Abstract

Here we present the results of detailed (1:24,000 scale) mapping of the Spanish Point 7.5 Minute Quadrangle, Wyoming, which occurs along the western flank of the Bighorn Mountains. The Quadrangle includes Archean crystalline rocks of the Wyoming Province Beartooth-Bighorn Magmatic Terrane, Paleozoic miogeoclinal clastic and carbonate strata (Flathead, Gros Ventre, Gallatin, Big Horn, Madison, and Amsden formations, and Quaternary (Pinedale Formation) glacial deposits. Isotopic age determinations (LA-ICPMS U-Pb on zircon) of the basement crystalline rocks, which occupy the higher elevations in the eastern part of the Quadrangle, were conducted at the University of Arizona Laserchron Center. Seven samples of grey foliated to massive granodiorite were analyzed, all yielding ages of about 2880–2885 Ma, which is correlative to the Lookout Mountain granodiorite to the north. The dominant basement fabric orientation is ∼N75W90. The Lookout Mountain Granodiorite was uplifted along the N-S striking, west verging Laramide Spanish Point Reverse fault, which dips steeply to the east. Paleozoic strata in the footwall are steeply inclined to overturned. Displacement along the fault is as much as 150 m. To the west, the Paleozoic strata roll into a homocline that dips ∼10 degrees to the west. The Pleistocene Pinedale Formation glacial deposits, which are as much as 50 m in thickness, occur as ground, end, and lateral moraines along the principal drainages.