Abstract

In the Uinta Mountain area, the Popo Agie Formation is conformable on the underlying Gartra Formation. The Popo Agie consists, in ascending order, of the purple, ocher, and upper carbonate units. In northwestern Colorado, the ocher unit is largely replaced by the sandstone and conglomerate unit. Locally, the ocher unit is thin or absent because of thickening of the underlying purple unit. An angular unconformity separates the Popo Agie from the overying Nugget Sandstone. The Popo Agie sequence in the Uinta Mountain area differs from that of western Wyoming only in the absence of a carbonate unit below the purple unit and the addition of one sandstone and conglomerate unit.

The Gartra and Popo Agie were deposited in a vast fluvial-lacustrine complex that covered most of western Wyoming and adjacent parts of Idaho, Utah, and Colorado. In general, the Gartra was deposited by high-gradient streams that flowed from the east and southeast, the purple unit by low-gradient streams, and the ocher and upper carbonate units in a large, saline lake. The sandstone and conglomerate unit is the product of local high-gradient streams and deltaic deposition. During deposition of the purple and ocher units, much of the sediment was derived from volcanic sources.