ABSTRACT: The South Fork Fault Revisited: Implications for Exploration, Bighorn Basin, Park County, Wyoming

Timothy L. Clarey

In recent years, explorationists in the Bighorn basin have focused increased attention upon the Shoshone River drainage subbasin, an area dominated by volcanic deposits and shallow faulting phenomena. An understanding of the extent and developmental history of the South Fork fault is a vital precursor to subsurface exploration in this area.

A new interpretation of the South Fork fault is proposed in light of thin-skinned thrust theory. Cross sections and seismic data indicate that the South Fork fault is an allochthonous salient, emplaced in the Bighorn basin in one deformational event during the early to middle Eocene. All observed structural geometries can be interpreted as developing under a compressional regime, similar to the Wyoming-Utah-Idaho thrust belt. Faults either follow bedding-plane surfaces, cut up section in the direction of tectonic transport, or form backthrusts. Other identified or interpreted structures, such as tear faults and triangle zones, further support the conclusion that this area underwent thin-skinned shortening. A single decollement within the Jurassic Gypsum Spring Formation appears to con rol structural development. Tectonic transport was approximately southeast, parallel to tear faults in the allochthonous plate.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990