Abstract

The Pass Peak Formation is a Tertiary basin-flank deposit exposed throughout much of the Hoback Basin; a depression physiographically distinct from, but structurally continuous with, the northern Green River Basin. Two lithofacies of the Pass Peak Formation are identified. A northern quartzite conglomerate-sandstone facies, possibly as thick as 3200 ft, intertongues southward with a sandstone-siltstone facies 1500 ft thick. This in turn extends southward and intertongues with the Wasatch Formation. Arkosic sandstone and conglomerate intertongues with the Pass Peak on the east.

Trends in grain size and sandstone composition, and directional structures indicate that: (1) a sedimentary source to the north supplied quartzite cobbles and associated garnet-bearing sand and (2) an igneous or metamorphic source, or both, to the northeast supplied abundant arkosic debris. Regional geology suggests that the Pinyon Conglomerate to the north was the source for Pass Peak sediments and the Precambrian core of the Wind River Range to the northeast was the source for the arkosic sediments to the east.

Uplift in the Mt. Leidy Highlands, the Gros Ventre Mountains, and the Wind River Range during the early Eocene resulted in the deposition of fanglomerate in the northern part of the basin and coarse arkosic alluvial plain sediments in the eastern part. The finer debris from the north was carried southward onto a flood-plain where it mixed with the finer sediments from the east.

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