TECTONIC AND EUSTATIC CONTROLS ON THE CHARACTER OF THE MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY AND EARLY PENNSYLVANIAN INCISED VALLEY-FILLS

This study utilizes outcrops and well logs and cores in Ohio and Kentucky to flesh out the nature of paleodrainage systems and their depositional architecture during the late Mississippian and earliest Pennsylvanian in the central Appalachian Basin. The Mississippian-Pennsylvanian boundary is marked by a prominent glacioeustatic unconformity that separates the Kaskaskia and overlying Absaroka cratonic sequences. As Lower Pennsylvanian strata onlapped this unconformity, a complex depositional history evolved that was that was strongly influenced by tectonics related to the early stages of the Allegheny Orogeny. This regional unconformity has up to 137 m of erosional relief due to deep paleovalleys cut by Early Pennsylvanian streams. In Lawrence County, Ohio, the Sharon Sandstone overlies this unconformity and is thickest in the subsurface along a paleovalley which trends south into Kentucky. Thrust loading during the Early Pennsylvanian produced a migrating forebulge that forced longitudinal drainage to onlap the craton margin, while creating a narrow, tide-dominated seaway during glacioeustatic highstands.

A wide range of facies occurs in the IVFs, including red bed sequences between the Mississippian Pennsylvanian unconformity and the Olive Hill Clay. The red-bed IVF along KY Route 10, located 11 km west of Lloyd KY is 12.5 m deep and 366m wide contains thin-bedded, gently inclined red mudstone with both clast and mud-supported lenses of white chert clasts from Slade Formation. The spores from the Anthony coal here indicate these strata are likely from the lower part of the Grundy formation. In Ohio Rt. 823 roadcuts northeast of Sciotoville contain a redbed IVF 15 m deep and 300+ m wide. Red bed colluvial facies interfinger with black silty shales and lenticular quartzose sandstones with tidal bundles and clay-draped ripple bedding. Wherever the Olive Hill Clay is found in this area, it occurs as lenses occupying paleotopographic lows and is probably of lacustrine origin. The red-bed valley-fills are likely to have been river valleys that were SE flowing tributaries to the major South flowing trunk stream. These tributary valleys became deactivated, lacking through-flowing drainage. Mass wasting of soils partially filled margins of valleys and rising sea level created tidally influenced backwater areas.