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Lacustrine / Palustrine Carbonate – Mudrock Cycles of the Upper Pennsylvanian Monongahela Group of the Northern Appalachian Basin: Lithofacies, Architecture and Geochemistry

Abstract

The Upper Pennsylvanian (Gzhelian) Monongahela Group in the northern Appalachian Basin of the upper Ohio River Valley is composed of sandstones, mudrocks, and coals along with carbonates, which characterize a significant portion of the succession. While a large volume of work has been produced describing the range of carbonate facies present in these units and interpreting the environments of deposition, little work has focused on: 1) delineating the vertical stacking of lithofacies and identification of significant facies transitions, 2) recognizing key surfaces and stratal geometries, 3) placing this facies architecture into a regional stratigraphic framework. This work examines the vertical succession of facies and cyclicity observed in the Monongahela in large, fresh road cut exposures along Ohio Route 7 south of Wheeling, West Virginia. Preliminary examination of these units indicate that the thick (multiple bedset) carbonate – dominated units are characterized by sub – meter, carbonate – mudrock cycles. Mudstones are dark gray to pale gray – green and laminated while carbonate lithofacies range from barren mudstones to skeletal (dominantly ostracode) and lithoclast wackestone, packstone and rare grainstones. Although within – bed scour and juxtaposition of carbonate facies is not uncommon, overall bed thickness and geometries are sheet-like. The tops of carbonate beds can display apparent exposure features such as loaded desiccation cracks and rhizo-mottling / rhizoliths. Mudstones often display abundant stained, high angle and bed – parallel fractures. These facies together with petrographic observations support a lacustrine to palustrine interpretation. These bedding cycles can be arranged into bedset successions that resemble cleaning – upward parasequences, ranging from lower thin – bedded wackestone facies to thick mudstone; however the nature of these “parasequences” requires further analysis. Although “parasequence” stacking has not yet been recognized, some of the Monongahela carbonate units display significant, multiple bedset truncation and fill. These features indicate significant loss of accommodation with associated truncation and bypass, representing sequence boundaries. This hierarchy of facies cyclicity and bounding surfaces, supplemented with isotopic and major element geochemistry, will provide a robust dataset to interpret the tectonic and climatic dynamics controlling deposition in this large foreland lake system.