A PETROGRAPHiC CORE ANALYSES OF “SHE LOWER AND MiDDLE OREGVWEAN ROCKS. PULASK! FIELD... JACKSON COUNTY, MKCWGAN Thesis fair flu Dogs“ of M. S. MICHIGAN STATE UNW’ERSWY WEEEEam H. Kirschke i962 MICHIGAN STATE UNNET‘SITY DEE‘T’ARTI‘I’LNT Cr-F ClOLCGY EAST LANSING, MICHIGAN dirt—$414098— I‘LL-3‘34 \\. If) A R; 4:21 3730?, ABSTRACT A PETROGRAPHIC CORE ANALYSIS OF THE LOWER AND MIDDLE 0RDOVICIAN ROCKS, PULASKI FIELD, JACKSON COUNTY, MICHIGAN .17 S‘J A detailed petrographic analysis was made onia core taken from the Lower and Middle Ordovician rocks in the Pulaski field, Jackson County, Michigan. The core was studied by thin sections and observations on the stained and etched surface of the entire core. A comparison between the Gamma Ray-Neutron log and the core was made to determine if any petrofabric types give destinctive deflections on the log. No relationship existed between sedimentary fabric and deflections on the log. The Black River is a microcrystalline limestone containing dolomite of four different occurrences. Dolomitization is penicontemporaneous and only that dolomite occurring as an interlocking mosaic of crystals could be traced stratigraphically. The Oneota dolomite has an epigenetic origin. A zone of dolomite in the Black River limestone could be traced throughout Calhoun, Jackson, and Hillsdale Counties. It might be inferred that this is one of the few stratigraphically significant zones in the Black River. A PETROGRAPHIC CORE ANALYSIS OF THE LOWER AND MIDDLE ORDOVICIAN ROCKS, PULASKI FIELD, JACKSON COUNTY, MICHIGAN BY William H. Kirschke A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1962 ACKNOWLEDGEMENTS The author wishes to thank Dr. C. E. Prouty, under whose direction this problem was undertaken. The author also wishes to thank Dr. J. H. Fisher and Dr. J. E. Smith, the other members of his guidance committee, for their suggestions and criticisms. All fossil identifications were made by Dr. C. E. Prouty. Recognition is also given to Martin Horowitz for his constructive criticism of this work. ii; a! lp-osl‘ ‘ TABLE OF CONTENTS I. INTRODUCTION . . . . . . . . . . . Purpose . o o o . . o . . . . 0 Previous Work . . . . . . . . . Location . . . . . . . . . . . . II. STRATIGRAPHY . . . . . . . . . . . Introduction . . . . . . . . . . Prairie du Chien . . . . . . . . St. Peter . . . . . . . . . . . Glenwood . . . . . . . . . . . . Black River . . . . . . . . . . Trenton . . . . . . . . . . . . III. LABORATORY TECHNIQUES . . . . . . . Sampling . . . . . . . . . . . . Staining . . . . . . . . . . . . Preparation of Thin Sections . . IV. MEGASCOPIC DESCRIPTION . . . . . . Introduction . . . . . . . . . . Descriptive log . . . . . . . . V. MICROSCOPIC DESCRIPTION . . . . . Classification and Terms Used . Use of Formula . . . . . . . . . Descriptive log . . . . . . . . VI. OBSERVATIONS AND CONCLUSIONS . . . Petrography Comparison with Gamma Ray-Neutron log . . . . . . . . . .. iii. PAGE 10 12 12 12 16 16 17 17 32 32 33 PA GE Key Hor izons O O O O O O O O O O O O O O O O O O O O I O 34 VII. REFERENCES CITED 0 O O O O O O O O O O O O O O O O O O O O O 36 iv. Figures 1. Location Map . . . . . 2. Outcrop Map . . . . o 3. Stratigraphic Column . 4. Gamma Ray-Neutron Log Table 1. Folk's Classification ILLUSTRATIONS Page 35 18 - n . o u a o . o q . o . . . . . . . . a a a - u o o a . . . 0 _ o O INTRODUCTION Purpose of Study A comprehensive study is currently being prepared of a core given to the Geology Department by the Sun Oil Company. The interval cored is Ordovician, beginning in the Black River limestone, passing through the Glenwood shale, and ending in the Oneota dolomite which is Lower Prarie du Chien. Paleontological and calciumdmagnesium ratio in- vestigations are being prepared by other students. The purpose of this study is to make a detailed petrographic analysis of this core, examin- ing the lithology, fabric, and general composition with the aid of thin sections and examination of stained surfaces. This includes any informa- tion concerning the occurrence of dolomite. It is hoped that this data will then be added to existing information gained solely from well cutting studies and megascopic examinations of cores in this interval. This analysis will then be compared to the Gamma Ray-Neutron log of the well to determine if any petrofabric types give distinctive de- flections on the log. In addition, the writer will try to determine whether those types, if present, can be used as marker horizons for future well log correlations. Previous Work In recent years much interest in the Middle Ordovician rocks of Michigan has developed with the discovery of new oil reserves in these strata. Regional stratigraphic studies have been made by a number of workers. Much of their work is cited in the discussion of the general stratigraphy of Michigan. After a thorough search of the literature it 1. 2. became evident to the writer that present descriptions of this interval are based entirely on well cutting analyses and megascopic core de- scriptions. Published petrographic studies of a detailed nature have not been prepared and detailed descriptions of these rocks do not exist. The limited area of this study provides an opportunity for making such a detailed description, possibly as a guide to future work. Location This well, the Peterson-Howard Well #1, was drilled to a total depth of 4395 feet and cored from 4100 feet to 4395 feet. Its location is the NW% NE% SW% of Sec. 17, T45, R3W, Pulaski Twp., Jackson County, Michigan. See Figure I. Figure l - Location of well GENERAL STRATIGRAPHY OF THE LOWER AND MIDDLE 0RDOVICIAN ROCKS IN MICHIGAN The lower and middle Ordovician rocks can be seen in outcrop in the Northern Peninsula of Michigan, Wisconsin, Illinois and in Ontario eastward to the Frontenac axis (Fig. 2). Regional thinning and thick- ening and erosional unconformities control the thickness of the form- ations in the Michigan basin (Cohee, 1948). A discussion of the general stratigraphy of this interval in Michigan is presented in this paper. No attempt has been made to correlate these strata on a regional basis. Lower Ordovician The Prairie du Chien has a threefold division. The members, in ascending order, are the Oneota dolomite, the New Richmond sandstone, and the ShakOpee dolomite. Oneota dolomite Overlying the Trempealeau formation (Upper St. Croixan) in the Southern Peninsula (Fig. 3) is the Oneota which is a buff to light brown dolomite. Sand and chert are abundant with some oolitic chert. Associ- ated with the dolomite are minor amounts of light green to green-gray shale. In southwestern Michigan the Oneota dolomite is principally sandstone with subordinate amounts of chert and dolomite (Cohee, 1948). New Richmond sandstone This unit has been recognized in drill cuttings from some wells in southwestern Michigan and is from 5 to 10 feet thick. 0.10 _ (2452, _ nirlllil‘ulnn. . .u.ol . 246.30.: sonata 3:223qu 22 asterrug ._ couches 24.92.30 .3 gaitg OQUOWJ Amama .oonoo nouwav axoou ceaua>ovuo mo monouso u N shaman 5. Figure 3 - Generalized section of Cambrian and Ordovician systems of Michigan modified from Cohee, 1945a, and Ehlers, Landes, and Cohee, 1947 Queenstown Big Hill RICHMOND Stonington Bills Creek CINCINNATIAN MAYSVILLE Lorraine EDEN Utica Collingwood z Trenton SI TRENTON Black River E MOHAWKIAN BLACK RIVER Glenwood é CHAZYAN St. Peter Northern Peninsula Southern Peninsula PRAIRIE Shakopee CANADIAN DU CHIEN Ne“ R1°hm°nd Oneota HERMANSVILLE Jordan TREMPEALAU Lodi r”””””,,,»«r”” St. Lawrence G LAKE Franconia w SUPERIOR E ”2 Dresbach M 5 MUNISING g . Eau Claire u IUJ Mt. Simon JACOBSVILLE Jacobsville 6. Shakopee dolomite The ShakOpee dolomite consists of a buff, brown and gray dolo- mite which may be sandy, cherty, and shaly in part. Northward and eastward in the Southern Peninsula this unit is largely sandstone, shaly dolomite, and dolomitic shales. St. Peter sandstone The St. Peter in Michigan is a white to brown, predominately pure, friable sandstone. Chert fragments and pyrite are locally common. The quartz in the St. Peter is fine to medium grained, well-rounded to sub-rounded, frosted and pitted (Horowitz, 1961). This unit conform- ably overlies Lower Ordovician beds in southwestern Michigan. The un- conformable relationship is due to pre-St. Peter erosion of the Prairie du Chien. Middle Ordovician Glenwood shale In southwestern Michigan the Glenwood consists principally of sandstone and sandy dolomite. In many areas it is green-gray, sandy, pyritic shale and dolomite. In southeastern Michigan a sandy and some- times pyritic green, brown, or gray shale, 5 feet or more in thicknes,. has been found below the Black River limestone where the St. Peter sandstone is absent. In places quartz grains are abundant, particularly at the contact with the underlying dolomites. Black River limestone The Black River is a brown and gray fossiliferous limestone con- taining interbedded dolomite in some places. There are areas where it a; is dolomite and limestone or entirely dolomite. The basal part of this unit may be a fine grained dark gray to black argillaceous limestone or a limestone and shale. The Black River ranges from 150 to 517 feet in thickness. Trenton limestone The Trenton overlies the Black River limestone and is similar to it in lithology. The boundary between these two units is a very argil- laceous limestone and shale section at the base of the Trenton. The total thickness of the Trenton ranges from 203 to 479 feet. Where the Trenton-Black River becomes largely dolomite the contact is not easily recognized. The regional thickening of the Trenton-Black River rocks in the Chatham sag in Kent County, Ontario suggests that this was a struc- turally low area between the Michigan Basin and the Black River seas to the east. LABORATORY TECHNIQUES Method of Sampling The core was initially sampled at regular intervals of 5 feet through each megascopically similar "unit". Any changes in color, dolomite content, abundance of partings, or fabric were regarded as a "unit". Only samples of the dominant lithology within these "units" were taken. After the core had been stained it was examined with a binocular microsc0pe. Additional samples were cut at microscopic changes in lithology, fabric, and at any unique lithology within a previously defined "unit". Staining Techniques The entire limestone section was stained to determine its gross lithological character. Staining of the least abundant constituent of the limestone was desired, which in most instances was dolomite. Dolomite can be stained blue by immersing the samples in a five percent solution of hydrochloric acid, to which a few draps of a freshly prepared concentrated solution of potassium ferricyanide have been added (Steidtmann, 1917). The sedimentary calcite remains unaffected. The blue reaction of the dolomite is due to its FeO content, which Steidt- mann found to be present in all the samples he studied. The same was true for all samples in this study. The samples were not etched by conventional methods as described by Lamar (1950). The sections of core, which had been previously sliced, were placed in the staining solution with the surface to be etched facing down. The sections were supported so that the surfaces 9. 10. which were submerged in acid did not rest on the bottom of the contain- er. No appreciable channeling due to etching developed from this method. All the samples were immersed in the staining solution from one to two minutes (increasing the time as the solution became weaker). This produced ample relief in the unstained constituents making their identification and distribution as readily apparent as the dolomite. The stained surfaces were studied with particular emphasis on the following: 1. Texture, grain size, distribution and amount of dolomite. 2. Type, distribution, and amount of insoluble material. 3. Fossil content. 4. Relation of insolubles to bedding. Preparation of Thin Sections All thin sections were made perpendicular to the bedding. To keep the orientation of the thin sections with respect to the top of the core an arrow was penciled on the flat surface of the core produced by the first cut. A second cut produced an oriented slab approximately 1/16 of an inch thick which was perpendicular to the bedding. The position at which the thin section was to be made was marked and the slab stored in a sample envelope until all sections were cut. Blanks were made from the slabs by the following method: 1. Lines were inscribed around the desired area with a knife. 2. The inscribed lines were gently tapped with a hammer and chisel. 3. The blanks were then cut out and trimmed with tile cutters. 11. The blank was ground by hand (#600 carborundum powder on a glass plate) on one side to provide a smooth and flat surface for mounting on a glass slide. The equipment used for mounting consists of a bunsen burner, ring stand, metal plate, mounting medium (Aroclor), eraser pencil, and clean frosted glass slides. The glass slides were frosted with #180 grit on a lap wheel. The mounting process is as follows: The blank and glass slide were laid with the clean side up on the heated metal plate. The temp- erature of the plate was adjusted so that the Aroclor, when applied to the glass slide, melted in approximately one minute. Most of the bubbles were "cooked" out in three to five minutes. When the Aroclor was prOperly "cooked", the slide and blank were removed. The blank was then placed on the slide, care being taken to first place one side down and let the rest of the blank gradually settle into place. This method removed most of the remaining bubbles. The next step was to work out the excess Aroclor and bubbles by pressing down on the blank with the eraser pencil. The slide was than marked to show the depth and tap of the core orientation. The final step in the preparation was grinding the blanks to an approximate thickness of .03mm. Initial grinding was on a lap wheel with #180 carborundum powder. Grinding was continued with this grit until the edges of the slide began to wear away. The grinding was then continued with #400 powder until more of the edges were worn away. This step minimized the amount of hand grinding required. The final grinding was done by hand on a glass plate with #800 powder until the desired thickness was obtained. The completed sections were then studied in detail as to fabric, composition, and occurrence of dolomite. MEGASCOPIC DESCRIPTION OF CORE Included under this heading are the observations made on the stained surfaces with a binocular microscope. Arenaceous, as used in this description is a term indicating size and not composition. Any unit requiring a compositional term for detrital quartz grain content will be prefixed by sandy. The numbers in parentheses refer to the gamma-ray-neutron curve (Fig. 4), Depth from BLACK RIVER LIMESTONE Surface Unit 1 - Limestone, light brown and gray-brown aphanitic 4100.0 with zones and patches of arenaceous limestone; clusters of sparry calcite with some dolomite, scattered dolomite rhombs, dolomitic partings with argillaceous and silty material; some pyrite, styolitic parting, fossil fragments - bryozoans, brachiOpods, corals, ostracods..................... 4107.8 Unit 2 - Limestone, light brown (taffy colored), aphanitic with many clusters of sparry calcite. Calcite filled vertical fractures, profusely scattered , dolomite rhombs, few partings, styolites, some pyrite. Dark gray-brown 4108.9 - 09.9 feet, (1) arenaceous in patches with abundant dolomitic partings. Occasional fossil fragment becoming very fossiliferous at 4122, corals, brachiopods.... 4122.0 Unit 3 - Limestone, gray and brown-gray, aphanitic with many zones and patches of arenaceous limestone; clusters of sparry calcite with some dolomite - not present 4122 - 22.6 feet; dolomite rhombs Sparse, dolomitic partings with argillaceous and silty material; some fossil fragments - brachi0pods, ostracods, with corals being most abundant......... 4127.5 Unit 4 - Limestone, light brown at change grading into brown gray, aphanitic with some arenaceous limestone; clusters of sparry calcite with little dolomite (more abundant at change continuing to 4129); sparsely scattered dolomite rhombs, dolomitic partings with argillaceous and silty material, chert nodule at 4134.3, styolites, pyrite; some fossil fragments, corals in abundance 4129.2 - 32.9 feat " biOlithite at 4131.4, braChLOPOdaeoeeeeeeaee 414209 12. L; 7.. Unit 5 - Unit 6 - Unit 7 - Unit 8 - Unit 9 - Unit 10- Unit 11- 13. Limestone, brown-gray, aphanitic with clusters of aparry calcite, very little dolomite - confined mostly to scattered rhombs; some pyrite, fossil fragments - mostly corals, some brachiopods........ Limestone, brown-gray, aphanitic with clusters of aparry calcite - some dolomite; dolomite confined to what few partings are present, calcite filled vertical fractures; no fossils observed............ Limestone, gray-brown, aphanitic with many zones and patches of arenaceous limestone; clusters of Sperry calcite with dolomite; scattered dolomite rhombs - most abundant in arenaceous zones, dolo- mitic partings with caly and silt - thick regular bands 4170.8 - 73.2 feet (2); some styolites, dissem- inated pyrite, vertical fractures; cross laminae 4171.2 - 71.7 - dolomite rhombs and detrital quartz grains; scattered fossil fragments - brachiopods (zone 4164.4), abundant bryozoans, ostracods....... Limestone, gray to light gray-brown, aphanitic with few clusters of aparry calcite; increase in dolomite rhombs over last unit; styolites, some pyrite; occasional fossil fragment, brachiopods.... Dolomite, light gray to gray-brown; finely crystal- line, some Pyrite (3)eeeeeeeeeeeeeeeeeeeeeeeeeeeeee Limestone, gray to light gray-brown, aphanitic with clusters of sparry calcite, profusely scat- tered dolomite rhombs, thick bands of dolomite; alternating dolomite and carbonaceous partings - with pyrite 4180.4 - 80.7 feet. Limestone becomes darker with sparsely scattered dolomite rhombs and partings 4180.0 - 80.3 feet and 4182.0 - 82.6 feet. Some pyrite; fossil fragments - brachiopods, OSCEECOOB, One Tetradium at 4186.3................. Limestone, gray and brown-gray, aphanitic with some arenaceous limestone; clusters of aparry cal- cite and dolomite, profusely scattered dolomite rhombs, dolomitic partings with some clay and silt - dolomite content increases at 4193.0 - 97.0 4143.6 4145.6 4175.4 4176.0 4177.8 4186.5 feet (4) and 4201.0 - 04.0 feet (5). Some pyrite, cross laminae at 4187.1, scattered fossils and fossil framents "' braChioPOds, 08tr8c0d8.ooggooo000...... 4205.5 14. Unit 12 - Limestone, dark gray and brown-gray, aphanitic Unit 13 Unit 14 - Unit 15 - Unit 16 - Unit 17 - Unit 18 - with arenaceous and rudaceous zones and patches; clusters of aparry calcite and minor amounts of dolomite; sparsely scattered dolomite rhombs, dolomitic partings with abundant clay and silt; styolites, calcite filled vertical fractures near base; fossil fragments - brachiopods, ostracods, crinoid, bryozoans, and a few Tetradium........... Limestone, medium brown, aphanitic; clusters of sparry calcite - some dolomite; dolomite rhombs aparse, dolomite filled vertical fractures, some pyrite. Change to unit below is gradational...... Limestone, dark brown-gray, aphanitic with many zones and patches of arenaceous limestone; few clusters of sparry calcite; scattered dolomite rhombs, dolomitic partings with clay and silt; chert nodule at 4230.4, cross laminae at 4228.9 and 4230.6 feet. Sparse fossil fragments......... Limestone, dark brown-gray, aphanitic; profusely scattered dolomite rhombs having a vague align- ment to bedding, thick irregular bands of dolomite with very little argillaceous and silty material; only an occasional brachiopod observed............ 4277.0 4228.4 4230.9 (6) 4239.0 Limestone, dark brown-gray, aphanitic with arenaceous and rudaceous zones and patches; clusters of sparry calcite and dolomite; scattered dolomite rhombs - more abundant at base, partings with abundant clayey material, zones containing rounded detrital quartz grains; styolites, thin stringers of cal- cite, fossils - brachiopods, ostracods, bry- ozoanaaeeeeeeaeeeeeeeeeeeeeeeeeeeeeeeeeieeeeeeeeee As the previous unit but with an increase in argillaceous and carbonaceous material in the Partings (7)eeeeeeeeeeeeaeeeeeeeaeaeeeeeeeeeeeeeoo Alternating limestone and dolomite (9). Limestone, brown, aphanitic; clusters of dolo- mite crystals; scattered dolomite rhombs, very few partings except at base where they are highly bituminous; some fossil fragments, possible meta- bentonite 4290 - 93.(8). Dolomite, brown, fine crystalline................. Section 4292.1 - 97.0 feet missing. 4262.7 4290.0 4314.6 O 15. GLENWOOD SHALE Unit 19 - Unit 20 - Unit 21 - Unit 22 - Shale, black bituminous grading to gray-green in bottom 6 inches (10).eeeeaeeeeeeeoeeeeaoeaeeeeeoaa Dolomite, light green, very fine crystalline; argillaceous, numerous Sand grainS................ Dolomite, very light gray (almost white), very fine crystalline - numerous sand grains........... Shale, gray-green, brittle, with.many small pyrite inclusions becomming very abundant in last 2 feet, numerous sand grains, greater concentration in bottom 10 inches (11). Dolomite, light gray, very fine crystalline, 4322.0 - 22.4 feet.............. ONEOTA DOLOMITE Unit 23 - Unit 24 - Unit 25 Unit 26 Unit 27 - Dolomite, light brown, fine to medium crystal- line, thin gray-green shale partings, great amount of pyrite in upper 2 feet, sand grains, Breccia at 4329, mud CIaCkB 43300200000000.0000... Dolomite, alternating light brown and gray-brown, very fine to fine crystalline, numerous white impure cherty inclusions; sandy, some glauconite.. Dolomite, brown, medium crystalline, few oolites and 83nd graiDS.............................o..... Dolomite, buff with greenish cast at points, fine crystalline, some white impure cherty material and some glauconite, sand grains, banding caused by zones of higher content of carbonaceous mat- erial, conglomerate at 4357.2, fossils at 4354 - 54.5 feet and 8t 4357.00eeaeeaeeeeeeeaeeeeeeeeease Dolomite, buff-gray to white, fine to medium crystal- line, increase in glauconite over previous section, sand grains, cherty, carbonaceous partings at 4373.1 4313.6 4314.7 4317.1 4320.7 4326.7 4332.7 4344.1 4345.1 4367.0 and 4378 - 84.1 feet (12); appears to be coarse crystal- line 4380.2 - 82.3, 011 stains 4378.7 - 79.3 feet.. 4389.0 MICROSCOPIC DESCRIPTION The classification used in this description is that of Folk (1959). The reader is referred to this publication for a complete description of the classification and to Table l for details compiled into table form. Percentages were approximated using the method de- scribed in Johannsen (1917). Allochems - Dismicrite - Fossils - Discussion of terms (Folk, 1959) detrital constituents (intraclasts, pellets, etc.) literally a disturbed micrite; a sack term for rocks of diverse and obscure origin, caused by one or more of the following conditions: 1. Animals (worms, mollusks, etc.) that burrow in and chew up the homogeneous carbonate mud. The void formed is later filled with spar. Ancient algal mats - if these are definitely algal plates the roch should be called an algal biolithite. Soft sediment slumping or mud cracking. Openings caused by passage of gas bubbles. Soft, unconsolidated carbonate mud is partly torn up by an increase in current velocity and rapidly redeposited. This causes irregular patches of spar. Partial recrystallization in patches would simulate a desmicrite. sedentary and tranSported fossil excluding coral or alga structures growing in situ and forming relatively immobile resistant masses . 16. 17. Intraclasts - fragment of penecontemporaneous, usually weakly consol- Micrite Oolites Pellets Sparite BLACK RIVER.LIMESTONE ‘ Unit 1 idated sediment that have been eroded from adjoining parts of the sea bottom and redeposited to form a new sediment. - microcrystalline calcite ooze. - particles showing either radial and/or concentric struc- ture. - rounded, spherical to elliptical or ovoid aggregates of microcrystalline calcite ooze, devoid of any internal structure. - sparry calcite cement and calcite formed by recrystal- lization of finer carbonate grains. Use of formula GroupI Dolomitized sparry calcite (101) cement Ii:DL8-—L* ._:arenaceous A110Chem / \Limestone Intraclast Description - Dominant lithology: Micrite (IIIm: L1); 5% dolomite, dolomite rhombs show no relation to bedding. Un- identifiable fossil fragments. . - . 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GLENWOOD SHALE This is predominantly a greenish-gray shale containing abundant detrital quartz and some detrital chert. There is some question as to the origin of the quartz and chert in the Glenwood, possibly being in- diginous to the Glenwood or reworked from the sandstones and cherts in the Oneota or St. Peter. The reader is refered to Horowitz (196D for a discussion on this problem. ONEOTA DOLOMITE The Oneota dolomite is composed of a complete mosaic of fine to medium interlocking dolomite crystals containing sparsely scattered quartz grains, some glauconite, and several beds containing oolite ghosts. These oolites are not of the siliceous variety generally found and described in the Oneota dolomite. They are dolomitized and the crystal size does not very from oolite to matrix. The dolomite crystals cut across the oolite boundaries and only a color variation outlines the oolite ghosts. Dolomitization appears to be epigenetic even through abundant vuggy porosity as seen in most epigenetic dolomites is absent. Comparison with Gamma Ray-Neutron log The deflections on the radioactive log are due to the presence of dolomite, silt, and clay, and not to sedimentary fabric types. Generally each unit is defined by a plateau with irregular variations within the plateau. Not all units are well defined, due to lithologic similaries within the Black River limestone and the Oneota dolomite sections 34. respectively. The negative "kick" at 4177 (Unit 9) and 4235 (Unit 15) indicate dolomite in large quantities (Fig. 4). Unit 9 is entirely dolomite whereas Unit 15 has thick irregular bands of dolomite. This dolomite is composed of an interlocking mosaic of dolomite crystals. The negative neutron and positive gamma-ray "kicks" at 4282 are persistent throughout the area but the cause is unknown. This is possibly a meta- bentonite or metabentonitic shale. The argillaceous banding and porosity in the Oneota dolomite is believed to account for the major deflections. Key Horizons Radioactivity logs from Calhoun, Jackson, and Hillsdale Counties were examined to determine if deflections were traceable. Unit 15 could easily be picked in all logs examined. The extent beyond the Albion- Scipio trend was not determined. Due to this regularity of occurrence Unit 15 could be used for well log correlations. The deflection on this log is believed due to the porosity in the dolomite. It might then be inferred that this zone represents one of the few stratigraphically significant zones in the Black River. us-b‘l‘ ’1 d J 5‘ ’..- 1 - A d Figure 4 - Gamma Ray-Neutron log, Howard-Peterson #1 well Numbered rectangles represent thin-sections. Gamma_Ray Neutron s I 8 d 1 L d ' 1 P d b- q . J z 3 r 1 vs 4 .. I; a 5 O 35. REFERENCES CITED Cohee, George V., 1948, "Cambrian and Ordovician Rocks in Michigan Basin and Adjoining Areas", Bull. Amer. Assoc. Petrol. Geol., Vol. 32, NO. 8, pp. 1417 "' 48. Folk, Robert L., 1959, "Practical Petrographic Classification of Limestones“, Bull. Amer. Assoc. Petrol. Geol., Vol. 43, No. 1, Pp. 1 " 38. Horowitz, M., 1961, "The St. Peter-Glenwood Problem in Michigan", un- published Master's Thesis, Michigan State University. Johannsen, A., 1917, "Suggestions for a Quantitative Mineralogical Classification of Igneous Rocks", Jour. Geol., Vol. 25, No. 1, P. 67s . Lamar, J.E., 1950, "Acid Etching in Study of Limestones and Dolomites", Ill. Geol. Sur. Circ. 156, p. 1 - 47. Steidtmann, E., 1917, "Origin of Dolomite as Disclosed by Stains and Other Methods", Bull. Geol. Soc. Amer., Vol. 28, pp. 431 - 50. 36. a! GEZU ? '5 .3“!- n. l'i‘ rflilf‘1 ‘ - . "IWil‘flilli‘lWilli