Abstract

Lower Paleozoic sedimentary rocks of the central Appalachian Orogen of eastern Pennsylvania accumulated in tectonically diverse environments. In particular, rocks of the parautochthonous Great Valley sequence reflect stabilization and subsequent foundering of the proto-North American carbonate platform, whereas partly coeval rocks of the allochthonous Hamburg klippe reflect sedimentation in continental margin and oceanic (abyssal plain and trench) environments. Stratigraphic relationships among these varied units are consistent with a foreland basin evolution model involving flexure of a viscoelastic plate (proto-North American craton) under a northwest-migrating thrust terrane. Uplift and exposure of the carbonate platform in Middle Ordovician time was a result of the advance of the thrust terrane composed of oceanic and trench deposits toward proto-North America. Flexural upwarping of the platform was accompanied by load-induced subsidence of the continental margin to the southeast of the exposed forebulge, and accumulation of black mudstone and carbonate turbidites. Uplift and exposure of the carbonate platform was greatest at the inferred New York continental promontory and it is likely that this continental headland was partially subducted. Rapid subsidence of the exposed platform, apparently in response to emplacement of the overthrust terrane onto continental crust, is recorded by an Upper Ordovician transgressive sequence of black shale and limestone (Jacksonburg Limestone) that disconformably overlies platform carbonate rocks (Beekmantown Group). As the tectonic wedge continued to move to the northwest, its weight was distributed over thicker continental crust, resulting in widespread subsidence. A conformable shallowing-upward clastic sequence (Martinsburg Formation; Bald Eagle-Juniata sequence and equivalents) accumulated in the elongate foreland basin in Late Ordovician time. Cessation of crustal convergence and the resultant heating of the thickened continental crust in the area of the partially subducted New York promontory resulted in uplift and exposure of the overthrust terrane and parts of the immediately adjacent foreland basin in eastern Pennsylvania and New Jersey. The entire foreland basin, including the exposed area, was subsequently covered by an Upper Ordovician-Lower Silurian transgressive sequence of coastal marine and fluvial deposits (Tuscarora and Shawangunk formations).