ESTIMATING TIMING AND MAGNITUDE OF SURFACE UPLIFT IN THE GREATER BIGHORN MOUNTAINS AREA, WYOMING, USING BASIN SUBSIDENCE MODELING

Previous geologic studies suggest that uplift of the Bighorn Mountains commenced during Paleocene time. These structural and stratigraphic studies indicate the minimum time of initiation of uplift but not necessarily the beginning of mountain building. Here we use flexural analysis of Powder River basin subsidence to reconstruct the timing, magnitude and location of tectonic loading by surrounding ranges.

Our data comes from newly constructed isopach maps of latest Cretaceous and Paleocene deposits (Lance and Fort Union formations) based on well-log data and recently published structure maps. From flexural modeling we find combinations of load heights and flexural rigidities that best-fit subsidence profiles along different basin cross sections. Assuming that lithospheric rigidity is unlikely to strongly vary across such a limited area, we can further constrain the height of mountain loads needed to fit the subsidence profiles.

Tentative results suggest that subsidence of the Powder River Basin during Maastrichtian time was primarily driven by uplift of the northern Laramie Range and, to a lesser extent, the Bighorn Mountains. During Paleocene time loading by the northern Bighorn Mountains became dominant, shifting the Fort Union depocenter to the north. The magnitude of surface uplift during the entire studied interval is in excess of 3.5 km. With additional constraints placed by reported unroofing histories, the total rock uplift exceeded 6.4 km. If uplift is along a discrete bounding fault with 30° dip, this requires ~12 km of shortening across the Bighorn Mountains. These results are consistent with previously published estimates, however we find that a significant amount of this uplift took place prior to Paleocene time, indicating that Laramide deformation in the Greater Bighorn region began somewhat earlier than previously assumed.