The Mount Watson Formation, an Interpreted Braided-Fluvial Deposit in the Uinta Mountain Group (Upper Precambrian), Utah

Abstract

The Mount Watson Formation, a member of the upper Precambrian Uinta Mountain Group, is here proposed as a formal lithostratigraphic unit. It consists of predominantly very light gray or almost white interbedded quartz arenite and subarkose with minor pale red or grayish-red arkosic arenite and grayish-green shale. This name was initially proposed by Wallace (1972) but not established formally. Type area and type section are at Mount Watson, Utah, a major peak in the western part of the Uinta Range. The depositional environment is interpreted as braided-fluvial based on several characteristics. Evidence for a braided-fluvial environment includes: (1) the paucity of fine-grained elastic material either as matrix or as mudrocks, (2) range of sand grain size from very line to very coarse with frequently-associated granules and pebbles, (3) predominance of omicron (planar) and pi (trough) cross-stratification in sets ranging from 4 in (10 cm) to 16.5 ft (5 m) thick, (4) presence of desiccation cracks and terraced ripples indicating subaerial exposure, (5) unimodal character of paleocurrent indicators within cosets of strata, and (6) recognition of a vertical facies sequence that closely matches the Platts Type depositional model of Maill (1977, 1978, 1982) for braided-stream deposits. A major river flowed westward through the Precambrian basin of the Uinta Mountain Group and deposited multicyclic quartzose sediment derived from the craton to produce arenite. Tributary streams carrying feldspathic sediment entered the basin from the north. This sediment was deposited and lithified to arkosic arenite or was intermixed with multicyclic quartzose sand in the trunk stream to produce subarkose. This paleoenvironmental interpretation contrasts fundamentally with that of Wallace (1972), who completed his interpretation before workable process-response facies models were developed for the braided-stream environment.