Fossil Distortion and Penetrative Rock Deformation in the Appalachian Plateau, Pennsylvania

Pennsylvanian age rocks of the eastern Appalachian Plateau Philipsburg quadrangle have undergone up to 10 per cent penetrative shortening in northwest-southeast directions as proven by distortion of the fossil Lingula carbonaria. Fossil distortion is manifested by changed width/length ratios, microjoints, and minute N. 44° E. to N. 58° E. oriented crinkles which do not appear in surrounding clay and silt shales. Shales have apparently adjusted to shortening shown in the fossils by intergranular movements. Fossil distortion and penetrative rock shortening is interpreted as the first episode of lateral tectonism during which rocks underwent lateral compaction before beginning major folding. Associated structures are systematic joints which intersect crinkles at 75°-90°.