FACIES ARCHITECTURE AND ALLOCYCLES IN THE GLENSHAW FORMATION (UPPER PENNSYLVANIAN), UPPER POTOMAC BASIN, MARYLAND/WEST VIRGINIA

A stratigraphic and sedimentologic analysis was carried out using outcrop and drill core data from the Glenshaw Formation in the Davis, Table Rock and Gorman 7.5-minute quadrangles. The study area occurs along the eastern portion of the Dunkard Basin where the Glenshaw reaches its maximum thickness of over 100 m. Sedimentary facies include fluvial-estuarine channel fills, splays, coastal plain lakes and swamps, and shallow marine deposits. Roof shales above the Upper Freeport, Mahoning, and Lower Bakerstown Coals contain well-preserved flora, and freshwater invertebrates, and graded, clay-draped silt laminations that exhibit thick-thin couplets and bundles suggestive of upper estuarine conditions.

A high resolution, statigraphic framework was developed using coal beds, paleosols and marine units. Eleven to twelve paleosol-bounded units are distinguishable which can be correlated with glacioeustatic allocycles described by previous workers elsewhere in the Dunkard Basin. Laterally persistent paleosols may have formed during falling base level, at times when the rate of sea level fall exceeded the rate of basin subsidence. River incision accompanying relative sea level fall would have resulted in sediment bypassing of interfluves, allowing extensive pedogenesis to take place. Rising relative sea level caused a rise in the water table in the coastal plain, which in turn provided for peat accumulation, though it’s extent and thickness were influenced by variations in topography. The link between sea level rise and peat accumulation is supported by the presence of shallow marine to tidal facies in roof rocks that overlie most, if not all, all significant coal beds.