Atwood Fault

Part of the Ste. Genevieve Fault Zone

Location

Southwestern Union County (K-5)

References

J. Weller and Ekblaw 1940, Heyl et al. 1956, Segar 1965, Nelson and Lumm 1985, Nelson and Devera 1994)

Description

J. Weller and Ekblaw (1940) defined the Atwood Fault based on surface mapping. Recent remapping (Nelson and Devera 1994) confirms the Atwood Fault to be much as depicted by Weller and Ekblaw. The fault is about 3.5 miles (5.5 km) long and strikes north-south at its northern end, curving to N15°W at its southern end. The east side is downthrown throughout the length of the fault. Maximum displacement is 300 to 500 feet (90-150 m) in Section 33, T12S, R2W, where the Clear Creek Formation on the east is juxtaposed with Bailey Limestone on the west. The fault surface is not exposed, but evidently dips steeply. The lack of strong drag folding and the parallelism of minor faults and joints with the main fault imply that the Atwood Fault, like the nearby Delta Fault, is a high-angle normal fault.

Segar (1965) mapped large positive gravity anomalies on the west side of the Atwood Fault, conditions suggesting that basement rocks are upthrown west of the fault. The time of faulting and its relation to regional tectonics are unclear.