STRATIGRAPHY ACROSS THE JURASSIC/TRIASSIC BOUNDARY ALONG THE SOUTH FLANK OF THE UINTA MOUNTAINS, NORTHEAST UTAH: THE BELL SPRINGS MEMBER OF THE NUGGET SANDSTONE

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. In our study area, 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 have been previously correlated to the Glen Canyon Group of the Colorado Plateau, even though no obvious Wingate or Kayenta Formation equivalents have been recognized. The Bell Springs Member contains abundant ripple-laminated, fine-grained sandstones, occasional mudcracks and salt casts, and medium to coarse, cross-bedded sandstones probably formed by migrating sand bars. The overall environment was a marine tidal flat, 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.