FACIES ASSOCIATION AND STRATIGRAPHIC RELATIONSHIP OF THE MARTINSBURG FORMATION AND THE REEDSVILLE FORMATION ALONG THE NORTHWESTERN EDGE OF THE GREAT VALLEY IN PENNSYLVANIA

The Reedsville and Martinsburg Formations are clastic units, primarily shale and siltstone, which occupy the same stratigraphic interval. They represent the Late Ordovician influx of clastic sediment over the Laurentian carbonate bank during the Taconic Orogeny. Along the western front of the Great Valley Section with the Blue Mountain and the Appalachian Mountain Sections of the Ridge and Valley Province, their character differs, due to differences in depositional environment. The Martinsburg Formation is primarily deep water turbidites, and pelagic marine life (e.g., graptolites) dominates the fauna. The Reedville Formation consists primarily of contourites, tempestites, and sediments deposited by other shallow marine currents. The marine fauna is dominantly benthic invertebrates – brachiopods, bryozoans, crinoids, and trilobites. Despite these differences, the units in Pennsylvania are mapped essentially based on their location. The name Martinsburg is used for this interval in the Great Valley Section. The name Reedsville is used in the remainder of the Ridge and Valley physiographic province. No published geologic maps properly address the transition between the two units.

Lehman (1989) first used the name Reedsville for rocks on the Great Valley side in Swatara Gap, Lebanon County, that had previously been mapped as upper Martinsburg Formation, based on the lithologic characteristics described above. Mapping by Ganis and Blackmer (2010) in Lebanon and Dauphin Counties confirmed this interpretation. They also found a time gap between the units, with the youngest Martinsburg in the C. americanus graptolite zone, and the Reedsville in the C. spiniferus zone. Geologic mapping and coring by Bierly farther to the southwest in Franklin County found a similar sequence of Martinsburg overlain by Reedsville. These occurrences indicate that the Reedsville extends at least into the northwestern edge of the Great Valley. It may have been even more extensive to the south and east prior to removal by erosion. Although we know that the deep-water Martinsburg facies thins dramatically across the Blue Mountain/Appalachian Mountain front, further mapping is necessary to determine its extent north and west of the Great Valley Section.