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 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 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 4.5 meter sequence of channel sandstones. An additional outcrop located 15 km northeast of the primary outcrop is also examined in this study.

Structures indicating rapid deposition of sediment by debris flows, sliding, and detachment of unlithified to semi-lithified beds are found within the lower deformed units. Tilted and disaggregated beds, inconsistent foliation orientations in fine-grained clastic rocks, folds, pinch and swell structures, 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 to be made available for public access. Microscopic analysis will determine composition and provenance, as well as provide structural evidence. This research will add to the understanding of depositional processes, including landslides and similar geologic hazards, following tectonic events.