William Beard
Shallow Seismic Reflection Profiling of Holocene Faults Near Monticello, Arkansas

William C. Beard, III, Department of Geology and Geological Engineering, University of Mississippi, University, MS 38677, James B. Harris, Department of Geology, Millsaps College, Jackson, MS 39210, and Randel T. Cox, Department of Geological Sciences, University of Memphis, Memphis, TN 38152

A shallow shear-wave seismic reflection profile was acquired near Monticello, Arkansas, across surface faults that displace Holocene sediments. Part of the profile is coincident with a paleoseismologic trench excavated during the summer of 1997. The seismic data were collected using common-depth-point (CDP) techniques which provided six-fold subsurface coverage over a 210 m-long line. The seismic data were processed using a standard sequence for shallow CDP data. The processed section shows coherent reflection energy to depths in excess of 150 m. The most coherent reflection is approximately 110 m deep and, based on correlation with local borehole data, is interpreted to represent the top of the Eocene Cockfield formation. All reflections appear to be displaced in a normal sense by two south-dipping faults that correlate updip with faults mapped in the paleoseismologic trench. The principal fault is interpreted to have ~20 m of down-to-the-south structural relief on the top of the Cockfield Formation and displacements decrease up-section, indicating that the fault may have been an active down-to-the-basin fault during Eocene subsidence of the Gulf of Mexico. Anticlinal folding in the footwall block of the principal fault suggests that the fault was reactivated during Quaternary E-W compression. Studies of this type in the central U. S., outside the New Madrid seismic zone, can help locate and characterize other possible earthquake source zones.