MAPPING THE UPPER TRIASSIC BELL SPRINGS FORMATION ALONG THE SOUTH FLANK OF THE UINTA MOUNTAINS, UTAH

Rocks that lie between the Upper Triassic Chinle Formation and Upper Triassic/Lower Jurassic Nugget Sandstone along the flanks of the eastern Uinta Mountains in northeastern Utah are distinctively different from both of those formations but have been mapped as one or the other in the past. These strata appear to be equivalent to the Bell Springs Member of the Nugget Sandstone as defined in Wyoming and to the Rock Point Formation of the Chinle Group near the Four Corners region. We have chosen to call this package of rocks the Bell Springs Formation following the usage of Lucas (1993). The unit is regionally mappable in northeastern Utah and has been separated out on our recent 7.5-minute quadrangle maps along the south flank of the eastern Uinta Mountains. The Bell Springs Formation is also exposed along the north flank of the Uinta Mountains in the Sheep Creek-Flaming Gorge area but may be attenuated by faults associated with the Uinta-Sparks fault zone. Correlative strata also seem to be present in the uppermost Ankareh Formation in the western Uintas. The Bell Springs consists primarily of (1) planar, ripple-laminated sandstone that often contains mud drapes (flaser beds) up to 10+ m thick, (2) cross-bedded sandstone that contains scoured channels filled with mudstone or sandstone with beds generally less than 1 m thick, and (3) mudstone beds that are commonly mottled and contain desiccation cracks. Both mudstone and sandstone beds have rip-up clasts, occasional bioturbation, and small salt casts. Some of the sandstone beds contain crinkly beds suggesting algal growth. The thinly bedded mudstone and siltstone beds are grayish red to dark reddish brown, and the sandstone layers vary from moderate orangish pink to moderate reddish orange with green and gray mottling. The Bell Springs is about 27 to 45 m thick and is conformable with underlying and overlying formations. Previous interpretations suggest that the Bell Springs Formation was deposited in a tidal flat environment; however, we now interpret the formation to have been mainly deposited in a semi-arid fluvial environment by meandering streams and includes sediments accumulated in point bar, levee, flood plain, and splay deposits.