MAPPING OF PROBABLE ALLEGHANIAN THRUST FAULTS WITH THE USE OF STRATIGRAPHIC ANALYSIS IN THE MILLBORO QUADRANGLE, BATH COUNTY, VIRGINIA

Previously unmapped, locally significant thrust faults cross-cut an anticline along State Highway 42 in Bath County, VA about 6 miles south of Millboro Springs. Preliminary observations show deformation that includes large-scale folds and micro-folds in the Clifton Forge Sandstone Member of the Keyser Formation, possibly caused by or associated with faulting. The Tonoloway Limestone, which underlies the Keyser, may be truncated along a fault, as it is locally present and then disappears abruptly. The Healing Springs Sandstone, which stratigraphically underlies the Licking Creek Limestone in this region, is thrust over the Licking Creek. The Clifton Forge and underlying limestones of the Byers Island Member of the Keyser, also on the hanging wall, appear to wedge out along the thrust. Thin sections show that the quartz grains in the upper Licking Creek have multiple fractures, evidence of the intensity of deformation along the thrust plane. One mile south, an apparently different thrust that dips shallowly at the surface has placed chert-free quartzose limestones of the upper Licking Creek over the stratigraphically younger Needmore Shale. None of these structures are shown on existing geologic maps.

Investigation of the structural and stratigraphic relations in nearby Porter’s Cave shows that the cherty lower Licking Creek is present in normal sequence, indicating that the subhorizontal fault exposed along SH 42 by the cave dips successively more steeply to the southeast, and cuts downsection. This provides some constraint on the lateral extent of the fault, hypothesized to dip southeast towards the Rough Mountain syncline which is cored by clastics of the Devonian Brallier Formation. Our mapping efforts also include photomeshing/gigapanning the exposures to obtain detailed pictures of the outcrops and the faults. From these we will produce a detailed structural cross section and measured stratigraphic section showing the units involved in the deformation.