The alluvial to eolian transition of the Chinle and Nugget Formations in the Southern Uinta Mountains, northeastern Utah

The Uinta Mountains of northeastern Utah preserve the northernmost outcrops of classic early Mesozoic stratigraphic units on the Colorado Plateau, and document the transition to equivalent strata further north in Wyoming. In this region, the predominantly fluvial Upper Triassic Chinle Formation is overlain by the predominantly eolian Upper Triassic-Lower Jurassic Nugget Sandstone, which may be partially or wholly equivalent to the Glen Canyon Group further south. Previous work on these units has reached little consensus on where the boundary between the Chinle and Nugget should be placed, whether this boundary is conformable, and what lithostratigraphic nomenclature should be used. Our stratigraphic and sedimentologic investigation of outcrops in and around the western portion of Dinosaur National Monument and north of Vernal, Utah, indicate that the orange sandstone interval and overlying purple interval are assignable to the upper member of the Chinle Formation. This purple interval is interpreted as ephemeral sheet-flow deposits dominated by mud flats. The Chinle-Nugget contact appears conformable in many areas, with no evidence of the J-0 unconformity. The lowermost 10-20 meters of the Nugget Sandstone show extensive evidence of subaqueous deposition and/or modification, with only occasional thin cross-bedded eolian units, and these transitional layers commonly contain vertebrate tracks, some of which suggest a Triassic age. Taken together, these data indicate the gradual drying of the Chinle river and floodplain system at the same time as the encroachment of the Nugget/Wingate erg. Trace and body fossils indicate this erg margin was populated by a wide variety of invertebrate and vertebrate organisms, including early dinosaurs