EOCENE AND YOUNGER FAULTING WITHIN THE SOUTHEASTERN MARGIN OF THE REELFOOT RIFT

Detailed subsurface mapping of the Eocene Upper Claiborne Group of west Tennessee, northwest Mississippi, and part of Arkansas shows this section is deformed by dextral movement on the southeast margin of the Reelfoot Rift. Structure contour maps prepared from over 1000 electric logs of the tops of the Flour Island, Memphis Sand, and the Eocene section indicate three deformation styles. Four east-west oriented grabens are mapped above the southeast rift margin. These grabens were active during deposition of Memphis Sand. Clay units in the Memphis Sand are generally thicker in the grabens. Facies in the overlying Cockfield Formation are also influenced by this graben/intergraben pattern. Deformation by these grabens may continue into the Holocene. The modern stream valleys of the Coldwater, Wolf, Hatchie, and Forked Deer Rivers are wholly or in part controlled by these grabens.

A second group of faults strike north-south in southwest Tennessee and northwest Mississippi. These include the Ellendale, Memphis, and Hurricane Creek Faults. These faults were active as growth faults during Memphis and Cook Mountain deposition. The Ellendale Fault was subsequently reactivated as a reverse fault. Deformation has apparently continued into the Holocene as evidenced by control of the Hurricane Creek valley. On their north ends, these faults merge with northeast trending faults above the southeast Reelfoot Rift margin. These margin faults include the Crittenden County fault zone and the Bluffline Fault. Rift margin faults displace Precambrian basement and have seismogenic activity. Originally normal faults, they have been reactivated as transpressive faults as exemplified by a positive flower structure in the Cockfield Formation in Lauderdale County.

This study suggests deformation is occurring on the southeastern Reelfoot Rift margin as a part of the New Madrid seismic zone activity. The seismic hazard of the region, particularly the greater Memphis area, is directly impacted by this deformation.