The Break-Away Point of the South Fork Superfault: A Catastrophic Gravity Slide, Wyoming, USA

Exposures of the South Fork Fault (SFF) system extend for over 35 km along the South Fork Shoshone River, southwest of Cody, Wyoming. Earlier workers have demonstrated the presence of tear faults, a triangle zone and other thin-skinned geometries along the leading edge of the allochthonous slide mass. Transport was to the southeast, down a relatively flat slope, in middle Eocene time, approximately coeval with the Heart Mountain Fault (HMF). Well data, seismic data and surface exposures indicate the detachment surface is in the Jurassic Sundance Formation and/or the underlying Gypsum Spring Formation.

Cross sections involving surface and well control extend the SFF 35-40 km northwestward to the break-away point and have identified an area of tectonic denudation. The SFF slide mass apparently broke free, under the influence of gravity, along an east-west-trending monocline created by the confluence of the Pat O'Hara Mountain and the Rattlesnake Mountain anticlines. The exposed denuded surface was almost instantaneously buried by additional Eocene-age volcanics. Sliding was initiated immediately following emplacement of the HMF mass. Rapid loading by HMF carbonate blocks may have assisted in initiating the break-away. Several large HMF remnants, including Logan Mountain and Sheep Mountain, were apparently carried “piggy-back” style to the southeast, enlarging the extent of the HMF. Tear faults in the rapidly moving SFF allochthon created different structural geometries within the slide mass and caused transport distance to vary between 5 km and 10 km. Catastrophic emplacement in minutes or hours seems likely, as has been suggested for the HMF. The total areal extent of the SFF allochthon exceeds 1400 km², nearly one-half the total size of the HMF system. The rapid emplacement of the SFF system in a single, continuous event, and the extensive transport distance, have defined the SFF as a superfault.