EOCENE BASINS IN NORTHEASTERN GREAT BASIN, UTAH AND NEVADA, AND THEIR SIGNIFICANCE

Eocene sedimentary rocks from near the Wasatch front in Utah to east-central Nevada record areas of subsidence prior to the onset of volcanism at 43-42 Ma. As part of our work on USGS mineral resource projects, we are compiling information on Eocene sedimentary rocks from digital state maps and recent fieldwork. The results are being used to develop a regional geologic setting of the Eocene in northeastern Great Basin.

Eocene units dominated by shallow lacustrine and wetland limestone and clastic rocks are exposed along the front of the Sevier thrust belt in central to southern Utah and in the hinterland of the thrust belt in central Utah and eastern Nevada. Units include the Flagstaff, Colton, Fowkes, White Sage, Sheep Pass, and Elko Formations as well as scattered, unnamed areas of outcrop. Typically, fluvial conglomerate and sandstone near the base of the Eocene sequence are overlain by ostracod limestone, shale, and volcaniclastics. Clasts in the fluvial units are locally derived from underlying or nearby units and indicate restricted drainage. Near the Wasatch front, Eocene rocks overlie Paleogene orogenic sedimentary rocks derived from the Sevier thrust belt; in western Utah, they overlie Cambrian rocks; and in most of eastern Utah and Nevada, they overlie Paleocene to upper Paleozoic rocks. This distribution of rocks underlying similar Eocene rocks reflects the structural relief during the Cretaceous to Tertiary Sevier orogeny and subsequent erosion of the Sevier highlands (Armstrong, 1968). The predominantly low-energy-derived basin rocks indicate low topography, but at least locally deposition started with uplift or (and) collapse of Sevier thrust faults. Accumulation of up to 600 m of shallow-water sediments indicates slow but significant subsidence. We suggest that the Eocene basins formed as sag basins at the onset of Tertiary extension.