In total, 1249 geophysical well logs were interpreted to map the stratigraphy and structure of the Eocene Memphis Sand in west Tennessee, northwest Mississippi, and Crittenden County, Arkansas, where it overlies the southeastern margin of the Reelfoot rift. The Memphis Sand is a 300-m-thick reworked fluvial sand deposited on a low-relief subsiding plain with limited accommodation space. The Memphis Sand has experienced syn- and postdepositional faulting of four types in the region. (1) West-trending grabens were active during Eocene Claiborne deposition. These grabens also appear to control modern drainage and thus may extend to the surface. (2) The ∼N33°E Shelby faults pass through Shelby County, Tennessee, into northwest Mississippi and extend to or near the surface. They were right-lateral normal faults during Eocene Memphis Sand deposition, but they became inverted reverse faults in the late Eocene. (3) The ∼N67°E-striking Gibson faults are high-angle normal and reverse faults with a component of right-lateral strike slip. These faults were active during Memphis Sand through lower Cockfield deposition north of the Hatchie graben and appear to be due to faulting above an outboard fault of the Reelfoot rift. (4) The ∼N43°E-striking Lauderdale faults are high-angle normal and reverse faults that reflect right-lateral strike-slip reactivation of the southeastern margin of the Reelfoot rift.
The structure and stratigraphy revealed in this mapping indicate that the geology of the Memphis Sand is more complex than previously recognized. Faulting of the Memphis Sand and overlying sediments reveals potentially seismogenic faults in western Tennessee, eastern Arkansas, and northwestern Mississippi. This study provides fundamental data for future groundwater modeling and seismic hazard analysis.