GEOLOGIC MAP OF THE SKULL RIDGE 7.5-MINUTE QUADRANGLE, SHERIDAN COUNTY, WYOMING, USA

The Bighorn Mountains trend North from central Wyoming (WY) into southern Montana and are positioned to the northeast of the Rocky Mountains. The Bighorns and other surrounding mountain ranges in the Wyoming Province were uplifted during the Laramide Orogeny around the Late Cretaceous. The Archean basement rocks are divided into two sections, a northern granitic batholith and southern gneiss complex, and were formed ~2900-2500 Ma during a series of geologic events that included regional metamorphism, plutonism, and a deformational period. The margin areas of the Bighorn Mountains are characterized by an abundance of deposited sedimentary rocks units that overlie the crystalline basement rock. Geologic units from Precambrian to Quaternary in age. The entirety of the Bighorn Mountains are currently mapped (Scale: 1:500,000) for the Wyoming State Geological Survey. The previous map of WY lacks any subdivision of granitoid rock units and classifies the northern granitic batholith as ranging from quartz diorite to quartz monzonite, all of similar ages. This project involved construction of a detailed bedrock geologic map of the Skull Ridge, WY 7.5-Minute Quadrangle (Scale 1:24,000). This research further advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques and Field Move Clino were used in concert with geochronological and geochemical analyses. Our goal is to continue to subdivide the various phases of the ~2.8-3.0 Ga Archean rocks based on their lithology, age and structure. Throughout the mapping, four granitoids were found within the area, some cross-cut by aplitic and pegmatitic dikes. The granitoids displayed subtle but perceptible planar fabric that was variable in orientation. This variability is interpreted as flow during emplacement within the pluton. Within the sedimentary rocks of the Skull Ridge quad, an anticline plunging to the northwest was discovered encompassing the mapping area north of Tongue Canyon. Preliminary geochemistry indicates an enriched source of magma from a subduction zone setting as the petrogenesis of the Archean basement rocks. Zircons collected from the samples are euhedral to subhedral and range in color from pink to yellow.