The Brownsville paleovalley in Edmonson County, Kentucky and the Evansville paleovalley in southwestern Indiana are but two examples of major valley systems associated with development of the Kaskaskia-Absaroka Sequence Boundary in the Illinois Basin region. Other workers have produced subcrop maps of the Mississippian-Pennsylvanian unconformity surface in Illinois, Indiana and Kentucky. Little regional work elucidating the detailed nature of tributary valley systems however, has been completed in Kentucky. Correlation of the outcrop with mostly electrical logs from the outcrop belt along the Brownsville paleovalley and into the subsurface of Butler, Edmonson, Grayson, Ohio and Warren counties shows the dissected nature of the pre-Pennsylvanian surface. Local relief mapped along this sequence boundary commonly exceeds 30 meters, particularly in the lower reaches of tributary valleys proximal to trunk systems. Preliminary petrographic work on rocks in this area has revealed feldspar and fine-grained sedimentary rock fragments to be more abundant in Chesterian siliciclastics versus basal Pennsylvanian siliciclastics. Outcrop investigation of Pennsylvanian and Chesterian sandstones suggests several depositional environments ranging from fine-grained tidally influenced channel deposits to coarse-grained, bedload dominated fluvial systems. Future work will entail establishing petrofacies characteristics versus environments of deposition and well-log signature to better aid in differentiating Chesterian and basal Pennsylvanian sandstones in problematic areas.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana