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Clay mineral analyses of Pennsylvanian sediments from the Illinois basin indicate that three major processes account for the wide range of clay mineral composition in these sediments: preservation of source material, soil formation, and diagenesis. Most shales, underclays in areas of rapid sedimentation, and insoluble residues of limestones contain detrital illite and lesser amounts of chlorite, kaolinite, and mixed-layer illite/smectite (I/S). Source material composition does not seem to have varied significantly during the Pennsylvanian era. Most underclays, clays associated with coal, and some shales and claystones consist of source materials modified by soil-forming processes. Variation in clay mineral composition results from alteration in two distinct soil-forming environments. In areas of relatively low subsidence and sedimentation rate, the prolonged action of plant roots and/or percolation of water resulted in the formation of underclays and claystones containing poorly crystallized “soil” kaolinite, mixed-layer ka-olinite/smectite (K/S), and minor I/S and vermiculite. The extraction of K+ and Mg2+ by growing plants under saturated soil conditions (i.e., in underclays, coals, shales, and claystones) in other areas of the basin resulted in the formation of I/S and well-crystallized kaolinite with calcite, ferroan dolomite, siderite, pyrite, and/or marcasite. Sandstones commonly contain abundant authigenic kaolinite formed by post-depositional alteration of source materials. The lack of evidence of diagenetic formation of illite and chlorite suggests that the source detritus was derived by mechanical erosion in highland areas and that, similar to modern tropical basins, the intervening lowlands were protected from erosion by lush vegetation.

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