DOES THE PENNSYLVANIAN-AGED MORGANTOWN SANDSTONE RECORD RAPID TRANSPORT INTO THE DEPOSITIONAL BASIN FORMED DUE TO UPLIFT OF THE APPALACHIAN MOUNTAINS? DEPOSITIONAL AND STRUCTURAL FEATURES OF THE BASAL MORGANTOWN SANDSTONE, PITTSBURGH, PA

The Morgantown Sandstone (Pennsylvanian) of the Casselman Formation of the Greater Pittsburgh Region has traditionally been classified as a fluvial channel deposit (Shultz and Harper, 2002). Recent work suggests a different interpretation for deposition (Orsborn, 2014). Unique characteristics found at multiple outcrops suggest an alternate hypothesis of deposition such as debris flow and rapid transport and deposition of semi-lithified beds during tectonic uplift of the Appalachian Mountains to the east.

This study continues the research of Orsborn (2014) and closely examines the structural features and composition of the basal Morgantown Sandstone. The primary outcrop being studied and mapped is a 315-meter long exposure of the sandstone located in the Appalachian Foreland Basin in southwestern PA. This outcrop displays an approximately meter-thick lower clast- to matrix-supported pebble conglomerate that locally contains clasts of platy shale and isoclinally folded fine sandstone. Above this basal unit is an undulating, highly deformed sandstone with abundant stringers and lenses of coal, and chaotic beds of foliated shale, siltstone and fine sandstone that contain cobbles and boulders of coarser-grained nonfoliated sandstone. These deformed beds are succeeded by a thick sequence (4.5m) of channel sandstones. Structures indicating rapid deposition of sediment by debris flows, sliding and detachment of semi-lithified beds are found within the lower deformed units. Tilted and disaggregated beds, inconsistent foliation orientations in fine-grained clastic rocks, folds, large sandstone clasts, and lack of sedimentary structures indicative of fluvial deposition provide evidence of a different mechanism of deposition. Cross-sections of this outcrop are being created and will be uploaded to Google Earth. In addition, detailed descriptions of the deformed basal beds will be made by studying thin sections. This work will help document rapid deposition of the sandstone into the foreland basin due to tectonic uplift of the Appalachians and will contribute to the geologic understanding of the depositional processes following uplift of mountain belts.