Structural and seismic-reflection evidence for development of the Simpson Ridge anticline and separation of the Hanna and Carbon Basins, Carbon County, Wyoming

The eastern border of the Hanna Basin, Wyoming, is defined by Simpson Ridge anticline, a Laramide structure that separates the Hanna Basin from the more easterly Carbon Basin. New geologic mapping along with interpretation of well logs and seismic-reflection data suggest that this structural feature was created through a combination of thick-and thin-skinned deformation during the contractional Laramide orogeny. Major west-directed, basement-involved faulting occurs beneath Simpson Ridge, suggesting that this structural feature is not related to the east-vergent Elk Mountain anticline. Although major basement-involved thrusts underlie Simpson Ridge, the development of the anticline also involved thin-skinned, out-of-the-basin thrusting related to the deeply rooted basement faults by a trishear deformational zone. An unconformity between the Cretaceous-Paleocene Ferris Formation and Paleocene Hanna Formation marks initiation of fault-controlled uplift along the Simpson Ridge anticline. On the eastern side of Simpson Ridge, the Hanna Formation was faulted westward onto the Lewis Shale, thereby yielding a younger-on-older structural relationship. Palynological data suggest that the age of the Hanna Formation in the southwestern Carbon Basin is late Paleocene. These data thus provide a maximum age for later deformation along Simpson Ridge anticline and for the consequent definition of the western edge of Carbon Basin.