PRELIMINARY GEOLOGIC MAP OF THE HUNTER MESA 7.5 MINUTE QUADRANGLE, JOHNSON COUNTY, WYOMING

The Hunter Mesa 7.5 Minute Quadrangle occurs along the east-central summit of the Bighorn Mountains. The coordinates of the Quadrangle range from 44o15’00” and 44o22'30” north latitude and 106o52'30' and 107o00'00" west longitude. The Quadrangle occurs entirely in the Bighorn National Forest and is in the drainage of Clear Creek. Elevations range from 6,800-9,000 ft. The Quadrangle is dominated by Archean crystalline rocks of the Wyoming Province Beartooth-Bighorn Magmatic Terrane, but also includes Oligocene gravels of the White River Formation and Quaternary glacial deposits of the Bull Lake and Pinedale Formations. The Archean rocks are mainly granodiorite and tonalite gneiss, with lesser granite, pegmatite, and migmatite. Several mafic dikes of varying ages and orientations crosscut the host quartzofeldspathic rocks. Fabric orientations for the host rocks range from N10W to N20E and are steeply inclined. One sample of gneiss yielded an age (U-Pb LA-ICPMS on zircon) of ~2910 Ma, which is consistent with other ages available for the Bighorn Mountains southern gneiss terrane. Additional isotopic ages are forthcoming. More than 150 m of flat-lying gravel of the Oligocene White River Group cap Hunter Mesa and occur as valley fill in flat-lying areas known locally as parks. The gravel is massive bedded and includes boulder-size clasts of local basement rocks. Pinedale Formation glacial deposits, which are as much as 50 m in thickness, occur as ground and terminal moraines along the principal drainages and overlie older glacial deposits of the Bull Lake Formation.The glacial deposits consist of matrix-supported boulder gravels as well and form classic kettled alpine glacial topography.