Abstract

Recent mapping in the Swallow Canyon quadrangle, northeastern Utah and northwestern Colorado, has resulted in the division of part of the eastern Uinta Mountain Group into three informal formations and further subdivision into facies associations. Four facies associations are identified: the low-angle and trough cross-bedded facies association, the thickly bedded to massive facies association, the interbedded facies association, and the fine-grained facies association. The first three facies associations are medium- to coarse-grained sandstones, with architectural characteristics of braided fluvial environments. The fine-grained facies association contains sedimentary structures characteristic of a low-energy to stagnant environment, likely within a delta plain. Paleocurrent analysis of the sandstone facies yields an overall southwesterly paleoflow direction, and stratigraphic and facies analyses show southwestern progradation of the eastern Uinta Mountain Group fluvial-deltaic system. The informal formations are: the formation of Diamond Breaks, the formation of Outlaw Trail, and the formation of Crouse Canyon. The formation of Outlaw Trail is a distinct, fine-grained, laterally continuous (10s of kms) interval (~50–70 m thick). The formation of Diamond Breaks contains all Uinta Mountain Group strata exposed stratigraphically below the formation of Outlaw Trail (~500–1000 m thickness) within the Swallow Canyon quadrangle, and the formation of Crouse Canyon contains at least ~1000 m of strata above the formation of Outlaw Trail. The formations of Diamond Breaks and Crouse Canyon both contain packages of quartz-rich cross-bedded sandstone, whereas the formation of Outlaw Trail contains arkosic-rich laminae of siltstone to fine-grained sandstone.