THE RELATIONSHIP BETWEEN GROUNDWATER AVAILABILITY AND DIABASE DIKES IN THE DEEP RIVER TRIASSIC BASIN, NORTH CAROLINA

Among geologists, extension agents, and well drillers, the Deep River Triassic basin is well known for its low groundwater yields. Poor sorting and high clay content of the basin’s sedimentary rocks limit infiltration and lead to low hydraulic conductivity; wells drilled in older crystalline rocks outside the basin are usually much more successful. Further, Triassic soils are poorly suited for septic systems. Unless there is access to a public water supply and sewage system, the geologic boundary of the Triassic basin poses an impediment to residential and commercial development, as can be seen today in many rural areas, including northernmost Wake and southern Granville Counties.

For water wells in the basin, a preferred drilling target is a steeply dipping diabase dike. Dikes vary greatly in thickness, but even thin dikes can be good targets. Yields of 10 to 20 gpm are common for wells in diabase, while wells in sedimentary rock seldom exceed 2 gpm, and many are dry holes.

Dikes can commonly be traced on the surface on the basis of spheroidally weathered residual cobbles and boulders. Larger dikes can be located on aeromagnetic maps, and even small ones can be mapped easily and in detail using ground magnetic surveys. Diabase outcrops typically exhibit strongly developed orthogonal joint systems. The higher yield of wells in diabase dikes has been variously attributed to jointing within the dikes, damming of groundwater, or fractures in adjacent baked zones of country rock. Unlike dikes, sills do not seem to be associated with higher yields.

Most diabase dikes are unmapped, as only the most obvious are shown on existing geologic maps. In the parts of the basin we have surveyed, we encounter at least one dike per km on a typical east-west traverse. Geophysical methods could be employed to great advantage in order to produce high quality, detailed maps showing dikes in areas under consideration for development. Such maps would also be useful in addressing important environmental questions, including the prediction of groundwater behavior at sites of proposed hydraulic fracturing or wastewater injection or irrigation.