PRELIMINARY GEOLOGIC MAP OF THE FORSYTH RESERVOIR 7.5’ QUADRANGLE, SEVIER COUNTY, UTAH

The Forsyth Reservoir quadrangle is located in Sevier County, Utah and is part of the Fish Lake Plateau. The bedrock geology of the mapping area is dominated by thick, densely welded ash flow sheets derived from the Marysvale Volcanic Field located to the W of the Fish Lake Plateau. Volcanic units include (from base to top) the Johnson Valley trachyandesite (⁴⁰Ar/³⁹Ar plateau age on plagioclase: 26.18 ± 0.09 Ma), the Lake Creek trachyite (whole rock ⁴⁰Ar/³⁹Ar age: 25.15 ± 0.14 Ma), and the Osiris trachyite (mean ⁴⁰Ar/³⁹Ar total fusion ages of sanidines: 23.03 ± 0.08 Ma). Patches of remnant basalt flows crop out in the eastern part of the quad. Recent geochronology on these flows yielded an ⁴⁰Ar/³⁹Ar plateau age of 5.02 ± 0.05 Ma and an isochron age of 4.98 ± 0.04 Ma. In a few locations small outcrops of Tertiary (Flagstaff Formation or perhaps younger) to Jurassic (Carmel Formation) sedimentary rocks are exposed under the volcanics. The structural geometry of the quad is characterized by a suite of grabens and normal faults of varying size, orientation, and age. Most of the grabens and normal faults in the W part of the quadrangle strike NNW while the master fault of the whole Fish Lake Plateau, the Thousand Lakes Fault, strikes NNE to NE. Partially lithified sediments, composed of boulder to silt-sized volcanic fragments, have filled the Fremont River and UM creek grabens. These sediments are older than boulders exposed on straths cut into the sedimentary fill which have cosmogenic exposure ages of 600-900 ka. Surficial deposits in the quadrangle include alluvium and colluvium. A small palustrine carbonate deposit crops out along Salt Gulch near the trace of the Thousand Lakes fault in the SE most part of the quadrangle. Our preliminary geologic map was constructed using ArcGIS and is based on data from extensive field inspection and air photo interpretation.