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Abstract: Petrographic Differentiation of Basal Pennsylvanian and Mississippian (Chesterian) Siliciclastics in Western Kentucky

Department of Geography and Geology, Western Kentucky University, Bowling Green, KY

The Mississippian-Pennsylvanian unconformity (Kaskaskia Absaroka Sequence Boundary) in the Illinois Basin represents the development of a series of paleovalleys that incised into a myriad of geologic units including shales, limestones, and sandstones. Stratigraphic relationships associated with the unconformity can be traced in outcrop but are much more problematical in the subsurface. This is especially true if there is no pronounced lithologic contrast between Pennsylvanian and underlying units based on electric-log signature. In western Kentucky, the basal Pennsylvanian (Caseyville) is typified as either conglomerate, conglomeratic sandstone or sandstone. Confusion on the correct stratigraphic placement of the Mississippian-Pennsylvanian contact in the subsurface has been somewhat exacerbated by the lack of systematic petrographic characterization and limited floral and faunal zonation of relatively coarse-grained, Chesterian and basal Pennsylvanian siliciclastics. Few petrographic studies comparing and contrasting Chesterian and basal Pennsylvanian rocks have been conducted. The efforts of this study in Butler, Edmonson and Warren counties suggests that there are important textural and compositional differences that should be scrutinized in cores in existing fields. Properly locating the contact via petrographic methods should enhance exploration and development of hydrocarbon and groundwater resources along the southeastern flank of the Illinois Basin.

AAPG Search and Discovery Article #90930©1998 AAPG Eastern Section, Columbus, Ohio