Upper Triassic strata that lie between the Chinle Formation and Nugget Sandstone along the south flank of the Uinta Mountains in northeastern Utah are distinctive. In the past, these rocks have been lumped together with the overlying or underlying units. These strata are equivalent to the Bell Springs Member of the Nugget Sandstone as defined in Wyoming and perhaps to the Rock Point Formation of the Chinle Formation near the Four Corners region. In this study, these rocks will be called the Bell Springs Formation following the usage of Lucas (1993) in Wyoming. The unit is regionally mappable in northeastern Utah, and 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 Formation is comprised of interbedded fine- to medium-grained sandstone and siltstone, as well as planar laminated mudstone. The unit varies from planar laminated sandstone with abundant ripple marks, to cross-bedded sandstone that contains scoured channels filled with mudstone or sandstone. The mudstone beds are commonly mottled and contain desiccation cracks while both the mudstone and sandstone beds have rip-up clasts, occasional bioturbation, and small salt crystal casts. The thinly bedded mudstone and siltstone beds are purple to red to brown, and the sandstone beds 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. The presence of desiccation cracks, plant root traces, small eolian sand dunes, gypsum casts, crinkly algal mat beds, and bioturbation indicate intermittent subaerial exposure. Fluvial deposits by meandering streams, including point bar, levee, and splay deposits comprise a large part of this formation. Rocks of the Bell Springs Formation have previously been interpreted as either tidal flat or fluvial/lacustrine deposits. A tidal flat environment certainly may produce some of the features found in these deposits, such as, alternating erosion and deposition of interfingering channels and scours with rip-up clasts, ripples, flaser bedding, desiccation cracks, and bioturbation; however, these rocks lack some of the most important characteristics of tidal flat deposits such as herringbone-cross-stratification, general fining upward successions, and regionally associated sediments that would typically be found in shallow marine environments. We conclude that the sedimentary characteristics and regional setting of these rocks fit best with a fluvial environment interpreted as a meandering system being deposited on a broad floodplain in an arid to semi-arid climate. This depositional environment existed between the expanding Nugget Sandstone erg and the shrinking Chinle Formation as desertification increased during the Late Triassic and Early Jurassic in what is now the western United States. This study not only helps solidify the understanding of the depositional history of these strata, it also clarifies the nomenclature of these formations for future mapping and research.



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Physical and Mathematical Sciences; Geological Sciences



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Bell Springs Formation, Unita Mountains, Nugget Sandstone, Chinle Formation

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Geology Commons