EXTENDED ABSTRACT: Relation Between Holocene and Tertiary Normal Faults: A Comparison of Shallow Seismic and Gravity Data with Deep Well Data Across the Baton Rouge Fault System, Northern Gulf of Mexico Coast, Louisiana, USA

The Tepetate-Baton Rouge fault system traverses Louisiana from west (Tepetate system) to east (Baton Rouge system), and continues east and south of the Pearl River. This fault system is part of a larger, regional, down-to-the-basin fault system along the northern Gulf of Mexico that extends into eastern Mexico (Murray, 1961). Within our study area (Fig. 1) productive hydrocarbon accumulations occur principally south of the Baton Rouge fault-line scarp in deep (~5-10,000 feet) rollover structures, downthrown to the fault. Immediately to the north of the fault there are no equivalent structural traps. Shallow (<1000 ft) hydrogeology studies suggest that fluids can migrate across the fault zone. Extensive, but unpublished well data from oil and gas exploration has generally suggested the existence of E-W striking subsurface growth fault trends but correlation with much lesser studied near-surface faults is lacking. By comparison, the location of shallow (< 1500 ft) growth faults, their geophysical characterization and the natural moderators that control their rates of movement in the southern Gulf Coast region are poorly known. We show for the first time that fault-line scarps are parallel to subsurface growth fault traces that are mapped within productive hydrocarbon intervals. At depth, this fault system exhibits late Eocene to Oligocene synextensional growth strata. Maps of surface, fault-line scarps (Durham, 1964; McCulloh, 1991 and 1996) indicate reactivation of these growth faults during at least the Quaternary. New laser altimetry data (www.atlas.lsu.edu) (Fig. 1) helps verify and modify prior interpretation of fault-line scarp locations. Overlapping normal fault segments along the central Baton Rouge fault system, in Livingston Parish, may develop ramps that serve to divert local stream flow from a general N-S direction into a more NW-SE direction. We use new, high-resolution gravity data (+/- .01 milligal), digital elevation models (LiDAR, +/- 1 ft; Light Detection and Ranging), and borehole data (<100 ft depth), to investigate the effects of ramp evolution on sediment history. Associated shallow (<300 ft) sedimentary bodies can be discerned in gravity models. Gravity data reveals there is no consistent spatial relation between the northern limits of Bouguer gravity anomalies and location of the fault-line scarp. However, gravity anomalies are probably associated with denser (sand) elongated units oriented parallel-to-subparallel to the strike of the fault-line scarp and within the overlap zone. Interpreted sand bodies increase in width (~500-3,000 ft) and thickness (~150-250 ft) toward the east where the fault offset and accommodation, created by the rollover are expected to be larger. A fault zone ~300 ft-wide extends from the northern fault-line scarp southward, as interpreted from high-resolution (~100-350 Hz), seismic data. Together with forced folds and late-stage multiple fracture directions that are expected from competent rock models of overlapping normal fault zones (Peacock and Sanderson, 1991), a complex sediment distribution pattern is predicted.