Geologic map of the Bald Mountain Quadrangle, northern Bighorn Mountains, Wyoming

Abstract

Here we present the results of detailed (1:24,000 scale) mapping of the Bald Mountain 7.5 Minute Quadrangle Wyoming, which includes Archean basement rocks of the Laramide Bighorn uplift. These basement rocks are mantled by Paleozoic cratonic strata of the Flathead, Gros Ventre, Gallatin, Bighorn, Madison and Amsden formations. This region was explored for gold associated with the basement rocks, and Th in the basal Flathead sandstone. Isotopic age determinations (LA-ICPMS U-Pb on zircon) of the basement rocks were conducted at the University of Arizona Laserchron Center. Three phases of Archean rocks were defined, all of which range from ∼2880-2890 Ma, and include a prominent, pink, strongly foliated (WNW trending) alkali feldspar granite with pegmatite, a weakly foliated purple granite, and a yellow, poorly exposed adamellite. These rocks were uplifted during the Paleogene Laramide orogeny, and now form a southwest verging breached drape fold over a steeply inclined, basement-cored reverse fault. Dips of footwall strata range from 10–70° to the SW. Hanging wall rocks dip gently to the NE. Quaternary landslide deposits occur along steep slopes in the Gros Ventre Shale and alluvium occurs along the principle streams.