6 ^€e\ 5 UAO «4J STATE OF ILLINOIS DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION FRANK G. THOMPSON. Director DIVISION OP THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chi^ URBANA REPORT OF INVESTIGATIONS— NO. 114 MARINE POOL, MADISON COUNTY A New Type of Oil Reservoir in Illinois H. A. LOWENSTAM AND E. P. DuBoiS PRINTED BY AUTHORITY OP THE STATE OF ILLINOIS ffP^ p«!^'%r URBANA, ILLINOIS 1946 FEB 2 5 1999 li&t^ ^ia »*>«*»'U^ STATE OF ILLINOIS D WIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION FRANK G. THOMPSON, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS— NO. 114 MARINE POOL, MADISON COUNTY A New Type of Oil Reservoir in Illinois H. A. LOWENSTAM AND E. P, DuBoiS PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1946 FEB 2 5 1999 IL yit% 5 Development of the pool 5 Stratigraphic summary 7 Structure 9 Devonian and Silurian stratigraphy . 13 Devonian system 13 Undifferentiated Devonian siltstones 13 Cedar Valley formation 14 Wapsipinicon formation 14 Fissure system 15 Silurian system 18 Reef and associated deposits of post-Bainbridge Niagaran age 19 Interreef and associated deposits of post-Bainbridge Niagaran age 23 Interfingering of reef and normal facies .... '.". 25 Age of uppermost deposits 25 Interpretation of the post-Bainbridge Niagaran facies 26 Bainbridge formation 27 Alexandrian series 28 Sexton Creek formation 28 Edgewood formation 28 Silurian-Devonian history 28 Oil potentialities and migration 29 ILLUSTRATIONS FIGURE PAGE 1. Index map showing location of the Marine area 5 2. Drilling and production curves, Marine pool 6 3. Sequence of strata in the Marine pool 7 4. Silurian structure in the Marine pool 9 5. Devonian structure in the Marine pool ; 10 6. Chouteau structure in the Marine pool 11 7. Ste. Genevieve structure in the Marine pool 12 8. a-d, Lithologic character of Silurian reef deposits 19 8. e-h, Lithologic characters of Silurian interreef deposits 20 9. Characteristic electric logs of Niagaran interreef (A) and reef (B) deposits 22 10. Diagrammatic cross-section of Marine reef 23 11. Cross-section showing Silurian reef and interreef facies relationships 24 12. Map showing regional structure of the top of the Silurian and present known distribution of the upper Silurian reef detritus 16-17 MARINE POOL, MADISON COUNTY A New Type of Oil Reservoir in Illinois BY H. A. LOWENSTAM AND E. P, DuBoiS INTRODUCTION THIS REPORT describes the occur- rence of oil in a Silurian coral reef, the first commercial occurrence of this type in Illinois, Because there is a considerable area within the State where stratigraphic traps of this sort may be found, it is believed that the description and dis- cussion of the Marine pool is of special interest. Broad regional studies will be needed to evaluate the possibilities for addi- tional oil occurrences of this sort. The area concerned includes T. 4 N., R. 6 W., and portions of adjacent townships in Madison County, Illinois (fig. 1). The Marine pool proper is located in sees. 9, 10, 15, and 16 with some outlying production in sees. 3 and 8. The principal producing zone is the upper portion of the Silurian strata. The pool lies on the western side of the Illinois basin in a region where the surface of the Devonian strata dips to the southeast at a rate of about 30 feet to the mile. The geologic strata present are of Pleistocene, Pennsylvanian, Mississippian, Devonian, and Silurian age. The object of this study is to describe the character of the oil reservoir in the Marine pool and to point out its significance in rela- tion to future oil prospecting in Illinois. The report is based on data available as of May 10, 1945, During the seven-month period, May through November, 16 pro- ducing wells were completed in the Marine pool making a total on December 1, 1945, of fifty-three producing wells in the pool. Cumulative production to November 1, the latest date available, was approximately 1,163,000 barrels. Data from the new wells do not change the interpretations in the report in any essential way. DEVELOPMENT OF THE POOL Seismograph and gravity surveys indi- cated the presence of the structure here designated as the Marine dome. The dis- covery well was the Eason and Company — Mayer No. 1, in the SE. i, NW. i, SW. i, sec. 15, T. 4 N., R. 6 W,, completed in July, 1943. This well, which was drilled on the southeastern flank of the structure, had an initial production of 14 barrels of oil and 3 barrels of water per day. It was abandoned almost immediately because of the low production. Fig. 1, -Index map showing location of the Marine area. Development proceeded slowly, averag- ing about two completions a month, with a maximum of five completions in November, 1944 and in January, 1945 (fig. 2). By May 1, 1945, there were 37 producing wells in the field with a cumulative produc- [5] MARINE POOL, MADISON COUNTY u. 20 ■ J~ 1 / / ■ -^ / / /^i — / .'^ / V '/^ ■ ■^ V y f> ^ ^ y SON 1943 MJJASONDJF 1944 1945 ^ ■^ / ^ / ( ..°> ^ oJ> // f^ / ^ L ^^ ^ Y HUME ^e^c^ ./ '"■-^ y^ JASONOJFMAMJJASONDJFM 1943 1944 1945 Fig. 2. — Drilling and production curves, Marine pool. STRATIGRAPHIC SUMMARY tfon of more than 757,000 barrels of oil and an average daily production for the field of about 2,200 barrels. Most of the wells were drilled by rotary tools to a depth a few feet below the top of the Devonian limestone, where casing was set and cemented. The wells were then completed with cable tools and were either shot or acidized. One, two, or occasionally three "breaks" have been reported at vary- ing depths below the top of the Silurian. A few wells appear to be producing from fis- sures in the Devonian strata. Initial pro- ductions have averaged about 150 barrels per day (table 1). STRATIGRAPHIC SUMMARY Figure 3 The strata below the Pleistocene till or glacial drift include about 500 feet of sand- stones and shales of Pennsylvanian age, and about 200 feet of sandstones, shales, and limestones of Upper Mississippian (Chester) age (fig. 3). The Golconda formation is generally the uppermost unit of the Chester series, but in a few places it has been cut through by post-Chester erosion so that the Pennsylvanian strata rest upon lower beds. The Lower Mississippian strata include about 40 feet of Ste. Genevieve limestone, 160 feet of St. Louis limestone, 120 feet of fossiliferous Salem limestone, and about 475 feet of limestones, shales, and siltstones of Osage age. The Chouteau limestone aver- ages about 25 feet in thickness and in some wells is separated from the brown and black shales of Upper Devonian-Kinderhook age by 10 feet, more or less, of green and red shales. The dark colored shales of Upper Devonian-Kinderhook age are about 70 feet thick. The Devonian strata are represented by an undifferentiated upper shaly and silty zone and by the Cedar Valley and Wapsi- pinicon limestones. The total thickness of the Devonian beds ranges from 25 to 35 feet. The underlying Silurian formations range from 430 to 570 feet in total thick- ness, with the thicker sections being near the reef. It is presumed that even greater thicknesses of Silurian beds exist within the LLJ LJ-I :^3COLUMNAR o2 SECTION Q- ■•.o.-..-.- •.•■.••■.• :■••;.■•.: •;o-::o.-. Z < z z z LU CL 1 1 1 . . . ■ — — — — 1 1 [ ^-^- — - — ^^ — = — — ^ •.•.■A\\-.-.;',>-;-.v"X •• •■■::■■ z < Q. CL (/) (/) '.. L .'_L^^ ' 1 ' p^ 1 \a^ 1 1 CP^ 1 ' ^^^ — 1 — ' — t!^C^^^ ' IT' ^ I- ' 1 I ' III III — \ — ' — I — ' — 1 — "— ALEX I 1 1 1 1 z < § Z z o z o 1 '1 ' I ' 3 cr 2 — ^ \ ■— . ' r- MOH __ .L._., , ...L,..., 1. Gal. ' 1 ' 1—^ Fig. 3. — Sequence of strata in the Marine pool. MARINE POOL, MADISON COUNTY Table 1.- -Well Data for Marine Pool May 10, 1945 Company and Farm Location IP' Treatment Rock Hill et al— John Becker 1 Obering — Bird 1 Hubbard— Kolb 1 Obering et al. — Becker-I.C. RR. 1 Obering et al. — Becker-I.C. RR. 2 Obering — John E. Becker Comm. 1 Obering, MacDowell, Lord- Becker Comm. 2 Obering — Becker-Pence 3 . . . . Obering— Becker-I.C. RR. 3 . Obering — Becker-Pence 4. . . . Obering — Grimm 1 Obering — Grimm 2 Ohio— E. H. Grotefendt 1 . . . Ohio— Grotefendt-I.C. RR. Comm. 1 Shell Oil— Kettler 1 Rock Hill O. & G.-Pence 1 . . Rock Hill O. & G.— Pence 2. . Rock Hill and Obering — Pence 3 RockHill— W.H. Pence4... Sinclair-Wyoming — Hess 1.. . Sinclair-Wyoming — Hess 2. . . Sinclair-Wyoming — Ben Hess 2-A Sinclair-Wyoming — Hess 3. . . Rock Hill Oil (Eason & Co.)- L. Mayer 1 Ohio Oil— Pence 1 Ohio — Pence Comm. 2 Rock Hill— Horn 1 Kacalieff — Illinois Central RR. 1 Rock Hill— Mayer 2 H. P. Hubbard— Kolb 2 H. Hubbard— Kolb 3 Rock Hill— Pence 1 Rock Hill— Pence 1-D Rock Hill— Pence 1-Elbring- I.C. RR. 1 Rock Hill— Pence-Mayer 3 . . Rock Hill— Pence-Mayer 4 . . Rock Hill — Pence-Gericke 1 . Rock Hill— Pence-Gericke 2. Rock Hill— Pence-Elbring 2. Shell Oil— I. Elbring 1 Rock Hill— Ed. Grimm 1 . . . . Sohio Pet. — Keown 1 T. 4N., R. 6 W. Section NWSWSE3 SE NW SE 8 SE SW SW 9 SESWSE9 SENWSE9 SESESE9 SENESE9 NW SE SE 9 NWSWSE9 NWNESE9 SESESW9 SENESW9 SESENE9 SESWNE9 SENWSW9 SE SW SW 10 SESESWIO NW SW SW 10 NW SE SW 10 930 N 330 ENWSW 10. SE SW NW 10 290 S IIOOWSWNWIO NWNWSWIO SENWSW15 SENWNW15 NWNWNW 15 SESESW15 SE SE NW 16 300S. 360ESESE'nE16 SENWNW16 SESWNW 16 SENENE 16 710 S 125ESENE 16... SENWNE16 NWNENE16 NWSENE16 SESWNE16 NWSWNE16 NWNWNE16 SENENW16 SENESE17 NWNENE17 52BOP+150BW*...... 45 BOP 34BOP+300BW (Worked over 28 BOP) 80 BOP 58BOP+10BW 115 BOF (W.O. 264BOP+10 BW) 236 BOF 84 BOP 877 BOF 150 BOP 101 BOP 164 BOP 128 BOP 38 BOP+15 BW D & A (Show oil) 241 BOF 75BOP+10BW 224 BOP 249 BOP 373 BOF + 17 BW 22BOP + 7BW SWD 45 BOP 240 BOS (OWWO) 14BOP + 3BW 27 BOP 152 BOP D&A 30 BOP + 10 BW 50BOP + 750BW 150 BOP + 5BW 30 BOP + 2 BW 168 BOP + 103 BW SWD 190 BOF 350 BOP 165 BOP + 33 BW 15 BOP + 150 BW 94 BOP 124 BOP 40 BOP D&A 103BOP+3BW Acidized 200 gals. Acidized 750 gals. Acidized 250 gals. Shot 10 qts. Shot 30 qts. Acidized 2000 Acidized 300 gals. Acidized 200 gals. 100 gals. 1000 gals. Shot 10 qts. Acidized 3000 gals. Shot 20 qts. Shot 20 qts. ♦Abbreviations : D & A — dry and abandoned IP — Initial production BOP - — Barrels oil pumping BOF — Barrels oil flowing BOS — Barrels oil swabbing BW — Barrels water WO — Worked over OWWO— Old well worked over SWD — Salt water disposal STRUCTURE structure. Data from nearby wells indicate that the Ordovician system is represented by 175 feet of Maquoketa shale and lime- stone, 90 feet of Galena (Kimmswick) lime- stone, and about 475 feet of Plattin, Joachim, and Dutchtown limestones. The St. Peter sandstone lies just below the Dutchtown formation at a depth of nearly 3,000 feet. STRUCTURE In the upper part of the Silurian (fig. 4) and in the Devonian (fig. 5) and Lower Mississippian (fig. 6) strata, the structure at Marine is an elongate dome trending northeast-southwest, with its highest point near the northeast where there is nearly 100 feet of closure, and a lower slightly LEGEND ILLINOIS STATt GfOcOG/CA, .<<>' CONTOUR ON TOP OF SILURIAN LIMESTONE, INTERVAL-IO FEET • PRODUCING OIL WELL ^ ABANDONED OIL WELL -(J)- DRY HOLE Fig. 4. — Silurian structure in the Marine pool. 10 MARINE POOL, MADISON COUNTY MAY 10, 1945 LEGEND ILLINOIS STATE G EOLOCICAL SURVEY /'< -\* CONTOUR ON TOP OF DEVONIAN LIMESTONE INTERVAL - 10 FEET • PRODUCING OIL WELL ^ ABANDONED OIL WELL -6- DRY HOLE Fig. 5. — Devonian structure in the Marine pool. larger area to the southwest where there is about 70 feet of closure. The Eason and Co.— Mayer No. 1 well, SE. \, NW. SW. J, sec. 15, is located high on structure, which is interpreted to indicate a south- eastward extension of the structure (figs. 4, 5, 6, and 7), although it might possi- bly represent a separate structure of lesser magnitude. Other wells located structur- ally high in the immediate area are the Gins- berg-Brooks No. lA in the SW. i, SE. i, NE. i, sec. 10, the Rockhill-Becker No. 1 in the NW. i, SW. \, SE. i, sec. 3, and the Obering-Bird No. 1 in the SE. i, NW. \, SE. -J, sec. 8. These wells may indicate small subsidiary structures or a single more or less continuous flanking structure. The structure on the top of the Ste. Gen- evieve limestone (fig. 7) reflects the struc- ture of the underlying horizons but indicates STRUCTURE 11 /U/4K /O, 1945 LEGEND ILLINOIS STATE GEOLOGICAL SURVEY .x^ CONTOUR ON TOP OF CHOUTEAU LIMESTONEy INTERVAL - 10 FEET • PRODUCING OIL WELL ^ ABANDONED OIL WELL -^ DRY HOLE Fig. 6. — Chouteau structure in the Marine pool. less deformation. The northeast-southwest trend is less pronounced than in the lower beds, and the amount of closure is somewhat less. Little information has been obtained on the relations of the Ste. Genevieve and Pennsylvanian structures. That which is available indicates a close conformity be- tween the Ste. Genevieve and a thin lime- stone about 400 feet above it in the Penn- sylvania strata. The upper surface of the Chester series is irregular and reflects only to a limited extent the surface of the Ste. Genevieve limestone. The inequalities developed upon the Chester surface are in part compensated for by the deposition of a variable thickness of the lower Pennsylvanian sediments. Only two wells in the immediate vicinity of the pool reach the Galena (Kimmswick) formation. These are the Eason and Co. 12 MARINE POOL, MADISON COUNTY MAr lOy IS45 LEGEND / CONTOUR ON TOP OF ST. GENEVIEVE LIMESTONE , INTERVAL- 10 FEET ILLINOIS STATE GEOLOGICAL SURVEY • PRODUCING OIL WELL f ABANDONED OIL WELL -(J)- DRY HOLE (O) SALT WATER DISPOSAL WELL Fig. 7. — Ste. Genevieve structure in the Marine pool. —Mayer No. 1 well in the SE. \, NW. i, SW. i, sec. 15 and the Ryan Oil Co.— Kesner No. 1 well in the NW. i, SW. i, SW. J, sec. 2. The elevations of the Galena in these wells appear to be relatively con- sistent with the regional elevations and sug- gest the possibility that the structure in the Galena is considerably less than it is in the overlying beds. DEVONIAN SYSTEM 13 DEVONIAN AND SILURIAN STRATIGRAPHY The following discussion of stratigraphy is confined to the Devonian and Silurian deposits because they form the immediate capping rocks and main reservoir of the pool. Cores from five rotary vi^ells and one cable-tool well form the main source of information for the following lithologic descriptions. Additional data were secured from core chips and shot fragments, and from rotary- and cable-tool cuttings. The six cores represent a continuous sequence from about two feet below the top of the Devonian to about 63 feet below the top of the Niagaran. Devonian System The Devonian deposits consist of three distinct units which, in descending order, include a zone of undifferentiated shaly siltstones and the Cedar Valley and Wapsi- pinicon limestones. The thickness of the Devonian deposits ranges from 25 to 35 feet. UNDIFFERENTIATED DEVONIAN SILTSTONES The highest deposits referred to the Devonian include 7 to 10 feet of massive firm calcareous and argillaceous siltstones of which the upper 3 to 6 feet are grayish- brown and the lower 4 feet are dark brown in color. The upper grayish-brown siltstones are usually argillaceous, glauconitic, and highly calcareous but with occasional thin dolo- mitic lenses. Incorporated within this unit are thin laminae and lenses of dark brown argillaceous siltstones that are similar in lithology to the underlying siltstones. Fish remains and phosphatic pellets have been found near the base of the upper grayish- brown siltstone. The top of the grayish-brown siltstone is indicated in electric logs by a characteristi- cally small but sharp increase in impedance. A narrow zone in which semi-consolidated grayish-brown siltstones appear to have been incorporated in a matrix of dark brown siltstone marks the contact with the under- lying beds and seems to represent a rapid intraformational facies change. The lower dark brown siltstones are more argillaceous, less calcareous, and more highly pyritic than the upper but contain some lenses and laminae of grayish-brown siltstone and characteristically contain sand in the lower portions. The sand appears in thin lenses of grayish-brown siltstone or occasionally scattered through the dark brown siltstone but becomes more abundant with depth. Sometimes it is concentrated in silty, calcareous slightly glauconitic lenses which are inclined and suggest cross lamination. The individual sand grains are medium to coarse, frosted, and well rounded. Chert is also occasionally present near the base and is gray or brownish-gray in color and is slightly calcareous, argilla- ceous, glauconitic, and pyritic. The chert may be a silicified organic sand which in places is predominantly made up of sponge spicules and subordinately of ostracods, tentaculites, and echinoderm fragments. The individual fragments are medium to coarse in size and are uniformly sorted. Scattered pebbles of chert occur above the main zone of development. Small aggre- gates of sphalerite and galena are locally present. Data on the contact relations with the overlying Upper Devonian - Kinderhook shale are lacking. The lower beds of the overlying shale are lithologically similar to the undifferentiated Devonian siltstones but are less calcareous and contain lenses of fine sand. The Sporangites* in the Upper Devonian-Kinderhook beds are noticeably larger and more abundant than those in the Devonian siltstones. The contact with the underlying Cedar Valley limestone is represented in the core of the Ohio Oil Co.— Grotefendt, I. C. R. R. Community No. 1 well in the SE. \^ SW. i, NE. i, sec. 9. The contact is sharp and irregular. The basal four inches of the undifferentiated Devonian siltstone is very calcareous and sandy and contains scattered crinoid columnals as well as frag- ments of the underlying Cedar Valley lime- stone. *Sporangit£s is now considered a nomen ambiguum, and Tasmanites is proposed to replace Sporangites. Schopf, Wilson and Bentall, Illinois Geol. Survey Rept. Inv. 91, 1944. 14 MARINE POOL, MADISON COUNTY A small variety of Sporangites occurs commonly in these siltstones. Megascopic fossils are confined to two species of bra- chiopods, one of which is referable to Lingula. Individuals of both species are fairly abundant. Ostracods from the basal chert suggest a post-Cedar Valley pre- Cerro Gordo age/ CEDAR VALLEY FORMATION The Cedar Valley formation in the Marine pool area includes 3 to 10 feet of sandy argillaceous limestone beds which are interlaminated with sandy shales. Depend- ing upon the proportion of sand and shale and upon the degree of segregation of the limestone and shale fractions, the forma- tion is either a more or less continuous sequence of slightly argillaceous limestones with occasional discontinuous shale part- ings or a thin-bedded to thinly laminated series of limestone beds with interbedded sandy shales. The limestone is gray, white, brown, or buff, sometimes with dark gray or buff mot- tling, and represents coarse, occasionally medium to fine-grained coquinas or "semi- coquinas" of skeletal debris. The organic constituents consist predominantly of disso- ciated crinoid columnals and to a lesser ex- tent of fragmentary corals, brachiopods, and bryozoa. Secondary replacement by silica, glauconite, and pyrite is common. How- ever, replacement by glauconite and silica is seldom complete. Large scale replace- ment by silica is rare and confined to aggre- gates which are free of argillaceous impu- rities. Selective pyritization of bryozoan fragments is apparent. The interstitial spaces of the coquina may contain sand, gray or brown clay, silt, glauconite, pyrite, or secondary calcite in any proportion or combination. The sand is commonly medium to coarse, rarely fine- grained, and is well rounded and frosted. The pyrite is usually finely disseminated and is found in the shaley portions. A small variety of Sporangites occurs in the argillaceous and silty portions and is simi- 1 C. L. Cooper, personal communication. lar to that in the overlying undifferentiated siltstones. The shale laminae within the limestone are gray to black, rarely light brown, and are locally glauconitic, pyritic, or sandy. Their composition is similar to that of the argillaceous portions of the coquina matrix. Because the sand content is variable the laminae may actually range in lithology from a shale to an argillaceous sandstone. Conglomeratic zones occur locally at the top and bottom of the formation. The peb- bles in the conglomeratic phase near the top consist of chert and medium to dark gray finely crystalline limestone, both of which are foreign to the Cedar Valley limestone in this area. The pebbles in the basal beds were derived from the underlying Wapsi- pinicon, and light to dark gray chert peb- bles and abraded corals and bryozoa are present. The limestone matrix is free from shale, contains less silt and more sand than the rest of the formation. Very thin lenses of silty sand are commonly associated with the conglomeratic zone. Ill-defined patches of dark brown pyritic siltstone containing abundant small Sporangites are present and may represent pebbles which were only semi- consolidated at the time of their incorpora- tion into the sediment. The irregular contact with the overlying beds has been described. A sharp lithologic break marks the contact of the Cedar Val- ley and underlying Wapsipinicon lime- stone. From the evidence presented below in connection with the development of joints and fissures it appears probable that the Cedar Valley limestone rests unconform- ably upon the Wapsipinicon formation. Correlation of these beds is based upon lithologic similarity and continuity with beds of Cedar Valley age in western Illi- nois. WAPSIPINICON FORMATION The Wapsipinicon is represented by 11 to 15 feet of limestones and occasionally dolomites. The limestones are dense and litho- graphic to finely, rarely medium, crystal- line, and are brown or buff, occasionally DEVONIAN SYSTEM 15 dark gray in color. Isolated medium to coarse sand grains which are well rounded and frosted occur throughout the limestone. Toward the bottom of the formation the sand content increases slightly and the lime- stone becomes more silty. Chert, which occurs at random throughout the formation, is usually limited but may make up as much as 60 percent of the rock. In color it ranges from brown to gray or black. Discontin- uous fractures of microscopic size are com- mon in the limestone. The fractures and minute vugs and fossil cavities are often lined or filled with secondary calcite and sometimes with glauconite or chalcedony. Minute glauconite spots and dendrites of manganese oxide occur rarely in the top of the formation. Fossils are occasionally silicified. Stylolites are locally developed. The dolomites are buff to brown in color and finely to medium crystalline. Fossil casts and molds are characteristic. Some of the fossil cavities are filled with second- ary calcite. Sand grains similar to those in the limestones are locally present. Patches of dolomite within the limestone are com- mon, particularly along the margins of fis- sures and joints which have been enlarged by solution. A very sandy conglomeratic phase about seven feet below the top of the Wapsipin- icon is represented in the core of the Shell Oil Co.— Elbring No. 1 well (sec. 16, T. 4 N., R. 6 W.). The rock is a sandy dol- omitic limestone which is dark brown with light brown mottled areas, dense to finely sucrose, and is penetrated by trail-like oval shaped worm tubes. Sand is unevenly dis- tributed through the limestone matrix but less concentrated than in the tube fillings. Commonly the light brown areas contain more sand than does the darker brown dol- omitic limestone. Unsorted well rounded pebbles of dark gray chert and of buff and light brown finely crystalline limestone with a few scattered sand grains are common. The material in the worm tubes consists of about 60 percent sand grains in a matrix of white to buff finely crystalline sucrose lime- stone which is occasionally glauconitic. The sand is similar to that in the rest of the zone and ranges from rounded to rarely subangu- lar in shape and is fine to coarse, frosted and unsorted. The entire conglomeratic zone in the Shell-Elbring well is 2^ feet thick. These beds are assigned to the Wapsi- pinicon formation on the basis of lithologic similarity and subsurface continuity with similar beds in western Illinois. Information on the contact relations of the Devonian and Silurian deposits is lim- ited to cores from three wells. That from the Sohio— Imbs No. 1, NW. i, NE. i, SE. ^, sec. 7, shows the contact of the Wap- sipinicon and the interreef facies of the Silurian. The surface of the contact is very irregular and has been enlarged by solution accompanied by sand and clay infiltration (fig. 8a). The basal 12 inches above the contact plane appears to be gradational in lithology. Lenses in the basal few inches suggest incorporation of Niagaran material in the basal beds of the Wapsipinicon. The core of the Sohio— Imbs No. 2 (SE. i, SE. i, SE. i, sec. 1, T. 4 R, R. 7 W.) does not show the actual contact, but the basal Wapsipinicon deposits are again more or less transitional with the Niagaran. The cable-tool core of the Shell — Elbring No. 1 (SE. i, NE. i, NW. i, sec. 16) does not show the actual plane of contact but there is an abrupt change from the typical Wap- sipinicon lithology to that of the underlying detrital reef sand. FISSURE SYSTEM The rocks of the Wapsipinicon formation and of the upper parts of the Niagaran series contain many fissures (fig. 8a). This is known from recovery of a number of filled crevices in cores, as well as recovery of sand and green shale in most well cut- tings from the Niagaran and Wapsipinicon formations. The fissure system is most strongly developed in the Wapsipinicon and in the upper beds of the Silurian pink detrital limestone and is less pronounced at greater depths. The interreef beds of the Niagaran show few fissures, and those which are present are confined to the upper- most few feet. MARINE POOL, MADISON COUNTY The intricate system of fissures intercon- nects with bedding planes and occasionally with joints, stylolites, and worm-borings. The fissures are commonly irregularly en- larged by solution and may attain a width of two inches. The walls are smooth or may be sculptured in such a way as to indi- cate solution accompanied by movement. The main fissures and solution-enlarged bed- ding planes are always tightly filled with clay or sand and commonly by both. A direct relationship appears to exist between the width of the fissure and the proportion- ate filling of sand. The widest fissures are as a rule filled with tightly packed cal- careous sand. Fragments of wall rock with solution resorbed margins occur commonlly within the sand matrix and are occasionally arranged in discontinuous bands. What little clay occurs in the large fissures is con- centrated along the wall and around the fragments of wall rock. With decrease in width, the fill of the main fissures consists primarily of clay and to a lesser extent of sand which is scattered at random through it. Secondary calcite is sometimes present along the fissure walls and is most promi- nent in the zone of deepest penetration. In the reef dolomite, clay fillings are still com- mon but sand is rare. Smaller fissures com- monly branch off the main ones and pene- trate the wall to a variable depth. In the pink detrital limestone they interconnect with the pores of the interstitial matrix and with the vugs. The fill of these secondary fissures is similar to that of the main fissures but clay is quantitatively dominant. In physical character the sand in the fis- sures is coarse to fine and occasionally grades to silt. It is well rounded, frosted, and only occasionallly sorted. The clay is light to dark green, less commonly gray, and rarely white. Finely disseminated pyrite occurs adjacent to the wall rock and as megascopic crystals in the clay and occa- sionally in the sand. Marcasite and sider- ite is also of rare occurrence in the sand. Since the fissures do not extend into the Cedar Valley formation it is assumed that the fissure system was developed in post- Wapsipinicon pre-Cedar Valley time. Sur- face connection for the fissures appears to have been necessary to account for the sand and clay fill. The absence of clay in the Wapsipinicon formation and in the Niaga- ran^rocks indicates that derivation was not from these sediments. In addition, the Wapsipinicon does not appear to be suffi- ciently sandy to provide the amount of sand present in the fissures. The source of clay and sand is at present unknown. It is inter- esting to note however that the Hoing sand at Colmar-Plymouth oil field, and the shale of the Independence formation of Iowa were laid down in the same interval between Cedar Valley and Wapsipinicon time and may have some mutual relation. Silurian System The Silurian rocks, in contrast to the meager development of the Devonian depos- its, are represented in thicknesses which range from 430 to 570 feet. The post- Bainbridge Niagaran^ rocks display consid- erable variations in thickness and lithology. Two well developed facies realms are pres- ent, one of which coincides with the area of reef development, and the other with the normal or interreef area. The electric log characteristics of these two realms are depicted in figure 9. Below these two facies, the Bainbridge, Sexton Creek, and Edge- wood formations extend through both areas, and, except for differences in thickness, are presumably equally developed in both. In the following treatment, the litholo- gies of the two facies realms of the post- Bainbridge Niagaran are dealt with sepa- rately. The uppermost 63 feet of the reef and the uppermost 20 feet of the normal or interreef facies are represented by cores. Description of the underlying Silurian deposits is based upon electric logs and sample studies. i^The term Bainbridge is here used to include only those dominantly red rocks and associated deposits at the base of the Niagaran although it seems probable that some of the lower strata of the reef and possibly of the normal or inter- reef strata may also be of Bainbridge age. The term Niagaran is similarly used to include all the Silurian above the Sexton Creek formation although it is possible that some of the higher beds referred to the Niagaran in this area may ultimately prove to be of younger Silurian age. SILURIAN SYSTEM 19 Fig. 8.— Lithologic character of Silurian reef deposits. Scales are in centimeters. a. Clay-filled fissure in the reef core rock. c. Inclined reef flank. b. Reef-capping detrital limestone. d. Typical reef core. REEF AND ASSOCIATED DEPOSITS OF POST- BAINBRIDGE NIAGARAN AGE Pink detrital limestone. — Throughout the pool area the highest Niagaran deposits consist of a variable thickness of coarse white, pink, or buff detrital limestone (fig. 8b). Near the base of the zone the rocks become slightly dolomitic and the color changes to various shades of gray. The buff, white, and gray limestone is charac- teristically mottled with pink. The rock is made up of calcareous skele- tal elements and fragments, mostly dissoci- ated crinoid columnals and bryozoa, brachi- opods, corals, stromatoporoids, and rarely gastropods, and calyx plates and brachial ossicles of crinoids, which are packed into a loose coquina and cemented with calcite. 20 MARINE POOL, MADISON COUNTY Fig. 8, e-h. Lithologic characters of Silurian interreef deposits. Scales are in centimeters. Occasionally bryozoa and corals are the most common faunal elements. The size and ratio of the fossil constituents varies somewhat horizontally and vertically. The grain size of these strata made up domi- nantly of corals and bryozoa is generally larger than is that of the crinoid columnal coquinas. Large heavy shelled brachio- pods, colonial corals, and stromatoporoids occur at random throughout the coquina. The interstitial spaces of the crinoidal limestone are largely filled with secondary calcite and less commonly with light green clay. These limestones made up of the larger bryozoan and coral fragments some- times contain small fragments of other forms in the interstitial spaces. Vugs of irregular size and shape occur throughout the rock. These are commonly located within brachiopods, in the interskeletal spaces of the stromatoporoids and corals, along discontinuous fissures, and in the inter- stitial spaces of the coquina. Many of the vugs are more or less filled with secondary calcite. Stylolites and stylolite seams are common, and bedding planes are frequently interlocked in styolite fashion, suggesting solution along those surfaces. Bedding appears to be massive but where bedding planes are present in cores they are more or less horizontal in the upper part of the section and inclined in the lower. Dips SILURIAN SYSTEM 21 range from 10 to 20 degrees but occasion- ally up to 45 degrees where they have been altered by solution. Toward the base of the pink detrital lime- stone the strata grade into and interfinger with the underlying reef dolomite and dolo- mitic limestone. The colors of the rocks are darker in this zone, and poorly defined areas of medium to fine-grained darker slightly dolomitic limestone are present. In these patches, partial recrystallization and dolo- mitization has altered the rock so that the individual constituents of the coquina are less distinct. Inasmuch as only two wells in the pool area extend through the detrital limestone, reliable figures on thickness are not avail- able. The two wells which penetrate the unit show thicknesses of 6 and 22 feet, while another well penetrated 39 feet of the limestone without going completely through it. Because there is apparent gra- dation with the underlying beds, it is possi- ble that these figures simply reflect depth to the zone of recrystallization and dolo- mitization. Reef Core and reef flank beds. — (figs. 8c and d). The following description of the Niagaran beds which underly the detrital limestone of the pool area is based upon 40 feet of core and 372 feet of rotary samples from the Eason— Mayer No. 1 well (SE. \, NW. i, SW. i, sec. 15). The core con- sists of dolomite, dolomitic limestone, and some pure detrital limestone. The rocks appear to be massively bedded but occasion- ally show bedding planes that are inclined with dips ranging from 20 to 45 degrees, usually less than 30 degrees. The bedding planes are occasionally grooved and stri- ated parallel to the plane of dip and suggest stylolitic interlocking. A film of green to gray clay and a thin zone of secondary cal- cite sometimes occurs along the bedding plane contacts. Normal stylolites are also present and are capped by a thin film of green clay. Dolomites and dolomitic limestone form most of the core. Clastic limestones are relatively rare and are developed most com- monly in the upper portion of the core where they are interbedded with dolomitic limestone. Small dolomitized areas are present in practically all the limestone. It appears likely that poor recovery in the lower part of the core was associated with the occurrence of pure vesicular dolomite. The color of the dolomitic rocks is domi- nantly gray, the darker shades being associ- ated with the purer dolomites. Mottled zones reflect random areas of dolomitiza- tion. The limestones are light gray, white, or buff and are rarely mottled with pink. They are medium to coarsely crystalline and re- semble in composition the uppermost pink detrital limestone. One flat buf¥ pyritic chert nodule represents the only occurrence of chert found in the core. Localized brec- ciation is suggested by the occurrence of angular areas of light gray fine-grained dolomite within the limestone. The large fossils consist of solitary corals, bryozoa, crinoids, and brachiopods. The dolomitic limestone varies from dense to vesicular, from medium to fine- grained or sucrose, and may enclose coarsely crystalline beds. Fossils in the more dolo- mitic portions are represented only by casts and molds. The large fossils include rep- resentatives of the same groups which occur in the limestone. The dolomites are dense to finely sucrose and vesicular. Discontinuous intersecting fissures occasionally produce a brecciated appearance. Random distribution of fos- sils of various sizes is common. The orien- tation of the fossils is variable, and angles up to 80 degrees with horizontal were noted. The percentage of recognizable fos- sils is smaller than in the limestone and dolomitic limestones. The average size of the organisms also appears to be larger than those in the other two lithologies. The fos- sil casts sometimes show resorbtion and irregular enlargement. Deposition of sec- ondary calcite in the casts and molds and in the fissures, vugs, and pore spaces ranges from a thin film to complete filling. In con- trast, secondary calcite in the limestones is largely confined to fossil cavities and inter- stitial spaces of the coquina. ifwnnnv FEB 2 5 1999 22 MARINE POOL, MADISON COUNTY F-R LIMESTONE DETRITAL LIMESTONE , REEF-FLANK DOLOMITIC LIMESTONE LEGEND CHERTY LIMESTONE SILTY DOLOMITIC LIMESTONE, INTER-REEF DOLOMITE REEF- ROCK [^ SHALE OR SILTSTONE r^-ri CALCAREOUS SILTSTONE ^^-^ INTER REEF Fig. 9. Characteristic electric logs of Niagaran interreef (A) and reef (B) deposits. SILURIAN SYSTEM 23 r"r~n ^ n - '^^■^^^ v^y ^-r-J^ ^pinicon= Maquoketa P^ LIMESTONE ^g LIMESTONE , ARGIL LACEOUS ^ LIMESTONE , CHERTY Ft^ DOLOMITE ^^ SHALE [Hj]] SAND Fig. 10. Diagrammatic cross-section of Marine reef. Rocks similar to those described above extend downward for an additional 372 feet below the base of the core. Pure dolo- mites are less commonly represented and are concentrated in narrow zones with the greatest development near the base where there is a dolomite zone 35 feet thick. Pure limestones and to a lesser extent dolomitic limestones form the bulk of the sequence. A recurrence of the pink mottled coarsely crystalline limestone is present 90 feet be- low the top of the Niagaran. Below the zone of reef development there is a 10-foot zone of transition into the underlying red facies of the Bainbridge. This zone consists at the top of greenish to brownish-gray and gray dolomitic fine- grained argillaceous and silty limestones. Downward the dolomite content is reduced and the silt and carbonate fractions are more sharply segregated. White partly pinkish mottled crinoidal limestone and greenish- gray calcareous siltstone with scattered crinoid columnals suggest the underlying Bainbridge. The fossil assemblages of the reef facies, including both the reef and reef flanks, comprise stromatoporoids, colonial corals of the genera Favosites and Halysites, Pycno- stylus guelphensis, and a number of uniden- tified large solitary corals. Among the extremely abundant crinoidal remains, all of which are dissociated, the genera Crotalo- crinites and Gissocrinus alone have been recognized. Heavy shelled pentameroid brachiopods, bryozoa, gastropods, and pele- cypod fragments are present. INTERREEF AND ASSOCIATED DEPOSITS OF POST-BAINBRIDGE NIAGARAN AGE The deposits of the normal or interreef realm are variable in their lithologic com- position. The diagrammatic cross-section (fig. 10) which represents an interpreta- tion of available well records expresses the variable lithologic character of these depos- 24 MARINE FOOL, MADISON COUNTY In fer- ree f- -Reef- LEGEND CHERTY LIMESTONE DOLOMITE REEF [E3 SILTSTONE Fig. 11. Cross-section showing Silurian reef and interreef facies relationships. (See fig. 12.) its. Recurrence, interfingering, and grada- tion of the several lithologies reflect rapid lateral and vertical change in facies (figs. 8e, f , g, and h and 11). The main constitu- ents of the Niagaran interreef beds above the Bainbridge are limestones, dolomites, and calcareous siltstones. In color the beds are usually gray, green or white, although occasionally they are brown. Clay and silt are almost invariably present in the carbon- ate rocks to a greater or lesser extent. The limestones are white, buff, gray or greenish-gray in color, finely to coarsely crystalline, and may be silty, argillaceous, and cherty. The coarse inclusions in the limestone include aggregates of skeletal ele- ments, mostly crinoid columnals and less commonly bryozoan and coral remains. The fossil density commonly approaches but rarely attains that of a coquina. The organic remains are small and suggest derivation from fragile skeletal elements. Clay occurs in finely disseminated form or is concentrated in thin beds of shale. Where it is disseminated the clay content is com- monly indicated by a greenish color. Finely disseminated pyrite is present in the argilla- ceous zones. Numerous bryozoan remains characterize the shaly portions of the lime- stone. Chert is white and bluish-gray and commonly encloses dissociated spicules of siliceous sponges. Complete sponges are present but rare. The dolomites are usually porous, white, buf¥, and light gray and are finely crystal- line. They are usually silty and may be cherty and argillaceous. The silt, chert, and argillaceous constituents are similar to SILURIAN SYSTEM 25 those in the limestone. Diagenetic removal of the carbonates of the original fossil skele- tons is demonstrated by the presence of casts and molds. The same phenomenon is also noticeable in the very silty limestones. Re- placement by secondary calcite and less com- monly by silica is of frequent occurrence. The interstitial pore spaces and scattered small vugs are either lined or filled with secondary calcite. The siltstones are dark brown and brown- ish-gray in color, pyritic, and commonly highly calcareous. They grade into and apparently are interlaminated with very silty light brown to brownish-gray lime- stones. Minute chitinous fossil fragments are present in the siltstones. The lithologic types usually grade into one another, and sharp contacts are rare. Clay is concentrated in lenses, in nodules of argillaceous limestone, or in partings be- tween limestone beds. In some cases the distribution of the clay suggests that the rock represents an intraformational con- glomerate. The thickness of the Niagaran deposits in the interreef areas is 395 feet in the Bayer— Grimm No. 1 well (SE. i, NE. i, SW. J, sec. 20). Thicknesses of 425 feet are reached two to three miles north of the pool area and are interpreted in terms of regional thickening. In the Ryan — Kisner No. 1 well (NW. i, SW. i, SW. i, sec. 2) which is intermediate with respect to both reef and interreef areas, the Niagaran deposits are 502 feet thick. The fossil remains of the normal or inter- reef facies comprise, in order of decreasing abundance, crinoidal remains, isolated spi- cules and rarely complete skeletons of tetractinellid sponges (Astylospongidae) , fragile solitary and less commonly heliolitid corals, and bryozoa. INTERFINGERING OF REEF AND NORMAL FACIES Although most of the wells and cores studied conform to the general patterns stated above for reef and interreef litholo- gies, there are several instances in which there are mixtures of reef and interreef lithologies within a given well (fig. 11). Thus in the marginal Eason — Mayer No. 1 well (SE. i, NW. i, SW. i, sec. 15), the beds which immediately overly the abnormally thin Bainbridge sediments con- sist of a fine silty argillaceous dolomitic limestone which is typical of the interreef or normal sediments in the upper part of the Silurian. In the Obering — Bird No. 1 well (SE. i, NW. i, SE. i, sec. 8), the pink detrital limestone (which normally directly overlies the reef rock) is underlain by a very finely crystalline gray dolomite of the interreef type. The entire post-Bainbridge Silurian sequence in the Ryan Oil Co. — Kisner No. 1 well (NW. i, SW. i, SW. i, sec. 2), is poorly understood. The highest deposits consist of white, medium to fine dolomites which are mottled with pink and red. The larger part of the Niagaran section between the upper pink detrital limestone and the Bainbridge below consists of white, medium to fine, crystalline limestone with a few lenses of dolomite and occasional shale part- ings. Inspection of the electric log indi- cates that the strata present are directly continuous laterally with normal interreef deposits, but differ from them by being characterized by a very considerably higher impedence. AGE OF UPPERMOST DEPOSITS A systematic study of the fossils in these deposits has not been undertaken. The few forms listed below were secured from lime- stone beds immediately below the pink detri- tal limestone in the core of the Eason — Mayer No. 1 well. The coral was identi- fied by Dr. R. S. Bassler of the National Museum, Washington, D. C. Crotalocrinites sp. Gissocrinus sp. Pycnostylus guelphensis The generic identification of Crotalo- crinites is based upon a characteristic por- tion of the root. In the Niagaran deposits of northeastern Illinois this form ranges from the upper beds of the Waukesha into 26 MARINE POOL, MADISON COUNTY beds of Guelph age.^ In northwestern Illi- nois it occurs in beds of Port Byron age. The genus Gissocrinus until recently has been thought to be restricted to the Laurel, Louisville, and Brownsport beds of southern Indiana, Kentucky, and Tennessee. Its presence in beds of Waukesha to Guelph age in northeastern Illinois and in the Liston Creek formation of northern Indiana has been demonstrated.^ The specimen of Gis- socrinus is closely related to Gissocrinus quadratus Springer. Pycnostylus guelph- ensis is confined to the youngest Niagaran deposits of Guelph age of northern and eastern North America. This faunal evi- dence, although meager, suggests a Guelph age for the uppermost reef rocks in the Marine area. It is hoped that a detailed study of the numerous forms yet unidenti- fied in deposits of both reef and interreef facies w^ill establish a firmer correlation. INTERPRETATION OF THE POST-BAINBRIDGE NIAGARAN FACIES An analysis of the data presented above furnishes certain criteria which lead to the following interpretation of the characters and relationships of the reef and normal or interreef facies. Contrasting features of the two facies are expressed by differences in lithology, thickness, bedding characteris- tics and fossils. Lithology. — The lithologies of the pool area, above the Bainbridge, can be readily distinguished from those of the surrounding area by the absence of silt, clay, and chert. Pure limestones and dolomites form the characteristic lithologies of these pool depos- its. Coarsely crystalline limestone in which the individual coquina constituents are also relatively coarse are prominent within the pool area. Thickness. — In the area under discussion, the Niagaran deposits display a regional thickening of approximately ten feet per mile toward the southeast. Locally how- ever the two facies complexes differ in thick- 3 Lowenstam, H. A., Biostratigraphy studies of the Niagaran interreef deposits of northeast- ern Illinois; State Museum Sci. Papers No. 3, in press. 4 Idem, ness by as much as 130 feet. The increase in thickness is confined to the pool area whereas those of the surrounding area are more or less consistent with the regional trend. Bedding. — A striking attribute of the deposits in the pool area is the presence of beds with dips ranging from 10 to 45 degrees. Fossils. — The fossils in the pool facies are characterized by large size, and robust to heavy skeletal development. In contrast, the fossils of the surrounding facies complex are uniformly fragile and predominantly smaller. The pool area forms an insular occur- rence in the surrounding facies complex (fig. 12) which possesses features diagnos- tic of deposition in a still-water environ- ment. The sediments represent a combi- nation of chemically precipitated carbon- ates, fine-grained elastics, and skeletal aggre- gates of autochthonous organisms. The chert represents localized concentrations of silica which may have been in part derived from the siliceous skeletal elements of autochthonous sponge colonies. The size and fragile character of the fossil constitu- ents in general, and the relative abundance of tetractinellid sponge fragments in cer- tain beds in particular, corroborate deposi- tion under still-water conditions.^ In contrast, the facies complex above the Bainbridge in the pool area reflects an are- ally restricted occurrence of rough-water environment. The large mound-like form and its variously inclined beds are character- istic features of reef structures. Thus it is recognized that the pool lithologies repre- sent an isolated reef in the surrounding normal or interreef facies. The reef litholo- gies are entirely made up of the remains of the reef population in the form of actual skeletal elements and water-abraded and selected derivatives. The physical aspect and composition of the fossil assemblages are in agreement with those from other Niagaran reefs. The fauna of the detrital fans of the inclined reef flank include abun- dant stubby and semiglobular colonies of 5 Idem. SILURIAN SYSTEM 27 Favosites, heavy shelled pentameroid brachi- opods, large fragmentary colonies of Haly- sites, and remains of large robust crinoids. It is noteworthy that representatives of the genus Crotalocrinites in the Niagaran de- posits of northeastern Illinois are confined to the reefs. In contrast, the previously known representatives of Gissocrinus were interreef dwellers. The occurrence of Gis- socrinus in the reef flank beds at Marine is similar to its occurrence in the contempora- neous reef at Thornton in northeastern Illi- nois in which Gissocrinus, together with other faunal elements of the interreef realm, encroaches on sheltered parts of the reef flank where environmental conditions approach those of the interreef. In the light of the interpretation offered it is concluded that in the Eason — Mayer No. 1 well (SE. i, NW. i, SW. i, sec. 15), the greatest part of the Niagaran sec- tion above the Bainbridge consists of reef flank deposits. This view is supported by the nature of the dissociated and broken skeletal elements which form the major con- stituents of the limestones and dolomitic limestones and by the presence of inclined bedding planes and reef breccias. Only massive dolomites and dolomitic limestones can be regarded as marginal parts of the reef core proper. The coarse commonly pink crinoidal coquinas which form a blan- keting layer across the top of the reef reflect the terminating phase of reef development and represent reworking and planation at the beginning of^, and subsequent modifica- tion to, rough-water conditions. The Eason — Mayer No. 1 well is marg- inally located with reference to the main body of the reef, as is indicated by the prev- alence of reef flank deposits. The main development of the reef core is located more centrally with respect to the pool structure. Since the detrital mantle of the reef is com- paratively thin, although variable in thick- ness, the structure on top of the Silurian seems to reflect roughly the major fea- tures of the reef. The highest part of the reef is located in the northeastern part of the pool where it is bordered by steeply dip- ping narrow flanks. The main axis of the reef body trends to the southwest where it is broad and platform-like. If the suggested interpretation of the reef structure is cor- rect, it is evident that the localized structure centered at Eason — Mayer No. 1 well must be regarded as a vertical series of recurrent forereefs which developed and were in turn buried by debris from the main body of the reef. A similar interpretation is suggested for the isolated small structure located north- west of the main reef body in the SE. ^ sec. 8. The overlap of the pink detrital limestone on the interreef in this area is not suflScient to account for the local Silurian high, but the presence of a small forereef may account for the local structure. In Ryan— Kisner No. 1 well (NW. ], SW. ^, SW. J, sec. 2), the dolomite which has been reported to underlie the pink detri- tal dolomite may represent a small forereef. The underlying Niagaran deposits, above the Bainbridge, are transitional in character between the reef and interreef lithologies. In the light of the interpretation offered, they appear to represent primarily reef- derived material of a selected grain size which was deposited in the adjacent inter- reef tract. Future drilling between the three fore- reefs may determine whether they were iso- lated, as thought at present, or were inter- connected in the form of a horseshoe-shaped barrier reef which in part surrounded the main reef. BAINBRIDGE FORMATION The typically developed Bainbridge com- prises about 70 feet of red and green silt- stones and shales with occasional thin beds of argillaceous limestone and about 60 feet of red to white massive coarsely crystalline limestone with occasional interbedded red or green shales and siltstones. Those limestones in the upper portion are commonly light gray or green and finely to very finely crystalline. Of less common occurrence are coarsely crystalline white limestones which contain isolated fossil fragments that are in part replaced by red ferruginous material. The limestone in the 28 MARINE POOL, MADISON COUNTY lower part of the formation consists of densely packed aggregates of pink and red, rarely white, medium- to coarse-grained crinoid fragments. Red ferruginous fossil fragments are especially common in these strata. The interstitial spaces of the fossil aggregates are filled with material which ranges in composition from clay to pure calcite. The Bainbridge averages about 130 feet thick in the normal or interreef area but beneath the reef flank is only 55 feet thick, a figure which includes 10 feet of transition beds at the top. The Bainbridge strata which underlie the reef and, to a variable extent the neighbor- ing interreef area, are comparatively thin, ranging from 40 to 60 feet in thickness. Farther from the reef this facies thickens rapidly and reaches a maximum of about 170 feet. Beds contemporaneous with the upper portion of the red facies of the Barn- bridge within the reef apparently consist of more or less typical reef deposits, and in the area immediately adjacent to the reef it con- sists of interreef deposits similar to those in the upper part of the Niagaran. In Eason Mayer No. 1 well, beds contemporaneous with the upper part of the red Bainbridge consist of greenish-gray interreef deposits overlain by typical reef rock. It is possi- ble that the thinning of the Bainbridge below the reef may be the result of squeeze and compaction caused by the load of the reef body. However, if so, it should be expected that typical Bainbridge would be developed uniformly in the whole interreef area up to the very edge of the reef. Fur- ther, if material had been squeezed out from beneath the reef the zone bordering the reef would have a greater thickness of Bain- bridge than its normal interreef thickness. Maximum compaction of 20 feet may be involved but does not explain the thinning of the entire Niagaran in the interreef area. Lateral gradation from the typical facies of the upper red Bainbridge into contempora- neous beds of reef lithology is therefore sug- gested. ALEXANDRIAN SERIES The Alexandrian deposits are uniformly developed in the pool area and its environs. Two formations, the Sexton Creek and the Edgewood, are recognized on the basis of their lithologic similarity to and conti- nuity with the surface exposures and on their stratigraphic relations. Sexton Creek Formation. — The Sexton Creek formation comprises 20 to 40 feet of white to buff finely crystalline to litho- graphic limestones or dolomites. More or less spherical grains of glauconite occur commonly in the upper portions of the for- mation and help to distinguish it from the immediately overlying Bainbridge beds. With increase in depth, the glauconite be- comes less common, and white, gray, or blue opaque chert makes its appearance. Edgewood Formation. — The Edgewood formation in the Marine pool area consists of 10 feet of light brown to buff finely sucrose dolomite which in electric logs is characterized by a high self-potential. The formation appears to grade into the over- lying Sexton Creek formation and in nearby areas can not be satisfactorily separated from it on the basis of gross lithology alone. Within the pool area it is distinguished by the absence of chert. SILURIAN-DEVONIAN HISTORY The broad features of Silurian and Devo- nian history may be summarized as follows. Little is known at present about Alexan- drian time beyond the fact that the sea which covered the area was first muddy, owing to reworking of Maquoketa material, and was later comparatively clear. Uni- form sedimentation conditions apparently prevailed. In early Niagaran time a depositional tract which extended across Illinois to the edge of the Michigan basin was influenced by the Ozark upland. Ozark-derived sedi- ments contributed to the Bainbridge forma- tion in southeastern Missouri and in south- western Illinois as well as to the so-called Osgood^ facies of the Joliet dolomite of *5 Dunn, Paul, Silurian foraminifera of the Mississippian basin. Jour. Paleo., May 1942, p. 317. OIL POTENTIALITIES 29 northeastern Illinois. The Bainbridge sedi- ments reflect a sea which was comparatively muddy from the fine elastics swept down from the Ozark upland. While similar sedimentation conditions still prevailed over most of the area, localized deviations pro- moted reef formation in the Marine area. Shortly thereafter, the Ozark-controUed sedimentation gave way gradually to the generalized offshore conditions which ex- isted in most of Illinois in the normal or interreef tract. In contrast to the rough- water conditions in the reef area, still-water conditions prevailed in the interreef tract. The sea there was still comparatively muddy. Fluctuations in the amount of detrital material laid down are indicated by the rapidly shifting subfacies of the inter- reef realm. Within the immediate vicinity of the reef, the fine material derived from it was incorporated as a detrital fraction of the carbonate portion of the interreef sedi- ments. In late Niagaran time, the reef building phase appears to have come to an abrupt end. The blanketing mantle of coquina indicates rough water conditions which accompanied cessation of reef build- ing. The uniform thickness and character of the Wapsipinicon deposits indicate a nearly featureless Silurian surface across both reef and interreef areas. The fact that the reef was not truncated suggests that while rough-water sediments were slowly built up on the top of the dead reef, sedi- ments must have accumulated rapidly in the interreef area. Probably the interreef sedi- ments received a considerable part of the rough-water sediments which were within the carrying capacity of the turbulent water along the edge of the two facies realms. As a result, the interreef tract was built approx- imately to the level of the reef before the recession of the Silurian sea. The Wapsipinicon sediments were de- posited in a shallow sea. Following Wap- sipinicon time the area was uplifted and the land surface was subjected to solution and development of fissures, after which the fis- sures were filled with clay and sand from an unknown source. This solution and sub- sequent filling of crevices extended down into the Silurian rocks. The deposits laid down in Cedar Valley time reflect a very shallow sea with a muddy bottom which was above wave-base much of the time. The final deposits of the Devo- nian, the undifferentiated siltsones, indi- cate that the muddiness of the sea increased considerably whereas little chemical precipi- tation of carbonates took place. OIL POTENTIALITIES AND MIGRATION The producing area of the Marine pool coincides with the distribution of the reef facies. No producing wells nor oil shows have been reported from the surrounding interreef areas. The "pay zones" of the pool are nearly all located in the upper zone of pink derital limestone. In a limited num- ber of producers the depth of production suggests that the pay is in the Wapsipinicon. Little is known about the petroleum possi- bilities of the reef core itself inasmuch as it is reached by none of the producers. Two feet of saturation in a thin zone of forereef facies were reported in the marginal Eason — Mayer No. 1 well. Testing to the base of the creviced zone within the main body of the reef is recommended, particularly after the decline of the present shallower production. This would require deepening to a depth of 50 to 100 feet below the top of the Silurian beds. As suggested previously, it appears possi- ble that the "Trenton" strata in the Marine pool area are structurally low. If this is true, a trap may be formed in the region of reversal of dip between the regional dip to the southeast and the margin of the pool where dips would be toward the center of the pool. Core studies have produced few data which would lead to a proper interpreta- tion of the accumulation of the oil. The fissures in the Silurian are suggestive as a means of oil migration. However, the fact that all of the fissures which have been recovered are tightly closed and packed with clay and sand would apparently restrict 30 MARINE POOL, MADISON COUNTY migration to only those which were incom- pletely filled, or were filled only with sand. This may account for scattered Wapsipini- con production. Some evidence was found in the core of the Rockhill — Pence-Mayer No. 1 well (NE. i, NE. i, sec. 16) for the develop- ment of zones of secondary porosity and permeability in the wall rock adjacent to the fissures. The pink detrital limestone, which is normally tight, is in this well soft and incoherent. The interstitial spaces of the coquina are in part open, or may be filled with white porous chalky carbonate material. The walls of the spaces are often thinly lined with minute crystals of calcite. In addition, some of the fossil elements show signs of solution. This porous zone was observed along the margins of a fissure and it seems probable that oil accumulation and migration may have been controlled by such porosity. These zones were probably developed at the time that the fissures were being subjected to solution. Zones of in- completely dissolved porous limestone were formed adjacent to the fissures, which after sand and clay filling had blocked the main crevices, were again subjected to ground water circulation which further increased their porosity and permeability. It seems probable, that during drilling the soft por- ous rocks of the "pay zone" usually disinte- grate into an incoherent lime-sand which is recovered only in part or not at all. The erratic distribution of initial productions appears to be dependent more on the size and number of porous zones penetrated than upon structure. The Wapsipinicon and Cedar Valley for- mations and the Devonian-Kinderhook shales have apparently acted as a caprock to prevent the vertical escape of oil from the pink detrital limestone. The discovery of a Niagaran reef at Marine, far removed from the reef belt which fringes the Michigan basin and from the reef cluster in northwestern Illinois and Iowa, appears to be significant. The size of the Marine reef, at least as large as that of any other known Niagaran reef, suggests that the Niagaran deposits adjacent to the Ozark upland are potentially sites of other reefs, and as such have additional oil poten- tialities. It is not known at present whether the Marine reef is isolated or is a part of a reef belt. It seems at least possible that it may be connected by a series of reefs to the known area of reef development in the north. At the present, it is not possible to dis- tinguish by surficial methods those struc- tures which represent Silurian reefs from those which are the result of deformation alone. The pink crinoidal limestone which is con- fined to the reef area in the Marine pool and can be used as a guide to the distribu- tion of the reef itself may not be similarly useful elsewhere. Depending upon the local history, a development of pink detrital lime- stone may be present over the top of the reef if the reef stopped growing while at or near wave base, or there may be an over- lap of interreef facies on the reef wherever sediments accumulated faster than the growth of the reef. In the latter case, pene- tration of interreef beds below the Devo- nian does not necessarily imply the absence of a reef. It is suggested that detailed analysis, probably by insoluble residue meth- ods, may produce criteria by which proxim- ity to reefs can be recognized. Illinois State Geological Survey Report of Investigations No. 114 1946