ABSTRACT

The Tuscarora Formation (Lower Silurian) of central Pennsylvania owes its origin to a combination of tectonic, eustatic, and depositional events. Tuscarora deposition began at the beginning of the Silurian Period at the time of both glacioeustatic sea-level rise and renewed tectonic elevation of the Taconic source terrain. Braided alluvial systems transported coarse sediment northwestward to a wave-dominated coast, yet sea-level rise was so rapid that the shoreline retrograded from western to eastern Pennsylvania. At the time of transgression, shoreline characteristics varied from high-energy beaches in central Pennsylvania to estuaries along the south-central border region of Pennsylvania. Seaward of the shoreline, sand was fashioned into shelf sand-wave complexes, analogous to moder shoreface-connected sand ridges. Much of the Tuscarora Formation west of the Susquehanna River accumulated in this shallow-marine shelf environment. Later in the Early Silurian, eustatic sea-level fall resulted in progradation of lower energy coastal sand/mud flats northwestward over the former shelf, shortly before a subsequent sea-level rise terminated Tuscarora development with the return of the sea in which the Rose Hill Formation was deposited.

The basis for these interpretations is the analysis of five major lithofacies in the exposed Tuscarora Formation of central Pennsylvania. The basal part of the Tuscarora consists of two lithofacies of coastal origin: the horizontally laminated lithofacies (high-energy beach) and the pink transitional lithofacies (estuary). The main body of the Tuscarora comprises the eastern cross-laminated lithofacies (braided fluvial) and the western cross-laminated lithofacies (shelf sand-wave complexes). Capping the Tuscarora is the red, Skolithos-burrowed lithofacies (coastal sand/mud flats).