9-6 Structural Interpretation and Geologic Map of the Mexican Hill Quadrangle, Northwestern Bighorn Mountains, Wyoming, USA

Session: Advances in Geologic Mapping, Databases, and Dissemination: Student Posters

Presenting Author:

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Abstract:

The Bighorn Mountains of north-central Wyoming show evidence of a long geologic history, beginning in the Archean. The formation of the mountains themselves is due to the Laramide Orogeny, a mountain building event which effected the landscape of the western North American Cordillera. This project involved geologic mapping of the Mexican Hill Quadrangle at the 1:24,000 scale and a structural analysis to find evidence of Laramide-style deformation.

The Mexican Hill Quad. is found in the northwestern Bighorn Mountains, bordering Montana, and contains Archean basement rocks of the Wyoming Province, along with sedimentary units from the Cambrian through Mississippian Periods. The crystalline rocks within the quadrangle are tonalite, quartz monzodiorite, granodiorite, monzogranite, and mafic dikes. The sedimentary units include the Flathead Sandstone, Gallatin-Gros Ventre (GGV) Fms, Bighorn Dolomite, Madison Limestone, and Amsden Fm. The Flathead, GGV, and Amsden formations were typically observed as grassy hills and shallow slopes whereas the Bighorn Dolomite and Madison Limestone were exposed as prominent, steep cliffs. Traditional field mapping methods combined with Field Move Clino were used to create a detailed geologic map of the quadrangle along with interpretive cross sections of the field mapping area.

Within the mapping area, the units demonstrate Laramide deformation, including a major thrust fault that was observed cutting through the field area. In the northern half of the map, basement rocks were mapped next to the GGV Fms which indicates there can be as much as 115m of missing data. In the southern half, the Madison Ls was mapped next to basement which indicates missing data up to 400m thick. West to this fault flatirons were observed dipping ~73°W which were later interpreted as monoclinal folding that can be observed in other areas in the Bighorn Mountains. The rest of the sedimentary units were dipping shallowly (~2°-34°). By plotting this bedding data on stereonets, almost all units west of the fault dip to the west while units to the east of the fault dip to the northeast/east. Continued studying of the structural deformation of the Bighorn Mountains will provide a clearer history of the range.

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