DEPOSITIONAL SYSTEMS AND FACIES ARCHITECTURE OF THE PITTSBURGH SANDSTONE AND ASSOCIATED FACIES, UPPER PENNSYLVANIAN MONONGAHELA GROUP, PUTNAM COUNTY, WEST VIRGINIA

The recent construction of an 8 mile segment of Rt. 35 between Winfield and Scott Depot has produced a series of exceptional exposures of the Monongahela Group. Stratigraphic sections were measured and described. Component sedimentary facies were identified based on lithology, sedimentary structures, fossils, paleocurrents and facies geometry. The elevation of key beds was measured by altimeter. Correlations were facilitated by comparison of section elevations to structural contours on the Pittsburgh Coal at the base of the Monongahela Group. Sandstone isopach maps were made by combining outcrop data with gamma ray logs from oil and gas wells. Interpretation of facies and depositional systems was made based on comparison with facies models and previous work. Sandstones formed in high sinuosity fluvial channels and as levees and splays. Single story channel-fills are typically about 6 m thick. The repeated development of shale-draped foresets in some cross-bed sets inidcate periodic slackwater events between episodes of bedform migration, and suggest the possibility of some tidal influence and upper estuarine conditions. Siltstones and shales formed in deactivated channels and in flood-basins. Hackly, calcareous mudstones with root structures, slickensides, and horizonization formed through pedogenesis during times of nondeposition or slow deposition. The considerable thickness of the Pittsburgh Sandstone in linear or bifurcating belts of up to 20 m or more both in northern West Virginia and in our study area, along with deep incision that has removed the Pittsburgh Coal suggests that it represents an incised valley-fill rather than simply a channel-fill. Well-developed paleosols in our outcrops represent significant amounts of time when sediment bypassing occurred perhaps accompanying valley incision. If they can be shown to be laterally persistent, these paleosols may represent interfluvial sequence boundaries. Our results are preliminary as fieldwork is ongoing.