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 Cambro-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 cannot be located with accuracy. Several geophysical methods were used to locate the faults, including ground penetrating radar (GPR), audio-magnetotellurics (AMT), electrical resistivity imaging (ERI), and borehole geophysics. 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.

GPR collected with a 25 MHz antenna was useful in mapping the thickness of the alluvial deposits to a depth of approximately 25 m, and could identify areas of possible subsidence of layered alluvium into underlying voids. The AMT method was capable of imaging broad resistivity contrasts along thrust faults at depths of up to 500 m. The ERI surveys were useful in identifying the precise position of inferred bedrock faults, as well as surface and subsurface karst features. 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-filled solutional conduit. The surface expression of the depression was approximately 50 m in diameter, while the low-resistivity anomaly was 5 to 15 m wide and appeared at 5 to 25 m depth.