Trentonian and Chazyan carbonates of eastern Pennsylvania provide important information regarding tectonic activation of the North American carbonate platform during the Middle Ordovician. In particular, the well-studied Lehigh Valley sequence records Chazyan-Blackriverian uplift and erosion of the Beekmantown carbonate platform (Black River hiatus) followed by rapid subsidence and sedimentation of the transgressive Jacksonburg Limestone in Trentonian time. A more detailed reconstruction of these events is gained from analysis of allochthonous Chazyan carbonates that tectonically overlie the Lehigh Valley sequence. These rocks, the Moselem Member of the Hamburg klippe sequence, include 230 m (755 ft) of ribbon limestone, black shale, slump deposits, and minor flint-bearin carbonate-clast conglomerates. Abundant gravity-flow deposits (ribbon limestones, conglomerates), widespread black-shale sedimentation, and slump and sediment-creep folding are indicative of deposition on a subsiding, low-angle depositional slope. Carbonate mud and flint pebbles were derived from the eroding Beekmantown platform, which palinspastic reconstructions place northwest of the Moselem depocenter. Synthesis of Chazyan-Trentonian stratigraphic relations of eastern Pennsylvania suggests the following scenario for this part of the orogene. During the Chazyan-Blackriverian, uplift and erosion of the Beekmantown platform were concomitant with sedimentation of the Moselem Member in a subsiding southeasterly foredeep. In the Trentonian, rapid collapse of the exposed platform was follo ed by deposition of the transgressive Jacksonburg Limestone. This scenario is similar to Middle Ordovician events in other parts of the orogene (e.g., Trenton Group, central New York) and accords well with sedimentation patterns associated with downbending of the Australian plate in the Timor Trough.

End_of_Article - Last_Page 1922------------