccia of Jefferson Canyon. Angular to subrounded fragments of quartzite are embedded in an indurated matrix of comminuted quartzite. schist. Tuff locally penetrates the limestone, which is brecciated and pulverized for about 20 ecm below the contact (fig. 15). Apparently the motion of the mega- breccia during emplacement was sufficient to disrupt the underlying bedrock. Penetration of ash as irregular seams into the disrupted limestone demonstrates a probable incipient stage in the plucking of wall rock and in its incorporation into the megabreccia. DISCUSSION An eruptive origin of the megabreccia units related to the Manhattan and Mount Jefferson calderas is in- dicated by several facts. Much of the clast material in the megabreccia units of the Manhattan caldera is not known outside the Manhattan caldera. For example, the clastic rocks of the Permian Diablo Formation in the megabreccia unit of Sloppy Gulch, the reddish-brown rhyolite and gray andesite of the middle member of the Round Rock Formation, and the light-gray rhyolite of the rhyolite megabreccia unit in the upper member of the Round Rock are not found outside the caldera. Nor does it seem likely that some of these rocks could have existed in caldera walls that were removed by erosion. That these rocks originated at depth beneath the caldera seems more plausible than that they originated as gravity slides into the caldera resulting from collapse of oversteepened topographic walls. The locally jumbled (mixed) association of clasts of a great variety of lithologies in both the megabreccia unit of Sloppy Gulch and the megabreccia of Jefferson Canyon suggests that the clasts were not derived by landsliding of masses of wall rocks, which should have left a megabreccia con- taining spatially associated groups of clasts of the same lithology, reflecting distribution of lithologies in the col- lapsed wall rocks. Many of the clasts, as in the mega- breccia unit of Sloppy Gulch, in the middle member of DISCUSSION 19 FIGURE 14.-Devitrified glass rind on rhyolitic welded ash-flow tuff clast in the megabreccia of Jefferson Canyon. Dark-gray devitrified glass rind at left is welded against light-gray rhyolite at right, part of a large clast about 3 m in diameter. (See text for additional ex- planation.) the Round Rock, in the rhyolite megabreccia in the up- per member of the Round Rock, and in the megabrec- cia of Jefferson Canyon are intensely brecciated in a manner unrecognized in any of the wall rocks of the calderas or reasonably inferred to have occurred in higher, now eroded, zones in the structural margins of the calderas. Many of these clasts were brecciated and indurated ("healed") before emplacement of the megabreccias. Such seems particularly true of brec- ciated clasts that have unbroken glass rinds. The sources of the clasts likely were at depth within the root zones of the calderas. The general size distribution of large clasts of the megabreccia units of Sloppy Gulch and of Silver Creek suggests point sources at the margin of the Manhat- tan caldera, probably vent zones along the ring fracture of the caldera. The similarity of the graded-size distribu- tion of large clasts to that of large clasts in "co- ignimbrite lag-fall"" deposits in the Acatlan Ignimbrite of west-central Mexico described by Wright and Walker (1977) and in "proximal-facies'' ash-flow deposits from Mount Mazama in Oregon described by Bacon (1984) is noteworthy. Maximum dimension of clasts in the Mount Mazama proximal-facies deposits is about 7 m. FIGURE 15.-Contact of light-colored tuff of the megabreccia of Jefferson Canyon on dark Paleozoic limestone. Limestone is brecciated and pulverized below contact, and tuff penetrates limestone in an apparent incipient stage of plucking of wall rock by an erupting ash flow. 20 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA A pyroclastic-flow lag deposit that formed during a 1980 phreatic explosion eruption at Mount St. Helens, Wash., contains blocks as long as 3 m near the lip of the explosion pit, and a lithic clast in a pyroclastic flow deposit "derived from great depth" is about 5 m in max- imum diameter (Rowley and others, 1981, p. 506). The megabreccias of the Manhattan caldera may be more akin to proximal-facies ash-flow deposits than to lag- fall deposits inasmuch as large clasts in the Manhattan caldera megabreccias also are interpreted by us to have been transported by pyroclastic flows. (See also Bacon, 1984.) Druitt and Sparks (1982) considered lag-fall deposits to be proximal pyroclastic-flow deposits. The interpretation that megabreccia outside the ring- fracture zones of the Manhattan and Mount Jefferson calderas constitutes outflow material in part depends on whether or not significant topographic walls were developed around the calderas. If initially steep topo- graphic walls can be demonstrated, then megabreccia that forms relatively flat sheets just outside the ring- fracture zones (structural walls) of the calderas could be interpreted as the result of caldera wall collapse that moved brecciated material on a listric surface from perhaps as far as 5 km outside the caldera structural margin onto this margin, as well as into the caldera. This question cannot be resolved unambiguously on the basis of present knowledge. The middle member of the Round Rock Formation oc- curs as apparent outflow lying upon a nearly flat sur- face of Paleozoic rocks and the megabreccia unit of Sloppy Gulch (pl. 1, cross section C-C"). This configura- tion suggests that collapse of an oversteepened caldera wall did not contribute to formation of the middle member of the Round Rock here. The presence of the megabreccia unit of Sloppy Gulch lying on the granite of Pipe Spring 4 km south of the Manhattan caldera seems impossible to account for by collapse of caldera walls. If rock had slid northward toward the caldera because of an oversteepened topo- graphic margin formed during caldera collapse, slide material in the patch of megabreccia would have con- sisted only of granite. Because part of the megabreccia of Jefferson Canyon has a matrix that is analogous to the common black vitrophyric basal part of many welded ash-flow tuffs, it is inferred to have been emplaced in like manner, as a hot ash-flow eruption. Presence of the vitrophyre on a relatively flat surface outside any known calderas in the southern Toquima Range implies deposition as outflow. Virtual confinement of the middle member of the Round Rock Formation within the Manhattan caldera and a more or less even distribution of the member within the caldera, except near the southwest margin of the caldera where the member is appreciably thicker (pl. 1, cross section B-B'), are analogous to the distribu- tion of some ash-flow tuff units within calderas. Rinds and other features that characterize the sur- faces of clasts in the megabreccia units provide impor- tant clues to the emplacement of the megabreccia. Clasts that have no apparent rinds probably indicate near equilibrium between the temperature of the clast and that of the enclosing ash matrix at the time of emplacement, probably by heating of the clasts before or during eruption. Equilibrium between the temper- ature of clasts and of enclosing ash matrix likely also existed during emplacement of the rhyolite megabrec- cia in the upper member of the Round Rock Formation and that of the megabreccia unit of Silver Creek. However, during eruption of other megabreccia units, erupting ash incorporated relatively cool wall-rock fragments, upon which the hot ash apparently formed chilled rinds. Perhaps in some eruptions, clast materials of the megabreccia were derived from wall rocks at dif- ferent levels within the vent, so that deep highly heated fragments became mixed with cool fragments incor- porated near the surface. As a result fragments both with and without rinds, and with rinds in various stages of development, were mixed in the erupting ash column and were deposited side by side when the megabreccia flow came to rest. Such a deposit is represented by the megabreccia of Jefferson Canyon. Probably eruption of the megabreccia unit of Sloppy Gulch took place dur- ing conditions under which deeper Paleozoic rocks were relatively hot and nearer surface volcanic rocks were relatively cool, with the result that generally only the volcanic clasts developed rinds. Likewise, at the time of eruption of the middle member of the Round Rock Formation, only reddish-brown rhyolite was near the surface and sufficiently cool to develop rinds in hot ash. After emplacement on a cool surface outside the caldera, the hot ash in basal parts of some ash flows fused into vitrophyre. Rhyolite blocks picked up during eruption of the rhyolite megabreccia in the upper member of the Round Rock were tumbled and eroded in the erupting column such that their surfaces were plucked, rounded, and in some cases polished. The large faceted blocks of rhyolite seen in the middle member of the Round Rock probably rose in the erupting column more slowly than ash and other small particles. Such a condition could abrade and facet lower sides of large clasts; spurts or lags in the erupting ash, or contact with other large clasts or with vent walls, might tumble the clasts and bring new sur- faces into position to be faceted. The presence of large isolated blocks within ash-flow tuff implies that the blocks became entrained in ash and transported by the ash flow to their present sites, rather DEEP-LEVEL, SUBVOLCANIC "EXPLOSION-BRECCIA" 21 than that they collapsed from an oversteepened caldera wall into tuff newly emplaced within the caldera. Nowhere is there evidence of large blocks "plowing" in- to and disrupting a previously emplaced ash flow. DEEP-LEVEL, SUBVOLCANIC «"'EXPLOSION-BRECCIA" The brecciated clasts of the megabreccia units of the Manhattan and Mount Jefferson calderas are quite similar to breccia described in the root zones of some deeply eroded Tertiary calderas and other igneous centers elsewhere in the world. This similarity suggests that the Manhattan and the Mount Jefferson brecciated clasts may have formed in a manner similar to that in- terpreted for formation of the deep-level subvolcanic breccia. In this report we make no distinction between "gas-explosion'' breccia and "hydrothermal" breccia formed in deep levels, inasmuch as criteria for their distinction have not yet been defined. Bowes and Wright (1961) described a breccia pipe about 200 X 500 m across within the Kentallen, Argyll, Scotland igneous complex and attributed it to a deep- level gas explosion origin. They suggested (1961, p. 310-311) that "sudden increase in gas pressure *** caused cushioned explosions in the joints and fractures of *** quartzite" to form a pipe. "Gas streaming through the pipe caused erosion and transport of the boulders." Subsequent intrusion of magma, possibly ex- plosive, incorporated large blocks of breccia into the magma. Similar subvolcanic breccias in other of the British Tertiary volcanic centers have been described by Richey (1940), Richey and Thomas (1932, p. 805-811), Tyrell (1928), and Bailey and others (1925). Gilluly and Gates (1965, p. 51) described a breccia associated with granodiorite in the northern Shoshone Range, Nev., that appears to have formed as a precursor to granodiorite emplacement, the granodiorite having invaded the breccia locally. They stated, "The breccia is made up of angular fragments and blocks, some as much as 10 feet long, of the immediately adjacent sedimentary rocks ***. The breccia is cemented by cream-colored quartz and chalcedony ***. Perhaps it formed during the intrusion by rock bursting along a crack opened by granodiorite magma." Tweto (1951, p. 527) described wall-rock breccia at the edges of sills near Pando, Colo., that he interpreted to have formed just in advance of sill emplacement by "ex- plosive introduction of fluids or tenuous magma." At the Grizzly Peak cauldron complex in the Sawatch Range, Colo., '"exotic'' breccia described by Cruson (1972) is similar to brecciated clasts of the mega- breccia units in the southern Toquima Range. Cruson attributed formation of the breccia both to cauldron subsidence and to degassing of an underlying magma chamber (gas explosion). Much of the breccia deep in the Grizzly Peak cauldron, presumably formed by gas explosion, consists of brecciated Precambrian rocks devoid of volcanic matrix; some breccia is cemented with "rock flour" and other with coarse-grained quartz. A nearly circular, pipelike body 1,000 m in diameter is choked with large rounded boulders of brecciated Precambrian rocks surrounded by a jacket of similar boulders in a tuff matrix. At the Mt. Lewis cauldron in the Northern Shoshone Range, Nev., subvolcanic breccia pipes 1-2 km in diameter are exposed at a level of erosion that has removed all but local remnants of the cauldron fill (Wrucke and Silberman, 1975; Gilluly and Gates, 1965). The breccia pipes, some of which contain ash-flow materials, are eroded to a depth probably no more than 100-200 m below the pre-caldera surface; they exhibit features of ash-flow vents in the upper reaches of the caldera root zone. The breccia pipes within the Mt. Lewis cauldron demonstrate a sequence of development (Gilluly and Gates, 1965, p. 66-75) consistent with that inferred for the emplacement of some of the megabrec- cia and ash-flow tuff units of the Manhattan and Mount Jefferson calderas. Gilluly and Gates stated, "The primary factor in forming the pipes was the rise of a plug of volatile-rich magma" (p. 75). Initial activity con- sisted of the development of coarse breccia that occurs now as remnants in discontinuous patches along the margins of pipes. The breccia is a mixture of small to large fragments, ranging from microscopic size to "acre size'" and consisting chiefly of material of the adjacent wall rocks (mostly Paleozoic sedimentary rocks). The large blocks commonly are internally fractured. Some of the large blocks may have slumped into position from wall rock as much as 100 m higher. Gilluly and Gates (1965, p. 73-75) suggested that the breccia may have formed through the explosive action of eruptive volatiles and a process of rock bursting into a rising gas stream. Following its formation, the coarse breccia was in- vaded and largely displaced by fine breccia that con- sists of abundant fragments of Paleozoic sedimentary rocks and of Tertiary volcanic rocks, particularly intru- sive porphyries, presumably derived at depth. In addi- tion to Paleozoic rocks and Tertiary volcanic rocks, the fine breccia in one pipe contains abundant fragments of collapsed pumice and perlitic glass. Following development of the fine breccia, pumiceous vitrophyre was emplaced in one of the pipes (Gilluly and Gates, 1965, p. 71-72). The vitrophyre is charged with fragments of Paleozoic rocks, Tertiary porphyritic rocks, and tuffaceous and pumiceous Tertiary volcanic 22 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA rocks. According to Gilluly and Gates (1965, p. 72), the vent probably "was once largely filled with breccia, both coarse and fine, much of which became incorporated in the pumiceous vitrophyre, while the remainder was car- ried to the surface and there deposited as tuffs and extrusive breccia." A quartz latite breccia unit lying on Paleozoic rocks adjacent to a breccia pipe on Mt. Lewis, described by Gilluly and Gates (1965, p. 75-77) as intrusive, was described by Wrucke and Silberman (1975, p. 13) as containing pumice lapilli and as closely resembling a welded ash flow. The pumiceous vitro- phyre in the breccia pipe probably represents magma in a stage of eruption just prior to fragmentation into a gas-charged column of pumice and glass shards, loaded with fragments of wall rocks, that was discharged at the surface as an ash flow or ash-flow breccia. Large blocks of Tertiary conglomerate and bedded tuff, one almost 1,000 m long, have foundered in the breccia pipes perhaps as far as 300-400 m from their original positions (Gilluly and Gates, 1965, p. 68, 72-73). No evidence indicates whether the blocks subsided as a result of magma withdrawal at depth in the pipe or as a result of sinking within a rising column of vesiculating magma or gas-charged ash. Some of the bedded tuff contains lithic blocks as much as 2 m in diameter that according to Gilluly and Gates (1965, p. 73) "clearly record explosive eruptions older than the pumiceous vitrophyre." Gilluly and Gates concluded that the pipes demonstrate alternating intrusion, erup- tion, and subsidence in a "two-way pump action." Clasts in the megabreccia of the Manhattan caldera and of the Mount Jefferson caldera that consist of monolithologic indurated breccia are believed to be analogous to the coarse breccia of the breccia pipes in the root zone of the Mt. Lewis cauldron. Megabreccia in the Manhattan and Mount Jefferson calderas that has sparse matrix is similar to erupted equivalents of the fine breccia of the Mt. Lewis pipes that carries large clasts of coarse breccia, and megabreccia with signifi- cant to dominant ash matrix is similar to erupted equiv- alents of the pumiceous vitrophyre that carries large clasts of coarse breccia. At the localities cited, deep-level subvolcanic "explosion-breccia"' developed in a series of steps: initial fracturing of country rock by streaming gases derived from invading magma; rotation of breccia fragments (in some cases, disruption by a process of fluidization); cementation by rock flour produced by fluidization or subsequently by precipitated minerals (for example, quartz and chalcedony); further fracturing; and incor- poration of breccia clasts either in erupting tuff or in intruding magma. These steps are the same as those inferred for formation of brecciated clasts and their incorporation in ash-flow tuffs during development of the Manhattan and the Mount Jefferson calderas. The mechanism of fracturing, brecciation, and fluidization of solid rocks by gas or other fluid streaming was described in detail by Reynolds (1954). Myers (1975) clarified some of the applications of fluidization. PHYSICS OF MEGABRECCIA ERUPTION The mechanism of emplacement described here can account for the presence of megabreccia clasts with typical dimensions of 10-300 m within a matrix that commonly makes up less of the total rock unit volumetrically than do the clasts. The primary question to resolve is whether or not clasts could have been emplaced in a megabreccia as a result of upward dis- placement, rather than solely as a result of downward, gravity-induced displacement. Previous theoretical work has considered fragments smaller than those observed at Manhattan and at Mount Jefferson: frag- ments less than 10 m long in explosive, maar-type erup- tions (McGetchin and Ullrich, 1973) and fragments less than 0.5 m long in sustained eruptive column events (Sparks and others, 1978). Expulsion of material at the scale that occurred at Manhattan and at Mount Jeffer- son has never been observed in historic times (Francis, 1983). Earlier workers have made simplifying assump- tions and estimated eruption parameters given certain initial conditions. Some of these assumptions are not strictly appropriate to the ejection of abundant large blocks. Volcanic eruptions can generally be divided into two types: explosive eruptions that consist of very low density, gaseous matrix material or fluid, and gas- streaming eruptions that consist of denser, spatially more continuous fluid material. In explosive eruptions (McGetchin and Ullrich, 1973), rock fragments have limited ballistic ranges. A rock fragment with an aver- age density of 2,400 kg/m}, upon which the force of gravity is exactly balanced by the upward pressures ex- erted by the erupting gas column, is characterized as follows: mass Xgravity=pressure X area. Then, L=p/(D,g)=480 m if L is the dimension of the fragment, Df is the frag- ment's density, g is gravity, and p is the pressure ex- erted, taken here as 100 bars. Any fragment within the eruptive blast smaller than 480 m would be accelerated upward while acted upon by a pressure of 100 bars. For example, if L=100 m, Force=pL*-D.L'g=7%X10" nt PHYSICS OF MEGABRECCIA ERUPTION 28 The acceleration, a, neglecting all drag, would be a=Force/D.L}=30 mis" McGetchin and Ullrich (1973) considered explosive, maar-type volcanic eruptions more rigorously than the foregoing analysis, but calculated parameters for en- trained fragments only as long as 10 m. By consider- ing friction and dynamic conditions, they found typical conditions to include a maximum dynamic pressure of 96 bars at 500 m depths, decreasing to 9 bars at the surface where the exit velocity was 330 m/s. At these pressures a fragment 100 m in dimension would no longer be accelerating when it left the vent. A fragment 10 m in dimension would accelerate at about 30 m/s and have an exit velocity of about 265 m/s and a vacuum ballistic range of about 6 km (McGetchin and Ullrich, 1973, table 4). No calculations were made for fragments larger than 10 m, but extrapolations of their exit- velocity curves indicate that low velocities (<100 m/s) would be expected. With an increase in the amount of large fragments in an eruptive mass, assumptions about friction with the vent wall, dynamic drag, buoyancy, and turbulent flow become less certain. On the basis of the calculations, the large blocks would not be expected to travel far. Several other exit-velocity calculations have been based on observed ejecta. Minakami (1980) derived velocities of 180-210 m/s from observations of 1-m blocks, 3.5 to 4.5 km from Asama, Japan. Melson and Saenz (1968) calculated a 220-m/s ejection velocity for fragment impacts 5.5 km from the 1968 eruption site of Arenal, Costa Rica. Kieffer (1981) used an initial reservoir pressure of 125 bars and an initial fluid veloci- ty of 100 m/s to model the May 18, 1980, blast of Mount St. Helens. Maar-type eruptions are considerably smaller than the voluminous eruptions of tuff envisioned at the site of the Manhattan caldera. Rather than small gas (maar) eruptions, massive, ash-charged eruptive columns of pyroclastic material, with subsequent collapse and basal outflow, are hypothesized. The kinematics of an entrain- ed block fragment in a large eruptive column would be more akin to those of a golf ball in a fountain. The massive scale and catastrophic nature of large volcanic eruptive columns and their gravitational collapse re- quire considerations different from those of small gas eruptions. Fragments 100 m long might be only a minor component of the total eruptive column, and their in- fluence on boundary effects, such as friction along the vent walls, would not be a critical factor in dynamics of the eruption. Sparks and others (1978) modeled the generation, movement, and emplacement of pyroclastic flows by column collapse, and their results provide estimates of conditions surrounding the movement of large blocks in these eruptions. Their calculations in- dicate that a fragment 0.5 m in radius and 2,500 kg/m in density can remain suspended in a fluid matrix of 500-kg/m' density if it moves at 200 m/s; the en- trained solids-and-gas mixture resembles a high-velocity jet with a height of as much as several kilometers above the vent, above which the column becomes a thermally driven convective plume (Walker, 1973). The upward velocity of the column was found by Sparks and others (1978) to decrease quickly after leaving the vent. A column leaving a 200-m-radius vent decelerates from 200 m/s to almost 0 m/s about 3 km above the surface and then re-accelerates slightly (fig. 16). In this exam- ple, the initial, postcollapse flow velocity would be about 120 m/s and could sustain fragments 0.1 m in radius. An initial vent velocity of 400 m/s would produce an initial flow velocity of about 180 m/s and could sustain fragments 0.4 m in radius. The large blocks observed in the megabreccia units at the Manhattan and Mount Jefferson calderas appear to have been emplaced by a process similar to that described in the preceding paragraph, but their dimen- sions are orders of magnitude greater than those con- sidered above. The larger blocks undoubtedly did not rise very far in the eruptive column, but probably were pushed out of the vent area and entrained in the ash flows when they formed immediately after column col- lapse (fig. 16; pl. 1, cross section C-C"). The hindered-settling ratio for particles in suspension is useful for analysis of an ash eruption that contains large clasts, if the eruptive column over short vertical distances is considered to be a non-accelerating fluid with turbulent flow. The hindered-settling ratio is fac- tored into the equation for the settling velocity of a suspended fragment (Gaudin, 1939) to obtain the hindered-settling velocity, v, in a fluid composed mostly of fragments, v=(8gLI8Q((D,. D )-1))" where L is the dimension of the fragment, Df is the density of the fragment, D, is the density of the slurry matrix fluid, Q is the coefficient of resistance (varies be- tween 0.4 for a sphere and 5.0 for a prism), and g is grav- ity. Figure 17 indicates hindered-settling velocities for spherical rock fragments with a density of 2,400 kg/m® in slurries of varying density. A 100-m diameter spherical block in a slurry matrix with a density of 800 kg/m? would settle at 114 m/s; if the column were moving upward at 200 m/s, the block would move up- ward relative to the surface (fig. 16). This approximation is not applicable at low slurry densities (<10 kg/m'), inasmuch as the fluid is no 24 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA 500 METERS 0 1000 FEET FIGURE 16.-Generation of a pyroclastic flow and megabreccia. Sketch is at same scale as cross section C-C", plate 1; rock unit symbols are as on plate 1; Pzs, Paleozoic rocks; Trl, lower member of the Round Rock Formation; Tris, megabreccia unit of Sloppy Gulch. If hindered settling velocity of an entrained fragment, v (=114 m/s, see text), is exceeded by velocity of erupting matrix material, v,,, (=200 m/s), net velocity of fragment, Vp (=86 m/s), is upward. (Modified from Francis, 1983, fig., p. 66-67). longer truly a suspension, but rather two suspensions, one with large dense particles and one with fine par- ticles. For reference, air at room temperature has a den- sity of about 1.3 kg/m?. McGetchin and Ullrich (1973) calculated a density of 17 kg/m} for material immedi- ately after its exit from the vent, and they calculated densities of 240-250 kg/m? for magma several hundred meters beneath the surface. Sparks and others (1978) modeled densities between 1 and 500 kg/m? for mate- rial in collapsing columns. Large blocks 100 m in dimension would be expected to move upward while incorporated in the erupting fluid and gas several hundred meters beneath the surface (figs. 16, 17). Upon ejection these blocks would rapidly decelerate, fall out of the rising column, and come to rest near the vent. If deposition were from voluminous ash flow moving across a surface sloping gently away from the vent, large clasts presumably could be trans- ported considerable distances within the ash flow. SUMMARY Early in the history of the Manhattan caldera, follow- ing emplacement of a large body of magma in the up- per part of the crust, initial degassing of the magma SUMMARY 25 100 - FRAGMENT SETTING VELOCITY (v), IN METERS PER SECOND | | | | | _L _] 0 200 400 600 800 1000 SLURRY DENSITY (D,), IN KILOGRAMS PER CUBIC METER FIGURE 17.-Plot of hindered-settling velocities of entrained fragments as a function of fragment size and matrix/slurry densi- ty. Fragment sizes of 1, 10, 100, and 500 m are considered. McGet- chin and Ulrich's (1973, table 4) calculations of dynamic slurry velocity and density are plotted (dashed line) for comparison; 100-m curve intersects this plot where the density is defined at depths of 200 m from surface. resulted in gas streaming upward along fracture zones. The main zone of streaming gas and entrained particles was along what was to become the south margin of the caldera. A general west-northwest trend of this margin, parallel with the main strands of the Walker Lane struc- tural zone, hints that initial localization of the Manhat- tan caldera was determined by the regional structural grain. Other fractures may have been induced by magma emplacement. Violent, even explosive stream- ing of gas from the magma chamber resulted in local development of "explosion-breccia'"' of the type recog- nized as characterizing some deep parts of intrusive and volcanic igneous centers. The breccia became indurated either through cementation by hot pulverized and fluid- charged (fluidized) rock or through mineralization subse- quent to the violent brecciation. The zones of gas- explosion brecciation ultimately reached the surface and allowed venting of ash flows that were deposited as the lower member of the Round Rock Formation. In the in- itial "throat-clearing'"' phase of eruption, much vent wall rock was incorporated in the volcanic ash. The wall-rock material consisted of a variety of lithologic types, displaying varied degrees of gas-explosion brecciation depending on how faithfully eruption of this phase followed the earlier-brecciated fracture zones of gas leakage. Also, clasts acquired from wall rocks at deep levels had been preheated because of proximity to the underlying magma chamber and because of gas transfer of heat, whereas clasts from high-level wall rocks were much cooler but perhaps also in part brecciated. Prod- ucts of these early eruptions were deposited as the megabreccia unit of Sloppy Gulch and the megabrec- cia unit of Mariposa Canyon. The largest clasts were deposited close to vent zones and smaller clasts were carried farther from vents, as is typical of co-ignimbrite lag fall and proximal-facies ash-flow deposits. At the Mount Jefferson caldera initial temperatures were higher than at the Manhattan caldera. Thus the ash matrix of the megabreccia of Jefferson Canyon, derived from this caldera, became somewhat welded on emplacement. Temperature was high enough, in fact, that locally the lower part of the megabreccia unit fused after emplacement to form black vitrophyre encompass- ing large blocks of the megabreccia. During emplacement of the remainder of the lower member of the Round Rock Formation above the initial megabreccia deposit, the main subsidence of the Man- hattan caldera took place. Ash-flow and megabreccia eruptions continued periodically thereafter. After the lower member was emplaced, a volcano developed at the south margin of the caldera at the locus of what is believed to have been the main vent of the lower member of the Round Rock. This was the first event leading to the formation of the middle member of the Round Rock. Initially, andesitic magma was in- truded in the vent zone, perhaps with only minor ex- plosive eruption at the surface. At least two varieties, one hornblende bearing and the other biotite bearing, were emplaced as separate intrusions. Parts of the andesitic intrusions were solidified and even autobrec- ciated(?) by the time of emplacement of rhyolitic magma in the volcano. Some of the rhyolite was erupted local- ly at the site of the volcano as small welded ash-flow eruptions, and autobrecciated domes may have formed. At depth part of the rhyolite magma became mixed with andesitic magma, resulting in rocks that are chemical- ly transitional between the end members. Following solidification of rhyolite magma, a cataclysmic explo- sion destroyed the small compound volcano near the south margin of the Manhattan caldera, blasting the mixed andesite-rhyolite across most of the caldera to form a widespread relatively thin layer of megabreccia. A thicker layer of megabreccia fell back in the vicinity of the explosion vent, and near the vent the older megabreccia unit of Sloppy Gulch became incorporated with the megabreccia of the middle member of the Round Rock. The cataclysmic eruption may have been induced by renewed gas buildup from the continuing degassing of the large magma chamber underlying the caldera. The eruption marked the beginning of a phase that led ultimately to emplacement of the upper member of the Round Rock Formation. Materials that 26 ASH-FLOW ERUPTIVE MEGABRECCIAS, NYE COUNTY, NEVADA were thrown the farthest from the blast site at the volcano-the initially erupted material-contained no volcanic ash, accounting for the apparent absence of tuff matrix within the middle member of the Round Rock in the northern third of the Manhattan caldera. As the eruption progressed, ash became mixed with clasts of andesite and rhyolite of the small volcano; ash is evident and locally even abundant in the middle member in the southern two-thirds of the caldera. The inference that andesite dominated in the subsurface and that rhyolite dominated near the surface of the volcano could account for the apparent dumping of andesite locally on top of rhyolite in the erupted megabreccia member and for the apparently cooler temperature of rhyolite than andesite when emplaced (as evidenced by the ash rinds on rhyolite clasts). As a final event in this phase of caldera volcanism, a small plug and related dikes of rhyolite were intruded into the vent of the volcano. Eruption of ash-flow material then continued and the upper member of the Round Rock Formation formed. Midway through emplacement of the upper member of the Round Rock, a vent zone at the northeast edge of the caldera apparently became clogged with rhyolitic magma. Either gas-explosion brecciation or autobrec- ciation disrupted part of this rhyolite, and then subse- quent violent eruption "cleared the throat" of the vent zone to form the relatively small unit of rhyolite megabreccia within the upper member of the Round Rock. Following emplacement of most of the upper member of the Round Rock, a final gasp of explosive activity of the Manhattan caldera caused eruption of the mega- breccia of Silver Creek. Granite wall rocks of the vent had not previously been brecciated by gas explosion, inasmuch as the clasts of the unit are virtually unbrec- ciated. However, because much of the matrix of the granite clasts of the unit is comminuted granite, we infer that eruption involved gas explosion. Again, size sorting of larger clasts close to the inferred vent zone of the megabreccia suggests a co-ignimbrite lag fall or proximal facies of ash-flow tuff (Bacon, 1984). Presence of both thin outflow and thin intracaldera facies of the megabreccia unit of Silver Creek at the structural margin near the inferred vent zone suggests that the caldera at the time of eruption was not depressed much below the surrounding surface (pl. 1, cross sections D-D' and E-E"). After the ash-flow eruption phase of the Manhattan caldera had been completed, small plugs of rhyolite and quartz latite were emplaced along the east margin of the Manhattan caldera. Also, after completion of the ash-flow eruption phase of the Manhattan caldera- within probably no more than about 500,000 years- the Diamond King Formation, the Bald Mountain Formation, and the tuff of Peavine Creek were laid down across the dormant caldera. These units, of course, were derived from sources outside the Manhattan caldera. We believe that the next event in the evolution of the caldera was intrusion of the olive-brown andesite as a stock and associated sills and dikes. Some uncertainty surrounds this assessment, however. The caldera ap- parently was resurgently uplifted: the preserved volcanic units in the central part of the caldera that are younger than the Round Rock Formation stand higher than those in the marginal parts and outside the caldera. Resurgence appears to be the most likely mechanism to have elevated the core of the caldera suf- ficiently to allow gravity sliding of a large mass of volcanic rocks westward to lower terrain within the western part of the caldera (pl. 1). We note that this slide mass is not internally brecciated, as slide masses resulting from oversteepened topography commonly are considered to be. This large slide mass clearly was in place at the time of irruption in the western part of the caldera of the domes or plugs of dacite at about 24.5 Ma (pl. 1), marking the end of caldera activity. The olive- brown andesite, because of its widespread occurrence within the caldera, might be part of a magma body (shallow pluton?) that caused resurgence of the caldera. It remains unresolved, however, whether the andesite was emplaced about 24.5 Ma or whether it was em- placed nearer to its determined age of 22 Ma, and thus was unrelated to caldera activity. Intrusion of andesite now exposed within the Manhattan caldera was at a shallow level, as attested to by the occurrence of abun- dant vesicles in some of the sills. If andesite of this phase of igneous activity were responsible for resurgence, it must have been emplaced as a large, broad mass at depth, below the levels depicted in the geologic cross sections (pl. 1). Resurgence of the Manhattan caldera was not a proc- ess of doming, as can be seen in the geologic cross sec- tion A-A' through the caldera (pl. 1). Instead, the volcanic rocks within the core of the caldera were lifted as a more or less undeformed slab, cylindrical in form. But as the slab was raised, the margins beyond the slab slumped and tilted inward, apparently because the in- ward dip of the ring-fracture zone caused the diameter to increase upward; extension of the intracaldera rocks thereby was required. Thus everywhere in the peripheral zone the volcanic units dip inward toward the core of the caldera. The process is somewhat analogous to that described by Bonham and Noble (1982) for resurgence both of the Mount Jefferson caldera and of the Big Ten Peak caldera about 10-35 km southeast of Manhattan. We conclude that calderas of this region of the Great Basin are of a genre somewhat different from those of REFERENCES CITED 27 the San Juan Mountains and perhaps of some other localities of the world. 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FROELICH U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1472 A clarification of the Triassic-Jurassic stratigraphic sequences, sedimentation, and depositional environments UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1989 DEPARTMENT OF THE INTERIOR MANUEL LUJAN, Jr., Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Library of Congress Cataloging in Publication Data Lee, K.Y. Triassic-Jurassic stratigraphy of the Culpeper and Barboursville basins, Virginia and Maryland. (U.S. Geological Survey professional paper ; 1472) Bibliography: p. Supt. of Does. no. : I 19.16:1472 1. Geology, Stratigraphic-Triassic. 2. Geology, Stratigraphic-Jurassic. 3. Geology-Culpeper Basin (Va. and Md.) 4. Geology-Virginia-Barboursville Basin. I. Froelich, A.J. (Albert Joseph), 1929- II. Title. III. Series. QE676.L44 1989 87-600318 For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 CONTENTS Page Page ABSLTACE lleno 1 | Stratigraphy-Continued INtrOUUCtON ...... 2 Newark Supergroup-Continued Regional setting...... . ao 2 Culpeper Group-Continued Previous investigations a 3 Mount Zion Church 28 Scope, purpose, and method Of Study ...... 9 Midiand FOPMALEIGR. :...... ;.. nl nene neenee rn ies 24 ACKNOWI@USM@NES ...... ccc 9 Hickory Grove Basalt .. . 28 10 Turkey Run Formation ...... iF move revels tones 26 Newark Supergroup 10 Sander Basalt Culpeper Group m .. 10 Waterfall FOFMAbION ..................s ll.... inin rn 28 Manassas slice 12 Millbrook Quarry Member ...... 29 Reston Member General discussion of the depositional mode 30 Rapidan Member _. Di&D&S@ ...... 31 Tuscaror® Creek MeMD@P ...... 14 Distribution and mode of OCCUrrence 31 Poolesville Member ...... . 15 Description Of FOCK ...... 31 Balls Buff SiltStOM@ cem 16 Geochemistry 82 Leesburg Member 19 | Thermally metamorph0S@d POCKS ...... 82 TibbStOWN ...... 20 | Summary .... . 34 Mountain Run MeMbD@P ...... 20 | References cited ...... 865 Haudricks Mountain Member ...... 21 | Appendixes: Catharpin Creek Formation ...... A; Measured SEELIONE soln. leone ber lero nein n 38 B: Drill and core hole descriptions and columns ...... 44 ILLUSTRATIONS PLATE 1. Triassic-Jurassic stratigraphy of the Culpeper and Barboursville basins, Virginia and In pocket A. Evolution of stratigraphic nomenclature in the Culpeper and Barboursville basins, 1928 to present B. Generalized correlation diagram of the Newark Supergroup in the Culpeper, Newark, and Hartford basins C. Bedrock map of the Culpeper and Barboursville basins D. Simplified stratigraphic correlation diagram summarizing age and lithology of the Culpeper Group in the Culpeper and Barboursville basins Page FIGURE 1. Map showing principal Triassic-Jurassic basins containing the Newark Supergroup strata exposed in Eastern NOPth A MEMICE .;. ...... .... ..... 00 ihe. a ote denon coun dea ver Te above a eee dive vae va atea in s Ten becn beh an heen dec aFe Sid i nes aas Tr ea dee d a an des dB a a te aut 3 2. Sketch map of the age of strata, Culpeper Group, Culpeper basin, Virginia and Maryland, showmg the location of diagnostic palynologic samples and palynofloral zones and paleontolO gi¢ 10C@Iiti@$ ...... 5 3. Sketch map of the Culpeper basin, Virginia and Maryland, showing the location of measured sections and selected Aho a o - e S Photographs showing: _____________________________________________________________________________________________________________________________ 6 4. Outcrop of the Reston Member of the Manassas Sandstone in the northwestern part of the Independent Hill 7.5-min Quadrangle, town Of nn cnc cerns 13 5. Outcrop of the Poolesville Member of the Manassas Sandstone in the western part of the Independent Hil] 7.5-mIn QUAATAR&IG® ...... ...... .... .l. Nil. nb ll onne ine he ce evie neon rhan n tn enas on cine aie neun ane en 15 6. - Upper surface of the Poolesville Member of the Manassas Sandstone showing carbonate nodules and BOMeRE ...... ... 2 oo nl cnc o cove Pees £0,300 Bede LEAH de Pho ddan h wena Bh inne nie de chee ns ce 0024 das $e i date ag ae an be rhea ae ane deans e eved e mee tea aoe 15 7. Outcrop of the Poolesville Member of the Manassas Sandstone exposed on the west bank of the Potomac River near the end of Virginia State Route 656, LOUudOUn COUNtyY, e 16 II IV CONTENTS Page FIGURES 8-14. Photographs showing: 8. Bedding plane exposure of ripple marks and mud cracks in the Balls Bluff Siltstone on a quarried slab at the Culpeper Crushed Stone quarry, Stevensburg, CUIPeper COUNtY, V&A. ...... inns 17 9. _ Balls Bluff Siltstone exposed in north quarry face at the Culpeper Crushed Stone quarry near Stevensburg, Culpeper East 7.5-min Quadrangle ie ceded tev ain darned onn 1 fre an th 17 10. Bedding plane exposure of dinosaur tracks in mud-cracked Balls Bluff Siltstone at the Culpeper Crushed Stone quarty, Stevensburg. Culpeper COUNMLY, VA. .. .. ....2...... 02... 2.00. .002. ... ... Ac IL cn ier Ave reer ored rae raves 18 11. Exposure of a limestone conglomerate in the Leesburg Member of the Balls Bluff Siltstone in a road cut of U.S. Route 15, north of Leesburg, Va...... 19 12. Outcrop of indurated greenstone conglomerate of the Mountam Run Member of the Tibbstown Formatlon exposed at the west end of Chandler Street, CUIDEDET, neenee. 21 13. Outcrop of interbedded sandstone and siltstone of the Catharpin Creek Formation in the east-central part of the ThOrOUghfare G&P 7.5-MiN QUAUTANGIG neenee. 22 14. Mount Zion Church Basalt exposed on the north cuts of the Southern Railroad about 50 m northwest of the intersection with U.S. Route 15, in the east-central part of the Thoroughfare Gap 7.5-min QUAGFAR@IG -. ...... ¢, .... 5%... .... ... 2. ... i Bel eon bere on aree n a a win a ins be h ve ie eosin wv sacs a hone n aa a 23 15. Photomicrograph of the Mount Zlon Church Basalt from an exposure in Virginia State Route 600 east of BUL IMN 2... ... 19h... en Foie en ca cnc conned n ine oor ne ie c tie Polen coi ie AR A ea cs acai tt a ee cdo ve eve vae ots afa 24 16, 17. Photographs showing: 16. Exposure of the Hickory Grove Basalt on the south bank of Broad Run in the southern part of the Thoroughfare Gap 7.5-min im. Arin. m Anns onn honom Anoop nve tulane 25 17. Sander Basalt exposed in the northwestern part of the Sander quarry, 7 km southeast of Warrenton, FAIGUWEF COUNTY, V 000... emi ogc iw pare 18. Photomicrograph of zeolitic amygdules in the upper sequence of the Sander Basalt at the Sander quarry, o o ah at eaid a s a aoe aa / - 27 19. Diagrammatic sketch showing stratigraphic relations of an alluvial fan to lacustrine deposits in a closed basin, the facies distribution, and the rAdi@l f2N nnn 30 20. Photograph showing diabase at MOUunt PONY, CUIPEDEP COUNTY, V &...... cnn cnn nn nii. 82 21-25. Photomicrographs of: 21. - Mount Pony diabase, CUHIPEDBF COUNEY;, V ill ole leni nne nle eme nnn cen cen de hone neon oon ronson gaan 32 22. Pegmatitic phase of a diabase exposed in the Luck quarry, LOUudoun COUNtY, V&A... 33 23. Granophyric phase of diabase along the south side of Mountain Run, Culpeper County, 33 24. Cordierite-hornfels from the Chantilly Crushed Stone quarry, eastern Arcola 7.5-min Quadrangle, Toudoun County, VA..) 2... oll hina ac iaa a eats i t e aat n ae d 33 25. Granulite (granofels) from the east bank of Little Rocky Run, Fairfax COUNtY, 34 TABLES Page 1. Summary of measured sections in the Culpeper b@8in, Virginia @Ad MArYIANG ...... 7. TABLE 2. Summary of core holes and selected drill holes in the Culpeper basin, Virginia and Maryland ...... 8 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VIRGINIA AND MARYLAND By K.Y. LEE and A.J. FROELICH ABSTRACT The Culpeper basin of northern Virginia and adjacent Maryland is an elongate, north-northeast-trending, fault-bounded trough con- taining a thick sequence of Upper Triassic to Lower Jurassic nonmarine sedimentary rocks. The similar but much smaller Barboursville basin, a few kilometers to the south, contains only Upper Triassic clastic sedimentary rocks. The Lower Jurassic strata of the Culpeper basin are interbedded with a series of basalt flows, and both Upper Triassic and Jurassic rocks are intruded by Early Jurassic tholeiitic diabase; the Barboursville basin contains no known igneous rocks. The Culpeper Group of the Newark Supergroup is herein used for the entire sequence of lower Mesozoic strata in both basins. The term "lower part of the Culpeper Group" is used informally for the mainly Upper Triassic sequence of continental sedimentary rocks occupying the entire Barboursville basin and the southern quarter and eastern half of the Culpeper basin; the "upper part of the Culpeper Group" includes the Lower Jurassic sedimentary rocks and intercalated tholeiitic basalt flows restricted to the west-central Culpeper basin. The lower part of the Culpeper Group, mostly of Late Triassic age, is subdivided into four formations: the Manassas Sandstone, the Balls Bluff Siltstone, the Tibbstown Formation, and the Catharpin Creek Formation. The Manassas Sandstone contains three distinctive lenticular cobble and boulder conglomerate members that lie at the base of the Culpeper Group in different areas along the eastern margin of the basins. These are the Reston Member, the Rapidan Member, and the Tuscarora Creek Member. The conglomerates each grade upward and laterally into the widespread Poolesville Member, the main arkosic red sandstone unit of the Manassas. The Balls Bluff Siltstone conformably overlies, grades into, and intertongues with sandstone of the Manassas and occupies the medial part of both basins. The Balls Bluff is predominantly red-brown calcareous siltstone and fine-grained sandstone intercalated with greenish-gray to dark-gray fossiliferous mudstone; it intertongues with the Leesburg Member, which is predominantly limestone conglomerate, in the northwestern Culpeper basin. The Balls Bluff Siltstone is conformably overlain by the Tibbstown Formation in the Barboursville basin and the southern Culpeper basin, and by the Catharpin Creek Formation in the central Culpeper basin. The Tibbstown Formation, predominantly arkosic sandstone at the base, contains the Mountain Run Member, mainly greenstone cobble conglomerate, and the Haudricks Mountain Member, primarily quartz, phyllite, and schist pebble conglomerate. These conglomerates are present at the top of the formation on the west side of the basin, Manuscript approved for publication February 19, 1987. and this upward-coarsening sequence constitutes the youngest sedimentary rocks of the Culpeper Group in the Barboursville and southern Culpeper basins. Similarly, the Catharpin Creek Formation in the central Culpeper basin, mainly arkosic sandstone at the base and containing rocks of both Late Triassic and Early Jurassic age, grades laterally and upward into the Goose Creek Member, a coarse conglomerate. The upper part of the Culpeper Group of Early Jurassic age consists of the Mount Zion Church Basalt at the base, locally containing lenses of sandstones and siltstones that separate two basalt flows. The upper basalt flow is overlain by the Midland Formation, mainly reddish-brown sandstone and siltstone with gray, fossiliferous, calcareous shale zones near the base, succeeded by the Hickory Grove Basalt. The Hickory Grove contains lenticular sandstone and siltstone bodies that separate three or more basalt flows. It is overlain by the Turkey Run Formation, predominantly sandstone interbedded with red-brown and gray-green siltstone, capped by the Sander Basalt. The Sander Basalt comprises the greatest number and thickest series of flows, and contains the thickest and most extensive lenticular sandstone and siltstone inter- calations. The overlying Waterfall Formation consists of interbedded sandstone, siltstone, conglomerate, and several fossiliferous, calcareous shale zones. The Waterfall displays several local and at least one regional unconformity near Thoroughfare Gap, where it is overlain by the Millbrook Quarry Member, a coarse boulder and cobble conglomerate that is the youngest Jurassic sedimentary unit of the Culpeper Group. Fossil flora and fauna are present but generally sparse in the "red beds" of the Culpeper and Barboursville basins. Phytosaur bones of Late Triassic age have been identified from the Balls Bluff Siltstone; dinosaur tracks are well preserved in the same formation and in the Manassas Sandstone. Dinosaur tracks of Early Jurassic age occur in the Turkey Run and Waterfall Formations. More important, shales of the Midland and Waterfall Formations have yielded well-preserved freshwater fish. Conchostracans, ostracodes, fish scales, insect parts, and diagnostic spores and pollen are also present in the Manassas Sandstone, the Balls Bluff Siltstone, and the Tibbstown, Catharpin Creek, Midland, Turkey Run, and Waterfall Formations. The presence of Late Triassic and Early Jurassic spores and pollen throughout the exposed stratigraphic section has enabled the systematic palynofloral zonation of the entire Culpeper Group, supported in places by characteristic fish zones. Strata of the Culpeper Group range in age from the Late Triassic (at least late Carnian, about 225 Ma) to the Early Jurassic (late Sinemurian or early Pliensbachian, about 193 Ma), with the Triassic-Jurassic boundary (about 208 Ma) a short distance below the base of the Mount Zion Church Basalt in the upper part of the Catharpin Creek Formation. +A TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. The distribution of strata in the Culpeper Group is explained by accumulation of clastic sediments in a closed basin flanked by fault- block mountains that were episodically uplifted and eroded. Deposi- tion within the basin was controlled by subsidence accompanying extensional tectonics during a dominantly semiarid climate punctuat- ed by periods of abundant precipitation. The Manassas, Tibbstown, Catharpin Creek, and Waterfall Formations and the Leesburg and Goose Creek Members of the Balls Bluff Siltstone reflect deposition near basin margins by braided streams and debris flows of alluvial fan complexes. The principal part of the Balls Bluff Siltstone, on the other hand, contains primary structures that suggest deposition in playa lakes, on subaerial silty fluvial mudflats, or at the distal parts of alluvial fans. The siltstone and shale cycles of the Balls Bluff, Midland, and Waterfall indicate deposition in lakes. Some of the lakes of the Balls Bluff were playas; others in the Midland and Waterfall Formations were probably perennial. Turbidite deposition in the lakes was locally important. Numerous local unconformities, commonly overlain by conglomerate, attest to the tectonic instability of the mountainous source areas flanking the Early Jurassic lakes. Tectonic instability in the Early Jurassic, indicated by upward- coarsening sequences in the Catharpin Creek Formation, was accompanied by fissure flows of basalt. During the late stages of Jurassic deposition of the Culpeper Group, tholeiitic diabase stocks, sills, and dikes (about 195 Ma) extensively intruded the sedimentary rocks and basalt flows and thermally metamorphosed them. The strata in the Culpeper and Barboursville basins were regionally tilted to the west, particularly along the western border faults, which produced steep dips, drag folds, gravity slumping, and slippage along bedding planes; broad, gently southwest plunging folds, and en echelon and strike-slip faults along the western basin margin; and folds and transverse faults within the basins. INTRODUCTION The original stratigraphic nomenclature of the Cul- peper basin was introduced by Roberts (1922, 1923, 1928) in Virginia. Jonas and Stose (1938) later intro- duced nomenclature from the Gettysburg basin into the Maryland part of the Culpeper basin; these units did not match those of Roberts. For about half a century there were no significant advances in the stratigraphy of any of the early Mesozoic basins of Eastern North America. The systematic palynological studies of Cornet (1977) provided precise new temporal data in previously undatable rocks that proved the presence of Early Jurassic as well as Late Triassic strata in the northern early Mesozoic basins of Eastern North America. Widespread stratigraphic revisions have followed. In the Culpeper basin, four systems of stratigraphic nomenclature have recently been proposed (Cornet, 1977; Lee, 1977, 1979, 1980; Lindholm, 1979). Objections have been raised to each, and no single scheme is in common use. Plate 1 (A4) summarizes the evolution of stratigraphic nomenclature in the Culpeper basin. None of these systems, which describe the same sequence of rocks, is without shortcomings, whether based on recent geologic mapping (Lindholm, 1977, 1978, 1979; Lee, 1979, 1980; Gore, 1983), recent studies of fossil fish and dinosaur tracks and bones (Olsen, 1978, 1984; Olsen and others, 1982), or studies of spores and pollen (Cornet, 1977). Furthermore, none of these systems is compatible with recent revisions of the Newark Supergroup in Eastern North America (Froelich and Olsen, 1984), with usage in the Hartford basin as shown on the State geologic maps of Massa- chusetts (1983) and Connecticut (1983), or with the most recent revisions in the Newark basin of New Jersey and Pennsylvania (Olsen and others, 1982; Olsen, 1984). A reexamination of the Late Triassic and Early Jurassic stratigraphy of the Culpeper basin indicates that it can be correlated with the newly revised stratig- raphy of the Hartford and Newark basins to the north. In each of these major basins, two clastic sedimentary successions of Early Jurassic age occur between three major basalt flow sequences. The strata between the flows are now considered formations: the Shuttle Meadow and East Berlin Formations in Connecticut and Massachusetts and the Feltville and Towaco For- mations in New Jersey. This pattern of nomenclature is herein extended to the southernmost exposed Jurassic strata in Eastern North America by defining and naming the Midland and Turkey Run Formations in the Culpeper basin. All of the remaining stratigraphic revisions stem logically from (1) considering the sedi- mentary sequences that are separated by basalt flows to be formations, (2) elevating the Newark Group of Eastern North America to a supergroup, and (3) establishing the entire section of Triassic and Jurassic strata in the Culpeper and Barboursville basins as a group. REGIONAL SETTING The Culpeper and Barboursville basins are centrally situated in a belt of Late Triassic to Early Jurassic fault-bounded troughs exposed in Eastern North America from Nova Scotia to the Carolinas. These troughs, containing nonmarine Newark Supergroup strata, are generally in alignment with the structural grain of enclosing upper Precambrian and lower Paleozoic erystalline and sedimentary rocks, and are located mostly within the Piedmont province of the Appalachian orogenic belt (Rodgers, 1970, p. 203-207; fig. 1, this paper). The Culpeper and Barboursville basins originated and evolved during the early Mesozoic, a time of continental rifting that preceded Coastal Plain deposition and the development of the modern Atlantic continental margin (Dickinson, 1974; Van Houten, 1977b, p. 83, 89-96). The Culpeper basin occupies about 2,750 km*, most of which is in northern Virginia. At the southern end of the Culpeper basin, Conley and Johnson (1975) delineated a small, isolated INTRODUCTION 3 EXPLANATION . Wadesboro (N.C. -S.C.) . Sanford (N.C.) . Durham (N.C.) . Davie County (N.C.) . Dan River and Danville (N.C. - Va.) . Scottsburg (Va.) . Basins north of Scottsburg (Va.) . Farmville (Va.) . Richmond (Va.) . Taylorsville (Va.) . Scottsville (Va.) . Barboursville (Va.) . Culpeper (Va. -Md.) . Gettysburg (Md. -Pa.) . Newark (N.J.-Pa. - N.Y.) . Pomperaug (Conn.) . Hartford (Conn. - Mass.) . Deerfield and Northfield (Mass.) . Fundy (Nova Scotia- New Brunswick, Canada) . Chedabucto (Nova Scotia, Canada) 0 100 200 300 MILES 0 _ 100 200 300 400 KILOMETERS FIGURE 1.-Principal Triassic-Jurassic basins containing the Newark Supergroup strata exposed in Eastern North America. trough, separated from the main basin to the northeast by about 2.5 km of Precambrian metamorphic rocks. This smaller trough, occupying about 51 km, was named the "Barboursville basin" (Lee, 1980) after the town of Barboursville, Orange County, Va. The Culpeper and Barboursville basins contain fluvial and lacustrine clastic sedimentary rocks, chiefly "red beds" and conglomerates. A large number of Lower Jurassic fissure basalt flows are present throughout the west-central part of the Culpeper basin. These basalt flows are generally conformable with bedding and, in places, are intercalated with lenses of unaltered, fine-grained red beds and conglomerates, but neither the enclosed strata nor the overlying and underlying beds apparently contain volcanic ash. Locally, fresh basalt cobbles and boulders in polymict conglomerate are intercalated with Hickory Grove or Sander Basalt flows, and in places basalt forms the matrix of marble, basalt, and quartzite cobble conglomerate. These characteristics indicate that fissure eruption and flows were syndepositional with continental clastic sedimen- tation. Toward the end of basin filling, the sedimentary rocks and basalt flows in the basin were tilted toward the west and northwest. The tilting may be related to subsidence flanking the uplift of the central axis of the Appalachian deformed belt to the east (McKee and others, 1959, p. 24, pl. 9). This uplift may have been caused by crustal thinning and mafic magmatic intrusion at depth (Ballard and Uchupi, 1975), ac- companied by extensive intrusion of tholeiitic diabase dikes and sheets in the Culpeper basin. The tilting was accompanied by large-scale movement along the normal western border fault, as contrasted with faults along the eastern border which locally show minor amounts of displacement, and by the development of broad, gentle, southwesterly plunging folds, strike-slip faults, and high-angle normal faults within the basins. The Culpeper Group thus retains a partial record spanning some 30 million years of Late Triassic to Early Jurassic continental sedimentation, climatic change, basaltic volcanism, and mafic intrusion ac- companying the incipient rifting of the North American and African plates, an event that culminated farther to the east with the opening of the Atlantic Ocean. PREVIOUS INVESTIGATIONS Studies related to the stratigraphy of the Culpeper basin began when W.B. Rogers' (1854) "New Red Sandstone of the Atlantic Slope" was designated the "Newark Group " in Virginia by Redfield (1856, p. 357). Roberts (1922, 1923, 1928) carried out the first systematic study of early Mesozoic geology in Virginia. He divided the rocks of the Culpeper basin into the Border Conglomerate, the Manassas Sandstone, and the Bull Run Shales and described the diabase and adjacent thermally metamorphosed red beds in the basin. He did not clearly define the stratigraphic units, identify the basalt flows, realize that conglomerates of many ages were present, or recognize the Jurassic age of part of the stratigraphic sequence. He also did not fully understand the importance of the regional structure or the significance of sedimentary facies changes in relation to depositional environments. In Maryland, Dorsey (1918) made a regional study of the stratigraphy and structure of the Triassic rocks. Later, Jonas (1928) and Jonas and Stose (1938), during geologic mapping in Frederick and Carroll Counties, Md., adopted the name "New Oxford Formation," named by Stose and Bascom (1929) for the exposures at New Oxford, Adams County, Pa. The New Oxford Formation represents the lowest stratigraphic unit of 4 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. the Triassic rocks in the Gettysburg basin of Pennsylvania and Maryland, and it was equated with all of the preserved Triassic section in the adjacent Culpeper basin of Maryland. Jonas and Stose (1938) mapped the early Mesozoic geology of the north end of the Culpeper basin adjacent to the Frederick Valley and separated limestone conglomerate from the adjacent sandstone, siltstone, and silty shale of the New Oxford Formation. Since the late 1940's the geology of the Culpeper basin has been studied in greater stratigraphic detail, in part because of discoveries and studies of fossil fish and spores in other basins, and in part because of the discovery of fossil fish in the west-central and western parts of the basin by Baer and Martin (1949, fig. 2) and subsequent detailed studies of other fossil remains in the strata of the Culpeper basin. Schaeffer and others (1975) and Schaeffer and McDonald (1978) identified Ptycholepis marshi Newberry and Redfieldius gracilis recovered from a site in Licking Run northwest of Midland (fig. 2), Fauquier County, Va., and at the bridge abutment of U.S. Route 15 over Catharpin Creek (fig. 2) northwest of Haymarket, Prince William County, Va. Nicholas Hotten III (unpub. data, 1959) and Robert E. Weems (1979) identified Late Triassic phytosaur bones found by R.E. Eggleton at what is now the Dulles International Airport during his mapping of the Herndon quadrangle in the late 1950's (Eggleton, 1975). Cornet (1977) made a detailed palynostratigraphic study in the northern part of the basin and first recognized and proved that Lower Jurassic as well as Upper Triassic sedimentary rocks are present in the basin. He placed the Triassic-Jurassic boundary just below the lowest basalt flow (Cornet, 1977, Palyno- florule 8/K2, p. 124, 175-183). From 1951 to 1978, studies of the areal geology of small parts of the basin by several geologists of the U.S. Geological Survey (USGS) contributed unpublished and open-filed geologic data and maps (Bennison and Milton, 1954). Bain (1959) provided new petrographic data about the diabase in the Nokesville 7.5-min Quad- rangle, and Lindskold (1961) studied the geology of the Gainesville 7.5-min Quadrangle and described the occurrence and petrography of the diabase there. Fisher (1964) provided detailed information on petro- graphy and chemistry of the Triassic rocks in Mont- gomery County, Md. Toewe's (1966) geologic map of the Leesburg 7.5-min Quadrangle, Loudoun County, Va., delineated and described the stratigraphically lowest basalt flow in the basin and contributed detailed analyses of sedimentary facies in the area. However, Toewe interpreted thermally metamorphosed red beds as a sequence of pyroclastic rocks and mapped a second, in part porphyritic, basalt flow sequence as intrusive diabase in the southwestern part of the quadrangle. McCollum (1971) recognized and briefly described the basalt flows in the west-central part of the Culpeper basin. Froelich published data on the physical proper- ties (1975a) and mineral resources (1975b) of the Triassic rocks in Montgomery County, Md. Hazlett (1978) made a field and petrographic study of the limestone conglomerate in the vicinity of Leesburg, Loudoun County, Va. Nutter (1975) studied and documented regional hydrogeologic relations in the Maryland part of the Culpeper basin and provided a description of drill cuttings in Montgomery County (Nutter, 1975, table 10, p. 32, 33). Otton (1981, table 12, p. 46-52; fig. 3, this paper) continued hydrogeologiec studies in Montgomery County and described two new drill holes. Lindholm (1977, 1978, 1979) studied the Triassic- Jurassic geology of the Culpeper basin in Virginia by using soil maps, proposed a revised stratigraphy, and made a comprehensive interpretation of the geologic history of the basin. He described the basalt flows and intercalated strata exposed along the road cuts of U.S. Route 29-211, Prince William and Fauquier Counties, Va. Lindholm and others (1979) published a com- prehensive summary and petrologic study of the diverse conglomerates in the basins, with an interpretation of their significance and origin. Lee (1977) carried out detailed geologic mapping of the Triassic and Jurassic rocks of 7.5-min quadrangles in Fairfax and Loudoun Counties, Va., and reconnais- sance geologic mapping in Montgomery and Frederick Counties, Md. He provided eight new measured sections in the northern part of the Culpeper basin (Lee, 1977; table 1, this paper). Lee found that the complex strati- graphic relations in northern Virginia could not be adequately portrayed by the existing formation names of Roberts (1928) and proposed changes in the nomen- clature of the sedimentary rocks in the basin. These rocks were redefined on the basis of lithostratigraphic sequence and his interpretation of the depositional environments. A geologic map of the Arcola 7.5-min Quadrangle, Va., was published by Lee (1978), followed by two USGS open-file reports that contained 34 geologic maps of the remaining quadrangles of the Culpeper and Barboursville basins (Lee, 1979, 1980). Hentz (1981, 1982, 1985) studied the structure and stratigraphy of the Jurassic Waterfall strata near Thoroughfare Gap in great detail, subdividing the lacustrine beds into eight subunits and showing that turbidites and unconformities are important in this area. He provided three new measured sections and two core hole descriptions (Hentz, 1981; tables 1 and 2, this paper). Upon completion of Lee's quadrangle INTRODUCTION 18° 77°30' I I Palynofloral Zones Age Sample (After Cornet, 1977) Corollina PI MBK torosus ? TH & 3 WEB g LEES 8,9 0:5) M/D 7A > M/D 4,5, 6, 9, 10 Corollina 2; | Sin- M/D 2,3 meyeriana & | Het BR 7 aad BR 1,2, 6 8 H 8 E HAYM 8 K2 L 8D Nor 8 P & "Upper Passaic" | a 8 62,3 2 8 J2 w- € | E-M 8 C2,3 CE go-2 - u, | Nor CR 80-1 & 8 B CR 80-2 iss "New Oxford- Ctr BAA7 Lockatong" > EXPLANATION m Diabase, metamorphic rocks, TH or crystalline basement rocks MBK x - Palyniferous locality ® Dinosaur footprint locality 1. Oak Hall 2. Culpeper Quarry 3. Broad Run 4. Dulles Airport 5. Comptons Corners * [] Fossil fish locality CR 80-2_A Nor A. Haymarket Sint! B CR 80-1 CL 3 B. Midland - - 0 at" C. Millbrook Quarry M/D 2-10 A - Megafloral locality } CC Comptons Corners L "* <_. z.. A R 60. Senses Guarry . ' Culpeper basin OH - Oak Hall ~ ¢] L --- Boundary between ages, _| approximately located; dashed where in doubt L Nor 0 5 10 MILES 0 5 10 KILOMETERS I l FIGURE 2. -Sketch map of the age of strata, Culpeper Group, palynologic samples and palynofloral zones and paleontologic localities. PI, Pliensbachian; Sin- Late Norian; E-M Nor, Early to Middle Norian; L Car, Late Carnian; ?, age in doubt. Culpeper basin, Virginia and Maryland, showing the location of diagnostic Het, Sinemurian-Hettangian; L Nor, TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. 78° 77°30" I I MEASURED SECTIONS MANASSAS SANDSTONE TIBBSTOWN FORMATION 1 Reston Member 23 Mountain Run Member 2 Do. 24 Catharpin Creek Formation and 3 Do. MountZion Church Basalt 4 Rapidan Member 25 Catharpin Creek Formation 5 Tuscarora Creek Member 26 Do. 6 Poolesville Member 27 - Mount Zion Church Basalt 7 Do. 28 Midland Formation 29 Do. BALLS BLUFF SILTSTONE 30 Do. 8 Balls Bluff Siltstone 31 Do. 9 Do. 32 Hickory Grove Basalt 10 Do. 33 Turkey Run Formation 11 Do. 34 Do. 12 Do. 35 Sander Basalt 13 Do. 36 Waterfall Formation 14,19 Do. 37 Do. 15 Do. 38 Do. 16 Do. ~ 39 Do. 17 Do. 18 Do. 20 - Balls Bluff Siltstone and Leesburg Member s 21 Do. a $9 :(- 22 Balls Bluff Siltstone ® - Selected core (c) and drill (d) holes, with total depth in feet (c)-140 (d)-262 (d)-600 (d)-857.5 (d)-1,002 mf (d)-857.5 (d)-1,000 (d)-767 (d)-650 (c)-500 (c)-500 (c)-57 (c)-65 (c)-125 wp Diabase, metamorphic rocks, Culpeper basin or crystalline basement rocks Short section X 38 e Measured section 38°30' |- 10 MILES 0 5 10 KILOMETERS | | FIGURE 3.-Sketch map of the Culpeper basin, Virginia and Maryland, showing the location of measured sections and selected drill holes. INTRODUCTION TABLE - 1.-Summary of measured sections in the Culpeper basin, Virginia and Maryland [Keyed to pl. 1. Do., ditto] Map Thickness No.1 Field No. Formation and Member Meters Feet Reference 1 KYL-77-1 Manassas Sandstone, Reston Member (Partial) 5.5 18 _ Lee, 1977, p. C-9, see. 1. 2 KYL-77-2 Do. 4.2 14 Lee, 1977, p. C-10, see. 2. 3 KYL-77-3 Do. 3.6 12 - Lee, 1977, p. C-10, 11, see. 8. 4 KYL-84-1 Manassas Sandstone, Rapidan Member (Partial) 75.4 249 _- This paper, app. A, see. 1. 5 KYL-84-2 Manassas Sandstone, Tuscarora Creek Member (Partial) 6.0 20 - This paper, app. A, see. 2. 6 KYL-77-4 Manassas Sandstone, Poolesville Member (Partial) 20 66 _- Lee, 1977, p. C-11, see. 4. 7 KYL-77-5 Do. 37 122 - Lee, 1977, p. C-12, see. 5. 2g PG-8/B Balls Bluff Siltstone (Partial) 39.15 129.2 Gore, 1983, app. A, B, p. 297-343. 29 _ PG 8/G Do. 3.6 11.9 Do. 10 PG G-3 Do. 24.4 80.5 Do. 11 PG G-7 Do. 5.32 17.5 Do. 12 PG G-8 Do. 10 33 Do. "1s / PG YD Do. 12 39.6 Do. "41 - PGG-1 Do. 6.82 22.5 Do. 16 PG G-5 Do. 4.15 13.7 Do. PGsK Do. 47.3 156 Do. 17 PG C-1 Do. 11 36.3 Do. 18 AJF-82-1 Do. 5.5 18 _ Froelich and others, 1982, stop 1, p. 59-62. 19 _- AJF-82-8 Do. 7.6 25 - Froelich and others, 1982, stop 1, p. 59-62. 20 KYL-77-6 Balls Bluff Siltstone (Partial) 40 131 _ Lee, 1977, p. C-13, 14, see. 6. 21 KYL-77-7 Leesburg Member (Partial) 76 251 - Lee, 1977,p. C-15, see. 7 22 JPS-84-1 Balls Bluff Siltstone (Partial) 65.6 215 - This paper, app. A, see. 5. P3 KYL-84-3 Tibbstown Formation, Mountain Run Member (Partial) 215.8 712 - This paper, app. A, see. 3. 24- '(AJF-82-7 Catharpin Creek Formation and Mount Zion Church 18.2 60 _ Froelich, and others, 1982, Basalt (Partial) stop 7, p. 75-76. 25 PG-T-3 Catharpin Creek Formation (Partial) 17.5 57.8 Gore, 1983, app. A, B. p. 297-343. 26 PG-T-1 Do. 10.45 34.5 Do. 27 KYL-84-4A - Mount Zion Church Basalt (Partial) 9.1 30 - This paper, app. A, see. 4A. 28 KYL-84-4B - Midland Formation (Complete) 378.8 1,250 - This paper, app. A, see. 4B. 29 ECT-66-1 Do. (Partial) 180 591 - Toewe, 1966, app. I, p. 32-85. 30 PG-MID-1 Do. 9.9 32.7 Gore, 1983, app. A, B, p. 297-348. 31 PG-MID-2 Do. 10.43 34.4 Do. 82 KYL-84-4C __ Hickory Grove Basalt (Complete) 211.1 695 - This paper, app. A, see. 4C. 88 KYL-84-4D - Turkey Run Formation (Complete) 216.6 715 - This paper, app. A, see. 4D. 34 KYL-77-8 Do. (Partial) 37 122 - Lee, 1977, p. C-15, 16, see. 8. 35 KYL-84-4E - Sander Basalt and sandstone and siltstone members (Partial) 805.7 2,659 _ This paper, app. A, see. 4E. 36 - TH-82-TC _ Waterfall Formation (Partial) 141 465 - Hentz, 1981, app. B. 87 TH-82-DB Do. 10 33 Do. 38 - TH-82-BR Do. 34.5 114 Do. 30 - AJF-82-6 Do. 14 46 _ Froelich, A.J. and others 1982, stop 6, p. 73-75. 1Map numbers correspond to numbers in figure 3. 2Cornet (1977) section measured and described by Gore (1983). Same section measured and described. TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. TABLE -2.-Summary of core holes and selected drill holes in the Culpeper basin, Virginia and Maryland [Keyed to pl. 1. Do., ditto] Map Well No. LD.. _ (WRD-ID) A F-52V-1D Formation member interval (ft) (Partial) Manassas Sandstone, Poolesville Member, 0-75 ft. (Complete) Manassas Sandstone, Reston Member, 75-125 ft. Peters Creek Schlist, 125-140 ft. Cored (c) or Drilled (d) Total depth Meters 42.5 Feet Reference 140 _ Larson, 1978, app., p. 31-35. B Mo-DC-59 (Partial) Manassas Sandstone, Poolesville Member, 0-50 ft. (Complete) Manassas Sandstone, Reston Member, 50-105 ft. Peters Creek Schist, 105-262 ft. 79.4 262 - Otton, 1981, table 12, p. 50-52. C _ Mo-Do-47 (Partial) Manassas Sandstone, Poolesville Member, 0-600 ft. (Partial) 181.8 600 _ Otton, 1981, table 12, p. 46-49. D Mo-Chb-26 (Partial) Manassas Sandstone, Poolesville Member, 0-770 ft. Manassas Sandstone, Tuscarora Creek Member, 770-808 ft. (Complete) Frederick Limestone (Cambrian), 808-857.5 ft. 260 857.5 Nutter, 1975, table 10, p.32, 33. E F-51V-14F (Partial) Manassas Sandstone, Poolesville Member, 0-545 ft. Manassas Sandstone, Reston Member, 545-610 ft. (Complete) Peters Creek Schist, 610-1,002 ft. 301 1,002 - This paper, app. B. EC-10 F Mo-EG1O0 (Partial) Balls Bluff Siltstone, 0-857.5 ft. 260 857.5 Otton, 1981. G _ 51V-23H (Partial) Manassas Sandstone, Poolesville, Member, 0-1,000 ft. 305 1,000 _ This paper, app. B. H 51V-24H (Partial) Balls Bluff Siltstone, 0-767 ft. 233.8 767 Do. I 51V-13A (Partial) Balls Bluff Siltstone, 0-280 ft. Hornfels (thermally metamorphosed siltstone), 280-650 ft. 198 650 Do. J - Man-1 (Partial) Balls Bluff Siltstone, 0-550 ft. 165 550 _ Sobhan, 1985. K - Man-2 (Partial) Balls Bluff Siltstone, 0-550 ft. 150 500 Do. L - Mid-1 (Partial) Midland Formation, 0-56.4 ft. 17 56.4 Smoot, this paper, app. B. M TGC-1 (Partial) Waterfall Formation, 0-65 ft. 19.7 65 - Hentz, 1981, app. 2. N TGC-2 (Partial) Waterfall Formation, 0-125 ft. 37.8 125 « Bo: 1Map identification letters correspond to letters in figure 3. INTRODUCTION 9 mapping, Froelich and others integrated the geologic map coverage with newly acquired hydrologic and geophysical data and prepared a series of topical reports and regional maps at a scale of 1:125,000 (Leavy, 1980, 1984; Wise and Johnson, 1980; Froelich and Leavy, 1982; Johnson and Froelich, 1982; Leavy and others 1982, 1983; Posner and Zenone, 1983; Froelich, 1985; Zenone and Laczniak, 1985). Some of the newly acquired chemical and isotopic data on the igneous rocks had a significant bearing on the age and stratigraphic interpretation (Sutter and Arth, 1983; Lee and others, 1984), and this paper incorporates much of the data acquired by many workers in the Culpeper and Barboursville basins over the past decade. SCOPE, PURPOSE, AND METHOD OF STUDY Systematic mapping for this investigation was under- taken by Lee from the fall of 1973 to the spring of 1977. The purposes of that study were to define and map the stratigraphic units and to determine the geologic factors controlling the origin of the sedimentary and igneous rocks and the thermal metamorphism of country rocks adjacent to intrusive diabase in the Culpeper and Barboursville basins. Geologic mapping at a seale of 1:24,000 was completed throughout most of the two basins (Lee, 1979, 1980). Individual lithologic units were identified on the basis of mineralogic composition, texture, structure, and other physical characteristics. Geologic contacts between stratigraphic units were located only approxi- mately, because in most areas outcrops are very sparse and critical stratigraphic contacts are commonly covered. A sand/shale ratio of two to one was arbitrarily used to delineate the gradational contacts between deposits of shale and (or) siltstone and deposits of sandstone and (or) conglomerate. Fresh rock exposures were sampled for laboratory investigation on the basis of lithostratigraphic facies changes, areal distribution, and variation of texture and mineralogy of rock types. Twelve typical sections across the basins were examined in detail to determine regional structure, lithostratigraphic facies, and contact relations between intrusive diabase and country rock with thermal metamorphism. Four sections were measured in the field, and thicknesses in the remaining eight sections were calculated as composite sections from the geologic quadrangle maps. In addition, thick- ness of individual stratigraphic units was measured at selected localities in the field (app. A, table 1) to supplement those measured previously and reported elsewhere (Toewe, 1966; Lee, 1977; Froelich and others, 1982; Hentz, 1982). Measurements of crossbeds and other primary sedi- mentary structures were made in siltstones and sand- stones to determine directions of paleocurrents. Pebble counts were made in conglomerates to ascertain the sediment provenance, and the rock fabric was studied to determine conditions of transportation and deposi- tion. Such determinations enable an understanding of the physical characteristics of fluvial and debris-flow deposits; the areal distribution of fluvial fan-segments, which consist of the fanhead, midfan, fan base, and distal facies; and the nature of coalescence among the fans distributed throughout the basins. Subsequently, we have gathered new age and cor- relation data and have assessed and compiled selected lithologic logs of deep water wells and core holes (tables 1 and 2, this paper; Sobhan, 1985). ACKNOWLEDGMENTS We are grateful to the residents in the Culpeper and Barboursville basins for their courtesy and hospitality. We are deeply indebted to many colleagues at the USGS with whom we have discussed various problems. Sincere thanks are due the following persons for their help during the course of this study: G.V. Cohee thoughtfully provided information on fossil fish in the Culpeper basin; E.C.T. Chao provided his unpublished data on Triassic and Jurassic geology in Fairfax and Loudoun Counties, Va.; N.F. Sohl and John Pojeta, Jr., identified freshwater crustacean fauna from limestone in Prince William County, Va.; S.H. Mamay identified plant remains from sandstones southwest of Frederick, Md.; Frank C. Whitmore, Jr., and Robert E. Weems identified the dinosaur tracks at the Culpeper Crushed Stone quarry in Culpeper County, Va.; E.I. Robbins made a palynologic study of siltstone in Culpeper County, Va.; Dr. Nicholas Hotten III, U.S. National Museum of Natural History, Smithsonian Institution, Washington, D.C., discussed the age of phytosaur remains in siltstone at Dulles Airport; D. Chaney of the Smithsonian Institution provided access to the Midland fish bed locality; Melody Hess determined clay minerals in siltstone and sandstone collected from U.S. Route 50 by X-ray diffraction methods; RH. Johnston, J.D. Larson, Chester Zenone, and Alex Posner carried out ground-water quality and quantity appraisals, and Randall Laczniak prepared computer models of the ground-water flow system throughout the Culpeper basin. Brooks Ellwood, Carol Raymond, and others completed paleomagnetic studies of selected outcrops of basalt and diabase; John Sutter and Joseph Arth of the USGS determined isotope ratios and 4°Ar/3°Ar age spectrum dates of the same basalts and diabases; and John Puffer and Brian Leavy completed chemical and isotopic analyses of the basalts. David Gottfried con- tributed stimulating discussions, guidance, and chemical analyses of diabase and basalt. Joseph P. 10 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. Smoot measured and described sedimentologiec aspects of the Midland core hole and the thick cyclic succession of Balls Bluff Siltstone at the Culpeper Crushed Stone quarry and provided many helpful suggestions in discussions of stratigraphic models. STRATIGRAPHY NEWARK SUPERGROUP CULPEPER GROUP General statement.-Upon completion of geologic mapping in the Culpeper and Barboursville basins it became apparent that the complex stratigraphic rela- tions in the basins could not be adequately portrayed by existing stratigraphic nomenclature (pl. 14). Con- sequently, a change in the nomenclature of both the sedimentary and basaltic rocks has been made to depict the stratigraphic relations on the basis of our under- standing of the lithostratigraphic sequence, age rela- tions determined on fossil content, depositional en- vironment, provenance, areal structural relations, and isotopic age dating of igneous rocks. The name "Culpeper Group" (Lee, 1979, 1980) is adopted herein for the distinctive complete lithostratigraphic sequence of Upper Triassic and Lower Jurassic rocks in the basins (pl. 14). This name was proposed informally by Cornet (1977, p. 119) and was used by Olsen (1978) and Lindholm (1979). It supersedes the term "Newark Group" (Lee, 1977, p. C2, C3), which was recently raised to Supergroup (Froelich and Olsen, 1984). The term "lower part of the Culpeper Group" is used informally for the mainly Upper Triassic sequence of continental sedimentary rocks occupying the entire Barboursville basin and the southern quarter and eastern half of the Culpeper basin; the "upper part of the Culpeper Group" includes the Lower Jurassic series of tholeiitic basalt flows restricted to the west- central Culpeper basin and the intercalated sedimen- tary rocks. Correspondingly, for correlations between the Culpeper, Newark, and Hartford basins, as in plate 1B, the term "lower part of the Newark Supergroup" is used for the mainly Upper Triassic sequences of sedi- mentary rocks and the term "upper part of the Newark Supergroup" includes the Lower Jurassic basalts and intercalated sedimentary rocks in all three basins. The Border Conglomerate of Roberts (1928) occurs on both flanks as well as in the interior of the Culpeper basin and spans the entire Triassic-Jurassic section. Most of the conglomerates are lenticular, isolated, and not coeval. They are separable into geographically and lithologically distinct members of several different formations of several ages. Therefore, the term "Border Conglomerate" is hereby abandoned (see pl. 14). The name "New Oxford Formation," extended by Jonas and Stose (1938) into the Maryland portion of the Culpeper basin from the Gettysburg basin, is replaced by the Manassas Sandstone and is no longer recognized in the Culpeper basin (see pl. 14). The Manassas Sandstone (Roberts, 1928) is retained, but several newly named conglomerate members are included within it (see pl. 1A). The type Bull Run Shales of Roberts (1928) is mainly siltstone, and the area he mapped within this unit in the Culpeper basin includes lithologies as diverse as conglomerate, sandstone, and basalt. The term "Bull Run Formation" has been used by Lindholm (1979) for a medial unit of mainly Upper Triassic calcareous siltstone; Lee (1977, 1979, 1980) expanded the unit to include Triassic and Jurassic strata and basalt flows. In all previous definitions, this hetero- geneous unit lacks distinctive boundaries and is hereby abandoned. Cornet's informal usage of formations "A" through "K" (pl. 14) is hereby abandoned, although the palynofloral zones that he established are incorporated and adapted to the appropriate named formations. Some of Lee's earlier informal proposals and nomen- clature are adopted as presented, such as Mount Zion Church Basalt, Hickory Grove Basalt, Sander Basalt, and Poolesville, Reston, Tuscarora Creek, and Rapidan Members; some are redefined and retained, such as Catharpin Creek Formation, Balls Bluff Siltstone, Leesburg Member, and Mountain Run Member (see pl. 1A). Some of Lindholm's (1979) terminology is adopted as proposed or redefined and incorporated herein, such as Goose Creek Member and Waterfall Formation, whereas some stratigraphic names are abandoned, such as Buckland Formation, Cedar Mountain Con- glomerate Member, and Barboursville Conglomerate Member (see pl. 14). Thus the redefined stratigraphy of the Culpeper basin is an amalgam of previous usage and modified informal names, adapted to reflect our present understanding of age and complex lithofacies relationships. Some of the names in the Barboursville basin are carried over from the Culpeper basin, and some in the Barboursville basin carried over to the southern Culpeper basin. Stratigraphic units and correlation.-The lower part of the Culpeper Group, mainly of Late Triassic age, is subdivided, in ascending order, into the Manassas Sandstone, the Balls Bluff Siltstone, the Tibbstown Formation, and the Catharpin Creek Formation. The Manassas Sandstone consists of the Reston Member, the Rapidan Member, and the Tuscarora Creek Member at the base, all chiefly conglomerates, overlain by and intertonguing laterally with arkosic sandstone of the Poolesville Member. The Balls Bluff Siltstone inter- tongues with its Leesburg Member, predominantly STRATIGRAPHY 11 limestone conglomerate and with arkosic sandstones of the Manassas, Tibbstown, and Catharpin Creek Forma- tions. The Tibbstown Formation, mainly arkosic sand- stone, includes the Mountain Run and the Haudricks Mountain Members at the top, both conglomerates. The Catharpin Creek Formation, mainly arkosic sandstone, contains two fan-shaped lobes of the Goose Creek Member, a conglomerate that may contain lowermost Jurassic beds at the top. The upper part of the Culpeper Group of Early Jurassic age consists of the Mount Zion Church Basalt and associated sandstone and siltstone members; the Midland Formation; the Hickory Grove Basalt and associated sandstone and siltstone members; the Turkey Run Formation; the Sander Basalt and associated sandstone and siltstone members; and the Waterfall Formation, mainly sandstone, siltstone, and calcareous fossiliferous shale, with conglomerate of the Millbrook Quarry Member at the top. The bulk composition of the conglomerates of the Culpeper Group shows considerable diversity of proven- ance (Lindholm and others, 1979). Their distinctive lithologic characteristics and restricted areal and stratigraphic distribution permit their differentiation into individual members and their correlation with specific source areas. The Lower Jurassic basalts are thought to be synsedimentary surface flows, as no pillow structures are evident and the flows are interbedded with relatively unweathered sandstone, siltstone, and conglomerate; they are believed to be fissure flows because of their great areal extent and because pyroclastic debris is absent from overlying, underlying, and intercalated sedimentary rocks. Stratigraphic correlation of the formations has been established throughout the basins by (1) detailed field investigations of the sedimentary succession and of the contained fossil flora and fauna, (2) interpretation of the conditions of transportation and deposition of individual stratigraphic units, and (3) lithostrati- graphic analysis of the sedimentary rocks, particularly in relation to the stratigraphic position of the locally fossiliferous Balls Bluff Siltstone, the Midland and Turkey Run Formations, and the enclosing basalt flows (fig. 2 and pl. 1.) Thickness.-The thickness of the Culpeper Group varies widely throughout the basins, in part because of highly variable rates of subsidence and continental sedimentation and in part because of erosion. In the northern Culpeper basin of Frederick County, Md., the measured or calculated thickness of this group is about 405 m (1,330 ft) about 2.3 km (1.4 mi) west of the intersection of Market and Patrick Streets, Frederick, Md., westward to east of Fuller, Md., and 627 m (2,057 ft) about 1.3 km (0.8 mi) N. 66° W. of Tuscarora, Md., westward to Point of Rocks, Md. It increases to 2,962 m (9,718 ft) across the basin from west of Dickerson, Montgomery County, Md., to the western border about 1.3 km (0.8 mi) west of Lucketts, Loudoun County, Va., and to 4,965 m (16,290 ft) in the area from north of Pender to north of Aldie, Va., and reaches a maximum of 7,900 m (25,920 ft) in the area south of Centreville to south of Antioch, Va., then decreases to 2,180 m (7,153 ft) in the area southeast of Mountain Run at Culpeper, Va., to the extreme southeastern corner of the Culpeper East 7.5-min Quadrangle, Orange County, Va. The thickness of the Culpeper Group in the Barboursville basin varies from 330 m (1,083 ft) to 576 m (1,890 ft). The preserved thickness at any point in the basins is indeterminate because of rapid lateral facies changes, widespread diabase intrusives, and poor exposures that may conceal major faults and unconformities. Age.-Fossil fauna and flora, though generally sparse throughout the Culpeper basin, represent a diverse assemblage of continental varieties ranging from dinosaurs and well-preserved fish to microscopic spores and pollen. Important and diagnostic fossils and their localities and stratigraphic position are given in figure 2. Applegate (1956) indicated that further study was needed to determine the specific age of fossil fish found in the Culpeper basin. The paleontologic affinity of the Midland Formation with the Feltville Formation of the Newark basin, New Jersey, and the Shuttle Meadow Formation of the Hartford basin, Connecticut (pl. 1B), first established on the occurrence of Piycho- lepis marshi Newberry (Schaeffer and others, 1975, p. 207, 208), was independently supported by palynologic studies of Cornet (1977, p. 134, 183) and by restudy of fossil fish by Olsen (1984) and Olsen and others (1982). These fossil fish were recovered from carbonaceous shale at Licking Run (fig. 2), about 2 km (1.25 mi) northwest of Midland, Fauquier County, Va. (Baer and Martin, 1949, p. 685; Schaeffer and others, 1975, p. 226-230), and at Catharpin Creek (fig. 2), at the bridge of U.S. Route 15 about 4.2 km (2.6 mi) north-northwest of Haymarket, Prince William, Va., by Schaeffer and others (1975, p. 229), who assigned a Late Triassic to Early Jurassic(?) age to the fish beds. Olsen and others' (1982) restudy of the fish fauna of the Culpeper basin confirmed the Early Jurassic (Hettangian) age of the Midland beds at the Licking Run and Catharpin Creek localities and supported the Sinemurian and possibly Pliensbachian age of the younger Waterfall sequence in the vicinity of Millbrook quarry in the Thoroughfare Gap quadrangle. Palynological studies by Cornet (1977) identified a Lower Passaic-Heidlersburg palynoflora of Late Triassic age (early and middle Norian) in the lower part of his formation K, now the Manassas Sandstone, 12 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. and a Manmassas-Upper Passaic palynoflora (middle and late Norian) in the upper part of his formation K, now the Manassas Sandstone and Balls Bluff Siltstone. Cornet (1977, p. 134, 183; fossil location Mid-3-6, pl. 1B, this paper) indicated an Early Jurassic (Hettangian to early Sinemurian) age for the Corollina meyeriana palynoflora in the dark-gray, fish-bearing Midland beds (his formation I) at Licking Run. He placed the boundary between the Upper Triassic and Lower Jurassic above the stratigraphic level of his palynofloral 8-K2 (Cornet, 1977, p. 124; pl. 1B, this paper) that occurs in his formation K near Interstate Route 66 in about the center of the Gainesville 7.5-min Quadrangle. This locality is stratigraphically in the uppermost part of the Balls Bluff Siltstone; thus the Triassic-Jurassic boundary apparently is above the Balls Bluff and below the Mount Zion Church Basalt, and probably lies within the barren sandstones and conglomerates of the Catharpin Creek Formation. Raymond and others (1982) determined the mag- netization of core from representative sites within the intrusive and extrusive rocks of the Culpeper basin (pl. 1C). Measurements of six diabase sills, four diabase dikes, and three basalt flows yield a remanent mag- netization after alternating field demagnetization that exhibits good within-site and between-site directional consistency. Paleopoles calculated from the remanent magnetization directions of these units correspond to the apparent polar wander path for North America at 200 Ma. Sutter and Arth (1983) determined the age spectrum dates and strontium isotope geochemistry from the same six diabase sill localities sampled by Raymond and others (1982). Whole-rock samples yield total gas 4°Ar/3°%Ar apparent ages that range from 187 to 206 Ma. 4°Ar/3°Ar age spectra from the same six samples define plateau ages that range from 192 to 200 Ma and yield a mean age of 197+4 Ma, which they interpret as the best estimate of the age of intrusion and erystallization of the sills. Whole-rock samples from four of these sills have initial "Sr/°5Sr ratios ranging from 0.7060 to 0.7066 and a rather constant rubidium/strontium ratio of 0.10 to 0.13, values consistent with published values for Mesozoic diabase dikes and sills of Eastern North America. The strontium isotopic signature of these sills is clearly more characteristic of continental tholeiite than of normal midocean ridge basalts. Cornet's (1977) palynologic study, Olsen and others' (1982) restudy and reevaluation of the fish fauna, the paleomagnetic pole positions of the basalts established by Raymond and others (1982), and finally the 4°Ar/3}Ar ages of the diabase sills and dikes by Sutter and Arth (1983) clearly establish the Early Jurassic age of the intercalated sedimentary and igneous rocks in the Culpeper basin. We, therefore, propose a Late Triassic age for the Manassas, Balls Bluff, and Tibbstown Formations, a Late Triassic and Early Jurassic age for the Catharpin Creek Formation, and an Early Jurassic age for the Mount Zion Church Basalt and all overlying formations and the intrusive diabase (pl. 14). MANASSAS SANDSTONE The Manassas Sandstone was named for rocks exposed in the vicinity of Manassas, Prince William County, Va. (Roberts, 1923, 1928). This formation was revised by Lee (1977, p. C3-C5, C11, C12; 1979, 1980). As revised in this report (pl. 14, 1C), the Manassas generally contains three discrete and separate len- ticular lower conglomerate sequences, formerly part of the Border Conglomerate of Roberts (1928), each with pebbles and cobbles of distinctive lithologic types, such as schist and quartzite, greenstone, or limestone. These conglomerates are the Reston Member, which occurs in the east-central part of the Culpeper basin (Lee, 1977, p. C3, C4 C9-C11), the Rapidan Member (new name), which is exposed in the southeastern parts of the Culpeper and Barboursville basins, and the Tuscarora Creek Member (new name), which is located in the extreme northern part of the Culpeper basin. Each member unconformably overlies or is locally in fault contact with pre-Triassic crystalline rocks, and each grades into or interfingers with the overlying sandstone of the Poolesville Member (new name). These con- glomerate members were probably more or less contemporaneous during the Late Triassic; the condi- tions of fluvial and debris-flow sedimentation were rapid and very similar, but the provenance was strikingly different for each. At the northern and southern extremities of the Culpeper and Barboursville basins, the basal outcrops are poor, conglomerate is absent, and the units consist of fine-grained sandstones and red-brown siltstones not typical of the Poolesville Member; at these localities the Manassas Sandstone is not divided (pl. 1C). The thickness of this restricted unit was not measured, but it probably does not exceed 250 m (820 ft). RESTON MEMBER The Reston Member is the lower stratigraphic unit of the Manassas Sandstone in the drainage areas of the Potomac and Rappahannock Rivers, including Bull Run and the Goose Creek-Seneca Creek areas (pl. 1C). Lee (1977, p. C3, C4; measured sections 1, 2, and 3, p. C9-C11) defined this member for the typically weathered road-cut exposures of loose to semicompact sand and pebble and cobble gravel east of the junction of the south ramp of the Dulles Airport Access Road to Reston Avenue in the northwestern part of the Vienna STRATIGRAPHY FIGURE 4.-Outcrop of the Reston Member of the Manassas Sandstone on a runway of the Old Dominion Speedway about 300 m (1,000 ft) southeast of Grant Avenue, town of Manassas, in the northwestern part of the Independent Hill 7.5-min Quadrangle. A lens of tightly packed imbricate quartzite-cobble conglomerate with a sandstone matrix is intercalated with planar-bedded conglomeratic sand- stone. Outcrop is a nearly horizontal cross section of strata dipping gently away from the field of view (northwest). Lens cover is 5 em (2 in) in diameter. 7.5-min Quadrangle (Lee, 1979, 1980). Drilled sections of this basal unit are documented in appendix B and table 2 of this paper and by Larson (1978), Nutter (1975), and Otton (1981). Distribution.-The Reston Member is exposed along the eastern border of the Culpeper basin in Fairfax, Prince William, and Fauquier Counties, Va., and in Montgomery County, Md. Excellent exposures are along the road and creek cuts, but most are deeply weathered and disaggregated. Where penetrated in several wells along the eastern margin of the Culpeper basin, the Reston Member is firmly cemented by iron oxide and clays. Lithology.-The Reston Member is composed of dusky-red, very dark red, and light-gray intermixtures of micaceous quartz and feldspar sand and angular to subangular boulders, cobbles, and pebbles of crystalline rock fragments in an interstitial clay-silt matrix (fig. 4). Generally, throughout this poorly sorted unit, the sand and silt are more abundant in the upper part of the unit than in the lower part. Clay minerals of two samples from Pender, Va., consist chiefly of illite, with subordinate amounts of kaolinite and halloysite. Quartz and feldspar contents in the sand fraction vary con- siderably; feldspar is dominant locally. The sand grains are predominantly subangular and, where sandstone is dominant, form beds that are thick to very thick. Cut- and-fill structures and gently inclined crossbeds are locally present in these sand beds. Rock fragments in 13 the conglomerate beds are chiefly angular to subangular phyllite, schist, quartzite, and vein quartz, similar to bedrock exposed in the adjacent Piedmont to the east. The size and composition of these fragments differ from place to place. Rock fragments are as large as 0.5 m (1.6 ft) in length at the type locality and along U.S. Route 50 at Pender, Fairfax County, Va. Conglomerate beds within this unit show imbricate pebble structures along road cuts near Reston, Fairfax County, Va. The Reston Member locally contains cross-laminated and compact lenses of micaceous, feldspathic siltstone and fine-to medium-grained arkosic sandstone, in addition to the main conglomeratic sandstone and conglomerate with schist, phyllite, quartzite, vein quartz, pebbles, and cobbles. Fining upward cycles of cobbles, pebbles, and granules occur locally. Thickness.-The Reston Member ranges from less than 3 m (10 ft) to an estimated maximum thickness of about 100 m (330 ft). It is 5.5 m (18 ft) thick at the type section and 3 m (10 ft) thick along the south cut of Compton Road, Fairfax County, Va. Based on widely separated outcrops in Prince William County, Va., the composite thickness is calculated to be 85 m (280 ft). It ranges from 16.5 to 19.5 m (54-65 ft) thick in drill holes in Montgomery County, Md., and Fairfax County, Va. (table 2, app. B). Relation to adjacent stratigraphic units.-The Reston Member unconformably overlies pre-Triassic Piedmont crystalline rocks and locally is truncated by steep faults exposed along the eastern border in Fairfax and Prince William Counties, Va. (pl. 1C, this paper; Leavy, 1980). The contact of the Reston with the Poolesville Member is laterally and vertically gradational and locally inter- fingering. A gradational contact is well displayed 1.3 km (.75 mi) southeast of Centreville, in Fairfax County, Va. Deposition.-The Reston Member consists mainly of coarse clastic materials derived from adjacent eastern highlands. The sediments were apparently transported and deposited by means of high-gradient streams and debris flows along the base of highlands. The present outcrops are the remnants of coalescing fluvial fan deposits in which the lithofacies vary from part of the fanhead to midfan facies. These coarse clastic deposits grade into arkosic sandstone of the Poolesville Member, which locally contains fine gravel lenses. The imbricate pebble structure of conglomerates and cross-lamination and crossbedding of sandstones generally indicate eastern and southeastern source areas. RAPIDAN MEMBER The Rapidan Member is the lowest stratigraphic unit of the Manassas Sandstone in the drainage area of 14 the Rapidan River and Mountain Run (pl. 1C, and measured section 1, app. A, this paper; table 1, Lee, 1980). It is herein named from typical exposures of greenstone conglomerate along the southern bank of the Rapidan River near Raccoon Ford, Unionville 7.5- min Quadrangle, in Orange County, Va. Distribution.-The Rapidan Member is well exposed on the western slope of "The Ridge" in the east-central part of the Culpeper East 7.5-min Quadrangle, Culpeper County, Va., and the type section is along the southern side of the Rapidan River in the extreme southeastern corner of the Culpeper East 7.5-min Quadrangle and the northern part of the Unionville 7.5-min Quadrangle (measured section 1, app. A). It is widespread but poorly exposed in the northern and southeastern parts of the Barboursville basin. Lithology.-The Rapidan Member is characterized by indurated conglomerates containing rock fragments of grayish-green and grayish-olive-green to dusky- green, Late Proterozoic metabasalt (Catoctin Forma- tion) and minor amounts of rock fragments consisting of light-gray and bluish-gray, metamorphosed, felds- pathic sandstone, quartzite, vein quartz, and schist (probably Lower Cambrian Chilhowee Group rocks). These rock fragments are intermixed with greenstone granules and greenish-gray clay sand and silt, which are cemented firmly by clay and silica and locally by secondary calcite. The rock fragments are angular to subangular or rounded; they show imbricate structure and, locally, cut-and-fill features. The average length of these fragments is 10 em (4 in); some are as large as 30 em (12 in) in the lowest part of this unit along cuts on the southern bank of the Rapidan River. Thickness.-The thickness of this member ranges from 70 m (230 ft) to 140 m (460 ft) on the southern bank of the Rapidan River in Orange County, Va. It pinches out to the northeast along the eastern margin of the basin between the Rapidan and Rapahannock Rivers. No accurate thickness could be determined in the Barboursville basin. Relation to adjacent stratigraphic units.-The Rapidan Member rests unconformably on and is locally in fault contact with pre-Triassic crystalline rocks (pl. 1C). It is laterally and vertically gradational and interfingers with sandstones of the Poolesville Member. These relationships are well shown along the southern bank of the Rapidan River in the extreme southeastern corner of the Culpeper East 7.5-min Quadrangle, Orange County, Va., where conglomerates of the upper part of the Rapidan Member are interbedded with sandstone of the lower part of the Poolesville Member. Deposition.-The outcrop patterns, primary struc- tures, and size distribution of clasts in the Rapidan Member suggest that this is a part of fanhead and TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. midfan deposits and indicate that the sequence rep- resents deposition of an isolated fluvial fan deposited along the northwestern front of Clark Mountain which spread to the Rapidan River and extended to "The Ridge" in Orange and Culpeper Counties, Va. (pl. 1C). TUSCARORA CREEK MEMBER Limestone and dolomite clast conglomerate, exposed on the southeastern bank of Tuscarora Creek in Frederick County, Md., is herein named the "Tuscarora Creek Member" of the Manassas Sandstone (Lee, 1979). The type section is exposed about 0.8 km (0.5 mi) S. 35° E. of the bridge over Tuscarora Creek on Maryland State Road 28. This member is the lowest stratigraphic unit of the Manassas Sandstone in the Frederick Valley of Maryland (pl. 14; measured section 2 in app. A, table 2, this paper; Nutter, 1975, table 20, p. 32, 33). Distribution.-The Tuscarora Creek Member under- lies much of the northeastern border area of the Culpeper basin north of the Potomac River, Frederick County, Md. It is well exposed in road cuts along Maryland State Road 28, 1.5 km (0.9 mi) N. 55° W. of Tuscarora and about 0.6 km (0.4 mi) S. 55° E. of the type section near the lower reach of Tuscarora Creek, along the banks of Tuscarora Creek about 0.25 km (0.15 mi) N. 30° E. of Churchill, and along the road cuts 0.8 km (0.5 mi) due west of the State Police barracks west of Frederick, Md. Lithology.-The Tuscarora Creek Member is well-to poorly sorted, thick-bedded to massive conglomerate composed of angular to subangular and subrounded pebbles and cobbles of light to dark gray and pinkish- red limestone, dolomite, and dolomitic limestone. The clasts were probably derived mainly from the Frederick and Grove Limestones (Upper Cambrian and Lower Ordovicion) that crop out nearby. The limestone in the clasts is very fine grained to very coarse grained, argillaceous and (or) quartzose; some clasts are flaggy and planar-laminated. The clasts are embedded in a calcite-cemented matrix chiefly composed of limestone and dolomitic limestone granules and dusky-red to grayish-red clayey sand and silt. Thickness.-This member ranges in thickness from 21 m (70 ft) at the type section near the lower reaches of Tuscarora Creek to 67 m (220 ft) west of Frederick, Md., and thins and pinches out to the south. It is 12 m (39 ft) thick where penetrated in a water well drill hole south of Dickerson, Montgomery County, Md. (Nutter, 1975, table 10, p. 32, 38). Relation to adjacent stratigraphic units.-The Tus- carora Creek Member unconformably overlies the Frederick Limestone. The contact is well exposed in a field north of Maryland State Road 28, about 1.8 km STRATIGRAPHY FIGURE 5.-Outcrop of the Poolesville Member of the Manassas Sandstone in the western part of the Independent Hill 7.5-min Quadrangle. Exposure is on the northwestern side of Cedar Run about 200 m (656 ft) southwest of the Virginia State Route 619 bridge. Poorly stratified, very coarse to coarse-grained sandstone is interbedded with pebble conglomerate lenses. Lens cover is 5 ecm (2 in) in diameter. (1.1 mi) N. 55° W. of Tuscarora, Frederick County, Md., and along Tuscarora Creek a short distance northeast of Churchill. The Tuscarora Creek Member is laterally and vertically gradational to and interfingers with the Poolesville Member. These relationships are well shown in road cuts west of Frederick, Md. Deposition.-Outcrops of the Tuscarora Creek Member in the Frederick Valley represent the re- maining portion of coalesced fan deposits ranging from part of the fanhead to midfan facies. As indicated by preserved imbricate pebble structures, these fans were formed from fluvial debris of streams that emerged from the adjacent uplands on the east and northeast that were underlain by Upper Cambrian carbonate rocks (pl. 1C). POoOLESVILLE MEMBER The Poolesville Member of the Manassas Sandstone is herein named for the exposures of arkosic sandstone interbedded with minor siltstone and mudstone near Poolesville, Md. (Lee, 1979). Lee (1977, p. C4, C5) previously called this unit the sandstone member and designated it as the uppermost stratigraphic unit of his Manassas Sandstone. The type section of this member is well exposed along the road cuts north and northeast of the city of Poolesville, Montgomery County, Md. A reference section for the Poolesville Member is along the northern bluffs of the Potomac River extending westward for 2 km (1.25 mi) from the mouth of Seneca Creek, west-central Seneca 7.5-min Quadrangle, 15 FIGURE 6.-Upper surface of the Poolesville Member of the Manassas Sandstone showing carbonate nodules and cement. Outcrop is in the northern part of the Independent Hill 7.5-min Quadrangle along a road cut on Liberia Avenue about 250 m (820 ft) north of the junction with Virginia State Route 663. The light-gray irregular patches of cement and the abundant granule-size spherules of carbonate intermixed in the muddy sandstone are interpreted as paleosol caliche. Lens cover is 5em (2 in) in diameter. Montgomery County, Md. Additional excellent outcrops are along the valley sides of Bull Run near the towns of Manassas and Yorkshire, Prince William County, Va., and along the banks of the Potomac River in Loudoun County, Va. (Lee, 1977, measured sections 4 and 5, p. C11, C12). This unit constitutes the bulk of the Manassas Sandstone (pl. 1B, 1C; table 2; app. B; figs. 5, 6). Distribution.-The Poolesville Member is extensive throughout the eastern part of the Culpeper basin and the southeastern part of the Barboursville basin. It is well exposed along the valleys of major streams in the Culpeper basin and along the valley of Blue Run in the Barboursville basin. Lithology.-The Poolesville Member is composed mostly of pinkish-gray, very fine grained to very coarse grained feldspar and quartz sand in a very dark red to dusky-red-purple clayey silt matrix, cemented mainly by silica and locally by calcite. It is micaceous and, in part, highly feldspathic. Clay-sized minerals from a sample collected at a road cut on U.S. Route 50, Fairfax County, Va., are chiefly illite and minor kaolinite. The unit is generally thick bedded to massive and planar-to cross-laminated, and locally contains large-scale "festooned" or trough cross-stratification (fig. 7). At places, pebble pavement showing imbrication of the larger fragments is a common feature. Red silty shale 2 METERS 0 1 FIGURE 7.-Outcrop of the Poolesville Member of the Manassas Sandstone exposed on the west bank of the Potomac River near the end of Virginia State Route 656, Loudoun County, Va. Broad trough cross-stratification is shown with paleocurrent flow direc- tion toward the viewer. chips forming thin to thick intraformational con- glomeratic sandstone lenses commonly occur in the upper part of this sequence in the Culpeper basin. An exposure of light-gray paleosol caliche is present in road cuts along Liberia Avenue southeast of Manassas (fig. 6). The caliche occurs as masses of spherules and irregular patches with sand grains in the lower part of this unit. The Poolesville Member is generally intercalated with lenticular bodies of light-gray to gray, highly feldspathic sandstone and quartzite-pebble con- glomerate. Calceareous sequences are commonly found in the transitional zones between the fluvial sandstone deposits and the overbank flood plain, mudflat, or playa lacustrine siltstone and shale of the Balls Bluff Siltstone. Thickness.-The total thickness of the Poolesville Member ranges from about 200 m (656 ft) at the northern and southern limits of outcrop to more than 1,000 m (3,280 ft) in the east-central parts of the Culpeper basin. Fossils and age.-Cornet (1977) included the rare palyniferous zones in the Poolesville Member in his Manassas-Passaic (shown as "lower Passaic" on fig. 2) palynofloral zone of Late Triassic (middle Norian) age. Rare plant fossils discovered in carbonaceous gray to green shale and siltstone east of Comptons Corner in the Manassas 7.5-min Quadrangle may be early Norian in age (Cornet, 1982, oral commun.). The fossil footprint Chetirotherium was recently discovered by P. Kimmil in red sandy siltstone a few hundred meters east of the plant fossil locality near Comptons Corner (R.E. Weems, 1984, oral commun.). Recently, spores collected from TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. dark-gray carbonaceous siltstone in the prominent bluffs along the Potomac River have been tentatively identified as indicating a late Carnian age (Hugh Houghton, oral commun., 1984). Relation to adjacent stratigraphic units.-The contact of the Poolesville Member with the underlying con- glomerates (pl. 1B, 1C) is everywhere gradational and intertonguing. The Poolesville is laterally and vertically gradational with the lower part of the Balls Bluff Siltstone, indicating that these units are, in part, contemporaneous sedimentary facies. Deposition.-The arkosic sandstones and pebbly sandstones of the Poolesville Member are the con- solidated detritus that was derived from adjacent highlands and was deposited mainly by braided streams. The preserved primary structures in some outcrops indicate sand deposition to be chiefly confined to the areas of midfan and fan base (pl. 1C). The dip of cross-lamination in sandstone, the imbrication of quartzite pebbles, and the depositional patterns of quartzite-pebble conglomerate lenses indicate eastern and southeastern source areas along the eastern basin border. Evidence for a southeastern source is well displayed west of Adamstown and north of Tuscarora Creek in Frederick County, Md. Eastern and north- eastern sources are indicated along the railroad cuts east of Dickerson and road cuts of Maryland State Road 28 in the vicinity of Poolesville, Md., and along road cuts of Maryland State Roads 107 and 28 in the vicinity of Dawsonville, Md. In the Barboursville basin, paleocurrent directions indicate that the Poolesville Member was derived from southeastern highlands and was deposited by a north- westerly flowing drainage system. BALLS BLUFF SILTSTONE The Balls Bluff Siltstone was named for the rocks exposed near Balls Bluff National Cemetery on the west bank of the Potomac River, Loudoun County, Va. (Lee, 1977, p. C5 and measured sections, p. C13, C14; Lee, 1979, 1980; Froelich and others, 1982, p. 59-62, 76-78; pl. 1B and 1C, tables 1 and 2, figs. 2 and 3, and section 5 in app. A, this paper). This unit constitutes the bulk of the Upper Triassic deposits in both the Culpeper and Barboursville basins, and many of its rocks were previously mapped as the Bull Run Formation. In the northwestern part of the Culpeper basin, it includes the Leesburg Member, a lenticular limestone conglomerate that intertongues with calcareous sandstones and siltstones of the upper Balls Bluff. Distribution.-The Balls Bluff Siltstone occurs throughout the central portions of the Culpeper and Barboursville basins. In the Culpeper basin, it crops out extensively along the west and south banks of the STRATIGRAPHY 17 FIGURE 8.-Bedding plane exposure of ripple marks and mud cracks in the Balls Bluff Siltstone on a quarried slab at the Culpeper Crushed Stone quarry, Stevensburg, Culpeper County, Va. Symmetrical ripple marks were probably produced by gentle waves, and the polygonal cracks are interpreted as subsequent desiccation features. Hammer is 30 em (12 in) long. Potomac River in Loudoun County, Va., along the valley of Bull Run in Prince William and Fairfax Counties, Va., and along the south side of Mountain Run; it is well exposed at the Culpeper Crushed Stone quarry in Culpeper County, Va., and along road cuts of U.S. Routes 15, 29-211, and 66. In the Barboursville basin, excellent exposures of this unit are in the Webster Brick Company quarry in the Gordonsville 7.5-min Quadrangle and along the valley of Blue Run in Orange County, Va. Lithology.-This unit is composed mostly of grayish- red and dusky-red, very fine grained to very coarse grained calcareous, clayey, micaceous, and feldspathic siltstone. Clay-sized minerals determined from three typical samples from road cuts of U.S. Route 50 in Fairfax County, Va., are mostly illite, with subordinate amounts of kaolinite and halloysite and a minor amount of montmorillonite and chlorite. The Balls Bluff is thin bedded to massive and extensively bioturbated, with irregular or convolute bedding, ripple marks (fig. 8), and planar or climbing-ripple cross-laminations. Mud cracks (fig. 8), rip-ups, animal and insect burrow casts, and plant rootlets are common. Siltstone is locally intercalated with thin lenses (1.0 to 20 em; 0.25 to 8 m) of gray to bluish-gray, argillaceous, locally odlitic limestone and gray dolomite. Brownish-gray carbonate concretions, as well as lenses of odids that contain minute caliche clasts and pellets, are also present in the middle part of the Balls Bluff Siltstone. Sobhan (1985) has described two 500-ft-deep core holes in the lower part of the Balls Bluff southwest of Manassas, Prince 10 METERS (APPROXIMATE) FIGURE 9.-Balls Bluff Siltstone exposed in north quarry face at the Culpeper Crushed Stone quarry near Stevensburg, Culpeper East 7.5-min Quadrangle. Cyclic lacustrine sequences are 3 to 4 m (10-14 ft) thick, dip gently westward, and consist of laminated siltstone and shale overlain by massive, bioturbated, mud-cracked mud- stones with dinosaur tracks. William County, Va. (table 2). Gray to light-gray and dark-red, calcareous, feldspathic, micaceous, fine-to medium-grained, thin-to thick-bedded, lenticular flu- vial sandstone and dusky-red, calcareous, silty, micro- micaceous shale are interbedded in the lower and upper parts of the unit. In the south-central part of the Culpeper basin, the Balls Bluff commonly contains two principal varieties of related lithology in addition to the typical dusky-red siltstone: (1) medium- to dark-gray, carbonaceous, fossiliferous, thin- to thick-bedded, massive clayey siltstone, silty shale, fissile to micro- laminated shale and minor fine- to medium-grained sandstones, and (2) light-greenish-gray, very fine to medium-grained, thin- to thick-bedded, massive silt- stone, silty shale, and fossiliferous shale. The latter unit represents either preserved remnants of marginal lacustrine or shallow playa lake deposits, whereas the former may be deep, less oxidized, possibly euxinic lacustrine sediments. The dark-gray carbonaceous sedimentary sequence locally contains lenses of argillaceous and dolomitic limestone and lenses of reworked dids (Carozzi, 1964). Euhedral grains and aggregates of authigenic pyrite are scattered throughout the rocks. J.P. Smoot (table 2, app. B) has measured and described a thick cyclic succession of lake sediments and marginal deposits exposed at the Culpeper Crushed Stone quarry near Stevensburg, Culpeper County, Va. (fig. 9). Fossils and age.-The Balls Bluff Siltstone locally contains fossil evidence of freshwater animals, such as tracks, trails, bones, and shells, mainly conchostracans, 18 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. FIGURE 10.-Bedding plane exposure of dinosaur tracks (light patches) in mud-cracked Balls Bluff Siltstone at the Culpeper Crushed Stone quarry, Stevensburg, Culpeper County, Va. Poorly defined tracks show movement across waterlogged mud from the lower left to the upper right. A 1-m (8.3-ft) tape is in the right- central part of the photo. Calcite veinlets fill intersecting joints in foreground. notostracans, ostracodes, and rare stromatolites and fish teeth and seales. Froelich and others (1982, p. 77) described such a fossiliferous Late Triassic lacustrine sequence exposed on U.S. Route 29 in the Gainesville 7.5-min Quadrangle near Manassas Battlefield. E.I. Robbins (oral commun., 1977) made a preliminary study of filamentous algae and plant spores from samples collected from siltstone along the south side of Mountain Run due east of Culpeper City, Culpeper County, Va., and gave a Late Triassic age for the siltstone. Amphibious phytosaur remains were re- covered at a site in the southeastern part of the Dulles International Airport property in Fairfax County, Va. (Nicholas Hotten III, unpub. data, 1959; Eggleton, 1975; Weems, 1979). Frank C. Whitmore, Jr., and Robert E. Weems (unpub. data, 1979) identified Upper Triassic dinosaur footprints exposed at the Culpeper Crushed Stone quarry near Stevensburg in the east- central part of the Culpeper East 7.5-min Quadrangle, Culpeper County, Va. (figs. 2, 10). Cornet (1977) identified freshwater crustaceans, ostracods, mollusks, fish seales, and plant spores (Carnmisporites granulatus Schulz, 1967, and Conbaculatisporites mesogoicus Klaus, 1960) (fig. 3), in the east-central part of the Gainesville 7.5-min Quadrangle, Prince William County, Va. Cornet included all palyniferous zones in the Balls Bluff in his Manassas-Passaic palynofloral zone (shown as "lower Passaic' in fig. 2) of Late Triassic (middle to late Norian) age, but recent restudy of the lowest paly- niferous gray-green silty shale zones indicate a late Carnian age (Hugh Houghton, written commun., 1984). Thickness.-The total thickness of Balls Bluff Siltstone is estimated to range from 80 m (262 ft) near the northern border of the basin west of Frederick, Md., to 1,690 m (5,545 ft) in the central portion of the Culpeper basin. Within this unit, the dark-gray or greenish-gray sequences in the southern part of the Culpeper basin generally average 3 m (10 ft) thick, but locally as much as 45 m (148 ft) of rhythmic eyclic deposits are continuously exposed at the Culpeper Crushed Stone quarry near Stevensburg, with neither base nor top of the lacustrine sequence present (table 2, fig. 9, app. A; J.P. Smoot, oral commun., 1984). In the Barboursville basin the maximum thickness of the Balls Bluff is estimated to be 120 m (894 ft) in the area surrounding the town of Somerset in the Gordons- ville 7.5-min Quadrangle, Orange County, Va. Relation to adjacent stratigraphic units.-The Balls Bluff Siltstone is gradational with the underlying Manassas Sandstone on the northeastern, eastern, and southeastern sides of the Culpeper basin and on the southeastern side of the Barboursville basin; with the overlying Catharpin Creek Formation on the south- western, western, and northwestern sides of the Culpeper basin; and with the Tibbstown Formation in most of the Barboursville basin. The contacts of the Balls Bluff with adjacent stratigraphic units generally are vertically gradational and laterally intertonguing throughout both basins (pl. 1C). In the Culpeper basin, tongues of the Balls Bluff Siltstone in the Rapidan and Poolesville Members of the Manassas Sandstone are well exposed on the southern side of the Rapidan River in the northern part of the Unionville 7.5-min Quad- rangle, Culpeper and Orange Counties, Va. Similar tongues in the Poolesville Member are well exposed on the western side of the Potomac River in Loudoun County, Va. In the Barboursville basin, the gradational stratigraphic relationship of this unit with underlying sandstone of the Poolesville Member is well exposed along Blue Run southeast of Barboursville. A similar relationship with the overlying sandstone of the Tibbstown Formation is well shown elsewhere in the tributary drainage areas of Blue Run. Deposition.-The Balls Bluff Siltstone was deposited chiefly in the medial parts of the closed basins. When the rates of deposition and basin subsidence were approaching balance, the deposits consisted of alter- nating layers of siltstone and shale that grade into impure pelleted limestone and dolomitic limestone. This depositional environment is similar to that reported by Hooke (1968, p. 614) for modern alluvial fans and in playa lakes. Most of the sediment deposition of this sequence may have occurred within or immediately adjacent to playa lakes where most of the distal facies of fluvial fans are STRATIGRAPHY localized. Sedimentary features indicate that during the lifespan of lakes in the Culpeper basin, little significant erosion of the lake sediments occurred, except possibly during periods of drought, when desic- cation took place, or when caliche accumulated, or when fluvial channels seoured and reworked soil zones. The odlitic layers of the dark-gray carbonate rocks found locally in the Culpeper basin consist of reworked odids (Carozzi, 1964, p. 231-241). These odids consist of simple or compound nuclei enveloped by several sets of concentric or nonconcentric rings with intercalations of argillaceous limestone and were formed in a shallow lake environment, probably near the margins. LEESBURG MEMBER The Leesburg Member of the Balls Bluff Siltstone is here redefined. It was previously called the Leesburg Limestone Conglomerate Member of the Bull Run Formation by Lee (1977, p. C6) and the Leesburg Conglomerate Member of the Bull Run Formation by Lindholm (1979, p. 1718) and Lindholm and others (1979, p. 1249). It is named for the exposures near the town of Leesburg, Loudoun County, Va. This unit forms the lenticular upper member of the Balls Bluff Siltstone in the northwestern part of the Culpeper basin. The type section is exposed in road cuts southeast of the junction of U.S. Route 15 bypass and the entrance road to Balls Bluff National Cemetery in Loudoun County, Va. (pl. 1C). Distribution.-The Leesburg Member underlies the northwestern part of the Culpeper basin in Virginia and Maryland. It is well exposed north of Leesburg along U.S Route 15 (Toewe, 1966, p. 5; Hazlett, 1978) and along the valley of the Potomac River in Loudoun County, Va., and Frederick County, Md. Lithology.-The Leesburg Member is composed chiefly of angular to subangular and subrounded to rounded cobbles of lower Paleozoic (probably mostly Cambrian) light-gray to grayish-black and pinkish-red limestone and dolomitic limestone and contains minor clasts of dolomite, quartzite, vein quartz, schist, slate, and greenstone embedded in a matrix of carbonate- rock granules, red sand, and clayey silt that is firmly cemented by calcite (fig. 11). Pebble counts of con- glomerate from 29 selected locations within this unit show 94 percent limestone and dolomitic limestone, 2 percent dolomite, 1 percent greenstone, 2 percent quartzite and metamorphosed feldspathic sandstone, and 1 percent slate, schist, chert, and siltstone. Each count represents a conglomerate area of 1 m*. Generally, dolomite fragments increase in abundance north- westerly from the northeast suburbs of Leesburg. The Leesburg Member contains both matrix-supported 19 FIGURE 11.-Exposure of a limestone conglomerate in the Leesburg Member of the Balls Bluff Siltstone in a road cut of U.S. Route 15, north of Leesburg, Va. Very poorly sorted subangular limestone and dolomitic limestone clasts, ranging in size from granules to small boulders, are randomly distributed in a red muddy matrix. The conglomerate is firmly cemented by calcite. Lens cover is 5 em (2 in) in diameter. and clast-supported conglomerates, as well as inter- bedded pebbly red sandstone and massive red mudstone (Lindholm, 1979, p. 1722; Lindholm and others, p. 1254, 1255). Limestone clasts in the Leesburg Member are very fine to medium grained and holocrystalline. Some of these rock fragments are argillaceous, quartzose, flaggy, and planar laminated. Rock fragments generally increase in size westerly and northwesterly from about 6.35 em (2.5 in) in the vicinity of Leesburg to as large as 1.2 m (4 ft) near Limestone Branch, north of Leesburg, Loudoun County, Va. The limestone conglomerate is metamorphosed into a light-gray marble in contact with intrusive diabase in the southeastern part of Leesburg. Shannon (1926) called this rock Potomac marble, a name popularly used for the unmetamorphosed limestone conglomerate exposed near Point of Rocks, Frederick County, Md., and formerly used as dimension stone. Thickness.-The Leesburg Member ranges in thick- ness from 40 m (131 ft) in the northwestern part of the Culpeper basin, Frederick County, Md., to 1,070 m (3,510 ft) west of the Potomac River in Loudoun County, Va. The deposits are thickest at the base of Catoctin Mountain, and the prism of conglomerate pinches out abruptly to the southeast into calcareous siltstone of the Balls Bluff and lower sandstones of the Catharpin Creek Formation in the vicinity of Leesburg, Loudoun County, Va. Relation to adjacent stratigraphic units.-The Lees- burg Member interfingers with the upper part of the 20 Balls Bluff Siltstone in the northwestern part of the Culpeper basin. The gradational and interfingering contact with the lower part of the Catharpin Creek Formation is well exposed along the road cuts southeast and south of Leesburg, Loudoun County, Va. (pl. 1C). Deposition.-The sediments of the Leesburg Member were derived by erosion from adjacent highlands to the west and northwest, as indicated by crossbeds and imbricate pebble structures. The lower Paleozoic car- bonate rock fragments were transported and deposited by means of swift streams and debris flows that accumulated along a scarp at the base of the highlands. The present outcrops are the remnants of debris flows and coalescing fluvial fan deposits in which lithofacies vary from fanhead to midfan facies. These coarse clastic deposits contain sceattered streamflow channel sandstone lenses in the eastern part of Leesburg. Lindholm and others (1979, p. 1254, 1255) indicate that debris flow, mudflow, and sheet flood deposits are also important environments of deposition represented in the Leesburg Member. TIBBSTOWN FORMATION The Tibbstown Formation is herein named for the sequence of predominantly arkosic sandstone and conglomerate that crops out in Tibbstown at the southern foothills of Haudricks Mountain less than 1 km (0.6 mi) northeast of the town of Barboursville, Orange County, Va., which is designated the type section. The unit overlies the Balls Bluff Siltstone in the Barboursville basin and in the southwestern Culpeper basin. This formation is equivalent to the lower part of the informal basaltic-flow bearing clastics member of the Bull Run Formation of Lee (1977, p. C7, C8). It locally includes two conglomerate members, the Mountain Run and Haudricks Mountain Members, which occur at the top of the formation in different localities. Distribution.-The Tibbstown Formation crops out extensively in the Barboursville basin and in a narrow arcuate belt between Culpeper and Brandy Station in the southwestern Culpeper basin. It is present dis- continuously south of Culpeper, where most of the unit is incorporated in the thermal metamorphic aureole adjacent to intrusive diabase. Lithology.-The Tibbstown Formation is predomi- nantly reddish-brown, fine- to medium-grained, felds- pathic, micaceous sandstone interbedded with red- brown, medium- to coarse-grained pebbly arkose con- glomerate, and dark-red siltstone and minor beds of gray, fine-grained sandstone, siltstone, and shale. Thickness.-The thickness of the sandstone part of this formation (exclusive of the conglomerate members) TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. is estimated to average 450 m (1,475 ft) in the Barboursville basin and about 300 m (1,000 ft) in the southwestern part of the Culpeper basin. Fossils and age.-Although almost all of this forma- tion is believed to be barren of diagnostic fossils, samples from a gray carbonaceous shale interbedded with fine-grained sandstone and siltstone east of the town of Culpeper contained sporomorphs characteristic of Late Triassic, early Norian age (Cornet, oral commun., 1982). Relation to adjacent stratigraphic units.-The arkosic sandstone of the Tibbstown Formation overlies the siltstone of the Balls Bluff Formation in apparent conformity, and intertongues with and is overlain conformably by the conglomerate members. MOUNTAIN RUN MEMBER The Mountain Run Member of the Tibbstown Forma- tion, named herein, was defined informally by Lee (1980) for greenstone conglomerate exposed along the eastern and southeastern sides of Mountain Run at the city of Culpeper, Culpeper County, Va. The type section is south of the filtration plant and on the eastern bank of Mountain Run in road cuts of Virginia State Road 3, south and southeast of Culpeper, and in the vicinity of the Culpeper water tower (pl. 1C; app. A, measured section 3). This unit was called the trap conglomerate by Roberts (1928, p. 20), who included it in his Border Conglomerate, and the Cedar Mountain Conglomerate Member of the Bull Run Formation by Lindholm and others (1979, p. 1249-1251). Distribution.-The Mountain Run Member under- lies much of the southwestern parts of the Culpeper basin, occupying a north-northeast-trending belt about 28 km long and about 3 km (1.9 mi) wide along the western basin margin between Locust Dale and Brandy Station. Excellent exposures of this unit are along Mountain Run at Culpeper, at Cedar Mountain, and in the area north of Brandy Station, Culpeper County, Va. Lithology.-The Mountain Run Member consists of two principal sequences of greenstone conglomerate. The lower sequence is composed of conglomerate with angular to subangular fragments of dusky-yellowish- green to dark-yellowish-green greenstone in a dusky- red to pale-green clayey sand and silt matrix (fig. 12). The upper sequence consists generally of conglomerate with more than 60 percent angular to subangular greenstone fragments, a subordinate amount of angular to subangular quartzite and feldspathic sandstone clasts, and minor vein quartz fragments. Cut-and-fill features and pebble imbrication within this member are well shown in Culpeper and at Cedar Mountain. STRATIGRAPHY 21 weld FIGURE 12.-Outcrop of indurated greenstone conglomerate of the Mountain Run Member of the Tibbstown Formation exposed at the west end of Chandler Street, Culpeper, Va. Angular to subangular greenstone cobbles and boulders with scattered vein quartz, quartzite, and schist pebbles are randomly intermixed with a predominantly greenstone granule, sand, and silt matrix, firmly cemented by clay and silica. Typical closely packed, matrix-rich, poorly sorted texture of this member near the source area. Lens cover is 5 em (2 in) in diameter. Thickness.-The Mountain Run Member ranges in thickness from a featheredge to 640 m (2,099 ft) near Culpeper, Culpeper County, Va. Relation to adjacent stratigraphic units.-The con- glomerates of the Mountain Run Member occur at the top of the formation, and lower and lateral contacts of the conglomerate with sandstones of the Tibbstown Formation are gradational or intertonguing, as shown along Mountain Run and in railroad cuts in the south- eastern part of Culpeper. The upper contact relations are unknown as erosion has removed all overlying units. Deposition.-The Mountain Run Member is a series of fluvial fan deposits derived from erosion of the adjacent Precambrian terrane on the west, northwest, and southwest (pl. 1C). The apexes of fans were located near the present basin border, and the detrital materials were transported and deposited by streams heading in the adjacent highlands. Outerops show that debris-flow deposits of the lower sequence of this member are less extensive than those of the upper sequence. In the southern part of the Culpeper basin, the upper sequence of this member at Cedar Mountain coalesced with the lower sequence in the vicinity of Cedar Run. The distribution and lithology of this member indicate an increased rate of uplift of the adjacent highlands in the later stages of the deposition of conglomerates of the Tibbstown Formation. HAUDRICKS MOUNTAIN MEMBER The Haudricks Mountain Member of the Tibbstown Formation is herein named for the pebble and cobble conglomerate interbedded with arkosic sandstone that conformably overlies and intertongues with the sand- stone of the Tibbstown Formation in Haudricks Mountain north of the town of Barboursville, Orange County, Va. The type locality of the Haudricks Mountain Member is along the secondary road across the crest of Haudricks Mountain 2 km (1.25 mi) north-northwest of the town of Barboursville in the Barboursville 7.5-min Quadrangle. These rocks were included by Roberts (1928, p. 17, 18) in his "schist conglomerate" of the Border Conglomerate. Lee (1980) informally included the Haudricks Mountain Member in the Mountain Run Member while recognizing the regional lithologic difference in clast components. Distribution and provenance.-The Haudricks Mountain Member crops out only in the Barboursville basin, where good outcrops are sparse and commonly deeply weathered to saprolite. The main source of the clasts in the conglomerate is the Fauquier and Catoctin Formations exposed in the adjacent Blue Ridge province. Lithology.-Sandstone, quartzite, and fine-grained metasiltstone clasts are the chief components of the conglomerate interbedded with fine-to coarse-grained arkosic sandstone at Haudricks Mountain. The weathered sandstone clasts are mainly light to dark gray and rounded to subrounded, and consist of medium to coarse quartz and feldspar grains. Minor clast components include vein quartz, phyllite, gneissic granite, and greenstone, and the matrix is generally loose, dark-grayish-maroon clayey sand and silt. Thickness.-It is difficult to estimate an accurate minimum thickness, but at least 500 m (1,640 ft) are calculated to be preserved on the flanks and crests of Haudricks Mountain. Relation to adjacent stratigraphic units.-The basal and lateral contacts of the Haudricks Mountain Member with the upward-coarsening sequence of sandstones of the Tibbstown Formation appear to be gradational, but exposed contacts are rare and deeply weathered. The upper relations are unknown as erosion has removed all overlying units. Deposition.-The lower part of the conglomerate at Haudricks Mountain was deposited by streams having headwaters in the adjacent northwestern highlands, as indicated by clast lithology, imbrication, and crossbeds; the upper part of the conglomerate in the southwestern part of the Barboursville basin suggests deposition from an east-northeasterly flowing stream. 22 CATHARPIN CREEK FORMATION The Catharpin Creek Formation is herein formally named for the well-exposed sequence of clastic sedi- mentary rocks that overlie the Balls Bluff Siltstone and underlie the Mount Zion Church Basalt along Catharpin Creek north and northwest of Haymarket, Prince William County, Va., which is designated the type section. In the west-central part of the Culpeper basin, it includes the Goose Creek Member (Goose Creek Conglomerate Member of the Bull Run Formation of Lindholm and others, 1979, p. 1249). The rocks of the Catharpin Creek Formation were originally included as the lower units of the informal basaltic-flow-bearing clastics member of the Bull Run Formation (Lee, 1977, p. C7), and later as the informal Catharpin Creek Member of the Bull Run Formation (Lee, 1979, 1980). Distribution.-The Catharpin Creek Formation under- lies a belt along the western part of the Culpeper basin, but it is not recognized in the Barboursville basin. Excellent exposures are along Bull Run, Broad Run, the Rappahannock River, Mountain Run, and road cuts of U.S. Routes 15, 29-211, and 50 in the Culpeper basin. Lithology.-The lower part of this formation consists of very dark red to dusky-red, micaceous, feldspathic fine-to coarse-grained sandstone and clayey siltstone, locally containing conglomerate lenses (fig. 13). These rocks grade upward into a sequence of dark-red to gray, micaceous, feldspathic sandstone; thin-bedded clayey siltstone; and laminated, fissile, silty shale. Thickness.-Because of poor exposures and extreme thickness variation, it is difficult to estimate the thickness, but the unit may be as much as 500 m (1,640 ft) thick excluding the conglomerate member. Relation to adjacent stratigraphic units.-The contact of the Catharpin Creek Formation with the Balls Bluff is gradational and intertonguing. This relationship is well shown along Bull Run and in road cuts in Loudoun, Prince William, and Fauquier Counties, Va. The contact of this formation with the Leesburg Member of the Balls Bluff in the northwestern part of the Culpeper basin is gradational or intertonguing south, east, and southeast of Leesburg, Loudoun County, Va. The upper contact with the Mount Zion Church Basalt is a sharp disconformity. Deposition.-Deposition of the Catharpin Creek Formation was contemporaneous with adjacent strati- graphic units. The detritus is thought to have been derived from the rocks of the adjacent western high- lands, and was deposited by braided streams and debris flows on fans. The outcrops indicate deposition to be chiefly confined to the area of midfan and fan base. The fanheads are deeply buried downdip to the west or were partly located outside the present basin TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. FIGURE 13.-Outcrop of interbedded sandstone and siltstone of the Catharpin Creek Formation in the east-central part of the Thoroughfare Gap 7.5-min Quadrangle. Exposure is along a road cut of U.S. Route 15, about 0.7 km (0.4 mi) northeast of the intersection with Virginia State Route 55. Sandstone beds are 5-15 em (2-6 in) thick and lenticular, while siltstone beds are thinner but more continuous; both show faint internal planar-lamination. Lens cover is 5 em (2 in) in diameter. border. On the basis of the trends of gravel trains and imbrication of pebbles, this unit was deposited by streams draining the northwestern, western, and southwestern highlands. During the processes of fan development, the areas of fan base and distal facies probably formed as a series of deltaic and overbank stream deposits which graded into finer materials in the lowland areas. The presence of conglomerate lenses and layers throughout this unit indicates that fluvial fans were active and that tectonic movements played a major role during deposition. GOOSE CREEK MEMBER The Goose Creek Member of the Catharpin Creek Formation is redefined and adopted herein. It was named the "Goose Creek Conglomerate Member of the Bull Run Formation" by Lindholm (1979, p. 1721), who identified this unit from "outcrops on the south side of Goose Creek 2 km (1.2 mi) east of the confluence of Goose Creek and Little River in Loudoun County." Distribution.-The outcrop belt of the Goose Creek Member, which is less than 3 km (1.9 mi) wide, strikes northerly for about 28 km (17.5-mi) from Catharpin Creek north of Haymarket, Prince William County, Va., to Sycolin Creek south of Leesburg, Loudoun County, Va. The lenticular conglomerate bodies that characterize the member are discontinuous and ir- regular, separated from one another laterally by sandstones and siltstones. Lindholm states (1979, p. STRATIGRAPHY 28 1722), "This pattern may reflect the presence of several different alluvial fans at the time of deposition." Lithology and provenance.-The predominant litho- logy consists of lenses of grayish-green to red-brown cobble and pebble conglomerate that fill scoured channels and grade laterally and vertically to coarse- and fine-grained reddish-brown arkosic sandstone and sandy siltstone. Lenses of conglomerate within this sequence consist chiefly of rounded to subrounded and subangular cobbles and pebbles of greenish-gray, fine- to coarse-grained quartzite, metasiltstone, greenstone and metabasalt, vein quartz, limestone, and granitic pegmatite. According to Lindholm (1979, p. 1722), most of the clasts were derived from the Catoctin Formation and Chilhowee Group, metamorphic rocks now exposed in the Blue Ridge province just west of the Culpeper basin. Thickness.-Lindholm stated (1979, p. 1722), "Although the paucity of outcrops precludes an accurate determination, the thickness exceeds 900 m" (2,952 ft). The lenticular unit tongues out abruptly, north and south along the strike. Relation to adjacent stratigraphic units.-The Goose Creek Member grades laterally and vertically into arkosic sandstones of the Catharpin Creek Formation; where the Goose Creek Member forms the top of the formation, it is overlain by the Mount Zion Church Basalt at a locally irregular but regionally paracon- formable contact. MOUNT ZION CHURCH BASALT The basalt exposed at Mount Zion Church, on U.S. Route 50 in the Arcola Quadrangle, Loudoun County, Va., is herein named the Mount Zion Church Basalt (Lee 1979, 1980). It is the lowest basalt flow and was first described in the Leesburg 7.5-min Quadrangle by Toewe (1966). It is equivalent to Lindholm's basalt flow unit I, the lowermost unit of his Buckland Formation (Lindholm, 1979, p. 1724, 1725, and fig. 8, p. 1729), and to formation J of Cornet (1977). Distribution.-The Mount Zion Church Basalt ex- tends discontinuously for more than 55 km (34.4 mi) in the west-central part of the Culpeper basin (pl. 1C). The basalt apparently pinches out southwest of Haymarket in the northeastern part of the Catlett 7.5-min Quad- rangle, Prince William County, Va., and is truncated by the western border fault southwest of Leesburg in the western part of the Leesburg 7.5-min Quadrangle, Loudoun County, Va. It occurs as two separate thin flowsheets separated by a sequence of red sandstone and siltstone to the west of the town of Catlett, and to the northeast and north of the city of Remington, Fauquier County, Va., but individual lentils are too thin to show FIGURE 14.-Mount Zion Church Basalt exposed on the north cuts of the Southern Railroad about 50 m (165 ft) northwest of the intersection with U.S. Route 15, in the east-central part of the Thoroughfare Gap 7.5-min Quadrangle. Prismatic jointed weathered flow surface is shown. Lens cover is 5 ecm (2 in) in diameter. at this seale (pl. 1C). In places the basalt flows crop out as discontinuous strike ridges, the gaps resulting from Early Jurassic erosion or nondeposition over paleo- topographic highs (Lindholm, 1979, p. 1725). A shallow USGS exploratory core hole located to intercept the lowest basalt flow horizon downdip of its surface occurrence recovered less than 10 m (83 ft) of basalt cobble conglomerate sandwiched between red arkosic sandstone, and no intact basalt, suggesting that Early Jurassic erosion accounts for the absence of basalt at this locality. Description of rock.-The Mount Zion Church Basalt is medium to dark gray, very fine to medium erystalline, porphyritic in part, mostly equigranular, and holo- crystalline (fig. 14). Augite and plagioclase (chiefly labradorite and andesine) display ophitic or subophitic texture (fig. 15). Vesicles are scattered but are mostly concentrated in the lower and upper parts of the sequence. Columnar joints are well developed in places. The basalt locally encloses poorly exposed, irregular lenses of dusky-to grayish-red, very fine to medium- grained, feldspathic and micaceous sandstone and siltstone, an example of which is exposed at a road cut along U.S. Route 15 about 100 m (328 ft) S. 50° W. of the intersection of U.S. Route 15 and the Southern Railroad southwest of Haymarket in the east-central part of the Thoroughfare Gap 7.5-min Quadrangle, Prince William County, Va. Geochemistry.-Based on chemical analyses of several samples of unweathered but altered basalt (Puffer and others, 1981; Leavy and Puffer, 1983; Lee and others, 1984), the Mount Zion Church Basalt is a high-Ti0;, 24 FIGURE 15.-Photomicrograph of the Mount Zion Church Basalt from an exposure in Virginia State Route 600 east of Bull Run. Augite (au), serpentinized clinopyroxene (epy), and laths of plagioclase (pl) (chiefly labradorite) make up an ophitic intergrowth with accessory magnetite and ilmenite (black). The field of view is 2.2 mm (0.08 in) across (plane-polarized light). quartz-normative tholeiite chemically similar to the Orange Mountain Basalt of the Newark basin in New Jersey and the Talcott Basalt of the Hartford basin in Connecticut (Puffer and Hurtubise, 1983). Thickness.-The Mount Zion Church Basalt ranges in thickness from 3 m (10 ft) north and northeast of Remington, Fauquier County, to 85 m (279 ft) west of Haymarket, Prince William County, Va., and from 90 m (295 ft) to about 140 m (459 ft) southwest of Leesburg, Loudoun County, Va. Lindholm (1979, p. 1730) calcu- lated thicknesses at 17 localities that ranged from 10 to 180 m (83 to 590 ft); he also showed that his basalt flow unit I, the Mount Zion Church Basalt, thickens to the north. Siltstone and sandstone lenses are poorly exposed and generally less than 10 m (83 ft) thick. MIDLAND FORMATION The Midland Formation is herein named for the locally well exposed succession of clastic sedimentary rocks that overlies the Mount Zion Church Basalt and is succeeded by the Hickory Grove Basalt along Licking Run about 2 km (1.25 mi) north of the town of Midland in the Midland 7.5-min Quadrangle, Fauquier County, Va. This section, which contains the Midland Fish Bed, is designated the type locality of the Midland Formation. This unit is equivalent to sedimentary unit I-II of the Buckland Formation of Lindholm (1979, p. 1724, 1725, and fig. 8, p. 1729), to the lower sedimentary unit of Lee's informal basaltic-flow-bearing clastics member of the Bull Run Formation (Lee, 1977, p. C7), and to formation I of Cornet (1977). TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. Distribution.-The Midland Formation occupies a well-defined north-trending arcuate belt averaging 1.0 km (0.625 mi) wide and slightly concave to the west between the Mount Zion Church and Hickory Grove Basalts. It extends for about 67 km (42 mi) from about 5 km (83 mi) north of Brandy Station to Sycolin Creek, about 6 km (4 mi) south-southwest of Leesburg, Loudoun County, Va. Lithology.-The Midland Formation consists of dark- red to reddish-brown micaceous, feldspathic, cross- bedded, ripple-laminated, fine- to medium-grained sandstone interbedded with reddish-brown, ripple- laminated, micaceous siltstone, dark-red, greenish- gray, and dark-gray to nearly black calcareous, silty microlaminated fossiliferous shale, and thin-bedded argillaceous limestone. Some thin beds of dark-gray to black shale are carbonaceous and pyritic, and some display desiccation cracks. In places, lenses of con- glomerate and conglomeratic, coarse-grained, red, brown, and gray arkosic sandstone are abundant, particularly near the northern limit of outcrop. Thickness.-The thickness of the Midland Formation ranges from about 300 m (984 ft) along the north side of Goose Creek, where it contains many conglomerate layers, to about 150 m (500 ft) northwest of Remington, where the underlying Mount Zion Church Basalt is absent or replaced by diabase intrusive rocks (pl. 1C). A fossiliferous sequence containing the fish-bearing beds was recently cored at the type locality; it consists of approximately 10 m (83 ft) of fossiliferous gray shale overlain and underlain by reddish-brown siltstone and sandstone (app. B). Relation to adjacent stratigraphic units.-Although both upper and lower contacts of this formation are poorly exposed, the contacts are either slight discon- formities or paraconformities, based on local and regional structural relations. Along Goose Creek in Loudoun County where the upper contact with the Hickory Grove Basalt is well exposed, the contact is baked for about 10 em (4 in) and is locally irregular with as much as 1 m (8 ft) of local relief. Conglomerates in the lower part of the Midland Formation contain no recognizable basalt cobbles or boulders; however, where the Mount Zion Church Basalt is discontinuous, the equivalent horizon in a USGS core hole near Midland is marked by a basalt cobble and boulder conglomerate, confirming that Early Jurassic erosion was locally important. ___ _ Fossils and age.-The Early Jurassic fossil fish Ptycholepis marshi Newberry was identified from the dark-gray, microlaminated shale and impure limestone sequence along Licking Run (Schaeffer and others, 1975; Schaeffer and McDonald, 1978). Olsen and others (1982, p. 36) subsequently identified Redfieldius sp, ‘u/ STRATIGRAPHY Semionotis micropterus, and Diplurus longicaudatus and assigned them to his Semionotis micropterus zone. Cornet (1977) identified diagnostic plant spores Ali- sporites grandis (Cookson), Verrucosisporites cheneyi, and Convolutispora klukiforma (Wilson) from these beds. Cornet (1977) assigned the spores to his Corollina meyeriana palynofloral zone of Early Jurassic (Het- tangian to Sinemurian) age. Deposition.-Although the bulk of the Midland Formation is reddish-brown siltstone, fine-grained, ripple laminated sandstone, and medium- to coarse- grained, crossbedded sandstone and lenticular con- glomerates, indicating deposition by streams, the most distinctive unit is the widespread dark-gray to black, fish-bearing, fossiliferous, calcareous shale and impure limestone sequence which is of lacustrine origin. Although these beds cannot be traced continuously along the depositional strike because of sparse outcrops, they mark widespread, possibly recurrent, lacustrine conditions in the Early Jurassic of the Culpeper basin. Another ostracod-bearing greenish-gray shale zone was exposed during recent excavations at the type locality about 10 m (83 ft) above the Mount Zion Church Bas..!t; however, the exposure was deeply weathered and has not been found elsewhere. HICKORY GROVE BASALT The middle sequence of basalt flows in the upper part of the Culpeper Group is herein named the "Hickory Grove Basalt" for the exposures at the type locality along Virginia State Road 701, about 180 m (590 ft) N. 82° W. of the intersection of U.S Route 15 and Virginia State Road 701, near Hickory Grove in the southeastern part of the Middleburg 7.5-min Quadrangle, Prince William County, Va. (Lee, 1979, 1980; measured section 4C, app. A, this paper). It is equivalent to basalt flow unit II of the Buckland Formation of Lindholm (1979, p. 1724, 1725, and fig. 8, p. 1729) and formation H of Cornet (1977). Distribution. -The Hickory Grove Basalt extends more than 62 km (50 mi) in the western parts of the Culpeper basin (pl. 1C). This basalt occurs as two separate flows separated by dark-red, fine-grained sandstone and siltstone north of Casanova Junction and pinches out near the southwestern border of the basin in Culpeper County, Va. To the north it occurs as at least three flows separated by fine-to coarse-grained sandstone and conglomerate beds southwest of Lees- burg, Loudoun County, Va., where it is truncated by the western border fault. Description of rock.-This basalt is medium to dark gray, very fine to coarse erystalline, mostly equi- granular and holocrystalline (fig. 16). Euhedral or 25 FIGURE 16.-Exposure of the Hickory Grove Basalt on the south bank of Broad Run in the southern part of the Thoroughfare Gap 7.5-min Quadrangle. Polygonally jointed flow surface contains numerous vesicles and amygdules. Lens cover is 5 em (2 in) in diameter. subhedral crystals of plagioclase, chiefly labradorite and andesine, are embedded in a ground mass of augite crystals, forming ophitic or subophitic textures. Acces- sory minerals are chiefly magnetite and ilmenite. Vesicles are present mainly in the upper part of the sequence. Compared with the Mount Zion Church and the Sander Basalts, the Hickory Grove is the least intensely altered and mineralized. The upper part of this basalt is interbedded with a polymict marble and basalt-cobble-bearing conglomerate at an abandoned quarry on the southern side of Goose Creek in Loudoun County, Va. This indicates that the basalt eruption was contemporaneous with fluvial deposition of the con- glomerate, as the basalt occurs both as matrix to and as subrounded cobbles within the conglomerate. 26 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. Geochemistry.-Based on chemical analyses of several samples of unweathered but altered basalts (Puffer and others, 1981; Leavy and Puffer, 1983; Lee and others, 1984), the Hickory Grove Basalt is a high-FegOq, high-TiO;, quartz-normative tholeiite chemically similar to the Preakness Basalt of the Newark basin in New Jersey and the Holyoke Basalt of the Hartford basin in Connecticut (Puffer and Hurtubise, 1983). Sandstone and siltstone members.-The sandstone and siltstone members consist of two or three poorly exposed lenticular units sandwiched between three or more separate basalt flows. The sandstone is dark red to red brown, arkosic, micaceous, fine to coarse grained, conglomeratic, and poorly sorted. It is interbedded with dusky-red siltstone, which is argillaceous, micaceous, and sandy. Thickness.-The Hickory Grove Basalt ranges in thickness from 80 m (262 ft) near the southwestern basin border, Culpeper County, Va., to 212 m (695 ft) (measured section 4C, app. A) in the west-central part of the basin, Prince William County, Va. Lindholm (1979, p. 1730) calculated thicknesses for his basalt flow unit II, the Hickory Grove Basalt, at 17 sites, where it ranged from about 50 to 380 m (165 to 1,246 ft), and he showed that the flows thicken to the north. The thickness of the sandstone and siltstone members is estimated to be less than 120 m (394 ft) collectively, but individual lentils are less than 50 m (165 ft) thick. TURKEY RUN FORMATION The Turkey Run Formation is herein named for the predominantly sandstone, siltstone, and shale sequence that overlies the Hickory Grove Basalt and is overlain by the Sander Basalt at the type locality along Turkey Run northwest of Casanova Junction in the southwest part of the Catlett 7.5-min Quadrangle, Fauquier County, Va. This unit is equivalent to sedimentary unit II-III of the Buckland Formation of Lindholm (1979, p. 1724, 1725, and fig. 8, p. 1729), to the middle sedi- mentary unit of Lee's informal basaltic-flow-bearing clastiecs member of the Bull Run Formation (Lee, 1977, p. C8, and measured section 8, p. C15, C16; measured section 4D, app. A, this paper), and to formation G of Cornet (1977). Distribution.-The Turkey Run Formation occupies a sinuous north-trending swale from 0.5 to 1.5 km (0.3 to 0.9 mi) wide, generally concave to the west between subdued strike ridges formed by the Sander and Hickory Grove Basalts. It extends for about 63 km (39.5 mi), from about 5 km (3 mi) north of Brandy Station, Culpeper County, to Goose Creek, about 11 km (7 mi) south-southwest of Leesburg, Loudoun County, Va. It crops out extensively in the west part of the Catlett 7.5-min Quadrangle and is especially well exposed in the vicinity of the community of Balls Mill along Licking Run, along Turkey Run 1 km (0.6 mi) northwest of Casanova, and 0.7 km (0.4 mi) northwest of Auburn, Fauquier County, Va. Lithology.-The Turkey Run Formation at its type locality consists largely of dark-red to medium-dark- greyish green, micaceous, feldspathic, laminated, ripple-laminated, and crossbedded, thin- to thick- bedded to massive, very fine to coarse-grained sand- stone, siltstone, and silty shale. Near the south end of its outcrop belt, along the Rappahannock River, the Turkey Run consists of upward-fining and upward-coarsening cyclic sequences of red-brown to greenish-gray, very fine to medium-grained sandstone, siltstone, and shale 5 to 10 m (16.5 to 33 ft) thick. Fossils.-Dinosaur tracks have been found at a quarry in this unit on the northern bank of the Little River about 0.7 km (0.4 mi) S. 30° W. of U.S. Route 15 in the extreme southwestern part of the Leesburg 7.5-min Quadrangle, Loudoun County, Va. Fine-grained, greenish-gray sandstone, siltstone, and carbonaceous shale along the Rappahannock River contain seattered plant fragments. Dark-gray to black, laminated, lacustrine shale and siltstone beds are present near the base of the formation east of Broad Run on U.S. Route 15-29-211 in Prince William County and at Cedar Run near Auburn in Fauquier County, where it contains black, phosphatic fish seales. Thickness.-The thickness of the Turkey Run Formation ranges from less than 150 m (492 ft) near the south end of the outcrop belt north of Culpeper to 218 m (715 ft) at State Route 701 (measured section 4D, app. A); it increases to as much as 330 m (1,082 ft) near Sander quarry and the village of Casanova; from there to the north end of the outcrop belt south of Leesburg, it averages about 300 m (1,000 ft). According to Lindholm (1979, fig. 8, p. 1729), this unit averages about 250 m (820 ft) in the vicinity of Buckland along U.S. Route 15-29-211 in Prince William County. Relation to adjacent stratigraphic units.- Both upper and lower contacts of this formation are poorly exposed; however, based on local and regional structural rela- tions, the contacts with both overlying and underlying basalts are either slight disconformities or paracon- formities. Deposition.-The abundant crossbedded arkosic sandstone suggests deposition under fluvial conditions; however, the climbing-ripple-laminated siltstone and silty shales, although predominantly red brown in color, locally contain some gray and green fissile shales similar to fossiliferous lacustrine strata of the Midland Formation. The Turkey Run Formation is everywhere overlain and underlain by subaerial fissure basalt flows, suggesting that the continental sediments are predominantly fluvial in origin; the sands were STRATIGRAPHY FIGURE 17.-Sander Basalt exposed in the northwestern part of the Sander quarry, 7 km (4.3 mi) southeast of Warrenton, Fauquier County, Va. View looking south at west-dipping sequence of several basalt flows with amygdaloidal tops and well-developed columnar joints. < deposited by mainly east-flowing streams, based on crossbed vector means (Lindholm, 1979, fig. 5, p. 1717). The alternating upward-coarsening and upward-fining cycles suggest at least a local deltaic origin, and the basal beds between Cedar Run and Broad Run clearly indicate deposition under lacustrine conditions. SANDER BASALT The basalt exposed at the Sander quarry, Fauquier County, Va., is herein named the "Sander Basalt" (Lee, 1979, 1980; measured section 4E, app. A, this paper). It is the uppermost sequence of basalt flows (fig. 17) and is equivalent to basalt flow units III, IV, and V of the Buckland Formation of Lindholm (1979, p. 1724, 1725, and fig. 8, p. 1729) and to formations B, D, and F of Cornet (1977). The Sander quarry is located about 7.2 km (4.6 mi) S. 35° E. of Warrenton, on the northeastern side of Virginia State Road 643 in the western part of the Catlett 7.5-min Quadrangle, Fauquier County, Va. The type section, in the northwestern part of the Sander quarry, consists of a lower sequence of mostly medium-grained, but partly coarse-grained, basalt and an upper sequence of aphanitic to medium-grained and in part porphyritic basalt, characterized by zeolite-filled almond-or pea-shaped vesicles (fig. 18). Distribution.-The basalt flows extend for more than 60 km (37.5-mi) in the western part of the Culpeper basin and are separated by intercalated sandstones and siltstones into several flowsheets in Loudoun, Prince 21. FIGURE 18.-Photomicrograph of zeolitic amygdules in the upper sequence of the Sander Basalt at the Sander quarry, Fauquier County, Va. Zeolite-filled amygdules (ze) in a holocrystalline ground mass showing primary ophitic intergrowth of augite (au) and labradorite (1a) with accessory magnetite and ilmenite (black euhedra). Late-stage calcite (ca) occurs in center of the larger zeolite amygdule. The field of view is 2.2 mm (0.08 in) across (plane-polarized light). William, and Fauquier Counties, Va. (pl. 1C). The Sander Basalt terminates near the southwestern border of the basin in the extreme southeastern part of the Brandy Station 7.5-min Quadrangle, Culpeper County, Va. It is truncated by the western border fault along the western basin margin in the Remington, Warrenton, Catlett, Middleburg, and Lincoln 7.5-min Quadrangles, Va. An isolated, shattered, altered, and steeply tilted basalt flow extends in a narrow belt for more than 4 km (2.5 mi) south-southwesterly from near Mill book quarry and is truncated by the normal western border fault; it is probably a fault sliver or horse of the Sander Basalt. A similar basalt is interposed with steeply dipping limestone, pebble-bearing conglomerate, and shattered sandstone at the abandoned Millbook quarry near the western border of the basin in the west-central part of the Thoroughfare Gap 7.5-min Quadrangle, Prince William and Fauquier Counties, Va. Description of the rock.-The Sander Basalt is dark to bluish and grayish black and mostly holocrystalline and equigranular, but in part microcrystalline and porphyritic. Crystals of augite and plagioclase (chiefly andesine and labradorite) exhibit ophitic or subophitic texture. The coarsest zone of the lower sequence of this basalt flow contains patchy streaks of micropegmatite, as much as 152 mm (6 in) thick. These coarse-grained streaks consist of blades of plagioclase and clino- pyroxene with minor vermicular quartz. Vesicles and amygdules are commonly present in the upper part of the flows (fig. 18). In places the Sander Basalt is intensely hydrothermally altered and locally min- 28 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. eralized with copper and iron sulfides, as well as zeolites. Geochemistry.-Based on chemical analyses of several samples of unweathered but altered basalts (Puffer and others, 1981; Leavy and Puffer, 1983; Lee and others, 1984), the several Sander Basalt flows are diverse and very complex. Some flows are high-Feg03, high-TiO;, quartz-normative tholeiites, while others are high-Fe;0s, low-TiO;, quartz-normative tholeiites. They are generally chemically different from the Hook Mountain Basalt of the Newark basin in New Jersey and the Hampden Basalt of the Hartford basin in Connecticut (Puffer and Hurtubise, 1938). Sandstone and siltstone members.-The poorly ex- posed sandstone and siltstone members of the Sander Basalt are composed of three or more lenticular units sandwiched between separate flows (pl. 1C), as well as several minor lentils too thin to portray at map scale. They consist of predominantly very dark red to grayish-red, greenish-gray to olive-gray, feldspathic, micaceous, fine-to coarse-grained and pebbly sandstone, thin to thick bedded, and in places interbedded with silty dark-red shale and locally with dark-gray cal- careous shale and silty shale. The sandstones are crossbedded in places and fine upward to climbing- ripple-laminated siltstone. The siltstone is red brown, micromicaceous, and interbedded with dark-red silty shale and locally with dark-gray calcareous shale. Thickness.-The thickness of the Sander Basalt is 242 m (794 ft) southeast of Warrenton and northwest of Catlett, Fauquier County, Va., and ranges from 140 m (459 ft) to more than 600 m (1,970 ft) west of U.S. Route 15, Prince William County, Va. (measured section 4E, units 1, 3, 5, and 7, app. A). Lindholm (1979, p. 1730) calculated thicknesses for his basalt flow units III, IV, and V, the Sander Basalt, at 25 sites. It is difficult to estimate the collective thickness at any given site because some flows are absent owing to structural truncation along the border fault and some flows contain thick lenses of sedimentary rocks; however, a partial minimum thickness of the lower flow is about 150 m (500 ft), and a maximum composite thickness of the three combined flows of Lindholm is 690 m (2,264 ft). Collectively and individually, the three major flow sequences and the numerous individual flows consti- tuting the Sander Basalt apparently thicken to the north. The three Sander sandstone and siltstone members west of U.S. Route 15 are about 205 m (674 ft) thick collectively (measured section 4E, units 2, 4, 6, with 55 m (181 ft) covered; app. A). WATERFALL FORMATION The Waterfall Formation, named by Lindholm (1979, p. 1725, 1726) for outcrops "in the vicinity of the community of Waterfall (Prince William County) especially in the fields north of Route 630, 0.3 km (0.2 mi) northwest of Waterfall" is here adopted. As revised herein, the upper conglomerate unit of the Waterfall Formation is named the "Millbrook Quarry Member." The Waterfall Formation overlies the youngest flow of the Sander Basalt and is the uppermost formation of the Culpeper Group. Distribution.-Lindholm (1979, p. 1731) stated: "The Waterfall Formation lies adjacent to the western border fault and extends northward from just south of the community of Broken Hill in Fauquier County to the Bull Run Estates in Prince William County. This area is 18 km (11 mi) long and has a maximum width of 2.3 km (1.4 mi)." Lithology.-The Waterfall Formation consists mainly of interbedded sandstone, siltstone, mudstone, shale, and conglomerate. Sandstone ranges from fine- to coarse-grained and pebbly, reddish-brown arkose to fine-to coarse-grained light- to dark-gray and bluish- gray calcareous graywacke; in places the light-gray sandstone is well sorted, crossbedded, quartzose, and slightly to moderately porous (less than 5 to 25 percent) and permeable (as much as 260.0 millidarcies). The siltstone and shale interbedded with the reddish-brown arkose are also red brown, in places mottled with grayish-green patches, commonly micromicaceous and slightly calcareous; siltstone, shale, and mudstone intercalated with the graywacke are light to dark gray, greenish and bluish gray, calcareous, fossiliferous, and phosphatic in places; six or more fish-bearing, gray to black, calcareous lacustrine shale beds (described by Baer and Martin 1949; Hentz, 1981, p. 20; 1985, p. 95-98, fig. 4; Olsen and others, 1982, p. 36) are present in the Waterfall Formation. According to Lindholm (1979, p. 1731), the conglomerate beds "are dominantly composed of clasts of fine-grained silicates and quartzite...." Greenstone metavoleanic and weathered basalt clasts are locally abundant, indicating that nearby Jurassic basalt flows were eroded and in- corporated into the Waterfall Formation. The con- glomerates are usually soft and deeply weathered, with clasts and matrix commonly altered to saprolite. Hentz (1981, p. 16, 17) has presented strong evidence indicating that many of the gray shales, siltstones, sandstones, and conglomerates are part of a complex lacustrine turbidite sequence. He also recognized at least four major angular unconformities in the Waterfall succession (Hentz, 1981, p. 13; 1985, p. 106). Fossils.-A small assemblage of probable freshwater biota from calcareous mudstones of the Waterfall Formation in the Middleburg 7.5-min Quadrangle is reported in an unpublished (1976) study of fossils by John Pojeta, Jr., and by Hentz (1981, p. 20; 1985, p. 95). At least two taxa of conchostracan arthropods are STRATIGRAPHY 29 present, one identified as belonging to the family Vertexiidae; ostracodes and a single phytosaur tooth were also recovered. At Thoroughfare Gap, con- chostracans, ostracodes, gastropods, and a variety of vertebrate remains were also found (Hentz, 1981, p. 20, and 1985, p. 95; Olsen and others, 1982, p. 36), including nonmarine fish fauna consisting of Semionotus elegans, Redfieldius cf. G. gracilis, Diplurus ef. D.longicaudatus, and Ptycholepis sp. (Schaeffer and others 1975; Schaeffer and McDonald, 1978). Dinosaur tracks were reported from recent road cuts of U.S. Route 66 about 2 km (1.25 mi) N. 80° W. in Prince William County, Va. (R.E. Weems and T.F. Hentz, pers. commun., 1979); Hentz (1981, p. 20; 1985, p. 95) documented two other sets of tracks, one identified as Hubrontes(?) by R.E. Weems (oral commun., 1981), on steeply dipping sandstones along Broad Run about 2 km (1.25 mi) south of Route 66. Abundant carbonized plant remains, including cycadophyte and lignite fragments, have been found in the area around Millbrook quarry. Cornet (1977) assigned the abundant palyniferous gray beds in his formation A, the Waterfall Formation, to the Corollina torosus palynofloral zone of Early Jurassic (Sinemurian to Pliensbachian) age. Thickness.-According to Lindholm (1979, p. 1782), "The maximum thickness of the Waterfall Formation (including the Millbrook Quarry Member) is calculated to be 1,500 m (5,000 ft) in the vicinity of Waterfall." Lee (1977, pl. 1b) calculated a thickness of 1,718.5 m (5,638 ft) in the Antioch area north of Thoroughfare Gap. Along Interstate Route 66 and elsewhere in the vicinity of Thoroughfare Gap, Hentz (1981, p. 13) measured and calculated a composite section of 1,150 m (8,773 ft). Relation to adjacent stratigraphic units.-Although the contacts are generally covered, the Waterfall Formation apparently overlies the Sander Basalt either conformably or in a paraconformity. The predominantly sandstone, siltstone, and calcareous shale of the Water- fall Formation is overlain by conglomerates of the Millbrook Quarry Member, locally disconformably and elsewhere in a gentle, locally obscure, angular unconformity apparently no more profound than other angular unconformities within the Waterfall Forma- tion. MILLBROOK QUARRY MEMBER The Millbrook Quarry Member of the Waterfall Formation is herein named for the uppermost con- glomerate and sandstone unit of the Waterfall Forma- tion exposed at and near the type locality, Millbrook quarry, south of Virginia Route 55, 1.2 km (0.7 mi) east of Thoroughfare Gap, in the Thoroughfare Gap 7.5-min Quadrangle, Prince William County. The Millbrook Quarry Member is a conglomerate unit that includes what Roberts (1928, p. 15, 16) called the arkose conglomerate of his Border Conglomerate, and is the uppermost part of Lee's (1977) informal basaltic-flow- bearing clastics member of the Bull Run Formation. Both Lindholm (1979, p. 1731) and Hentz (1981, p. 13) included strata assigned to the the Millbrook Quarry Member in the Waterfall Formation, while Lee (1980) included this unit in his informal Mountain Run member of the Bull Run Formation. Detailed mapping by Hentz in the Thoroughfare Gap area revealed a profoundly "unconformable relationship between the thick conglomerate on the western side of the field area and the underlying finer grained deposits to the east" (Hentz, 1981, p. 116); this conglomerate is the Millbrook Quarry Member, and the underlying deposits are the balance of the Waterfall Formation. Both matrix- supported and clast-supported pebble and cobble conglomerates are present, the latter type filling large channel forms (Hentz, 1981, p. 79). Lithology.-The conglomerate of this member con- tains abundant cobbles of weathered greenstone and lesser amounts of quartzite, gneiss, marble, limestone, basalt, and vein quartz in a clayey sand and silt matrix generally firmly cemented by calcite or silica. The clasts average 9 em (4 in) in diameter but are locally as large as 1 m (3.3 ft). The conglomerate is intercalated with lenses of medium -to coarse-grained reddish-brown arkosic sandstone and sandy micaceous dusky-red- brown siltstone. In places, as at Bull Run Mountain Estates and near the village of Waterfall, the unit is deeply weathered to saprolite. Distribution.-The Millbrook Quarry Member crops out in the foothills of Bull Run Mountain along the western margin of the central part of the Culpeper basin, with good exposures of the conglomerate at Millbrook quarry; along Broad Run and its western tributaries to the south of the quarry; in the vicinity of the village of Waterfall; in the stream valley 0.3 km (0.2 mi) southeast of Beulah Church in Fauquier County; and along road cuts west of Route 600 in Prince William County. These latter outcrops are deeply weathered to saprolite and are less than 50 m (164 ft) east of the border fault (Froelich and others, 1982, p. 71, 72). Thickness.-It is extremely difficult to estimate an accurate thickness because of the discontinuous nature of outcrops and ill-defined lenticular bedding; however, Hentz (1981, p. 144) calculates an approximate thickness of 450 m (1,476 ft) in the area south of Millbrook quarry. Relation to adjacent stratigraphic units.-Although outcrops are generally poor and the lower contact generally covered, the conglomerates of the Millbrook Quarry Member overlie the Waterfall beds in an apparent disconformity in most places; locally, as shown by detailed mapping along Broad Run, the 30 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. contact is a gentle angular unconformity. The commonly steep and characteristically erratic dip of the crudely bedded conglomerates of the Millbrook Quarry Member compound the problem of deciphering local relations. The upper contact is obscure, as erosion has long since removed any overlying units. Deposition.-The conglomerate deposits near Water- fall and seattered outcrops in the foothills of Bull Run Mountain suggest deposition as fluvial fan deposits; primary structures and clast lithologies indicate that the apex of the fan lay in the adjacent highlands to the west-northwest. GENERAL DISCUSSTON OF THE DEPOSITIONAL MODEL The great thickness of discrete conglomerate lenses that are largely restricted to narrow zones along the basin margins is strongly suggestive of alluvial fans. This marginal distribution is well explained in the model of equilibrium between fan sedimentation and basin area (fig. 19). The generally poor sorting, coarse- ness, and paucity of well-defined sedimentary sequences suggest episodic sedimentation from areas of high Mountain a ,,/,_I [YB f F—r,‘ till/l/ all Wt“ K “V 7, f; / / f @ % ? IU/ 14], / [fs ‘fl [K {if ( .' sa. s ;Q%x:’fixfifi pa 'A:f} I R pesto. satin (ULS - » 0 {os ® ode Net G - \\ Overbank flood/ relief, probably by debris flows, rock avalanches, and steep, shallow streams. The angularity of the clasts and the lack of weathering of boulders and cobbles despite labile mineralogies indicate either very rapid erosion and burial or arid conditions, or both. Bull (1964, p. 105) pointed out that some fans are segmented owing to repeated periods of uplift and erosion, and subsequent redeposition of gravels in lower segments at reduced gradients; commonly in intermontane basins, the lower fan segments coalesce with other fans, which may be the case along the eastern margin of the Culpeper basin. Conglomerates commonly intertongue with sandstone, a condition that might result from perturba- tions of Hooke's (1968) steady-state equilibrium model by fan segmentation or by changes of average discharge (Bull, 1964, p. 101-106, 128; 1972). Arkosic sandstone and lithic graywacke are in- terpreted primarily as fluvial deposits because of unidirectional crossbedding, upward-fining sequences from pebbly sands to silts, and the presence of channel- like scours. Many of the grayish-red arkosic sandstones were probably deposited by small streams, because the basal scours are shallow, sandstones are relatively thin Subsidence - Channel floor (gravel) bplain (silt) Point bar (sand) :; +/ Fanhead Midfan Fanbase Debris-flow Sieve Debris-flow Stream-flow Stream-flood and Lacustrine deposits deposits levee deposits channel deposits channel deposits deposits FIGURE 19.-Diagrammatic sketch showing stratigraphic relations of an alluvial fan to lacustrine deposits in a closed basin, the facies distribution, and the concave-upward radical fan profile (modified after Spearing, 1974, fig. 1; Bull, 1964, p. 105, 106; Denny, 1967; Hooke, 1967, 1968; Selley, 1978; and Blatt and others, 1980). DIABASE 31 and medium- to fine-grained, and crossbeds are small in scale. Associated sandy micaceous siltstones con- taining abundant tubes (interpreted as root casts and burrows), mud cracks, dinosaur tracks, and carbonate nodules of reworked caliche are interpreted as sub- aerial, overbank, and flood-plain deposits, in places modified by soil-forming processes. Dusky-red mudstone and silty mudstone containing abundant mud cracks, dinosaur footprints, and carbon- ate pellets are interpreted as the deposits of subaerially exposed playa mudflats. Light-gray and ripple- laminated greenish-gray silty mudstone, sparsely fossiliferous gray calcareous shale, and carbonaceous shale are interpreted as probable shallow lake deposits, while dark-gray, silty, calcareous, varved, or laminated, phosphatic fish-bearing carbonaceous shales are in- terpreted as relatively deep lakebeds. The cyclic alternations of these fine-grained lithologies are believed to reflect expansions and contractions of perennial lakes, primarily in response to climatic fluctuations. The presence of graded beds from coarse to fine, flute casts and sole marks, slumped and convolute bedding, and lenticular pebble conglomerates within laminated dark-gray shale sequences has been interpretated as evidence of turbidites in the Jurassic strata near the western basin margin (Hentz, 1982, 1985, p. 102). Evidence of climatic cyelicity in the deposits of the Late Triassic to Early Jurassic basins of eastern North America has been repeatedly documented (Reinemund, 1955; Hubert and others, 1976; Cornet, 1977; Hubert, 1977; Van Houten, 1977a 1977b; Wheeler and Textoris, 1978; Olsen and others, 1982; Olsen, 1984). Hubert (1977) convincingly documented the origin of carbonate nodules in the New Haven Arkose of the Connecticut Valley as caliche, produced under semiarid conditions, and the abundant carbonate nodules in the Culpeper basin were probably formed under similar conditions. Conversely, the presence of rich and diverse palyno- floral remains in the gray silty mudstones and carbon- aceous shales intercalated in the red bed sequences of the Culpeper basin are believed to reflect intermittent humid conditions. The cumulative depositional thicknesses, the inter- tonguing relationships of extremely diverse lithologies, the presence of turbidites in lake strata, the distribution of conglomerates, and the presence of local uncon- formities all support the interpretation of an episod- ically subsiding, intermontane, fault-bounded con- tinental basin flanked by periodically uplifted mountains. The cyclicity of some of the sedimentary packages, the diversity of flora and fauna, and the primary structures and mineralogy of the con- glomeratic and red bed strata all indicate a fluctuating but prevailing semiarid climate punctuated by episodic humid conditions in the Late Triassic and Early Jurassic of the Culpeper and Barboursville basins. In the Early Jurassic, the Culpeper basin erosional and depositional enivironment was punctuated by extensive fissure eruptions and subaerial basalt flows fed by deep-seated intrusions. DIABASE DISTRIBUTION AND MODE OF OCCURRENCE Diabase is confined to the Culpeper basin and apparently intruded the Triassic-Jurassic sedimentary rocks shortly before or concurrent with westerly tilting of the basin during the latest stages of sedimentation. Outerops of diabase extend from Boyds, Montgomery County, Md., to south of the Robinson River, Madison County, Va. Diabase occurs chiefly as stocks, sills, saucer-shaped sheets, and dikes in the northern basin, and as sills, sheets, and dikes in the south. Small dikes with north, northwest, and northeast trends are scattered throughout the basin and locally cut through both Mount Zion Church and Hickory Grove Basalt flows in Prince William County, Va. (pl. 1C), although they may also be exposed feeders to some of the flows. DESCRIPTION OF ROCK The diabase is medium and medium-dark gray, chiefly equigranular and locally coarse to very coarse crystalline, but aphanitic at chilled intrusive margins. It consists of dark-grayish-green to black discrete crystals of pyroxene, mostly augite with lesser amounts of pigeonite, which, with seattered granules or aggregates of magnetite and ilmenite, fill the interstices between light-gray plagioclase laths, chiefly labradorite (figs. 20, 21). The more quickly cooled dikes and other small intrusions and the borders of large bodies are generally darker in color, finer grained, and more dense than diabase in the interior of the large bodies. Some narrow dikes and small pluglike features in the central part of the basin are olivine-bearing. The diabase locally shows a textural change from normal diabase to granophyre associated with syenite, fer- rogabbro, and a pegmatitic facies of the diabase (fig. 22). Generally, the diabasic pegmatite facies evolved because of slower cooling and an increase in volatiles within the central portion of the magma, and occurs as irregular bodies, bands, and lenses in the diabase masses. The pegmatite is composed of light-gray to pinkish-gray plagioclase feldspar, minor potassium feldspar, and sparse grains of quartz occurring in the interstices of bladelike pyroxene associated with minute 32 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. 0 1 2 METERS FIGURE 20.-Diabase at Mount Pony, Culpeper County, Va. Equi- granular and medium-grained orthopyroxene-bearing diabase shows well-developed, widely spaced joints and subhorizontal sheets. crystals of ilmenite and magnetite. The sheets at Boyds, Md., and north of Rapidan, Va., also carry abundant orthopyroxene phenocrysts, mainly hypersthene and bronzite, concentrated in thick layers or zones of gabbro and norite. During late stages of magmatic differentiation, and partly as a result of assimilation of wall rock and contamination, the composition of the magma had changed sufficiently to permit the crystallization of granophyre (fig. 23). The granophyre is pale pink to pink, medium to coarse grained, and in part porphyritic. It consists of sodic plagioclase, potassium feldspar, and sparse micropegmatitic quartz, associated with discrete crystals of hornblende and clinopyroxene, and minor biotite, actinolite, magnetite, ilmenite, chlorite, and apatite. A large body of granophyre covers about 3 km? (1 mi?) on the south side of Mountain Run in Culpeper County, Va., and smaller differentiated bodies of granophyre, syenite, ferrogabbro, and pegmatite extend in a linear belt of podlike segregations almost to the town of Nokesville. GEOCHEMISTRY Chemically, most of the Early Jurassic diabase sheets of this basin are high-TiO;, quartz-normative tholeiites, similar to the York Haven diabase of Pennsylvania (Smith and others, 1975). Some of the dikes and the sheets at Gainesville, Nokesville, and Catlett, however, are low-TiO;, quartz-normative FIGURE 21.-Photomicrograph of the Mount Pony diabase, Culpeper County, Va. Augite (au) and labradorite (1a) in subophitic inter- growth with scattered sericitized plagioclase (pl), magnetite, and ilmenite. The field of view is 2.2 mm (0.08 in) across (plane- polarized light). tholeiites chemically similar to the Rossville diabase of Pennsylvania (Smith and others, 1975). Some thin dikes and small pluglike intrusives are olivine-bearing and rich in MgO. The quartz-normative basaltic magma that supplied the principal sheets and dikes is probably genetically related to that which was extruded in the west-central portion of the basin as surface flows during the Early Jurassic. The orthopyroxene-rich sheets at Boyds, Md., and Rapidan, Va., are considerably higher in MgO than the sheets and sills in the central part of the basin. THERMALLY METAMORPHOSED ROCKS The contacts of diabase with Triassic and Jurassic country rocks are generally sharp, but in some areas of wholesale assimilation and magmatic contamination around large sills and stocks, contacts are transitional across tens of meters; nevertheless, thermal meta- morphism of country rocks is extensive throughout the basin, and the thermal aureole surrounding the larger bodies is on the order of one-fourth to one-third the thickness of the intrusive. Metamorphic rock types are gray to dark-gray, medium-bluish-gray, and olive- black hornfels and light-gray granulite (granofels), metaconglomerates, and quartzite. These rocks were chiefly derived from feldspathic, micaceous, argil- THERMALLY METAMORPHOSED ROCKS FIGURE 22.-Photomicrograph of the pegmatitic phase of a diabase exposed in the Luck quarry, Loudoun County, Va. Elongated labradorite (1a) and augite (au) form an intergrowth with verm- icular quartz (q) grains. The field of view is 2.2 mm (0.08 in) across (plane-polarized light). laceous, arenaceous, ferruginous, and (or) calcareous sandstone and siltstone and minor conglomerate and shale of the Culpeper Group. Light-gray to gray marble was derived from contact metamorphism of limestone conglomerate of the Leesburg Member, and the Mount Zion Church, Hickory Grove, and Sander Basalts were slightly thermally altered by diabase dikes, some of which may be part of a near-surface feeder system. Hornfels is the dominant type of metamorphosed argillaceous rock (fig. 24, this paper; Lee and Froelich, 1985). In contact aureoles, an inner zone, generally characterized locally by cordierite, biotite, quartz, and plagioclase, is succeeded by a middle zone of cordierite, andalusite, plagioclase, and quartz, which is followed by an outer zone of chlorite, epidote, and quartz (Lee, 1982). Granulite and quartzite form fused lenses, bands, and irregular masses (fig. 25). The inner zone in these rocks is characterized either by a zone of decussate biotite, plagioclase, and fused quartz, or by a zone of hornblende, sodic plagioclase, titanite, and myrmekite- quartz. This zone is succeeded outward either by a zone of cordierite, andalusite, plagioclase, and fused quartz, or by a zone of fine black tourmaline, plagioclase, and quartz, which is generally followed by an outer zone of chlorite, epidote, and locally spotted aggregates of recrystallized feldspar (Lee, 1982). Marble derived from metamorphosed limestone conglomerate consists of calcite, lime-garnet, diopside, and serpentine, associated with minor amounts of vesuvianite, mag- netite, fluorite, and wollastonite. FIGURE 23.-Photomicrograph of the granophyric phase of diabase along the south side of Mountain Run, 2 km (1.25 mi) east of The Ridge, Germanna Bridge 7.5-min Quadrangle, Culpeper County, Va. Holocrystalline intergrowth of hornblende (h) and turbid potash feldspar (f) with albite (al) laths, minor quartz grains, and scattered ilmenite (i) and magnetite. The field of view is 2.2 mm (0.08 in) across (plane-polarized light). FIGURE 24.-Photomicrograph of a cordierite-hornfels from the Chantilly Crushed Stone quarry, in the eastern part of the Arcola 7.5-min Quadrangle, Loudoun County, Va. It is probably a meta- morphosed argillaceous, calcareous, and ferruginous siltstone of the Balls Bluff Siltstone. Cordierite (co) shows a well-developed hexagonal outline, cut by a sericite-quartz veinlet (v). Cordierite crystals are mostly altered to sericite, chlorite, biotite, and an isotropic substance (shimmer aggregate) with inclusions of quartz, magnetite, specularite, and ilmenite. The crystals are also partially rimmed by opaque iron and titanium oxides. The field of view is 2.2 mm (0.08 in) across (plane-polarized light). 34 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. FIGURE 25.-Photomicrograph of a granulite (granofels) from the east bank of Little Rocky Run, Fairfax County, Va. This is probably a metamorphosed feldspathic, ferruginous sandstone of the Poolesville Member of the Manassas Sandstone. Recrystallized mosaic of quartz (q) and turbid potash feldspar (f) contains euhedral to anhedral tourmaline (t). The field of view is 2.2 mm (0.08 in) across (plane-polarized light). SUMMARY The Triassic-Jurassic Culpeper and Barboursville basins were initiated and evolved during the early Mesozoic period of continental fragmentation and rifting that preceded continental drifting in the development of the modern Atlantic continental margin. Detrital sedimentary rocks of the Culpeper Group were derived by erosion from adjacent steep highlands and were deposited as alluvial fans along the base of adjacent highlands by streams and debris flows that in some cases bordered playa lakes. This sedimentation was controlled by extensional tectonics and fluctuating, generally semiarid conditions. The composition of the red beds of the Culpeper Group suggests that relief in the source areas was great and that rapid erosion and deposition resulted. The red color of the detritus is most likely allogenic, pre- diagenetic, and in places diagenetic. It is estimated that red beds make up 90 percent of the Manassas Sandstone and Balls Bluff Siltstone and at least 30 percent of the overlying formations. A dry climate of long duration is indicated by the development of extensive paleosol caliche, abundant desiccation cracks, bioturbation, and the searcity of bone remains. A wet climate cycle is marked by plant remains and the cyclic accumulation of laminated fossiliferous shales in lakebeds, but semiaridity probably was dominant, as even the dark-gray lake sediments show mud cracks and fossil soils. At the beginning of basin fill, the adjustments of the Earth's crust caused by intrusion at depth (Ballard and Uchupi, 1975) accelerated the rate of uplift of the adjacent highlands, accompanied by initiation of uplift along border faults. In early Manassas time, the rate of deposition was greater than the rate of basin subsidence and coarse fans and debris flows accumulated along the base of fault-block highlands. Fragments of Piedmont schist, gneiss, and quartz of the Reston Member ac- cumulated in the east-central part of the basin, and at about the same time deposition of the limestone detritus of the Tuscarora Creek Member in the northeast and greenstone fragments of the Rapidan Member in the southeast accumulated. Fluvial sands of the Poolesville Member gradually accumulated as source areas were worn down. Toward the middle of deposition of the Balls Bluff Siltstone, the movement along border faults reached a steady-state relation with deposition of fine- grained fluvial and lacustrine clastic sediments in the basin. During late Balls Bluff time, uplift of the western highlands was renewed by movement at the west border normal fault, resulting in deposition of the limestone clasts of the Leesburg Member in the northwestern Culpeper basin. This episode of coarse clastic deposition continued intermittently throughout Catharpin Creek time, with large aprons of Goose Creek gravels carried far out into the basin in latest Triassic and earliest Jurassic time. Tectonism along the western margin in Early Jurassic time was apparently more intense and somewhat more extensive than that along the eastern margin in the Late Triassic. The different style and the magnitude of the Early Jurassic episode is indicated by the widespread and repeated eruption of basalt flows, by the presence of turbidites, by local unconformities, and also by the size of the preserved Early Jurassic fans, which are apparently larger in the western portion of the basins than fans of the Late Triassic in the east and southeast. Perhaps relief in the western source area was greater than that in the Piedmont on the east, and it is possible that the western border faults of the basin migrated progressively westward with time, in part accounting for the asymmetry of lithofacies preserved in the present basins. Basin subsidence was accompanied by widespread episodic outpouring of basalt flows in the Early Jurassic, punctuated by long periods of fluvial and lacustrine deposition. Mesozoic deposition es- sentially ceased after regional basin tilting adjacent to the western border fault commenced. Toward the end of basin filling, large-scale westward monoclinal tilting of the rocks in the basin occurred, with the Jurassic rocks in the western part of the basin tilted steeply toward the border faults, accompanied by extensive intrusion of tholeiitic diabase sills, stocks, and dikes. A REFERENCES CITED 35 long period of uplift and erosion followed, causing deep denudation and exposure of the interior of both the Culpeper and the Barboursville basins. REFERENCES CITED Applegate, S.P., 1956, Distribution of Triassic fish in the Piedmont of Virginia [abs.]: Geological Society of America Bulletin, v. 67, no. 12, p. 1749. Baer, F. M., and Martin, W. H., 1949, some new finds of fossil ganoids in the Virginia Triassic: Science, v. 110, no. 2869. p. 684-686. Bain, G. L., 1959, The geology of the intrusives and associated country rocks of the Nokesville 7%4' Quadrangle: M.S. thesis, West Virginia University (Morgantown), 50 p. Ballard, R.D., and Uchupi, Elazar, 1975, Triassic rift structure in Gulf of Maine: American Association of Petroleum Geologists Bulletin, v. 59. no. 7, p. 1041-1072. 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Leavy, B.D., Grosz, A.E., and Johnson, S.S., 1982, Total count aeroradioactivity map of the Culpeper basin and vicinity, Virginia: Virginia Division of Mineral Resources Publication 40, scale 1:125,000. Leavy, B.D., and Puffer, J.H., 1983, Physical and chemical characteristics of four Jurassic basalt units in the Culpeper basin, Virginia: Geological Society of America, Southeastern Section, Abstracts with Programs, p. 92. Lee, K.Y., 1977, Triassic stratigraphy in the northern part of the Culpeper basin, Virginia and Maryland: U.S. Geological Survey Bulletin 1422-C, 17 p. 1978, Geologic map of the Arcola Quadrangle, Loudoun and Fairfax Counties, Virginia: U.S. Geological Survey Miscel- laneous Field Studies Map MF-973, scale 1:24,000. 1979, Triassic-Jurassic geology of the northern part of the Culpeper basin, Virginia and Maryland: U.S. Geological Survey Open-File Report 79-1557, 19 p., 16 oversize sheets, scale 1:24,000. 1980, Triassic-Jurassic geology of the southern part of the Culpeper basin and the Barboursville basin, Virginia: U.S. Geological Survey Open-File Report 80-468, 19 p., 18 oversize sheets, scale 1:24,000. 1982, Thermal metamorphism of Triassic and Jurassic sedimentary rocks in the Culpeper basin, Virginia: Geological Society of America Abstracts with Programs, p. 34. Lee, K.Y., and Froelich, A.J., 1985, Geochemical data for Triassic sedimentary and thermally metamorphosed rocks of the northern Culpeper basin, Virginia: U.S. Geological Survey Open-File Report 85-217, 19 p. Lee, K. Y., Leavy, B.D., and Gottfried, David, 1984, Geochemical data for Jurasasic diabase and basalt of the northern Culpeper basin, Virginia: U.S. Geological Survey Open-File Report 84-771, 20 p. Lindholm, R.C., 1977, Geology of Jurassic-Triassic Culpeper basin, north of Rappahannock River, Virginia [abs.]; American Association of Petroleum Geologists Bulletin, Association Round Table, v. 61, no. 5, p. 809. 1978, Tectonic control of sedimentation in Triassic-Jurassic Culpeper basin, Virginia [abs.]; American Association of Petroleum Geologists Bulletin, Association Round Table, v. 62, no. 3, p. 537. 1979, Geologic history and stratigraphy of the Triassic- Jurassic Culpeper basin, Virginia: Geological Society of America Bulletin, pt. 2, v. 90, p. 1702-1736. Lindholm, R.C., Hazlett, J.M., and Fagin, S.W., 1979, Petrology of Triassic-Jurassic conglomerates in the Culpeper basin, Virginia: Journal of Sedimentary Petrology, v. 49, no. 4, p. 1245-1262. Lindskold, J.E. 1961, Geology and petrography of the Gainesville Quadrangle, Virginia: M.S. thesis, George Washington Uni- versity (Washington, D.C.), 50 p. McCollum, M.B., 1971, Basalt flows in the Triassic Culpeper basin, Virginia: Geological Society of America Bulletin, v. 82, p. 2331-2332. McKee, E.D., Oriel, S.S., Ketner, K.B., MacLachlan, M.E., Goldsmith, J.W., MacLachlan, J.C., and Mudge, M.R., 1959, Paleotectonic maps, Triassic system: U.S. Geological Survey Miscellaneous Geologic Investigations Series Map 1-300 [1960]. Nutter, L.J., 1975, Hydrogeology of the Triassic rocks of Maryland: Maryland Geological Survey Report of Investigations no. 26, 37 p. Olsen, P.E., 1978, On the use of the term Newark for Triassic and Early Jurassic rocks of Eastern North America: Newsletter of Stratigraphy, v. 7, no. 2, p. 90-95. 1984, Comparative paleolimnology of the Newark Super- group: A study of ecosystem evolution: Ph.D. dissertation, Yale University (New Haven, Conn.), 726 p. Olsen, P.E., McCune, A. R., and Thompson, K.S., 1982, Correlation of the Early Mesozoic Newark Supergroup by vertebrates, prin- cipally fishes: American Journal of Science, v. 282, p. 1-44. Otton, E.G., 1981, The availability of ground water in western Montgomery County, Maryland: Maryland Geological Survey Report of Investigations no. 34, 76 p. Posner, Alex, and Zenone, Chester, 1983, Chemical quality of ground water in the Culpeper basin, Virginia and Maryland: U.S. Geological Survey Miscellaneous Investigations Series Map I- 1313-D, scale 1:125,000. Puffer, J.H., and Di Placido, A.M., Hurtubise, Donlon, and Leavy, B.D., 1981, Chemical composition of the igneous flow units of the Culpeper basin, Virginia: Geological Society of America Abstracts with Programs, v. 13, no. 1, p. 33. Puffer, J.H., and Hurtubise, D.0., 1983, Eastern North American Jurassic basalts: An interbasin petrologic model: Geological Society of America Abstracts with Programs, v. 15, no. 6, p. 665. Raymond, C. A., Elwood, B.B., Chaves, Lisa, and Leavy, B.D., 1982, Paleomagnetic analyses of lower Mesozoic diabase and basalt from the central and southern Appalachians: Geological Society of America Abstracts with Programs, v. 14, nos. 1 and 2, p. 76. Redfield, W.C., 1856, On the relations of the fossil fishes of the sandstone of Connecticut and other Atlantic States to the Liassic and Ookolitic periods: American Journal of Science, v. 22, ser. 2, p. 357-363. Reinemund, J. A., 1955, Geology of the Deep River coal field, North Carolina: U.S. Geological Survey Professional Paper 246, 159 p. Roberts, J.K., 1922, The Triassic of northern Virginia: Ph.D. dissertation, Johns Hopkins University (Baltimore, Md.), 272 p. 1923, Triassic basins of northern Virginia; Pan American Geologist, v. 39,, no. 3, p. 185-200. 1928 The geology of the Virginia Triassic: Virginia Geological Survey Bulletin 29, 205 p. Rodgers, John, 1970, The tectonics of the Appalachians: New York, Wiley-Interscience, 271 p. Rogers, W. B., 1854, Geological relations of the New Red Sandstone of the Middle States and Connecticut Valley to the coal-bearing rocks of eastern Virginia and North Carolina: Boston Society of Natural History Proceedings 5, p. 14-18. Schaeffer, Bobb, Dunke, D.H., and McDonald, N.G., 1975, Ptycholepis marshi Newberry, A chondrostean fish from the Newark Group of Eastern North America: Fieldiana Geology, v. 33, no. 12, p. 205-233. Schaeffer, Bobb, and McDonald, N.G., 1978, Redfieldiid fishes from the Triassic-Jurassic Supergroup of Eastern North America: American Museum of Natural History Bulletin, v. 159, article 4, p. 129-174. Selley, R.C., 1978, Ancient sedimentary environments [2d ed.]: Ithaca, N.Y., Cornell University Press, 287 p. Shannon, E.V., 1926, Mineralogy and petrography of Triassic limestone conglomerate metamorphosed by intrusive diabase at Leesburg, Virginia: U.S. National Museum Proceedings, v. 66, no. 2565, p. 1-31. Smith, R.C., III, Rose, A.W., and Lanning, R.M., 1975, Geology and geochemistry of Triassic diabase in Pennsylvania: Geological Society of America Bulletin, v. 86, p. 943-975. REFERENCES CITED 37 Sobhan, A.N., 1985, Petrology and depositional history of Triassic red beds, Bull Run Formation, eastern Culpeper basin, Virginia: M.S. thesis, George Wasington University (Washington, D.C.), 224 p. Spearing, D. R., 1974, Alluvial fan deposits, in Spearing, D.R., comp., Summary sheets of sedimentary deposits: Boulder, Colo., Geological Society of America, sheet 1 of 7, MC-8. Stose, G. W., and Bascom, Florence, 1929, Fairfield Gettysburg, Pa.: U.S. Geological Survey Geologic Atlas of the United States, Folio 225, 23 p. Sutter, J.F., and Arth, J.G. 1983, A Ap/ S Ar age spectrum dating and strontium isotope geochemistry of diabase sills from the Culpeper basin, Virginia: Geological Society of America Abstracts with Programs, v. 15, no. 2, p. 92. Toewe, E.C., 1966, Geology of the Leesburg Quadrangle, Virginia: Virginia Division of Mineral Resources Report of Investigation 11, 52 p. Van Houten, F.B., 19772, Cyclic lacustrine sedimentation, Upper Triassic Lockatong Formation, central New Jersey and adjacent Pennsylvania, in Van Houten, F.B., ed., Ancient continental deposits, Benchmark Papers in Geology, v. 43: Stroudsburg, Pa., Dowden, Hutchinson and Ross, Inc., p. 144-157. 1977b, Triassic-Liassic deposits of Morocco and Eastern North America: Comparison: American Association of Petrol- eum Geologists Bulletin, v. 61, no. 1, p. 79-99. Weems, R.E., 1979, A large parasuchian (Phytosaur) from the Upper Triassic portion of the Culpeper basin of Virginia: Proceedings of the Biological Society of Washington, v. 92, no. 4, p. 682-688. Wheeler, W.H., and Textoris, D.A., 1978, Triassic limestone and chert of playa origin in North Carolina: Journal of Sedimentary Petrology, v. 48, no. 3, p. 765-776. Wise, M.A., and Johnson, S.S., 1980, Simple Bouguer gravity anomaly map of the Culpeper basin and vicinity, Virginia: Virginia Division of Mineral Resources Publication 24, scale 1:125,000. Zenone, Chester, and Laczniak, R.J., 1984, Ground-water resources of the Culpeper basin, Virginia and Maryland: Simulation of the ground-water system: U.S. Geological Survey Miscellaneous Investigations Map I-1313-F, seale 1:125,000. 38 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. APPENDIX A: MEASURED SECTIONS SECTION 1.-RAPIDAN RIVER, VIRGINIA [Type section of the Rapidan Member of the Manassas Sandstone. Measured by K.Y. Lee using hand level and tape, 1976, along the south side of the Rapidan River in the southern part of the Culpeper East 7.5-min Quadrangle; about 720 m (2,362 ft) due north of a spot (elevation 339 {t), on Virginia State Road 681, in the northeast Unionville 7.5-min Quadrangle, Orange County, Va.] Thickness Meters Feet Upper Triassic and Lower Jurassic: Culpeper Group (in part): Upper Triassic: Manassas Sandstone (in part): Rapidan Member: 2. Conglomerate, grayish green' (5G 5/2), dusky-green (5G 3/2), and grayish- olive-green (5GY 3/2), subangular to angular greenstone pebbles and cobbles averaging 100 mm (4 in) in diameter; scattered gray quartzite and vein quartz fragments within a matrix of greenstone granules and grayish-red sand and silt; firmly cemented by clay and silica. Contains grayish-red and dark-red, fine- to coarse-grained, feldspathic and micaceous sandstone lenses ...... 33 108 1. Conglomerate, dusky-green (5G 3/2) and grayish-olive-green (5GY 3/2), sub- angular to angular greenstone pebbles and cobbles averaging 128 mm (5 in) in diameter, as large as 300 mm (12 in); scattered gray quartzite and vein quartz fragments in matrix of green- stone granules, dusky-red sand and silt; cemented firmly by clay and §HICB . :. 2252s. .in on vine evie 43 141 Total thickness of Rapidan Member .... 76 249 Angular unconformity. Pre-Triassic gray to grayish-black mica-tale-shist (not measured). SECTION 2.-TUSCARORA CREEK, MARYLAND [Type section of the Tuscarora Creek Member of the Manassas Sandstone. Measured by K.Y. Lee using hand level and tape, 1976, along Maryland State Road 28; about 800 m (2,624 ft) southeast of the bridge of Maryland State Road 28 over Tuscarora Creek in the Buckeystown 7.5-min Quadrangle, Frederick County, Md.] Thickness Meters Feet Upper Triassic and Lower Jurassic: Culpeper Group (in part): Upper Triassic: Manassas Sandstone (in part): Tuscarora Creek Member (incomplete): 2. Conglomerate, light-gray (N7) to medium-gray (N5), grayish-red (5R 4/2) subangular limestone cobbles and pebbles averaging 26 mm (1 in) in 1Color description are based on the "Rock-Color Chart" of the Geological Society of America (Goodard and others, 1948). Thickness Meters Feet diameter; limestone fragments are lithographic to medium-grained; scat- tered vein quartz, quartzite, and chert fragments in matrix of limestone granules and dusky-red silt; firmly cemented by calcite. Grades into or intertongues with dusky-red and grayish-red feldspathic sand- BEONC .. 2. eis. i lvoe unl ane aver sda iee 4 13 1. - Conglomerate, medium-gray (N5), light- gray (N7), subangular to rounded limestone pebbles and cobbles, averaging 28 mm (1 in) in diameter, as large as 200 mm (8 in); limestone fragments are lithographic to medium- grained; some clasts are vein quartz, quartzite, and chert within matrix of limestone granules and dusky-red clayey silt; firmly cemented by CBlGILe !. .33.. ..on revs .. orr n el oe is ate, bo -a Total thickness of Tuscarora Creek Member onset (3 20 Angular unconformity. Upper Cambrian gray to blackish-gray lithographic to fine-grained Frederick Limestone (not measured). SECTION 3.-CULPEPER, VIRGINIA [Type section of the Mountain Run Member of the Tibbstown Formation. Measured by K.Y. Lee using hand level and tape, 1976, about 640 m (2,099 ft) southeast of the overpass bridge of Virginia State Route 3 over the Southern Railroad in the northwestern part of the Culpeper East 7.5-min Quadrangle, Culpeper County, Va.] Thickness Meters Feet Upper Triassic and Lower Jurassic: Culpeper Group (in part): Upper Triassic: Tibbstown Formation (in part): Mountain Run Member (incomplete): 4. Conglomerate, dusky-yellow-green (5GY 5/2), grayish-olive-green (5GY 3/2), and grayish-green (10G 4/2), sub- angular to angular greenstone pebbles and cobbles averaging 128 mm (5 in) in diameter, as large as 600 mm (24 in); scattered vein quartz, gray quartzite, and schist fragments in matrix of greenstone granules, dusky- red and dark-red sand and silt; firmly cemented by clay and silica. Commonly intercalated with dark-red, felds- pathic, micaceous, clayey sandstone and siltstone. Scattered epidotization of 6 20 3. Conglomerate and sandstone, inter- lensed; greenstone conglomerate, same as unit 4. Sandstone, very dark red (5R 2/6), grayish-red (10R 4/2); very fine to very coarse grained, and in part, conglomeratic; feldspathic, clayey, micaceous, and slightly cal- careous; thick-bedded to massive; clayey, silty matrix; firmly cemented by clay, silica and locally minor CRIGEEC ... .}. ...,) 2. 0 nl neer rine ine 2, / COYGPOQ ...... ...sa. %.. olo eels nln t eries 1. Conglomerate, grayish-olive- green (5GY 3/2), grayish-green (10G 4/2), angular to subangular greenstone pebbles and cobbles averaging 210 mm (8.5 in) in diameter, as large as 900 mm (86 in); scattered vein quartz, quartzite, and schist fragments in matrix of green- stone granules, grayish-olive-green and grayish-red sand and silt; firmly cemented by clay and silica.... 'Tota1 thickness of Mountain Run MERADET -...... 0000000 APPENDIX A Thickness Meters Feet 7T 26 52 171 151 495 217 712 SECTION 4.-HICKORY GROVE, VIRGINIA [Section of the Mount Zion Church Basalt (4A), Midland Formation (4B), Hickory Grove Basalt (4C), Turkey Run Formation (4D), and Sander Basalt (4E). Measured by K. Y. Lee using hand level and tape, 1975; about 576 m (1,889 ft) east of the intersection of Virginia State Route 701 and U.S. Route 15 at Hickory Grove, Va.; and from the base of the Mount Zion Church Basalt in the southwest Arcola 7.5-min Quadrangle and southeast Middleburg 7.5-min Quadrangle, Prince William County, Va.] Upper Triassic and Lower Jurassic: Culpeper Group (in part): Lower Jurassic: 4A. Mount Zion Church Basalt (incomplete): 1. Basalt, medium-dark-gray (N4); weath- ered light brown (5YR 5/6) to dark- yellowish-brown (10YR 4/2); fine- to medium-crystalline; chiefly equi- granular and holocrystalline; porphy- ritic in part; plagioclase, chiefly labra- dorite, and augite show ophitic texture. Scattered aggregates of magnetite and ilmenite. Total thickness of Mount Zion Church Basalt (partial) }.................._._______._.___. Covered. Upper Triassic and Lower Jurassic: Culpeper Group (in part): Lower Jurassic: 4B. Midland Formation: (complete): 1. Sandstone, dusky-red (5R 3/4) tograyish- red (5R 4/2); very fine to medium- grained; feldspathic, micaceous, and clayey; thin-bedded to very thick bedded and planar-laminated. Inter- calated with layers of medium-dark- gray (N4) and dark-gray (N3) clayey siltstone and silty shale in the lower part. Locally intercalated with dark- gray, fissile, silty shale, such as at the Thickness Meters Feet 9 30 intersection of Virginia State Route 701 and -U.S: Route 15 :-..:....;........... Covered (This interval contains fish- bearing shale as at Licking Run) ...... Total thickness of Midland Formation (@OMPI@H@) ln. Upper Triassic and Lower Jurassic: Culpeper Group (in part): Lower Jurassic: 4C. Hickory Grove Basalt (complete): 1. - Basalt, medium-dark-gray (N4); weather- ed light-brown (5YR 5/6); very fine to very coarse crystalline; equigranular and holocrystalline; euhedral to sub- hedral crystals of plagioclase, chiefly labradorite, embedded in an augite groundmass; scattered grains of magnetite and ilmenite. Zeolite-filled vesicles present mainly in the upper part. Total thickness of Hickory Grove Basalt ((Coal) o o ae Upper Triassic and Lower Jurassic: Culpeper Group (in part): Lower Jurassic: 4D. Turkey Run Formation (complete): 1. Siltstone, dark-reddish-brown (10R 3/4), blackish-red (5R 2/2), and grayish- red (5R 4/2); weathered light-brown (5YR 6/4); very fine to very coarse grained; feldspathic, clayey, and micaceous; very thin to very thick bedded and planar-laminated. Inter- calated with layers of sandstone and silty shale; feldspar epidotized near the contact with basalt. Total thickness of Turkey Run Formation fCOMIDIGLG).......... ... ...Ace i nese Upper Triassic and Lower Jurassic Culpeper Group (in part): Lower Jurassic: 4E. Sander Basalt (incomplete)(Units: 1, 3, 5, 7-basalts; units 2, 4, 6-intercalated sandstone and siltstone lentils): 7. Basalt, medium-dark-gray (N4), dark- greenish-gray (5GY 4/1), and medium- bluish-gray (5B 5/1); weathered pale brown (5¥R 5/2), dark-yellowish- brown (10 YR 4/2), and light-brown (5YR 5/6); aphanitic to medium ery- stalline; equigranular and in part por- phyritic; plagioclase, chiefly labra- dorite, laths intergrown with augite; scattered grains of magnetite and ilmenite in a holocrystalline lcci COVereQ |... ela ne ec lean dead 39 Thickness Meters Feet 93 305 288 945 381 1,250 212 695 218 715 112 367 55 181 40 6. Sandstone, greenish-gray (5GY 6/1) to olive-gray (5Y 4/1), and moderate- yellowish-brown (10YR 5/4); very fine to medium-grained; feldspathic and micaceous; thin- to thick-bedded and planar laminated. Contains subor- dinate amounts of clayey siltstone and gilty Shale -Art... 75 5. Basalt, sameas unit 7, except porphyritic texture common; columnar jointing well developed 76 4. Sandstone, very dark red (5R 2/6) to grayish-red (10R 4/2); very fine to coarse-grained; feldspathic, micaceous, silty, and clayey; very thin bedded to very thick bedded, in part massive. Intercalated with subordinate amounts of siltstone and silty SHAC . 2.22. 002.0701 .t e ohare deedee rons 55 3. Basalt, same as unit 7, except mostly medium-crystalline; contains scattered zeolite-filled vesicles. Porphyritic texture common ..................l......... 55 2. Siltstone, grayish-red (5R 4/2), very dusky red (10R 2/2) to medium- dark- gray (N4); very fine to very coarse grained; clayey, feldspathic, mi- caceous, and calcareous; bedding very thin to massive. Contains beds of silty shale and fine-grained sandstone. Epidotization of feldspar common .... _ 20 66 1. Basalt, medium-dark-gray (N4), dark- greenish-gray 5GY 4/1), and medium- bluish-gray (5B 5/1); weathered dark- yellowish-brown (10Y R 4/2) and light brown (5Y R 5/6); aphanitic to coarse- crystalline, in part pegmatitic in texture; equigranular to porphyritic; plagioclase, chiefly labradorite, laths intergrown with augite; scattered magnetite and ilmenite in holocrystal- line groundmass. Zeolite-filled vesicles 246 249 181 181 common in the upper part ...... 303 994 Total thickness ; of Sander: Basalt measured (Partigl) T5l 2465 SECTION 5.-STRATIGRAPHIC SECTION OF THE BALLS BLUFF SILTSTONE AT THE CULPEPER CRUSHED STONE QUARRY, STEVENSBURG, CULPEPER COUNTY, VIRGINIA Joseph P. Smoot The quarry exposure is primarily composed of about 65.5 m (215 ft) of slightly thermally altered mudstones and siltstones. These rocks are well indurated and have a slabby to blocky parting. The sandstone and coarse siltstone layers appear to be quartzose with a dolomitic TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. cement, based on their weathering characteristics and examination under a hand lens. Carbonate minerals are apparently altered to epidote in a few zones. The carbonate minerals include a tan-weathering cement, which is interpreted as a ferroan dolomite, and a white calcite cement. The cements are intergranular in coarser layers and occur as tube- and crack-fillings and as nodules in mudstones. Most of the mudstones and siltstones have been divided into eight lithologic types on the basis of their primary depositional features (fig. A-1). These litho- logic types occur repeatedly throughout the section, defining cycles. Portions of the exposure were inac- cesible for examination owing to the steepness of the quarry wall. These sections (from about 1 to 7 m (8.3 to 23 ft) and from 10 to 15.5 m (383 to 51 ft) below the uppermost layer) were measured by dropping a metric tape over the ledge and making observations with binoculars. The lithologic types are indistinguishable over significant portions of these sections and are labeled lithologic type 9. Five additional lithologic types were recognized, but each of these occur only once in the section. These are labeled a-d. LITHOLOGIC TYPES 1 Dark-gray to black, laminated shaly mudstone: Laminae are defined by alternations of silt layers with silty clay layers. The fine silt layers are flat and continuous, and the coarse silt layers form thick laminae which are lenticular or which pinch and swell rhythmically. Sole markings resembling trails are common on bedding planes, as are sand- sized peloidal structures, which may be fecal pellets or internal molds of ostracode shells. Mud cracks are absent or rare. 2 Dark-gray and purplish-red platy mudstone that is laminated to thin-bedded: Layering is defined by alternations of muddy silt layers with silt to very fine sand layers. The coarser layers pinch and swell rhythmically or are lenticular, forming oscillatory ripple marks on bedding planes. The thin silt layers may have internal low-angle inclined lamination, and the thicker layers, partic- ularly near the tops of this lithologic type, contain internal asymmetric, sinusoidal, cross-laminae indicating transport to the E.-SE. The sand layers may have scoured basal contacts and load casts. Polygonal mud cracks are common and more abundant toward the tops of these units. Cracks near the base are 15-20 ecm (6-8 in) deep and 40-50 em (16-25 in) apart, while cracks near the top are around 5 ecm (2 in) deep and 10-15 em (4-6 in) apart. APPENDIX A 41 CYCLE LITHOLOGIC COLOR CYCLE LITHOLOGIC COLOR NUMBER TYPE NUMBER TYPE Sigeie? 2 £ 8 9m" o > a" Evan = $05 awmaein 0 L1 1// £5 11/1/ EXPLANATION ) Yy VY ps3 Z f 12 NV y | P RJ 1 Laminated shaley mudstone 5 4 es é ag 7 g 2 Laminated to thin-bedded mudstone 11 6 9 3 P @ 3 Mudstone containing cracks Me, 1 c* 9 4 Siltstone containing mudstone lenses g Erice 1 5 Mudstone containing cracks and vugs 7 10 b* 9 6 Mudstone and siltstone containing ov ) v cracks and tubes 9 eas 1 a * 7 Siltstone and mudstone containing 3 tubes and large cracks 8 Mudstone or siltstone containing tubes 9 6 and disseminated sulfide minerals d* g D 9 Undifferentiated silty mudstone 7 2 4 m a Tubes and cracks; silt lenses 8 6 y ves 4 % ADDITIONAL LITHOLOGIES 7 a* Black peloidal sandstone containing i 6 mud clasts 2 q 7 b* Sandy mudstone containing sandstone 4 § 5 lenses g 3 c* Lenticular odlitic calcarenite 5 % 7 d* Mudstone containing flaggy partings 3 and siltstone lenses 2 6 6 1 8 5 7 (s 4 7 9 5 6 4 5 pere 4 3 4 2 6 1 4 5 8 4 ke 3 h 2 3 E 1 5 = 4 Mk 4 0 7 7 2 6 e 4 9 3 1 5 5 -z 4 FIGURE A-1.-Stratigraphic section of the Balls Bluff Siltstone at the Culpepper Crushed Stone quarry, Stevensburg, Va. 42 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. 3 Gray and purplish-red mudstone disrupted into breccia-like blocks by abundant red or gray, silty mudstone-filled, polygonal cracks: The cracks are typically narrow and jagged in cross section, forming crosscutting polygons, each 5-10 em (2-4 in) in diameter, in plan view. The mudstone blocks may have internal lamination as in lithology 2, and the layering in adjacent blocks is parallel, showing no evidence of rotation. 4 Tan-weathering, thin beds of siltstone occurring as layers of concave-upward, curling lenses in red and gray silty mudstone: The siltstone beds are commonly disrupted by broad (to 20 em (8 in)), flat-bottomed areas of silty mudstone which define polygonal cracks. The siltstone beds, have internal asymetric, sinusoidal, ripple cross lamination or horizontal planar-lamination and mud partings. Small scours and internal grading are common in the siltstone beds, and sole markings resembling small trails, dinosaur tracks, and prod marks are also common. The silty mudstone in the cracks contains abundant spherical to very flattened elliptical blobs of dolomite or calcite which are interpreted as cement-filled vugs. The silty mud- stone between the siltstone beds contains narrow, jagged cracks, some filled with dolomite or calcite, and fewer of the "vugs." The siltstone lenses near the base of these units are closely spaced and show little curvature, while those near the top are widely spaced and strongly curled. The upper siltstone layers also contain numerous narrow, angular internal cracks. 5 Massive, red and gray, silty mudstone to muddy siltstone with abundant narrow, jagged cracks and caleite- or dolomite-filled spheroidal to flattened el- liptical "vugs": The cracks defineirregular, cross- cutting polygons 3-10 em(0.6-4 in) indiameter and also occur as horizontal cracks connecting the "vugs." Small tubes(diameterslessthan1 mm(.04)) may be present in the crack fillings. 6 Massive, red and gray, silty mudstone to muddy siltstone with abundant narrow, jagged cracks and dolomite- or caleite-filled tubes: The tubes are 0.2- 2.0 mm (.008-0.8 in) diameter, sinous, and mostly oriented perpendicular to bedding and may bifur- cate and taper. The cracks are similar to those of lithology 5 except in the upper parts of the units where the cracks are wider and the polygons have greater diameters (as much as 20 ecm (8 in)). The tubes appear to preferentially occur within poly- gonal crack fillings near the bases of the units and are more randomly distributed near the tops. 7 Massive, red and gray, muddy siltstone to silty mudstone with abundant dolomite-or mud-filled tubes and 10-20 em (4-8 in) deep, narrow, sinuous cracks: The tubes range in diameter from sub- mm-to em-seale, commonly branch and taper, and are randomly distributed. Some of the smaller tubes are filled with calcite, and the larger tubes are commonly filled with coarse silt and nodular dolomite. The cracks are mostly filled with muddy silt containing dolomitic cement and define poly- gons 20-40 em (8-16 in) in diameter. 8 - Massive, gray to black, sandy mudstone or siltstone with abundant dolomite- and calcite-filled tubes and disseminated sulfide minerals: The tubes range in diameter from sub-mm- to em-seale and may branch and taper or remain the same diameter for their exposed length. Dolomitic nodules are common in the larger tubes and are also randomly scattered (the largest reaching 15 cm (6 in) in diameter). Narrow, sinuous cracks 20-30 em (8-12 in) long are common and define polygons 30-40 em (12-16) in diameter. Small, irregular patches of sandstone are probably remnants of bedding disrupted by bioturbation. Low-amplitude, long-wavelength (30-40 em (12- 16 in)), symmetric ripple structures are present on some of the upper bedding planes. 9 Undiffentiated, massive, red and gray, silty mud- stone: The sedimentary features are probably the same as either lithology 5 or lithology 6. a Black, peloidal sandstone with mud clasts to granule size: The layer is 1-7 em (0.4-2.8 in) thick lying on a sharp, erosional basal contact with as much as 4 em (0.8 in) relief. Thin bedding is defined by changes in grain size and thin, shaly partings. The overall unit appears to be coarse-tail graded. Dissem- inated sulfide minerals are abundant. b Dark-gray, tan-weathering, sandy mudstone with sandstone lenses and pods: Sandstone lenses are 1-5 ecm (0.4-2.0 in) thick and 5-10 cm (2-4 in) long and have internal flat lamination to cross-lamination. The lamina- tion is commonly deformed into concave- convex folds which appear to be coincident with dinosaur tracks on the bedding surface. Polygonal cracks 10-20 ecm (4-8 in) in diameter are common, as are tubes filled with sand, mud, and dolomite. Sulfide minerals are abundant in the sandstone lenses. c A lenticular oolitic calcarenite overlying the irregularly humpy bedding surface of lithology 8: The upper surface of lithology 8 is dom- inated by nodules of dolomite and epidote, large tubes, polygonal ridges, and trails of poorly defined dinosaur tracks. Some of the APPENDIX A 43 humpiness may be due to superimposed tracks. The humpiness appears to decrease downdip, and the polygons have larger diameters and are less ridgelike in that direction (west). The odlitic calcarenite varies in thickness from 0 to 20 em (8 in). It contains flat-lying, pebble-sized clasts of mudstone and laminar micrite. The laminar micrite also occurs as humpy layers within the calcarenite. Disseminated sulfide minerals are abundant. d Mudstone with flaggy partings and beds of curled siltstone lenses, viewed through bi- noculars: The siltstone lenses appear to be present at the basal contact and a few other horizons within this lithology. The lower meter appears to have thinner parting than the upper meter. The weathering style of this zone is similar to the lower meter of cycle 16. e Shaly to flaggy mudstone interval, viewed through binoculars: Large mud cracks are visible in the upper portion, whereas none were seen in the lower part. Therefore, the basal part is assigned to lithology 1 and the top is assigned to lithology 2. INTERPRETATION OF THE ENVIRONMENT OF DEPOSITION Lithology 1 is interpreted as the bottom sediments of a perennial lake that may have been very deep at some times. Lithology 2 is interpreted as a shallower, wave- reworked lake deposit that was intermittently sub- aerially exposed and desiccated. Lithology 3 is interpreted as lake sediments that were subaerially exposed and intermittently rewetted. The brecciation is believed to be the result of repeated desiccation with very little sediment accumulation. Lithology 4 is in- terpreted as the deposits of a temporary lake which were disrupted by desiccation cracks during prolonged periods of subaerial exposure. Lithology 5 is inter- preted as heavily desiccated deposits of an aggrading, playalike mudflat. The vugs are believed to be vesicles formed during flooding events over the dry flats. Lithologies 6 and 7 are interpreted as root-disrupted deposits of vegetated subaerial flats. Lithology 6 is believed to represent drier conditions than lithology 7 because the mud cracks are smaller and narrower and the proposed root structures preferentially occur in the cracks. Lithology 8 is interpreted as the deposits of a vegetated, intermittently desiccated, shallow lake margin. The cycles are defined by the systematic vertical changes from rocks having well defined layers to rocks that are internally massive. The contacts between the cycles are abrupt, while the boundaries between litho- logic types within the cycles are gradational. Two end- member types of cycles are proposed: 2-1-2-3-4-6-7-8 and 4-5-6-7. The cycles are interpreted as representing transgressions and regressions of a lake, probably due to climatic changes. Cycles of the longer type, which are predominately gray colored, are interpreted as representations of overall wetter conditions than the shorter cycles, which are predominately red. Trans- gressions are believed to be from the west-northwest toward the east-southeast. This interpretation is based on the cross-lamination in asymmetric oscillatory ripple structures in lithologies 2 and 4 and the observed lateral transitions within beds of lithology 4 from better layered on the western side of the quarry to more mud cracked on the eastern side. The shorter cycles appear to be more abundant at the base of the section. The cycles gradually change to the longer variety toward the middle portions of the section, and then start to change to the shorter variety toward the top. This may represent a longer term climatic eyclicity. 44 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. APPENDIX B: DRILL AND CORE HOLE DESCRIPTIONS AND COLUMNS (Geologic logs of selected water wells and core holes, Culpepper basin, Va.) WELL E.-U.S. GEOLOGICAL SURVEY TEST HOLE F-51V 14F Fox Mill Subdivision, Fairfax County, Va. Quadrangle: Herndon 7.5-min Total depth: 305.5 m (1,002 ft); -202 m (-662 ft) Logged by J.D. Larson and A.J. Froelich, Oct. 20-26, 1978 Well no. 51V-14F Elevation: 340 ft Location: lat 38° 5528" N., long 77° 23'27" W. Formation: Triassic Manassas Sandstone: Poolesville Member, Reston Member; Peters Creek Schist Elefiglt. Lithology Remarks 0-3 Soil and Sand and silt, yellow brown; loose alluvium quartz pebbles. Culpeper Group, Manassas Sandstone (partial), Poolesville Member (partial) 3-10 Siltstone and Red-brown, soft, micromicaceous, shale fractured, noncalcareous. 10-60 Siltstone and Red-brown to grayish-red '(10R 4/2); shale in part very calcareous matrix and fracture filling; firm. 60-100 Sandstone and Red-brown to purplish-brown; siltstone interbedded, micromicaceous, firm, platy, arkosic, in part calcareous; first water influx at 75 ft. 100-110 Sandstone Fine- to medium-grained, reddish brown and gray, with scattered coarse subangular quartz and feldspar grains to 2.0 mm. 110-130 Siltstone and Reddish-brown (10R 3/4 to 10R 4/2); shale interbedded, firm, platy, noncalcareous. 130-180 Sandstone Fine- to medium-grained, with medium, coarse, and very coarse grains of quartz and feldspar at 170-180 ft, reddish-brown (10R 4/2), in part with calcareous matrix; slight water in flux. 180-200 Siltstone and Very fine grained, grayish-red (10R sandstone 4/2), platy, micromicaceous, brittle. 200-240 Sandstone Medium- to coarse-grained and conglomeratic, with abundant quartz and schist fragments, gray, arkosic, partly calcareous. 240-280 Siltstone and Very fine grained, grayish-red (10 R sandstone 4/2), in part mottled green and gray at 280-290 ft, nonealcareous except for rare calcite veinlets. 280-290 Sandstone Reddish-brown (10R 3/4) to gray, mottled; medium- to coarse- Depth. in feet Lithology Remarks grained and conglomeratic, in part slightly calcareous. Grayish-red (10R 4/2), fine-grained, arkosic, micromicaceous, noncalcareous. Gray, medium- to coarse-grained and conglomeratic; arkosic, in part with calcareous cement. Reddish-brown (10R 3/4), very fine to fine-grained, micaceous, silty and clayey matrix, noncaleareous. Gray, very coarse grained to conglomeratic, abundant quartz and schist fragments 1.0 to 2.5 cm. Dusky-red-brown (5R 3/4), very fine grained, micromicaceous, mostly noncalcareous. Dusky-red-brown (5R 3/4) and gray, very coarse to medium-grained, scattered quartz pebbles, blocky. Red brown (5R 3/4), very fine to medium-grained, arkosic, micromicaceous, platy. Grayish-red-brown (5R 4/2), in part mottled gray and green, interbedded, with calcite and quartz fragments; impure mottled silty limestone at 470-480 ft. Siltstone and sandstone 290-320 320-330 Sandstone 330-350 Sandstone 350-360 Sandstone Siltstone and sandstone 360-390 390-400 Sandstone Sandstone 400-450 Sandstone and siltstone 450-545 Top Reston Member (complete, 19.8 m (65 ft)) 545-590 Conglomerate Red-brown (5R 4/2), abundant - angular quartz and schist fragments to 2.5 em; matrix is medium- to coarse-grained arkose; malachite stain at 580-590 ft. Brownish-gray (5YR 4/1), abundant quartz and schist pebbles and fragments to 2.5 em; matrix is fine-grained sandstone and siltstone. 590-610 Conglomerate Base Reston Member, Unconformity, Peters Creek Schist 610-620 Saprolite Weathered schist, reddish-brown and greenish-gray, mottled, polyfoliated. Greenish-gray, chloritic, slightly weathered at 620-630 ft; abundant vein quartz at 650-660, 700-710 ft, with traces of pyrite and magnetite. Silvery-gray-green (5GY 4/1), chloritic, sericitic(?), polyfoliated; abundant vein quartz at 730-740, 790-840 ft, with traces of fresh pyrite and magnetite. Silvery-gray-green (5GY 4/1), chloritic, micaceous, in part quartzose, in part pelitic with 620-710 Schist Schist 710-840 840-1002 _ Schist APPENDIX B 45 Elefiteht. Lithology Remarks 3,65; Lithology Remarks fine-grained metagraywacke, 70-90 Sandstone and Sandstone, fine grained, pale brown polyfoliated; abundant vein siltstone to brown-gray, calcareous, quartz at 840-850, 880-900, 930- micaceous; siltstone, pale to 950 ft; with traces of pyrite, moderate brown, calcareous, magnetite, and garnet(?), pyrite micaceous; scattered copper common from 930-1,002 ft; minerals: malachite, azurite, increase in water inflow to 50 cuprite, chrysocolla at 85-92 ft (X- gal/min between 840 and 900 ft. ray LD.) 90-100 Siltstone and Moderate-brown, calcareous, NoTE.-The section of the Reston Member and partial Poolesville shale micaceous. Member of the Manassas Sandstone penetrated apparently con- | 100-170 Sandstone and _ Sandstone, fine- to medium-grained, stitutes a stacked succession of eight upward-fining fluvial siltstone moderate-gray brown, scattered sequences, with the bases at 610, 400, 360, 330, 290; 240, 180, and coarse-grained sandstone and 110 ft. pebbles at 160-170 ft; calcareous, micaceous siltstone, as above; 'Color descriptions are based on the "Rock-Color Chart" of the ziaggéognga?fi§zrafaggu{éfi’ft Geological Society of America (Goodard and others, 1948). sestiored ealeite erystals at 180. 150, 160 ft. 170-200 Sandstone and Sandstone, fine- to medium-grained, siltstone pale- to moderate-brown, scattered coarse-grained sandstone and pebbles at 190-200 ft, abundant WELL G.-FAIRFAX COUNTY WATER loose sand; calcareous, micaceous; AUTHORITY WELL NO. TW-1 siltstone, as above. Brookfield, Va., U.S. Route 50 and Flatlick Branch 200-280 Siltstone and Siltstone, pale- to moderate-brown - sandstone and gray-red, calcareous, micaceous, fissile; sandstone and USGS well no.: 51V-23H Quadrangle: Herndon 7.5-min loose sand, as above; copper at 230 Elevation: 280 ft Total depth: 305 m (1,000 ft); ft, calcite crystals and veinlets at Location: lat 38°53'20" N., -219.5 m (-720 ft) 240-270 ft. long 77°25'17" W. Logged by J. Carey and S. 280-390 _ Siltstone Gray-red, micromicaceous, Formation: Triassic Manassas Morsches, Aug. 10-24, 1979 calcareous matrix and veinlets at Sandstone (partial); 290, 320 ft; copper minerals Poolesville Member (azurite, malachite) at 320, 350 ft. (partial) 390-450 Siltstone and Siltstone, as above; sandstone and sandstone loose sand, fine- to medium- Depth, Lithology Remarks grained, as above; coarse-grained in feet sand at 410-420 ft. 0-3 Soil and Clay, silt, sand, quartz pebbles; 450-480 Siltstone As above; in part, fine sandy at 460- alluvium yellow-brown, micaceous, loose. 470 ft. 480-520 Siltstone and Siltstone, as above; sandstone, as Culpeper Group, Manassas Sandstone (partial) Poolesville Member sandstone above; medlu.m-c0arse-gra1ned (partial) sapdsbong, friable, at 490-500 ft with calcite crystals. (Note: 3-20 Sandstone and __ Sandstone, very fine grained, light- Casing run at 500 ft; flowing 15 siltstone blue-gray to light-olive-gray, very gal/min at surface prior to micaceous; interbedded with installation of casing) siltstone, moderate-brown, 520-540 Siltstone and Siltstone, as above; shale, moderate- calcareous, micromicaceous, platy shale brown, calcareous, micromicaceous, to fissile. fissile, in part silty, scattered clacite 20-30 Siltstone and Moderate-brown, calcareous, crystals and veinlets at 520-530 ft. shale micromicaceous, platy- fissile, 540-670 Sandstone and Siltstone, fine- to medium-grained, interbedded. siltstone moderate-gray-brown, calcareous, 30-70 Siltstone and Sandstone, fine- to medium-grained, micaceous; siltstone, as above; sandstone scattered coarse grains and shale, moderate-brown, silty at conglomeratic at 60-70 ft; green- 580-590 ft. gray to light-gray, abundant loose | 670-690 Sandstone Medium- to fine-grained, moderate- quartz grains, in part micaceous, brown, calcareous, micaceous, calcareous; siltstone as above; silty. scattered calcite crystals at 50-60 | 690-710 Sandstone and As above. ft. siltstone 46 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. £522? Lithology Remarks agile}: Lithology Remarks 710-730 Sandstone As above, fine- to medium-grained, 180-200 Shale Gray-red, micaceous, calcareous with cuprite(?) and chrysocolla matrix. and malachite, scattered calcite 200-250 Siltstone and Gray-red and moderate-brown, crystals as above. shale micaceous, calcareous matrix, 730-750 Sandstone and As above. sparse calcite fracture fillings. siltstone 250-290 Siltstone Moderate-brown, micaceous, 750-760 Siltstone As above. calcareous matrix and vein 760-790 Siltstone and As above. filling. sandstone 290-330 Siltstone and As above. 790-820 _ Sandstone Medium- to fine-grained, moderate- shale brown, calcite crystals and 330-470 _ Shale Moderate-brown, argillaceous, gray- veinlets. red, micaceous, calcareous, silty; 820-840 Sandstone and As above. with scattered calcite veinlets. siltstone 470-510 Siltstone As above. (Note: Intermediate 840-880 Missing casing run at 500 ft) 880-970 _ Sandstone Fine-grained, as above, calcite 510-700 _ Siltstone Moderate-brown, micaceous, crystals at 880-900 ft. calcareous, argillaceous, fissile to 970-990 Missing platy; trace copper mineral 990-1,000 - Sandstone Fine-grained, as above. malachite (at 560, 630, 660, 690 ft) calcite crystals and veinlets 1,000 Total depth At total depth, well flowed at 25 common. gal/min prior to pump test; 11-hr 700-767 Siltstone and Moderate-brown, micaceous, fissile, pump test at 350 gal/min with 95- shale calcareous. ft drawdown. NoTE.-Geophysical logs available: Gamma ray to 656 ft; caliper to 654 ft; multipoint electric to 656 ft (16 in and 64 in normal resistivity). WELL H.-FAIRFAX COUNTY WATER AUTHORITY WELL NO. TW-2 U.S. Route 50 1.5 mi west of Chantilly, Va. at Friendly Village (of Dulles) Trailer Park USGS well no. 51V-24H Elevation: 260 ft Location: lat 38°53'50" N., long 77°27'25" W. Formation: Triassic Balls Bluff Quadrangle: Herndon 7.5-min Total depth: 233.8 m (767 ft); -154.6 m (-507 ft) Logged by J. Carey and S. Morsches, Aug. 7-8. 1979 NoTE.-Geophysical logs available: Gamma ray to 503 ft; caliper to 503 ft; multipoint electric to 503 ft (16 in and 64 in normal Siltstone 13525: Lithology Remarks 0-3 Soil Clay, silt, and sand, yellow-brown, loose, scattered pebbles. Culpeper Group, Balls Bluff Siltstone (partial) 3-40 Siltstone Moderate-brown to gray-red, micaceous, calcareous, soft. 40-100 Siltstone Gray-red, micromicaceous, in part with very calcareous matrix and calcite fracture filling. 100-150 Siltstone and Gray-red, micromicaceous, shale calcareous matrix and calcite vein fillings. 150-170 Siltstone As above. 170-180 Shale and As above. siltstone _ resistivity). Well yield while drilling (air pumped with compressor on rig): Depth (feet) Est. yield (gal/min) 300 200 325 400 (picked up a lot of water between 310 and 320 ft) 500 425 680 150 (Cased to 500 ft) WELL I. -FAIRFAX COUNTY WATER AUTHORITY WELL NO. TW-3 Braddock Road and Flatlick Branch USGS well no.:51V-13A Elevation: 230 ft Location: lat 38°52'05" N., long 77°27'55" W. Formation: Triassic Balls Bluff Quadrangle: Manassas 7.5-min Total depth: 198 m (650 ft); -128 m (-420 ft) Logged by J. Carey and S. Morsches, July 11-30, 1979 Siltstone and hornfels-thermally metamorphosed siltstone £1652; Lithology Remarks 0-3 Soil and Clay, silt, sand, gravel, yellow- alluvium brown; quartz and siltstone pebbles, loose. Culpeper Group, Balls Bluff Siltstone (partial) 3-50 Siltstone Gray-red, calcareous, micromicaceous, argillaceous, APPENDIX B 47 Depth. in feel Lithology Remarks platy, fissile; scattered calcite crystals and veinlets. 50-110 Siltstone and Siltstone, pale-red to gray-red, as shale above; shale, gray-red, calcareous, fissile, silty, scattered claystone. 110-190 Siltstone As above, minor shale-claystone, gray-red, calcareous, micaceous; scattered calcite crystals and veinlets. 190-210 Siltstone, shale, _ Siltstone and shale; as above; and sandstone sandstone, very fine grained. 210-220 _- Siltstone and As above. shale 220-230 Siltstone, shale, _ Siltstone and shale; as above; and sandstone sandstone, very fine grained, gray-red, calcareous. 230-250 Siltstone Gray-red, calcareous, micromicaceous (Note: Flow gaged at 24 gal/min) 250-280 Siltstone and As above. shale HORNFELS-Thermally metamorphosed siltstone 280-290 Siltstone and shale, as above; hornfels, grayish-green, silicious, abundant epidote and calcite. As above. Siltstone, gray-red to dusky-brown, gray-brown, argillaceous, micaceous, calcareous; hornfels, greenish-gray and red, hard, laminated with abundant (30 percent) epidote, calcite, quartz and feldspar crystals. Gray-red, argillaceous, micaceous, calcareous; abundant epidote, copper mineral (malachite?). Siltstone, gray-red, laminated, calcareous, very hard; interbedded with hornfels, dark- gray, green, abundant epidote, feldspar. (Note: Flow gaged at 35 gal/min, installed casing to 500 ft) Hornfels, dark-gray, yellow-gray, olive-gray, with epidote, calcite, felspar, biotite; siltstone, gray- brown, slightly calcareous. Siltst ne, brown to dusky-brown, hard, brittle, laminated, calcareous, micaceous; hornfels, dusky-brown to light-olive-gray; abundant epidote and feldspar; well flowed at 5 gal/min at total depth. Siltstone, shale, and hornfels Siltstone Siltstone and hornfels 290-300 300-350 350-370 Siltstone Siltstone and hornfels 370-580 Hornfels and siltstone 530-600 Siltstone and hornfels 600-650 CORE L OF THE MIDLAND FORMATION AT LICKING RUN Joseph P. Smoot This core was taken through the Midland Fish Bed, a fossiliferous Lower Jurassic calcareous shale, formerly exposed at the Licking Run dam site in the Midland 7.5-min Quadrangle. It starts in a sandstone overlying the shale and ends in red mudstones beneath it. About 15 m (50 ft) of core was recovered and is illustrated in the measured section (fig. B-1). The thicknesses presented here and their relative depths were deter- mined by direct measurement of the recovered material, which probably has resulted in some inaccuracies. The relationships presented here are a reconstruction made by matching the ends of broken core segments, by determining the orientation by sedimentary structures, and by matching similar sedimentary features or trends where necessary. Two important contacts, which were not well determined, are the red-to-gray color transition and the diabase-sandstone contact. The color transition appears to be very sharp, but the lower contact of the gray mudstone is a drilling spinout; however, a matching spinout was not observed on top of the underlying red siltstone. The base of the sandstone overlying the diabase appears to be thermally altered and, thus, is probably correctly oriented. However, the pieces do not fit, suggesting that there may be some material missing between the two lithologies. The lowest portion of the core consists of silty mud- stones that contain abundant mud cracks and bio- turbation and thin sandstones that form sharp-based, graded units dominated by ripple cross-lamination. These are followed by silty mudstones that have larger, better defined burrows and deeper, wider cracks. Sandstones associated with these mudstones are dom- inated by ripple cross-lamination like the sandstones below, but they have load casts and foundered ripples at their bases and soft-sediment deformation is common. The red-to-gray transition occurs within this portion of the core. Above this is a fine-grained silty mudstone containing abundant carbon-filled tubes and zones of thin, flat silt laminae. A brownish, organic-matter- rich, calcareous shale containing fish fossils abruptly overlies an ostracodal sand that fills a scour contact in the underlying silty mudstone. The brownish shale grades back into the gray silty mudstone with an increase in bioturbation. The mudstone becomes coarser grained upward by a gradual increase in the thickness and number of silt laminae. This culminates in a medium-grained sandstone with dune-scale crossbeds fining upward into rippled siltstone. After the thin diabase intrusive, a thick sequence of flat-laminated, fine-grained sandstone marks the top of the core. METER -Q 48 TRIASSIC-JURASSIC STRATIGRAPHY OF THE CULPEPER AND BARBOURSVILLE BASINS, VA. AND MD. GRAIN SIZE IN mm 7 LITHOLOGIC SUBUNIT T GRAIN SIZE (See Appendix B) IN mm mmtrm 0 p FIGURE B-1.-Core of the Midland Formation at Licking Run. 7 LITHOLOGIC UNIT (See Appendix B) 14 13 COLOR | Brown | Gray | Reddish tan Gray Red METER APPENDIX B 49 EXPLANATION (f Poor recovery Flat lamination Wavy lamination Massive mudstone 21 17. Sutures of Neocardioceras uptonense CObb&N, M. Sp 22 18. Suture of Neocardioceras laevigatum Cobban, M. SP 23 19. Scatter diagrams showing umbilical ratios and number of ribs per half whorl of Neocardioceras minutum Cobban, e A e o e ae a e ae ae a aaa a c 24 20. Suture of Neocardioceras minutum CObb&M, M. SP 24 21. Suture of NeOC@rdiOG@r@s SP keer eee ee a a a a a a a a eee e a a a e ae ee e e a a a ee e eee 24 TABLES Page TABLE 1. Localities at which fossils were collected kkk aaa alla alll ll. 3 2. Zonation and ranges of Cenomanian ammonites 26 TARRANTOCERAS STEPHENSON AND RELATED AMMONOID GENERA FROM CENOMANIAN (UPPER CRETACEOUS) ROCKS IN TEXAS AND THE WESTERN INTERIOR OF THE UNITED STATES By WILLIAM A. COBBAN ABSTRACT Tarrantoceras, Eucalycoceras, Pseudocalycoceras, Sumitomoceras, and Neocardioceras are closely related genera of ammonites confined to rocks of middle and late Cenomanian age. Tarrantoceras, repre- sented by T. sellardsi (Adkins) and T. flexicostatum, n. sp., was prob- ably derived from Calycoceras (Gentoniceras) leonense (Adkins). Eucalycoceras appeared abruptly near the end of middle Cenomanian time and then disappeared until the middle of the late Cenomanian. The genus is represented by E. templetonense, n. sp., and E. pen- tagonum (Jukes-Browne). Neocardioceras, which was probably derived from Tarrantoceras, is known by the new species N. uptonense and N. laevigatum of early late Cenomanian age, N. minutum, n. sp. and N. sp. of middle late Cenomanian age, and N. juddii? (Barrois and de Guerne) and N. densicostatum, n. sp., of late Cenomanian age. Pseudocalycoceras and Sumitomoceras, known only from the later part of middle late Cenomanian time, were probably derived from early forms of Eucalycoceras pentagonum. Pseudocalycoceras is represented by P. angolaense (Spath), and Sumitomoceras is known from S. con- lini Wright and Kennedy and S. bentonianum (Cragin). INTRODUCTION Stephenson (1955, p. 59) proposed the genus Tarran- toceras for laterally compressed ammonites that have ribs which cross the flank and venter and bear umbilical, inner and outer ventrolateral, and siphonal tubercles. The genus was named for Tarrant County, Tex. As pointed out by Stephenson (1955, p. 59), the genus close- ly resembles the English genus Eucalycoceras Spath (1923, p. 144) in form and ornament, but the American genus has a simpler suture with a shallow lateral lobe. Other genera are closely related to Tarrantoceras and Eucalycoceras. Pseudocalycoceras Thomel (1969, p. 650) differs from Tarrantoceras and Eucalycoceras mainly in having conspicuous rursiradiate ribbing. Neocar- dioceras Spath (1926, p. 81) differs from the other genera discussed chiefly in having ribs that cross the venter as chevrons. Sumitomoceras Matsumoto (in Matsumoto and others, 1969, p. 280) loses its siphonal tubercles at an early growth stage. All of these closely related genera are confined to rocks of middle and late Cenomanian age, and most are widely distributed over much of the world. One or more of the genera have been found in Mississippi, Texas, New Mexico, Arizona, Utah, Colorado, Oklahoma, Kan- sas, Wyoming, South Dakota, and Montana. Species of these genera have very restricted time spans and provide a means for the correlation of thin rock units. This report was prepared to bring our knowledge of these genera in the United States up to date, to describe important new species, and to show the biostratigraphic sequence of species. All specimens described and illustrated in this report are kept in the National Museum of Natural History in Washington, D.C., and have USNM catalog numbers. Plaster and plastic casts of the holotypes of new species as well as casts of some of the other figured specimens are in the reference collections of the U.S. Geological Survey at the Denver Federal Center in Lakewood, Colo. These casts were prepared by R.E. Burkholder and Robert O'Donnell of the U.S. Geological Survey, and Burkholder made all the photographs. The author made the drawings of the sutures. LOCALITIES AT WHICH FOSSILS WERE COLLECTED The fossils described in this report came from 67 localities in the Western Interior of the United States and from several localities in northeast Texas. Localities of collections from the Western Interior are shown on figure 1, and data concerning the locality number, names of collectors, year of collection, locality, and stratigraphic assignment are given in table 1. The prefix D indicates Denver Mesozoic locality numbers; the rest are Washington, D.C., Mesozoic locality numbers. SYSTEMATIC DESCRIPTIONS In the descriptions and illustrations of sutures, E stands for the external or siphonal lobe, L stands for the lateral lobe, and E/L stands for the saddle (first lateral saddle) that separates the external and lateral A 2 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. 113° 109° 105° 101° §§ __ TT mn mommas tew mee sme me se | x | NORTH K DAKOTA [ [ 45C} 3-4 y | ~ =- -- --* $, SOUTH x 5-6 DAKOTA \x 10—13)< | U 27 __) x x r UAR 21-26 i COLORADO | 28-31 \ x y 32 _ i x33 34 f KANSAS 37° ._ X I I I, NEW MEXICO I el 20 , 45-46 t , 47-48 \ 1 \ | | R ~ -_ UNITED | ~~ _ stares! _ [C ___ STATES | _ MEXICO \ 0 100 200 300 400 500 KILOMETERS --- --- 0 100 200 300 MILES FiGURE 1.-Map of part of the Western Interior of the United States showing localities of fossil collections. Numbers refer to U.S. Geological Survey Mesozoic localities in table 1. lobes. No further divisions of the sutures are described. In the drawings of sutures, the heavy straight line represents the middle of the venter, the evenly curved dashed line represents the umbilical shoulder, and the evenly curved solid line represents the umbilical seam. Phylum MOLLUSCA Class CEPHALOPODA Order AMMONOIDEA Suborder AMMONITINA Superfamily ACANTHOCERATACEAE Grossouvre, 1894 Family ACANTHOCERATIDAE Grossouvre, 1894 Genus TARRANTOCERAS Stephenson, 1955, p. 59 Type species.-By original designation, Tarrantoceras rotatile Stephenson, 1955, p.59, pl. 5, figs. 1-10 (=Mantelliceras sellardsi Adkins, 1928, p. 239, pl. 25, fig. 1; pl. 26, fig. 1). Diagnosis.-Small to medium-sized, somewhat evolute, compressed ammonites ornamented by dense, mostly rectiradiate, slightly flexuous primary and secondary ribs that cross the venter transversely and that, at most growth stages, bear small inner and outer ventrolateral and siphonal tubercles. Primary ribs arise from bullate to nodate tubercles located on the umbilical shoulder. Sutures are fairly simple and acanthoceratid with broad, bifid first lateral saddles and much smaller, shallow, bifid lateral lobes. Remarks.-Tarrantoceras is closely related to Eucaly- coceras Spath (1923, p. 144) from which it differs main- ly in having a more simplified suture with a short lateral lobe instead of a long, narrow one. The narrow lobe of Eucalycoceras has been illustrated for several species from Madagascar (Collignon, 1937, pl. 4, fig. 3; pl. 8, fig. 5; pl. 9, figs. 3, 4) and for one from India (Kossmat, 1895, pl. 25, fig. 3c). In addition, some examples of Eucalycoceras have umbilical tubercles that project into the umbilicus (Kossmat, 1895, pl. 25, fig. 3a), a condi- tion not observed in Tarrantoceras. Ammonites pen- tagonus Jukes-Browne (in Jukes-Browne and Hill, 1896, p. 156, pl. 5, figs. 1, la), the type species of Eucalyco- ceras, differs further from Turrantoceras in having broad, flattened ribs with steep adoral faces on the last half of the body chamber. Sumitomoceras Matsumoto (in Matsumoto and others, 1969, p. 280), regarded as a subgenus of Tarrantoceras by Wright and Kennedy (1981, p. 38), differs from that genus in having siphonal tubercles on the innermost whorls and in losing the ven- trolateral tubercles at a smaller diameter. Occurrence.-Tarrantoceras is best known and best preserved in rocks of late middle to early late Cenoma- nian age in Texas, New Mexico, Colorado, and Wyoming. Crushed fragments occur in rocks of early late Cenoma- nian age in Montana, South Dakota, and Kansas. Out- side the United States, the genus has been recorded from Mexico (Powell, 1965, p. 522), Colombia (Biuirgl, 1957, p. 137), and Morocco (Collignon, 1966, p. 30-32, 59). GENUS TARRANTOCERAS STEPHENSON TABLE 1.-Localities at which fossils were collected Locality (fig. 1) U.S. Geological Survey Mesozoic locality Collector, year of collection, description of locality, and stratigraphic assignment 10 11 12 13 D556 24615 D8463 21355 D12630 21850 12740 23060 23064 D4462 D5940 D5947 D11780 W.A. Cobban, 1955. SE% sec. 13, T. 22 N., R. 1 W., Teton County, Mont. Marias River Shale, from limestone concretions 4.2 m (14 ft) above base of Cone Member. W.A. Cobban, 1953. North side of Mill Coulee in the SE% sec. 21, T. 21 N., R. 1 W., Cascade County, Mont. Marias River Shale, from limestone concretions associated with a 1-m-thick bed of ben- tonite 4.2 m (14 ft) above base of Cone Member. E.A. Merewether, 1972. SW% sec. 13, T. 9 S., R. 34 E., Big Horn County, Mont. Base of Greenhorn Calcareous Member of Cody Shale, from limestone concretions in ben- tonite bed M of Knechtel and Patterson (1956, p. 21). W.A. Cobban, 1948; E.A. Merewether and Cobban, 1983. SE% sec. 14, T. 9 S., R. 34 E., Big Horn County, Mont. Base of Greenhorn Calcareous Member of Cody Shale, from limestone concretions in ben- tonite bed M of Knechtel and Patterson (1956, p. 21). N.H. James, 1962. Northeast of Greybull in the NW % T. 53 N., R. 92 W., Big Horn County, Wyo. Near top of Frontier For- mation. J.B. Reeside, Jr., and D.A. Andrews, 19838. East of Herren Gulch in the SE% sec. 9, T. 53 N., R. 92 W., Big Horn County, Wyo. Cody Shale, from silty concretions 24.4 m (80 ft) above base. W.A. Rubey, 1924. NEV sec. 6, T. 9 S., R. 59 E., Carter County, Mont. Greenhorn Formation, from a limestone concretion in the lower part. J.B. Reeside, Jr., H.R. Christner, and W.A. Cobban, 1950. North of Belle Fourche in the E% sec. 14, T. 9 N., R. 2 E., Butte County, S. Dak. Greenhorn Formation, from a thin layer of calcarenitic limestone. J.B. Reeside, Jr., H.R. Christner, and W.A. Cobban, 1950. Southeast side of peninsula in Belle Fourche Reservoir in W! sec. 31, T. 9 N., R. 4 E., Butte County, S. Dak. Basal limestone bed of Greenhorn Formation. W.A. Cobban, 1964. About 5.7 km (3% mi) south-southeast of Upton in the NEV sec. 24, T. 47 N., R. 65 W., Weston County, Wyo. Greenhorn Formation, from a lime- stone concretion in the lower part. W.A. Cobban, 1967. About 4 km (2% mi) west of Upton in the NW! sec. 33, T. 48 N., R. 65 W., Weston County, Wyo. From an ironstone concretion in the Belle Fourche Shale. W.A. Cobban, 1961. About 4.8 km (3 mi) south of Upton in the NW % sec. 14, T. 47 N., R. 65 W., Weston County, Wyo. Belle Fourche Shale, from a limestone concretion 19.8 m (65 ft) above a 0.6-m-thick bed of bentonite. E.A. Merewether, 1964. About 8.8 km (5% mi) northwest of Upton in the NEM 8 TABLE 1.-Localities at which fossils were collected-Continued Locality (fig. 1) U.S. Geological Survey Mesozoic locality Collector, year of collection, description of locality, and stratigraphic assignment 14 15 16 17 18 19 20 21 22 23 24 25 26 27 D6962 D9798 D8918 D9894 D9337 D5900 D7530 D7388 D7389 D7390 D7395 D7402 D7403 D7410 sec. 13, T. 48 N., R. 66 W., Weston Coun- ty, Wyo. Greenhorn Formation, from a fer- ruginous concretion in the concretionary facies. W.A. Cobban, 1962. SW % sec. 30, T. 41 N., R. 80 W., Natrona County, Wyo. Frontier Formation, from a 2-m-thick bed of orange- brown-weathering sandstone in Belle Fourche Member. E.A. Merewether, 1975. NW! sec. 1, T. 42 N., R. 82 W., Johnson County, Wyo. Frontier Formation, from Belle Fourche Member. E.A. Merewether, 1973. Ervay Basin in the SW % sec. 19, T. 34 N., R. 88 W., Natrona County, Wyo. Frontier Formation, from upper part of Belle Fourche Member. W.A. Cobban, 1976. SEV sec. 13, T. 35 N., R. 84 W., Natrona County, Wyo. Frontier Formation, from a limestone concretion 2.4 m (8 ft) below base of a middle Turonian sandstone bed. E.A. Merewether and W.A. Cobban, 1974. Emigrant Gap Ridge in the NW! sec. 32, T. 34 N., R. 81 W., Natrona County, Wyo. Frontier Formation, from a lenticular bed of conglomerate in the upper part. W.A. Cobban, 1967. Head of Elm Creek in the W% sec. 14, T. 36 N., R. 62 W., Niobrara County, Wyo. Belle Fourche Shale, from limestone concretions 3.6 m (12 ft) below bentonite marker bed. E.A. Merewether and W.A. Cobban, 1970. Olsen Basin in the sec. 14, T. 23 N., R. 88 W., Carbon County, Wyo. Frontier Formation, from brown-weathering sand- stone concretions above the lowest sand- stone unit. G.A. Izett, G.R. Scott, and W.A. Cobban, 1969. NEM sec. 31, T. 1 N., R. 81 W., Grand County, Colo. Benton Shale, 64.3 m (211 ft) below base of Juana Lopez Member. G.A. Izett, 1969. Same locality as D7388. Benton Shale, 59.7 m (196 ft) below base of Juana Lopez Member. G.A. Izett and W.A. Cobban, 1969. Same locality as D7388. Benton Shale, 58 m (191 ft) below base of Juana Lopez Member. G.A. Izett, 1969. Same locality as D7388. Benton Shale, 38.4 m (126 ft) below base of Juana Lopez Member. G.A. Izett and W.A. Cobban, 1969. Center of sec. 31, T. 1 N., R. 80 W., Grand Coun- ty, Colo. Benton Shale, 62.5 m (205 ft) below base of Juana Lopez Member. G.A. Izett and W.A. Cobban, 1969. Same locality as D7402. Benton Shale, 60.6 m (199 ft) below base of Juana Lopez Member. G.R. Scott and G.A. Izett, 1970. South side of highway at east edge of Eldorado Springs in the SW! sec. 30, T. 1 S., R. 70 W., Boulder County, Colo. Greenhorn Limestone, from a shale bed underlying a 15-em-thick bed of bentonite in the upper part of the Hartland Shale Member. 4 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. TABLE 1.-Localities at which fossils were collected-Continued Locality (fig. 1) U.S. Geological Survey Mesozoic locality Collector, year of collection, description of locality, and stratigraphic assignment 28 29 30 31 32 33 34 35 36 37 38 39 40 41 D6472 D6998 D12558 D12629 D12627 18686 22899 D12452 D12493 D5268 D12356 D11587 D11584 D5848 W.H. Birchby, 1968. Boggs Creek in the NW% sec. 1, T. 21 S., R. 66 W., Pueblo County, Colo. Greenhorn Limestone, from lower part of Bridge Creek Member (Cob- ban and Scott, 1972, p. 24, bed 67). W.H. Birchby, 1969. SW! sec. 30, T. 20 S., R. 65 W., Pueblo County, Colo. Greenhorn Limestone, from 3.6 m (12 ft) above base of Bridge Creek Member (Cobban and Scott, 1972, p. 23, bed 84). W.A. Cobban, 1984. West of Pueblo in the SW% sec. 25, T. 20 S., R. 66 W., Pueblo County, Colo. Greenhorn Limestone, from Lincoln Member. W.A. Cobban, 1985. West of Pueblo in the NW % sec. 25, T. 20 S., R. 66 W., Pueblo County, Colo. Greenhorn Limestone, from lower part of Bridge Creek Member (Cob- ban and Scott, 1972, p. 23, bed 79). W.A. Cobban, 1985. Thompson Arroyo in the SW! sec. 5, T. 24 S., R. 54 W., Otero Coun- ty, Colo. Greenhorn Limestone, float on lower part of Bridge Creek Member (prob- ably from same bed as bed 84 of section at Pueblo, Colo., described by Cobban and Scott, 1972, p. 23). N.W. Bass, 1941, G.R. Scott and W.A. Cob- ban, 1966. El sec, 15, T. 30 S., R. 60 W., Las Animas County, Colo. Greenhorn Limestone, from basal bed of Bridge Creek Member. J.B. Reeside, Jr., H.R. Christner, and W.A. Cobban, 1950; G.R. Scott and Cobban, 1966. SW % sec. 12 and NW % sec. 13, T. 30 S., R. 60 W., Las Animas County, Colo. Greenhorn Limestone, from basal bed of Bridge Creek Member. W.A. Cobban, 1984. NW % sec. 8, T. 33 S., R. 60 W., Las Animas County, Colo. Greenhorn Limestone, from lowest concre- tionary limestone bed in Bridge Creek Member. F.B. Zelt, 1984. Northeast of Henrieville in the $% sec. 14, T. 37 S., R. 2 W., Garfield County, Utah. Tropic Shale, from 7.5 m (24 ft) above base. Fred Peterson, 1965. E% sec. 11, T. 43 S., R. 2 E., Kane County, Utah. Tropic Shale, from 11 m (36 ft) above base. F.B. Zelt, 1983. East of Wahweap Creek in the NW % sec. 2, T. 48 S., R. 2 E., Kane County, Utah. Tropic Shale, 5.5 m (18 ft) above base. S.C. Hook, J.I. Kirkland, and W. A. Cobban, 1981. Ha Ho No Geh Canyon in the NW % sec. 25, T. 30 N., R. 13 E., Coconino Coun- ty, Ariz. Mancos Shale, from gray limestone concretions 8.5 m (18 ft) above base. S.C. Hook and W. A. Cobban, 1981. SW! sec. 17, T. 10 N., R. 21 E., Navajo County, Ariz. Unnamed Cretaceous sandstone. G.R. Scott, 1967. Old railroad grade south of Taylor Springs in sec. 15, T. 24 N., R. 23 E., Colfax County, N. Mex. Greenhorn Limestone, from Lincoln Member. TABLE 1.-Localities at which fossils were collected-Continued Locality (fig. 1) U.S. Geological Survey Mesozoic locality Collector, year of collection, description of locality, and stratigraphic assignment 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 D10734 D5810 D5309 D5784 D5795 D5776 D5777 D9031 D10533 D10996 D11004 D11510 D11529 D11533 D11538 D6842 S.C. Hook and J.R. Wright, 1979. Near Taylor Springs in the NW! sec. 22, T. 24 N., R. 23 E., Colfax County, N. Mex. Greenhorn Limestone, from Lincoln Member. W.A. Cobban, 1967. Soda Spring on San Ysidro 15-minute quadrangle, Sandoval County, N. Mex. Mancos Shale, from sand- stone bed in lower part. W.A. Cobban, 1966. Near railroad cut at southeast edge of Lamy, Santa Fe Coun- ty, N. Mex. Paguate Tongue of Dakota Sandstone. E.R. Landis and W.A. Cobban, 1967. Southeast of Puertecito in the SEV sec. 33, T. 3 N., R. 5 W., Socorro County, N. Mex. Mancos Shale, 17 m (56 ft) above base. W.A. Cobban, 1967. West of Riley in the SW % sec. 16 and NW % sec. 21, T. 2 N., R. 4 W., Socorro County, N. Mex. Mancos Shale, about 27 m (90 ft) above base. W.A. Cobban, 1967. sec. 8, T. 5 S., R. 2 E., Socorro County, N. Mex. Mancos Shale, 10.7 m (35 ft) above base. E.R. Landis and W.A. Cobban, 1967. SE% sec. 8, T. 5 S., R. 2 E., Socorro County, N. Mex. Mancos Shale, from thin beds of very fine grained sandstone in lower part. W.A. Cobban, 1970. Wild Horse Mesa in the center of the north line of sec. 11, T. 18 S., R. 17 W., Grant County, N. Mex. Colorado Formation, from a limestone concretion in lower part. S.C. Hook and W. A. Cobban, 1978. South of center of north line of sec. 11, T. 18 S., R. 17 W., Grant County, N. Mex. Colorado Formation, from limestone concretions in lower part. S.C. Hook and Gerald Stachura, 1976. Fox- tail Creek in the SE% sec. 31, T. 17 S., R. 17 W., Grant County, N. Mex. Colorado Formation, from a limestone concretion in shale member. S.C. Hook, 1976. Redrock Canyon in the NW% sec. 27, T. 18 S., R. 15 W., Grant County, N. Mex. Colorado Formation, from a limestone concretion in the shale member. S.C. Hook, 1981. SEV sec. 31, T. 17 S., R. 17 W., Grant County, N. Mex. Colorado Formation, from a limestone concretion in the shale member. S.C. Hook and W.A. Cobban, 1981. NEV sec. 11, T. 18 S., R. 18 W., Grant County, N. Mex. Colorado Formation, from limestone concretions in lower part. S.C. Hook and W.A. Cobban, 1981. SE sec. 5, T. 18 S., R. 16 W., Grant County, N. Mex. Colorado Formation, from a lime- stone concretion in the shale member. S.C. Hook and W. A. Cobban, 1981. NEV sec. 11, T. 18 S., R. 18 W., Grant County, N. Mex. Colorado Formation, from shale member. E.R. Landis and W. A. Cobban, 1968. NEV sec. 30, T. 20 S., R. 8 W., Luna County, N. Mex. Colorado Formation. GENUS TARRANTOCERAS STEPHENSON 5 TABLE 1.-Localities at which fossils were collected-Continued Locality (fig. 1) U.S. Geological Survey Mesozoic locality Collector, year of collection, description of locality, and stratigraphic assignment 58 59 60 61 62 63 64 65 66 67 D10112 S.C. Hook, 1976. NEW sec. 13, T. 21 S., R. 9 W., Luna County, N. Mex. Colorado For- mation, from about 53 m (174 ft) above base. S.C. Hook and W. A. Cobban, 1976. NEV sec. 13, T. 21 S., R. 9 W., Luna County, N. Mex. Colorado Formation, from limestone con- cretions near top of Bridge Creek Lime- stone Member (Hook and Cobban, 1981, fig. 3). S.C. Hook and W.A. Cobban, 1976. NW % sec. 30, T. 20 S., R. 8 W., Luna County, N. Mex. Colorado Formation, from upper part of Bridge Creek Limestone Member. W.A. Cobban, 1981. South of center of north line of sec. 30, T. 20 S., R. 8 W., Luna Coun- ty, N. Mex. Colorado Formation, from Bridge Creek Limestone Member. S.C. Hook, E.R. Landis, and W.A. Cobban, 1977. North of Sierra De Cristo Rey in the SW sec. 9, T. 29 S., R. 4 E., Dona Ana County, N. Mex. Base of Boquillas Formation. S.C. Hook and W.A. Cobban, 1979. About 0.6 km south of Love triangulation station on Eagle Mountains SW quad- rangle, Hudspeth County, Tex. Chispa Sum- mit Formation, 30 m (98 ft) above base. W.A. Cobban, 1979. Chispa Summit, Jeff Davis County, Tex. Chispa Summit Forma- tion, 1.5 m (5 ft) above base. S.C. Hook and W.A. Cobban, 1979. Same locality as D10741. Chispa Summit Forma- tion, 4.6 m (15 ft) above base. S.C. Hook and W.A. Cobban, 1979. Same locality as D10741. Chispa Summit Forma- tion, from a chalky limestone bed 39 m (128 ft) above base. S.C. Hook and W. A. Cobban, 1979. About 1.4 km southwest of railroad cut at Chispa Summit, Jeff Davis County, Tex. Chispa Summit Formation, from yellowish, soft marl bed about 40 m (131 ft) above base. D10114 D10196 D11483 D10142 D10946 D10741 D10742 D10746 D10898 1928. 1942. 1955. 1955. 1955. 1955. 1971. 1972. 1976 1977. 1978. 1984. Tarrantoceras sellardsi (Adkins) Plate 1; plate 2, figures 1-22, 27, 28; text figure 4 Mantelliceras sellardsi Adkins, p. 239, pl. 25, fig. 1; pl. 26, fig.4. Mantelliceras sellardsi Adkins. Moreman, p. 207. Tarrantoceras rotatile Stephenson, p. 59, pl. 5, figs. 1-10. Tarrantoceras stantoni Stephenson, p. 60, pl. 5, figs. 11-21. Tarrantoceras lillianense Stephenson, p. 60, pl. 5, figs. 22-27. Tarrantoceras multicostatum Stephenson, p. 61, pl. 6, figs. 21-23. Eucalycoceras sellardsi (Adkins). Kennedy, p. 84. Tarrantoceras rotatile Stephenson. Cobban and Scott, p. 64, pl. 10, figs. 1-11; text fig. 25. [1978]. Utaticeras? sellardsi (Adkins). Young and Powell, fig. 5. Tarrantoceras rotatile Stephenson. Cobban, p. 23, pl. 6, figs. 8-10, 28, 29; pl. 11, figs. 7, 8, 11-16; pl. 12, figs. 13, 14; text fig. 4. Tarrantoceras rotatile Stephenson. Cooper, p. 92, text fig. 20. Tarrantoceras sellardsi (Adkins). Cobban, p. 78. Diagnosis.-A compressed, moderately evolute species that ranges from rather sparsely and strongly ornamented forms to more compressed and fairly dense- ly ribbed forms. The species also ranges from forms that retain ventrolateral tubercles to the aperture (micro- conchs) to larger forms that lose the ventrolateral tubercles farther down on the body chamber (macroconchs). Description.-The holotype, from the basal Eagle Ford Group of east-central Texas, is a large broken specimen that appears to be part of a body chamber and part of the last septate whorl (pl. 1, figs. 8, 9). The specimen was only briefly described by Adkins (1928, p. 239), who gave no dimensions other than the um- bilical ratio. A plaster cast reveals half a whorl about 55.5 mm in diameter with an umbilical width of 13.5 mm and an umbilical ratio of 0.24. Offset against the half whorl is a fragment that probably is the older part of the body chamber. The half whorl and the fragment have high, narrow whorl sections with flattened flanks and venter. The umbilicus has a narrowly rounded shoulder and steep walls. Ornament consists of closely spaced, rectiradiate to rursiradiate, slightly flexuous ribs as wide as or a little wider than the interspaces and umbilical, ventrolateral, and siphonal tubercles. Ribs are mostly of alternate lengths; the longer ones begin from umbilical bullae on the umbilical shoulder, and the shorter ones arise at midflank or a little lower. The ribs on the fragment of the presumed body chamber have steeper adapical faces than adoral ones. Faint inner ven- trolateral tubercles are present on the older part of the half whorl. Much stronger, clavate outer ventrolateral tubercles border the flattened venter, which is crossed transversely by the ribs. Each rib bears a weak, clavate siphonal tubercle. Part of the venter may have been con- cealed by matrix when Adkins described the specimen, because he mentioned that siphonal tubercles were ab- sent (and, hence, his assignment to Mantelliceras). Adkins did not note the faint inner ventrolateral tubercles on the older part of the half whorl and mistook the outer ventrolateral and siphonal tubercles as "a pair of shoulder tubercles." The outer ventrolateral tubercles number about 22 per half whorl. According to Adkins, no sutures are preserved. Specimens of Tarrantoceras sellardsi from northeast- ern Texas occur in silty limestone and tend to be frag- mented and crushed. Farther north, in the Fort Worth-Dallas area (fig. 2), excellent specimens have been found in limestone concretions near the base of the Eagle Ford Group. At least four major finds have been made: one by T.W. Stanton on Walnut Creek northeast of Mansfield (fig. 2, loc. 11740), a second by W.J. Ken- nedy and J. M. Hancock from the same general area (fig. 2, loc. D12626), a third by the late J.P. Conlin near 6 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. 33° MAW—m"—Th_“mm"m“7 TARRANT . DALLAS ~ L ._.____..l / / / 11740 p -- 1 | | t L_ -__ _/ D12626 | | z I I | Fort Worth -, i / ( I ~ Man*~[1( ld o 24510 \ (imam 5 S & JOHNSON 14583 Aivargdo Ox i OCleburne _ - - / [ | § AA _ -- \ a 25 50 0 75 KILOMETERS | 1 | 0 25 | 50 MILES FiGUuRE 2.-Map of part of northeastern Texas showing localities (x) and their USGS locality numbers where Tarrantoceras sellardsi (Adkins) was collected from near the base of the Eagle Ford Group. Lillian (fig. 2, loc. 24510), and a fourth by T.W. Stan- ton, L. W. Stephenson, and J.B. Reeside, Jr., near Alvarado (fig. 2, loc. 14583). All collections, except the recent one by Kennedy and Hancock, were made before 1955. Stephenson (1955) described as new species Tar- rantoceras rotatile, T. stantoni, T. lillianense, and T. multicostatum from the concretions near Mansfield, Lillian, and Alvarado. Stephenson's forms were later in- terpreted as normal variants within a single species (Cobban and Scott, 1972, p. 64). These variants center about Stephenson's T. lillianense. His T. rotatile and T. multicostatum represent the slenderer and more weakly ornamented end of the variation series, and his T. stantoni represents the more robust and more strong- ly ornamented end. The smallest complete whorl examined by me (pl. 2, figs. 18-20) has a diameter of 4.8 mm. At this size, the whorl section is broader than high, with flattened flanks and broadly rounded venter. Ornament consists of weak, prorsiradiate ribs and strong, nearly equal sized ventrolateral and siphonal tubercles. Nodate inner ventrolateral tubercles are already present at the beginning of this whorl at a diameter of about 2.2 mm. Outer ventrolateral and siphonal tubercles arise at a diameter of 3.5 mm. Larger growth stages have been well described by Stephenson (1955). In general, whorls become slenderer and more densely ribbed with growth (fig. 3), and the inner ventrolateral tubercles weaken with growth and disappear on the body chamber. 30 o _ ® © © < o g 20 |- @ 44 s o ° & o e s < o o -e o o a s e ® a ® .e o { 10 |- o __] & t o | | | | | | | 10 20 30 40 50 60 70 80 90 DIAMETER, IN MILLIMETERS FiGuRE 3.-Scatter diagram showing number of ribs per half whorl of 25 specimens of Tarrantoceras sellardsi (Adkins) from the Eagle Ford Group from USGS Mesozoic localities 11740, 14583, 24510, and D12626 in Tarrant and Johnson Counties, Tex. The diamond encloses the rib count of the holotype from Williamson County, Tex. GENUS TARRANTOCERAS STEPHENSON 7 The species is dimorphic with adults ranging from about 40 to 90 mm in diameter. Smaller adults (micro- conchs) retain the flattened, tuberculated venter to the aperture (pl. 1, figs. 1-3; pl. 2, figs. 9-11), whereas the larger ones (macroconchs) lose the flattening and tuber- culation (pl. 1, figs. 10-13; pl. 2, figs. 27, 28). The aper- ture is normal, and none of the specimens has crowded or reduced ribbing at the aperture. Body chambers oc- cupy one-half to two-thirds of a whorl (pl. 1, figs. 2, 7, 11; pl. 2, figs. 8, 10, 22, 27). The suture is fairly simple with deep, narrow exter- nal lobe, very broad bifid first lateral saddle, and much narrower and short bifid lateral lobe (fig. 4). Occurrence.-The holotype is from the Tarrant For- mation of the Eagle Ford Group near Round Rock, Williamson County, Tex. A fragment of Acanthoceras amphibolum Morrow is in the matrix. This ammonite also occurs with Tarrantoceras sellardsi in the collec- tions from the limestone concretions in the Fort Worth-Dallas area (fig. 2). Other fossils in these con- cretions include JInoceramus arvanus Stephenson, Pseudomelania? basicostata Stephenson, Lispodesthes lirata Stephenson, Puzosia sp., Desmoceras (Pseudouhli- gella) aff. D. japonicum Yabe, Acanthoceras johnsonanum Stephenson, Cunningtoniceras lonsdalei (Adkins), Borissiakoceras orbiculatum Stephenson, Anisoceras plicatile (J. Sowerby), and Turrilites acutus americanus Cobban and Scott. Tarrantoceras sellardsi occurs farther west in Trans-Pecos Texas (fig. 1, locs. 63-65), where crushed fragments are present in the U.S. Geological Survey's collections from the lower part of the Chispa Summit Formation. A fragment of a very large specimen recorded as Pseudocalycoceras cf. P. har pax (Stoliczka) by Hook and Cobban (1983, p. 51) from the flaggy limestone member of the Boquillas Lime- stone is probably an unusually large 7. sellardsi. In the El Paso area (fig. 1, loc. 62), the species occurs at the base of the Boquillas Formation, where 7T. sellardsi was reported as T. rotatile (Strain, 1976, p. 82). Here the species is associated with many of the same mollusks found in the Fort Worth-Dallas area and also with Ostrea beloiti Logan and Pseudacompsoceras landisi Cobban. Tarrantoceras sellardsi, recorded as T. rotatile (Cobban, 1977, p. 23), occurs at many localities in New Mexico, especially as flattened fragments in the Man- cos Shale (fig. 1, locs. 45, 46) or in thin beds of calcarenitic limestone in the Mancos Shale (fig. 1, loc. 47). Better preserved specimens occur in limestone concretions in the Clay Mesa Tongue of the Mancos Shale and in sandy concretions in the Paguate Tongue of the Dakota Sandstone in west-central New Mexico (Cobban, 1977, pl. 6, figs. 8-10, 28, 29; pl. 11, figs. 7, 8, 11-16). In southeastern Colorado, T. sellardsi is found in skeletal limestone near the top of the Graneros Shale FIGURE 4.-External sutures of Tarrantoceras sellardsi (Adkins) from near the base of the Eagle Ford Group at USGS Mesozoic locality D12626 (fig. 2). A, Hypotype USNM 400771. B, Hypotype USNM 400772. C, Hypotype USNM 400760 (pl. 1, figs. 4-7). (Cobban and Scott, 1972, p. 7, 65, pl. 10, figs. 1-11; text fig. 25). Here the species, recorded as T. rotatile, is associated with Inoceramus rutherfordi Warren, Ostrea beloiti Logan, Desmoceras (Pseudouhligella) sp., Acan- thoceras amphibolum Morrow, Borissiakoceras sp., and Turrilites acutus americanus Cobban and Scott. North of Colorado, T. sellardsi becomes scarce, and only an occasional specimen has been collected from limestone concretions near the top of the Belle Fourche Shale in eastern Wyoming (fig. 1, loc. 19). Types.-Hypotypes USNM 400759-400772. Tarrantoceras flexicostatum Cobban, n. sp. Plate 2, figures 23-26; text figure 5 Diagnosis.-A medium-sized, very compressed, high- whorled species ornamented by closely spaced, narrow, flexuous ribs. Umbilical tubercles are conspicuous, 8 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. FIGURE 5.-Inner part of the seventh-from-last external suture at a diameter of about 31 mm of Tarrantoceras flexicostatum Cobban, n. sp., from the lower part of the Cody Shale at USGS Mesozoic locality 21850 in Big Horn County, Wyo. (fig. 1, loc. 6). Holotype USNM 400773 (pl. 2, figs. 25, 26). whereas ventrolateral and siphonal tubercles are small and weak. Description.-The holotype (pl. 2, figs. 25, 26) is a complete adult 80.8 mm in diameter with an umbilical width of 28.0 mm and an umbilical ratio of 0.35. Whorl is subrectangular in cross section, with the greatest width low on the flattened flank. The venter is narrow, flattened to rounded, and bordered to the aperture by small, clavate outer ventrolateral tubercles. A narrow- ly rounded umbilical shoulder merges evenly into a ver- tical umbilical wall. Inner whorls are crushed, whereas the body chamber, which occupies about three-fourths of a whorl, is undeformed. Ribs are numerous, flexuous, mostly prorsiradiate, and narrower than the inter- spaces. Longer or primary ribs rise singly or in pairs from nodate to bullate tubercles low on the umbilical shoulder. Shorter or secondary ribs arise at midflank or a little lower. Primary and secondary ribs tend to alternate, and all ribs curve forward a little on the ven- trolateral shoulder to the position of the inner ven- trolateral tubercle, where they straighten and cross the venter transversely. Forty-nine ribs are present on the last complete whorl. Faint inner ventrolateral tubercles disappear at the end of the phragmocone, but small, clavate outer ventrolateral tubercles and siphonal tubercles persist to the aperture. The shape of the aper- ture follows the form of the ribs. Only the inner part of the suture is visible; it has a broad, bifid lateral lobe (fig. 5) like that on most specimens of Tarrantoceras sellardsi (fig. 4). The species is apparently dimorphic. A paratype has a diameter of only 60 mm (pl. 2, figs. 23, 24). Like the holotype, the body chamber is ornamented by numerous narrow, flexuous ribs that bear small outer ventrolateral and siphonal tubercles to the aperture. The younger half of the penultimate whorl is almost smooth, whereas it is well ribbed on the holotype. Remarks.-Tarrantoceras flexicostatum differs from T. sellardsi chiefly in its more compressed whorls and in its more flexuous ribbing. Eucalycoceras rowei (Spath), as figured by Kennedy (1971, p. 83, pl. 49, figs. 2-7; pl. 50, figs. 3, 4a, b, 6a, b, 7a, b), resembles T. flex- icostatum in its dense ribbing, some of which is flex- uous, but the ribs on E. rowe? do not curve forward on the ventrolateral shoulder, and the suture (Kennedy, 1971, pl. 49, figs. 4b, 6b) has the long, narrow lateral lobe typical of Eucalycoceras. Occurrence.-The types are from silty limestone con- cretions in the lower part- of the Cody Shale at USGS Mesozoic locality 21850 in the SE% sec. 9, T. 53 N., R. 92 W., Big Horn County, Wyo. (fig. 1, loc. 6). Associated fossils include Inoceramus prefragilis stephensoni Kauffman and Powell, Dunveganoceras pondi Haas, Calycoceras canitaurinum (Haas), Metoicoceras praecox Haas, and Idiohamites n. sp. Tarrantoceras flexicostatum is an uncommon fossil. Some poorly preserved fragments from the base of the Greenhorn Formation in the Black Hills area of western South Dakota may represent this species (fig. 1, loc. 9). Types.-Holotype USNM 400773, paratype USNM 400774. Genus EUCALYCOCERAS Spath, 1923, p. 144 Type species.-Ammonites pentagonus Jukes- Browne (in Jukes-Browne and Hill, 1896, p. 156, pl. 5, figs. 1, la). Diagnosis.-Medium-sized, generally compressed, somewhat evolute to somewhat involute ammonites that have, at most growth stages, flattened flanks and venter ornamented by dense tuberculate ribs that cross the venter transversely. Ribs tend to be alternately long and short. The long (primary) ribs originate from con- spicuous tubercles located low on the umbilical shoulder or even projected into the umbilicus. At most growth stages, all ribs bear small inner and outer ventrolateral and siphonal tubercles. Ribs on the younger part of the body chamber may become flattened, have steep adoral faces and more sloping adapical faces, and lack all tubercles except the umbilical ones. Suture is charac- terized by a long, narrow lateral lobe and broad first lateral saddle (fig. 6). Remarks.-According to Kennedy (1971, p. 81), the genus has its origin in a group of compressed middle Cenomanian Calycoceras. The few American specimens may be migrants from other regions. GENUS EUCALYCOCERAS SPATH 9 Occurrence. is widely distributed in rocks of middle to late Cenomanian age in Europe, Africa, Madagascar, North America, Japan, and India. Eucalycoceras pentagonum (Jukes-Browne) Plate 3; text figures 6, 7 1864. 1896. Ammonites harpax Stoliczka, p. 72 (pars), pl. 38, fig. 2, 2a. Ammonites pentagonus Jukes-Browne, in Jukes-Browne and Hill, p. 156, pl. 5, fig. 1, la. Acanthoceras pentagonum (Jukes-Browne). Kossmat, p. 14, pl. 4, fig. Sa-c. Acanthoceras pentagonum (Jukes-Browne and Hill). Choffat, p. 71, pl. 4, fig. 4a-c; pl. 6, figs. 3a, b, 4. Acanthoceras pentagonum (Jukes-Browne and Hill). Pervinguigre, p. 271. Eucalycoceras pentagonum (Jukes-Browne). Spath, p. 144. Acanthoceras pentagonum (Jukes-Browne and Hill). Diener, p. 164. Eucalycoceras pentagonum (Jukes-Browne). Wright and Wright, p. 26. Eucalycoceras pentagonum (Jukes-Browne and Hill). Collignon, p. 138, pl. 370, figs. 1610, 1611. Eucalycoceras aff. pentagonum Jukes-Browne and Hill. Col- lignon, p. 12. Eucalycoceras pentagonum (Jukes-Browne). Il'in, p. 13, pl. 2, fig. 2a, b; pl. 3, fig. 1. Eucalycoceras pentagonum (Jukes-Browne). Kennedy, p. 81, pl. 48, figs. 1-6; pl. 49, fig. la-c. Eucalycoceras (Eucalycoceras) pentagonum (Jukes-Browne and Hill). Thomel, p. 83, pl. 28, figs. 1, 10. Eucalycoceras pentagonum (Jukes-Browne and Hill). Pop and Szasz, p. 189, pl. 10, fig. la, b; pl. 11, fig. 1. Eucalycoceras pentagonum (Jukes-Browne). Matsumoto, p. 106, fig. 1a, b. 1897. 1898. 1907. 1923. 1925. 1951. 1964. 1965. 1970. 1971. 1972. 1973. 1975. 1976 [1978]. Eucalycoceras pentagonum (Jukes-Browne). Kennedy and Hancock, pl. 11, fig. Ta, b. 1981 [1983]. Eucalycoceras pentagonum saharense Collignon and Roman, p. 101, pl. 14, fig. la, b. | / Diagnosis.-A large, compressed to stout species that has numerous closely spaced ribs that may be effaced at midflank on the body chamber. Ventrolateral and siphonal tubercles disappear on older part of body chamber, and ribs become flattened with steep adoral faces on younger part of body chamber. Description.-The species has been well described and illustrated by Kennedy (1971, p. 81) who observed that the holotype is a poorly preserved, phosphatic internal mold of which nearly one-half of the last whorl is body chamber. Jukes-Browne (in Jukes-Browne and Hill, 1896, p. 156) gave its diameter as 4 in. (102 mm). Ken- nedy noted that the holotype is moderately involute, that the whorl section of the phragmocone is com- pressed with flat sides and high, arched venter, and that the venter becomes broadly rounded on the body chamber. Ribs are numerous, fairly straight, and mostly rectiradiate. Long ribs begin from conspicuous umbilical tubercles, and short ones arise near midflank. All ribs on the phragmocone bear slightly clavate inner ven- trolateral tubercles, slightly clavate but stronger outer ones, and slightly bullate siphonal ones. All ventro- lateral and siphonal tubercles disappear on the older part of the body chamber, and ribbing is effaced there at midflank. On the younger part of the body chamber, ribs become flattened and wide on crossing the broad- ly rounded venter. The ribs are wider than the inter- spaces and have steep adoral faces. The effaced ribbing at midflank is shown very well on two large specimens illustrated by Kennedy (1971, pl. 48, fig. 1b; pl. 49, fig. 1c). Some specimens of com- parable size from other regions do not show this (Kossmat, 1897, pl. 4, fig. 3a; Thomel, 1972, pl. 28, figs. 1, 10). A few incomplete specimens from the base of the Bridge Creek Member of the Greenhorn Limestone of L_ BF ___ FIGURE 6.-Most of external suture of Eucalycoceras pentagonum (Jukes-Browne) at a diameter of about 80 mm from a phragmocone from bed C of the Cenomanian limestone at Whitlands along the Devon coast between Pinhay Bay and Humble Point, England. Hypotype USNM 400910. Specimen furnished by C.W. Wright and has his number 22430. 10 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. southeastern Colorado are probably referable to Eucalycoceras pentagonum (pl. 3, figs. 4, 5, 7, 8, 10-13). The Bridge Creek specimens have body chambers with broad, flattened ribs that have steep adoral faces and more sloping adapical ones. Conspicuous, clavate, near- ly equal sized inner and outer ventrolateral tubercles and nodate siphonal tubercles are present on the older part of the body chambers. Primary ribs begin from bullate tubercles well down on the umbilical shoulder, and secondaries arise low on the flank. In most cases, long and short ribs alternate. Ventrolateral and siphonal tubercles weaken and disappear on the younger part of the body chamber, where the ribs become flattened and slablike. On one fragment, ribs become narrow and more closely spaced near the aperture (pl. 3, figs. 4, 5). The specimens from the Bridge Creek Member differ from the holotype in their smaller size and in their lack of conspicuous effaced ribbing at midflank. In their size and general appearance, the Greenhorn specimens resemble forms assigned to E. pentagonum by Il'in (1970, p. 13, pl. 2, fig. 2a, b; pl. 3, fig. 1) and by Col- lignon and Roman (1981, p. 101, pl. 14, fig. la, b). Crushed fragments from a thin bed of calcarenitic limestone in the Greenhorn Formation in western South Dakota (fig. 1, loc. 8) are apparently referable to E. pen- tagonum. One of the larger specimens has effaced rib- bing on the older part of the body chamber (pl. 3, fig. 6). Only part of the phragmocone is preserved, but it has a suture much like that of E. pentagonum (fig. 7A). Other fragments from this bed may represent micro- conchs of E. pentagonum. The younger parts of body chambers lack tubercles, and the ribs are flattened with steep adoral faces (pl. 3, fig. 9). Similar specimens oc- cur at the base of the Bridge Creek Member of the Greenhorn Limestone in southeastern Colorado (pl. 3, figs. 7, 8). Small specimens from limestone concretions in the lower part of the Colorado Formation of south- western New Mexico (fig. 1, loc. 61) may also be inter- preted as microconchs (pl. 3, figs. 1-3). The last two septa of one specimen have an estimated diameter of 34.3 mm. Another specimen, which has an estimated diameter of 43 mm at the base of the body chamber, has flank ribbing effaced on the older part of the body chamber (pl. 3, fig. 3). Occurrence.-The holotype of Eucalycoceras pen- tagonum came from the remanié phosphatic fauna of bed C of the Cenomanian limestone near Lyme Regis in Dorset, England (Kennedy, 1971, p. 81), from the Calycoceras guerangeri Zone of late Cenomanian age. The specimens compared to E. pertagonum from southeastern Colorado are from the basal bed of limestone in the Bridge Creek Member of the Greenhorn Limestone at USGS Mesozoic localities 18686 and 22899 on the east flank of the Model anticline (fig. 1, \ \ L | ) L 5 mm __| A B FIGURE 7.-External sutures of Eucalycoceras pentagonum (Jukes- Browne). A, Hypotype USNM 400778 (pl. 3, fig. 6) at a diameter of about 43 mm, from the Greenhorn Formation at USGS Mesozoic locality 23060 near Belle Fourche, S. Dak. (fig. 1, loc. 8). B, Hypo- type USNM 400775 (pl. 3, figs. 1, 2) at a diameter of 32.3 mm, from the Bridge Creek Limestone Member of the Colorado Formation at USGS Mesozoic locality D11483 in the Cooke Range in south- western New Mexico (fig. 1, loc. 61). locs. 33, 34; Bass and others, 1947). A large collection of Calycoceras naviculare (Mantell) was described from this bed at this locality, as were specimens of More- manoceras scotti (Moreman) and Anisoceras plicatile (J. Sowerby) (Cobban, 1971). Fragments of specimens refer- able to E. pentagonum from the Black Hills area came from the Greenhorn Formation at USGS Mesozoic locality 23060 (fig. 1, loc. 8). These fragments were not associated with other fossils. The specimens from south- western New Mexico (fig. 1, loc. 61) were found with Euomphaloceras (Euomphaloceras) euomphalum (Sharpe), Metoicoceras sp., Vascoceras diartianum (d'Orbigny), and other small ammonites. Types.-Hypotypes USNM 400775-400781, 400910. Eucalycoceras templetonense Cobban, n. sp. Plate 4; text figure 8 1975. Eucalycoceras sp. A. Hattin, Kansas Geological Survey Bulletin 209, pl. 2, figs. S-U. Diagnosis.-A large, moderately robust species that has fairly sparse ribbing. Innermost whorls tend to have smooth or nearly smooth flanks. GENUS EUCALYCOCERAS SPATH 11 Description.-The holotype (pl. 4, figs. 2-4) is a com- plete, uncrushed adult from the Templeton Member of the Woodbine Formation of north Texas. The specimen is 83 mm in diameter, the umbilical width is 25 mm, and the umbilical ratio is 0.30. Whorls are higher than wide with flattened flanks, narrow flattened to rounded venter, narrowly rounded umbilical shoulder, and ver- tical umbilical wall. The body chamber occupies three- fourths of a whorl. The parts of the inner whorls expos- ed are almost smooth except for small, closely spaced, nodate tubercles on the umbilical margin. Eighteen of these umbilical tubercles occur on the last whorl of the phragmocone and 12 are present on the three-fourths of a whorl of body chamber. Other ornament consists of primary and secondary ribs that have inner and outer ventrolateral and siphonal tubercles on all but the youngest part of the body chamber. Primary ribs originate singly or in pairs from the umbilical tubercles, and secondary ribs arise low on the flank. Most ribs alternate in length. On the last septate whorl, ribs are weak and prorsiradiate at first and then gradually become stronger, slightly flexuous, and rectiradiate. Each bears a weak, bullate inner ventrolateral tuber- cle; a much stronger, nodate outer ventrolateral tuber- cle that rises above and bounds the narrow, flat venter; and a weak, clavate siphonal tubercle. Ribs bend for- ward a little at the site of the inner ventrolateral tuber- cle and then cross the venter transversely. On the body chamber, ribs remain strong and, at midflank, they curve back to the position of the inner ventrolateral tubercle and then straighten slightly and cross the venter transversely. On the outer part of the flank and on the venter, the ribs become broad and flatten a lit- tle. There are 40 ribs on the last complete whorl. Inner ventrolateral tubercles disappear on the older part of the body chamber, and the outer and siphonal ones weaken and disappear about at the middle, where the venter becomes rounded. The aperture is normal. The suture has a deep, moderately narrow external lobe and a smaller, narrow, bifid lateral lobe (fig. 8). A much broader, asymmetrical saddle separates these lobes. In its general form and complexity, the suture is much like that of E. pentagonum (fig. 6). Innermost whorls are best observed on a crushed paratype (pl. 4, fig. 10). At a diameter of 13.5 mm, the whorls are compressed with flat flanks, narrowly rounded umbilical shoulder, and vertical umbilical wall; the venter is not preserved. Ornament is absent except for conspicuous nodate tubercles low on the umbilical shoulder; these number eight per whorl. Larger inner whorls can be seen on a phragmocone (pl. 4, figs. 5, 6) of 36 mm in diameter from almost the same locality. This specimen has an umbilical ratio of 0.33 and a whorl section slightly higher than wide, with [ 5 mm | FiGURE 8.-Most of the fourth-from-last external suture at a diameter of about 40 mm of the holotype (USNM 400782) of Eucalycoceras templetonense Cobban, n. sp., from the Templeton Member of the Woodbine Formation at USGS Mesozoic locality 20314, along Templeton Branch of Cornelius Creek 2% mi north of Bells, Grayson County, Tex. flat flanks and rounded venter. The ribs are straight, rectiradiate to prorsiradiate, number 31 per whorl, and cross the venter transversely. Umbilical tubercles are bullate to nodate and number 18 per whorl. Inner ven- trolateral tubercles are nodate and somewhat smaller than the clavate outer ones. Siphonal clavi are smaller and lower than the outer ventrolateral ones on the older part of the outer whorl and about equal in size and height on the younger part. The species attains a large size. Crushed specimens have diameters of 100-110 mm. Remarks.-This form differs from other described species in its smooth-flanked inner whorls and in its thick ribs that cross the venter of the body chamber. A crushed fragment from eastern Wyoming (pl. 4, fig. 1) may have had spinose umbilical tubercles projected into the umbilicus like those of E. gothicum (Kossmat, 1895, p. 69, pl. 25, fig. Sa-c). Occurrence.-The types are from silty limestone con- cretions 6.0-6.7 m (20-22 ft) above the base of the Templeton Member of the Woodbine Formation at USGS Mesozoic locality 20314 along Templeton Branch of Cornelius Creek 4.4 km (2% mi) north of the center of Bells, Grayson County, Tex. (Stephenson, 1952, p. 41, loc. 164; pl. 1, loc. 164). The holotype and a paratype (USNM 400788) were collected by Frank Crane, Fort Worth, Tex., who gave them to the late James P. Con- lin of Fort Worth. Mr. Conlin kindly donated his remarkable collection of fossils to the U.S. Geological Survey. Stephenson described many molluscan fossils from the Templeton Branch locality and listed 29 species from there (Stephenson, 1952, locality 164 on unnumbered table in pocket). Ammonites described by Stephenson as Mammites? bellsanus and Metoicoceras crassicostae have their type localities here. Stephen- son's Mammites? bellsanus is probably better assigned 12 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. to Plesiacanthoceras owing to the resemblance of its phragmocone to that of P. wyomingense (Reagan). Eucalycoceras templetonense may also be present far- ther south in Dallas County, where a part of a body chamber (unfigured paratype 400911) that was at least 100 mm in diameter was found in the Six Flags Lime- stone Member of the Woodbine Formation of Norton (1965). This specimen, from the J.P. Conlin collections, has Conlin's catalog number 14988. In the Western Interior region, crushed fragments of E. templetonense have been found in the Belle Fourche Shale on the west flank of the Black Hills uplift (fig. 1, loc. 11), in the Frontier Formation of east-central Wyoming (fig. 1, loc. 16) and south-central Wyoming (fig. 1, loc. 20), in the Lincoln Member of the Greenhorn Limestone of northeastern New Mexico (fig. 1, locs. 41, 42) and south-central Kansas (Hattin, 1975, pl. 2, figs. S-U), and in the lower part of the Mancos Shale in west- central New Mexico (fig. 1, loc. 43) and south-central New Mexico (fig. 1, loc. 48). Inoceramus prefragilis Stephenson is present at most of these localities. Plesiacanthoceras wyomingense (Reagan) occurs at lo- cality 20 and Dunveganoceras pondi Haas at locality 11. Types.-Holotype USNM 400782, paratypes USNM 400783-400788, 400911. Genus PSEUDOCALYCOCERAS Thomel, 1969, p. 650 Type species.-By original designation, Ammonites harpax Stoliczka, 1864, p. 72 (part), pl. 39, fig. 1, la, 1b. Diagnosis.-Stout, fairly evolute ammonites that have strong, thick ribs, which are usually conspicuously rur- siradiate on the body chamber. Ribs generally alternate between long and short. Long ribs begin from strong, nodate tubercles or bullae low on the umbilical shoulder; shorter ones arise near midflank. All ribs cross the venter transversely, and all bear inner and outer ventrolateral and siphonal tubercles on the inner whorls and on the older part of the outer whorl. The suture is like that of Eucalycoceras. The scope of the genus has been treated in detail by Cobban and Scott (1972, p. 63). Pseudocaly- coceras seems to have been derived from Eucalycoceras, from which it differs in its more robust shell, its sparser ribbing, and its conspicuous rursiradiate ribbing. Remarks.-On the holotype of the type species, Am- monites harpax Stoliczka (1864, pl. 39, fig. 1, 1a, 1b), um- bilical bullae on the younger part of the body chamber twist back initially and then curve forward before merg- ing into ribs that bend forward on the lower and middle parts of the flank and finally bend back on the outer part of the flank. This results in a distinctive S shape for the ribs. Stoliczka's illustration is a good drawing, and Wright and Kennedy (1981, text fig. 14A, B) have recently shown good photographs of the type. Most American specimens of Pseudocalycoceras lack twisted umbilical bullae and have straighter ribs. Occurrence.-The genus is known only from upper Cen- omanian rocks. Wright and Kennedy (1981, p. 37) listed it from England, France, Spain, Romania, Syria, Israel, Madagascar, Angola, India, and the United States. In addition, the genus occurs in Tunisia and Japan. Pseudocalycoceras angolaense (Spath) Plate 5; plate 6, figures 1, 2, 13, 14, 18, 19; text figure 10 1920. Acanthoceras rhotomagense Brongniart. Taubenhaus, p. 12, pl. 1, fig. 3a, b. Acanthoceras sp. A. Moreman, p. 95, pl. 15, fig. 2. Acanthoceras lyelli (Deshayes) Leymerie. Douvillé, p. 31, pl. 1, fig. la, b; text fig. 1. Protacanthoceras angolaense Spath, p. 316. 1927. 1931. 1931. 1940. Protacanthoceras angolaense Spath. Basse, p. 448, pl. 6, fig. 3a, b. 1942. Eucalycoceras dentonense Moreman, p. 205, pl. 33, figs. 4, 5; text fig. 2k. 1942. Eucalycoceras indianense Moreman, p. 206, pl. 33, figs. 9, 10; text fig. 21. 1942. Eucalycoceras lewisvillense Moreman, p. 206, pl. 33, figs. 6, 7; text fig. 2n, u. 1959. Eucalycoceras dentonense Moreman. Matsumoto, p. 97, text fig. 51. 1959. Eucalycoceras indianense Moreman. Matsumoto, p. 98. 1962. Protacanthoceras angolaense Spath. Avnimelech and Shoresh, p. 531. 1963. Eucalycoceras underwoodi Powell, p. 315, pl. 31, fig. 17; text fig. Je, g. 1969. Pseudocalycoceras (Neocalycoceras) angolaense (Spath). Thomel, p. 651. Eucalycoceras (Proeucalycoceras) dentonense Moreman. Thomel, p. 650. Eucalycoceras (Proeucalycoceras) lewisvillense Moreman. Thomel, p. 650. "Protacanthoceras" angolaense Spath. Kennedy, p. 115. [1973]. Pseudocalycoceras dentonense (Moreman). Cobban and Scott, p. 63, pl. 13, figs. 11-29; pl. 15, figs. 1-7, 10-13. Pseudocalycoceras (Haugiceras) angolaense (Spath). Thomel, p. 97. Pseudocalycoceras sp. aff. P. dentonense (Moreman). Mat- sumoto and Kawano, p. 13, pl. 1, fig. la-e; text fig. 3. Pseudocalycoceras dentonense (Moreman). Hattin, pl. 6, figs. F, G. [1978]. Pseudocalycoceras angolaense (Spath). Cooper, pl. 4, figs. 1, j. Pseudocalycoceras dentonense (Moreman). Hattin, text fig. 5 (13). Pseudocalycoceras dentonense (Moreman). Kauffman, pl. 18, figs. 5, 6. Pseudocalycoceras angolaense (Spath). Cooper, p. 96, text figs. 4A-C, H-K, 61, J, 10F, G, 14A, 18E, F, 19A, B, 23-25, 26F-K. Pseudocalycoceras dentonense (Moreman). Hattin and Siemers, text fig. 6 (13). Pseudocalycoceras dentonense (Moreman), Wright and Ken- nedy, p. 37, pl. 5, fig. 4a-c; pl. 6, figs. Sa-e, 6a, b, Ta, b; text figs. 15A, B, E-H, 198, T. 1969. 1969. 1971. 1972 1972. 1975. 1975. 1976 1977. 1977. 1978. 1978. 1981. GENUS PSEUDOCALYCOCERAS THOMEL Diagnosis.-A stout to moderately compressed, small to medium-sized, highly variable species that has straight to flexuous, usually rursiradiate ribs that have steeper adoral faces than adapical ones on the younger part of the body chambers. Outer ventrolateral tubercles are closer to the siphonal ones than to the in- ner ventrolateral ones. Description.-The well-preserved specimen from the upper Cenomanian of Angola that was referred to Acan- thoceras lyelli Leymerie by Douvillé (1931, p. 31, pl. 1, fig. la, b; text fig. 1) and then assigned to the new species Protacanthoceras angolaense by Spath (1931, p. 316) represents the sparsely ribbed, robust end of the variation series characteristic of this species. The specimen is a complete adult 75 mm in diameter with an umbilical width of 28 mm and an umbilical ratio of 0.37. Douvillé described the body chamber as having a circular section. Ribs on the body chamber are straight, slightly rursiradiate, barlike, but narrower than the interspaces; ribs are mostly long and number 12 on the last half whorl and 13 on the preceding half whorl. Longer ribs begin from umbilical tubercles, and all ribs cross the venter transversely as thickened ridges. Ribs on most of the outer whorl have small, nodate to clavate, inner ventrolateral tubercles and slightly larger, clavate outer ventrolateral and siphonal tubercles. Cooper (1978) illustrated several specimens that show the ornament on the inner whorls. Ribs are more numerous on some of these whorls, and they are flexuous on some whorls. The smallest whorls tend to be weakly ornamented or nearly smooth. The suture of P. angolaense has a broad, bifid first lateral saddle and a narrow, long, bifid lateral lobe (Douvillé, 1931, text fig. 1; Cooper, 1978, text fig. 25). 13 American specimens of Pseudocalycoceras angolaense are usually slenderer and more costate than the Douvillé's Angolan specimen. Cobban and Scott (1972, p. 63) combined as one variable species (P. dentonense) the forms described by Moreman (1942) as the new species Eucalycoceras dentonense, E. indianense, and E. lewisvillense. Larger collections now at hand and the investigations of Cooper (1978) reveal that all of More- man's species can be referred to P. angolaense. One of the best collections of P. angolaense is from limestone concretions in the Colorado Formation at USGS Mesozoic locality D11529 in southwestern New Mexico (fig. 1, loc. 54). Thirteen specimens are suitable for rib counts, which range from 10 to 20 per half whorl (fig. 9). Early whorls can be observed on several specimens. The innermost whorls of one specimen (pl. 5, figs. 8, 9) have ventral ornament at a diameter of only 3 mm. On some specimens, the outer ventrolateral tubercles appear first, but on others the inner and outer ventrolateral and siphonal tubercles arise together. Ven- tral ribs usually form a little later. One individual already had inner ventrolateral tubercles at a diameter of 5.5 mm, whereas the outer ones formed at 7.0 mm, the umbilical ones at 9.4 mm, and the siphonal tubercles and ventral ribs at 10.4 mm. As the shell enlarges, the ventral ribs and tubercles tend to occur in groups of three or four separated by smooth areas (pl. 5, figs. 3, 4, 12-15). This grouping of ventral ornament may per- sist to a diameter of as much as 33 mm. Larger specimens show considerable variation from coarsely or- namented forms (pl. 5, figs. 10, 11, 16, 17) somewhat resembling P. haugi (Pervinquigre, 1907, p. 270, pl. 14. fig. 1a, b) to finer and more densely ribbed forms (pl. 5, figs. 23, 24). The largest adult from southwest New x o \ o a o o x-----!/@\--x x g o < bilhd o C u T 10 -e -] S ee C LL & W co fra o | I | | | | 10 20 30 40 50 60 70 80 DIAMETER, IN MILLIMETERS FIGURE 9.-Scatter diagram showing number of ribs per half whorl (.) of 13 specimens of Pseudocalycoceras angolaense (Spath) from a bed of limestone concretions in the Colorado Formation at USGS Mesozoic locality D11529 in southwestern New Mexico (fig. 1, loc. 54) as well as rib counts (X) on three examples of P. angolaense from Angola (Cooper, 1978, text figs. 23, 26). Lines connect counts on the same specimen. 14 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. Mexico has a diameter of 93.3 mm and an umbilical ratio of 0.32 (pl. 5, figs. 25-27). Ventrolateral and siphonal clavi disappear at about the end of the older half of the body chamber. The ribs then become asymmetric with the steep side forward as in Eucalycoceras Spath. The suture (fig. 10) of specimens from southwestern New Mexico is fairly simple with broad, bifid E/L sad- dle, and narrower, bifid L. The holotype of P. harpax (Stoliczka, 1864, pl. 39, fig. 1, la, 1b), on which Pseudocalycoceras was based, has a little more digitate suture. Pseudocalycoceras angolaense is dimorphic. Cobban and Scott (1972) illustrated several adults of different sizes. On all adults, ventrolateral and siphonal tubercles weaken and disappear on the younger part of the body chamber. Occurrence.-In Angola, Pseudocalycoceras angolaense is associated with a varied ammonite fauna that includes Calycoceras naviculare (Mantell), Euom- phaloceras (Kanabiceras) septemseriatum (Cragin), Metoicoceras geslinianum (d'Orbigny), and Scipono- ceras gracile (Shumard). The same fossils are found with P. angolaense in southwestern New Mexico and in southeastern Colorado. In the Western Interior region, P. angolaense occurs in the basal part of the Bridge Creek Member of the Greenhorn Limestone in eastern Colorado, northeastern New Mexico, and southwestern Kansas; in the middle of the Hartland Member of the Greenhorn Limestone in central Kansas; in the Bridge Creek Limestone Beds in the lower part of the Mancos Shale in south-central New Mexico; in the Bridge Creek Limestone Member of the Colorado Formation in southwestern New Mexico; in the lower part of the Tropic Shale in southern Utah; and in the lower part of the Mancos Shale in northeastern Arizona. In north- eastern Texas, P. angolaense occurs in the Britton For- mation, and the species is found in the Boquillas Limestone in Terrell and Val Verde Counties in south- western Texas. In Trans-Pecos Texas, P. angolaense was found in the lower part of the Chispa Summit For- mation at Chispa Summit (fig. 1, loc. 66). Associated fossils include EFuomphaloceras (Kanabiceras) septem- seriatum, Metoicoceras geslinianum, Sciponoceras gracile, and Worthoceras gibbosum Moreman. Types.-Hypotypes USNM 400789-400802. Genus SUMITOMOCERAS Matsumoto, in Matsumoto and others, 1969, p. 280 Type species.-By original designation, Sumitomoceras faustum Matsumoto and Muramoto, in Matsumoto and others, 1969, p. 283, pl. 38, figs. 1-4; text fig. 8. Diagnosis.-Small to moderate-sized, compressed, fairly evolute ammonites with long and short ribs that | 5 mm | FIGURE 10.-External suture of Pseudocalycoceras angolaense (Spath) at a whorl height of 16.5 mm. Hypotype USNM 400792 from USGS Mesozoic locality D11529 (fig. 1, loc. 54) from the lower part of the Colorado Formation. cross the venter transversely. Long ribs begin from um- bilical tubercles; all ribs bear small inner and outer ven- trolateral tubercles in most growth stages, but these disappear near the beginning of the adult body chamber. Weak siphonal tubercles are present on the earliest whorls. A few weak, shallow constrictions are present on the larger whorls. Sutures are like those of Eucalycoceras with long, narrow, bifid lateral lobes. Remarks. (1978, p. 93) considered Sumitomo- ceras to be a junior subjective synonym of Tarran- toceras. However, Wright and Kennedy (1981, p. 38) believed that enough differences were present to justify regarding Sumitomoceras as a subgenus of Tarran- toceras. The very early loss of siphonal tubercles, the presence of constrictions, and the Eucalycoceras-like suture easily distinguishes Sumitomoceras from Tarran- toceras; they are treated herein as separate genera. Occurrence. -Sumitomoceras is known only from up- per Cenomanian rocks of England, France, the United States, Japan, and possibly India. In the United States, the genus has been found in Texas, Colorado, and Arizona. Sumitomoceras conlini Wright and Kennedy Plate 7, figures 1-15, 26-28; text figure 11 1981. Tarrantoceras (Sumitomoceras) conlini Wright and Kennedy, p. 39, text fig. 16A. Diagnosis.-A moderately large species that has whorls higher than wide with flattened flanks and numerous ribs. Description. -The holotype (pl. 7, figs. 26-28) was described briefly by Wright and Kennedy (1981, p. 39), although no dimensions were given. The specimen is a well-preserved adult that consists of most of the body GENUS SUMITOMOCERAS MATSUMOTO 15 chamber and part of the phragmocone. Because the venter at the adoral end of the body chamber is miss- ing, the diameter at that end of the specimen can only be estimated at 70 mm. Its umbilical width is 23.9 mm, and the umbilical ratio is 0.36. Whorls are much higher than wide with flattened flanks, well-rounded venter, narrowly rounded umbilical shoulder, and vertical um- bilical wall; greatest width is at the umbilical shoulder. Ribs are mostly rectiradiate and cross the venter transversely. Long ribs arise singly or in pairs from sharp, nodate umbilical tubercles located well down on the umbilical shoulder or even projected slightly into the umbilicus. Shorter ribs arise at midflank or slight- ly lower. Ribs number 33 on the last half whorl of the phragmocone and 31 on half of a whorl of the body chamber. Each rib on the outer whorl of the phragmocone supports a small nodate inner ven- trolateral tubercle and a similar-sized slightly clavate outer one. These tubercles weaken greatly on the older part of the body chamber, and the inner ones disappear there. The outer ones probably disappear on the younger part. Only part of the suture is visible, but it is much like that of a paratype of Sumitomoceras faustum Matsumoto and Muramoto (in Matsumoto and others, 1969, text fig. 8) in that the lateral lobe is long and narrow (fig. 11B). Remarks.-Sumitomoceras conlini is an uncommon fossil. Most specimens are from southern Texas and southwestern New Mexico. The species seems to be di- morphic with the holotype representing the macroconch. The diameter at the base of the body chamber of the type is 46 mm. Microconchs are best represented by several specimens from southwestern New Mexico (pl. 7, figs. 5-8, 11-13). These have diameters at the base of the body chamber of less than 30 mm. At these small sizes, ventrolateral tubercles weaken and disappear on the older part of the body chamber. Occurrence.-The holotype was collected by the late J.P. Conlin of Fort Worth, Tex., from the Britton For- mation on the east bank of a creek 0.15 mi west of the old Britton-Midlothian road 4 km (2% mi) south of Britton in Ellis County, Tex. Several specimens present in the collections of the U.S. Geological Survey are from the lower part of the Boquillas Limestone in Terrell and Val Verde Counties in southwest Texas (USGS D7443, D7466) and in Uvalde County farther east (USGS 15344). Associated ammonites include Sciponoceras gracile (Shumard), Pseudocalycoceras angolaense (Spath), and Metoicoceras geslinianum (d'Orbigny). In southwestern New Mexico, S. conlini occurs in lime- stone concretions in the Colorado Formation. The best locality (fig. 1, loc. 54) has a large variety of molluscan fossils that includes Inoceramus pictus J. de C. Sower- by, Exogyra levis Stephenson, Sciponoceras gracile l 2 mm ) FIGURE 11.-External sutures of Sumitomoceras conlini Wright and Kennedy. A, Hypotype USNM 400807 (pl. 7, figs. 9, 10) at a whorl height of 11.5 mm, from the Bridge Creek Limestone Member of the Colorado Formation at USGS Mesozoic locality D10196 (fig. 1, loc. 60). B, Holotype USNM 400803 (pl. 7, figs. 26-28) at a diameter of 20 mm, from the Britton Formation 4 km (2% mi) south of 'Britton, Ellis County, Tex. (Shumard), Worthoceras gibbosum Moreman, Euom- phaloceras (Kanabiceras) septemseriatum (Cragin), Pseudocalycoceras dentonense (Moreman), and Metoi- coceras geslinianum (d'Orbigny). A few specimens of S. conlini have also been found in the lower part of the Bridge Creek Member of the Greenhorn Limestone in southeastern Colorado (fig. 1, locs. 28, 34). Types.-Holotype USNM 400803, hypotypes USNM 400804-400809. Sumitomoceras bentonianum (Cragin) Plate 6, figures 3-12, 15-17; plate 7, figures 16-25; text figure 12 1893. 1931. 1932 1942. Pulchellia bentoniana Cragin, p. 239. Eucalycoceras bentonianum (Cragin). Adkins, p. 63. [1933]. Eucalycoceras bentonianum (Cragin). Adkins, p. 408. Eucalycoceras bentonianum (Cragin). Moreman, p. 207, text fig. 2E. Eucalycoceras bentonianum (Cragin). Adkins and Lozo, pl. 6, figs. 9, 10. 1951. 16 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. 1971. 1976 "Eucalycoceras" bentonianum (Cragin). Kennedy, p. 121. [1978]. Eucalycoceras bentonianum (Cragin). Young and Powell, fig. 2. Tarrantoceras bentonianum (Cragin). Cooper, p. 96. Tarrantoceras (Sumitomoceras) bentonianum (Cragin). Wright and Kennedy, p. 39. Tarrantoceras (Tarrantoceras) aff. lilianense (sic) Stephenson. Wright and Kennedy, p. 41, text fig. 15C, D. 1978. 1981. 1981. Diagnosis.-A moderately evolute species that has whorls somewhat higher than wide with sharp, mostly rectiradiate ribs separated by wider interspaces. Description.-The holotype (pl. 6, figs. 15-17) is almost half a whorl that appears to be about half liv- ing chamber and half phragmocone. Cragin (1893, p. 239) gave its diameter as about 57 mm. A plaster cast at hand has an umbilical ratio of 0.39. Inner whorls are not preserved. The half whorl has a subovate section with slightly flattened flanks, well-rounded venter, nar- rowly rounded umbilical shoulder, and steep umbilical wall. The greatest width is at the umbilical shoulder. Ribs, which number about 20 per half whorl, are rec- tiradiate, very narrow, sharp, and nearly straight. They are narrower than the interspaces and are both long and short. Long ribs begin from umbilical bullae or at the umbilical shoulder. Short ribs begin at midflank or lower. All ribs are highest where they cross the venter transversely. Ribs on the older end of the whorl have faint inner ventrolateral tubercles and stronger, nodate, outer ones. Siphonal tubercles are absent on the plaster cast and were not mentioned by Cragin. Moreman (1942, text fig. 2E) indicated the presence of siphonal tubercles in a cross-sectional drawing of the whorl, but this is probably an error. Wright and Kennedy (1981, text fig. 15C, D) illus- trated an interesting specimen assigned by them to Tar rantoceras. This specimen, from the Britton Formation in Ellis County, Tex., seems to me to be an aberrant form of S. bentonianum. The specimen consists of a complete adult body chamber 61.7 mm in diameter and most of the inner whorls. Umbilical width is 21.8 mm, and the umbilical ratio is 0.35. Diameter at the base of the body chamber is about 23 mm. All visible whorls are higher than wide with flattened flanks. Inner whorls have long and short ribs that are sparse, weak, and rec- tiradiate. The older half of the body chamber has strong, narrow, slightly flexuous, prorsiradiate ribs that are narrower than the interspaces. Long ribs begin from small, sharp bullae on the umbilical shoulder, and short ribs begin at midflank or lower. Most ribs have faint, bullate inner ventrolateral tubercles and larger, clavate outer ones. Ribs remain high and narrow where they cross the venter transversely. Ornament changes abruptly on the younger half of the body chamber. Here the ribs are flexuous, irregular in height, and nodeless; they are conspicuous only on the venter, where they are high, narrow, and flexed forward. The aperture is nor- mal and follows the trend of the ribbing. Several incomplete but well-preserved specimens have been found in southwestern New Mexico. One of the bet- ter specimens (pl. 6, figs. 3-5), from the Colorado For- mation at USGS Mesozoic locality D6842 (fig. 1, loc. 57), has a diameter of 42 mm, an umbilical width of 14.5 mm, and an umbilical ratio of 0.35. The specimen consists of half a whorl of body chamber and most of the last septate whorl. The diameter at the base of the body chamber is 28.4 mm. Whorls are robust and about as high as they are wide with flattened flanks. The venter on the body chamber is high and arched. Ribs are strong, narrow, and rectiradiate to prorsiradiate. Most ribs are long and usually arise in pairs from pointed nodate umbilical tubercles. The few short ribs arise near midflank. Ribs are flexuous, bend forward a little on the ventrolateral shoulder, and arch forward upon crossing the venter. Ribs are flattened a little on the venter of the phragmocone but, on the venter of the body chamber, ribs become narrow or even sharp. Nodate inner and outer ventrolateral tubercles are con- spicuous on the phragmocone. The outer ones weaken and disappear on the older part of the body chamber, whereas the inner ones disappear on some ribs and reap- pear on others. The complete external suture has a broad, bifid lateral saddle and long, narrow, bifid lateral lobe (fig. 12). Occurrence.-The holotype came from "Hackberry creek, Dallas county, in clay-ironstone concretions of the Eagle Ford shales" (Cragin, 1893, p. 240). Rocks at this locality in northeastern Texas are now assigned to the Britton Formation. Cragin's locality may be the same as locality 11 or 15 of Moreman (1942, p. 197). In having sparse, thin ribs, Sumitomoceras bentonianum resembles Calycoceras (Gentoniceras) leonense (Adkins) from the older Tarrant Formation farther south in Texas. This may have led Young and Powell (1976, FIGURE 12.-Last external suture at a diameter of 28.6 mm of Sumitomoceras bentonianum (Cragin) from the Bridge Creek Limestone Member of the Colorado Formation at USGS Mesozoic locality D6842, Luna County, N. Mex. (fig. 1, loc. 57). Hypotype USNM 400810 (pl. 6, figs. 3-5). GENUS NEOCARDIOCERAS SPATH 17 p. XXV.15-XXV.23) to regard S. bentonianum (as Eucalycoceras bentonianum) as a guide to rocks of C. leonense age. Specimens referable to S. bentonianum have been found at a few localities in the Western Interior region. The best localities are in southwestern New Mexico (fig. 1, locs. 57, 60), where the species occurs in limestone concretions in the lower part of the Colorado Forma- tion. Associated fossils include Sumitomoceras conlini, Euomphaloceras (Kanabiceras) septemseriatum, and Metoicoceras geslinianum. In southeastern Colorado, S. bentonianum occurs sparsely in the lower part of the Bridge Creek Member of the Greenhorn Limestone (fig. 1, locs. 33, 35). Types.-Hypotypes USNM 400810-400817. Genus NEOCARDIOCERAS Spath, 1926, p. 81 Type species.-Ammonites juddii Barrois and de Guerne, 1878, p. 46, pl. 1, figs. la, b, 2a, b. Diagnosis.-Neocardioceras is a fairly small, moder- ately evolute, generally compressed genus that usual- ly has numerous rectiradiate to prorsiradiate, narrow ribs that bend forward at the ventrolateral shoulder and cross the venter as chevrons. Long ribs begin from um- bilical tubercles or on the umbilical wall; short ribs begin farther out on the flank. All ribs support small inner and outer ventrolateral tubercles and siphonal clavi. The sutures of the European specimens have not been il- lustrated. American specimens have simple sutures with broad, bifid first lateral saddles and much smaller, shallow, bifid lateral lobes. Remarks.-The cotypes of Neocardioceras juddii (Bar- rois and de Guerne, 1878, p. 46, pl. 1, figs. la, b, 2a, b) were very small, pyritized inner whorls that apparent- ly have been lost. These specimens came from the Plenus Marls of the Paris Basin at Novy-Chevriéres (Département de Ardennes, France). Considerable confusion as to the scope of Neocar- dioceras followed the assignment of the Angolan am- monite Prionotropis echinatus Douvillé (1931, p. 34, pls. 3, 4) to Neocardioceras by Spath (1931, p. 316). The Angolan species is a typical Huomphaloceras (Kanabi- ceras) septemseriatum (Cragin), which is widely distrib- uted in Texas and the Western Interior. Examples of the assignment of E. septemseriatum to Neocardioceras are Adkins (1932, p. 437), Moreman (1942, p. 213), and Cob- ban and Reeside (1952, p. 1017). Some specimens of Kamerunoceras were referred to Neocardioceras before that genus was established (Cobban and Reeside, 1952, p. 1018; Repenning and Page, 1956, p. 268). Occurrence.-Neocardioceras is known only from up- per Cenomanian rocks in England, France, Germany, Czechoslovakia, Brazil, and the United States (Texas, New Mexico, Colorado, Arizona, Utah, Wyoming, and Montana). Neocardioceras juddii (Barrois and de Guerne) Plate 8; text figure 14 1875. 1878. Ammonites neptuni Geinitz, pl. 62, fig. 4a, b. Ammonites juddii Barrois and de Guerne, p. 46, pl. 1, figs. 1a, b, 2a, b. Ammonites [Prionocyclus] neptuni Geinitz. Jukes-Browne and Hill, p. 443, 449. Neocardioceras juddi (sic) (Barrois and Guerne). Spath, p. 81. Gauthiericeras aff. bravaisi (d'Orbigny). Moreman, p. 96, pl. 14, fig. 2. [1978]. Neocardioceras juddi (sic) (Barrois and Guerne). Ken- nedy and Hancock, p. V.17, pl. 15, fig. 2a, b, 6a, b only. Neocardioceras juddii juddii (Barrois and Guerne). Wright and Kennedy, p. 50, pl. 8, fig. la, b; pl. 9, figs. 4, 12-20; text fig. 19J, L. Neocardioceras juddii (Barrois and Guerne). Hook and Cob- ban, p. 9, pl. 1, figs. 6-8. 1903. 1926. 1927. 1976 1981. 1981. Diagnosis.-A variable species ranging from com- pressed forms with thin, fine ribs and sharp ventro- lateral tubercles (N. juddii juddii) to more robust forms with fewer and more rounded ribs and blunter tubercles (juddii barroisi). Description.-The cotypes of Neocardioceras juddii, which apparently were lost, were only 4 and 5 mm in diameter with umbilical ratios of 0.38 and 0.40, respec- tively (Barrois and de Guerne, 1878, p. 46, pl. 1, figs. 1a, b, 2a, b). Drawings of the specimens by Barrois in- dicate 18 to 23 ribs in half a whorl. Wright and Kennedy (1981, p. 50) divided their English and French specimens of N. juddii into two groups-N. juddii juddii and N. juddii barroisi (n. subsp.). The eight examples of N. juddii juddii from England illustrated by Wright and Kennedy range in diameter from 13 to 40 mm. Ornament consists of nar- row, rectiradiate to prorsiradiate primary and second- ary ribs, thin umbilical bullae, and small bullate inner and outer ventrolateral tubercles and siphonal clavi. About 20-28 siphonal clavi are present per half whorl. Wright and Kennedy's N. juddii barroisi differs from N. juddii juddii in having stouter whorls and broader and more rounded ribs that do not project as strongly where they cross the venter. Ribs are also fewer; the eight English specimens illustrated by Wright and Ken- nedy for which half-whorl rib counts can be made or estimated have counts that range from 14 to 22. The ammonite figured by Geinitz (1875, pl. 62, fig. 4a, b) as Ammonites neptuni and assigned to N. juddii barroisi by Wright and Kennedy combines the stoutness of that form with narrow and abundant ribs similar to those of N. juddii juddii. 18 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. Ammonites referable to N. juddii are abundant in limestone concretions in the Colorado Formation in the Cooke Range and in the Little and Big Burro Mountains of southwestern New Mexico. Preservation is good, and all growth stages can be observed. The first 32-4 whorls are very evolute and smooth, and whorl sections are about as broad as high with well- rounded flanks and venter. Ribbing, outer ventrolateral tubercles, and siphonal tubercles appear together at some diameter between 3.5 and 4.5 mm. By a diameter of 6 or 7 mm, the flanks have flattened considerably, and the whorl section becomes higher than wide. Ribs on these early whorls are closely spaced, sinuous, and slightly prorsiradiate; each begins at the umbilicus. These ribs become bullate at the ventrolateral shoulder, where they bend forward and rise again into conspicuous bullate or nodate outer ventrolateral tubercles. Ribbing differentiates into primaries and secondaries at an ear- ly growth stage and continues in this manner through- out ontogeny. On many specimens, single primary ribs arise from narrow, sharp umbilical bullae but, on other individuals, a pair of primaries may arise from a bullate umbilical tubercle. Secondary ribs are long and usually begin close to the umbilical shoulder. Umbilical tubercles are conspicuous only on the larger specimens and number 4-10 per half whorl. These tubercles are usual- ly of irregular height (pl. 8, figs. 28, 36), but they may be more uniform and conspicuous on some individuals (pl. 8, fig. 25). Ribs and tubercles persist on the adult body chamber to the aperture. Each rib has a uniform- ly sized nodate outer ventrolateral tubercle and a slight- ly smaller nodate to clavate siphonal tubercle. Inner ventrolateral tubercles are nodate to bullate and usual- ly of irregular height, with the larger ones located on the primary ribs. The body chamber occupies about half of a whorl (pl. 8, figs. 13, 33, 38). The largest adult has a diameter of 69 mm (pl. 8, figs. 37-39). N. juddii is present in 11 fossil collections from southwestern New Mexico, and two collections have many specimens suitable for either rib and tubercule counts or determination of umbilical ratios. USGS Mesozoic locality D10533 (fig. 1, loc. 50) in the Big Burro Mountains has 28 specimens suitable for measurements, and USGS Mesozoic locality D10114 (fig. 1, loc. 59) in the Cooke Range has 17 measurable specimens. These measurements are shown on figure 13. The suture of N. juddii is fairly simple (fig. 14). E is moderately wide and deep; the E/L saddle is broader than E and bifid; L is shallow and bifid and about half as wide as the E/L saddle; and the second lateral sad- dle is high, bifid and smaller than L. Remarks.-Wright and Kennedy (1981, p. 119) listed the following ammonites that occur with Neocardioceras juddii in the Neocardioceras Pebble Bed in Devon, England: Neocardioceras tenue Wright and Kennedy, Thomelites serotinus Wright and Kennedy, Thomasites cf. gongilensis lautus (Barber), Allocrioceras annulatum (Shumard), and Sciponoceras bohemicum anterius Wright and Kennedy. At another locality in England (Haven Cliff), Wright and Kennedy also found Spathites (Jeanrogericeras) cf. subconciliatus (Choffat) and Thomasites cf. rollandi (Peron) with N. juddii. In the United States, the ammonites associated with N. juddii represent other species. At Chispa Summit in Trans-Pecos Texas, N. juddii occurs with poorly pre- served specimens of Kamerunoceras cf. K. eschii (Solger), Thomelites n. sp., Pseudaspidoceras sp., Vascoceras cf. V. cauvini Chudeau, and V. silvanense Choffat. In southwestern New Mexico, N. juddii is found with Kamerunoceras n. sp., Pseudaspidoceras pseudonodosoides (Choffat), Vascoceras proprium (Rey- ment), Thomasites sp., Sciponoceras gracile (Shumard), Anisoceras plicatile (J. Sowerby), and Worthoceras ver- miculus (Shumard). Occurrence.-Wright and Kennedy (1981, p. 49) recorded abundant Neocardioceras juddii from the Neocardioceras Pebble Bed at the base of the Middle Chalk along the Devon coast in southern England. These authors also recorded the species from France, Belgium, Germany, Czechoslovakia, and the United States. The American specimens are from Texas, New Mexico, Arizona, Colorado, Utah, and Montana. Texas records are based on crushed specimens in the Britton Formation in north Texas that were recorded as Gauthiericeras aff. bravaisi (d'Orbigny) by Moreman (1927, p. 96, pl. 14, fig. 2) and on uncrushed specimens in the collections of the U.S. Geological Survey from the lower part of the Chispa Summit Formation in Trans-Pecos Texas (fig. 1, loc. 67). New Mexico occur- rences are in the southwestern part of the State, where the species is abundant near the top of the Bridge Creek Limestone Member of the Colorado Formation (Hook and Cobban, 1981, p. 8, 9, pl. 1, figs. 6-8). In Arizona, N. juddii is present in collections of the U.S. Geological Survey from the lower part of the Mancos Shale of the Black Mesa area (fig. 1, loc. 39) and from unnamed sandstone farther south in the Show Low area (fig. 1, loc. 40). Colorado occurrences are sparse. A few frag- ments have been found in a bed of limestone in the lower part of the Bridge Creek Member of the Greenhorn Limestone in the southeastern part of the State (fig. 1, locs. 29, 31, 32). In southern Utah, N. juddii occurs as impressions in the lower part of the Tropic Shale (fig. 1, locs. 36-38). Small specimens have been found in lime- stone concretions associated with a widespread bed of bentonite (bed M of Knechtel and Patterson, 1956, p. 21) in south-central and northwestern Montana (fig. 1, locs. 1-4). GENUS NEOCARDIOCERAS SPATH 19 50 I | | [— o § o & Eli 40 |- J Ooo 0 __] & g & & & e O o o o [e] ® e o O.------------0 E o (e] o o s a: LU o _ e S A m 30 |- & __] 3 5 e A | 20 | I | | 80 > I | I w (e] x z 30 |- ( o - g 0 o x p< o - o x o lar e Fe o Z o o & x o o 5; o eo co el a. U x o + (é? a% el © g 20 |- ee o x (e] + - 0 eee o + + + #e ea ® to o o o + + B 16 1 | | | | 0 10 20 30 40 50 60 DIAMETER, IN MILLIMETERS FIGURE 13.-Scatter diagrams of Neocardioceras juddii (Barrois and de Guerne) from USGS Mesozoic locality D10533 (closed circles) and USGS Mesozoic locality D10114 (open circles) plotted against diameter. Circles connected by lines represent measurements on a single specimen. A, Umbilical ratios in percentages. B, Number of ribs per half whorl of specimens of N. juddii juddii (x) and N. juddii barroisi (+) from England and France illustrated by Wright and Kennedy (1981, pl. 8, fig. 1b; pl. 9, figs. 1b, 2a, 4a, 5, 6, 9a, 10a, 12a, 14a, 16a, 17, 18, 19b). Types. -Hypotypes USNM 83078357, 356889, 400818-400831. Neocardioceras densicostatum Cobban, n. sp. Plate 9, figures 1-31 Diagnosis.-A densely ribbed species that has nar- row, threadlike, flexuous ribs. Ventrolateral and siphonal tubercles are small and nodate and usually in- conspicuous. Description. -The type lot consists of flattened specimens from the Hartland Shale Member of the Greenhorn Limestone at USGS Mesozoic locality D7410 northwest of Denver, Colo. (fig. 1, loc. 27). At this locality, the strata are steeply inclined and the fossils are slightly distorted. The holotype (pl. 9, fig. 23) is an adult 43.7 mm in 20 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. E \ L \ Ju ) l 5 mm | FIGURE 14.-External suture at a diameter of 28 mm of Neocar dioceras juddii (Barrois and de Guerne) from the Colorado For- mation at USGS Mesozoic locality D10533 (fig. 1, loc. 50). Hypotype USNM 400826 (pl. 8, figs. 23, 24). diameter with an umbilical width of 13.2 mm and an um- bilical ratio of 0.30. Only the lateral view of the specimen can be seen. Septa are not preserved and, accordingly, the diameter at the base of the body chamber could not be determined. All whorls down to a diameter of about 4 mm are densely ribbed. The ribs are narrow, prorsira- diate, and flexuous, and they number 75 on the last com- plete whorl. Long ribs begin singly or in pairs on the umbilical shoulder or from bullae on the shoulder. Long ribs are usually separated by a shorter rib that arises low on the flank. All ribs bend forward on the ven- trolateral shoulder. Weak, nodate inner and outer ven- trolateral tubercles are present on the older half of the outer whorl, but the inner ones weaken and disappear on the younger half. The aperture is normal and is preceded by a nearly smooth area that has a few close- ly spaced, faint ribs. Details of the venter can be seen on some of the paratypes. Small, nodate siphonal tubercles seem to per- sist to the aperture (pl. 9, figs. 18, 20, 25, 27). Inner ven- trolateral tubercles usually disappear on the last half whorl, but exceptions exist (pl. 9, figs. 18, 29). Both rows of ventrolateral tubercles disappear on some individuals (pl. 9, figs. 11, 28, 30). Ribs per half whorl can be determined on 62 specimens in the type lot. Counts range from 19 to 48 and show great scatter (fig. 15). The species is dimorphic. The smallest specimens on plate 9 are probably adults, and some a little larger (pl. 9, figs. 8, 18) are certainly adults. Fragments of much larger individuals (pl. 9, figs. 30, 31) indicate that the species attained diameters of at least 50 or 60 mm. Remarks.-Neocardioceras densicostatum resembles N. juddii in its thin, flexuous ribs, but the sparser rib- bing and persistence of tuberculation to the aperture of the latter separates the two species. Occurrence.-The types are from the Hartland Shale 3 w is w | U « e 40 |- * & * - Pol e &C e co U « U e e] o wal * * g U A as ® ® u L " U a] 30 |- A* U z. to. § O ea 4 e e e C e e LL ® ee «*e o. ee e U 9) c E ® * 20 |- s e to | | | | 0 10 20 30 40 50 DIAMETER, IN MILLIMETERS FIGURE 15.-Scatter diagram showing number of ribs per half whorl of Neocardioceras densicostatum Cobban, n. sp., from the Hartland Shale Member of the Greenhorn Limestone at USGS Mesozoic locali- ty D7410 (fig. 1, loc. 27). Member of the Greenhorn Limestone at USGS Mesozoic locality D7410 at Eldorado Springs, Colo. (fig. 1, loc. 27). Associated fossils include impressions of Inoceramus cf. I. pictus J. de C. Sowerby, Wor- thoceras sp., and Anisoceras sp. Types.-Holotype USNM 400832, paratypes 400833-400862. Neocardioceras uptonense Cobban, n. sp. Plate 10, figures 36-70; text figure 17 1975. Eucalycoceras sp. B. Hattin, pl. 2, fig. K only. 1977. Acanthoceras group "B." Kauffman and Powell, p. 103, pl. 10, fig. 7 only. Diagnosis.-A fairly small species characterized by effaced ribbing on the flank of the phragmocone and by conspicuous umbilical, ventrolateral, and siphonal tubercles. Description.-The holotype (pl. 10, figs. 64-66) is a small adult 31.2 mm in diameter with an umbilical width of 9.5 mm and an umbilical ratio of 0.30. Intercostal whorl sections are a little higher than wide, with flat flanks and broadly rounded venter. The umbilicus has a narrowly rounded shoulder and vertical wall. Flanks on the phragmocone are smooth to faintly ribbed. The visible parts of the earliest whorls appear to be entire- ly smooth. Slightly bullate umbilical and nodate ven- trolateral tubercles first appear near the beginning of GENUS NEOCARDIOCERAS SPATH 21 the penultimate whorl at a diameter of about 6.5 mm. On the last complete whorl of the phragmocone, um- bilical tubercles number 11 and ventrolateral tubercles number 22. Clavate siphonal tubercles are well developed on that whorl. On the younger half of that whorl, very prorsiradiate, weak ribs arise singly or in pairs from twisted, bullate umbilical tubercles or arise higher on the flank. All ribs rise into sharp, nodate in- ner ventrolateral tubercles and then bend forward and cross the venter as chevrons, where they support equal- sized nodate outer ventrolateral and clavate siphonal tubercles. The body chamber occupies about three- fourths of a whorl and has a diameter of 18.3 mm at its base. Ribs are more conspicuous on the body chamber and cross the entire flank. Umbilical tubercles are strong, closely spaced, nodate to bullate and are located on the shoulder. Bullate tubercles have flat, adoral faces. Inner ventrolateral tubercles are bullate on the body chamber and gradually weaken toward the aperture. Outer ventrolateral tubercles become clavate and, with the siphonal clavi, remain strong to the aper- ture. The last complete whorl of the holotype has 28 siphonal clavi. The type lot is from a limestone concretion in the Belle Fourche Shale near Upton on the west flank of the Black Hills uplift in eastern Wyoming (fig. 1, loc. 12). Thirty-one specimens are suitable for measurements of diameter, umbilicus, and number of ribs per half whorl. The bulk of the specimens are 10-20 mm in diameter and have 9-16 ribs per half whorl (fig. 16). Diameters at the base of body chambers can be determined for 15 of the specimens. Although the holotype is the largest of these 15 specimens, fragments of 8 specimens exist that are as large as or larger than the type, which sug- gests that the species is dimorphic. Three of these specimens are shown (pl. 10, figs. 61-63). Diameters at the base of the body chambers of microconchs range from 10 to 16 mm, and those of macroconchs range from 18 mm to an estimated 25 mm. The holotype is a small macroconch. Whorls less than 3 mm in diameter are smooth and have well-rounded venters and broadly rounded flanks. Nodate inner ventrolateral tubercles appear at some diameter between 3 and 4 mm. Siphonal clavi form by a diameter of 4 mm, and nodate umbilical tubercles arise between 5 and 6 mm. At a diameter of about 6 mm, flanks flatten, and ventral ribs and outer ventrolateral tubercles appear. On some individuals, ventral orna- ment occurs as groups of ribs and tubercles separated by smooth or weakly ornamented areas, and weak con- strictions may separate some ribs. This condition may be present at diameters of as much as 10 or 11 mm (pl. 10, fig. 45). Umbilical, outer ventrolateral, and 40 | 30 |- UMBILICAL RATIO, IN PERCENT 20 20 RIBS PER HALF WHORL 0 I | I | 0 10 20 30 40 DIAMETER, IN MILLIMETERS FIGURE 16.-Scatter diagrams of Neocardioceras uptonense Cobban, n. sp., from a concretion in the Belle Fourche Shale at USGS Mesozoic locality D5947 (fig. 1, loc. 12). A, Umbilical ratios. B, Ribs per half whorl. Lines connect measurements on single specimens. siphonal tubercles usually persist to the aperture (pl. 10, fig. 57). Inner ventrolateral tubercles weaken on the body chamber and disappear near the aperture. Ventral ribbing is conspicuous on the body chamber and may be crowded at the adoral end (pl. 10, fig. 57). The aper- ture is normal and follows the form of the ribs. The suture is simple and characterized by a shallow, bifid lateral lobe (fig. 17). Neocardioceras uptonense shows normal intraspecific variation of ammonites in that it ranges from com- pressed, finely ornamented specimens (pl. 10, figs. 48, 49) to robust, coarsely ornamented specimens (pl. 10, figs. 51-53). In addition, occasional specimens have or- nament of irregular height (pl. 10, figs. 61, 67). A few large, crushed adults from the Benton Shale in north- central Colorado (fig. 1, loc. 25) reveal a loss of tuber- culation on the younger part of the body chamber (pl. 10, figs. 68, 70). Remarks.-Neocardioceras uptonense differs from N. juddii (Barrois and de Guerne) in its sparser ornament and in its effaced ribbing on the flank of the phragmo- cone. The effaced ribbing also distinguishes it from N. tenue Wright and Kennedy (1981, p. 50, pl. 8, figs. 2-6, 8). Occurrence.-The types are from a limestone con- cretion in the Belle Fourche Shale at USGS Mesozoic 22 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. E B FIGURE 17.-External sutures of Neocardioceras uptonense Cobban, n. sp., from the Belle Fourche Shale at USGS Mesozoic locality D5947 (fig. 1, loc. 12). A, Paratype USNM 400876 at a diameter of 13 mm. B, Paratype USNM 400877 at a diameter of 12.4 mm. locality D5947 near Upton in the NW! sec. 14, T. 47 N., R. 67 W., Weston County, Wyo. (fig. 1, loc. 12). Associated ammonites include Moremanoceras n. sp., Metoicoceras aff. M. latoventer Stephenson, Boris- siakoceras aff. B. orbiculatum Stephenson, Hamites simplex d'Orbigny, and Allocrioceras sp. The collection is from 6.4 m (21 ft) above a 3.6-m-thick unit of shale that contains Dunveganoceras pondi Haas. A few fragments of Neocardioceras uptonense have been found in the Frontier Formation farther southwest near Casper, Wyo. (fig. 1, loc. 18), where they were found with a late form of Calycoceras canitaurinum (Haas), Dunveganoceras problematicum Cobban, and Metoicoceras frontierense Cobban. Specimens of N. up- tonense also occur in the Frontier Formation farther north in Wyoming (fig. 1, loc. 15). Neocardioceras uptonense occurs in the middle of the Lincoln Member of the Greenhorn Limestone in central Kansas (Hattin, 1975, pl. 2, fig. K) and near the base of the Hartland Shale Member in western Oklahoma (Kauffman and Powell, 1977, p. 103, pl. 10, fig. 7). The species also occurs in the Lincoln Member in the Pueblo area in southeastern Colorado (fig. 1, loc. 30) where it is associated with Inoceramus prefragilis Stephenson and Calycoceras canitaurinum (Haas). Impressions of N. uptonense are abundant in the Benton Shale 60-64 m (196-211 ft) below the base of the Juana Lopez Mem- ber in the Middle Park area in north-central Colorado (fig. 1, locs. 21, 22, 25, 26). Some of these impressions are shown on plate 10. Types.-Holotype USNM 400863, paratypes USNM 400864-400888. Neocardioceras laevigatum Cobban, n. sp. Plate 9, figures 32-38; text figure 18 Name.-Latin levigatus, smooth, slippery; in refer- ence to the smooth, polished appearance of the holotype. Diagnosis.-A moderately involute, compressed form that has smooth or faintly ribbed flanks. Description.-The holotype (pl. 9, figs. 35, 36) is an adult that consists of the uncrushed phragmocone and the older part of the crushed body chamber. The specimen has a diameter of 25.1 mm, an umbilical width of 5.5 mm, and an umbilical ratio of 0.22. The diameter at the base of the body chamber is 20.4 mm. Whorls are much higher than wide with the greatest width at the narrowly rounded umbilical shoulder. Flanks are flattened, and the narrow venter is rounded. Ornament on the last complete whorl of the phragmocone consists of eight conspicuous, nodate tubercles low on the um- bilical shoulder; equal-sized, nodate to slightly clavate outer ventrolateral and siphonal tubercles located on low, chevronlike ventral ribs; a few flexuous striae of irregular strength; and, on the older half of the whorl, a few small, widely spaced, nodate inner ventrolateral tubercles. On the older part of the whorl, the outer ven- trolateral and siphonal tubercles are in groups of twos or threes separated by smooth areas on the venter (pl. 9, fig. 36). Ornament on the preserved part of the body chamber consists of faint, closely spaced, flexuous ribs, and small, nodate, closely spaced inner ventrolateral tubercles. The venter on the body chamber is not preserved. The last two septa are approximated. The suture is characterized by a broad, shallow, bifid lateral lobe (fig. 18). Remarks.-Neocardioceras laevigatum is a rare am- monite from the Black Hills area. The holotype was the only representative of this species in the concretion that produced the type lot of N. uptonense. In other areas, N. laevigatum is more common (pl. 9, fig. 38). In its smooth or nearly smooth flanks, the subspecies resembles impressions of small ammonites from Okla- homa and Kansas referred to Acanthoceras group "B" by Kauffman and Powell (1977, p. 103, pl. 10, fig. 2 only) and to Eucalycoceras sp. B by Hattin (1975, pl. 2, fig. L only). However, specimens of N. laevigatum from the Black Hills and Colorado are more involute than are the ammonites from Oklahoma and Kansas. Occurrence.-The holotype is from a limestone con- cretion in the Belle Fourche Shale 4.8 km (3 mi) south GENUS NEOCARDIOCERAS SPATH 23 | 2 mm FigurE 18.-Part of the third-from-last external suture of the holotype of Neocardioceras laevigatum Cobban, n. sp., at a diameter of 18.7 mm, from the Belle Fourche Shale near Upton, Wyo. (fig. 1, loc. 12). USNM 400889 (pl. 9, figs. 35, 36). of Upton, Wyo., at USGS Mesozoic locality D5947 (fig. 1, loc. 12). Associated ammonites included Neocar- dioceras uptonense, Metoicoceras aff. M. latoventer Stephenson, Moremanoceras n. sp., Borissiakoceras aff. B. orbiculatum Stephenson, Hamites simplex d'Or- bigny, and Allocrioceras sp. Parts of two specimens were found at another locality on the west flank of the Black Hills uplift (fig. 1, loc. 10). Impressions of N. laevigatum occur in the Benton Shale at several localities in the Middle Park area of north-central Col- orado (fig. 1, locs. 23-25). Types.-Holotype USNM 400889, paratypes USNM 400890-400893. Neocardioceras minutum Cobban, n. sp. Plate 10, figures 1-35; text figure 20 1984. Neocardioceras spp. Cobban, p. 19, pl. 4, figs. 3, 5. Diagnosis.-A small, compressed, rather densely ribbed species that has ribs usually effaced on the mid- dle of the flanks of the phragmocone. Description.-The type lot, from a limestone concre- tion in the Greenhorn Formation at USGS Mesozoic locality D4462 near Upton, Wyo. (fig. 1, loc. 10), con- sists of 10 small, well-preserved specimens. The holotype (pl. 10, figs. 1, 2) is a complete adult 13.7 mm in diameter with an umbilicus of 3.4 mm and an um- bilical ratio of 0.25. The whorl section is rectangular, higher than wide, with flat flanks, a broadly rounded to flattened venter, and a narrowly rounded umbilical shoulder. Considerable shell material is preserved on the specimen. The body chamber probably occupies the last two-thirds or three-fourths of the outer whorl. Ribs on the outer whorl number 32 and cross the venter as chevrons. Long ribs begin singly or in pairs from con- spicuous bullae located on the umbilical shoulder. Short ribs arise at midflank. All ribs are greatly reduced at midflank, and all bend forward on the ventrolateral shoulder. Umbilical bullae number 14 on the last com- plete whorl, and each is twisted backward a little. On the older part of this whorl, nodate to bullate inner ven- trolateral tubercles are about equal in size to nodate to clavate outer ones, but, on the younger part of the whorl, the inner ones weaken and disappear. The outer ventrolateral tubercles weaken and become elongated following the trend of the ribs and, near the aperture, the tubercles bound the narrow, flattened venter. Slight- ly smaller, nodate to somewhat clavate siphonal tubercles persist to the aperture. A faintly sinuous lateral outline and a conspicuous ventral lappet characterize the aperture. A specimen 7.4 mm in diameter (paratype USNM 400907) was taken apart to observe the early whorls. Its first two or three whorls are wider than high with broad, rounded venter that merges evenly into rounded flanks. By a diameter of 4 mm, the whorl section has become squared with flattened flanks and slightly flat- tened venter. Other specimens show an increase in height over width as the shells enlarge beyond 4 mm in diameter. The specimens of Neocardioceras minutum in the type lot are all small. Most have diameters of 6.6-7.8 mm at the base of the body chambers. The species ranges from compressed, rather densely and finely ornamented forms (such as the holotype) to forms that are more evolute, more robust, and more coarsely and sparsely ornamented (pl. 10, figs. 5-7, 32-35). Even within the compressed forms, rib density varies (fig. 19). One specimen (pl. 10, figs. 26, 27) has 27 ribs per half whorl in contrast to 18 at a comparable diameter on the holotype. Similar-sized robust forms have 11 to 15 ribs per half whorl. Sutures are not visible on the holotype. A paratype (pl. 10, figs. 8, 9) has a simple pattern with a moderate- ly broad, shallow lateral lobe (fig. 20). Remarks.-Neocardioceras minutum was derived from N. uptonense by a reduction in size and by an increase in rib density. In addition, N. minutum shows a greater degree of variation. Some flattened specimens from Col- orado (fig. 1, loc. 23) are larger than the types from the Black Hills and may be macroconchs (pl. 10, fig. 28). Occurrence.-The type lot is from a limestone concre- tion in the concretionary facies of the Greenhorn For- mation (Robinson and others, 1964, p. 62) on the west side of the Black Hills uplift near Upton, Wyo., at USGS Mesozoic locality D4462 (fig. 1, loc. 10). Associ- ated ammonites include Metoicoceras aff. M. latoventer Stephenson, Borissiakoceras aff. B. orbiculatum Stephenson, and Allocrioceras sp. The specimens of Metoicoceras are slenderer than those occurring with Neocardioceras uptonense. Neocardioceras minutum also has been found in a ferruginous concretion at 24 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. 40 | '— = 8 9 xx LL o. 2 / o E 30 |- P 3 ox.o\o I x 5 o m x o®--e S o--e 5 o A 20 | 30 | o g 20 |- x - 6 x x C = V u ad T x E x [est LL & o f 4 . 10 - o o -I o B o | 0 10 20 DIAMETER, IN MILLIMETERS FiGurE 19.-Scatter diagrams of Neocar- dioceras minutum Cobban, n. sp., from the Greenhorn Formation at USGS Mesozoic localities D4462 (.) and 12740 (x). A, Umbilical ratios. B, Ribs per half whorl. Lines connect measurements on single specimens. | 1 mm | FIGURE 20.-Last external suture at a diameter of 7.3 mm of Neocar- dioceras minutum Cobban, n. sp., from the Greenhorn Formation at USGS Mesozoic locality D4462 near Upton, Wyo. (fig. 1, loc. 10). Paratype, USNM 400897. roughly the same stratigraphic level at a nearby locality (fig. 1, loc. 13). A little farther southeast, specimens were found in the lower part of the Greenhorn Forma- tion in a borehole in the Osage oilfield (Cobban, 1984, p. 19, pl. 4, figs. 3, 5). On the northern flank of the Black Hills uplift, N. minutum was recorded from the Green- horn Formation as "Ammonite, n. gen., sp. B" (Robin- son and others, 1964, p. 63, loc. 12740). In north-central Colorado, N. minutum has been found as impressions in shale in the Benton Shale in the Middle Park area (fig. 1, loc. 23). Two small fragments that are probably N. minutum were found in the Frontier Formation in east-central Wyoming (fig. 1, loc. 17). Types.-Holotype USNM 400894, paratypes 400895-400908. Neocardioceras sp. Plate 10, figures 71, 72; text figure 21 A phragmocone from the Frontier Formation of north-central Wyoming (fig. 1, loc. 14) reveals the presence of a large species of Neocardioceras in the up- per part of the zone of Metoicoceras mosbyense (sub- zone of Dunveganoceras conditum). The specimen has a diameter of 45.5 mm, an umbilical width of 13.3 mm, and an umbilical ratio of 0.29. A bit of attached body chamber indicates that the diameter is also that of the base of the body chamber. The whorl section is rec- tangular and higher than wide with flattened flanks and venter. Ribs on the outer whorl are narrow, straight, and prorsiradiate; they tend to alternate in length and number 19 on the last half whorl. Ribs bend forward on the ventrolateral shoulder and cross the venter as chevrons. Long ribs begin from inconspicuous umbili- cal bullae. All ribs bear nodate inner and outer ven- trolateral tubercles. Siphonal tubercles are small and clavate. Only part of the sutures are visible (fig. 21); they have a broad first lateral saddle and a narrower " / | \ | 10 mm E FigurE 21.-Part of the second-from-last external suture at a diameter of 42 mm of Neocardioceras sp., from the Frontier For- mation at USGS Mesozoic locality D6962 (fig. 1, loc. 14). Figured specimen USNM 400909 (pl. 10, figs. 71, 72). ORIGIN OF TARRANTOCERAS AND RELATED GENERA 25 bifid lateral lobe in which one branch is much longer than the other. Remarks.-This specimen is the size of large in- dividuals of N. juddii from southwestern New Mexico. However, the Wyoming specimen differs in its more robust shell and in its sparser ribbing. Type.-Figured specimen USNM 400909. ZONATION OF CENOMANIAN AMMONITES IN THE WESTERN INTERIOR The sequence of ammonites of Cenomanian Age in the Western Interior of the United States has been recent- ly summarized (Cobban, 1984), and newer data have been added (Cobban, in press). Zones and subzones now recognized are shown in table 2, which also shows the zonal position of the ammonites treated in this report. At all localities where Tarrantoceras sellardsi occurs, Acanthoceras amphibolum is present. However, the younger part of the zone of A. amphibolum (subzone of A. amphibolum fallense) has not yielded any specimens of T. sellardsi. Tarrantoceras flexicostatum may be restricted to the zone of Dunveganoceras pondi, but specimens of T. flexicostatum are too few to establish this with certainty. Eucalycoceras pentagonum is present throughout the zone of Sciponoceras gracile. The oldest specimens oc- cur in the subzone of Vascoceras diartianum in south- western New Mexico (fig. 1, loc. 61), and the youngest in the subzone of Euomphaloceras (Kanabiceras) septemseriatum in Colorado and South Dakota (fig. 1, locs. 8, 34). Eucalycoceras templetonense is found in Wyoming in the zones of Plesiaqcanthoceras wyo- mingense (fig. 1, loc. 20) and Dunveganoceras pondi (fig. 1, loc. 11). Pseudocalycoceras angolaense seems to be confined to the upper part of the Sciponoceras gracile zone (subzone of Euomphaloceras septemseriatum). All speci- mens of Sumitomoceras conlini and S. bentonianum col- lected to date are from this subzone. The types of Neocardioceras uptonense and N. laevigatum are from the zone of Dunveganoceras pon- _ di, but these species of Neocardioceras range as high as the zone of D. problematicum in Wyoming (fig. 1, loc. 18). Neocardioceras minutum occurs still higher in Wyoming (fig. 1, loc. 17), probably in the lower part of the zone of Metoicoceras mosbyense. Neocardioceras densicostatum is probably from high in the zone of M. mosbyense because of its occurrence high in the Hartland Member of the Greenhorn Limestone. ORIGIN OF TARRANTOCERAS AND RELATED GENERA The oldest of the species treated herein, Tarrantoceras sellardsi, was probably derived from the slightly older ammonite described by Adkins (1928, p. 240, pl. 28, fig. 1; pl. 29, fig. 3) as Eucalycoceras leonense. Adkins species is a robust, round-whorled form that seems best assigned to Calycoceras (Cobban and Scott, 1972, p. 60) and even to the subgenus C. (Gentoniceras) (Cobban, in press). Calycoceras leonense is similar to T. sellardsi in its size, umbilical ratio, suture, and ornament of conspicuous ribs and umbilical, inner and outer ven- trolateral, and siphonal tubercles. The main differ- ence between them is in the compressed whorls of 7. sellardsi, with its denser and more flexuous ribbing. Tarrantoceras flexicostatum was apparently derived from T. sellardsi through further compression of the whorls and probably through a slight increase in rib density. Eucalycoceras templetonense may be a migrant from outside North America. The species appears abruptly in the zone of Plesiaqcanthoceras wyomingense and ranges up into the zone of Dunveganoceras pondi. Eucalycoceras has not been found in the overlying zones of Dunveganoceras problematicum and Metoicoceras mosbyense, but the genus appears again as E. pen- tagonum in the zone of Sciponoceras gracile. No records of the genus are present in younger rocks. American specimens of Pseudocalycoceras angolaense seem to be derived from American specimens of Eucalycoceras pentagonum by a change to sparser and. rursiradiate ribbing. In addition, the ventral ornament on the early whorls changed from the uniform ribs and tubercles of E. pentagonum (Kennedy, 1971, pl. 48, figs. 4a, b) to one of ornamented areas separated by smooth areas (pl. 5, figs. 14, 15). Pseudocalycoceras angolaense is known only from the zone of Sciponoceras gracile and has no known descendents. Sumitomoceras has been found only in the upper part of the zone of Sciponoceras gracile and has no known descendents. The genus may have been derived from an early form of Eucalycoceras pentagonum in the sub- zone of Vascoceras diartianum through early loss of siphonal tubercles. Neocardioceras first appeared abruptly in the zone of Dunveganoceras pondi. The genus was probably de- rived from Tarrantoceras through a change in the ven- tral ribbing from a transverse style to one of forwardly inclined chevrons. Neocardioceras uptonen se, the oldest species, ranges through the zones of Dunveganoceras pondi and D. problematicum. Neocardioceras minutum, which occurs in the lower part of the overlying zone of 26 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. TABLE 2.-Zonation of Cenomanian ammonites in the Western Interior of the United States and ranges of species treated herein STAGE ZONE SUBZONE AMMONITES IN THIS REPORT Neocardioceras juddii Vascoceras cauvini ; 7 septemseriatum Sciponoceras gracile P 2 Euomphaloceras (Kanabiceras) --- Neocardioceras sp. Neocardioceras juddii Neocardioceras laevigatum Neocardioceras uptonense --- Neocardioceras minutum Eucalycoceras templetonense ? Tarrantoceras flexicostatum Sumitomoceras conlini Pseudocalycoceras angolaense Sumitomoceras bentonianum - --- ? Eucalycoceras pentagonum Tarrantoceras sellardsi Neocardioceras densicostatum --- Acanthoceras muldoonense Calycoceras (Conlinoceras) tarrantense r Vascoceras diartianum a. a D Dunveganoceras conditum & Metoicoceras mosbyense m Dunveganoceras albertense & 5: Dunveganoceras problematicum z g Dunveganoceras pondi 9 w Plesiacanthoceras wyomingense o Acanthoceras amphibolum fallense Acanthoceras amphibolum i Acanthoceras amphibolum amphibolum a C: s | Acanthoceras bellense Metoicoceras mosbyense (subzone of Dunveganoceras albertense), descended from N. uptonense through a reduction in size and an increase in rib density. Higher in the zone of M. mosbyense, Neocardioceras is scarce, but a phragmocone (pl. 10, figs. 71, 72) from the Fron- tier Formation of Wyoming indicates the presence of some large species that descended from N. uptonense. 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(10... 01 c. oul s ioe a ees pae de d) 17 LP O ee 17 peMHAGOMUS .... ...ll 2.2 scene eee ee ees 2, 8, 9 [Prionocyclus] neptuni . ................. 17 amphibolum, Acanthoceras ................. 7, 25 fallense, Acanthoceras .................. 25 angolaense, Protacanthoceras ............... 12 '"Protacanthoceras" ..................>> 12 Pseudocalycoceras ...... .. 12, 15, 25; pls. 5, 6 (Haugiceras) \ 9} >> 12 (Neocalycoceras) 12 Anisoceras plicatile ..................... 7, 10, 18 8D . cloe cls baga dv les aaa e bpi red aa e 20 annulatum, Allocrioceras ................... 18 anterius, Sciponoceras bohemicum ........... 18 arvanus, Inoceramus ......................> 7 B barroisi, Neocardioceras juddit .............. 17 basicostata, Pseudomelania ..... T Belle Fourche Shale ................. 7, 12, 21, 22 bellsanus, Mammites . 11 beloiti, Ostrea 2.2 ee eke ees d Benton Shale .....................> 21, 22, 23, 24 bentoniana, Pulchellia ..... .. bentonianum, Eucalycoceras . "Eucalycoceras" Sumitomoceras ... Tarrantoceras (Sumitomoceras) bohemicum anterius, Sciponoceras . Boquillas Formation ...... .... Boquillas Limestone ...... .. Borissiakoceras orbiculatum .. . " e e a bravaisi, Gauthiericeras Bridge Creek Limestone Beds, Mancos Shale .. 14 Bridge Creek Limestone Member, Colorado For- M@UION 22222222222 cscs ee ees 14, 18 Bridge Creek Member, Greenhorn Limestone . . 9, 14, 15, 17, 18 Britton Formation .................. 14, 15, 16, 18 C Caly@OGCera® \ less seee ee een enne ne n> 8, 25 (Gentonmiceras) ................... 0.9 25 IeOMeMS@ ...l 222222 scene nene ee eee e> 16 canitaurinum | fad nan aie 8, 22 Foc o r o 10 deonenge 2.2 0. .s. illi ness 25 on ae 10, 14 INDEX [Italic page numbers indicate major references] Page canitaurinum, Calycoceras .................} 8, 22 cauvini, Vaseoceras 2.0 ece ees 18, 26 Cenomanian ammonite zonation ............. 25 Chispa Summit Formation ..............> 7, 14, 18 Clay Mesa Tongue, Mancos Shale ...... .... 7 Cody Shale ...... 8 Colorado Formation .......... 10, 13, 14, 15, 16, 18 conditum, Dunveganoceras ................. 24 conlini, Sumitomoceras ............ 14, 17, 25; pl. 7 Tarrantoceras (Sumitomoceras) .......... 14 crassicostae, Metoicoceras .................. 11 Cunningtoniceras lonsdalei ............. 7 D, E Dakota Sandstone . l.} >> 7 densicostatum, Neocardioceras ...... ... 19, 25; pl. 9 dentonense, Eucalycoceras .................> 12 (Proeucalycoceras) ................>> 12 Pseudocalycoceras ...................}> 12, 15 Desmoceras japonicum ....................> 7 (Pseudouhligella) .... .... ...>>> 7 diartianum, Vascoceras ................}}}> 10, 25 Dunveganoceras albertense ................> 26 Conditum lle e eee ee ee ee ne 24 [pOMb ...ll 2222 ece ee eee ee ee ne 8, 12, 22, 25 problematicum sls. .> > 22, 25 Eagle Ford Group . lessee 5, T Eagle Ford shales . 16 echinatus, Prionotropis ....................} 17 eschii, Kamerunoceras ...................>} 18 Eucalycoceras 9+} 1, 2, 8, 12, 14 bentomianum ...... le cece nees 15 dentomense cscs cnn e> gothicum .. indianense . leonense ........... lewisvillense .. pentagonum ... S@haremse ...l... 2.2.2.2 seee ee eens sellardsi ...... templetonense .... underwoodi (Eucalycoceras) pentagonum ............ 9 (Proeucalycoceras) dentonense ........... 12 lewisvillense =...... .... (Eucalycoceras) pentagonum, Eucalycoceras ... 9 '"Eucalycoceras" bentonianum .............. 16 Euomphaloceras (Euomphaloceras) evomphalum 10 (Kanabiceras) septemseriatum ..... 14, 15, 17, 25 (Euomphaloceras) euomphalum, Euomphaloceras 10 euomphalum, Euomphaloceras (Euomphaloceras) 10 Exogyra levis 202k ec eee ee nes 15 F, G fallense, Acanthoceras amphibolum ..... ..... 25 faustum, Sumitomoceras ..................} 14 flexicostatum, Tarrantoceras ............ 7, 25; pl. 2 Fossil localities lll. ece} 3 Frontier Formation ................> 12, 22, 24, 26 frontierense, Metoicoceras .................. 22 Gauthiericeras bravaisi .................}}> 17 (Gentoniceras), Calycoceras ................. 25 leonense, Calycoceras .................. 16 geslinianum, Metoicoceras .............. 14, 15, 17 Page gibbosum, Worthoceras .. 14, 15 gongilensis lautus, Thomasites ...... 18 gothicum, Eucalycoceras ................... 11 gracile, Sciponoceras ................ 14, 15, 18, 25 Graneros Shale ...}} ece >> 7 Greenhorn Formation 8, 10, 23 Greenhorn Limestone ................ 9, 12, 14, 15, 17, 18, 19, 22, 25 guerangeri, Calycoceras 10 H-K Hamites simplex .................. }} ccc} 22, 28 harpax, Ammonites 9, 12 Pseudocalycoceras 7, 14 Hartland Member, Greenhorn Limestone ..... 14, 25 Hartland Shale Member, Greenhorn Limestone 19, 22 haugi, Pseudocalycoceras ... ................ 13 (Haugiceras) angolaense, Pseudocalycoceras ... 12 Idiohamites M. SD csc ec 8 indianense, Eucalycoceras . 12 Inoceramus arvanus c+} 7 PAEUE | (lls ll clu vc ces ea srs eer raves 15, 20 prefragilis ...... ... 12, 22 stephensoni Mt 8 rutherfordi ............ seee T japonicum, Desmoceras .................... T (Jeanrogericeras) subconciliatus, Spathites ... .. 18 johnsonanum, Acanthoceras ................ 7 Juana Lopez Member, Benton Shale .. ... 2g 22 juddi, Neocardioceras .................. s 17 juddii, Ammonites 17 Neocardioceras ... ... 17, 20, 21, 25, 26; pl. 8 juddit ........ 17 barroish leke eee 17 juddii, Neocardioceras juddii ............ 17 Kamerunoceras eschit 17 Tho ccc cu ela cds ara vea deba eed 4 18 (Kanabiceras) septemseriatum, Euomphaloceras 14, 15, 17, 25 L laevigatum, Neocardioceras ............ 22, 25; pl. 9 landisi, Pseudacompsoceras ................. 7 latoventer, Metoicoceras .................... 22, 28 lautus, Thomasites gongilensis .............. 18 leonense, Calycoceras ..................}.}> 25 (Gentomiceras) 00 16 Eucalycoceras ...............2 csc ces 25 levis, Exogyra .............. 02.0 ce eee eee 15 lewisvillense, Eucalycoceras ................ 12 (Proeucalycoceras) ...>} 12 lilianense, Tarrantoceras (Tarrantoceras) .... .. 16 lillianense, Tarrantoceras ................... 5 Lincoln Member, Greenhorn Limestone ...... . 12, 22 lirata, Lispodesthes ... ...l} ..} .} > 7 Lispodesthes lirata 7 Localities, fossil }}} >> 3 lonsdalei, Cunningtoniceras ................. 7 lyelli, Acanthoceras .>} 12 M Mammites bellsanus }}} 11 Mancos Shale ...ll 220 7, 12, 14, 18 Mantelliceras ............. 5 sellardsi .............. 2, 5 Metoicoceras crassicostae .. 11 frontierense ...... 000 scc eer 22 29 30 TARRANTOCERAS AND RELATED AMMONOID GENERA, TEXAS AND WESTERN INTERIOR, U.S. Page geslinianum (2. onus .o. uity o oars 14, 15, 17 intfoventer "2-2 -On o. .s. on anal ren bait ae 22, 28 . Ale .t Ladies aoa oe od 24, 25 PMIECOX . . Adnan so De nea a 8 Bp L. 9.0 o mene toate sev ane . panne nes 10 Middle Chalk .............. 18 minutum, Neocardioceras .... . 23, 25, 26; pl. 10 Moremanoceras scotti ...................... 10 .e Lpc lel Pl du aii ae 22, 23 mosbyense, Metoicoceras ................... 24, 25 multicostatum, Tarrantoceras ............... 5 N, 0 naviculare, Calycoceras .................... 10, 14 (Neocalycoceras) angolaense, Pseudocalycoceras 12 Neocardioceras ......................... 1, 17, 26 densicostatum ................... 19, 25; pl. 9 . 2 is sun oy .a denen o t 17 Juddit ..... ..> 17, 20, 21, 25, 26; pl. 8 ...le. .. cyl pe shee nde a ues 17 snus . . il en eda on d 17 laevigatum .............2.2.2.2... 22, 25; pl. 9 minudut c cl a 23, 25, 26; pl. 10 Pemue o.. uso reins on oa eae wa 18, 21 uptonense ................ 20, 22, 23, 25; pl. 10 Bp ore neb s ned monn aaa aar 24; pl. 10 SUD 1.0 uur tae nnn oe mune ne n a male 23 Neocardioceras Pebble Bed ... .............. 18 neptuni, Ammonites ............... 17 [Prionocyelus] ..............2.2.2...... 17 orbiculatum, Borissiakoceras .. ......... .. 7. 28, 23 Ostrea beloiti ..............2.2..2.2.2.0... 7 P Paguate Tongue, Dakota Sandstone .... ..... 7 pentagonum, Acanthoceras ................. 9 Eucalycoceras ................. 9, 11, 25; pl. 3 (Eucalycoceras) .................... 9 saharense, Eucalycoceras ...... .. 9 pentagonus, Ammonites ............... ..2, 8, 9 pictus, Inoceramus ........................ 15, 20 Plenus Marls 17 Plesiacanthoceras 12 wyomingense 12, 25 plicatile, Anisoceras ..................... 7, 10, 18 pondi, Dunveganoceras ...... 8, 12, 22, 25 praecox, Metoicoceras ...................... 8 prefragilis, Inoceramus ..................... 12, 22 Page stephensoni, Inoceramus ................ 8 Prionotropis echinatus ..................... 17 [Prionocyclus] neptuni, Ammonites ...... ... 17 problematicum, Dunveganoceras ............. 22, 25 (Proeucalycoceras) dentonense, Eucalycoceras .. 12 lewisvillense, Eucalycoceras ... ...... .. 12 proprium, Vascoceras ...................... 18 Protacanthoceras angolaense ................ 12 '"Protacanthoceras' angolaense ..... ...... ... 12 Pseudacompsoceras landisi ................. 3 Pseudaspidoceras pseudonodosoides ..... .... 18 he 8p -. asc AAAE Ta re ed pre s ae antee . 18 Pseudocalycoceras ......................... 1, 12 angolaense ................ 12, 15, 25; pls. 5, 6 dentonense ................2.2.2.0.... 12, 15 harpa® .... .. .o suave psa s aan val s an ait T, 14 A@ugt (. uske gas ncs area dens ede aa 13 (Haugiceras) angolaense ................ 12 (Neocalycoceras) angolaense ...... .... ... 12 Pseudomelania basicostata ............ 22. 7 pseudonodosoides, Pseudaspidoceras ...... ... 18 (Pseudouhligella), Desmoceras ...... ...... ... Pulchellia bentoniana ... ................... 15 Puzosia Sp 22220 eee e> 7 R, S rhotomagense, Acanthoceras ................ 12 rollandi, Thomasites ....................... 18 rotatile, Tarrantoceras . 2, 5 rowei, Eucalycoceras ....................... 8 rutherfordi, Inoceramus .................... T saharense, Eucalycoceras pentagonum ... ..... 9 Sciponoceras bohemicum anterius ......... .. 18 gracile 2222222 14, 15, 18, 25 scott, Moremanoceras ..................... 10 sellardsi, Eucalycoceras .................... 5 Mantelliceras 2, 5 Tarrantoceras ............... 5, 8, 25; pls. 1, 2 Utaticeras 5 septemseriatum, Euomphaloceras (Kanabiceras) 14, 15, 17, 25 serotinus, Thomelites .. 18 silvanense, Vascoceras ..... Moen 18 simplex, Hamites ......................... 22, 23 Six Flags Limestone Member, Woodbine For- MAtION ...ll lll 22222 lls lees 12 Spathites (Jeanrogericeras) subconciliatus .... 18 stantoni, Tarrantoceras ........... - 5 stephensoni, Inoceramus prefragilis ...... ... 8 U.S. GOVERNMENT PRINTING OFFICE: 1989-673-047-86,055 REGION NO. 8 Page subconciliatus, Spathites (Jeanrogericeras) .. .. 18 Suinitamaceras . .... .l... ul .n u iad. 1, 2, 14, 25 benfonianum a: .us. n na oe 15, 25; pls. 6, 7 confinils o. .s cle dust bol sa 14, 17, 25; pl. 7 feu stun s. . . sut o do unl u aon oy eate ay 14 (Sumitomoceras) bentonianum, Tarrantoceras .. 16 conlini, Tarrantoceras .................. 14 T. Tarrant Formation ...............2.2..2... 7, 16 Tarrantoceras ...... .. 1. 2 14, 16. 25 bentontanum . ...s. ...u... acl 16 Rlexicostatum ................0.. .. 7, 25; pl. 2 ...i... cl .la enna nas nan ah 5 milticostatum ...... .. livens. 5 Fofafile |.. ...... .ll deal s 2, 5 sellardsi . 2 ... cual. clue cen deco erasers 1. 5 (Sumitomoceras) bentonianum ..... ...... 16 ...0.... .. ul on 14 (Tarrantoceras) lilianense .. ............. 16 (Tarrantoceras) lilianense, Tarrantoceras ...... 16 Templeton Member, Woodbine Formation .... 11 templetonense, Eucalycoceras ...... .... 10, 25; pl. 4 tenue, Neocardioceras ...................... 18, 21 Thomasites gongilensis lautus .............. 18 rollandt | ... .... cu eats bee savin sn 18 8p ll ata io Seite toe des oo anes 18 Thomelites serotinus ....................... 18 . oe . apon o ter H n Se 18 Tropic Shale .ll cave 14, 18 Turrilites acutus americanus ................ 7 U-Z underwoodi, Eucalycoceras ................. 12 uptonense, Neocardioceras .. ...... 20, 23, 25; pl. 10 Utaticeras sellardsi ........................ 5 Vascoceras cauvini .................2..2... 18, 26 diartianum | 2... 10, 25 [OPIUM .... 2222020202202 uuu uva u ues 18 silvanense 2.02. 18 vermiculus, Worthoceras ................... 18 Woodbine Formation ...................... 11, 12 Worthoceras gibbosum .. ................... 14, 15 vermiculus 18 §p 0200.) so . oe n enone n aoa e on ates 20 wyomingense, Plesiacanthoceras ............. 12, 25 Zonation, Cenomanian ammonites ........... 25 PLATES Contact photographs of the plates in this report are available, at cost, from the U.S. Geological Survey Library, Federal Center, Denver, CO 80225 PLATE 1 [All figures natural size. Arrows mark base of body chambers] FiGurES 1-13. Tarrantoceras sellardsi (Adkins) (p. 5). 1-3. Front, side, and rear views of hypotype USNM 400759, from USGS Mesozoic locality 24510 (text fig. 2). 4-7. Rear, top, front, and side views of hypotype USNM 400760, from USGS Mesozoic locality D12626 (text fig. 2). 8, 9. Bottom and side views of a plaster cast of the holotype from Williamson County, Tex. 10-13. Front, side, rear, and top views of hypotype USNM 400761, from USGS Mesozoic locality D5309 (text fig. 1, loc. 44). Note the injured ventral area on figure 12 that was later healed (fig. 13). U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 PLATE 1 TARRANTOCERAS PLATE 2 [All figures natural size except as indicated. Arrows mark base of body chambers] FiGurRES 1-22, 27, 28. Tarrantoceras sellardsi (Adkins) (p. 5). 1-3. 4-6. 7, 8. 9-11. 12-14. 15-17. 18-20. 21, 22. 27, 28. Hypotype USNM 400762, from USGS Mesozoic locality D12626 (text fig. 2). Hypotype USNM 400763, from USGS Mesozoic locality 24510 (text fig. 2). Hypotype USNM 400764, from USGS Mesozoic locality 24510 (text fig. 2). Hypotype USNM 400765, from USGS Mesozoic locality 24510 (text fig. 2). Hypotype USNM 400766, from USGS Mesozoic locality D12626 (text fig. 2). Hypotype USNM 400767, from USGS Mesozoic locality D12626 (text fig. 2). Hypotype USNM 400768, X3, from USGS Mesozoic locality D12626 (text fig. 2). Hypotype USNM 400769, from USGS Mesozoic locality 24510 (text fig. 2). Hypotype USNM 400770, from USGS Mesozoic locality 24510 (text fig. 2). Note injury on venter (fig. 28). 23-26. Tarrantoceras flexicostatum Cobban, n. sp. (p. 7). 23, 24. Paratype USNM 400774, from USGS Mesozoic locality D12630 (text fig. 1, loc. 5). 25, 26. Holotype USNM 400773, from USGS Mesozoic locality 21850 (text fig. 1, loc. 6). U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 _ PLATE 2 1 2 3 4 5 6 12 13 14 TARRANTOCERAS PLATE 3 [All figures natural size. Arrows mark base of body chambers] FIGURES 1-13. Eucalycoceras pentagonum (Jukes-Browne) (p. 9). 1, 2. Hypotype USNM 400775, from USGS Mesozoic locality D11483 (text fig. 1, loc. 61). See text figure 7B for suture. Hypotype USNM 400776, from USGS Mesozoic locality D11483 (text fig. 1, loc. 61). Hypotype USNM 400777, from USGS Mesozoic locality 22899 (text fig. 1, loc. 34). 6. Hypotype USNM 400778, from USGS Mesozoic locality 23060 (text fig. 1, loc. 8). 3 4, 5. 8. Hypotype USNM 400779, from USGS Mesozoic locality 22899 (text fig. 1, loc. 34). 9. 3. 7, Hypotype USNM 400780, from USGS Mesozoic locality 23060 (text fig. 1, loc. 8). Bottom, top, rear, and side views of hypotype USNM 400781, from USGS Mesozoic locality 22899 (text fig. 1, loc. 34). 10-1 U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 _ PLATE 3 12 EUCALYCOCERAS PLATE 4 [All figures natural size. Arrows mark base of body chambers] FIGURES 1-13. Eucalycoceras templetonense Cobban, n. sp. (p. 10). 1. Paratype USNM 400783, from USGS Mesozoic locality D5940 (text fig. 1, loc. 11). 2-4. Holotype USNM 400782, from USGS Mesozoic locality 20314 on Templeton Branch northeast of Bells, Grayson County, Tex. 5, 6. Paratype USNM 400785, from the same locality as figs. 2-4. 7, 8. Paratype USNM 400786, from USGS Mesozoic locality D7530 (text fig. 1, loc. 20). 9, 12. Paratype USNM 400784, from USGS Mesozoic locality D7530 (text fig. 1, loc. 20). 10, 13. Paratype USNM 400788, from the same locality as figs. 2-4. Figure 10 is the innermost whorls of figure 13. 11. Paratype USNM 400787, from USGS Mesozoic locality D10734 (text fig. 1, loc. 42). U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 PLATE 4 12 EUCALYCOCERAS PLATE 5 [All figures natural size. Arrows mark base of body chambers] FIGURES 1-27. Pseudocalycoceras angolaense (Spath) (p. 12). 1, 2. 3, 4. 5-7. 8, 9. 10, 11. 12-15. 16, 17. 18, 19. 20-22. 23, 24. 25-27. Hypotype USNM 400789, from USGS Mesozoic locality D11529 (text fig. 1, loc. 54). Hypotype USNM 400790, from the same locality. Hypotype USNM 400791, from the same locality. Hypotype USNM 400792, from the same locality. See text figure 10 for the suture. Hypotype USNM 400793, from USGS Mesozoic locality D10196 (text fig. 1, loc. 60). Front, side, rear, and bottom views of hypotype USNM 400794, from USGS Mesozoic locality D11538 (text fig. 1, loc. 56). Hypotype USNM 400795, from USGS Mesozoic locality D10112 (text fig. 1, loc. 58). Hypotype USNM 400796, from USGS Mesozoic locality D11529 (text fig. 1, loc. 54). Hypotype USNM 400797, from USGS Mesozoic locality D10112 (text fig. 1, loc.58). Hypotype USNM 400798, from USGS Mesozoic locality D11529 (text fig. 1, loc. 54). Hypotype USNM 400799, from USGS Mesozoic locality D10196 (text fig. 1, loc. 60). U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 _ PLATE 5 PSEUDOCALYCOCERAS PLATE 6 [All figures natural size. Arrows mark base of body chambers] FIGURES 1, 2, 13, 14, 18, 19. Pseudocalycoceras angolaense (Spath) (p. 12). 1, 2. Hypotype USNM 400800, from the Britton Formation at the spillway of the Garza-Little Elm Reservoir, Denton County, Tex. (USGS Mesozoic loc. D9439). 13, 14. Hypotype USNM 400801, from USGS Mesozoic locality 18686 (text fig. 1, loc. 33). 18, 19. Hypotype USNM 400802, from the same locality. 3-12, 15-17. Sumitomoceras bentonianum (Cragin) (p. 15). 3-5. Hypotype USNM 400810, from USGS Mesozoic locality D6842 (text fig. 1, loc. 57). See text figure 12 for suture. 6, 7. Hypotype USNM 400811, from USGS Mesozoic locality 18686 (text fig. 1, loc. 33). 8, 9. Hypotype USNM 400812, from USGS Mesozoic locality D12452 (text fig. 1, loc. 35). 10-12. Hypotype USNM 400813, from USGS Mesozoic locality D10196 (text fig. 1, loc. 60). 15-17. - Top, back, and side views of a plaster cast of the holotype from the Britton Formation at Hackberry Creek, Dallas County, Tex. U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 _ PLATE 6 G 17 18 PSEUDOCALYCOCERAS AND SUMITOMOCERAS PLATE 7 [All figures natural size except as indicated. Arrows mark base of body chambers] FIGURES 1-15, 26-28. Sumitomoceras conlini Wright and Kennedy (p. 14). 1-4. 5-7. 8. 9, 10. 11-13. 14, 15. 26-28. Hypotype USNM 400804, X1 and X2, from USGS Mesozoic locality D11529 (text fig. 2, loc. 54). Hypotype USNM 400805, from the same locality. Hypotype USNM 400806, from the same locality. Hypotype USNM 400807, from USGS Mesozoic locality D10196 (text fig. 1, loc. 60). See text figure 11A for suture. Hypotype USNM 400808, from USGS Mesozoic locality D11529 (text fig. 1, loc. 54). Hypotype USNM 400809, from the Boquillas Limestone at USGS Mesozoic locality D7466 along State Route 163 opposite entrance to Tom Brite road, Val Verde County, Tex. Holotype USNM 400803 (J. P. Conlin 4500), from the Britton Formation 4 km (2% mi) south of Britton, Ellis County, Tex. Figure 28 was taken after the older part of the body chamber was removed. See text figure 11B for the suture. 16-25. Sumitomoceras bentonianum (Cragin) (p. 15). 16, 17. 18-20. 21-28. 24, 25. Hypotype USNM 400814, from USGS Mesozoic locality 18686 (text fig. 1, loc. 33). Hypotype USNM 400815, from the same locality as figs. 14, 15. Side, top, and rear views of hypotype USNM 400816 (J.P. Conlin 4499), from the Britton Formation on a tributary to Newton Branch 6 km (3% mi) south of Britton, Ellis County, Tex. Hypotype USNM 400817, from the Boquillas Formation at USGS Mesozoic locality 15344 on the Nueces River 1 km upstream from the Southern Pacific Railway bridge, Uvalde County, Tex. U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 PLATE 7 26 SUMITOMOCERAS PLATE 8 [All figures natural size except as indicated. Arrows mark base of body chambers] FIGURES 1-39. Neocardioceras juddii (Barrois and de Guerne) (p. 17). 1-3. 4-6. 7-9. 10-12. 13, 14. 15-17. 18-20. 21, 22. 23, 24. 25-27. 28, 29. 30. 31, 82. 33, 34. 35, 36. 37-39. Hypotype USNM 400818, from USGS Mesozoic locality D10533 (text fig. 1, loc. 50). Hypotype USNM 400819, from the same locality. Hypotype USNM 400820, from the same locality. Hypotype USNM 400821, from USGS Mesozoic locality D9031 (text fig. 1, loc. 49). Hypotype USNM 400822, from USGS Mesozoic locality D10996 (text fig. 1, loc. 51). Hypotype USNM 400823, X2, from USGS Mesozoic locality D10533 (text fig. 1, loc. 50). Hypotype USNM 400824, from the same locality. Hypotype USNM 400825, from USGS Mesozoic locality D11510 (text fig. 1, loc. 53). Hypotype USNM 400826, from USGS Mesozoic locality D10533 (text fig. 1, loc. 50). See text figure 14 for suture. Side, rear, and top views of hypotype USNM 400827, from USGS Mesozoic locality D10114 (text fig. 1, loc. 59). Side and top views of hypotype USNM 356889, from the same locality. Hypotype USNM 400828, from the same locality. Hypotype USNM 307357, from the same locality. Hypotype USNM 400829, from USGS Mesozoic locality D11004 (text fig. 1, loc. 52). Hypotype USNM 400830, from the same locality. Hypotype USNM 400831, from USGS Mesozoic locality D11533 (text fig. 1, loc. 55). PLATE 8 PROFESSIONAL PAPER 1473 U.S. GEOLOGICAL SURVEY NEOCARDIOCERAS PLATE 9 [All figures natural size} FIGURES 1-31. Neocardioceras densicostatum Cobban, n. sp. (p. 19). From the Hartland Member of the Greenhorn Limestone at USGS Mesozoic locality D7410 (text fig. 1, loc. 27). Paratype USNM 400833. Paratype USNM 400834. Paratype USNM 400835. Paratype USNM 400836. Paratype USNM 400837. Paratype USNM 400838. Paratype USNM 400839. Paratype USNM 400840. 9. Paratype USNM 400841. 10. Paratype USNM 400842. 11. Paratype USNM 400843a, b. 12. Paratype USNM 400844. 13. Paratype USNM 400845a, b. 14. Paratype USNM 400846. 15. Paratype USNM 400847. 16. Paratype USNM 400848a, b. 17. Paratype USNM 400849. 18. Paratype USNM 400850. 19. Paratype USNM 400851. 20. Paratype USNM 400852. 21. Paratype USNM 400853. 22. Paratype USNM 400854. 23. Holotype USNM 400832. 24. Paratype USNM 400855. 25. Paratype USNM 400856. 26. Paratype USNM 400857. 27. Paratype USNM 400858. 28. Paratype USNM 400859. 29. Paratype USNM 400860. 30. Paratype USNM 400861. 31. Paratype USNM 4008622, b. 32-38. Neocardioceras laevigatum Cobban, n. sp. (p. 22). 32, 33. Paratype USNM 400890, from USGS Mesozoic locality D4462 (text fig. 1, loc. 10). 34. Paratype USNM 400891, from USGS Mesozoic locality D7395 (text fig. 1, loc. 24). 35, 36. Holotype USNM 400889, from USGS Mesozoic locality D5947 (text fig. 1, loc. 12). 37. Paratype USNM 400892, from USGS Mesozoic locality D7390 (text fig. 1, loc. 23). 38. Paratype USNM 400893 (a) associated with impressions of Neocardioceras uptonense Cobban, n. sp., from USGS Mesozoic locality D7402 (text fig. 1, loc. 25). p no ue to b m U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 1s Toan e ~ sik ie i. NEOCARDIOCERAS PLATE 10 [All figures natural size except as indicated. Arrows mark base of body chambers] FiGurESs 1-35. Neocardioceras minutum Cobban, n. sp. (p. 23). 1, 2. Holotype, USNM 400894, from USGS Mesozoic locality D4462 (text fig. 1, loc. 10). 3, 4. Paratype USNM 400895, from USGS Mesozoic locality 12740 (text fig. 1, loc. 7). 5-7. Paratype USNM 400896, from USGS Mesozoic locality D4462 (text fig. 1, loc. 10). 8, 9. Paratype USNM 400897, from the same locality as figures 5-7. See text figure 20 for suture. 10-12. Paratype USNM 400898, X2, from the same locality. 13-15. Paratype USNM 400899, X2, from the same locality. 16, 17. Paratype USNM 400900, from USGS Mesozoic locality 12740 (text fig. 1, loc. 7). 18, 19. Paratype USNM 400901, X2, from the same locality as figures 16 and 17. 20-22. Paratype USNM 400902, X2, from the same locality. 23-25. Paratype USNM 400903, X2, from USGS Mesozoic locality D4462 (text fig. 1, loc. 10). 26, 27. Paratype USNM 400904, X2, from the same locality as figures 23-25. 28. Paratype USNM 400908, from USGS Mesozoic locality D7390 (text fig. 1, loc. 23). 29-31. Paratype USNM 400905, X3, from USGS Mesozoic locality 12740 (text fig. 1, loc. 7). 32-85. Top, rear, side, and front views of paratype USNM 400906, X2, from USGS Mesozoic locality D4462 (text fig. 1, loc. 10). 36-70. Neocardioceras uptonense Cobban, n. sp. (p. 20). 36. Paratype USNM 400881, from USGS Mesozoic locality D7402 (text fig. 1, loc. 25). 37. Paratype USNM 400882, from the same locality as figure 36. 38-40. Paratype USNM 400864, from USGS Mesozoic locality D5947 (text fig. 1, loc. 12). 41, 42. Paratype USNM 400865, from the same locality as figures 38-40. 43, 44. Paratype USNM 400866, from the same locality. 45-47. Paratype USNM 400867, from the same locality. 48, 49. Paratype USNM 400868, from the same locality. 50. Latex cast of paratype USNM 400869, from the same locality. 51, 52. Paratype USNM 400870, from the same locality. 53. Latex cast of paratype USNM 400871, from the same locality. 54. Latex cast of paratype USNM 400878, from USGS Mesozoic locality D7402 (text fig. 1, loc. 25). 55. Paratype USNM 400879, from the same locality as figure 54. 56. Latex cast of paratype USNM 400880, from the same locality. 57. Paratype USNM 400872, from USGS Mesozoic locality D5947 (text fig. 1, loc. 12). 58. Paratype USNM 400873, from the same locality as figure 57. 59. Paratype USNM 400883, from the same locality as figure 54. 60. Paratype USNM 400884, from the same locality as figure 54. 61, 62. Paratype USNM 400874, from the same locality as figure 57. 63. Paratype USNM 400875, from the same locality as figure 57. 64-66. Holotype USNM 400863, from the same locality. 67. Paratype USNM 400885, from the same locality as figure 54. 68. Latex cast of paratype USNM 400886, from the same locality as figure 54. 69. Latex cast of paratype USNM 400887, from the same locality. 70. Paratype USNM 400888, from the same locality. 71, 72. Neocardioceras sp. (p. 24). Figured specimen USNM 400909, from USGS Mesozoic locality D6962 (text fig. 1, loc. 14). U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1473 PLATE 10 10x21 2 x2 x 2 X215x2/ f ~ 16 17 11 12 13 : 14 32 39 40 41 42 0 4s 440 as 46 {WIS - Uh. 33 38 sa & BYE} \ L nd a ty kel ¥ 9 l ' m 4 NEOCARDIOCERAS DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 117°07'30" 117°05. 117°U’30” C> 38°3730" 39°35" 117°00' FEET QC 75 P6 PROFESSIONAL PAPER 1471 PLATE 4 CORRELATION OF MAP UNITS METERS 117°0730" R 117°09 17802230" Base from U.S. Geological Survey 1908 SCALE 1:24 000 Manhattan, Round Mountain, 1 ¥ o A and Belmont West, 1971 m- -- -- r= p= - 3 1 5 0 1 KILOMETER CONTOUR INTERVAL 40 FEET APPROXIMATE MEAN NATIONAL GEODETIC VERTICAL DATUM OF 1929 DECLINATION, 1988 TRUE NORTH Diamond King Bald Mountain FEET 9000 8000 Manhattan Gulch & 7000 f 6000 5000 4000 3000 2000 r g g a B METERS C I C METERS FEET (- 2500 FEET 3s g fi -2500 8000 8000 Egg Trmr Trm ris Oal Tm - est m mt? ost - os) Tru __2g . C T 1000 -| Prs & v o 6000 6000 5000 -1500 5000 - e I- |-1500 4000 4000 - (- 1000 3000 3000- 2000 2000 F FEET 3000 E E" zig reer dk Trus sL 8000 Uu Tru? Qal NC Wi f- 4 mec 1000 Trm _ {9 Silver Creek 7000 i Ziz 8000 --3><__ _ Trur Tdk Qal Tr |-- Trus Tdk i S2 " Ti s <5 Ti aim“, w \ j f MILE FEET 8000 7000 6000 5000 4000 3000 117°00' Trl DI o § = 5 bs S C mo Wash 2000 F’ g) Silver Creek 6000 _ 6000 5000 5000 4000 L 4000 Kap: 3000 3000 GEOLOGIC AND GRAVITY MAP OF THE MANHATTAN CALDERA, NYE COUNTY, NEVADA, INCLUDING GEOLOGIC CROSS SECTIONS A-A' THROUGH G-G' NEVADA MAP LOCATION METERS r- 2500 |- 2000 - 1500 |- 1000 METERS |-2500 |- 2000 |-1500 |-1000 38°35" } QUATERNARY > TERTIARY CRETACEOUS PALEOZOIC DESCRIPTION OF MAP UNITS Kg Pd Oz Oza Est 38°32"30" 5 ‘ Mariposa Canyon 116°57'30" Geology by D. R. Shawe, 1973-82 Gravity measurements by D.B. Snyder, 1981-83 METERS - 2500 rim Qal [- 2000 - 1500 (- 1000 117°07"30" 3937-80" wtp ¢-Tris Granite of Pipe Spring 38°30" 117°0730" i Alluvium (Quaternary)-Includes minor ta- lus and landslide deposits Volcanic rocks (Tertiary)-Undivided on in- set map Dacite-quartz latite plug, domes, dikes, and sills (Tertiary)-24.5 Ma Crone Gulch Andesite (Tertiary)-Consists of stock, sills, and dikes Rhyolite and quartz latite plugs (Tertiary)- 24.8 Ma Younger volcanic rocks (Tertiary)-Consists of the Bald Mountain Formation andthe tuff of Peavine Creek (24.6 Ma) Diamond King Formation (Tertiary) Tuff-breccia dikes (Tertiary) Round Rock Formation (Tertiary)- About 25 Ma. Consists of: Upper member-Locally includes: Megabreccia unit of Silver Creek Rhyolite megabreccia Middle member-Locally includes: Rhyolite plugs and dikes Lower member-Locally includes: Megabreccia unit of Mariposa Canyon Megabreccia unit of Sloppy Gulch- Internal contacts bound bedded layers in the unit m Rhyolite dikes (Tertiary)-36 Ma Granite and sedimentary rocks, undivided (Cretaceous and Paleozoic) on inset map-Cretaceous granite and Paleo- zoic sedimentary rocks Porphyritic granite (Cretaceous) Nonporphyritic granite (Cretaceous)-95 Ma Marine sedimentary rocks (Paleozoic) MEGABRECCIA CLAST DESIGNATIONS Cretaceous granite Permian Diablo Formation Ordovician(?) Zanzibar Limestone Limestone Argillite Cambrian siltstone Cambrian quartzite Paleozoic limestone Paleozoic argillite Paleozoic schist Contact-Dashed where gradational or in- ferred; queried where uncertain -- ___ Strike and dip of beds --- High-angle fault-dashed where inferred, dotted where concealed, queried where uncertain; bar and ball on downthrown side; arrow shows direction of move- ment, in cross section -&A--&- Low-angle fault-Dotted where concealed; teeth on upper plate Margin of Manhattan caldera Contour of megabreccia clasts in the mega- breccia unit of Silver Creek-Show- ing maximum size, in meters 0 Data point-Locality where megabreccia clast -29.29 size was determined e Gravity data-Isostatic residual gravity map reduced at 2,670 kg/m. Isostatic con- ditions assume Airy-type compensa- tion with a crustal thickness at sea level of 25 km, density of topography 2,670 kg/m, and density contrast at the Moho of 400 kg/m. See also Bol and others (1983) and Healey and others (1981) --25- Gravity contour-Contour interval 1 milligal 2.32 | Specific gravity of surface sample Note: Trl/Pzs, Trl/Kg-Kgp contacts on cross sec- tions are based on two-dimensional gravity modeling and on geologic constraints. Den- sities (D) used in modeling are shown on the cross sections, and are based on densities determined from surface samples collected at localities indicated on map. No vertical exag- geration on sections 117°00' | 117°00° E) 1 2 1,3 MILES I I I T 0 1 2 3 KILOMETERS |38°37"30" 38°30" SUMMARY MAP SHOWING GENERALIZED DISTRIBUTION OF MEGABRECCIA UNITS AND ASSOCIATED INTRUSIVE ROCKS 38°45" 30°37'30" 38°30' 3 3+ 2 4 KILOMETERS ] CONTOUR INTERVAL 200 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAP LOCATION MAP SHOWING AEROMAGNETIC CONTOURS, CALDERAS, AND GRANITE PLUTONS OF PART OF THE SOUTHERN TOQUIMA RANGE, NYE COUNTY,NEVADA PCG DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Py ao g PAplEIPEAl'lfLI-Z7 é 117°0730" 117°00" | e = a \ 16°52'30 5 a 4 \ a e| e\e E4 ® ~2o A j 1 3 38°45 s Cp 2 ) & */* P0 HP)) *>, \ N | s O * % s § l‘ e y M N\ ¥ x L. $ P 'f 2:0 *% l e 710) | 7 i ' +, f j P Ly. T O o; # .' [ f ounp F til C | Q p < $ v. o % $ F 6 ig o e G Q 9 600 ** * O ( °. a EXPLANATION H (a s o & C % Outline of stock or granite pluton- / ® (a) Dashed where projected Outline of caldera-Dashed where e — projected; queried where uncer- Q @ tain a Magnetic contours-Showing total j / a 4,30? intensity of Earth's magnetic field, in gammas, relative to arbitrary C datum; contours interrupted * # C where too close to show; con- O Q) A S tour interval 20 gammas; L, mag- -200 [e] netic low; H, magnetic high 0 F, -32 O Aeromagnetic survey flown by Aerial 0 * o Surveys; Salt Lake Ciiy Utah in .' fi 1978, at 1,000 ft above ground; a L > original flightline spacing approx- a é a a imately 4 mile. From U.S. Geolo- gical Survey (1979) s O [ , 2 : o O A O O ® o O * Moss a % ® ® / o at O (3 G / A (o)] M U \ L C : o Id 44 x V fs 0, 0 g" & [€] e \ oJ G A «€ 0 (xz f 0 *. o _| - i & L \ 0 % H 0 0 0 ts ( . \ f 6 G h 2s E o' n . B E « # a \ ( ue - i 0 0 L”, as //450 "a C a O C>\ ¥ % » e e O a ® a $ Q 0 )- (€ Tile | L o ® | 0 F /. J" st Q / /// ~30 { -2 * o D g < { A1. cm ~ 16 are fit ( Biv i \ f ‘ M 58 h t a \ Q ® 0% s “a D h é Q3 G g / g I H t 3 ) s 1 % ( R ¥ * y as* Q L D -400 : s o B F \ § b lal o a s is & AYR U Riss b f O q =% g y o_» -x _ (& ® P @ o M a ® 2 9 m ->... | ar a ® 0 A a l n 1 o a 204g j % B) I TEN ; 0 e 0 0 p s o U 3 c A E A D £7 f k. @ © & 3 0 E0 I C3 * r- % D 0 Q o ~I @ s pas Q l $ s 0 > 0 ~ e * z 4 U A I T 0 4 ) fo MX / ® $ a o ¢ % ~ M A g 7 0 $200 ¢ * a / I s / as * a a ® f é & e $ * e ¢ e H e | j # 2 | "% 117°0730" a= i & | I 117°00 116°52'30" Egsspfgzlrm léguggglogéfil Survey SCALE 1:62 500 acs" < % : aks 1 DEPARTMENT OF THE INTERIOR ( J } U.S. GEOLOGICAL SURVEY 117°02"30" 30 "45 7 ing fracture 'and margin of \ Mbunti(Jefferson T) caldera A f u - k64645 al) l 38°42"30" 117°02'30" Base from U.S. Geological Survey, Round Mountain, 1971 CQC 5 P6 PROFESSIONAL PAPER 1471 PLATE 3 CORRELATION OF MAP UNITS DESCRIPTION OF MAP UNITS [MEGABRECCIA CLAST DESIGNATIONS ¢ K t i QUATERNARY Oal _| Alluvium (Quaternary) o granite €q Cambrian Gold Hill Formation quartzite Tuff of Mount Jefferson (Tertiary)- Pe] Paleozoic imestone 27 Ma TERTIARY Megabreccia of Jefferson Canyon Pra Paleozoic argillite (Tertiary) Contact G diorite stock (Tertiary)- 36 M f tenodiorite stock (Tertiaty) S3 y- Fault-Dotted where concealed; bar and CRETACEOUS Granite (Cretaceous) bfall on downthrown side; arrow shows direction of movement, in cross section PALEOZOIC Marine sedimentary rocks (Paleozoic) Margin of Mount Jefferson caldera METERS (- 2500 Jefferson Canyon 6000 5000 4000 FEET f I METERS : 2500 8000 V 7000 . - 2000 6000 | 5000 - 1500 "30°42930" 4000 117°00° Geology by D.R. Shawe, 1973-74 MAPPED AREA APPROXIMATE MEAN pecuination, 1988 GEOLOGIC MAP OF PART OF THE SOUTHWEST MARGIN OF THE MOUNT JEFFERSON CALDERA AND ADJACENT TERRANE, NYE COUNTY, NEVADA, INCLUDING GEOLOGIC CROSS SECTIONS H-H' AND I-T SCALE 1:24 000 0 1 MILE 0 1 KILOMETER CONTOUR INTERVAL 40 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929