Neogene Tectonics and Geomorphology of the Eastern Uinta Mountains in Utah, Colorado and Wyoming

Hansen, Wallace

A recent reevaluation of the Bishop Conglomerate in the Eastern Uinta Mountains region helps clarify how the conglomerate relates to the Browns Park Formation and, in turn, how both formations relate to middle Tertiary tectonic activity and late Tertiary and Quaternary drainage adjustments. Field relations indicate widely divergent histories for the two formations: They are separated in their type areas by great topographic discontinuity, but east of the Uinta Mountains, between Cross Mountain and Maybell, Colo., the Browns Park lies directly on the Bishop. The Bishop Conglomerate is roughly coextensive with the subjacent Gilbert Peak erosion surface; the Bishop was deposited on this broad pediment soon after the surface formed, perhaps while the surface was still forming. In many places, however, mountainward parts of the surface are bare rock. These places and the adjacent mountains were the sources of the conglomerate. The site of Browns Park, which lies in the axial region of the Uinta anticline, was part of the crest of the Uinta Mountains when the Gilbert Peak surface was forming. On the high proximal margins of the pediment, especially on the south slope of the range, headward parts of the surface penetrate deep into the range as flat-bottomed, alluviated remnants of dendritic valleys. Basinward, these valleys coalesced, and at one time they must have formed a continuous bajada around the north, east, and south margins of the range. A long period of crustal stability was needed to develop so broad and uniform a plain. Nearly flat to rolling uplands 180 m or more above the Gilbert Peak surface and the Bishop Conglomerate at various places in the Eastern Uinta Mountains are regarded as remnants of a truncation surface higher and older than the Gilbert Peak surface. Remnants of this surface, here called the Wild Mountain upland surface, are preserved on Wild, Diamond, Douglas, Blue, and Cross Mountains and perhaps on summits farther west. They consist of resistant bare rock, and they truncate subjacent formations without regard for rock structure. The Wild Mountain upland surface probably is Eocene in age. It probably formed in the mountains while the thick Eocene alluvial and lacustrine deposits of the Wasatch, Green River, Bridger, and Uinta Formations were accumulating in the adjacent basins.