STATE OF ILLINOIS William G. Stratton, Governor DEPARTMENT OF REGISTRATION AND EDUCATION Vera M. Binks, Director 1957 PETROLOGY OF THE PALEOZOIC SHALES OF ILLINOIS Ralph E. Grim Wmiam F. Bradley W. Arthur White REPORT OF INVESTIGATIONS 203 ILLINOIS STATE GEOLOGICAL SURVEY JOHN C. FRYE, Chief URBANA, ILLINOIS PETROLOGY OF THE PALEOZOIC SHALES OF ILLINOIS Ralph E. Grim William F. Bradley W. Arthur White Illinois State Geological Survey Report of Investigations 203 Urbana, Illinois 1957 PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS STATE OF ILLINOIS HON. WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION HON. VERA M. BINKS, Director BOARD OF NATURAL RESOURCES AND CONSERVATION Hon. Vera M. Binks, Chairman W. H. Newhouse, Ph.D., Geology Roger Adams, Ph.D., D.Sc, Ll.D., Chemistry R. H. Anderson, B.S., Engineering A. E. Emerson, Ph.D., Biology Lewis H. Tiffany, Ph.D., Pd.D., Forestry Dean W. L. Everitt, E.E., Ph.D., University of Illinois President Delyte W. Morris, Ph.D., Southern Illinois University GEOLOGICAL SURVEY DIVISION JOHN C. FRYE, Ph.D., D.Sc, Chief (61360— 3M— 7-57) CONTENTS Page Introduction 5 Definition of shale 5 Samples studied 6 Analytical procedure 6 Clay mineral composition 7 Pennsylvanian shales 14 Mississippian shales 16 Pre-Mississippian shales , 16 Analytical data 16 X-ray data 16 Differential thermal analysis 18 Chemical analysis . , . . , 19 Optical properties 21 pH and cation exchange capacity 23 Texture 24 Electron-microscopic analysis 27 Appendix A. — Location and description of shales 29 Appendix B. — Summary of data obtained from thin-section studies 33 ILLUSTRATIONS Figure Page 1. X-ray diffraction patterns showing criteria on which are based estimates of mix- tures of discrete crystallizations with mixed-layer assemblages 13 2. X-ray diffraction patterns illustrating chlorite and kaolinite-bearing specimens . 14 3. X-ray diffraction patterns of an illite-bearing shale . 14 4. Differential thermal analysis curves characteristic for all the Paleozoic shales used in this study 19 5. Electron micrograph of DeLong shale (978 RRR), representative of the shales that have abundant kaolinite 26 6. Electron micrograph of Renault shale (578), representative of the shales that do not contain kaolinite 27 TABLES 1. Mineral components of clay-size fraction 8 2. Chemical analyses of clay fractions 20 3. Optical data of clay fractions , 22 4. pH data of whole shales 23 5. Base exchange capacity of shales 24 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/petrologyofpaleo203grim PETROLOGY OF THE PALEOZOIC SHALES OF ILLINOIS RALPH E. GRIM, WILLIAM F. BRADLEY, and W. ARTHUR WHITE ABSTRACT A series of Paleozoic shales, ranging in age from Ordovician through the Pennsylvanian, from widely separated geographic localities in Illinois were inves- tigated by X-ray diffraction, differential thermal, chemical, and microscopic meth- ods, in order to determine their clay mineral composition and textural charac- teristics. Illite is the dominant clay mineral in most of the shales investigated, a chlo- ritic clay mineral is present in many of them in varying abundance, kaolinite is a component of many of them in varying abundance, and montmorillonite was found in only one of them in minor quantities. The clay mineral content of the shales ranges from less than 40 to almost 100 percent. An analysis of the data shows that certain clay mineral compositions are characteristic of certain parts of the Paleozoic section of Illinois. The clay min- erals in the Pennsylvanian shales are illite, kaolinite, chlorite, and mixed layer assemblages. Mississippian shales are characterized by little or no kaolinite. Pre-Mississippian shales are rich in illite and commonly contain only a small amount of chlorite. INTRODUCTION The object of this report is to pre- sent data on the mineralogical, tex- tural, and chemical composition of the Paleozoic shales in Illinois and such conclusions regarding the origin of these sediments as seem justified from the analytical data. Samples of shale from many stratigraphic horizons have been studied as well as samples from the same horizon at different geo- graphic locations. The work reported herein was done by the staff of the Illinois State Geo- logical Survey as part of the funda- mental study of the natural resources of Illinois to provide a basis for their use and future development. DEFINITION OF SHALE Shale is defined (Pirsson and Knopf, 1926; Twenhofel, 1950) as a minutely- grained argillaceous sedimentary rock which is layered or laminated parallel to the bedding and in which there has been no substantial change in mineral composition since accumulation other than changes that resulted from dia- genesis or compaction. The layering may be due either to a general parallel arrangement of flake-shaped or elon- gate particles, or to an alternation of beds of somewhat different composi- tion and texture. According to the foregoing defini- tions, shales differ from clays and silt- stones only in being bedded or lami- nated. Some students consider that the term shale indicates a material more indurated than clay (Pettijohn, 1949) and, therefore, that shale is the laminated equivalent of claystone or siltstone. At times shale has been de- fined solely on the basis of induration without any reference to lamination (Ries, 1927). In the authors' experi- ence materials generally called shales are more indurated than those gener- ally called clays. It has been stated frequently that there is an essential difference in the [5] ILLINOIS STATE GEOLOGICAL SURVEY compositions of clays and shales. Such statements have been based usually on assumption rather than on any actual analytical data. Some investigators, notably Harker (1895), Hutchings (1892), and Brammall (1921) have suggested that mica-type minerals are likely to predominate as the constitu- ents of shales. One of the objects of the present study was to determine if materials generally classed as shales tend to have a particular composition. All of the materials investigated in the present study have been classed as shales by geologists who have studied the Paleozoic section in Illinois. They are minutely grained argillaceous sedi- ments with more or less obvious lamin- ations parallel to the bedding and without any obvious post-depositional change except that which could be due to diagenesis and/or compaction. SAMPLES STUDIED The stratigraphic positions, geo- graphic locations, and megascopic characters of the samples studied are given in Appendix A. The samples represent materials from outcrops and cores. Most of the samples are from Penn- sylvanian horizons, but there are also a number from older Paleozoic beds. For the Pennsylvanian system an ef- fort was made to obtain fairly com- plete stratigraphic representation, and also samples from the same horizons in several widely scattered localities. Also at a few places samples were col- lected vertically through thick shale horizons. The samples were restricted to dis- tinct shales, that is, materials that could be classified as shaly sandstones or shaly limestones were not studied. ANALYTICAL PROCEDURE The primary objective was to study the clay mineral composition. Supple- mental information, such as pH and cation exchange capacity, and texture, as revealed by microscopic study of thin sections cut parallel and at right an- gles to the lamination, was obtained for a limited number of samples. The nonclay mineral components were not particularly studied. If mate- rials like quartz and organic material were prominent, their textural attri- butes and their associations with the clay minerals were observed in the thin sections. No attempt was made to study the so-called ''heavy" minerals. To minimize interferences of quartz and coarse-grained accessories with diffraction and thermal analyses of the actual clay minerals, less than 2-mi- cron (and in some cases less than 1- micron) fractions were collected. Ap- pendix B shows the percentage of quartz, muscovite, total clay clay min- erals, carbonate, feldspar, organic material, etc., based on microscopic study. Collections of minus 2-micron size fractions were made by wet sedimenta- tion. In most cases repeated washing of a crushed sample with distilled water accomplished adequate disper- sion, but for a few samples some am- monia water was necessary. Identification and estimation of the clay minerals were primarily based on X-ray diffraction, using powder and oriented aggregate techniques with un- treated samples. Glycol treatments or moderate heat treatments were em- ployed only when necessary. The prin- ciples used to arrive at quantitative in- terpretations of the diffraction data recently have been described by Johns, Bradley, and Grim (1954). The tabu- lated data have been accumulated in PALEOZOIC SHALES OF ILLINOIS intermittent periods over a span of years, and progressive improvements in technique have been adopted as they emerged, without prejudice. In many cases the X-ray data were supple- mented by differential thermal an- alyses, and in somewhat fewer in- stances complete chemical analyses were obtained of the minus 2-micron fractions. For the more complex mix- tures which include both regular clay minerals and associated mixed-layer assemblages, the supplemental infor- mation from differential thermal curves and chemical analyses can be interpreted in only the most general terms. The pH measurements and cation- exchange capacities were determined by standard methods. To determine cation-exchange capacity, the ammo- nium acetate technique was employed. In general, analyses other than dif- fraction were more frequent among the earlier specimens, and diffraction emphasis is greatest in the more re- cent collections. CLAY MINERAL COMPOSITION The clay mineral composition of the samples studied are given in table 1 in parts in ten. The column headings in table 1 are chosen to preserve as much continuity as possible through specimens of wide- ly varying crystallinity and degree of orientation and through diffraction data of inconstant quality. Under chlorite are entered all those components which have a fundamental period near 14A which do not swell markedly with glycol or collapse under mild heat treatment. Few, if any, com- pare in quality with chlorite as in a hand specimen. They include some samples which might be called vermic- ulites. Under kaolinite are included any ka- olin group minerals. Kaolinite itself is most common, and collapsed halloysite or ''fire-clay mineral" is frequent. A separate column contains entries for those samples in which it was not pos- sible to distinguish whether the com- ponent was of 14A or 7A period. Iron- or magnesium-bearing kaolin analogues would not be distinguished in specimens as heterogeneous as those here reported. Illite, as tabulated separately, refers to predominantly unswellable mica crystallites. In general these entries are more or less typical examples of the two-layer muscovite polytype. The mixed systems, which include mica and abundant swellable layers, vary widely. Some mixed systems are quite homogeneous, clearly tending to maintain a given ratio of swelling to nonswelling particles throughout an aggregate. Other mixed systems are widely heterogeneous, and include par- ticles of many degrees of swellability. In favorable cases the two situations may be distinguished, and may be re- solved from the more entirely illitic components. Other mixed systems composed of mica layers and over-size layers not subject to swelling are also occasion- ally identifiable. Fbr many samples the superposed (001) diffraction sequences of the sep- arate clay mineral groups are all re- solved and may be read from a single diagram. When this is not true, swell- ing an aggregate with ethylene glycol, or collapsing it with selected heat treatments of from 200° to 500° or 600° C, will usually move the position of some sensitive features to diffrac- tion angles where they are resolved. Figures 1-3 illustrate the nature of the diffraction criteria utilized. ILLINOIS STATE GEOLOGICAL SURVEY Table 1. — Mineral Components of the Clay-Size Fraction of Shale (in parts in 10) lUite-montmorillonite < c/2 ;■ T3 (U eterogene- is mixed stems 15 Chlo kaoli difFei o c o o .§ X u W ^ X g O Xo^ ^ =: 6 d Iowa Rosiclare 1086 1087 Ste. Genevieve 1095 Hannibal DS-38* DS-51* DS-45* 546* 1-2 2-3 1-2 1-2 — 1-2 — 1-2 5-6 5-6 6-7 5-6 1 1-2 New Albany 1093 547* DEVONIAN 1-2 1-2 Sexton Creek DS-7* 1-2 SILURIAN 8-9 Maquoketa 580* NF42* 1-2 ORDOVICIAN 7-8 9-10 — 1 * Outcrop samples; others are core samples. Fig. 1. — X-ray diffractometer curves that illustrate criteria upon which were based estimations of mixtures of discrete crystallizations with mixed-layer assemblages. Beneath the several traces, heavy lines erected on the base line represent the height above background of lines from monomineralic com- ponents, and more or less broken curves indicate the level above background of diffuse scattering from mixed-layer assemblages. The illustrations, in decreasing order of prominence of the mixed- layer matter show: la — mainly 10 A layers mixed with swellable 15 A layers, plus discrete illite and kaolinite; lb — same material swollen with ethylene glycol; 2a — 10 A layers mixed with swellable layers and 10 A layers mixed with non-swelling 14 A layers, plus discrete illite, chlorite, and probably kaolinite; 2b — same material swollen; 3a — 10 A layers mixed with swellable layers, plus discrete illite and chlorite; 3b — same material swollen; 4a — discrete illite, chlorite, and kaolinite with signi- ficant mixed-layer associates, partly swelling; 4b — swollen specimen; 5a — discrete illite, chlorite, and kaolinite with small but real mixed-layer associate; 5b — the same material with ethylene glycol, illus- trating the trivial degree to which the mixed assemblage swells. PALEOZOIC SHALES OF ILLINOIS 13 14 ILLINOIS STATE GEOLOGICAL SURVEY Fig. 2. — X-ray diffractometer curves for chlorite and kaolinite-bearing specimens arranged in in- creasing order of kaolinite content relative to chlorite. Top — chlorite abundance is more than twice kaolinite abundance. Second — chlorite abundance about twice that of kaolinite. Third — chlorite and kaolinite are nearly equal in abundance. Bottom — no chlorite is certainly identifiable. Jifove, X-ray diffractometer curve of a main- ly illite-bearing shale with some swellable mixed-layer and small chlorite and kaolinite accessories. Below, heating the specimen reduces the evidence of swellable mixed layers and noticeably alters intensities of the chlorite diffraction effects. Pennsylvanian Shales The Pennsylvanian shales are com- posed essentially of a mixture of ilhte, kaolinite, and chlorite. In many shales illite is the most abundant clay mineral component. The relative abundance of kaolinite and chlorite varies widely from sample to sample. Only one sam- ple (241) contains any considerable proportion of conventional montmoril- lonite, that is, in other than mixed- layer relationship with illite, and this sample is from a section that has distinct evidence of post-depositional weathering. A large percentage of the samples show illite-montmorillonite- mixed- layer assemblages. In most of the sam- PALEOZOIC SHALES OF ILLINOIS 15 pies for which any judgment could be made, the mixed-layer assemblages were heterogeneous. In general, sam- ples showing relatively large amounts of mixed-layer material also show rela- tively smaller amounts of illite. The tabulations tend to show greater influence of mixed layering in core samples than in outcrop samples. No significance can now be attached to this trend. The outcrop fractions stud- ied had a higher upper limit of particle size, were characteristically less well oriented than the preparations from cores, and were mainly examined by film registrations in which the dilution of diffraction criteria for mixing by the fixed diffraction effects of the larger more typical mica particles made the observations far more diffi- cult. The illite content ranges from about 80 percent to 30 percent. It seems to be slightly more abundant in shales from the lower part of the Pennsyl- vanian section. Chlorite ranges in abun- dance from to about 40 percent. In general, the upper part of a section contains unquestioned chlorite but to- ward the base of the Pennsylvanian the chlorite is frequently questionably present or in many samples definitely not a component. Kaolinite ranges in abundance from to about 60 percent. In the upper part of the section and in the Casey- ville beds it usually makes up about 20 to 30 percent of the clay minerals. In beds just above the Casey ville, the kao- linite is relatively most abundant in the Pennsylvanian shales. It is inter- esting that generally in samples with relatively high kaolinite, the chlorite is relatively low and vice versa. At first sight there appears to be no correlation between the stratigraphic position of Pennsylvanian shales and their clay mineral composition, and it would take considerably more detailed work than has been done to establish unequivocally such correlations. How- ever, there are many indications of such correlations. Thus, all of the Up- per McLeansboro samples from Ed- wards, Macoupin, and Bond counties have a relatively high uniform kaolin- ite content. All of the samples of Pur- ington shales (except 241 which has been weathered) have substantially the same clay mineral composition. In the DeLong beds there is a relatively high abundance of kaolinite and illite with generally little or no chlorite. The Babylon and pre-Babylon beds just above the Caseyville are characterized by high kaolinite contents and little or no chlorite. There appeared to be a distinct change in the clay mineral composition at the top of the Casey- ville. There are some noteworthy excep- tions to the correlation of stratigraphy with clay mineral composition. Thus sample 243, unlike other Trivoli sam- ples, appears in the table with a high kaolinite content. It is a brown shale, high in limonite. Either weathering or improper identification could account for the instance. All of the Brereton samples show large variations in clay mineral composition. Such examples are irregular, of course, only on the as- sumption that the correlations are ac- curate and that there has been no se- lective surficial alteration. The analytical data are not adequate to determine whether or not there is any correlation between the clay min- eral composition and the geographic distribution of any particular shale horizon. 16 ILLINOIS STATE GEOLOGICAL SURVEY MississiPPiAN Shales The Mississippian shales are char- acterized by either no kaolinite or very Httle (10-20 percent) . Illite or a mixed- layer assemblage of illite-montmoril- lonite is the dominant clay mineral component. Unlike the Pennsylvanian shales, the mixed-layer assemblage is either homogeneous or unspecified rather than definitely heterogeneous (with one exception, 1105-67A). Of particular interest is the relatively high abundance of homogeneous mixed-layer system in the upper part of the section including the Upper Clore to the Menard. This character- istic should have definite value for correlating strata. Chlorite in amounts of 20 to 30 per- cent is a common component of the Mississippian shales. A considerable number of Mississippian samples con- tain undifferentiated kaolinite-chlorite, that is, material which is either kaolin- ite or chlorite but for which absolute differentiation was not possible. Pre-Mississippian Shales The number of samples of pre-Mis- sissippian shales investigated is too few to permit broad generalities. All of these shales are rich in illite, con- tain no mixed-layer assemblages or kaolinite, and usually contain only a small amount of chlorite. ANALYTICAL DATA The analytical data from which the clay mineral determinations are de- rived are here discussed in some detail because their interpretation is impor- tant and often difficult. Such discus- sion also reveals additional character- istics of the shales. X-RAY Data X-ray diffraction analyses are flexi- ble enough to permit the analyst to choose conditions that will emphasize the features which he considers im- portant. In arriving at the procedures utilized here, we have considered sev- eral well known generalizations based on experience in these and other labor- atories. It is widely recognized that among the layer silicates, including clay min- erals, diffraction effects in the 001 order sequence afford clearer criteria for qualitative identification of phases than do the effects in other directions. Qualitative work takes advantage of the natural tendency of the fine- grained layer minerals to align them- selves from suspension into sub-par- allel oriented flaky aggregates. Such flakes segregate the important diag- nostic diffraction features into a nar- row region in reciprocal space near the 001 pole to the flake and greatly atten- uate all other features in the same re- gion. When the orientation of mono- mineralic flakes is good, the intensity measured at such a textural maximum may be as much as 50-fold the inten- sity that would have been afforded by the same feature spherically distrib- uted, as in the powder method. For analyses done by photographic registration, the flake technique was obligatory for even qualitative identi- fications in complex mixtures and mul- tiple exposures were sometimes neces- sary to observe specific ranges of dif- fraction angle. Direct-recording dif- fractometer instruments now in popu- lar use permit a natural continuous scanning of the same pole (001) which was scanned in sections and at some ef- fort by film methods. Analyses in ta- ble 1 include some specimens estimated from films and some from diffractom- eter records. Clay minerals are subject to: 1) sig- nificant degrees of isomorphous substi- PALEOZOIC SHALES OF ILLINOIS 17 tution of chemical entities within indi- vidual crystallizations; 2) wide lati- tudes of degree of perfection with which small crystallites conform to the ideal geometry of a crystal type, and 3) frequent epitactic intergrowth of layers of two or more separate specific crystallizations. In favorable cases sufficiently ho- mogeneous aggregates have been amenable to quite detailed study, and the laws relating intensities of succes- sive 001 diffraction features to such variabilities are known. The relation- ships are sufficiently complex, however, that in dealing with unknown mixtures there is but little hope of developing a set of standards suitable for construc- tion of the large number of working curves which would be required to cover all the composition and quality ranges encountered in natural sedi- ments. Our procedures are therefore aimed at taking the best possible ad- vantage of qualitative criteria and ad- mitting some latitude of error in the quantitative estimates. Estimates are tabulated only on the basis of apparent parts in tens, not in percentages. Two important factors limit the possible error in arbitrary estimates made as outlined above: 1) In an extensive series of speci- mens, some of the samples will be ei- ther nearly monomineralic or predom- inantly homogeneous mixed systems. Approximate upper limits are there- fore fixed for the diffracting power of flake aggregates of each mineral in one or more degrees of perfection of textural alignment. These limits, and the fact that no sum can exceed ten parts, fixes many compositions with a high degree of reliability. 2) There is a useful generality in the several crystallizations involved. Even though the chemical composi- tions are significantly variable, aver- age electron densities in each crys- tallization are near 0.8 electrons per A^, and each crystallization has one prominent diffraction maximum in the range of - — r — values near 0.14 to A 0.16 for which effective contributions are comparable fractions of the total electron density. For example, in Brown's recent tabulation (Brown, 1955) of calculated F-values in this an- gle range for micas and chlorites of varying ion contents, calculations within each set vary by a factor near two, but for equal ion-ratios and vol- umes the greatest ratio from chlorite to mica is 1.2. In the sediments here analyzed, if equal perfection of crystallites and equal degree of preferred orientation existed, the sum of all integrated in- sin B tensities in the above range should not vary by more than 2-fold. Actual ranges encountered, both in film estimates and in integrated dif- fractometer counts, run as much as 10- fold. Inasmuch as poor quality in crys- tallites and imperfect parallelism both contribute to too rapid decline of inten- sities with increasing diffraction angle, it is clear that even the poorest orien- tations afford counts of as much as five times those that would be available from powders. The tabulated relative importance of clay mineral components are interpre- tations of an inspection of the general level of intensities of successive orders for each separate component with at- tention directed at apparent degree of orientation, apparent degree of crys- tallinity, and relative importance of each order sequence with respect to the order sequences of other components. 18 ILLINOIS STATE GEOLOGICAL SURVEY In the best aligned flakes, illites and highly illitic mixed systems deliver 2,000 to 3,000 counts, integrated by counting at the first maximum and one- half degree on either side. Kaolinites deliver about 3,000 counts in the first maximum and about 200 in the third maximum. Chlorites are highly varia- ble in the first and third maxima, but have the potential in the fourth maxi- mum cited above as applicable to the sin ^ ^ . — r — range near 0.14. A In analysis all components are given a weight on the basis of proportional parts of these upper limits, then for any samples in which diffuse nature or poor orientation (or occasional high iron content) results in low totals, the abundance estimates are scaled up in- dividually according to the apparent degree of diffuseness or poor orienta- tion. Most sums fall from 8 to 11 parts. The practice of making allowances for low grades of crystallinity in diffused sequences precludes, in general, the ability to infer that separate non-crys- talline matter is present. In only a few instances does there appear to be any significant non-crystalline accessory. Differential Thermal Analysis The differential thermal analyses were made by the method described by Grim and Rowland (1942) on the less than 1- or 2-micron fractions. The less than 2-micron fraction was used for outcrop samples with numbers less than 800 (samples were collected and fractionated before 1945), and the less than 1-micron fraction was used for outcrop samples with numbers greater than 800, F series (samples were col- lected and fractionated after 1945), and for core samples. The analyses were first made in a DTA furnace in which the atmosphere of the furnace was oxidizing. Those samples which gave peaks showing or- ganic matter and pyrite were analyzed again in a DTA furnace with a nitro- gen atmosphere in order to suppress the oxidation reactions. Second an- alyses were carried only to 700 °C. (Curves run above 700° in the inert at- mosphere are influenced by the resid- ual combustibles normally already burned by the conventional procedure.) Composite thermograms were then made, using the thermograms derived in the nitrogen atmosphere for the portion up to 700 °C. and the thermo- grams derived in the oxidizing atmos- phere for the remainder. Examples of DTA curves for several of the samples studied are given in figure 4. Kaolinite clay minerals, if present in sufficient quantities, tend to lengthen the endothermic peak between 500° and 600° C. and tend to increase the sharpness and height of the exother- mic peak between 900° and 1000 °C. In general, kaolinite does not show un- equivocally on the curves until it makes up about 50 percent of the clay mineral fraction. At lower percentages of kaolinite, the dominant characteris- tics of the curves are those of the three-layer clay minerals, primarily illite. The factors determining whether or not small amounts of kaolinite influ- ence a DTA curve, are probably par- ticle size, degree of organization of the mineral, and the intimacy of its mix- ing with other clay minerals. The variable influence of small amounts of clay mineral components on the DTA curves of samples, such as these complex mixtures, renders it difficult or impossible to make precise clay mineral identifications based solely on DTA curves. The curves show many PALEOZOIC SHALES OF ILLINOIS 19 0°C 100° 200° 300° 400° 500° 600° 700° 800° 900° 1000° Fig. 4. — Differential analysis curves shown above are characteristic of the curves for all the Paleozoic shales studied. variations whose significance is not known and which do not seem to corre- late with the clay mineral composi- tions. Perhaps future work will show the cause of these variations and thereby permit more satisfactory in- terpretations of the DTA curves. As indicated above, the curves in general indicate the character of the dominant clay mineral component. Also in samples where the X-ray data indicate either chlorite or kaolinite, for example 978 RRR, the DTA data fre- quently suggest which is the proper in- terpretation. Also, it should be mentioned that DTA curves can frequently be used for correlation even though the causes of the distinctive characteristics are not completely understood. Thus, the lower Paleozoic shales tend to give dis- tinctive DTA curves because of shal- low endothermic peaks in the broad re- gion from 300° to 700 °C. This must be due, at least in part, to the general absence of kaolinite and perhaps also to a higher degree of crystallinity of the mica. Chemical Analyses The chemical composition of the less than 2-micron fraction of many of the shale samples is given in table 2. In mixtures of such variable composition, in which the component minerals may also vary in composition, no close cor- relation between mineral composition and chemical composition is to be ex- pected. In general, only the crudest kind of qualitative estimates of clay mineral composition could be made from the chemical composition of the less than 2-micron fraction, and sub- stantially nothing is revealed of the chemical nature of particular compo- nent clay mineral species. There are, 20 ILLINOIS STATE GEOLOGICAL SURVEY Table 2. — Chemical Analysis of Clay Fraction of Shale Less Than 2 Microns. Sample No. Si02 AI2O3 Fe^Os FeO CaO MgO NaaO K2O Ti02 Ign. H2O+ H2O- - P2O6 Total 262 51.50 24.51 2.73 3.89 0.96 1.70 0.93 3.77 1.49 8.38 7.82 0.99 — 99.86 229 47.89 25.99 8.61 0.90 0.94 2.48 0.41 3.81 1.60 7.76 7.75 1.98 — 100.39 280 52.37 23.62 2.84 4.93 0.76 2.82 0.80 3.69 1.33 6.81 6.70 0.57 — 99.97 243 46.68 27.74 7.37 1.18 0.86 2.35 0.20 3.75 1.35 8.45 8.52 1.89 — 99.93 270 51.57 23.79 2.88 5.07 0.66 2.80 0.74 3.83 1.54 6.96 6.94 0.66 — 99.84 239 48.44 26.68 6.76 1.64 0.80 2.65 0.35 3.41 1.40 7.62 7.57 1.42 — 99.75 202 49.50 27.60 2.10 3.37 1.48 2.66 0.51 4.57 1.17 7.74 7.66 1.32 — 100.70 285 50.90 25.80 2.39 3.76 0.63 2.63 0.84 4.64 1.61 7.29 7.17 0.69 — 100.49 272 57.08 21.93 2.10 3.78 0.66 2.61 0.76 2.72 2.15 6.21 5.71 1.19 — 100.00 254 51.85 25.60 2.16 3.49 0.81 2.65 0.64 4.35 1.49 6.73 7.17 0.43 — 99.77 1088 48.19 28.05 0.69 4.44 0.48 2.71 0.75 5.61 1.15 7.72 7.37 1.74 — 99.79 224 . 49.27 27.87 2.32 3.22 0.69 2.49 0.58 4.37 1.69 7.84 7.83 0.65 — 100.34 215 47.18 24.95 8.30 2.15 0.87 2.26 0.51 4.03 1.55 8.28 7.94 0.46 — 100.08 217 53.06 26.86 2.42 3.00 0.76 2.51 0.71 3.30 0.81 6.47 6.45 0.96 — 99.90 242 51.02 25.30 2.04 5.06 0.87 2.25 0.46 2.79 1.39 8.99 7.32 1.23 — 100.17 241 51.19 22.31 8.00 1.66 1.25 2.53 0.59 4.10 1.44 6.98 6.46 0.61 — 100.05 240 57.21 18.97 4.66 2.89 0.92 2.17 0.93 3.27 2.16 6.77 5.98 0.27 — 99.95 B36 58.27 21.27 1.63 1.73 0.94 1.23 0.93 3.93 3.37 5.91 — 0.25 0.80 100.01 204 54.61 26.75 1.45 2.08 0.48 2.16 0.83 4.84 0.90 6.69 6.06 0.85 — 100.79 978NNN 48.12 26.73 1.50 4.22 0.58 2.68 0.73 5.33 1.37 8.69 7.64 1.41 — 99.95 4-115 47.18 28.52 2.51 2.84 0.58 2.06 1.04 4.73 1.06 9.62 8.19 1.21 — 100.14 F119 48.05 29.24 1.36 2.97 0.59 1.99 0.55 4.65 1.42 8.95 8.39 1.75 — 99.77 213 47.59 30.04 4.53 1.66 0.84 2.08 0.56 3.19 0.62 8.81 8.34 1.13 — 99.92 F121 48.13 31.03 1.39 1.13 0.69 1.17 0.41 2.69 1.37 11.82 11.24 1.09 - 99.83 978UUU 43.12 23.09 9.80 0.77 0.80 1.69 0.73 5.31 1.53 12.71 8.00 2.07 — 99.55 F124 46.56 29.82 3.54 1.51 0.14 1.29 0.51 3.95 1.57 10.99 10.46 1.07 — 99.88 268 51.45 26.82 2.07 3.01 0.65 1.89 0.29 5.15 1.80 7.23 7.31 0.87 — 100.36 977DD 46.79 33.52 2.48 0.51 0.19 0.78 0.54 2.40 1.85 11.39 10.84 1.37 — 100.45 1105-57 49.16 29.32 1.43 1.84 0.27 1.68 1.11 5.87 1.11 8.21 7.56 1.40 — 100.00 1105-59 49.33 28.55 1.54 2.51 0.26 1.84 0.87 5.77 1.36 7.88 7.28 1.76 — 99.91 1105-60 48.82 28.79 2.05 1.78 0.39 1.74 0.99 6.11 1.33 7.86 6.92 1.49 — 99.86 1105-61 49.62 27.87 1.77 2.14 0.60 1.97 0.99 5.96 1.36 7.97 7.08 1.94 — 100.25 4-244 46.87 28.46 2.05 3.80 0.55 2.19 1.19 4.42 1.00 9.45 8.39 1.30 — 99.98 4-250 46.95 27.36 1.31 4.57 0.66 2.07 0.46 3.38 0.92 11.52 8.97 1.59 — 99.70 1105-62 49.13 29.17 1.37 1.78 0.21 1.77 1.16 6.28 1.49 7.58 6.74 2.31 — 99.94 1105-63 48.78 28.21 2.37 2.25 0.36 2.12 0.80 6.08 1.36 7.69 7.04 1.22 — 100.02 1105-64 47.39 27.54 2.78 2.57 1.14 1.99 0.83 5.51 1.37 8.64 6.94 1.26 — 99.76 1105-65 46.95 26.60 1.98 4.15 1.30 2.27 0.84 5.33 1.38 8.87 7.14 2.07 — 99.67 577A 51.92 23.63 2.49 2.39 2.19 3.96 0.19 4.57 0.58 7.99 7.06 4.74 — 99.91 579B 51.44 28.47 1.23 1.43 1.35 1.59 0.38 5.30 0.55 8.41 8.14 1.96 — 100.15 576 53.83 23.74 2.62 1.62 1.61 3.90 0.39 5.67 0.35 6.65 6.44 2.82 — 100.38 575B 51.23 27.34 4.27 0.89 1.35 2.09 0.44 4.05 0.50 8.23 8.18 2.17 — 100.39 575A 50.89 28.26 4.71 0.50 1.22 2.12 0.31 3.93 0.55 8.10 8.02 4.03 — 100.59 578 50.94 24.85 5.86 1.01 1.65 2.78 0.10 6.62 0.50 6.23 5.68 2.54 — 100.54 1095 51.86 20.70 4.21 2.28 0.61 5.46 0.56 5.94 1.01 7.20 6.45 3.06 — 99.83 DS38 50.23 21.66 7.34 0,89 0.92 5.79 0.37 5.97 0.53 6.83 6.68 2.48 — 100.53 DS51 50.97 23.97 4.24 1.70 1.20 4.23 0.08 6.92 0.57 6.44 6.24 2.26 — 100.32 DS45 51.28 22.91 4.04 1.69 1.71 4.29 0.27 6.85 0.53 7.08 6.81 2.22 — 100.65 546 64.22 15.03 3.06 1.38 3.06 2.42 0.47 4.33 0.29 6.40 5.08 1.02 — 100.66 1093 48.34 19.44 — 4.56 0.30 2.25 1.19 5.22 0.92 17.19 8.33 1.15 — 99.41 547 46.12 18.91 3.11 3.15 1.04 3.07 0.74 6.30 0.45 17.53 11.57 0.99 — 100.42 DS7 49.80 23.15 5.96 2.05 1.63 4.04 0.04 6.53 0.45 6.71 6.11 1.82 — 100.36 580 51.46 22.75 6.87 1.16 0.69 2.86 0.45 6.79 0.49 6.97 6.75 2.21 — 100.49 NF42 51.10 21.96 6.38 1.65 1.73 3.91 0.03 6.62 0.46 6.24 5.75 1.74 — 100.08 PALEOZOIC SHALES OF ILLINOIS 21 however, some significant points in these data that warrant discussion. In the Pennsylvanian shales, the less than 2-micron fraction of the sam- ples contains variable amounts of un- combined silica that accounts in part for the variable amount of Si02. The silica content is frequently high for those samples for which X-ray diffrac- tion estimates lead to low totals. If an amorphous phase is present, this im- plies that it is relatively rich in silica. In general, samples with particular- ly high contents of AUG., (30 it per- cent) are relatively rich in kaolinite. The amounts of Fe203, FeO, and total iron in the samples are extremely variable. High FeO is generally cor- relative with prominent chlorite con- tent and high Fe203 with kaolinite con- tent or possible enrichment in obvious- ly weathered outcrops. The highest iron contents all occur in oxidized form and probably derive from pyrite, which does not yield iron to be collected in the fine fraction unless it has been oxi- dized. In the fresher samples there is no particular significance to the total iron content. There is no correlation of MgO con- tent and clay mineral composition ex- cept that it is lowest where the kaolin- ite content is high. Actually the MgO content is unexpectedly low for the chlorite content shown by the X-ray data, but the FeO correlations noted above and the implication of frequent vacancies in the brucite portions of the structure drawn from the relatively poor crystallinity of the chlorites are probably important enough to properly rationalize the analyses. In general, a high K2O content ac- companies a high illite content. K2O contents of illite probably range from about 5 percent to 10 percent with higher values in this range more fre- quent than lower. In some cases, low percentages could be accidental, for ex- ample in sample 241 the clay fraction indicated illite abundance of 80 percent but yielded only 4.10 percent of K2O. This was an oxidized material, and may have had some mixed layer char- acter which was not observed because of the high iron content. In pre-Pennsylvanian shales the sil- ica content tends to be high and the aluminum content low, reflecting con- siderable amounts of uncombined sil- ica and the absence of kaolinite. The amounts of Fe203, FeO, and total iron are about the same in these shales as in those of Pennsylvanian age. In many samples the MgO content is higher than in the Pennsylvanian shales suggesting better developed chlorite and possibly less replacement in brucite positions. In general the content of K2O is higher in pre-Penn- sylvanian shales than in the Pennsyl- vanian shales, indicating a tendency for the illites to be relatively richer in potassium. Except for samples 1093 and 547, which contain much organic material, the values for loss on ignition and H^0+ are relatively low in the pre- Pennsylvanian shales. These shales tend to have better organized illite and less kaolinite contents. Vari- ation in chlorite organization can cause only a small variation in ignition loss, and in these samples its contribution was very minor. OPTICAL PROPERTIES Oriented aggregates of less than 2-micron fraction were studied micro- scopically in an effort to measure opti- cal properties. The maximum index of refraction among the samples ranged 22 ILLINOIS STATE GEOLOGICAL SURVEY Table 3. PENNSYLVANIAN -Optical Data of Clay Fractions Less Than 2 Microns McLeansboro Group 7 7 — a 2V U. McLean; iboro 262 1.572 .030 small 278 1.575 .025 « 264 1.574 .025 « 281 1.574 .025 ii 229 1.575 .025 i( Trivoli 243 1.576 .025 a 238 1.580 .025 « ^ 270 1.574 .025 " Exline 280 1.575 .025 a 279 1.570 .020 a Sparland 239 1.575 .025 medium 202 1.572 .025 small 237 1.579 .028 medium 227 1.575 .025 small 285 1.575 .025 a Carbondale Brercton 269 1.570 .025 a 272 1.572 .030 a 231 1.572 .025 a 254 1.574 .025 a 224 1.574 .023 u St. David 215 1.580 .025 a Lowell B36 1.572 .025 (( Liverpool 242 1.574 .025 u 240 1.572 .020 a 275 1.574 .025 u 241 1.574 .025 u 204 1.572 .025 u Tradewater 7 7 — a 2V Sign Greenbush 213 Macedonia 268 1.575 1.573 .020 .025 small u a " MISSISSIPPIAN « Kinkaid 577A 1.570 Chester .030 small It Clore 579B 1.570 .030 small a Menard 576 1.560 .030 5^ u Vienna 575B 575A 1.570 1.570 .030 .025 6° small a Renault 578 1.575 .030 6° 11 IOWA Hannibal DS38 DS51 DS45 546 1.580 1.575 1.575 1.572 .030 .030 .030 .022 4° 5° small ? « DEVONIAN " New Albany 547 1.572 .027 ? u u Sexton Creek DS7 SILURIAN 1.585 .030 ORDOVICIAN small u u Maquoketa 580 NF42 1.580 1.583 .030 .030 4° 6-8° Sign (-) (-) from 1.560 to 1.580. The birefringence that could be measured ranged from 0.030 to 0.035. The optic angle was small and the sign negative wherever these values could be determined. Table 3 gives optical data for a repre- sentative series of the samples. One would conclude from the optical data that a mineral with a moderately high birefringence and indices in the range 1.550 to 1.580 was a major com- ponent of these samples. Such a min- eral is illite. One would also conclude from these data that this illite was mixed with some other component, but in general the presence of the other components, quartz, kaolinite, chlorite, mixed-layer assemblages, etc., could not be established by the optical data. In view of the complex composition of these shales, no correlation between the optical data and the clay mineral composition is to be expected and in fact none is shown. PALEOZOIC SHALES OF ILLINOIS 23 Table 4. — pH Data of Whole Shales PENNSYLVANIAN Carbondale Grindstaff McLeansboro U. McLeansboro 262* pH 6.9 Brereton 1105-17 1105-19 269* 231* 254* 1105-22 1105-23 7.1 4.2 6.7 6.3 6.9 8.7 5.9 1105-49 1105-50 1105-51 1105-53 8.2 4.7 6.3 6.0 278* 7.0 1105-55 4.7 264* 6.5 , 1105-56 6.0 281* 229* 7.1 6.7 1105-57 1105-58 7.2 7.2 Carlinville St. David Caseyville 1105-1 1105-2 8.3 9.0 1105-25 Summum 7.9 Pounds 1105-59 7.3 Trivoli 1105-26 6.8 1105-60 7.6 1105-3 1105-4 243* 238* 9.2 3.9 6.7 7.2 Lowell 1105-28 1105-29B 7.1 7.2 Battery Rock 1105-61 1105-62 1105-63 7.6 8.1 6.9 270* 1105-6 7.2 9.0 Liverpool 242* 6.0 Lusk 240* 7.0 1105-64 8.5 Gimlet 275* 6.4 1105-65 9.0 1105-7 9.0 241* 7.0 1105-8 7.2 1105-31 6.1 MISSISSIPPIAN 1105-9 1105-12 7.1 7.1 1105-32 Tradewater 6.4 Chester Kinkaid Cutler 1105-13 1105-14b 5.1 6.4 Greenbush 213* Stonefort 6.2 1105-67 1105-67A 1105-68 1105-69 8.8 9.1 9.2 8.5 Bankston 1105-39 9.1 Clore 1105-15 1105-16 6.3 7.1 Macedonia 1105-43 9.1 1105-70 1105-72 7.5 8.0 268* 6.4 1105-73 8.3 Sparland 239* 6.8 Delwood Menard 237* 6.7 1105-45 5.1 1105-74 9.3 227* 5.4 1105-47 7.0 1105-75 9.6 285* 7.6 1105-48 6.4 1105-76 9.5 * Outcrop. pH and Cation Exchange Capacity Table 4 gives the pH values of a rep- resentative series of the whole shales. The samples from outcrops show a range 5.4 to 7.6. The core samples show a much greater range from 3.9 to 9.6. Because the pH value may be influenced by some of the nonclay min- erals as well as the clay minerals, some caution must be exercised in interpre- tation. Millot (1949) has experienced some success in utilizing the principle that the pH of a sediment reflects the pH of the environment in which it accumu- lated. The data here presented, and listed in table 4, do not seem to sup- port this correlation. However, the data do show that the low pH values are from samples in which the mixed layer component is prominent. Table 5 presents cation exchange ca- pacity values for a series of Pennsyl- vanian shales. The exchange capacity is contributed to largely by the illite and chlorite components with the other components acting largely as diluents. The data are given merely to show the order of magnitude of this property of these materials. 24 ILLINOIS STATE GEOLOGICAL SURVEY Table 5. — Base Exchange Capacity of Shales Base exchange Base exchange Number capacity Number capacity m.e./lOOgm. m.e./lOO gm. 262 9.9 227 3.7 278 10.9 285 6.4 264 4.3 254 5.5 281 8.7 231 5.1 229 11.3 269 4.6 238 4.8 242 6.2 243 9.5 240 4.4 270 4.2 275 3.5 280 7.7 241 18.5 279 5.9 213 11.9 239 11.1 268 5.4 237 6.5 Texture The results of the study of thin sec- tions of about 45 shales are summa- rized in appendix B. Sections cut both parallel and perpendicular to the bed- ding were studied with the petro- graphic microscope. The particle size values reported in the tables were determined microscop- ically. No attempt was made to obtain such values by wet sedimentation methods since particle size values ob- tained in this way reflect the method and intensity of disaggregation and dispersion rather than anything inher- ent in the shale. The clay mineral content of the shales studied varies from substan- tially 100 percent to 40 percent or less. In those samples with very high clay mineral contents, the clay mineral par- ticles are all very small (maximum size less than 5-10 microns and mostly less than 2 microns) and they show uni- form aggregate orientations parallel to the lamination. The maximum diver- gence from perfect parallelism is about 20°. There is no positive evidence of secondary growth of the clay mineral particles. In the very pure clay mineral shales, the maximum size of the nonclay min- erals is 10 microns or less. As the amount of nonclay minerals increases their maximum size also increases. Thus, in shales with 10 percent nonclay minerals, their maximum size is likely to be about 10 microns; with 30 per- cent nonclay minerals, the maximum size is about 50 microns; and with 50 percent nonclay minerals, it is likely to be as much as 100 microns. As the amount of nonclay minerals increases, the particle size of the clay minerals also increases. In shales with 50 percent or more nonclay minerals, the maximum size of the micaceous minerals may reach 150 microns. Mus- covite, biotite, and chloritic types of mica can be identified among these larger flakes. The various types of mica are present in distinct books; that is, each book homogeneous and apparently mono-mineralic. The large mica flakes in the shales with large percentages of nonclay min- erals show a general tendency towards uniform orientation parallel to the lam- ination. It is not perfect as many of the flakes are bent around quartz grains and some few are actually per- pendicular to the lamination. The boundaries between the large flakes of mica and the very small clay mineral particles is distinct. Frequently the books of mica show split or frayed ends. In most of the shales with large amounts of nonclay minerals the non- clay minerals are scattered through the clay minerals; that is, there is no tendency to concentrate the nonclay minerals and clay minerals in distinct layers, and the shale is composed of a heterogeneous assemblage of clay and nonclay minerals of all sizes. In some shales there is an occasional slight con- centration of nonclay minerals in small lenses or layers, and in a few samples PALEOZOIC SHALES OF ILLINOIS 25 studied there is a distinct layering of clay minerals and nonclay minerals. The very largest flakes of mica are found in the shales having a layering of components and in the layers com- posed of the nonclay minerals. The specific nature of the types of mica, the characteristics of the flakes themselves, and the association of large flakes with large and abundant nonclay minerals indicate that they have been deposited as such. There is no evidence of substantial secondary growth of the large flakes of the micas. The unsorted characters of many of the shales suggest fairly rapid accu- mulations of the shales showing this particular characteristic. All of the shales contain carbona- ceous organic material varying in amount from a trace to about 5 per- cent. In most of the shales studied, organic material makes up 1 percent or less of the total sample, and in general the amount is least in shales with the highest clay mineral content. The or- ganic matter is present in pigmentary material (that is, as particles too small to be recognized individually), in black opaque particles many of which are about 10 microns in diameter, and very rarely in waxy, yellow, translu- cent rounded or lenticular particles 10-30 microns in cross-section. The pigmentary material is most common in the purer clay mineral shales, and indeed is the only type of organic matter in many such shales. Very frequently the pigmentary organic matter is concentrated in streaks par- allel to the lamination. Also the larger discrete organic particles are very often concentrated in elongate patches parallel to the lamination. Quartz in rounded to angular grains is the most abundant nonclay mineral and frequently the only one that can be identified readily. Many of the larger grains show secondary growth and mutually accommodating boun- daries when they are concentrated in layers and lenses. In shales containing quartz in grains 30 microns or more in diameter and in abundance greater than about 30 percent, angular to rounded grains of feldspar can be iden- tified easily. The abundance of the feld- spar is probably less than one percent. Both plagioclase and orthoclase can be found. There is no indication that the feldspar is not depositional. A few of the shales studied are cal- careous. In almost all cases the car- bonate particles are fossil fragments. Frequently the fossil fragments have been broken and are strung out par- allel to the lamination. No attempt was made to investigate the possible ''heavy mineral" components of the shales. The major, and in many shales the sole, cause of the lamination is the ag- gregate parallel orientation of the clay mineral particles. Additional causes of the lamination are the concentration of organic material in streaks and the concentration of the quartz grains in distinct layers. In shales with appre- ciable amounts of organic matter the shale splits preferentially where the organic material is concentrated. Lamination is least well developed in the pure clay mineral shales with very small amounts of organic matter where the sole cause of lamination is the par- allel orientation of the clay minerals. In many such shales there is a ten- dency to a blocky fracture across the direction of parallel orientation of the clay mineral flake surface as well as parallel to the clay mineral surfaces. In such purer clay mineral shales, the clay mineral particles are very small. It seems that the clay mineral particles 26 ILLINOIS STATE GEOLOGICAL SURVEY Fig. 5. — Electron micrograph of DeLong shale (978 RRR), representative of the shales that have an abundance of kaollnite, X 10,000. should be larger than about 5 microns before their parallel orientation alone is likely to cause distinct fissility. No broad general correlation be- tween the textural data and the strati- graphic or geographic occurrence of the shales was found. Shales with large quantities of nonclay minerals as wxll as those with substantially no nonclay minerals appear to occur indis- criminately throughout the Paleozoic section and in various parts of the state. However, it cannot be stated, on the basis of the present work, that such a correlation does not exist with- in restricted stratigraphic horizons or within limited areas. Vastly greater detailed study of the texture of partic- ular horizons would be required to es- tablish such correlations. There appears to be no correlation between the clay mineral composition PALEOZOIC SHALES OF ILLINOIS 27 ■if'fM' Fig. 6.— Electron micrograph of Renault shale (578), representative of the shales that do not contain kaolinite X 10,000. and the textural characteristics. Thus the relative abundance of any of the specific clay minerals is not related to the total abundance of the clay min- erals. For example, illite is not neces- sarily the most abundant component of shales with the highest total clay min- eral content. Also there appears to be no relation between the clay mineral composition and the particle size and abundance of the specific clay min- erals. Electron-Microscopic Analyses Several Paleozoic shales were se- lected for study in order to determine if the electron microscope would show any difference in the morphology of different species of clay minerals. The shale specimens were cleaved along the partings and they were blown clean with a stream of air. These spec- imens were shadow cast by the carbon replica technique described by Collins (1955). 28 ILLINOIS STATE GEOLOGICAL SURVEY After shadow casting, the samples were placed in concentrated H2F2 for several minutes and then they were transferred to a dish containing water. Water was drawn up into an eye drop- per and this water was gently forced against the specimen lying under wa- ter. This action caused the carbon rep- lica to detach from the clay specimen and float. By means of a screen the replica was then changed from the water to H2F2 to dissolve out the re- mainder of the silicates. After remain- ing in the H2F2 for several minutes, the carbon replica was changed again to wash water and washed. After being washed for several min- utes, the stainless steel electron microscope screening was picked up by the edge with a pair of small forceps with weak tension. The screen was placed under a portion of the carbon replica and a section of the replica was torn off. The water was blotted from the forceps and from underneath the screen, which was allowed to dry be- fore being placed in the electron microscope. Micrographs were taken of the rep- licas, ranging in magnification from 1000 to 10,000 diameters. In general the electron micrographs show only that the shales are com- posed of flake-shaped units without re- solving them in particles of distinct outline or size, for example, figure 6. Occasionally, for example, as in fig- ure 5, the micrograph shows the pres- ence of fairly distinct hexagonal flakes in samples with a relatively high kao- linite content. REFERENCES Brammall, a., 1921, Reconstitution processes in shales, slates, and phyllites: Mineralog- ical Mag., v. 19, p. 211-224. Brown, G., 1955, The effects of isomorphous substitution on the intensities of (001) re- flections of mica- and chlorite-type struc- tures: Mineralogical Mag., v. 30, p. 657- 665. Collins, Barbara Schenck, 1955, Electro- microscopic study of clay minerals: Ph.D. thesis, Univ. of Illinois, Urbana. Grim, R. E., and Rowland, R. A., 1942, Dif- ferential thermal analysis of clay minerals and other hydrous materials: Am. Miner- alogist, V. 27, p. 746-761, 801-818. Harker, a., 1895, Petrology for students: Cambridge University Press, London. HUTCHINGS, W. M., 1892, Notes on the Ash Slates and other rocks of the Lake Dis- trict: Geol. Mag. v. 29, p. 154-161, 218-228. Johns, W. D., Bradley, W. F., and Grim, R. E., 1954, Quantitative estimation of clay minerals by X-ray diffraction methods: Jour. Sed. Petrology, v. 24, p. 242-251. MiLLOT, G., 1949, Relations entre la constitu- tion et la genese des roches sed'mentaire argileuses: Geologie Appliquee et Prospec- tion Miniere, v. 2, p. 350, Nancy, France. PiRSSON, L. v., and Knopf, A., 1926, Rocks and rock minerals: John Wiley and Sons, New York. Pettijohn, F. J., 1949, Sedimentary rocks: Harper Bros., New York. Ries, H., 1927, Clays, occurrence, properties, uses: 3rd ed., John Wiley and Sons, New York. Twenhofel, W. H., 1950, Principles of sedi- mentation: 2nd ed., McGraw-Hill Book Company, Inc., New York. PALEOZOIC SHALES OF ILLINOIS 29 APPENDIX A. Location and Description of Shales Number Formation County Megascopic description PENNSYLVANIAN SYSTEM McLeansboro Group 262 278 264 281 229 1105-1* U. McLeansboro U. McLeansboro U. McLeansboro U. McLeansboro U. McLeansboro Carlinville 35-55' ** Edwards t Edwards Edwards Bond Macoupin ] Williamson 1105-2 Carlinville 55'6 "-63' Williamson 1105-3 Trivoli 63'-148' Williamson ' 1105-4 Trivoli 150'-152'6" Williamson 243 238 270 Trivoli Trivbli Trivoli Sangamon Sangamon Madison i 1105-6 Trivoli 193-218' Williamson 4-10 280 279 1105-7 1105-8 Trivoli 349' Exline Exline Gimlet 239-281' Gimlet 283-294' White St. Clair St. Clair Williamson Williamson 1105-9 1105-12 1105-13 1105-14b Gimlet 294-315' Gimlet 349-361' Cutler 366-368' Cutler 372'6"-378'2" Williamson Williamson Williamson Williamson 4-39 4-45 1105-15 1105-16 239 202 237 Cutler 569' Bankston 585' Bankston 381-386 Bankston 397-400' Sparland Sparland Sparland White White Williamson Williamson Menard Livingston Menard 227 Sparland Marshall 285 Sparland Vermilion ' Carbondale 1105-17 269 1105-19 4-52 272 231 Brereton 407-434' Brereton Brereton 451-452' Brereton 610' Brereton Brereton Williamson Saline Williamson White Perry Bureau 254 Brereton Bureau 1096 1088 224 1105-22 1105-23 Brereton Brereton Brereton Brereton 462-488' Brereton 488-530' Christian Vermilion Tazewell Williamson Williamson Blue-gray, well laminated. Blue-gray, well laminated. Yellow-gray, sandy. Yellow-gray, laminated. Light gray, weathering yellow. Dark gray, gray, greenish gray, soft, sandy, uneven partings. Dark gray, soft, occasional thin lime streaks, uneven partipgs. Gray to dark gray, light gray, sand and lime streaks, soft to hard, even partings. Black gradihg into dark gray with soft even part- ings. Brown, laminated. Blue-gray, well laminated. Gray, the coarsely laminated beds have con- choidal fractures, others have shaly frac- tures. Gray to very dark gray; slightly sandy, even partings. Greenish uneven partings. Gray, massive to well bedded. Blue, poorly laminated. Gray to almost black, even partings. Gray, sandy, interlamination of dark gray shale and light gray sardstone uneven partings. Very dark to almost black, even partings. Very dark gray, even partings. Dark, slaty. Gray to dark gray — uneven partings, coal part- ings, grading to sandy at base. Dark gray, even partings. Gray, gritty, uneven partings. Gray to dark greenish gray, wavy partings. Gray, hard, siliceous, uneven partings. Gray-brown, poorly laminated. Blue-gray, poorly laminated, silty. Elue-gray, weathering in upper part, poorly lam- inated. Blue-gray, weathering brown, many concretions, well laminated. Gray, massive. Dark gray, sandy, hard, even partings. Blue, thinly laminated, lignitic. Dark gray almost black, slaty. Black, calcareous, fossiliferous, even partings. Gray grading to black, fissile at top. Light to dark olive green shale, sandy clay, poorly laminated. Light to dark blue, not sandy, plastic, poorly laminated. Dark gray, laminated. Gray, laminated. Gray, bedded, silty, full of concretions. Dark gray, sandy, uneven partings. Dark gray, sandy, rather hard, sand extremely fine-grained, even partings. * Compound numbers have same geographic location, may be either a core or outcrop. ** Depths indicate core samples. t Detailed locations can be obtained from Survey files. 30 ILLINOIS STATE GEOLOGICAL SURVEY Appendix A. — (Continued) Number Formation Countv Megascopic description 215 1105-25 St. David St. David 539-553' Knox Williamson 1105-26 Summum 553-602' Williamson 4-82 Summum 868' White 4-101 Lowell 862' White 1105-28 Lowell 625-652' Williamson B36 1105-29B Lowell Lowell 653-663' LaSalle Williamson 242 240 275 Liverpool Liverpool Liverpool Greene Knox Madison 241 1105-31 Liverpool Liverpool 665-682' Greene Williamson 1105-32 Liverpool 682-713' 4-105 977U 204 978NNN Liverpool Liverpool Liverpool Liverpool White Warren LaSalle Schuyler Blue-gray, weathering brown, well laminated. Light gray to greenish gray, calcareous at top, sandy at base, irregular partings. Dark gray to black, hard, even partings, lime buttons. Black with much pyrite with light bands near the middle, even partings. Dark gray, gritty, Lingula sp. common, even partings. Gray to dark gray, hard, slightly sandy, darker towards bottom, even partings. Dark gray, hard sandy but less than above, un- even partings. Gray black, well laminated. Gray, non-sandy, well laminated. Dark gray poorly bedded with hard streaks of sand. Yellow green. Dark gray to very dark gray, hard, slightly sandy in places, even parting. Very dark gray to black, hard, smooth, spar- ingly fossiliferous in lower three feet, even partings. Gray, even partings. Light gray, soft, laminated with sphenoidal fracture. Gray, well laminated. Gray to blue-gray, laminated. Tradewater 977W F117 213 977Y F119 4-115 977AA F121 4-124 Abingdon Abingdon Greenbush Greenbush Greenbush DeKoven 933' Wiley Wiley Davis 964' 1105-39 Stonefort 803-819' 4-128 Stonefort 982' 4-133 Stonefort 986' 978RRR Delong 977EE Delong 977GG Delong 978UUU Delong F124 Delong 977HH Delong 977JJ Delong 978XXX Delong F129 Delong 4-138 Macedonia 999' 1105-43 Macedonia 841-893' 4-167 Macedonia 1138' Warren Fulton Mercer Warren Fulton White Warren Fulton White Williamson White White Schuyler Warren Warren Schuyler Fulton Warren Warren Schuyler Fulton White Williamson White Light olive gray, weathering yellow-olive, very sandy near base, evenly bedded. Gray, iron-stained, laminated. Light gray, slightly bedded. Light olive gray, sandy at top, evenly bedded. Medium blue gray, silty, many concretions, ir- regular bedding. Dark gray, even partings. Gray, not sandy, finely bedded. Black, soft, clayey, poorly bedded. Dark gray, very carbonaceous, contains fine mica flakes, even partings. Clay shale with sandstone layers. Shale has waxy and pink-gray appearance, feels soapy, uneven partings. Black, carbonaceous, even partings. Black, fissile, calcareous in spots. Gray, clayey, weathered, poorly bedded. Light gray, sulfur stained, laminated. Yellow, hard, limonitic, laminated. Gray to yellowish green, clayey, poorly bedded. Light gray, soft, finely sandy, poorly bedded. Black, soft, iron and sulfur stained, concretion layers, evenly bedded. Black, sandy, grading down into sandstone, thinly laminated. Gray, evenly bedded, soft. Medium gray, thick-bedded, sandy, hard, ir- regular, iron stained. Black, fissile, fossiliferous, not calcareous. Dark gray, even partings for top 3^, bottom }/2 gray, sandy, interlaminated with gray sand- stone layers. Black, micaceous, contains pyrite replacements of plant remains, fissile, carbonaceous. PALEOZOIC SHALES OF ILLINOIS 31 Appendix A. — (Continued) Number Formation County Megascopic description 268 F130 F71 1105-45 1105-47 1105-48 4-181 4-195 4-208 977DD 978A4 977BB F133 1105-49 1105-50 1105-51 1105-53 1105-54 4-215 1105-55 1105-56 1105-57 1105-58 4-222 978C4 F135 F139 F140 1105-59 1105-60 1105-61 4-232 4-233 4-240 4-241 4-244 4-250 1105-62 1105-63 1105-64 1105-65 Macedonia Seville Jackson Fulton Seville Fulton Delwood 994-1011' Williamson Delwood 1023-1064' Williamson Delwood 1129-1135' Delwood 1243' Delwood 1349' Delwood 1373' Williamson White White White Pope Creek Tarter Tarter Tarter Warren Schuyler Warren Fulton Grindstaff 1185-1197' Williamson Grindstaff 1203-1207' GrindstafF 1207-1226' Williamson Williamson Grindstaff 1231-1234' Grindstaff 1240-1245' Williamson Williamson Grindstaff 1400' Grindstaff 1252-1266' White Williamson Grindstaff 1269-1343' Williamson Grindstaff 1344-1369' Williamson Grindstaff 1394-1426' Grindstaff 1456' Williamson White Babylon Babylon Pre-Babylon Pre-Babylon Schuyler Fulton Fulton Fulton Caseyville Pounds 1445-1475' Williamson Pounds 1483-1486' Williamson Battery Rock 1557-1604' Williamson Battery Rock 1584' White Battery Rock 1594' White Battery Rock 1630-1645' White Battery Rock 1663' White Battery Rock 1733' Battery Rock 1752' White White Battery Rock 1602-1611' Battery Rock 1618-1633' Lusk 1653-1670' Williamson Williamson Williamson Lusk 1670-1674' Williamson Gray, weathered to thinly bedded, concretions. Medium dark blue gray, well bedded, iron bands and concretions. Blue, gray, sandy, micaceous, thin sand layers alternate with shale layers. Very dark gray to gray, even partings at top, uneven partings near base. Shale, dark gray, sandy with light gray sand- stone streaks, hard, uneven partings. Dark gray, hard, sandy, uneven partings. Dark gray, very carbonaceous, laminated. Gray, laminated, micaceous, wavy partings. Gray, slightly micaceous, varve-like alternating light and dark beds. Yellow-gray to gray-yellow, evenly bedded. Blue-gray, thinly and evenly bedded. Gray, iron stained, sandy, poorly laminated. Dark gray, flaky, fossiliferous, leaves and small iron-stones. Dark gray with light gray sandstone streaks, even partings. Dark gray, wavy partings. Very dark gray to light gray, sandy, sandstone streaks, hard, even partings. Dark gray, sandy, uneven partings. Dark gray, irregular streaks of light gray sand- stone, uneven partings. Black, fossiliferous, poorly bedded. Dark gray and light gray sandstone interstrati- fied, slaty. Dark gray to light gray sandstone partings, hard, even partings. Dark to light gray sandstone layers interbedded, sandy, uneven partings. Black, hard, smooth, uniform, even partings. Gray, micaceous, sandy, very thin seams of sandstone, even partings. Blue-gray, thinly and evenly bedded. Dark gray, flaky, ferruginous. Dark gray to black, soft, flaky, ferruginous. Dark gray, flaky, slaty, ferruginous below, iron- stone lenses. Dark gray with light gray sandstone streaks and partings, wavy parting. Dark gray with light gray sandstone partings, wavy partings. Very dark gray with light gray sandstone streaks and partings, hard, wavy partings. Gray, sandy with minute lenses of sandstone evenly bedded. Gray, micaceous, a small proportion of thin streaks of sandstone evenly bedded. Gray with very thin layers of sandstone, uneven partings. Dark gray, very carbonaceous, plant remains, even partings. Dark gray, fine-textured, hard, even partings. Very dark gray, carbonaceous, composition Ir- regular, irregular partings. Dark gray, laminated. Dark gray, hard, sandy, uneven wavy partings. Gray, sandy, some limestone streaks, even part- Thinly stratified, dark gray sandy shale and light gray sandy limestone. 32 ILLINOIS STATE GEOLOGICAL SURVEY Appendix A. — (Continued) Number Formation County Megascopic description MISSISSIPPIAN 1105-67 1105-67a 1105-68 1105-69 1091 1090 577A 1105-70 1105-71 1105-72 1105-73 579B 1105-74 1105-75 1105-76 1105-77 576 1105-79 575B 575A 1092 578 1086 1087 1095 DS38 DS51 DS45 546 1093 547 DS7 580 NF42 Chester Kinkaid 1688-1711' Williamson i Kinkaid 1718-1746' Kinkaid 1777-1781' Williamson I Williamson Kinkaid 1790-1801' Williamson < Kinkaid Kinkaid Kinkaid Pope Pope ] Pope 1 Clore 1834-1848' Williamson Clore 1867-1872' Clore 1919-1944' Williamson ] Williamson Clore 1949-1951' Williamson ' Clore Menard 1976-2018' Saline Williamson < Menard 2019-2035' Williamson Menard 2041-2061' Williamson i Menard 2078-2088' Williamson Menard Waltersburg 2125-2136' Johnson Williamson ] Vienna Vienna Renault Renault Johnson < Johnson < Johnson < Hardin ( Iowa Rosiclare 2971' Rosiclare 2984' Ste. Genevieve 3252' Hannibal Hannibal Hannibal Hannibal Wayne Wayne Jefferson Calhoun ' Pike Pike Union DEVONIAN New Albany 2519' New Albany Clinton Union SILURIAN Sexton Creek Alexander Green to light gray, calcareous, interlayered with thin limestone beds, uneven partings. Same as above. Dark gray, laminated with limestone streaks, very hard, fossiliferous, occasional slicken- side. Gray to dark gray, dark gray portions even parting, gray portion wavy partings. Yellow. Red. Blue gray, interbedded with limestone and chert, even partings. Dark gray, laminated in upper portion, sandy streaks and partings, micaceous near base. Dark gray, fairly soft, wavy partings. Black, even partings near base, bituminous, cut by limestone beds. Very dark gray, fairly soft, light gray irregular sandstone and inclusions, fossils toward bottom, uneven partings. Dark gray, well laminated. Gray to very dark gray, sandy and calcareous, even partings, soft. Dark gray, soft, occasional lime streaks, wavy to even partings. Gray to dark gray, almost limestone, wavy part- ings. Light gray to very dark gray, even parting, soft, even partings. Blue. Light gray, fairly hard limestone and sandstone streaks, wavy partings. Green, wavy, poorly laminated. Green, wavy, poorly laminated. Greenish gray. Green, poorly laminated. Well laminated. Well laminated. Green, flaky. Green, siltstone. Green, soft. Blue, gray, calcareous, uneven partings. Black, even partings. Black, even partings, fissile. Gray to greenish gray, calcareous, gross lamina- tion. Maquoketa Maquoketa ORDOVICIAN Monroe Greenish gray, clayey, even partings. Kane Gray, calcareous, weathered, poorly laminated PALEOZOIC SHALES OF ILLINOIS 33 ."2 B o I gSnO9UB|p0SlJ^ SUOJDIUi UI 3ZIS iunuiixBj\i ^SDU^punqy to «-i_o > o OJ > ■l-l > o.t- > a S.S o C/3 o o o IZ) o c« O 3 3 3 3 3 u IS 16 IS CO o ^ o ^ Dh cx Ph a Cu Dh Vh S-H Oh O o o o o LO o o O oo O (N c/D O <-i (N OO -"f ON 1—1 ^ r-- ^ (N OO On C-1 ro O 34 ILLINOIS STATE GEOLOGICAL SURVEY «o (U Ul 3 +-< X ,*" H "rt gSn03UB]p3SIJ/^ (J OLh Oh Oh 0^ a S a^ c^O^ B hJ ^ ^ hJ H-1 O O O Lo o 1— I o hJ J J V V -^ U UU HU H^ HU< c XO 1-H O O ^ - V - ^ V < H U U H ^ ^ o -C o -C j5 ■1-1 bfl !? bfl ^ ^ !>. a >. ,° R rt tjj rt bO fcja XJ ^ >. T3 t^ '^ >> J S hJ rt C J H-1 -J ^ -n u Tl Td s t3 nzS'O 'Td'O G o <-) o LO o o o o u^ O o o •^ ro -o ^ Ci oo oo o CO Tt^ Tti >-n u-^ VO LO W-1 H lo H o O lo u-^ ^H iH ■> ii > > '> a o.- a^ o -t; o o O *-i CJ O IS IS 3 3 m CN oo ^D t^ CO U^ CN r- w-^ cN ^ ^ rJH CS . rt 3, 3. 3. 3 rt 3 rt 3, 3, 3, cS 3 rt 3 rt 3, a (yu cph u o* c a c^u au a a cu cu au a S2 K o o CO u, Ui O O Oh Oh g2 o o (N o o o CO W-1 O O o vO OO O OO OO OO CO w^ OO O OO O OO < OO ,— i CO ^ Q G CO CO Q OC en O .S'3 .0) g c^-;:: S • '^ 2 g «2' rt o oj g.2 2 5-1 a +-''^ a fi fl o O 5-1 -H u o c ^ >> q; g C 03 i . f> o o S •S-S^ II 5h':;j i= (-< 5h o — w 8°o5o II "i*"^ cu ' ' !3 CD j3 ® till I f^ d ^ d c^ c-^ g-S « •^ii^iit; II g Oj S^ I' CJ O O D" Illinois State Geological Survey Report of Investigations 203 35 p., 5 figs., 5 tables, app., 1957