HYDROGEOLOGIC CHARACTERIZATION OF DIABASE DIKE INTRUSIONS IN GRANITE

A hydrogeologic research station in central North Carolina encountered diabase intrusions into a granite pluton. Magnetic surveys identified the locations of diabase dikes, and these locations were confirmed by rock coring and monitoring well installations. A core boring fully penetrated a “thin”, nearly vertical diabase dike and a 342-foot deep open hole bedrock monitoring well was drilled through the approximate center of a “thick” diabase dike. Hydraulic characteristics of the diabase dikes varied greatly, and appear to be a function of dike thickness and the relative location within the dike and surrounding contact zone. Waterborne resistivity and temperature data collected in a nearby river suggests that the impact of a diabase dike on ground-water flow is evident. The contact between the diabase and granite results in the formation of relatively impermeable clay in saprolite. However, the less weathered diabase tends to be very highly fractured, resulting in a zone of very high hydraulic conductivity.