GEOLOGIC MAP OF THE EASTERN PART OF THE MOUNT POWELL 7 ½ MINUTE QUADRANGLE, HIGH UINTA WILDERNESS, NORTHEASTERN UTAH

Neoproterozoic Uinta Mountain Group stratigraphic units exposed in the eastern Mount Powell quadrangle include the newly discovered fm of Island Lake, and stratigraphically upward, the formations of Red Castle, Dead Horse Pass, Mount Agassiz, and Hades Pass .

The formation of Island Lake (> 25 m) is a purple to grey, medium to very coarse-grained quartz arenite in medium-thick lenticular to tabular beds, interbedded with green to orange, micaceous silty shale, siltstone and very fine quartz arenite. Sedimentary structures include mud-crack casts, trough crossbeds, starved current and mud-draped wave ripples, m-scale soft-sediment folds, and possible HCS. Paleocurrent data are polymodal and show flow to the SSW and NNE. Detrital zircons include 1000-1100 Ma Grenville grains, 1450-1500 Ma mid-continent granite-rhyolite grains, and 1700-1850 Ma Yavapai province grains.

A full section of the fm of Red Castle is exposed and divided into three members. The lower Red Castle member (~185 m) is a coarse-grained arkose with subordinate fine-grained sandstone and siltstone. Sand to silt ratio is > 10. It contains m-scale amalgamated channelforms filled with lateral accretion surfaces, and interbedded with tabular medium to thick beds. Paleocurrents are unimodal southward. The middle Red Castle (~400 m) contains laterally continuous upward-fining m-scale couples of medium to coarse, moderately sorted arkosic arenite overlain by very fine sandstone, siltstone, and shale. Medium beds of sandstone are trough crossbedded with mud-drapes on foresets. These beds are packaged within low-angle angle lateral accretion surfaces within subtle amalgamated channelforms. Sand to shale ratio is ~ 1. Paleocurrents are unimodal southward. The upper Red Castle (~220 m) contains shale (some organic-rich) to very fine quartzose sandstone and coarse grained, pebbly arkosic arenite. Isolated m-scale channelforms are characteristic. Sand to shale ratio is < 1. Paleocurrents are unimodal to the northwest.

The quadrangle straddles the east-trending Laramide-age Uinta Mountain anticline. On the north limb of the fold, several southwest-vergent thrust faults repeat the section. North- and south-dipping normal faults on the crest of the fold accommodated post-Laramide collapse of the anticline.