Geologic Map of the Skull Ridge Quadrangle, Sheridan County, Wyoming USA

This project involved the construction of a detailed geologic map of the Skull Ridge, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000) and supported the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The quadrangle includes parts of the Bighorn National Forest and the Amsden Creek Wildlife Habitat Management Area, which are popular recreation sites for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques and FieldMove Clino were used in concert with petrography, geochemistry and isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Skull Ridge Quadrangle has revealed the presence of three different Archean quartzofeldspathic units, eight sets of amphibolite and diabase dikes, and a Phanerozoic sedimentary section including the Cambrian Flathead Sandstone through the Pennsylvanian Amsden Formation deformed by two possible blind faults and forming an anticline along the northern section of the mapping area. The Archean rock units’ range in age from ca. 2.91–2.86 Ga, which correlates with other basement rocks reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement.