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Dead Horse Tuff








Densely welded, phenocryst-rich and phenocryst-poor, rhyolitic to dacitic ignimbrites of Eocene age are exposed in Elko County. These exposures are within the Tuscarora and Independence Mountains in the China Camp, Sugarloaf Butte, Lake Mountain, Stampede Ranch, Reed Station and Mahala West Quadrangles, to the north in the Bull Run, Wild Horse and Mount Velma and Mountain City Quadrangles, and to the northeast in the Jarbidge Wilderness and adjacent Marys River Basin Quadrangles (Coats, 1985; Coash, 1967; Decker, 1962).

These ignimbrites are similar or identical in the China Camp, Sugarloaf Butte, Lake Mountain, Stampede Ranch, Reed Station, Mahala West, and Mount Velma Quadrangles (Coats, 1985; Coash, 1967). They are composed of crystal and crystal-vitric, dacitic to rhyolitic tuffs. The tuffs are rich in hornblende and biotite, have been strongly altered to chlorite, and contain a recrystallized matrix of quartz and feldspar (Coats, 1985). Thicknesses have not been estimated for this unit. Hornblende-biotite andesite and rhyodacite tuffs are also present in the Reed Station and Mahala Creek West Quadrangles, and have been dated at 40.7 +/- 1.2 Ma (McKee and others, 1976).

In the Jarbidge Wilderness and adjacent Marys River Basin Quadrangle, Eocene ignimbrites are composed of two cooling units that appear to be unconformable in many places. The tuff itself contains more fresh glass and less hornblende than those exposed in the Mount Velma area (Coats, 1985).

In the Bull Run Quadrangle, D.I. Axelrod (in Coats, 1985) described a ash-flow tuff sequence whih includes rhyolitic, dacitic and latitic tuffs. The rhyolite welded tuff is composed of crystals and crystal fragments of quartz, plagioclase, sanidine, and biotite in a groundmass of glass, shards, and pumice fragments. It is exposed eastward in the Wild Horse and southern Mount Velma Quadrangles (Coats, 1985). The lower part of the sequence contains welded latitic lapilli tuff with biotite and plagioclase phenocrysts and lapilli up to 3 inches long (Coats, 1985). Also included in the lower part of the sequence, are green-gray to black andesite flows with phenocrysts of augite or diopside and plagioclase. The lower bioite latite welded tuff is dated at 42.5 +/- 2.1 Ma (Coats, 1985).

In the Mountain City Quadrangle, a series of Eocene ash-flow tuffs commonly overlie Cretaceous granite or Paleozoic rocks, and in a few places rest on undated air-fall tuff (Coats, 1985). The lowest unit in several localities, including the Rabbit Draw area, is a pumiceous biotite rhyolite welded tuff that has been dated at 40.0 +/- 1.2 Ma (Coats, 1985; McKee and others, 1976). In other areas, the lowest unit is composed of a sequence of dacitic tuffs with a high proportion of glass and varying phenocryst suites with differing percentages of plagioclase, biotite, hornblende, hypersthene, and augite. At least one of the overlying units is so biotite-rich that it is as fissile as a biotite schist (Coats, 1985). These dacitic tuffs have been dated as 43.5 +/- 2.2 Ma (Coats, 1985). The only Eocene units that have been named in this sequence within Elko County are the lapilli tuff of Packer Basin in the southern Independence Valley dated at 41.0 +/- 1.2 Ma, and the ignimbrite of Mount Tuscarora, which is about 1,000 feet thick and is exposed from the Mount Blitzen Quadrangle northward to the Owyhee Quadrangle (Coats, 1985). The tuff of Packer Basin is composed of biotite and hornblende biotite rhyolite and dacite tuff with pumice lumps and xenoliths in a glassy matrix. The ignimbrite of Mt. Tuscarora is a single cooling unit of rhyolitic tuff (Coats, 1985).

In the Jarbidge Mountains, Eocene ignimbrites were named the Dead Horse Tuff and are described below.

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Last modified: 09/12/06