Using Geophysics to Map Bedrock Faults, Dikes, and Surficial Geology in Relation to Karst Features in the Briery Branch Quadrangle, Rockingham County, Virginia

Several faults have been identified in the Cambrian and Ordovician carbonate sequence in western Rockingham County, Virginia. These faults were recognized by mapping offsets between stratigraphic units in the hanging wall of the North Mountain fault system. Portions of these faults are associated with karst features, but are covered by alluvium and thick soil development in the floodplains of Dry River and Briery Branch; therefore, the position of the faults could not be located with accuracy through surface mapping alone. In an earlier study, ground penetrating radar (GPR) and audio-magnetotellurics (AMT) geophysical methods were used to locate the expression of these faults at depths greater than ~5m. . Here, we employ electrical resistivity imaging (ERI), and borehole video to examine the influence the faulting may have on near-surface karst experssion. These geophysical datasets also help in the identification of igneous dikes and karst features present in the subsurface, and shed light on how these structures may affect the local groundwater flow regime and karst development. The ERI surveys were useful in identifying the precise position of inferred bedrock faults, as well as karst features in the shallow subsurface. On the ERI profiles, the faults inferred through geologic mapping appear as sub-vertical low resistivity zones, indicating increased fracture porosity and weathering along the fault surface. Enhanced fracturing in these zones was also observed in adjacent borehole video logs. Diabase dikes appear as sub-vertical high resistivity features, and were verified at the surface through outcrop observation. Sinkholes and other karst features were also identified in the ERI profiles, generally expressed as low-resistivity anomalies. A pseudo-3D ERI survey was completed over a closed depression in highly weathered alluvium overlying limestone bedrock that clearly images the outline of the depression in the subsurface, as well as a possible water or mud-filled solutional conduit. The surface expression of the depression was approximately 165 ft (50 m) in diameter, while the low-resistivity anomaly was 15 to 50 ft (5 to 15 m) wide and appeared at depth of 15 to 80 ft (5 to 25 m)