RECONSTRUCTION OF PALEOALTIMETRY AND TECTONIC HISTORY OF BIGHORN MOUNTAINS AND SURROUNDING RANGES IN NORTHEAST WYOMING WITH 3D BASIN MODELING

Most previous geologic studies suggest that Laramide Uplift in the Bighorn Mountains area took place and made significant surface relief in Paleocene time. These studies are based upon structural and stratigraphic data, such as cross-cutting relationships and provenance data, which are more indicative of the latest time of activity and not necessarily the initiation of mountain building. Here we use 3D flexural analysis to build the basin subsidence models of the Powder River Basin and the Wind River Basin in northeast Wyoming, attempting to estimate the timing and the magnitude of the major uplifts of this area.

Modified isopach maps of latest Cretaceous and Paleocene deposits (Lance and Fort Union Formations) based on well-log data and outcrops in both the Powder River Basin and the Wind River Basin are used in our research. We built a 3D flexural model that was very similar to the real pattern of the basin subsidence by changing the flexural rigidity and the height of mountains. With the Assumption that lithospheric rigidity is unlikely to have strongly difference in such a limited area, we are able to further constrain the location and the average height of mountain loads which are needed to form the final subsidence pattern in the whole basin.

Tentative results suggest that the Casper Arch, the Northern Laramie Range and the Hartville Uplift were active in Maastrichtian time, with a surface average height about 1km. The Powder River Basin Subsided most significantly in the southwest part because of the corner effect of these ranges. Meanwhile, the uplifting of the Casper Arch also leaded to the subsidence in the east part of the Wind River Basin. During Paleocene time, loading was changed to dominate the locations of the Owl Creek Range and the Bighorn Mountains. Therefore, the depocenter of the Fort Union Formation in the Powder River Basin shifted to the north. The depocenter of the Wind River Basin also moved slightly to the west because of the uplifting of the Owl Creek Range. There was about 2km of surface uplift along the Bighorn Mountains in the end of the Paleocene time. With additional constraints placed by reported unroofing histories, the total rock uplift exceeded 5km. No evidence shows the subsidence along the east edge of the Powever River Basin. Therefore, the Black Hills didn't come out until early Eocene.