FRACTURES, DIABASES, AND SEDIMENTS; HYDROGEOLOGIC CHARACTERIZATION IN THE LOWER PIEDMONT OF NORTH CAROLINA

A landscape-scale hydrogeologic assessment was conducted on 2000 acres in the lower Piedmont of North Carolina about 25 miles northeast of Raleigh. Poorly-exposed features that influence groundwater movement were accurately mapped using geophysical methods and confirmed by drilling. Diabase dikes were mapped using a proton free-precession magnetometer, and high-angle bedrock fractures were mapped using a very low frequency electromagnetic meter (VLF). Approximately 200 groundwater wells and 300 temporary, shallow boreholes were installed. VLF located significant high-angle bedrock fractures and fractures associated with the dikes. Drilling encountered fractures and associated mineralized zones and slickensides, and fractures provided significant increases in water yield. The magnetometer allowed very accurate location of the 2 dikes and identified differences in orientation. The eastern dike is linear while the western dike exhibits numerous en-echelon offsets. Both dikes act as barriers to groundwater flow. Two marine/fluvial sedimentary sequences have been identified. The older sequence is up to 15 feet thick with much of the landscape covered by truncated deposits. A complete sequence consists of coarse sand overlain by dense clay-rich deposits and capped by sandy to gravelly clays. Coarse crossbedding, clay balls, and wave-features are present in outcrop. A younger sedimentary sequence is inset into the older unit and lies on an erosional unconformity. Incipient soil formation is present in both sedimentary sequences. These clay-rich units act as confining layers for shallow groundwater along stream margins. The accurate location of key features allowed construction of a detailed hydrogeologic model for predicting future movement of groundwater. Details of the model development and calibration will be presented in a subsequent paper.