Shallow faulting of the southeast Reelfoot rift margin

Approximately 1900 geophysical well logs are used to map the shallow stratigraphy and structure of the Tertiary of west Tennessee, northwest Mississippi, and Crittenden County, Arkansas. The Eocene Memphis Sand is highly reworked sand deposited on a low relief subsiding plain with limited accommodation space. The overlying Eocene Cook Mountain Formation represents a change to predominantly silt and clay and an increase in accommodation. Above the Cook Mountain, the Eocene Cockfield Formation is a lignitic, deltaic sand and clay unit in north Tennessee and a thin silt and sand rich clay in southwest Tennessee and northwest Mississippi. All three formations were influenced by syndepositional faulting.;Faulting is divided into three types: (1) West-trending grabens developed over deep west-striking basement faults that were active during Claiborne deposition. These grabens also appear to control modern drainage and thus may extend to the ground surface; (2) The Shelby faults are north-striking basement faults that extend to or near the surface. Originally normal faults, they have inverted since the late Eocene. These faults trend under Memphis into northwest Mississippi; (3) The rift parallel Gibson faults are high angle normal and reverse faults with a component of right-lateral movement. They are subdivided into older Gibson group 1 faults, which were active during Claiborne deposition, and younger Gibson group 2 faults that continue to be active. Gibson Group I faults indicate movement on a currently unmapped basement fault whereas the group 2 faults represent movement on the mapped Reelfoot rift margin.;Deep faulting on the southeastern Reelfoot rift margin appears to be seismogenic and if this faulting extends to the surface then seismic hazard on this rift margin is significant. Shallow faulting that probably extends to the seismogenic basement depths is mapped under western Tennessee, northwest Mississippi and eastern Arkansas. These faults potentially raise the seismic hazard for the region and particularly the city of Memphis, Tennessee.