ARE THE REEDSVILLE SHALE AND MARTINSBURG FORMATION EQUIVALENT OR SEPARATE UNITS? SORTING OUT A STRATIGRAPHIC CONUNDRUM ACROSS THE EAST-CENTRAL APPALACHIAN BASIN

The type locality of the Upper Ordovician Reedsville Shale is located at Reedsville, Pennsylvania. The type locality of the Upper Ordovician Martinsburg Formation is located at Martinsburg, West Virginia. These two units have been variously mapped as equivalent or separate units in different states, or in different geographic locations within the same state. At Reedsville, only the Reedsville Shale is recognized; however, further to the southeast near Fannettsburg, Pennsylvania both the Reedsville and Martinsburg are mapped as separate units with the Reedsville overlying the Martinsburg.

Far to the south in Monroe County, West Virginia, a contact between two distinct lithologies historically mapped as all belonging within the Reedsville Shale is observed along Rte. 311 near to the Virginia-West Virginia state line. The lower unit is yellowish-orange shale and fine sandstone containing sparse fossils of brachiopods and graptolites. The upper unit is chocolate-brown calcareous shale with abundant fossils, including the Orthorhyncula brachiopod zone. Further north, this contact is again observed off Crooked Run Road just west of the state line in the Orkney Springs 7.5 minute quadrangle, and to the southwest both units are observed in fault contact within the North Mountain thrust fault zone at Brocks Gap, and at the base of Cooper Mountain southeast of Rawley Springs, Virginia. Further to the northeast near Front Royal, Virginia strata containing the Reedsville marker zone of Orthorhyncula brachiopods have been recognized, but Reedsville is not mapped separately from Martinsburg in the Shenandoah Valley of northern Virginia.

We propose that these formations should be mapped separately where the predominantly shale lithofacies is distinct from the overlying brown interbedded shale and fossiliferous limestone, siltstone and sandstone bearing the Orthorhyncula zone. Prior work has relied upon physiographic separation of the unit names in different structural blocks; however, we observe continuity of both units on either side of the North Mountain thrust zone. Tectonic structures have undoubtedly contributed to the difficulty in mapping the distinct lithologies. We address these conflicts and present a framework for seamless mapping that references prior work by Rick Diecchio.