INVESTIGATIONS INTO THE STRUCTURAL HISTORY OF SEDIMENTARY UNITS FOUND IN THE SHELL RESERVOIR QUADRANGLE OF THE BIGHORN MOUNTAINS, WY

The structural geology of the Bighorn Mountains is well documented. The area of interest is the southwestern portion of the Shell Reservoir Quadrangle. The surficial geology of the mapping area is characterized by 5 sedimentary units that overlay an intrusive Archean basement. These units are the Flathead Sandstone, Gros-Ventre Fm., Gallatin Fm., Bighorn Dolomite, and Madison Limestone. These units were all mapped using field observations and numerous geologic cross-sections. Due to field difficulties and the similarities of the units, the Gallatin and Gros-Ventre Fms. were mapped together. In the Bighorns, the most prominent structural feature is a monocline that can be observed along the walls of Shell Canyon towards the west of the Shell Reservoir Quadrangle. For the region being analyzed, all sedimentary units are observed to be relatively flat (< 25°) and the main structural focus is placed on a thrust fault that extends along the southwestern corner of the mapping area. Evidence of this fault was revealed by the lack of the full thickness of the Flathead Sandstone in places, and the lack of it entirely in other locations. In places the Gros Ventre Fm. directly overlays the Archean basement, omitting the Flathead Sandstone entirely. Therefore, in this southwestern area, multiple hypotheses involving a thrust fault were constructed to support these observations. Field observations suggest that a section of the granitic basement and Flathead Sandstone was thrust over younger material. Further analysis via cross-section of these structural features is underway.