EVIDENCE FOR MULTI-STAGE DEFORMATION WITHIN PENNSYLVANIAN ROCKS IN THE DUNKARD BASIN, SOUTHWESTERN PENNSYLVANIA

Pennsylvanian clastic rocks and minor carbonate units, which comprise the lower Casselman Formation of the Conemaugh Group crop out along an 800 ft railroad cut with topographic relief of up to 100 ft in southwestern PA. Rocks at this outcrop record tilted beds, listric normal faults, a reactivated angular unconformity and small-scale asymmetric folds. These structures are not typical of the general flat-lying rocks that characterize this region of the Appalachian Plateau and suggest at least 3 episodes of deformation that record sliding and/or extension along detachment faults. The lowest exposed units, tilted 30 - 40° degrees to the south, comprise decimeter scale alternating shale and lithic-quartz arenite beds with a 1 meter thick massive limestone unit. Locally developed within the tilted blocks are small-scale, gentle to open asymmetric detachment folds that record southward transport. A prominent angular unconformity separates the tilted blocks from overlying, sub-horizontal, medium to thick-bedded cross-stratified quartz arenite. Listric normal faults that presumably underlie the tilted section do not cut the unconformity. Stratigraphically above the small-scale detachment folds and just below the unconformity is a zone of variable thickness comprised of disrupted siltstones containing siderite nodules and small stringers of coal. Locally, foliation and fragmentation of the coal suggest that the erosional surface was reactivated during detachment of overlying beds. In the sandstone above the unconformity, clastic dikes that cut discontinuous thin, tabular, coal beds and conjugate shear fractures record injection of fluid-rich sandstone and brittle deformation. The unconformity and overlying subhorizontal sandstone are cut by a late brittle listric normal fault that dips to the north and records about 1 meter of displacement. We suggest that these structures record transport and deformation of semi-lithified beds into the Appalachian Basin following uplift.