Abstract

During Paleocene time, the Granite, Wind River, Washakie and Owl Creek Mountains were rising highlands that enclosed the Wind River Basin on the south, west, and north. Fluvial deltaic, lacustrine and paludal sediments of the Fort Union Formation accumulated in the subsiding basin. The streams flowed basinward, as evidenced by paleocurrent data for cross-bed sets in fluvial sandstones that crop out along the southern and western margins of the basin. The streams probably flowed across the southern margin of the present Powder River Basin to empty into the Cannonball Sea.

The initial detritus eroded from the rising highlands consisted dominantly of grains derived from Mesozoic and Paleozoic strata that mantled the Precambrian cores of the ranges. The sedimentary detritus was largely quartz, chert, sandstone-siltstone fragments, and round, spherical grains of zircon, tourmaline, and rutile. Further uplift and unroofing of the highlands led to extensive breaching of the Precambrian cores of the Granite Mountains and southern end of the Wind River Mountains by Middle to Late Paleocene time. The sandstones at Castle Gardens, Alkali Butte, and Hudson show marked mineralogical differentiation from cherty litharenite and sublitharenite in the lower parts of the sections to subarkose, lithic arkose, and arkose in the upper parts. The influx of Precambrian detritus was characterized by quartz, feldspar, igneous, and metamorphic fragments, and angular zircon, tourmaline, rutile, garnet, zoisite, staurolite, and sphene. The Precambrian cores of the Washakie, Owl Creek, and northern end of the Wind River Mountains were exposed only locally during the Paleocene.