- UW Department of Geology and Geophysics
The diverse orientations of Precambrian-involved and fault-bounded uplifts in the Rocky Mountain foreland region seem difficult to reconcile with the inferred Paleocene–Eocene shortening direction and plate convergence. Some have hypothesized that the shortening direction rotated as much as 90° over time, whereas others have hypothesized a relatively stable strain field involving concurrent oblique slip on faults with varied strikes.
The stratigraphic thicknesses of the synorogenic Paleocene–Eocene Fort Union, Wasatch, and Green River Formations in the southern Green River Basin of Wyoming are used here to define the pattern of basin subsidence and to interpret the timing of movement on two contractional structures with contrasting orientations. Isopach maps and cross sections of three sequential time-stratigraphic intervals show thinning around the north–south-oriented Rock Springs uplift and regional southward thickening of these three stratigraphic intervals. The southward thickening evidently resulted from flexural subsidence in response to loading from the adjacent east–west-oriented Uinta uplift. Stratigraphic thinning in the vicinity of the Rock Springs uplift demonstrates that growth of the underlying blind thrust system produced an area of comparatively low accommodation space within the subsiding Green River Basin.
The concurrent growth of the Rock Springs and Uinta uplifts does not support a rotation of shortening direction from east–west to north–south. The margin of a Neoproterozoic rift basin may have controlled the orientation of the thrust faults bounding the north flank of the Uinta uplift, and Late Cretaceous-through-Eocene contraction caused oblique slip on these faults during structural inversion. A north–south-striking blind reverse fault underlies the west flank of the Rock Springs uplift, but Phanerozoic strata cover the Precambrian core of the uplift, so the structure within the Archean basement rocks is unknown. Despite contrasting strikes and different structural histories, the Uinta and Rock Springs uplifts responded concurrently to northeast–southwest- to east-northeast–west-southwest-directed shortening.