SHEAR-WAVE SEISMIC REFLECTION IMAGING OF THE BIG CREEK ESCARPMENT NEAR HELENA ARKANSAS: EVIDENCE FOR A TECTONIC ORIGIN

The Big Creek Fault Zone (BCFZ), in eastern Arkansas, has been recognized and studied for over 50 years, although little detailed work regarding its geologic framework and potential earthquake hazard has been done. Shear-wave seismic reflection profiling across the northeast-trending Big Creek escarpment (southwest of Helena, Arkansas) shows the feature to be underlain by high-angle faults that extend upward into Quaternary sediments (shallower than 45 m). The structural style of the faults, and warping of shallow reflectors, indicates compression and suggests that the faults may have been reactivated as reverse or transpressional faults. In order to further investigate near-surface deformation associated with the Big Creek escarpment, additional S-wave reflection profiling was carried out east of the earlier survey, where the escarpment diverts from its parallel relationship with the Big Creek valley and bends anomalously to the east. The purpose of the profile was to determine if the east-trending scarp segment is tectonic or represents a fluvial terrace. Seismic data were acquired using a 1.8-kg sledgehammer impacting a 4.5-kg steel I-beam oriented perpendicular to the spread, and processed with special attention to static corrections and velocity analysis. The processed profile exhibits strong, continuous, reflections at depths between 30-50 m (probably associated with the Quaternary-Tertiary boundary). The reflections are disrupted by a 50-m-wide fault zone coincident with the south-facing scarp. The fault zone is interpreted based on seismic amplitude variations, offset reflections, and the presence of strong diffracted energy (coincident with the boundaries of the fault zone). Separation across the fault zone appears to be up-to-the-south, although topographic relief along the line is higher on the north. Therefore, the fault zone has likely experienced structural inversion during late Tertiary or Quaternary time.