PRELIMINARY RESULTS FROM A GEOPHYSICAL SURVEY ON A LARGE DEPRESSION IN THE PENNSYLVANIAN CLASTIC CAPROCK OF THE CUMBERLAND PLATEAU

At least eight, large (up to 300m diameter and up to 50m deep) circular to ovate depressions are present in the Pennsylvanian sandstone and conglomerate caprock of the Cumberland Plateau. Previous investigators have suggested that the formation of these features are either due to 1) roof collapse into large caverns within the stratigraphically lower Mississippian carbonates or 2) are the remnants of surface erosion related to plunge pool formation at waterfalls. Eight large depressions were identified on topographic maps and five of these were examined in the field. We applied two complementary geophysical methods across the surface of one of these depressions to identify vertical and lateral changes in the subsurface in an attempt gather information that might reveal their genesis. Electrical resistivity tomography (ERT) and P-wave seismic refraction (SR) methods were deployed along two perpendicular lines. One ~122m long line extended from the center of the depression to the upper rim and a second ~198m line crossed the first line at ~46m from the center and extended across the depression nearly rim to rim. Three distinct layers differentiated by variations in physical properties were identified in both the ERT and SR data sets. The upper ~6m thick layer consists of high resistivity/very low velocity material that generally parallels the topography of the depression. The ~6-24m-thick underlying second layer consists of moderately conductive/stiff material and the lowermost third layer generally consists of relatively-resistive/very stiff materials with pockets of higher resistivity/lower velocity. Our preliminary interpretations are that the upper layer is composed of highly weathered materials and a thick accumulation of organic matter. The middle layer is possibly rubble with large blocks of sandstone inter-filled with clays and weathered materials. The lowermost layer likely represents bedrock. Property variations observed in the data sets near the center of the feature might also represent a throat at depth. Future work including additional geophysical surveys on this feature and other similar features will help to determine their origin and development.