CROSS SECTION ACROSS PALEOZOIC PIEDMONT TERRANE, SOUTHERN LANCASTER COUNTY, PA

Southern Lancaster County marks the boundary between the Laurentian continental margin to the north and Paleozoic accreted terranes to the south. Recent excavation for a water pipeline for the Wildcat Point generation plant in Rock Springs, MD northwest to Peach Bottom, PA on the Susquehanna River provided temporary bedrock exposure of the contact between the accreted State Line serpentinite and adjacent, continental derived metasedimentary rocks. The 8 km excavation provided a unique opportunity to compare the previously mapped geologic contacts and interpretations based on rare surface exposure and float with direct subsurface observation.

The local geology is characterized by two belts of serpentinite, the southern, massive State Line body and the much smaller and thinner New Texas body to the north and west. Between the two serpentinite belts is a highly variable metadiamictite. The serpentinites are in fault contact with the metadiamictite and the serpentinite and metadiamictite were then thrust over the quartz-rich metasedimentary rocks of the Peters Creek Formation. However, in the vicinity of the excavation, the New Texas serpentinite was revealed to be a 250 m highly variable zone of mixed massive to sheared serpentinite, rubbly soapstone and minor greenschist distinguished by a near vertical, NNE trending foliation. The immediate contact of the serpentinite and the Peters Creek is distinguished by a typical metasomatic zonation from serpentinite to talc to greenschist to chlorite muscovite schist.