Piecing Together the Triassic/Jurassic Stratigraphy Along the South Flank of the Uinta Mountains, Northeast Utah: A Stratigraphic Analysis of the Bell Springs Member of the Nugget Sandstone Piecing Together the Triassic/Jurassic Stratigraphy Along the South Flank of the Uinta Mountains, Northeast Utah: A Stratigraphic Analysis of the Bell Springs Member of the Nugget Sandstone

by Jensen, Paul H

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 study 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 Bell Springs Member contains abundant fine-grained, ripple-laminated sandstones, red and green mudstones, occasional mudcracks and salt casts, evidence of burrowing and exposure, and some medium- to coarse-grained sandstones with small-scale (30-40 cm high) cross-beds. This member 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 appears to be entirely Upper Triassic, based upon dinosaur tracks, while the upper windblown unit’s age is unknown, but probably straddles the Triassic-Jurassic boundary.

During mapping in the Donkey Flat, Steinaker Reservoir, Dry Fork, and Lake Mountain quadrangles, the Bell Springs Member of the Nugget Sandstone was mapped as a separate unit.