HIGH-RESOLUTION SEISMIC REFLECTION IMAGING OF A REACTIVATED TRIASSIC BASIN BORDER FAULT: THE CHARACTER OF BASEMENT-INVOLVED FAULTING IN COASTAL PLAIN STRATA

The Triassic Dunbarton basin is buried by > 365 m of Late Cretaceous to Recent Coastal Plain deposits. The Dunbarton basin has been the subject of many studies over the years, along with its northwestern border fault, the Pen Branch fault in SC, because these structures underlie a Department of Energy facility. However, the exact location of the southwestern projection of the Pen Branch fault into GA has not previously been clearly documented. High resolution seismic reflection and collocated travel time inversion-refraction imaging of several seismic lines in Burke Co. GA tightly constrain the location of the fault in GA, provide constraints on the location of its projection from SC into GA, and help to resolve the upward termination of the Pen Branch fault within Coastal Plain strata.

Stratigraphic and structural relations visible in the seismic data indicate that the Pen Branch fault is a Triassic normal fault that was reactivated in a reverse sense in the Cenozoic. The reflection data (both prestack time and depth migrations) on at least one of the seismic lines show detailed images of the character and seismic expression of basement-involved faulting in the overlying Coastal Plain strata. The expression of the fault as a discrete fault surface is confined to the lower Coastal Plain section. In upper stratigraphic intervals (Cenozoic), the fault-related deformation is expressed as fault-propagation folding that decreases in amplitude and becomes broader upwards in the section. There are no discrete fault offsets of the seismic reflectors in the shallow section within the resolution limits of the data. Geomorphic analysis of Savannah River terrace surfaces (see accompanying abstract) preclude any broad deformation of the ground surface within the resolution of the data. This deformation style has clear implications for surface-related faulting studies in that basement-involved faults in Coastal Plain strata may not be expressed as discrete offsets in surface or near surface sediments.