- UW Department of Geology and Geophysics
The western border of the Hanna Basin is defined by the Rawlins uplift, a Laramide, basement-involved, faulted arch. This north-northwest–south-southeast-trending structure separates the Hanna Basin on the east from the Great Divide Basin (part of the greater Green River depositional basin) on the west. The Rawlins uplift is a west–southwest-vergent, macroscale, fault-related fold. Detailed geologic mapping, construction of serial cross sections, and the incorporation of data from a seismic-reflection profile indicate that displacement along the fault zone flanking the uplift's western margin cannot account for the net structural relief between the Hanna and Great Divide Basins (∼37,000 vertical ft [∼11,285 vertical m] and ∼27,000 vertical ft [∼8,235 vertical m], respectively). The exposed frontal fault traces are interpreted as high-angle (∼70°) splays off a shallowly dipping (∼25°), master fault zone developed within Archean granitic rocks of the Wyoming province. A low-dipping, braided, plastic-to-brittle thrust-fault zone in the Precambrian basement is inferred to accommodate much of the fault displacement and thus account for the structural relief between the core of the uplift and the adjacent basins. Within the study area, displacement along the exposed frontal fault zone decreases from south to north. Within the map area, bedding attitudes along the southwest limb (forelimb) of the uplift range from ∼30–90° with only local areas of overturned beds. However, southwest of the map area, Upper Cretaceous strata are sub-vertical to overturned. On the homoclinal backlimb, dips are ∼10–15° east-northeast into the Hanna Basin. Eastward structural bends at the southern and northern margins of the uplift suggest a component of left-lateral, oblique-slip displacement along the southern margin and right-lateral, oblique-slip displacement along the northern margin. Pre-existing basement anisotropies or discontinuities are likely responsible for these important changes in structural grain.