PRELIMINARY GEOLOGIC MAP OF THE CARIBOU CREEK 7.5 MINUTE QUADRANGLE, JOHNSON COUNTY, WYOMING

The Caribou Creek 7.5 Minute Quadrangle occurs along the east-central summit of the Bighorn Mountains. The coordinates of the Quadrangle range from 44o7’30” and 44o15'00” north latitude and 106o52'30' and 107o00'00" west longitude. The Quadrangle occurs almost entirely in the Bighorn National Forest and is in the drainage of Clear and Crazy Woman creeks. Elevations range from 6,800-9,400 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 Paleozoic strata of the Flathead, Gallatin, Gros Ventre, and Bighorn formations. The Archean rocks are mainly granodiorite and tonalite gneiss, with lesser granite, pegmatite, and aplite. Several orientations and ages of mafic dikes crosscut the host quartzofeldspathic rocks. One sample of gneiss yielded an age (U-Pb LA-ICPMS on zircon) of ~2954 Ma, which is consistent with other ages available for the Bighorn Mountains southern gneiss terrane. The dominant structural grain trends NE-SW. Additional isotopic ages are forthcoming. Paleozoic strata, which occur only in the southeastern part of the Quadrangle, dip moderately to the east and are more than 300 m in thickness. More than 150 m of flat-lying gravel of the Oligocene White River Group cap many of the flat-lying areas known locally as parks. The gravel is massive bedded and includes boulder-size clasts of basement rocks.