Mapping deformed Pennsylvanian Rocks in the Dunkard Basin, Southwestern, Pennsylvania using GigaPan Images

Patricia Campbell, Slippery Rock University
Matthew Graves, Slippery Rock University
Michael Stapleton, Slippery Rock University
Thomas Anderson, University of Pittsburgh

Panoramic images of an outcrop produced with a GigaPan robotic camera mount and Canon EOS 60D camera are used to enhance the quality of structural analysis of deformed Pennsylvanian rocks that crop out along a 250 meter-long railroad cut in southwestern PA. Clastic rocks of the lower Casselman Formation of the Conemaugh Group 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 three episodes of sliding and 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. Locally developed within the tilted blocks are small-scale, tight 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. The basal unit above the unconformity varies from 10's of centimeters to about a meter and comprises fractured siltstone and fine-sandstone, disrupted coal beds and rounded cobbles. The deformation recorded within this unit suggests reactivation of the erosional surface during detachment of the overlying sandstone. Above the unconformity, clastic dikes cut thin, discontinuous coal seams, recording the injection of fluid-rich sandstone. Conjugate shear fractures record brittle deformation of semi-lithified beds. A late north-dipping listric normal fault that records about one meter of displacement cuts both the angular unconformity and overlying quartz arenite

Although rocks of the Dunkard Basin rarely show the complex structures recorded at this outcrop, we suggest that the normal faults and detachments record transport into the Appalachian Basin penecontemporaneous with accumulation of the Casselman Formation.