A geophysical investigation of geologic structure and regional tectonic setting at the Savannah River site, South Carolina

Time-structure, isochron, and reflection amplitude maps were constructed from 270 km of reprocessed seismic reflection data recorded at the Savannah River Site (SRS), South Carolina. These maps indicate the presence of three major basement blocks bounded by northeast trending faults that penetrate upward from basement into the Atlantic Coastal Plain sediments. Most of these faults are interpreted to be Paleozoic and Mesozoic structures reactivated under compression.

The northernmost fault block is bounded on the southeast by the Tinker Creek fault (TCF), a high-angle southeast dipping reverse fault, that appears to be a reactivated splay from a major decollement (Augusta fault?) at 5 km depth. The TCF can be traced with certainty for a distance of 15 km across SRS where it offsets the top of basement and shallower reflections. The throw of the fault, as does its upward penetration into the sedimentary section, increases to the northeast where the shallowest reflections (≈ 20 m) appear to be disturbed. The northwest boundary of this block is unknown.

The central fault block, 9 km wide in a NW-SE direction, is bounded on the northwest by the Tinker Creek fault and on the southeast by the Pen Branch fault, the northwest border fault of the concealed Dunbarton Triassic basin, now reactivated as a southeast dipping high-angle reverse fault within the Coastal Plain sediments. The Pen Branch fault (PBF) is the main throughgoing fault in a 3 km wide, 25 km long, fault zone consisting of subparallel fault segments that are commonly down-to-the-northwest north of PBF and down-to-the-southeast south of PBF. The offset of time horizons by the PBF increases to the northeast and the shallowest resolvable horizons appear in time to be deformed across its length. The Steel Creek fault, an antithetic fault to PBF, controls the drainage of Pen Branch, a tributary stream to the Savannah River.

The third fault block is bounded by the Pen Branch fault on the northwest. The southeast boundary of this southernmost block is unknown, but could be defined by the Martin fault, a down-to-the-northwest fault located south of SRS that appears to be a major structure.

Interpretation of isochron maps and analysis of trend surfaces fitted to reflection time horizons suggest that faulting was ongoing through the Late Cretaceous and was accompanied by tilting and horizontal rotation of the fault blocks. Fault block movements in the Tertiary are uncertain as shallow time horizons cannot be correlated areally with confidence; however, isochrons and reflection horizons correlated across faults reveal deformation, but not offset, of the shallowest resolvable events. The comparison between time-structure and topography reveals areas of positive correlation that suggest either near surface velocity variations or possible Cenozoic uplift.

The reprocessed seismic data elucidate the geometry of the Dunbarton basin and the underlying crustal structure. The Dunbarton basin is essentially a half-graben bounded on the northwest by the near vertical Pen Branch fault. The maximum thickness of the basin fill, as determined by seismic modelling, is between 4 and 5 km. Minor intrabasinal faults are imaged within the basin as well as bright discontinuous reflections that probably represent sills. The crystalline crust below SRS is highly reflective and is distinguished by zones of southeast dipping reflections that are correlatable between seismic lines. Two major crustal reflections, tentatively identified as the Augusta fault and a mid crustal decollement, appear to correspond to events imaged in seismic lines collected by COCORP 60 km to the southwest where the latter event was interpreted to be the Appalachian Master decollement. The continuity of deep reflection events along geologic strike from Georgia to South Carolina supports models for evolution of the Appalachian orogen that incorporate large scale, presumably Paleozoic age, thrusts beneath the Piedmont and under the Atlantic Coastal Plain.