Geomorphic and geophysical evidence of a collapse origin for large sandstone depressions atop the western Cumberland Plateau in Tennessee

For over 70 years, closed topographic depressions developed in siliciclastic rocks of the Cumberland Plateau have been hypothesized to form via collapse of rock into caverns formed in the carbonate-rich rocks below. We made observations of surface characteristics and applied geophysical methods across two of these depressions on the western part of the plateau to collect subsurface information and test the cave collapse hypothesis. Electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) were conducted at both Sites 1 and 2 and two-dimensional (2D) multi-channel analysis of surface waves (MASW) was conducted at Site 2. Three zones of material are interpreted at both sites based on field observations, and variations in density (velocity) and resistivity characteristics; a relatively thin cover of organic matter and colluvium, sandstone bedrock primarily observed at the rims and upper slopes of the depression, and a possible boulder/rubble zone identified towards the centers of the depressions. Details derived from LiDAR data, field observations and collected geophysical data for Site 2 allow for the projection of a circular breccia pipe in the subsurface. A 3-stage model of development is proposed for these features: A Youthful Stage occurs after initial collapse of surface material into a subsurface cavity in which the depression has a cylindrical form. In the Mature Stage, the slopes have shallowed, and slope adjustments are made as the breccia pipe fills and settles. Bedrock cliffs are mechanically weathered and influenced by gravity, forming talus that covers slopes and the floor with colluvium mixed with organic matter. The Degraded Stage is entered when the breccia pipe has settled, the bedrock exposures are covered, and the depression begins to fill. The depressions studied for this research are classified as being in the Mature stage. Future geophysical surveys and additional geologic mapping and field work at these and other depressions on the plateau will add to our understanding of their development and evolution over time.