Abstract

Regional study of the Summum, St. David, and Brereton cyclothems of the Carbondale Formation of Illinois and the Marmaton Group, Des Moines Series, of the Mid-Continent is based on more than 1000 columns with spacing of one per township in Illinois, Indiana, western Kentucky, Missouri, Iowa, eastern Kansas, and northeastern Oklahoma. Environmental patterns for 29 depositional stages have been mapped by plotting the proper lithology at each control point. Lithologic patterns are interpreted in terms of environments of deposition, considering evidence from faunas, floras, and sedimentary structures.

The maps show fluviatile and delta environments with deposits from the northeast that formed level depositional plains which became coal swamps following a slight rise in regional water level. Marine waters from the west, advancing west of the Ozarks and north of the Mississippi River arch, inundated large areas of coal swamps in the Illinois basin. Additional deltas and coal swamps in southern Illinois and Indiana and western Kentucky divide the Summum into two cyclothems and the Brereton into three. Only one cycle is recorded in each in adjacent shelf areas. The maps show that marine conditions prevailed in Missouri and Kansas during times that fluviatile, deltaic, and coal swamp environments were in Illinois, Indiana, and Kentucky.

Isopach studies of the intervals show pronounced thinning on the Mississippi River arch and slight thinning on the LaSalle anticline. Accessibility of clastic sediment was probably a more important determinant of thickness of sediment accumulation than local tectonism during sedimentation.

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