GEOMORPHIC AND PALEOSEISMIC INVESTIGATION OF THE BIG CREEK FAULT ZONE, PIGTAIL POINT, PHILLIPS COUNTY, ARKANSAS

Located in east-central Arkansas, the Big Creek fault zone (BCFZ) is defined by a series of NE-trending lineaments between the White River and Helena, Arkansas. The BCFZ was first recognized over 70 years ago by Fisk (1944), yet it remains poorly characterized and rarely considered in the seismic hazard community’s understanding of potential earthquake hazard in the central U.S. Detailed geologic and geomorphic mapping, coupled with paleoseismic trenching and geophysical exploration at Pigtail Point, provides new information on the BCFZ.

The 6-km-wide BCFZ zone is comprised of the Big Creek escarpment and Kingtown lineament. Evidence for late Quaternary deformation along the BCFZ includes tectonic-related geomorphology such as NW-side-up linear escarpments and structurally-controlled drainages, persistent tonal lineaments, liquefaction-related deformation, and inferred faulting and folding from borehole and geophysical profiles. Geomorphic analysis indicates the Big Creek escarpment is largely a fault-line fluvial scarp with localized remnants of the fault zone preserved adjacent to the scarp. Offset geophysical reflectors suggest an overall NW-side-up vertical separation across steeply dipping faults coincident with the escarpment. However, multiple paleoseismic trenches and natural exposures excavated across the escarpment provide no evidence for discrete surface faulting or folding. Nonetheless, two trenches did reveal evidence for one paleo-liquefaction episode that cross-cuts the 25-27 ka Ash Hill braid belt deposits. Additional deformation may exist on lineaments immediately west of the crest of the escarpment or southeast beyond the escarpment where ponded water limits exploration capabilities.

Air photo analysis and initial geomorphic mapping of the Kingtown lineament shows the alignment of sharp linear tonal contrasts, subtle (<1m) escarpments, and deflected drainages that are laterally persistent for 11 km within late Pleistocene braid belt deposits. A concurrent seismic reflection survey (Harris, 2016) indicates these lineaments are associated with stratigraphic truncation on near-vertical faults at depth. It is unclear from this initial study whether the BCFZ ruptures the entire width of the escarpment and lineaments, or as discrete faults within this broad zone.