GEOLOGIC MAP OF THE PARK RESERVOIR 7.5-MINUTE QUADRANGLE, WY, USA

This project involved the construction of a detailed bedrock geologic map of the Park Reservoir, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000). The Quadrangle occurs entirely in the Bighorn National Forest, which is a popular recreation site 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 were used in concert with isotopic age determinations. Our goal is to further subdivide the various phases of the 2.8-3.0 Ga Archean rocks based on their lithology, age and structure.

The northern part of the Bighorn Mountains is composed of the Bighorn Batholith, a composite complex of intrusive bodies that were emplaced between 2.96-2.84 Ga. Our mapping of the Park Reservoir Quadrangle has revealed the presence of five different Archean quartzofeldspathic units, sets of amphibolite and diabase dikes, a small occurrence of the Cambrian Flathead sandstone, two Quaternary tills, and Quaternary alluvium. The Archean units range in age from ca. 2.96-2.75 Ga, the oldest of which are the oldest yet 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 flow during emplacement. The Granite Ridge tear fault, which is the northern boundary of the Piney Creek overthrust block, is mapped into the Archean core as a mylonite zone. This indicates that Piney Creek overthrust block bounding faults are controlled by weak zones within basement rocks.