Abstract

In early Tertiary time, the western Wyoming overthrust region underwent several stages of uplift accompanied by synorogenic deposition of thick sediments into subsiding intermontane basins. The Hoback Formation in the Hoback Basin is an examble of one of these thick, early Tertiary deposits. The Hoback Formation ranges in age from Middle Paleocene (Torrejonian) to early Eocene (Graybullian). In the center of the Hoback Basin, it is about 16,000 feet thick, but thins southward to about 8,000 feet before plunging into the subsurface in the north end of the Green River Basin, where it intertongues with the Chappo Member of the Wasatch Formation. A conformable, intertonguing and gradational contact separates the Hoback from the overlying Pass Peak Formation, except in a local area along the front of the Gros Ventre Mountains, where the Pass Peak unconformably overlies the Hoback.

Six facies within the Hoback Formation suggest three major environments of deposition. A thick sandstone facies, characterized by large-scale cross-bedded, tabular, fine-grained sandstones, delineates a flood plain stream belt. A conglomerate and sandstone facies and a pebbly sandstone facies, distinguished by stratified pebble deposits with upper flow regime bedding and lenticular form, suggest major stream deposition on an alluvial plain. A flood plain is indicated by three facies: thin sandstone-dark shale, thin sandstone-vari-colored shale, and limestone-dark shale. In these facies, thick clays accumulated as overbank deposits, enveloping thin, small-scale cross-bedded, lenticular sand deposits of ephemeral stream channels.

Hoback sandstones are composed chiefly of quartz, chert, and limestone fragments with distinctive assemblages of minor constituents including zircon, opaques (chiefly leucoxene), colorless epidote-clinozoisite and collophane. Hoback conglomerates are composed entirely of Paleozoic and Mesozoic sedimentary rocks. The pebbles and light and heavy minerals in lower Hoback strata are mainly Mesozoic derivatives, whereas in upper Hoback strata, Paleozoic derivatives are dominant. This vertical distribution implies progressively deeper erosion of the source area. Cross-stratification measurements indicate predominant west-to-east sediment transport throughout most of Hoback time. The Snake River Range west of the Hoback Basin was a likely source for most of the Hoback sediments for several reasons: (1) The range is located west of the Hoback Basin, (2) It was uplifted in Late Cretaceous time just prior to Hoback deposition, (3) The Mesozoic rocks have been stripped off along the axis of the Snake River Range, exposing Paleozoic rocks.