GEOPHYSICAL CHARACTERIZATION OF KARST IN A CARBONATE OUTLIER USING RESISTIVITY, ATLANTIC COASTAL PLAIN, LENOIR COUNTY, N.C.

Karst features are documented within carbonate rocks of the Atlantic Coastal Plain, such as the Castle Hayne Limestone (CHL). The CHL is distributed over approximately 130 km along a northeast-southwest trending outcrop belt in the North Carolina coastal plain. Numerous outliers of the CHL have been mapped, extending as far as 160 km west of the main outcrop area.

An outlier of CHL, the focus of this study, is in southern Lenoir County, covering approximately 1,400 acres. Within the outlier, one open sinkhole was identified along with several other features that could be the result of subsidence related to the dissolution of limestone in the shallow subsurface. In addition, conversations with landowners revealed occasional catastrophic collapses over the years, triggered by heavy equipment and, in some cases, livestock.

For this study, three features were examined: 1) an open sinkhole, 2) an area where collapse was triggered by the weight of a drilling rig (the resulting sinkhole was subsequently backfilled), and 3) a circular, partially water-filled, shallow depression.

Resistivity profiles were acquired using an IRIS Instruments SYSCAL Kid Switch, a 24-channel resistivity meter in a Wenner array. The electrode spacing was 5 m with profile lengths of 115 m. The measured apparent resistivity data were inverted using Res2DInv software. The resulting profiles imaged to depths of approximately 18 m below ground level.

The open sinkhole resistivity profiles showed a shallower (6-7 m depth) and possibly thicker (up to 9 m) low-resistivity zone compared to the other features examined. The other imaged features had deeper low-resistivity zones (12-14 m depth) with minimum thicknesses of 4-5 m and a wider area (30-50 m) of interpreted subsidence. The low-resistivity zones are possibly related to water-filled cavities within the limestone. The similarity between the resistivity profiles for the shallow depression and the backfilled sinkhole could indicate that the shallow depression is also a karst feature.

This study demonstrates the value of resistivity profiles in delineating potential karst features. While the overall hazard potential for sinkhole development in this area may be relatively low, such surveys can provide valuable information for future land-use planning and risk assessment.