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Regional mapping of the sedimentary environments of the different stages within the Liverpool, Summum, St. David, Brereton, Sparland, Gimlet, Exline, Trivoli, and Carlinville cyclothems of Illinois has been completed, or is in progress, for western Kentucky, western Indiana, Illinois, Missouri, Iowa, eastern Kansas, and northeastern Oklahoma. These cyclothems cover the upper half of the Des Moinesian Series and the basal part of the Missourian Series of the Mid-Continent region. The maps are based on approximately 1,100 control points which, where possible, have a preferred geographic spacing of one point per township. Lithofacies maps were prepared for each depositional stage by placing, at each control point, the lithology, thickness, and other characteristic features s ch as fossils--plant and animal, color, and sedimentary structures. After the lithofacies maps were completed, the sedimentary environments were inferred.
A sequential viewing of the environmental maps gives an idea of the dynamic changes that occurred within the area and illustrates the alternate predominance of the opposing forces of marine transgressions and deltaic growth. The maps show that the western part of the region experienced predominantly marine conditions while the eastern part was subjected to predominantly non-marine conditions. The non-marine sands and shales of the delta and fluviatile environments followed the north, northeast, and east paleoslope in the region and formed platforms on which coal swamps could develop later. The marine water transgressed from the Mid-Continent, north of the Ozarks, into the Illinois basin, depositing marine limestones and shales.
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