Abstract

Study of the early Cenozoic Hoback formation adds detail to the Laramide history of central western Wyoming. The recently discovered late Torrejonian, Battle Mountain vertebrate faunule, low in the formation, includes a new condylarth, Tetraclaenodon transitus. Additions to the intermediate Dell Creek faunule include a new insectivore, Elpidophorus minutulus. Additional mammals from three other localities are described. In combination with earlier studies, stratigraphic thickness, faunal ages, and faunal levels provide estimates of sedimentation rates and times of orogeny.

The Hoback formation is a thick deposit of fluviatile, palludal, and lacustrine materials that accumulated in a subsiding orogenic trough. Sedimentation was very rapid, probably beginning and accelerating in Torrejonian, culminating during late Torrejonian, then decelerating during Tiffanian, Clarkforkian, and Graybullian times prior to a late phase of orogeny. Sediment was derived locally from western, mid-Laramide highlands which began to rise in the early Torrejonian; the uplift culminated between the middle and end of the Torrejonian. Orogenic phases were relatively brief but intense. The area of deposition was much lower, forested, temperate, humid, locally swampy with some lakes, and largely inhabited by a forest-dwelling mammalian fauna. The Cliff Creek thrust which overrode the Hoback formation is internally complex, with distinct fold-fault sets trending parallel with, perpendicular to, or acute to the thrust trace. Fold axes in the overridden western part of the Hoback formation roughly parallel the thrust trace and are concealed locally beneath the thrust. Local deformation severely distorted the conglomeratic materials in the formation.

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