Abstract

Nomenclature for the Upper Triassic and Lower Jurassic strata along the south flank of the Uinta Mountains has been somewhat confusing because of the position of the area between southern Wyoming, where one set of names is used, and central/southern Utah where a different set of formation names is used. The Nugget Sandstone or Glen Canyon Sandstone of the eastern Uinta Mountains overlies the Upper Triassic Popo Agie or Chinle Formation. The nature of the contact between these two formations is unclear both in stratigraphic location and conformability. The Chinle Formation consists, in ascending order, of the Gartra Member, the purple unit, the ocher unit, and the upper red unit. The overlying Nugget Sandstone consists of two members, the lower Bell Springs Member(?) and the overlying unnamed cross-bedded member, typically believed to be Navajo Sandstone equivalent. These two units of the Nugget Sandstone are thought to represent the Glen Canyon Group of the Colorado Plateau, although no obvious Wingate or Kayenta Formation equivalents have been recognized. The lower unnamed unit was deposited in a marine tidal flat environment, quite different from the mainly eolian environment of the rest of the Nugget Sandstone. The Bell Springs Member(?) may be entirely Upper Triassic, based upon dinosaur tracks found within it, while the upper unit’s age is probably Jurassic. In our current mapping efforts in the Donkey Flat, Steinaker Reservoir, Dry Fork, and Lake Mountain quadrangles, we intend to include this lower Nugget member as a separate unit and call it the Bell Springs Member(?).