Delineation of mafic intrusions near Bedford (Virginia, USA) using geological and geophysical methods

Abstract

In the present work, geological and geophysical methods were used to delineate the locations of multiple mafic intrusions at the Claytor Nature Study Center (CNSC) near Bedford, VA. An investigation of the groundwater hydrology of CNSC was launched in 2007. As a first step in that project a preliminary geological survey revealed sparse evidence of a number of diabase intrusions in the area. While diabase intrusions are not particularly permeable features, contact metamorphism of the host rock could provide conduits for groundwater due to stress fractures and joints and high-temperature recrystallization of the rock matrix. Following the geological survey, geophysical surveys including seismic refraction, ground penetrating radar, and magnetic ground measurements were conducted to determine the location and extent of these multiple igneous intrusions. Seismic and radar surveys proved to be inconclusive, but the magnetic surveys showed strong magnetic anomalies. The magnetic data were obtained using a Geometrics G-856 proton precession magnetometer and were interpreted using the Mag2dc algorithm and SGeMS geostatistical software. The results show that the intrusions are dikes that cut across nearly perpendicular to the regional metamorphic structures and trend generally north–south with a dip of approximately 75°–90° to the west. These findings are consistent with one of the general directions of stresses associated with the North Atlantic seafloor spreading in late Triassic or early Jurassic period. Subsequent hydrologic testing and groundwater modeling confirm the role of the dikes in the overall hydrogeology of the site.