The Bates Mountain Tuff was named for exposures on Bates Mountain in Sec. 35, T.20 N., R.46 E. Simpson Park Mountains (Stewart and McKee, 1968c).

The Bates Mountain Tuff is the youngest formation exposed in many areas, resting on Paleozoic, Mesozoic and Cenozoic rocks in various exposures (Sargent and McKee, 1969; Stewart and McKee, 1977). K-Ar dating of the tuff in a variety of locations give a range from 22.1 to 25.7 Ma, with several analyses indicating an average age of about 23.7 Ma. The Bates Mountain Tuff is considered early Miocene in age, assuming an Oligocene-Miocene boundary at 26 Ma.

The Bates Mountain Tuff is generally a low cliff forming, crystal-poor, rhyolite ash-flow tuff with less than 10 percent sanadine and quartz phenocrysts, and very small amounts of plagioclase and biotite, in a shard-rich matrix (Sargent and McKee, 1969). In Lander County, two or three cooling units are represented in the Bates Mountain Tuff, with only one unit near the edge of the ash-flow sheet. At least four cooling units are present in the Toquima Range (Stewart and McKee, 1977; Sargent and McKee, 1969). The interval between eruptions of individual cooling units was on the order of 0.5 to 1 M.y. (Gromme and others, 1972).

These cooling units are typically 50 to 150 feet thick with a thin lenticular zone of black glass (vitrophere) at the bottom of a grayish pink to buff nonwelded basal portion, grading upwards into cliff-forming, pink to reddish brown, densely welded and devitrified tuff. This is locally overlain at the top of the unit by soft, brick-red, vapor-phase altered tuff containing light gray pumice up to 2 inches in length (Stewart and McKee, 1977).

Sargent and McKee (1969) broke out 5 cooling units in the Bates Mountain Tuff; the upper unit however is the separate Tuff of Clipper Gap (Gromme and others, 1972; McKee, 1976b; Stewart and McKee, 1977). Gromme and others (1972) lumped the four cooling units of Sargent and McKee (1969) into three units in the northern Toquima Range (McKee, 1976b). Unit B, the bottommost unit is about 160 feet thick at Clipper Gap Canyon and is composed of a nonwelded base, a middle densely welded zone of white to pale pink tuff, and an upper moderately to densely welded zone. About 10 percent of the tuff is composed of phenocrysts, 50 percent of which are sanadine, 40 percent plagioclase, and the remainder is quartz, biotite, and hornblende (McKee, 1976b).

The overlying cooling unit C, consists of a lower, soft, light gray tuff which grades upward into increasingly welded pink to orange tuff. A 10 foot thick zone of black glass is present near the base of the unit. Large flattened pumice fragments are scattered throughout the tuff (McKee, 1976b).

The highest and youngest cooling unit in the sheet, unit D, is particularly unique because of its ubiquitous flattened gas cavities in the lower portion giving it a "swiss cheese" texture. This unit has been recognized throughout Lander and Nye Counties (Sargent and McKee, 1969). The upper and lower portions of the cooling unit are weakly to nonwelded with two intervening zones of densely welded pink crystal-poor rhyolite tuff (McKee, 1976b).

In the Pritchards Station Quadrangle area of the Park Range, the Bates Mountain Tuff is a single cooling unit of rhyolitic ash flow tuff that is moderately welded, and grayish to pinkish-orange in color. 20 to 30 percent of the rock is composed of purple pumice lapilli (Dixon and others, 1972). Phenocrysts compose 2 to 8 percent of the rock with 50 to 90 percent alkali feldspar, 6 to 20 percent potassium feldspar, 3 to 16 percent quartz, 0 to 5 percent biotite, 0 to 2 percent hornblende, 0 to 2 percent clinopyroxene and 0 to 4 percent opaques.

Remanent magnetization studies on this ash flow tuff sheet in the Toquima Range indicate that each cooling unit has a unique remanent direction. The bottommost cooling unit B has a normal remanent magnetization, cooling units 2 and 3 of Sargent and McKee (1969) or Unit C of McKee (1976b) are reversed, while unit D and the "swiss cheese" lithology are normal.

The Bates Mountain Tuff is 600 feet thick at the type locality on Bates Mountain in the Simpson Park Mountains (Stewart and McKee, 1968), 50-300 feet in the Monitor Range, 725 feet at Clipper Canyon in the northern Toquima Range (Sargent and McKee, 1969), 50 to 100 feet in the Pancake Range (Hose and Blake, 1976), and 150 feet in the Hot Creek-Park Range area (Dixon and others, 1972).

The Bates Mountain Tuff has an areal extent of about 3,500 square miles (Stewart and McKee, 1977). It is exposed in the Toquima, Toiyabe, Shoshone, Monitor, Hot Creek, and Park Ranges, Simpson Park Mountains, and to the west of the evaluation area. The total volume of the Bates Mountain Tuff may be 500 cubic kilometers (Gromme and others, 1972).