STRATIGRAPHY AND FACIES ANALYSIS OF THE NEOPROTEROZOIC UINTA MOUNTAIN GROUP, SWALLOW CANYON QUADRANGLE, UTAH-COLORADO BORDER REGION

Mapping and measured sections of the Neoproterozoic Uinta Mountain Group (2.7 km exposed locally) in the Swallow Canyon Quadrangle, eastern Uinta Mountains, Utah-Colorado border area, indicate facies representing cycles of braided stream deposition punctuated by low-energy deposition during drowning of the braided system.

Three facies are present: 1) trough-crossbedded sandstone facies 2) thickly-bedded to massive sandstone facies, and 3) fine-grained sandstone-shale facies.

The trough-crossbedded sandstone facies is characterized by cosets of medium-bedded linear troughs within tabular to wedge-shaped composite sets (10's of m-scale thick) or wide (1 to 10's of m-scale) shallow scours. This facies comprises fine-grained to granule, moderately- to poorly-sorted, well to sub-rounded, silica-, hematite-, and carbonate-cemented quartz arenite with minor quartzite pebbles and mudstone intraclasts. Subordinate thin intervals (< 10 cm thick) of red fine-grained sandstone and shale create recessive intervals within the thicker sandstone composite sets. Paleocurrent data reveal paleoflow directions of 145° to 300°. This facies represents migration of sinuous dunes and linguoid bars, scour fill, and minor waning flow within a sandy braided river system. The thickly-bedded to massive sandstone facies is gradational with the trough-crossbedded facies. This facies comprises coarse-grained to granule, poorly sorted and subangular, hematite- silica-, and carbonate-cemented quartz arenite with minor quartzite pebbles and mudstone intraclasts. The sandstone composite sets are punctuated by thin recessive intervals (< 10 cm thick) of red fine-grained sandstone and shale. Paleocurrent data indicate south-southwesterly flow (190°-270°). This facies represents complex bar formation, scour fill, and minor waning flow within a sandy braided river system. The fine-grained sandstone and shale facies is characterized by thinly bedded, dominantly greenish-grey, micaceous silica- and carbonate-cemented sandstone and shale. Sedimentary structures include mudcrack casts, symmetric ripples, and ripple trough crossbeds. This facies is found in one laterally continuous (>10s of km.) interval (~50 m thick) in the map and represents drowning of the braidplain by sluggish marine or lacustrine incursion.