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Abstract: Subsurface Sequence Stratigraphy of Early Chesterian (Mississippian) Mixed Carbonates and Siliciclastics, Illinois Basin

TREWORGY, JANIS D., W. JOHN NELSON, LLOYD C. FURER and BRIAN D. KEITH

Nine 4th-order (400 k.y.), disconformity-bounded, mixed carbonate-siliciclastic sequences were deposited in the Illinois Basin during the early Chesterian. We constructed detailed lithologic cross sections based on wireline logs, sample studies, and a few cores of the Ste. Genevieve Limestone through Glen Dean Limestone interval.

The cross sections show an upward change from carbonate-dominated sequences to mixed carbonate-siliciclastic sequences. In the subsurface, the sequences are commonly capped by subaerial exposure surfaces, that are marked by 1) red and green mudstones, interpreted to be paleosols, or 2) deeply incised valleys. In the lower part of the stratigraphic interval, sandstone-filled valleys are narrow and limited in area. In the upper part of the interval, the valleys are broad, anastomosing systems, some of which are several 10s of km wide. Incised valleys occur mainly in the basin interior whereas thick paleosols developed on the basin margins. The siliciclastics have multiple source areas, including the southeastern Canadian Shield, the Wisconsin Arch, the Transcontinental Arch, and the Ozark Dome.

Subsurface mapping demonstrates that there was local tectonic activity during the early Chesterian. Active structures included the northeast-trending Fluorspar Area Fault Complex and Wabash Valley Fault System, minor structures on the western shelf, and the DuQuoin Monocline. Because the nine sequences can be traced basinwide, however, the major control on sequence development was probably eustasy.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky