SUBDIVIDING THE BIGHORN BATHOLITH: GEOLOGIC MAP OF THE WOODROCK QUADRANGLE, SHERIDAN COUNTY, WYOMING USA

Here we present the results of detailed (1:24,000 scale) mapping of the Woodrock 7.5 Minute Quadrangle Wyoming, which mainly consists of Archean basement rocks of the Laramide Bighorn uplift. Our focus was on the Archean geology of the Laramide age Bighorn uplift. Isotopic age determinations (U-Pb on zircon) were conducted at the University of Arizona Laserchron Center. Our work revealed the presence of four different components of the batholith. The oldest unit is a ~2880 Ma foliated Lookout Mountain Granodiorite that occurs in the southern part of the quadrangle in the vicinity of Bruce and Lookout Mountains. The ~2775 Black Mountain Tonalite occurs in the northeastern part of the quadrangle. The age of the Black Mountain Tonalite and Lookout Mountain Granodiorite overlap but the units are distinct in terms of structure and lithology, so they were mapped separately. The central part of the quadrangle is underlain by the massive, 2660 Ma Taylor Mine Granite. This unit is poorly exposed and variable in texture. The youngest unit is the ~2850 Ma Owen Creek Alkali Feldspar Granite, which occurs in the western part of the quadrangle. Mafic dikes of variable geometry, texture and age crosscut the quartzofeldspathic rocks. Less than 20 m of poorly exposed Cambrian Flathead sandstone occurs along the extreme western margin of the quadrangle. We discovered as much of 30 m of Oligocene White River Formation strata that occurs as terrace deposits along the western side of the Tongue River in the central part of the quadrangle. The White River strata is poorly exposed and consist of thin bedded tuffaceous sandstone and massive conglomerate that are light colors and include clasts of Paleozoic carbonate and Archean basement rocks. Less than 50 m of Pinedale-age glacial tills occur along the upper Tongue River in the southern part of the Quadrangle, forming hummocky, poorly drained topography. Quaternary alluvium occurs along the larger streams.