NEW INSIGHTS ON THE STRUCTURAL EVOLUTION OF THE EOCENE SOUTH FORK DETACHMENT, WYOMING

Deformed Jurassic Sundance Fm. through Cretaceous Cody Fm. sediments are in fault contact with Eocene Willwood Fm. in the valley of the South Fork of the Shoshone River based on recent remapping. The structure’s bounding faults offset Willwood strata (53 Ma) and are overlain by the Willwood Fm. and Wapiti Deer Creek debris flow (ca. 49.5 Ma). The large-scale structure is an eroded anticline (N30°E) with an overturned, internal SE limb (Morrison-Mowry Fms.) and a lower, multiply folded and repeated Sundance-Morrison section. The presumed shortening direction for the structure is ~NW-SE confirmed, in part, by strain analysis of mechanically twinned calcite in the Sundance Fm. Twinning strains also record a sub-vertical shortening stain in lower plate Sundance limestones. Fault kinematic observations are rare although there is abundant grooved, striated calcite-quartz-gypsum with occasional organics. The South Fork deformation in Jurassic-Cretaceous sediments is Laramide-aged, and a displacement of 10 km is postulated along the Jurassic Gypsum Springs Fm.