The Sand Wash Basin in northwestern Colorado is a southern sub-basin of the Washakie Basin of Wyoming. It contains several hundred meters of the middle Eocene Washakie Formation, overlying a thick sequence of the lacustrine Green River Formation. Late Bridgerian and earliest Uintan mammalian faunas have been recovered from several localities within the basin. Key biochronological indicators of the earliest Uintan age of the fauna include the earliest agriochoerids (Protoreodon), oromerycids (Oromeryx), as well as hyracodontids (Triplopus) and eomyids (Namatomys).
Abundant petrified wood, nonmarine stromatolites, gastropods, bivalves, fish, turtles, and crocodilians show that the Washakie Formation in the Sand Wash Basin was deposited in a marginal lacustrine-fluvial setting. Paleocurrents indicate sediment transport from the north, and the composition of the sandstones is mostly devitrified volcaniclastics derived from the Absaroka volcanic field of northwest Wyoming, or possibly the Challis volcanic field of Idaho. No sediments appear to be derived from the nearby Uinta uplift to the west.
The lower part of the sequence is all of reversed magnetic polarity, and the upper part is of normal polarity. Based on correlations with the revised magnetic stratigraphy of the Washakie Basin, the Sand Wash Basin sequence was deposited during Chrons C21r and C21n (47-48 Ma).
During the latest Cretaceous through early Eocene, the Laramide Orogeny caused Rocky Mountain intermontane basins to subside and collect thick piles of terrestrial sediments (Dickinson et al., 1988).