A GEOPHYSICAL INVESTIGATION OF THREE CLOSED TOPOGRAPHIC DEPRESSIONS ATOP THE CUMBERLAND PLATEAU, TENNESSEE

At least eight, large (up to 1200ft diameter and up to 160ft deep), circular to ovate, closed topographic depressions are present in the Pennsylvanian sandstone and conglomerate caprock of the Cumberland Plateau. Hypotheses on the formation of these features include roof collapse into large caverns within the stratigraphically lower Mississippian carbonates, and that they are the remnants of surface erosion related to plunge pool formation at waterfalls near the edge of the plateau. We explored three of these features with a total of five (up to 630ft long) transects using both the electrical resistivity tomography (ERT) and P-wave seismic refraction (SR) methods. Each transect exhibited variations in resistivity and velocity with depth that could represent changes in surface materials (soils and organics), rubble zones associated with collapse breccia, and hard sandstone bedrock. Few zones of relatively low resistivity may be interpreted as shale intervals in the sandstone, or clay-filled loose/void-type areas. Future work will include shear wave seismic surveys over the same five transects and detailed stratigraphic description of bedrock at the edge of the plateau to determine possible influence of shale intervals on the geophysical results. The results of this study add data to the perplexing question of genesis for these interesting features.