NEGATIVE EVIDENCE FOR TECTONIC SURFACE DEFORMATION BY REVERSE-REACTIVATED SUBSURFACE FAULTS ASSOCIATED WITH TRIASSIC RIFTING AND BASIN FORMATION: A GEOMORPHIC ASSESSMENT OF THE SAVANNAH RIVER SITE, GA

The Savannah River Site (SRS), a U.S. Dept. of Energy facility near Augusta, GA, overlies a Triassic Basin border fault and other subsurface faults associated with Triassic rifting. Previous studies indicate that some of these basement faults were reactivated in a reverse sense during Cenozoic time and extend upward into overlying Coastal Plain sediments. To support recent seismic reflection data that suggest the faults do not reach the surface, we performed geomorphic analyses to assess evidence for tectonic surface deformation at the SRS. Using Light Ranging and Detection (LiDAR) data that cover the SRS, we 1) developed and interpreted site topographic data; 2) refined mapping of Quaternary alluvial terrace surfaces at the SRS; 3) developed and interpreted longitudinal profiles of streams and fluvial terrace surfaces; and 4) developed and interpreted hypsometric integrals (E) of 12 tributary catchments of the Savannah River at and near the SRS using a 10 m DEM from the National Elevation Dataset.

Visual inspection of LiDAR-derived topographic data reveals no non-anthropogenic topographic lineaments. Furthermore, no anomalous drainage patterns such as drainage capture, deflected drainages, or variable incision upstream or downstream of a mapped subsurface fault were observed. Profiles constructed for six streams at the SRS generally exhibit minor perturbations from a graded profile, all of which are attributable to anthropogenic modification or contrasting along-profile lithology. Fluvial terrace mapping and terrace surface profiling reveal < 5 m of short-wavelength relief within best-preserved surface remnants, similar to relief within the modern Savannah River floodplain. Finally, a low standard deviation and error of E calculated for the 12 tributary catchments suggests that the hypsometry of each basin is influenced by similar factors of regional extent, and not local influence associated with mapped subsurface faults. Taken together, these results constitute negative evidence for tectonic surface deformation at the SRS, and support the conclusion that the faults imaged in the subsurface at the SRS do not appear to deform the site landscape, consistent with previous studies at the SRS and the nearby Vogtle Electric Generating Plant.