STATE OF ILLINOIS ADLAI E. STEVENSON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION NOBLE J. PUFFER, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS— NO. 145 NIAGARAN REEFS IN ILLINOIS AND THEIR RELATION TO OIL ACCUMULATION BY H. A. LOWENSTAM PRINTED BV AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1949 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/niagaranreefsini145lowe STATE OF ILLINOIS ADLAI E. STEVENSON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION NOBLE J. PUFFER, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS— NO. 145 NIAGARAN REEFS IN ILLINOIS AND THEIR RELATION TO OIL ACCUMULATION BY H. A. LOWENSTAM PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1949 ORGANIZATION STATE OF ILLINOIS HON. ADLAI E. STEVENSON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION HON. NOBLE J. PUFFER, Director BOARD OF NATURAL RESOURCES AND CONSERVATION HON. NOBLE J. PUFFER, Chairman W. H. NEWHOUSE, Ph.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry LOUIS R. HOWSON, C.E., Engineering A. E. EMERSON, Ph.D., Biology LEWIS H. TIFFANY, Ph.D., Forestry GEORGE D. STODDARD, Ph.D., Litt.D., LL.D., L.H.D. President of the University of Illinois GEOLOGICAL SURVEY DIVISION M. M. LEIGHTON, Ph.D., Chief (84780— 3M— 9-49) SCIENTIFIC AND TECHNICAL STAFF OF THE STATE GEOLOGICAL SURVEY DIVISION 100 Natural Resources Building, Urbana M. M. LEIGHTON, Ph.D., Chief ENID TOWNLEY, M.S., Assistant to the Chief Velda A. Millard, Junior Asst. to the Chief Elizabeth Stephens, B.S., Geological Assistant Helen E. McMorris, Secretary to the Chief Berenice Reed, Supervisory Technical Assistant GEOLOGICAL RESOURCES Arthur Bevan, Ph.D., D.Sc, Principal Geologist Coal G. H. Cady, Ph.D., Senior Geologist and Head R. J. Helfinstine, M.S., Mechanical Engineer George M. Wilson, M.S., Geologist Robert M. Kosanke, M.A., Associate Geologist John A. Harrison, M.S., Assistant Geologist Jack A. Simon, M.S., Assistant Geologist Raymond Siever, M.S., Assistant Geologist Mary Barnes Rolley, M.S., Assistant Geologist Margaret A. Parker, B.S., Assistant Geologist Kenneth E. Clegg, Technical Assistant Oil and Gas A. H. Bell, Ph.D., Geologist and Head Frederick Squires, A.B., B.S., Petroleum Engineer David H. Swann, Ph.D., Geologist Virginia Kline, Ph.D., Associate Geologist Wayne F. Meents, Assistant Geologist Richard J. Cassin, M.S., Assistant Petroleum En- gineer Lester W. Clutter, B.S., Research Assistant Industrial Minerals J. E. Lamar, B.S., Geologist and Head Robert M. Grogan, Ph.D., Geologist Donald L. Graf, M.A., Assistant Geologist James C. Bradbury, A.B., Assistant Geologist Raymond S. Shrode, B.S., Assistant Geologist Clay Resources and Clay Mineral Technology Ralph E. Grim, Ph.D., Petrographer and Head William A. White, M.S., Associate Geologist Herbert D. Glass, M.A., Associate Geologist Groundwater Geology and Geophysical Exploration Merlyn B. Buhle, M.S., Associate Geologist M. W. Pullen, Jr., M.S., Associate Geologist Richard F. Fisher, M.S., Assistant Geologist Margaret J. Castle, Assistant Geologic Draftsman Robert D. Knodle, M.S., Assistant Geologist John W. Foster, B.A., Assistant Geologist Engineering Geology and Topographic Mapping George E. Ekblaw, Ph.D., Geologist and Head Areal Geology and Paleontology H. B. Willman, Ph.D., Geologist and Head J. S. Templeton, Ph.D., Geologist Ruth Bickell, Technical Assistant Jane Teller, A.B., Technical Assistant GEOCHEMISTRY Frank H. Reed, Ph.D., Chief Chemist Grace C. Johnson, B.S., Research Assistant Coal G. R. Yohe, Ph.D., Chemist and Head Donald R. Hill, B.S., Research Assistant Joseph E. Dunbar, B.S., Research Assistant Industrial Minerals J. S. Machin, Ph.D., Chemist and Head Tin Boo Yee, M.S., Assistant Chemist Paulene Ekman, B.A., Research Assistant Grace C. Moulton, M.S., Research Assistant Fluorspar G. C. Finger, Ph.D., Chemist and Head Robert E. Oesterling, B.A., Special Research Assistant James L. Finnerty, B.S., Special Research Assistant Chemical Engineering H. W. Jackman, M.S.E., Chemical Engineer and Head P. W. Henline, M.S., Chemical Engineer B. J. Greenwood, B.S., Mechanical Engineer James C. McCullough, Research Associate X-ray W. F. Bradley, Ph.D., Chemist and Head Physics Kenneth B. Thomson, Ph.D., Physicist R. J. Piersol, Ph.D., Physicist Emeritus Janice Helen Howard, B.S., Research Assistant Subsurface Geology L. E. Workman, M.S., Geologist and Head Elwood Atherton, Ph.D., Associate Geologist Donald B. Saxby, M.S,, Assistant Geologist Robert C. McDonald, B.S., Research Assistant Lois E. Titus, B.S., Research Assistant Mineral Resource Records Vivian Gordon, Head Harriet C. Daniels, B.A., Technical Assistant Dorothy Gore, B.S., Research Assistant Dorothy A. Foutch, Technical Assistant Zora M. Kaminsky, B.E., Technical Assistant Elene L. Roberts, Technical Assistant Janice J. Pohlman, Technical Assistant Analytical Chemistry O. W. Rees, Ph.D., Chemist and Head L. D. McVicker, B.S., Chemist Howard S. Clark, A.B., Associate Chemist Emile D. Pierron, M.S., Assistant Chemist William F. Loranger, B.A., Research Assistant Annabelle G. Elliott, B.S., Technical Assistant Alice M. Helmuth, B.S., Research Assistant Ruth E. Koski, B.S., Research Assistant Charles T. Allbright, B.S., Research Assistant MINERAL ECONOMICS W. H. Voskuil, Ph.D., Mineral Economist W. L. Busch, Assistant Mineral Economist Nina Hamrick, A.M., Assistant Mineral Economist Ethel M. King, Research Assistant EDUCATIONAL EXTENSION Gilbert O. Raasch, Ph.D., Associate Geologist in Charge Margaret Ann Hayes, B.S., Research Assistant LIBRARY Anne E. Kovanda, B.S., B.L.S., Librarian Ruby D. Frison, Technical Assistant Marjorie Roepke, B.S., Technical Assistant PUBLICATIONS Dorothy E. Rose, B.S., Technical Editor M. Elizabeth Staaks, B.S., Assistant Editor Meredith M. Calkins, Geologic Draftsman Ardis D. Pye, Assistant Geologic Draftsman Wayne W. Nofftz, Technical Assistant Leslie D. Vaughan, Associate Photographer Beulah M. Unfer, Technical Assistant Consultants: Geology, George W. White, Ph.D., University of Illinois Ceramics, Ralph K. Hursh, B.S., University of Illinois Mechanical Engineering, Seichi Konzo, M.S., University of Illinois Topographic Mapping in Cooperation with the United States Geological Survey. This report is a contribution of the Areal Geology and Paleontology and the Oil and Gas Divisions. August 1, 1949 CONTENTS Page Introduction 7 Acknowledgments 8 Niagaran stratigraphy 9 Distribution 9 Thickness 9 Sedimentation belts 9 Differentiation into groups 12 Bainbridge group 12 St. Clair formation 13 Distribution 13 Thickness . 13 Lithology 16 Correlation 16 Moccasin Springs formation 16 Distribution 16 Thickness 17 Lithology 17 Correlation 18 Thorn group 18 Coe group 18 Upper contact of the Niagaran , 18 Lower contact of the Niagaran 20 Niagaran sedimentation belts 21 Reef-free high-clastic belt 21 Areal extent 21 Thickness 21 Reef-bearing belts 21 Interreef sediments of the low-clastic reef-bearing belt 22 Areal extent 22 Thickness : 22 Facies 22 Lower or southern (Bainbridge) wedge 23 St. Clair formation 23 Moccasin Springs formation 23 Upper or northern (Thorn) wedge 23 Rough-water deposits 24 Still-water deposits 24 Intermediate deposits 25 Relations of the two wedges 25 Interreef sediments of the clastic-free reef-bearing belt 25 Reefs ;••;•••. 26 Distribution 26 Origin and classification 27 Shape of the reefs 28 Size of the reefs 29 Internal reef structure 29 Lithology 29 Fossil characteristics 30 Structural expression of reefs 31 Reef border 31 Niagaran oil possibilities 33 ILLUSTRATIONS Figure Page 1. Niagaran sedimentation belts in Illinois showing line of cross-section (fig. 2) 8 2. Diagrammatic cross-section of Niagaran strata from northwestern to southeastern Illi- nois along line indicated in figure 1 9 3. Thickness of Silurian strata in Illinois 10 4. Thickness of Niagaran strata in Illinois 11 5. Thickness of predominantly red facies of Niagaran strata in Illinois 14 6. Niagaran reef occurrences and sedimentation belts in Illinois 15 7. Distribution of Silurian strata and location of known reefs 27 8. North-south cross-section of Silurian strata in northeastern Montgomery County show- ing reef proximity by reef outwash and by local thinning of Bainbridge (St. Clair and Moccasin Springs) strata in the opposite direction from the regional trend 32 9. Silurian oil possibilities 34 PLATE Electric log cross-section of Silurian and Devonian strata Inside back cover NIAGARAN REEFS IN ILLINOIS AND THEIR RELATION TO OIL ACCUMULATION BY H. A. LOWENSTAM INTRODUCTION Until the study of well cores late in 1943 revealed that the newly dis- covered Marine pool in Madison County was producing oil from rocks of Silurian age, there appeared to be little promise of oil in commercial quantities in the Silurian strata of Illinois. Detailed studies of the Marine pool 1 showed that the oil occurs in a Niagaran reef atoll, a type of oil reservoir not previously known in Illinois. Thus the Marine pool is an example of the strati- graphic-trap type of oil reservoir, in which the trap is formed by enclosure of a porous permeable reef lens in essentially imperme- able interreef deposits. After the nature of the reservoir in the Marine pool became known, exploration for similar Silurian reefs was a logical con- sequence. Several factors pointed to the existence of other buried reefs in Illinois, despite the fact that the Marine reef was separated by approximately 225 miles from the outcropping reefs in northeastern and northwestern Illinois. Observations on reef distribution in Silurian outcrops from Iowa to Ontario had long established the fact that Silurian reefs are not isolated but occur in groups. Also, the fact that the Marine reef was found to be as large as (if not larger than) any of the reefs in out- crops to the north indicated that environ- mental conditions favorable to reef devel- opment in Silurian time had existed as far south as the off-shore waters of the Silurian 1 Lowenstam, H. A., and DuBois, E. P., Marine pool, Madison County — a new type of oil reservoir in Illinois: Illinois Geol. Survey, Rept. Inv. 114, 1946. Lowenstam, H. A., Marine Pool, Madison County, Illinois, Silurian-reef producer: in Structure of Typical American Oil Fields, vol. Ill, Am. Assoc. Petrol. Geol. pp. 153-188, 1948; reprinted as Illinois Geol. Survey, Rept. Inv. 131, 1948. Ozark Island. On the basis of these obser- vations, it appeared reasonable to assume that the reef at Marine was possibly linked through other reefs, then unknown, to the reef archipelago in the outcropping areas to the north, 2 a view subsequently strength- ened through locating two other subsurface reef sites — not productive — northeast of the Marine reef. 3 The present study is a systematic exami- nation of the subsurface Silurian strata of Illinois to determine the occurrence and spacing of reefs, and to delineate their dis- tribution as a guide for further oil explora- tion. An area bounded on the northwest by the Illinois River, on the west by a line 10 to 20 miles east of the Mississippi River, on the south by a line extending across the state from Kaskaskia Island, Randolph County, northeastward to Lawrence Coun- ty, on the east by the Indiana border, and on the north by a line through Kankakee has been examined in detail. Within this area, practically all available sample sets and all electric logs of borings penetrating the entire Silurian system have been care- fully examined, and records of wells that stopped in the Silurian, or even in the Devonian system, have been used in critical localities. As the Silurian system elsewhere in Illinois appears to offer little possibility for oil production, it has been examined only in sufficient detail to outline regional sedi- mentational relations. Along the western border of the State, Silurian strata have been entirely stripped or greatly reduced in 2 Lowenstam, H. A., and DuBois, E. P., op. cit., p. 30. 3 Lowenstam, H. A., op. cit., p. 179, fig. 13. [71 NIAGARAN REEFS IN ILLINOIS thickness by repeated post-Niagaran erosion. South of the intensively studied area the Silurian strata apparently do not contain reefs (figs. 1 and 2), and north of it reef reservoirs have been found to contain fresh water rather than brine, minimizing the chance for oil in commercial amounts. Reefs were found only in Niagaran (Middle Silurian) strata. The search for buried Niagaran reefs relies essentially on an understanding of facies characteristics and facies relations. The basic tool for facies analysis is the study of the lithologic character of well cuttings, of their fossil constituents, and of their acid-insoluble residues. Electric log, gamma ray log, and structure studies, al- though useful tools, are usually of second- ary importance. Criteria for differentiation of reef, interreef , and reef-free deposits have been developed by a study of reef-bearing Silurian outcrops of northeastern Iowa, northern Illinois, northern Indiana, Wis- consin, Michigan, and Ontario, and of the reef-free outcrops of Oklahoma, Arkansas, Missouri, southern Illinois, Tennessee, Kentucky, and southern Indiana. The reefs are organic structures, and their distribution in both time and space is most clearly defined in terms of their fossil content. Although this paper is written in terms of lithology, patterned to serve in exploration for oil, it could not have been completed without basic studies of the ecology of the Niagaran faunas. 4 Because the physical factors, both in present and past times, are expressed equally in the sedi- ments deposited and in the character of the faunas and floras whose remains are en- closed in the sediments, studies of sediments and of organisms must be combined in order to achieve a satisfactory understanding of the environments which controlled both. The fundamental aspects underlying the results presented here are to be published separately in a paper that will deal with the environmental setting, both paleonto- 4 Lowenstam, H. A., Biostratigraphic studies of the Niagaran interreef formations in northeastern Illinois: Illinois State Mus. Sci. Papers, vol. 4, 1948. Fig. 1. — Niagaran sedimentation belts in Illinois showing line of cross-section (fig. 2). logical and physical, which produced the reefs. Acknowledgments The writer wishes to express his thanks to the following staff members of the Illinois Geological Survey: A. H. Bell, R. E. Grim, D. H. Swann, H. B. Willman, and L. E. Workman. Through discussions they ma- terially aided in clarifying many of the problems which arose during the present study. The writer is indebted in particular to D. H. Swann whose criticism of the paper from the point of view of oil explora- tion brought out more clearly the criteria which will aid in locating subsurface reefs. D. B. Saxby assisted in the preparation of the maps. NIAGARAN STRATIGRAPHY N.W. S.E.. 6 REEF- BEARING REEF- BEARING LOW- CLASTIC BELT REEF - FREE "clastic-free belt / /reefJ HIGH -CLASTIC BELT o lond, R I I 1 APPROXIMATE HORIZONTAL SCALE / INTERREEF / \\ Thorn group / \ yV^ a' aN. X I A REEF ^>r^ (J Coe group {j^T- Moccasin Springs formation qe group — = _ j _ J r * — * "St.Clair formation 2 Bainbric Fig. 2. — Diagrammatic cross-section of Niagaran strata from northwestern to southeastern Illinois along line indicated in figure 1. NIAGARAN STRATIGRAPHY Distribution. — Strata of Niagaran or Middle Silurian age are continuous over the greater part of the State. They have been removed by erosion only in north- central, west-central, and southwestern Illinois. Thickness. — As there is no Upper Silu- rian in Illinois and the Lower Silurian (Alexandrian) is relatively thin, the iso- pach maps of the entire Silurian (fig. 3) and the Niagaran only (fig. 4) are very similar. Niagaran strata are thickest in a north- east-southwest belt 30 to 50 miles wide ex- tending from easternmost St. Clair County through southeastern Kankakee County. Within this belt the thickness of the Niag- aran strata is consistently 500 feet or more, and locally it increases to as much as 1,000 feet where reefs are present. The strata thin abruptly northwest and southeast of this belt to about 300 feet, except along the west foot of the LaSalle flexure between Woodford and Bureau counties where the thickness locally increases to nearly 500 feet. The causes of thinning in either direction from the belt of maximum thick- ness are unrelated, as the thinning to the southeast is depositional whereas that to the northwest is primarily due to erosion. Sedimentation belts. — The most con- spicuous regional variation in Niagaran lithology is the progressive decrease of ter- rigenous clastic content from southern to northwestern Illinois. Although the de- crease in clastic content and concurrent changes in the carbonate rocks toward the northwest are continuous, the change is more rapid in two zones, and the rocks can best be visualized by describing them in three belts separated by the bounding zones. The bounding zones are actually narrow transitional zones with a steep clastic-content gradient, rather than the abrupt lines indicated on the map (fig. 1) and diagrammatic cross-section (fig. 2). In addition, a regional eastward decrease in clastic content from the Ozarks to the Cincinnati arch is evident but of relatively little importance within Illinois, becoming conspicuous only farther east in Indiana. The southernmost belt can best be called the high-clastic belt, as it is characterized by limestones whose clastic noncarbonate content averages 35 to 40 percent. The sediments are all of the type seen in outcrops of the Bainbridge limestone in southeastern Missouri and southwestern Illinois. Bordering it to the northwest lies a second northeast-southwest trending belt, the low-clastic belt, that is characterized by dolomitic limestone and dolomite averaging 15 to 20 percent clastic content. Rocks of the Bainbridge type are present in the lower part of the section in the entire belt, but are overlapped by, and grade laterally into, rocks of the kind seen in outcrops in the Chicago area and in the upper Wabash Valley. The Niagaran rocks in the third belt in the northwestern part of the State are dolomite which commonly contains less than 5 percent insoluble clastic material, such as that seen in outcrops near Savanna, Illinois, in eastern Iowa, and in north-cen- 10 N I AG A RAN REEFS IN ILLINOIS Fig. 3. — Thickness of Silurian strata in Illinois. NI AG ARAN STRATIGRAPHY 11 Fig. 4. — Thickness of Niagaran strata in Illinois. 12 NIAGARAN REEFS IN ILLINOIS tral Wisconsin, and in Michigan. This belt is called the clastic-free belt. The two outer belts are relatively simple in that each contains only a few lithologic types. The middle belt is complex, con- taining wedges and even isolated lenses of the sediments characteristic of both of the other belts, in addition to lithologic types not represented in the other belts. The two northern belts contain the remnants of a scattered archipelago of organic reefs and form the southwestern part of a large oval-shaped reef-bearing area which, extended in the Niagaran sea for several hundred miles northeastward from the Ozark Island (fig. 7). The southern front of the reefs coincides with the border between the southern and cen- tral belts. Outcrops in each of the sedimentation belts aid in clarifying the facies relations observed in well samples. The differences of the Niagaran deposits in the various out- crop areas are directly correlated to their position in the sedimentary belts outlined. The northwestern Illinois outcrops are located well within the northern belt of essentially pure carbonate rocks, those of northeastern Illinois within the northern portion of the central low-clastic belt. The erosional remnants of lower Niagaran de- posits in west-central Illinois near Ham- burg and in the Grafton area have the same relative position within the low-clastic belt as those of northeastern Illinois. Be- cause these remnants are largely eroded to levels beneath the main reef-bearing horizons, it is uncertain whether the en- tire outcrop area or only part of it is within the reef border. The Niagaran exposures of southwestern Illinois and ad- jacent southeastern Missouri are located well within the southern belt of high-clastic deposits. Differentiation into Groups The composition of the rocks in the wide- ly separated outcrop areas differs significant- ly and evidently reflects major differences in the sedimentary conditions that produced the varied belts. Minor environmen- tal fluctuations occurred in each area, resulting in lithologic variations, which are the basis for the division into formations. The formations in each area, therefore, comprise a distinctive sequence which for convenience is considered to be a group (figs. 1 and 2). Because of the complex inter- fingering of the groups, especially in their contact areas, lateral and vertical varia- tions in lithology are locally pronounced, and in well samples it is not generally possible to trace some of the individual for- mations far from the outcrop areas. Al- though the groups are contemporaneous and each group contains all of the Niagaran strata present in each outcrop area, it is only by recognizing them as separate units that the regional relations can be ade- quately discussed. Bainbridge Group The term Bainbridge, originally applied by Ulrich 5 to the entire Silurian sequence above the Cape Girardeau limestone in southeastern Missouri, was later redefined to include only the Niagaran strata. The Bainbridge strata have long been considered a formation but are herein redefined as a group. The Bainbridge group consists of the Niagaran strata of the belt east and south of the Ozarks, an area which was apparently strongly affected by the Ozark Island during deposition. The group is characterized by a simple succession of two lithologically well-differentiated formations. The lower formation, the St. Clair, is considerably thinner than the upper one and consists of comparatively pure, commonly pink crin- oidal limestone. The upper formation, "the Moccasin Springs, comprises a thicker se- quence of dominantly reddish and purplish high-clastic limestone and calcareous silt- stone, commonly with greenish mottling. These two formations extend considerably beyond the area under consideration, the lower formation occupying a crescentic belt south and east of the Ozarks from west- central Texas to the central part of the Michigan basin, and the upper formation 5 Ulrich, E. O., in Quarrying Industry of Missouri: Mis- souri Bur. Geol. and Mines, vol. 2, 2nd ser., p. 110, 1904. Revision of the Paleozoic system: Geol. Soc. Am. Bull., vol. 22, pi. 28, 1911. BAIN BRIDGE GROUP 13 covering a narrower and shorter belt from south-central Oklahoma to central Illinois. At their geographic limits both formations interfinger with other Niagaran formations. As a group the Bainbridge is distinguished from groups in the sedimentation belts to the north by its red, pink, and purplish colors, the predominance of limestone, the absence of chert, and the comparative per- sistence of lithologic types over wide areas. The group is also characterized by a large amount of interbedded shale, particularly in the Ozark-bordering zone, the generally high percentage of insoluble residues, aver- aging 35 to 40 percent, and the relatively large amount of clay in the residues. ST. CLAIR FORMATION The lower formation of the Bainbridge group, the pink crinoidal limestone, has been named the St. Clair limestone 6 in the Batesville district in Arkansas. The name is here applied regionally to the pink crinoi- dal limestone of early Niagaran age throughout its distribution in the mid- western states except where local names are applicable to separate tongues in the marginal areas, as the Lego and Laurel tongues in Tennessee and a tongue in the base of the Joliet formation in northeastern Illinois and southeastern Wisconsin. Distribution. — The St. Clair formation occupies an arcuate belt about one hundred miles wide, lying on the east and south sides of the Ozark highland, and extending discontinuously from western Oklahoma and west-central Texas through Arkansas, western Tennessee, western Kentucky, Illinois, and western Indiana to the central part of the Michigan basin. It can be seen in outcrops in the Arbuckles and Criner Hills in south-central Oklahoma and Arkansas, in southeastern Missouri and southwestern Illinois, and in the Tennessee Valley of western Tennessee, with fingers or tongues reaching the outcrop areas of the Nashville dome in Tennessee, south- eastern Indiana, northeastern Illinois, and southeastern Wisconsin. Thickness. — In the high-clastic belt of southern Illinois (figs. 1 and 6) the St. Clair limestone thickens progressively to the east and northeast from 20 or 25 feet in the outcrop area along the Ozark bor- der 7 to about 80 feet near the Illinois-In- diana boundary. It also thickens to the north against the boundary of the central low-clastic belt. This northward thickening is more pronounced within the low-clastic belt, and the formation reaches its maximum Illinois thicknesses near the middle of the belt, where it is 80 to 150 feet thick over a considerable area beneath and just beyond the wedge edge of the overlying Moccasin Springs formation (fig. 5). Just north of the edge of Moccasin Springs cover the St. Clair locally reaches thicknesses of 220 feet, as in Ford, Champaign, Douglas, and Coles counties, but the pattern of extreme thicknesses is very irregular. The north- ward thickening appears to be due in part to the general thickening of the entire Niag- aran and in part to the northward diminu- tion of the elastics in the lower part of the Moccasin Springs formation; consequently beds are included in the St. Clair whose southern equivalents are considered Moc- casin Springs. The St. Clair formation thins northwest- ward from its area of greatest development by interfingering and gradation of the upper beds into rocks typical of the Chicago area, with only the basal member maintaining its identity as far as the northwestern bound- ary of the central low-clastic belt. This thin basal St. Clair tongue can be seen in outcrops in the Mississippi bluffs of western Illinois near Hamburg and in the Chicago area where it is more argillaceous and is represented in dolomitized form by the basal Joliet strata — the Osgood of Dunn, 8 Zones A and B of Willman, 9 and the basal unit of Lowenstam. 10 Penrose, R. A. F., The Batesville region of Arkansas: Arkansas Geol. Survey, Ann. Rept., vol. 1, pp. 102- 174, 1891. Modified by Williams, H. S., The Paleo- zoic faunas of north Arkansas: Arkansas Geol. Survey, Ann. Rept., vol. 5, pp. 277-301, 1900. 7 Ball, J. R., Some Silurian correlations in Lower Mis- sissippi drainage basin: Bull. Am. Assoc. Petrol. Geol., vol. 26, p. 6, 1942. 8 Dunn, P. H., Silurian foraminifera of the Mississippi basin: Jour. Paleo., vol. 16, p. 318, 1942. 9 Willman, H. B., High-purity dolomite in Illinois: Illinois Geol. Survey, Rept. Inv. 90, p. 26, 1943. 10 Lowenstam, H. A., op. cit., Biostratigraphic studies, p. 19. 14 NI AG ARAN REEFS IN ILLINOIS Fig. 5. — Thickness of predominantly red facies of Niagaran strata in Illinois. BAIN BRIDGE GROUP 15 Fig. 6. — Niagaran reef occurrences and sedimentation belts in Illinois. 16 N I AG A RAN REEFS IN ILLINOIS Lithology. — The St. Clair formation con- sists of buff to flesh-colored, occasionally white limestones that contain comparatively small amounts of terrigenous elastics and are prominently mottled with pink to red patches. Dissociated remains of small frag- ile crinoids are the main rock-forming con- stituents, particularly in the lower part of the formation. Ostracods form a con- spicuous though less bulky element. Magnification shows that the characteristic pinkish to reddish mottling is caused largely by the color of the crinoidal remains and of part of the ostracod shells. The red fer- ruginous crinoidal remains are most prom- inent toward the base of the formation and decrease in density in the upper beds. In some localities they are partially recrystal- lized, appearing as irregular coarse calcite grains with only spots of the characteristic pink to red coloration. The matrix of the basal crinoidal co- quinas and the bulk of the upper beds con- sist largely of exceedingly fine-grained limestone that is light-colored, commonly buff, and slightly clastic. The terrigenous clastic content increases generally toward the top but averages about 10 percent. The elastics, predominantly silt, with less clay and exceptionally some very fine sand, are disseminated throughout the fine-grained limestone matrix, with some concentration in irregular patterns, laminae and thin layers, some of which are mottled red. A considerable proportion of the silt is con- centrated in the abundant skeletons of arenaceous foraminifera which are very characteristic of the St. Clair limestone. Scattered grains of dolomite occur occa- sionally in the St. Clair strata. Chert is absent except for small quantities apparent- ly introduced by groundwater in a narrow subsurface belt near the outcrop border in Monroe, St. Clair, and Madison counties. The chief distinguishing features of the St. Clair formation as a whole are: the de- trital character, the pink and red color of the fossil remains, the comparatively low clastic content, and the absence of chert. Correlation. — Faunal studies still in progress indicate that the St. Clair forma- tion in the southeastern Missouri and south- western Illinois outcrop area is equivalent to the entire Osgood-Laurel-Waldron-Lego sequence of western Tennessee, rather than only its lower portion as suggested by Ball. 11 The uppermost St. Clair strata of south- eastern Missouri are correlated with the Lego formation by A. R. Loeblich 12 of the United States National Museum on the basis of arenaceous foraminifera, and the writer's studies indicate that the basal beds of the Moccasin Springs formation, im- mediately above the St. Clair, are to be correlated with the Dixon formation which overlies the Lego. MOCCASIN SPRINGS FORMATION The upper formation of the Bainbridge group, here named the Moccasin Springs formation, includes all of the Niagaran strata that overlie the St. Clair formation in the outcrop area in Cape Girardeau and Ste. Genevieve counties, southeastern Mis- souri, and in Alexander and Union counties, southwestern Illinois. The most extensive exposure known, which shows all but the basal 5 to 15 feet of the Moccasin Springs formation, is located in a small box canyon in the Mississippi River bluff in the SE. 1/4 SE. 1/4 NW. 1/4 of sec. 24, T. 32 N., R. 14 E., Cape Girardeau County, Missouri (Jonesboro quadrangle), about three miles south of Moccasin Springs. It was de- scribed in detail by Ball. 13 The basal beds can be seen at Greither Hill, in the NW. 1/4 sec. 11 (interpolated), T. 36 N., R. 9 E., five miles southwest of St. Marys, and 2 1/4 miles southeast of Ozora, Ste. Gene- vieve County, Missouri, and in a road-cut in the SW. 1/4 SE. 1/4 SE. 1/4 sec. 11, T. 5 S., R. 3 W., two miles south of Thebes, Alexander County, Illinois. Distribution. — The Moccasin Springs formation is less extensive than the St. Clair, occupying a discontinuous narrower semicircular belt that extends from south- central Oklahoma to central Illinois. The formation crops out in the Niagaran belts 11 Ball, J. R., op. cit., Some Silurian correlations, p. 6. 12 Loeblich, A. R., personal communication. 13 Ball, J. R., Type section of the Bainbridge forma- tion of southeastern Missouri: Bull. Am. Assoc. Petrol. Geol., vol. 23, pp. S9S-601, 1939. BAINBRIDGE GROUP 17 immediately adjacent to the Ozarks, but is partly or entirely replaced laterally by other rock types where Niagaran strata again come to the surface in Indiana, Kentucky, and Tennessee, a few hundred miles south and east of the Ozarks. Tongues of the Moccasin Springs formation extend into western Tennessee as the Dixon formation and the red lenses of the Bob member of the Brownsport formation, and into the Batesville area of Arkansas where it has been named the Larrerty formation. How- ever, the Moccasin Springs formation is not represented in the outcrop belt along the Cincinnati arch from Indiana to the Nash- ville dome area in Tennessee. Thickness. — In the southeastern Mis- souri outcrops, the Moccasin Springs for- mation ranges in thickness from 100 to 130 feet. Thickening to the northeast, east, and southeast, it attains its greatest thick- ness of nearly 400 feet in a narrow belt along the northern border of the high-clas- tic, reef-free belt. Over most of southern Illinois the formation is 160 to 200 feet thick. The formation thins abruptly to 100-150 feet at the south boundary of the central low-clastic belt and then thins gradually to the northeast and northwest across the belt, disappearing by interfingering at about the middle of the belt (figs. 1 and 5). The pronounced local deviations in thickness that occur in the low-clastic belt (fig. 5) are re- lated to reef-induced turbulence during de- position, as at the Marine reef. 14 These anomalies in thickness distinguish the sedi- mentation relations of the Moccasin Springs formation in this belt from its uniform development in the southern reef-free belt. Lithology. — The Moccasin Springs for- mation consists predominantly of red and mottled red and gray to greenish-gray very fine-grained silty argillaceous limestone and calcareous argillaceous siltstone. Uniform brick-red coloration is most prominent in the basal portion, purple mottling in the upper portion. Limestones and siltstones similar to the red and mottled beds but uniformly greenish-gray to olive-gray are 14 Lowenstam, H. A., op. cit., Marine pool, p. 176. common, and gray siliceous limestones also occur, being most common in the upper part of the formation. The clastic fraction of the sediments ranges from 20 to 62 percent, averaging 40 percent, and consists of silt, clay, and muscovite, the silt being the dominant clastic constituent. Fossils, principally small fragile crinoidal remains, are sparingly present in the cut- tings. Dark brownish-gray to black, thinly laminated calcareous siltstone that contains sporangites and graptolites forms small lenticular bodies at several stratigraphic positions but mostly in the top portion of the formation. Rock of this kind is ex- posed in the type-section of the formation 15 where it contains Tasmanites and Cyrto- graptus ulrichi in abundance. Chert is ab- sent in the typical Moccasin Springs de- posits. Dolomite is occasionally present in subordinate amounts. The distinguish- ing characteristics of the Moccasin Springs formation may be summarized as high-clastic content, dominant red colora- tion, and the absence of chert. Deviations from the typical Moccasin Springs lithology occur near the northern and northeastern borders of the high-clastic reef-free belt. These deviations have two distinct causes, local reef influence on dep- osition along the reef archipelago front and termination of the formation in an interfingered contact eastward toward the Cincinnati arch. The net effect of both is similar in that purer carbonate rocks are introduced into the very silty argillaceous series. Along the reef archipelago front, green- ish-gray to light olive-gray slightly silty moderately fossiliferous limestone which contains small amounts of chert and occa- sional silicified fossils is intercalated with typical red silty poorly fossiliferous Moc- casin Springs limestone. At the very reef front occasional thin layers of light gray- to white detrital limestone occur. They are composed largely of sorted crinoidal remains in a matrix of very fine-grained limestone that contains only small quanti- ties of coarse silt and very fine sand. The 15 Ball, J. R., op. cit., Type section of Bainbridge, p. 597, bed, 12-13. 18 NIAGARAN REEFS IN ILLINOIS matrix locally contains glauconite. These layers are interpreted as reef-derived sorted outwash. The second type of variation, the gradual eastward regional change, is expressed like- wise by a decrease of the red coloration and clastic content and by the introduction of silicified fossils and small amounts of chert. Silicified fossils and chert, nowhere present in the typically developed Moccasin Springs near the Illinois-Indiana boundary, occur at various levels in greenish-gray to olive- gray more pure and more fossiliferous limestones that show occasional faint red and purple mottling, and are attributed to relatively clear-water conditions. Correlation. — The Moccasin Springs for- mation at its type locality includes the equivalents of the Dixon and of the entire Brownsport formation of western Ten- nessee. It may include even higher Niag- aran strata, as the fauna collected by the writer from the uppermost bed 16 includes a Lissatrypa and other fossils that suggest a late Silurian rather than Niagaran age. Thorn Group The Thorn group includes all of the Niagaran formations exposed in the Chicago region, in southeastern Wisconsin, and in the Wabash Valley of northern Indiana. It is the predominant group of the low- clastic sedimentation belt but it thins south- ward, forming a tapering wedge that over- lies the Bainbridge group (fig. 2). Essen- tially all of the lithologic types in the group are represented in the quarry at Thornton in the southern suburbs of Chicago, in sees. 28 and 33, T. 36 N., R. 14 E., about half a mile west of Thorn Creek. The interreef strata of the Thorn group are distinguished by their gray to greenish- gray color, the abundance of chert, and the presence of predominantly silty insoluble residues that average 15 to 20 percent. They also differ from the Bainbridge group in being predominantly dolomite and in containing reef-derived carbonate elastics. They differ from both the Bainbridge and Coe groups by their greater lithologic varia- bility. 13 Bail, J. R., op. cit., bed IS, p. 596. Local formation names are used in the Chicago area and in northern Indiana. Be- cause individual formations have not been traced south of the outcrop areas into the Illinois basin, they are not described separately. Coe Group The Coe group includes all of the Niag- aran formations in the clastic-free belt. Strata typical of this group are well ex- posed in Coe Township, Whiteside County, Illinois, especially in the bluff extending eastward from Cordova and crossing the township in sees. 1-4, T. 19 N., R. 2 E. The interreef rocks of the Coe group are distinguished by brownish-gray color, very low terrigenous clastic content, absence of chert and limited facies variations. They also differ from the Bainbridge group in being predominantly dolomite. The Coe group as here defined comprises the Waukesha, Racine, and Port Byron formations, as Savage 17 differentiated them in northwestern Illinois. This restriction of the Niagaran to Savage's post-Joliet de- posits is made because Savage's "Strick- landia pyriformis zone," recently relocated in the Savanna-Fulton area by H. B. Will- man and the writer, has been found to occur within Savage's Joliet formation and not at the top of the Kankakee formation, as he defined it in that area. As the "Strick- landia pyriformis zone" constitutes the top zone of the Kankakee and Sexton Creek formations, the so-called Joliet strata in northwestern Illinois are referred at present to the upper Alexandrian. It should be noted, however, that the Stricklandia from northwestern Illinois seems to differ con- siderably from S. pyriformis ss. of south- western and northeastern Illinois, and a future taxonomic restudy of the S. pyri- formis group will be required to determine whether one or two species are represented. Upper Contact of the Niagaran The Niagaran system in Illinois is over- lain by deposits that range in age from Lower Devonian to Pleistocene. In general 17 Savage, T. E., Silurian rocks of Illinois: Geol. Soc. Am. Bull., vol. 37, pp. 513-533, 1926. UPPER CONTACT OF NIAGARAN 19 the Niagaran is overlain by progressively younger formations from south to north, al- though there are exceptions. There is difficulty in recognizing the top of the Niag- aran only in those areas where it is over- lain by certain Devonian limestones and dolomites, or by the Fern Glen limestone of Osage (Lower Mississippian) age. In southern Illinois the Moccasin Springs formation is nearly everywhere overlain by the Bailey-Grassy Knob formations of Low- er Devonian age, whose distribution coin- cides essentially with the reef -free belt (fig. 1 ) . These Lower Devonian formations are characterized by very siliceous limestones and dolomites, prevalently brownish-gray, containing much dull opaque chert spread rather regularly throughout the forma- tion. Some thin beds in the Niagaran, chiefly north of the limits of the Bailey- Grassy Knob, appear similar to these Lower Devonian formations. However, such thin beds are minor units dispersed through a Niagaran sequence whose predominant lithologic types never occur in the monot- onous sequence of several hundred feet of uniform highly siliceous and cherty Lower Devonian limestones. Because of the resemblance of these cherty Silurian sediments of the low-clastic belt to the Bailey-Grassy Knob, and particularly because of the resemblance of the Little Saline limestone of Oriskany age which overlies the Bailey-Grassy Knob to the un- dolomitized detrital "pink crinoidal" lime- stone that flanks and caps the Marine reef, confusion has arisen regarding the differ- entiation of these sequences in the border area. Three groups of criteria for differ- entiating the Devonian and Silurian se- quences are: (1) The thickness of charac- teristic Bailey-Grassy Knob rocks as op- posed to the thinness of similiar rock types in the Silurian; (2) minor but consistent differences between the Little Saline and Niagaran crinoidal limestones; and (3) the presence of a relatively unvarying sequence in the Devonian sediments which is not duplicated in the Niagaran. The Little Saline limestone is a pink, light gray to white highly crinoidal lime- stone or dolomitic limestone. It may be differentiated from the reef-flanking and reef-capping detrital limestone of the upper part of the Niagaran because it contains glauconitic clay patches, chert, and silicified fossils, none of which are are known from the Niagaran pink detrital beds. The Niag- aran "pink crinoidal" beds are conspicuous- ly coral-bearing, chips of colonial corals such as Favosites being encountered in prac- tically all cutting samples. Colonial corals, although noted in extensive outcrops of the Little Saline, are so rare that they have not been seen in cuttings. The fauna of the detrital beds is much more varied than those of the Little Saline. The Middle and Lower Devonian se- quence which underlies the Middle De- vonian Geneva-Dutch-Creek-Wapsipinicon horizon south of the reef front, comprises a regular unvarying sequence of three major units. The uppermost is the Clear Creek formation, characterized by coarse-grained light-colored dolomite with much spore- speckled chert and with conspicuous coarse glauconitic pellets. It is underlain by the Little Saline, a relatively pure but chert- bearing crinoidal limestone, which in turn is underlain by the monotonous brownish- gray cherty siliceous limestone and dolomite of the Grassy Knob-Bailey formations. This sequence is in turn underlain by the olive- green or reddish mottled very argillaceous limestones and siltstones of the Bainbridge which may contain lenses of sporangites- bearing black shales, fingers of reef detritus, and minor amounts of chert. The chert is confined to the upper part of the formation in the immediate vicinity of the reef border. North of the reef front, the sequences, largely or entirely Niagaran, which under- lie the Geneva-Dutch-Creek-Wapsipinicon horizon are much more varied due to the shifting facies created by the reef environ- ment. The characteristic rock expressions north of the reef front are the prevalent gray to olive-gray colors of carbonate rocks, the common though irregularly distributed translucent bright chert, and particularly the localized replacement of large sections of this varied sequence by the pure car- bonate reef-rocks. 20 NI AG ARAN REEFS IN ILLINOIS It has been noted that the entire Lower Devonian sequence of southern Illinois pinches out in central Illinois. 18 The pres- ent study indicates that these deposits, 1,100 feet thick at the southern border of the State and still 500 feet thick at Salem, wedge out abruptly within a distance of 10 miles or less against the reef front. Dis- covery of the reef front, delimiting a zone of thick Niagaran to the north against thin, high-clastic, compactible Niagaran to the south, appears to present a satisfactory ex- planation for the abrupt wedging out of the southern Lower Devonian sequences. It is probable that the reef front, which acted as a shore line for the Lower Devonian sea, was irregular and that tongues of Lower Devonian sediments may have lapped up in embayments between the outer reef bastions. The picture of the sharp northern limits of the pre-Dutch Creek Devonian sediments of southern Illinois is supported by the fact that the Bailey-Clear Creek sequence can be traced in wells to the outcrop belt 19 where this sequence can be dated by paleon- tological evidence. The Niagaran age of the sequence north of the reef front is also determined by tracing it in wells, and partic- ularly by the recognition of Silurian fossils in both cuttings and cores from several localities in this belt. The uppermost fingers of reef outwash as determined from records of wells south of the reef border, occur in the upper part of the Bainbridge formation several hundred feet below the Geneva horizon, although the Dutch Creek- Geneva formation a few miles to the north rests directly on Niagaran reef-rock. The paleoecological relations as determined in outcrops provide the key to the under- standing of these complex relations, per- mitting the confident use of lithologic criteria from wells, after the major fea- tures have been worked out. Where the Niagaran is overlain by Middle Devonian deposits, as in much of the central and northern parts of Illinois, the almost universal occurrence of fine- to 18 Workman, L. E., Subsurface geology of the Devonian in Illinois: Illinois Geol. Survey, Bull. 68, p. 194, 1944. 19 Workman, L. E., op. cit., Subsurface geology, pp. 194- 195. medium-grained sandstone, of sandy lime- stone, or of floating sand grains in the basal few feet of the Middle Devonian sequence of carbonate rocks will serve to distinguish these rocks from the underlying Niagaran. Sand is quite uncommon in the upper part of the Niagaran, and where present (with rare exception) is very fine. In addition, many of the Middle Devonian limestones and dolomites are characterized by brown colors that are not duplicated in the Niag- aran sequence. In general there are no difficulties in differentiating the Cedar Valley limestone, the New Albany, or the Kinderhook for- mations from the Niagaran in the areas in which they overlie it. In certain parts of the Ozark border area in Illinois in western St. Clair and Monroe counties, and possibly in northwestern Jack- son County, the Fern Glen limestone of early Osage age directly overlies the Moc- casin Springs formation of the Niagaran. Both formations are impure limestones which are in large part red or mottled red and green. The Fern Glen may be dis- tinguished commonly by the presence of varicolored chert — red, yellow, brown, and green. The Moccasin Springs has no chert in this immediate vicinity. The basal bed of the Fern Glen in several wells is a very fossiliferous crinoidal deep red clay-shale which rests on reddish siltstone or very silty red limestones of the Moccasin Springs. The somewhat similiar lithology of the two formations may be explained in part by the incorporation into the Fern Glen of Moc- casin Springs sediments which have been entirely removed from some localities in this area by pre-Osage erosion. Lower Contact of the Niagaran Strata The Niagaran in Illinois is uniformly underlain by late Alexandrian formations equivalent to the Brassfield limestone of Kentucky and Indiana. These are the SEDIMENTATION BELTS 21 Sexton Creek limestone in the southern half of the State and its dolomitic equivalent, the Kankakee dolomite, in the northern half of the State. The upper part of these Alexandrian formations is characterized by honey- yellow to light buff or brown micro- crystalline limestone or fine granular dolo- mite which is quite pure but has scattered greenish argillaceous blotches. Glauconite pellets are common. Light-colored translu- cent chert containing sponge spicules is com- mon, except in and near the northeastern Illinois outcrops. Confusion of the Brassfield equivalents with basal Niagaran deposits is most likely in the southwestern portion of the State. Here the top beds of the Brass- field (Sexton Creek) may consist of maroon to weak pinkish crinoidal coquinas some- what similiar lithologically to the overlying St. Clair formation. The Brassfield can be readily differ- entiated in outcrop by the abundance of Stricklandia pyriformis, the charac- teristic zone fossil of the upper por- tion of the Sexton Creek and Kankakee formations. Although this fossil has been recognized in coarse well cuttings, the com- mon occurrence of glauconitic pellets in the Brassfield equivalents (but not in the St. Clair) and of disseminated pyrite in the basal St. Clair (but not in the uppermost Brassfield) are of greater use in distin- guishing the two. A varied fauna of arenaceous foramini- fera in the St. Clair serves as the most reliable identifying criterion. The fora- minifera, particularly the coiled form, Ammodiscus, may be released through acidizing even in one or two cutting chips. Although a number of arenaceous foramim- fera have been recorded from the Brass- field, 20 none have been found in the routine acidization of small samples of well cuttings, and Ammo discus, which is very abundant in the Niagaran, has not been found in the lower beds. 20 Dunn, P. H., op. cit., p. 319. NIAGARAN SEDIMENTATION BELTS Reef-free High-clastic Belt Areal extent. — Reef-free Niagaran sedi- ments extend across southern Illinois north- ward to a line that runs roughly from southern St. Clair County northeastward toward Vermilion County (figs. 1 and 6). At their northern border the relatively thin reef-free sediments abut against the thicker deposits which enclose the reef archipelago. Where the border can be traced in detail in two widely separate areas of limited extent, one in Washington, Clinton, and western- most Marion counties and another along the Coles-Edgar county line, it appears sinuous rather than a straight line. Else- where this line of demarcation has been drawn on the basis of the facies relations of the few widely separated deep tests. The border of reef-free sediments also coincides with the northern limits of lower Devonian deposits. Thickness. — The reef-free high clastic deposits are thinnest along the Ozark out- crop border in southeastern Missouri and southwestern Illinois. Represented here by the Bainbridge deposits, they are estimated to range in thickness between 150 and 180 feet, exact figures being unavailable owing to the lack of complete exposures. Thicken- ing only slightly eastward beyond the out- crop border, the reef-free deposits remain essentially uniform in thickness, averaging around 225 feet over most of southern Illinois. Toward the north the deposits thicken, rapidly reaching their maximum thickness of about 450 feet in a narrow zone along the reef-archipelago front. A slight thickening at the southern tip of the State continues into western Kentucky and west- ern Tennessee. The thinning in the St. Louis area is a result of post-Niagaran pre- Osage erosion. Reef-bearing Belts Reefs are the most obvious feature that distinguish the Niagaran rocks of the north- ern two-thirds of Illinois from those of the southern area. The reefs do not form a 22 NIAGARAN REEFS IN ILLINOIS continuous barrier. They are isolated bodies surrounded by interreef deposits which form the bulk of the Niagaran rocks of the reef- bearing area. The interreef sediments may be best described by dividing the reef-bearing area into two broad belts, one extending through central and northeastern Illinois characterized by impure limestones and dolomite with a moderate terrigenous clas- tic content, and the other occupying north- western Illinois, where the interreef beds are relatively pure dolomites. The central Illinois belt is distinguished as the low- clastic reef-bearing belt, and the one in northwestern Illinois as the clastic-free reef- bearing belt. The low-clastic belt in many respects is transitional between the reef-free high- clastic belt of southern Illinois and the clastic-free belt of northwestern Illinois. It contains representatives both of the im- pure red Bainbridge sediments which are characteristic of the reef-free belt and of the pure dolomites of the northwestern area. The environment of deposition of each of the outer belts was relatively stable ; the southern area had still but muddy water, probably comparatively deep, during much of Niagaran time ; the northwestern area had shallow, rough water, which carried little or no noncalcareous mud. The vary- ing sediments of the inner low-clastic belt reflect rapidly shifting conditions of sedi- mentation. The sediments of the reef-bearing belts are described in four sections. The inter- reef sediments of the low-clastic belt are described in detail in the first section and those of the clastic-free belt in the second. These interreef deposits are the normal, expected rocks within their respective areas. The reefs themselves are described in the third section. The last section is devoted to the effects of reefs on the immediate sur- rounding interreef sediments, for the dis- covery of more reefs will depend to a large extent upon the recognition of reef-modified interreef deposits and of the proximity of reefs as indicated by the modified sedi- ments. INTERREEF SEDIMENTS OF THE LOW- CLASTIC REEF-BEARING BELT Areal extent. — The southeastern bound- ary of the low-clastic belt is marked by the sudden increase in thickness of Niagaran sediments which coincides with and is deter- mined by the line of outermost reef bastions extending diagonally across the state from southwestern Washington County to Ver- milion County. The belt extends beyond the State boundaries north and east to in- clude much of Indiana, southeasternmost Wisconsin, and the southern part of the Michigan basin. The northwestern bound- ary of this depositional belt is marked by the last occurrences of appreciable amounts of noncarbonate or impure carbonate sedi- ments and approximates the present course of the Illinois River (fig. 1). Thickness. — Near the southeastern edge of this belt the Niagaran sediments have their greatest thickness within the State. The interreef deposits range from 450 to about 650 feet, and local increases up to about 1,000 feet are caused by the reef bodies. The gradual thinning along the western, northwestern, and northern bor- ders of this thick belt is due to post-Niag- aran erosion, but the thinning at the south- east margin of this belt is depositional in nature, and is correlated with the lack of reef-derived detritus in the thin highly com- pactible nonreef sediments of the southern belt. Here the thickness is reduced by as much as 150 to 250 feet within six to ten miles. Facies. — The low-clastic reef-bearing belt as a whole displays the most complex sedi- mentation relations and the greatest varia- tion in lithologic types and depositional en- vironment of any of the areas under con- sideration. This is due, first, to its buffer position between the two extremes of quiet- water muddy sediments on the southeast and of rough-water nonmuddy sediments on the northwest and, second, to the superim- posed effect of the reefs on the interreef deposits. Disregarding the pronounced local anomalies created by the reefs and con- fined to the immediate surroundings of the REEF-BEARING BELTS 23 reefs, we can visualize the interreef sedi- ments of this belt as consisting of two wedges. The lower wedge is thick at the southern edge of the area and thins to the northwest and northeast. It consists of ex- tensions of the Bainbridge group northward from the southern reef-free area. The com- plementary upper wedge, thickest in the central and northern parts of the belt, con- sists of sediments of the Thorn group. Rapid lateral changes in the succession ren- der the division of this wedge into separate formations of little value for the present discussion. The two wedges are largely or entirely contemporaneous. Bainbridge sedi- mentation, which covered nearly all the central low-clastic belt at the beginning of Niagaran time, was gradually restricted un- til by the end of the Niagaran it was con- fined to the southern belt, at which time the entire central belt was being covered by deposits of the Thorn type. The facies analyses presented in the second Marine Pool report 21 refer to the relations in this low-clastic belt, "Facies A" being the Bainbridge wedge and "Facies B" the Thorn wedge. Lower or Southern (Bainbridge) Wedge St. Clair formation. — The St. Clair for- mation at the base of the Bainbridge wedge retains the lithologic character typical of the outcrop area northward to its area of maximum thickness at the north boundary of the reef-free belt. The upper portion of the sequence from this area on northward becomes progressively less crinoidal and con- sists predominantly of buff microcrystalline to very fine-grained limestone. The pink to red color of the remaining crinoidal ele- ments is retained only in certain beds, and these too eventually change to beds with fewer crinoidal remains which are no longer pink. The basal portion maintains its formational identity over a larger area than the upper portion but tends to become more variable in that the typical pink or red crinoidal coquina with irregular silty or 21 Lowenstam, H. A., op. cit., Marine Pool, pp. 167-174. shaly partings occur only as lenticles in low- clastic lithographic limestone speckled with pink crinoids. This same type of change can be observed in the southwestern part of the St. Clair belt in the Arbuckle Moun- tains in Oklahoma. There the typical St. Clair pink coquina of the Marble City area can be traced westward to the correlative pink crinoidal member of the Chimney Hill formation which, in similar fashion, gradual- ly loses its identity from east to west. As a result of dolomitization, the red to pink crinoidal remains lose their skeletal identity, and fuse into pink to reddish colored blebs (seen in dolomitic limestone in well samples), and eventually produce the pink to reddish mottling which can be seen in the basal Joliet dolomite and as occasional lenses in the upper Joliet dolomite of the Chicago area. Moccasin Springs formation. — The domi- nantly red and purple high-clastic Moccasin Springs limestones and calcareous siltstones, which comprise the upper part of the Bain- bridge wedge, thin toward the north and northwest and disappear near the middle of the belt. The basal brick-red high-clastic limestones and siltstones persist to near the limits of the formation. Only marginally does the clastic content decrease, accom- panied by fading of the red colors into brown or changing to green. Deposits in the upper part of the forma- tion show more deviation from the typical rock types of the southern belt, the varia- tions being greatest in the border zone against the overlapping facies wedge. The characteristic red and purple colors become more sparingly and irregularly distributed through this part of the section, occurring mostly in the form of faint mottling. Green- ish-gray, olive-gray, and buff fine to micro- crystalline limestone, which generally has a lower clastic content than usual, is inter- laminated with greenish-gray to darker gray shaly calcareous siltstones. Upper or Northern (Thorn) Wedge The Thorn wedge thickens from a feather edge at the southern border of the low-clastic reef-bearing belt to at least 450 feet in the syncline immediately west of 24 NIAGARAN REEFS IN ILLINOIS the LaSalle anticlinal belt. The original maximum cannot be determined because of the progressive thinning of the Niagaran deposits to the northwest as a result of erosion. In contrast to the gradual regional changes within the underlying Brainbridge wedge, the lithologic succession in this wedge is marked by extreme variations that indicate rapidly shifting conditions of sedi- mentation. The changes in general are controlled by the reefs and vary in in- tensity with distance from the reefs. Only in the more extensive interreef areas, such as those surrounding the St. Jacob and Woburn South pools, is there a stable litho- logic succession with a minimum of horizon- tal shift in facies. In addition to the striking changes in vertical succession in this wedge, a wide lateral range from calcareous or dolomitic siltstone and shale to low-clastic limestone and dolomite is present, although pure clas- tic-free carbonate rocks are confined to the reefs. There are exceptions to any general statement concerning rocks as variable as these, but on the whole the interreef rocks of this wedge are characterized by an abun- dance of chert and silicified fossils, by pre- vailing gray to greenish color, lack of red and purple shades, and by moderate to low clastic content. Although extremely varia- ble, the terrigenous elastics average perhaps 15 percent for the entire wedge. They consist predominantly of silt with smaller amounts of clay, mica, and very fine sand. The majority of the lithologic compo- nents of this wedge may be found in any given stratigraphic position and in irregular distribution throughout the facies complex; and gradation from one lithologic type into another is a characteristic feature of the shifting facies. The lithologic types are genetically divisible into three major groups: rough-water deposits, still-water deposits, and intermediate types. The criteria for differentiating the different sedi- ment groups are those previously deter- mined from outcrop studies in northeastern Illinois. 22 22 Lowenstam, H. A., op. cit., Biostratigraphic 6tudies. Rough-water deposits. — The rough-water deposits are very fine- to medium-grained light gray to buff limestone and dolomite which have the lowest clastic content of any interreef beds in the complex. The terrigenous clastic fraction consists of coarse silt and very fine sand. Micas and clays as a rule are absent. The rough-water de- posits are fairly cherty and locally contain an abundance of poorly preserved silicified fossils. Glauconite is fairly common. Oolites can be observed locally where silici- fication preceded dolomitization, which suggests that they may have been more abundant before destructive dolomitization. The dolomitic phase is usually porous. The fossil density in the limestone is generally comparatively high. The fossils, predomi- nately crinoidal remains, are of medium size, fairly robust, and locally form sorted coquinas. Still-water deposits. — The still-water de- posits range from dark-colored calcareous or dolomitic siltstone, which may be shaly locally, to very argillaceous limestone or dolomite. The siltstone is dark brown, brownish- gray, dark gray or weak greenish-gray, in part very finely laminated, and conspicuous- ly micaceous. It contains few fossils, which are always very small and of fragile build. Sporangites and less commonly graptolites and scolecodonts are characteristic fossil constituents. Chert is commonly absent. The siltstone occurs both in well-defined bodies and interbedded and interlaminated with other sediments. It is quantitatively subordinate in the southwestern portion of the wedge where it forms small lenticles. In the northeast it is more prominently developed and forms irregularly shaped bodies that appear to be continuous over wider areas. Examples of these different still-water siltstone types can be found in outcrops in the Lecthaylus shale and the Astraeospongia meniscus facies west of Blue Island, 23 in the lagoonal deposits at the base of the interreef strata exposed in 23 Roy, S. K., and Croneis, C, A Silurian worm and associated fauna: Geol. Ser. Field Mus. Nat. Hist., pp. 229-47, 1931. Lowenstam, H. A., op. cit., Biostratigraphic studies, pp. 76-117. REEF-BEARING BELTS 25 Thornton quarry, in late Niagaran silt- stone at Momence and west of Kankakee, and the Mississinewa shale facies in the Wabash Valley of northern Indiana. 24 The still-water limestone and dolomite deposits are olive-gray to medium gray and greenish-gray, dense, very fine-grained, commonly micaceous, and either very sili- ceous or very silty and shaly. Fine sand is occasionally present. They are commonly abundantly cherty and contain silicified fossils. Fossils are mostly crinoidal re- mains, are moderately abundant, and are uniformly of small fragile types. The chert encloses separate spicules and even complete skeletons of siliceous sponges. The olive- gray dense siliceous limestone and dolomite is confined to the southwestern part of the belt near the reef archipelago front, being replaced to the northwest and northeast by the silty gray to greenish-gray limestone and dolomite. The silty dolomite may be seen in northeastern Illinois in the interreef section in Elmhurst quarry and in the bluff exposures north of Lemont. 25 Intermediate deposits. — The intermedi- ate types cover the entire range between the high-clastic and very low-clastic car- bonate rocks and make up the larger por- tion of the facies complex. In general these sediments are lighter in color than the high- clastic carbonate rocks, being light gray to light olive-gray, greenish-gray, and less commonly light brown and buff. The olive shades are confined to the southwestern por- tion of the wedge. Chert and silicified fossils are common. The clastic fraction commonly is concentrated in silty and shaly lenses and laminae, which may be irregular and wavy, producing nodular structures which can be recognized in coarse cuttings. The density and the physical character of the fossils in these intermediate sediments vary between the extremes of the end mem- bers. 24 Cumings, E. R., and Shrock, R. R., The geology of the Silurian rocks of northern Indiana: Indiana Dept. Conservation, Pub. 75, 1928. 25 Lowenstam, H. A., op. cit., Biostratigrpahic studies, pp. 33-38, pp. 42-58. Relations of the Two Wedges Along the slanting boundary separating the two facies complexes of the low-clastic reef-bearing belt, intertonguing and grada- tion are found over a considerable area. As the boundary is approached there is a notable although irregular decrease of the clastic fraction of the Moccasin Springs formation, accompanied by fading and dis- appearance of the typical red to purple colors. The conspicuous increase in thick- ness of the underlying St. Clair formation in the area of disappearance of the Moc- casin Springs formation may be the result of facies replacement of the former by the latter. Chert and glauconite, usually ab- sent in the two Bainbridge formations, may occur in the transitional beds of the border area. In the overlapping Thorn strata the transition region is marked by the scattered occurrence of an occasional pink crinoid grain in the St. Clair position and of red and pink mottled shaly or silty limestone lenticles in the position of the Moccasin Springs formation. INTERREEF SEDIMENTS OF THE CLASTIC- FREE REEF-BEARING BELT The interreef sediments of the clastic- free belt are characterized by less complex sedimentation relations and by more limited variation in composition than interreef de- posits in the low-clastic belt to the south. The distinguishing features of the sedi- ments as a whole are the high carbonate content with little chert and a terrigenous clastic content estimated to average less than 5 percent, decreasing proportionately to the northwest across the belt. These practically pure carbonate deposits are uni- formly dolomitized. The predominant sediment is very slight- ly argillaceous dolomite which is porous, fine-grained, crystalline to granular, buff to white, and less commonly light gray and light brown. The clastic fraction consists of silt, very fine sand, and rarely of clay. Chert, glauconite, and silicified fossils may be present and are more prevalent in the southern portion of the sediment belt. The fossils are generally large and robust. 26 NIAGARAN REEFS IN ILLINOIS Crinoidal remains and colonial corals form the common recognizable constituents in cuttings. Similar dolomite which is slightly more argillaceous, commonly greenish-gray or mottled and contains a few shaly laminae and lenses, is less common and occurs largely in the basal portion of the sequence. Oolitic dolomite, in which the oolites are obscure unless silicifled, is less common. Another rare type of interreef deposits consists of thinly laminated alternating light gray to huffish-gray exceedingly fine-grained dense granular to lithographic dolomite, evidently the dolomitized equiv- alent of the Solenhofen type of lagoonal lithographic limestone. These laminated dolomites carry a few exceedingly fragile fossils and appear to be the only still- water deposits represented in the area. They have been observed in outcrops only in the higher part of the sequence, as in the Anamosa deposits at Stone City, Iowa, and less extensively in pockets in the steeply dipping flank beds of the reef that is ex- posed in the quarry at Cordova, Illinois. The rather limited variation in sedimen- tary types in this belt implies nearly con- tinuous rough-water conditions, the varia- tion expressing the degrees of water agitation- REEFS Distribution. — Reefs are developed in the two northern sedimentation belts, extend- ing over the entire northern two-thirds of the State, but are discontinuous. There is evidence of small-scale, local reefs merging into atolls, as in the case of the Marine reef, and possibly in the Kankakee region, but there is no tendency of large-scale coa- lescence into barrier reefs, as in the Permian reefs of Texas and New Mexico. Instead the reefs comprise a loosely grouped wide- spread archipelago which extended regional- ly far beyond the borders of the State and formed a broad northeast-southwest trend- ing eliptical belt around the central por- tion of the Michigan basin. As shown on the reef distribution map (fig. 7), the Illinois portion comprises the southwestern extremity of the archipelago which was anchored off the northeastern shore of the Niagaran Ozark Island. Erosion has ob- scured the border relations to the Ozark Island. The southern border of the' reef archipelago, although imperfectly known, is distinguished by several frontal reefs as well as by differences in lithology which reflect the changes in sedimentation conditions from interreef to nonreef deposition. These reefs along the southern front are unusually large and tall; one is nearly 1,000 feet in height. They evidently represent the outer reef bastions which grew on the slope that connected the reef-bearing shelf to the north with the deeper reef-free bottoms to the south. The reefs are exposed in great numbers in the outcropping areas of the archipelago, along the Niagaran escarpment all around the borders of the Michigan basin and in the western outlier in northwestern Illinois and adjacent Iowa, and have been described by several geologists. Publications by Hall, 20 Chamberlin, 27 and Graubau 28 trace the development of knowledge of these out- cropping reefs. We owe much of our pres- ent knowledge on the reef structures to Cumings and Shrock 29 who systematically analyzed the reefs of the outcropping belt in general, with particular emphasis on northern Indiana, and to Shrock 30 who described the Wisconsin portion of the archipelago. Observations on the reefs of northeastern Illinois have been made by 20 Hall, J., Physical geography and _ general geology: in Rept. Geol. Survey, State of Wisconsin, by Hall, J., and Whitney, J. D., 1862. 27 Chamberlin, T. C, Geology of eastern Wisconsin: Geol- ogy of Wisconsin, vol. 2, 1877. 28 Grabau, A. W., Paleozoic coral reefs: Geol. Soc. Am. Bull., vol. 14, 1903: Principles of stratigraphy: 2nd ed., A. G. Seiler, 1921. 23 Cumings, E. R., and Shrock, R. R., op. cit., The geology of the Silurian rocks. Niagaran coral reefs of Indiana and adjacent states and their stratigraphic relations: Geol. Soc. Am. Bull., vol. 39, pp. 579-620, 1928. 30 Shrock, R. R., Wisconsin Silurian bioherms (Organic reefs) : Geol. Soc. Am. Bull., vol. 50, pp. 529-562, 1939. REEF-BEARING BELTS 27 Fig. 7. — Distribution of Silurian strata and location of known reefs. Cumings and Shrock, 31 Fenton, 32 Bretz, 33 and Lowenstam. 34 Origin and classification. — The reefs are uniformly of organic origin. Erected by reef-building organisms, they were rigid structures which formed topographically raised shoals of various heights above the surrounding interreef bottoms. In their 31 Cumings, E. R., and Shrock, R. R., op. cit., Niagaran coral reefs. 32 Fenton, C. L., Niagaran stromatoporoid reefs of the Chicago region: Am. Midland Nat., vol. 12, pp. 203-12, 1931. 33 Bretz, J H., Geology of the Chicago region: Illinois Geol. Survey, Bull. 65, Pt. I, 1939. 34 Lowenstam, H. A., op. cit., Biostratigraphic studies, pp. 31, 42-43, 131-133. fossil state of preservation they form well differentiated bodies, which are structurally, lithologically, and faunally distinct from the horizontally bedded interreef deposits that enclose them. The reef structures display little varia- tion in the essential features of their build- ing plans. They consist of one or two com- ponents. In the simplest form the reefs are represented only by the massive non- stratified reef proper, the reef core of Cum- ings and Shrock. 35 In other reef structures, 35 Cuming?, E. R„ and Shrock, R. R., op. cit., The geology of the Silurian rocks, p. 140. 28 N I AG ARAN REEFS IN ILLINOIS the core is flanked by reef-derived detritus, which is bedded and dips at various angles radially away from the core. The reef cores and flank beds are commonly sharply delineated. A distinct variation of the type with flank deposits is represented by the additional accumulation of reef detritus on top of the reef, as found in subsurface on the Marine reef in Madison County. 36 Reefs with flank deposits are prevalent in the central low-clastic sedimentation belt, and reefs without flank deposits predomi- nate in the northern clastic-free belt, al- though both types occur in both areas. Good outcrop examples of the reefs with flank deposits may be seen in the railroad-cut at the Big Four Station in Wabash, Indiana, 37 and in a partial section in the Thornton quarry south of Chicago. Several reefs without flank deposits are well exposed in the north-facing bluff east of Cordova in northwestern Illinois. Shape of the reefs. — Information on the exact shape of the outcropping reefs is limited. The exposures commonly consist of single random vertical cuts which seldom either extend horizontally across the entire reef or expose both the top and base. Also many of the surface reefs have been deeply scalped by erosion. In rare instances, the gross features of the reef shape can be deter- mined by a sufficient number of vertical exposures and by the topographic expres- sion of many reefs as bedrock highs or klin- tar. The characteristic tendency of the reefs to stand up above the bedrock surface as klintar is due to the greater resistance of the reef rock to weathering and glacial scour as compared with the interreef rock. Such partially excavated reefs give outline patterns of the reefs in the horizontal plane. 38 Information on the surface topography of the reefs in outcrops is lacking. Curiously enough, it is the surface topography which 36 Lowenstam, H. A., and DuBois, E. P., op. cit., p. 19. 37 Cumings, E. R., and Shrock, R. R., op. cit., Niagaran coral reefs, p. 588, fig. 7. 3:s Cumings, E. R., and Shrock, R. R., op. cit., The geol- ogy of Silurian rocks, p. 97, fig. 6. Bretz, J H., op. cit., p. 66, fig. 51. Shrock, R. R., op. cit., Wisconsin Silurian bio- herms, fig. 4. is the most accurately known feature of the Marine subsurface reef in Madison County. 39 As far as has been ascertained to date, the reefs can be ellipsoidal, hemispheroidal, cuboidal, dome-, mound-, or ridge-like. Most forms may be found among both types of reefs, although there is a definite tend- ency toward ellipsoidal, hemispheroidal, and cuboidal shapes among the reefs without flank deposits and toward dome-, mound-, and ridge-shaped forms among the reefs with flank deposits. The core of the reefs with flank deposits is ridge- to dome-shaped. The subsurface Bartelso reef in Clinton County and the postulated Ayers reef appear to be ridge-shaped. The Sandoval reef is irregular diamond-shaped as far as can be ascertained from the compactional structure in the overlying Chester and Devonian strata. The reefs are actually more complex in shape than indicated by their mapped out- lines. The reefs with flank deposits com- monly have serrated cores and flanks which reflect fluctuations in outward growth. The reefs without flank deposits generally have even borders. Fore-reef development comonly adds to the complexity of the reef forms. Fore-reefs developed on the flanks as well as in front of the main reef. Most of those developed on the reef flanks are small and many are drowned and over- ridden by the detrital flank beds. Promi- nently developed fore-reef ridges flank the outcropping Thornton reef and the sub- surface Marine reef. At Marine the fore- reef ridge appears to be a composite of re- current fore-reefs successively overwhelmed by flank deposits from the main reef. The fore-reef at the Thornton outcrop is like- wise a complex structure; in the exposed section the flank deposits south of the main reef enclose several small fore-reefs, most of which are apparently limited by a larger bounding fore-reef ridge. A composite reef form can be clearly traced only in the case of the Marine sub- surface reef, whose topography and outlines indicate a horseshoe-shaped atoll. 39 -Lowenstam, H. A., op. cit., Marine pool, fig. 11. REEF-BEARING BELTS 29 Size of the reefs. — The reefs are varia- ble in size, ranging from a few feet to several miles in diameter and from a few feet to almost 1,000 feet in height. There is a definite relation between type and size of reef. The reefs without flanks are al- ways small in area and particularly in height, being low-lying structures of prev- alent horizontal growth, ranging from a few feet to a hundred feet or so in diameter. The reefs with flank deposits are larger and may show pronounced tendencies to- ward upward growth. They range from a few hundred feet to a known maximum of 7>Yi miles in diameter and from several tens of feet to almost 1,000 feet in height. The largest one known is the Marine subsurface reef, which covers, approximately six square miles and is nearly 500 feet thick. The largest outcropping reef in Illinois is the Thornton reef south of Chicago, which is probably a little more than a mile in di- ameter. 40 The outer reef bastions along the southern edge of the reef archipelago are among the largest and are the highest reefs yet observed. These are the Bartelso reef in Clinton County, the McKinley reef in Washington County, and the Sandoval reef in Marion County, each of which has been entirely penetrated by a single boring. Each is estimated to have an area of a little more than a square mile, and is known to be between 800 and 1,000 feet thick. Internal Reef Structure. — In outcrops reef cores are readily distinguishable from flank beds and interreef deposits by their massive appearance and lack of bedding. In some outcrops faint interrupted irregular growth lines may be seen in the solid reef mass, indicating temporary cessation of the normal continuous growth of the reef. The flanking detrital fans are commonly but not always sharply delimited at their contact with the reef core. They are invariably bedded. The bedding-planes characteris- tically are inclined radially away from the reef core and have dips ranging from a few degrees to 65 degrees. 41 Although common- 40 Bretz, J H., op. cit., p. 63. 41 Cumings, E. R., and Shrock, R. R., op. cit., Geology of the Silurian rocks, p. 142. ly even-bedded, the beds are locally lentic- ular, particularly where they lens out by interfingering with the interreef de- posits. In other places there is perfect gradation of the reef-flank beds into the horizontally bedded interreef deposits. Reef-mantling detritus is definitely known only from the Marine subsurface reef. It is massive with a few indistinct bedding-planes that are horizontal in the upper part and inclined as much as 20 de- grees in the lower part of the section. The structural relations and bedding character of the reef components can be determined in subsurface only by means of coring. Lithology. — Reef-flank and reef-core rocks are nearly identical, both in composi- tion and small-scale texture, as would be expected because the flank rock is composed largely of fragments of the core constituents. They are differentiated primarily by their bedding and structural characteristics which are visible only in outcrops and well cores. Although the reef-flank and reef-core rocks to a certain degree can be distin- guished in the Marine reef, 42 this is in large part due to differential dolomitiza- tion, and the distinctions are not generally applicable. In other reefs both components may be entirely limestone or dolomite. Al- most all the reefs are entirely dolomite ex- cept in the southwestern part of the archi- pelago in Madison, Clinton, Washington, and Marion counties where limestone and dolomitic limestone are conspicuous and are probably the prevalent rocks. The reef-rocks as a whole are charac- terized by high carbonate purity, that is, by their small content of insoluble residue. They are, as a rule, 98 percent or more carbonates, the only common insoluble material being secondary pyrite. Their low insoluble content is in conspicuous con- trast with the 15 to 20 percent found in the surrounding interreef deposits in the low-clastic reef-bearing belt, and particular- ly at the southern reef archipelago border where the bordering nonreef deposits aver- age 40 percent insoluble. However, this 42 Lowenstam, H. A., op. cit., Marine pool, pp. 174-76. 30 NI AG ARAN REEFS IN ILLINOIS criterion of distinction between reef and interreef rock loses its value in the northern nonclastic reef-bearing belt, where the in- terreef deposits average 5 percent insoluble residue and quite commonly equal the reef- rock in purity. Other regional lithologic characteristics of the reef-rocks are the large grain size and light colors of the limestone and the preva- lent bluish-gray color and conspicuous vesicular texture of the dolomite. Chert is absent from the reef-rock. The absence of chert is an expression of environment, be- cause the sponge population, whose skele- tons supplied the silica for the chert of the interreef deposits, was confined to the still- water bottoms surrounding the reefs. Since still-water conditions were prevalent in the belt of low-clastic sedimentation but were the exception in the northern clastic-free belt, it is evident that chert as a criterion for reef and interreef distinction loses its value in the latter belt where chert is ex- ceedingly rare. These criteria, previously outlined in the description of the Marine reef, 43 remain valid in the search for oil in other Niagaran reefs, because the critical area is located within the low-clastic reef- bearing belt, particularly along the border against the high-clastic reef-free sedimenta- tion belt. The lithologic characters of the reef de- posits as compared to those of the normal Niagaran facies are distinguished in electric logs by consistently higher resistivity and particularly by consistently higher negative self-potentials in the low-clastic reef-bear- ing belt (pi. 1, wells 2, 3,8, 10). Fossil characteristics. — The reefs form the centers of maximum fossil density. This is to be expected because the core structure was erected as a loosely meshed frame by the skeletons of the reef-building organisms. The interstices of this frame were largely filled and the reef core was commonly flanked by the remains of the faunal associates of the reef builders, and by broken fragments of both. This is most noticeable in the reefs, largely limestone, 43 Lowenstam, H. A., and DuBois, E. P., op. cit., Marine pool, Madison County, p. 21. Lowenstam, H. A., op. cit., Marine pool, pp. 174-176. in the southwestern part of the archipelago, where the detrital fans and caps of the reefs consist almost entirely of fossil debris. Syngenetic and diagenetic recrystallization and superimposed dolomitization have des- troyed the identity of the fossils in large portions of the reefs, particularly in the cores, over most of the area of reef distri- bution. However, despite dolomitization, the outcropping reefs remain the sites of maximum fossil density in comparison with the surrounding interreef deposits. Casts and molds are the principal forms of fossil preservation in the dolomitized reefs. The diversity of the invertebrate fauna also forms a striking feature of the reef assemblages, rarely approached and never equalled in the interreef assemblages. Colonial corals and stromatoporoids, ap- parently the main builders of the reefs, are most abundant in the reef cores, but crinoi- dal remains outnumber them conspicuously in the reef-flank beds. In physical appear- ance, the reef assemblages are characterized by the prevalence of large heavy-shelled ro- bust forms which contrast sharply with the small fragile forms that characterize the still-water deposits. This criterion again is generally applicable within the low-clastic sedimentation belt but is not as useful in the clastic-free belt to the north, where the difference between the reef forms and those of the prevalent rough-water inter- reef is a matter of degree. The fossil re- mains on the reef-flank generally show random orientation except for colonial corals and stromatoporoids that grew in place and are parallel to the dip slope. In well samples and in outcrops these criteria enable one to recognize reef as- semblages. Numerical frequency, physical appearance such as large size and robust build, and assemblage characteristics such as relative abundance of colonial corals are determinable from well cuttings, but the dip-slope relations of colonial corals (an important clue in determining reef-flank deposits) are determinable only from well cores. The latter criterion could be utilized by means of oriented coring in wildcat borings that penetrate reef-flank deposits in REEF-BEARING BELTS 31 order to discover the direction to the reef core. STRUCTURAL EXPRESSION OF REEFS Studies at Marine have indicated that the Marine structure is largely controlled by the compaction of the interreef deposits around the relatively thick and rigid un- compactible reef body. 44 In general, the excess thickness of the Niagaran in the larger reefs produces dome-like structure in the beds that overlie the reef. This ap- pears to be true of the Bartelso, Sandoval, and McKinley reefs in the oil-producing area, and of the Gibson City reef on the Champaign-Ford County boundary. The structural relief above the Gibson City reef is sufficient to allow removal of the Penn- sylvanian strata by erosion, producing the Silurian inlier shown on the State geologic map. However, how much of this struc- tural relief is due to the presence of the reef and how much of it is due to deforma- tion has not been determined. Compactional structures over reefs may in some places be differentiated from de- formational structures by their patterns. As they are not related to the presumed lines of weakness in the crystalline base- ment which control most deformation structures, their position and the direction of their axes fall into the normal regional structural pattern only by coincidence. The shape of the individual structure is con- trolled by the reef mass and its topography. Narrow ridge-shaped reefs have rather narrow "anticlines" superimposed above them, but a horseshoe atoll such as Marine produces a crescentic structure. Any struc- ture which appears "out-of-line" with re- gional trends or which appears unusual in outline should be examined with the possibility of reef rather than structural control in mind. REEF BORDER Evidence of the influence of reefs on their enclosing sediments is given by outcrop and subsurface data which are in large part ^Lowenstam, H. A., op. cit., Marine pool, pp. 179-185. complementary. Outcrops show contacts of small reefs to the best advantage, and show the minor details of the contact re- lations of large reefs. Because of the lack of suitable outcrops, most of our knowl- edge of the three-dimensional relations of large reefs to their surroundings and of the effect of these reefs on adjacent interreef sediments has come from subsurface infor- mation. The well records, although spaced too far apart to show the fine detail given by outcrops, are admirably suited for out- lining the broader aspects of reef influence. The lithologic contrast at the reef border is more pronounced in the low-clastic cen- tral belt than in the purer carbonate belt of northwestern Illinois, and the contribution of the reef to the surrounding sediments is best recognized in that part of the low- clastic belt where the interreef deposits consist of the Moccasin Springs formation. Shifts in the character, and particularly in the size distribution, of the clastic portions of the sediments form sensitive indices of the effects of reefs on their environment ; the bright colors of the clay in the Moc- casion Springs formation allow ready rec- ognition and tracing of changes in the dis- tribution of the elastics. The contacts between small reefs of the flankless type and the surrounding inter- reef deposits are usually sharp. In the simplest examples the interreef deposits dip over and under the enclosed reef bodies with no obvious lithologic changes, as may be seen in an unnamed flagstone quarry half a mile north of Lemont, a short distance southwest of Chicago. 45 The larger reefs with flank deposits com- monly show more complex boundary rela- tions, the interreef deposits interfingering with flank beds or occasionally grading im- perceptibly into them. Interreef fingers may extend a considerable distance into the reef-flank. The first recognition of an ex- tensive zone of influence surrounding a Niagaran reef was at Marine. 46 The pres- ent study has indicated that the Bartelso, McKinley, and Sandoval reefs, several reefs 45 Lowenstam, H. A., op. cit., Biostratigraphic studies, p. 29. 46 Lowenstam, H. A., op. cit., Marine pool, p. 176, fig. 10. 32 NI AG A RAN REEFS IN ILLINOIS ALLEN No. I, Hard Sec. 23, T.I2N, R 5W. TEXAS No.l, Long CEN.RL.No.l, Cress GULF No.l, Brandon Sec. 27, T.IIN..R.5W. S«c.35,T. II N., R 5 W. Seel, T.ION., R.5 W. SILICIFIED FOSSILS h~ H LIMESTONE | I j | DOLOMITE DOLOMITIC LIMESTONE or CALCITIC DOLOMITE |~'/~ | SILTY TO ARGILLACEOUS LIMESTONE, DOLOMITE | '. 1 / ; | SANDY (very fine sond ), DOLOMITE or LIMESTONE R RED COLORATION Fig. 8. — North-south cross-section of Silurian strata in northeastern Montgomery County showing reef proximity by reef outwash and by local thinning of Bainbridge (St. Clair and Moccasin Springs) strata in the opposite direction from the regional trend. The strata above the Bain- bridge are the Thorn group. in Douglas and Champaign counties, and a postulated reef in northeastern Mont- gomery County (figs. 5, 6, 8) have had a noticeable effect on the surrounding sedi- ments. Reef influence has been observed as far as two to eight miles beyond the reef edge. The distance is to a certain extent a func- tion of reef size; larger reefs have larger zones of influence. It is also a function of direction. Winds and waves in Illinois during Niagaran deposition were apparent- ly predominately from the south, as indi- cated by the configuration of the Marine reef and by distribution of reef outwash and of terrigenous elastics which were carried beyond Marine, Bartelso, and the other less well-known reef occurrences. At Marine, recognizable reef detritus extends no more than half a mile south of the reef but at least a mile north of it, and some effects can be noted nearly eight miles north. A reef affects the surrounding sediments in two ways. First, the reef contributed to the surrounding sediments rather pure car- bonate fragments in the form of well graded organic debris, ranging from sand size adjacent to the flanks to clay size at the limit of reef influence, possibly a num- ber of miles distant. Second, the fine terrig- enous elastics which would normally have settled in the reef area were transported past it because of the turbulence caused by shallow reef waters. The by-passing land- derived sediments were then deposited to the lee of the reef in amounts which appear abnormally large. These elastics are also graded, the silt portion settling closer to the reef than the clay portion. As a result of the two processes described above, one can recognize two divisions in the zone of modified interreef sediments sur- rounding a reef. The inmost zone sur- rounds the reef but is most extensive on the northern lee side and is quite narrow on the windward southern side. This zone was characterized by considerable turbulence which is expressed by a maximum of reef- derived carbonate sand and by virtual absence of clay-size material, either reef- derived or terrigenous, except possibly floc- ulated clay. It can therefore be recog- OIL POSSIBILITIES 33 nized by being somewhat thicker than the normal regional interreef section, by having purer carbonate rocks, and by having little or no clay or (in the case of Moccasin Springs sediments) red color. There may be fingers of fine-graded coquina, principally crinoidal, but containing recognizable frag- ments of stromatoporoids and colonial corals of which Favosites is most common. Recog- nizable fragments of corals and stromato- poroids are practically never seen in cuttings from normal interreef sediments in this belt. Lapping around the lee edge of the inner zone is a semicircular zone that has unusual- ly large amounts of clay-size material, both that derived from the reef and the terrig- enous material carried past it. Wells in this zone show abnormally thick Moccasin Springs type sediments, as recognized by the trace red and bright green colors of the clay. Otherwise the sediments appear normal and it is unlikely that this zone of thickened clay deposition can be recognized when the interreef sediments are of types other than the Moccasin Springs. Intensive study of thin-sections or of heavy minerals in the insoluble residues of interreef carbonate rocks may develop criteria for recognition of this zone in other areas. The simplest method of recognition of strata near large reefs in the southern por- tion of the low-elastics reef-bearing belt, where the Moccasin Springs is present and the possibilities of oil in the Niagaran are greatest, is the preparation of an isopach map of the red-colored part of the Moccasin Springs formation. Beneath a reef and the adjacent inner zone of reef influence where the water was turbulent, the red sediments are abnormally thin. They thicken radial- ly, probably reaching the normal regional thickness abruptly to the south. To the north, northeast, and northwest, the area of abnormally thin red sediments is sur- rounded by a narrow semicircular fringe in which red sediments are abnormally thick. This thickening is the more noticeable be- cause it lies in the direction in which the red sediments are regionally thinning. The broad picture given above is prob- ably oversimplified. It is known from out- crops that beds containing rather large amounts of terrigenous elastics may abut directly against a big reef and even creep up on the flanks, as at Thornton and Elm- hurst in the Chicago region. Beds of pure dolomite, identical in texture and color to reef rock and representing the fine-grained fraction of reef detritus, are interbedded with high-clastic beds at Elmhurst and Thornton right up to the reef margin. 47 These occasional occurrences of high-clastic beds immediately adjacent to a reef probably are deposits in quiet-water pockets or semi- lagoons protected by reef arms. Because of the prevalence of rough-water conditions which scattered the reef-detritus widely over the interreef tract, there is little lithologic contrast between reefs and their surrounding interreef deposits in the clastic-free belt of northwestern Illinois. This is best illustrated in the bluffs a short distance east of Cordova. There is evidence that at least some of the Niagaran reefs settled into the nonreef substratum during their growth. The settling resulted in sagging of the interreef beds down toward the reef flank, as is shown at Elmhurst. That this sagging oc- curred during deposition of the sediments is indicated by black shale sediments which are confined to the sagged portion adjacent to the reef border and are indicative of stagnant conditions in localized depressions. The squeezing of the substratum out from under a reef has been described for a modern reef 48 and has been observed at a forereef at Thornton described earlier. Should this phenomenon occur in a subsur- face reef it would be recognized by an ex- tremely local bulging of Moccasin Springs type sediments right at the reef border. NIAGARAN OIL POSSIBILITIES In Illinois the Silurian rocks which show the greatest promise as oil reservoirs are the Niagaran reef lenses. Oil production from the Niagaran reefs of Illinois is of 47 Lowenstam, H." A., op. cit., Biostratigraphic studies, pp. 46, 131-133. 4S Cited in Cumings.E. R., and Shrock, R. R., The geol- ogy of the Silurian rocks, p. 145. 34 NIAGARAN REEFS IN ILLINOIS two distinct types. The reef rock itself may act as the reservoir, or overlying Mis- sissippian and Devonian formations may produce oil from the structures formed as a result of differential compaction over the rigid reef bodies. Oil has been produced from Devonian and Mississippian rocks above several known Niagaran reefs: Marine, Sandoval, Bartel- so, and McKinley. It appears quite possible from a study of the structural relations as shown on upper horizons that certain other pools, such as Tonti and Patoka, may be underlain by Niagaran reefs. The bulk of the oil produced at Marine has been directly from the Niagaran reef body. In addition, an undetermined pro- portion of the "Devonian" production at Bartelso has been from Niagaran reef rock, and some of the Sandoval production may have been from the underlying reef. Niag- aran oil is being produced from the reef at McKinley pool, Washington County. As the Niagaran reef production found to date has come from the large reefs near the reef front, and as there are considerable areas near the projected portion of the reef front in which the Niagaran is still un- tested, it appears that further major Nia- garan oil production is most likely to be found in the southeasternmost part of the reef -bearing area (fig. 9). It should be pointed out that there is little control for the location of the reef front between west- ern Marion County and Coles County. The actual front is presumably sinuous in this region, showing extensive embayments and prominent reef bastions as it does in the area to the southwest and northeast in which it can be mapped with greater ac- curacy. It is theoretically possible that some reef outliers occur south of the archipelago bor- der, as reef development might have started there during the relatively clear-water phase of St. Clair deposition. The northeastern limit of the area of greatest likelihood of Niagaran reef pro- duction is the LaSalle anticlinal belt, as it appears that all commercial oil has been flushed from reefs on and beyond this struc- Fig. 9. — Silurian oil possibilities. ture. That commercial oil at one time was present in the Niagaran reefs of the LaSalle anticlinal belt and of the Chicago outcrop area is indicated by shows of residual "dead" oil through 200 feet of the Niagaran reef penetrated three miles north of Tuscola in Douglas County, by similar shows in Cham- paign and Ford counties wells, and by the well known asphalt occurrences in the out- cropping reefs of the Chicago area. Northwest of the belt of greatest reef development and preservation is a parallel belt which presents fair possibilities for reef production (fig. 9). It seems likely that the reefs in this area were smaller than those closer to the front, and in addition they must have been truncated by pre-New Albany as well as pre-Middle Devonian erosion. OIL POSSIBILITIES 35 Deeply eroded reefs may be present and may produce oil in the central Illinois area with poor possibilities (fig. 9). In addi- tion, it is largely within this area that porosity and permeability related to ero- sional surfaces may be expected in various nonreef Silurian rocks, with the possibility of light oil production. The points of greatest significance in the search for further Niagaran reefs may be summarized as follows : The reef-rock proper always consists of practically pure carbonates, either limestone or dolomite, that have higher electrical resistivity than normal interreef rocks. Reef dolomites are invariably blue-gray in color, coarse-grained, and vesicular. Reef limestones are pink to white coquinas, formed almost exclusive- ly of coarse unsorted fossil debris in which corals and stromatoporoids are important constituents. Chert is never present. The two reef rocks are replaced by varied impure carbonate rocks in the common interreef areas. The Niagaran rocks are abnormal- ly thick at reef cores. In the critical area for oil production where the Moccasin Springs formation is present, reef proximity may be most easily recognized by an abnormally thin section of Bainbridge red-colored beds closest to the reef and by abnormally thick red- colored sections at some distance to the lee of the reef. Fingers of well sorted fossil debris, chiefly crinoidal but containing no- ticeable amounts of coral constituents, ex- tend some distance from the reef. Although certain interreef beds may approach these ringers in purity and in fossil density, the coral content is a reliable index for reef proximity. Relative purity of the Niagaran section in general strongly suggests nearness to a reef. All these criteria for reef prox- imity are applicable only within the south- ern two-thirds of the low-clastic reef-bear- ing belt indicated on figures 1 and 6, as they cannot be applied when the interreef sediments as a whole become relatively pure and the red Moccasin Springs formation becomes patchy toward its feather edge. Structural highs occur in the beds that overlie reefs, due to the thicker reef sec- tion and to the compactibilitv of the inter- reef sediments. These structural highs im- perfectly reflect the reef topography and outline, and therefore bear only coincidental relation to the normal regional pattern of deformational structures. In general, alignment of reef structures may be more or less parallel to the reef front. Known structures in higher beds which do not fall into the regional structural framework should be re-examined for possible reef in- fluence. In some instances there may be superposition of reef-controlled structures on deformational structures. As reefs would not influence the struc- ture of underlying beds, seismographic re- flections from the "Trenton" will not in- dicate the presence of Niagaran reefs, al- though seismographic records of horizons above the Niagaran should prove useful. Gravity anomalies have proved successful in locating Permian reefs in west Texas, but it is not known whether there is suffi- cient difference in the density of reef and interreef Niagaran sediments to produce recognizable anomalies. The physical prop- erty of reef-rock which appears to offer the greatest possibility for direct geophysical detection is its electrical resistivity, which averages several times greater than that of the typical interreef sediments. Figure 6 indicates all known reefs in the area critical for oil production, although it is not complete for some areas in western Illinois. In addition it indicates a number of occurrences of reef flanks and reef- influenced interreef deposits in wells in the critical area. The most important of these near-reef occurrences can be summarized. The Kingwood Oil Company 1 Gaffner well in the SW.J4, SE.1,4, NE.i/J, sec. 30, T. 6 N., R. 3 W., Bond County, penetrated reef outwash containing fragments of reef- building corals and well assorted crinoidal coquina. It therefore appears likely that the nearby gas-bearing Avers structure 49 is developed over a ridge-shaped Niagaran reef. Payne, J. N., Structure of Herrin (No. 6) coal bed in Madison County and western Bond, western Clinton, southern Macoupin, southwestern Montgomery, north- ern St. Clair, and northwestern Washington counties: Illinois State Geol. Survey, Circ. 71, 1941. 36 NIAGARAN REEFS IN ILLINOIS Three wells in northwestern Mont- gomery County (fig. 8), the Gulf 1 Bran- don well, sec. 1, T. 10 R, R. 5 W., the Central Pipe Line 1 Gees well, sec. 35, T. 11 N., R. 5 W., and the Texas 1 Long well, sec. 27, T. 11 N., R. 5 W., are all within the inner zone of reef influence as indicated by the fact that the Moccasin Springs formation has been replaced by reef outwash and unusually pure low-clas- tic dolomite and dolomitic limestone. Of the three wells, the Gulf 1 Brandon well appears to be closest to the reef because it has a thin St. Clair .section and contains the greatest amount of reef outwash. The presence of this postulated reef is strength- ened by the occurrence of 50 feet of normal Moccasin Springs strata in the Alan 1 Ward well in sec. 23, T. 12 N., R. 5 W., 8 to 9 miles from the other wells and in the direction in which the Moccasin Springs formation is regionally disappearing. This last well is in the outer zone of influence in which clay-sized elastics and reef out- wash are dumped in the lee of the reef. The Continental Oil Company 1 Beachy well, sec. 13, T. 15 N., R. 6 E., Moultrie County, penetrated 160 feet of reef core and reef outwash and an additional 20 feet of reef-influenced sediments beneath the reef material. This would indicate that the reef began its growth not at the loca- tion of the Beachy well, but at some other point nearby. This reef is indicated on the regional reef distribution map of the second Marine pool report. 50 The Carter 6 Brauer well, sec. 21, T. 8 N., R. 3 E., the deep test in the northern 50 Lovvenstam, H. A., op. cit., Marine pool, fig. 7. part of the Louden pool, penetrated inter- fingering reef outwash and core rock as well as interreef sediments. The Pray 1 Baker well, sec. 17, T. 18 N., R. 6 E., Piatt County, penetrated a section of reef core, with outwash above and below. Numerous wells in the LaSalle anticlinal belt from Coles County north penetrated reef core, outwash, or both. The map (fig. 6) showing reef distribu- tion indicates other occurrences in which the evidence of reef proximity is doubtful, consisting only of unusually pure limestone or dolomite, whose relative purity may be due to admixture of reef-derived carbonates, or may indicate only normal sedimentary variations. Possibilities for oil production from Silu- rian rocks other than the Niagaran reefs may be summarized briefly. Reservoirs of low permeability and consequent light oil or gas production may be developed in a number of Niagaran and Alexandrian rocks by post-Niagaran erosion and weathering. Suitable structural or stratigraphic trap conditions have localized pools within the belt where pre-New Albany erosion has laid bare the Silurian rocks. Oil produc- tion at Mount Auburn in Christian County, Collinsville in Madison County, oil shows in the Silurian of Sangamon and Morgan counties, and gas production at Pittsfield, Pike County, are of this type. It is to be expected that similar light production will be found in the future in the region of west- central Illinois in which the Niagaran is less than 250 or 300 feet thick (fig. 4). Illinois State Geological Survey Report of Investigations No. 145 1949 REPORT OF INVESTIGATIONS 145, PLATE I ELECTRIC LOG CROSS-SECTION OF SILURIAN AND DEVONIAN STRATA