NEW GEOLOGIC MAPPING IN MONROE COUNTY, WEST VIRGINIA: ORDOVICIAN TO MISSISSIPPIAN STRATIGRAPHY AND THE STRUCTURAL FRONT TRANSITION FROM THE SOUTHERN TO CENTRAL APPALACHIANS

New 1:24,000 to 1:40,000 scale geologic mapping of Monroe County, West Virginia was conducted by the U.S. Geological Survey in conjunction with a county-wide water resource investigation. The county crosses the Appalachian structural front within a portion of the Roanoke recess and includes stratigraphy from the Ordovician Beekmantown Formation to the Mississippian Hinton Formation of the Mauch Chunk Group.

New mapping shows the Ordovician Reedsville Shale overlies a distinct shale unit that may be equivalent to the Martinsburg Formation. The Silurian Williamsport Formation and Keefer Sandstone can be traced on the southeast slope of Peters Mountain from Paint Bank to Narrows, Virginia, and along the southeast slope of Potts Mountain. The Devonian Hampshire Formation and Helderberg Group limestones are not present in Monroe County and were presumably eroded or not deposited. The units of the Mississippian Greenbrier Group were mapped at the formation level, which had not previously been done for the entire county, despite it being the original type-region for these units. Problematic stratigraphic relations within the Mississippian Bluefield Formation were clarified and mapped using the top of the Reynolds Limestone Member as a contact between the upper and lower Bluefield Formation.

A distinct change in structural trend occurs from SW to NE across the county. In the SW, the ENE-striking St. Clair thrust places Ordovician carbonates and shales over siliciclastic Silurian and Devonian units within the overturned Glen Lyn syncline, while to the NE this footwall syncline becomes upright and opens into NNE trending folds which also exhibit a change of trend within the Roanoke recess. In the center of the county fold hinges trend NNE and are parallel to smaller top-to-west thrust faults. This pattern appears consistent with a counterclockwise rotation of the principal stress axis. Several horses occur within the NE portion of the St. Clair fault zone, the largest extending for 18 km. The hanging wall of the St. Clair thrust is strongly folded adjacent to these horses, and the more open footwall folds formed as the thrust zone displacement died out to the northeast. Smaller, unnamed thrusts occur east of the St. Clair thrust system on the southeastern flank of Peters Mountain and Potts Mountain.