POSSIBLE BELL SPRINGS FORMATION–CHARACTERIZATION AND CORRELATION OF UPPER TRIASSIC SEDIMENTARY ROCKS IN NORTHEAST UTAH

Upper Triassic rocks that lie between the Chinle Formation and Nugget Sandstone along the south flank of the Uinta Mountains in northeastern Utah appear to be stratigraphically equivalent to the Bell Springs Member of the Nugget Sandstone in Wyoming and perhaps to the Rock Point Formation of the Chinle Group near the Four Corners region. This unit is regionally mappable in northeastern Utah, and thus we believe it should be raised to formation status and be called the Bell Springs Formation. It is the sedimentologic transition from the fluvial-lacustrine environment of the Chinle Formation to the eolian depositional environment of the Nugget Sandstone. The Bell Springs is comprised of interbedded fine to medium-grained sandstone and siltstone, as well as planar laminated mudstone. The unit varies from planar laminated sandstone showing abundant ripple marks that fluctuate between asymmetrical and symmetrical flaser ripples with thin mud drapes, to cross-bedded sandstone that contains scoured channels filled with mudstone or sandstone. The mudstone beds are commonly mottled and contain mudcracks while both the mudstone and sandstone beds have rip-up clasts and occasional bioturbation and small salt casts. The thinly bedded mudstone and siltstone beds are colored purple to red to brown, and the sandstone layers vary in color from red to brown to orange or tan with green and gray mottling. The ripple structures with mud drapes indicate fluctuating deposition in low energy water and strong currents. The presence of features like desiccation cracks, plant roots and bioturbation indicate possible intermittent subaerial exposure. Previous interpretations state that the alternating erosion and deposition of the interfingering channels and scours with rip-up clasts, along with the previously mentioned sedimentary structures, can be considered diagnostic of clastic tidal flat deposition; however, these rocks are missing some of the most important characteristics of tidal flat deposits. While our study has made many of the same observations as previous researchers, we interpret the sedimentary characteristics of these rocks as being associated with a fluvial/lacustrine depositional environment.