Abstract: Origin of Cyclicity in Conemaugh Formation in Appalachians of West Virginia

Santosh K. Ghosh

The upper Conemaugh sequences (about 200 m thick), exposed along Interstate 77, north of Charleston, West Virginia, form part of the southern Dunkard basin (of late Paleozoic age). The cyclic pattern, determined by Markov chain analysis as well as by visual inspection, typically consists of alternating fining-upward and coarsening-upward sequences, a pair of which constitutes a cyclothem. The typical fining-upward sequence consists of conglomerate, sandstone, siltstone, silty shale, mud-clay rocks, and limestones, overlain by a coarsening-upward sequence of silty shale, sandy siltstone, and sandstone. The cyclicity is interpreted to be the result of lateral shifting of subdeltaic lobes, initiated by crevassing of levees. The coarsening-upward sequence arises from superposition of leve -splay assemblages consisting of silty shale, siltstone, etc. The sequence overlies interdistributary-bay mud rocks and is overlain by channel sandstone. The fining-upward sequence, however, represents either a succession of distributary-bar (or point bar) and associated top-stratum deposits or, more commonly, a progressive silting of crevasse channels as the sediment influx from the main channel decreases. Lack of basinwide persistence of cycles coupled with the extensive lateral juxtaposition of high- and low-energy environments throughout the sequences preclude any significant allocyclic control for these deposits.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma