The Needles Range Formation was named by Mackin (1960) for exposures on the east side of the Needles Range, along and south of the Garrison-Milford Highway of southwestern Utah.

The Needles Range Formation is Upper Oligocene based upon radiometric age dates in Utah and Nevada which indicate an age of 29.7 +/- 0.3 Ma. Biotite-rich welded tuff in the northern Toquima and Monitor Ranges, and Simpson Park Mountains, informally called the tuff of Bottle Summit (Stewart and McKee, 1968), is both lithologically and temporally (30.6 +/- 1.2 Ma) similar to the Needles Range Formation, and may be correlative according to Gromme and others, 1972). The Needles Range Formation overlies the Windous Butte Formation in the Grant Range.

At the type section in the Needles Range, Mackin (1960) divided the formation into two members, the Wah Wah Springs Tuff and the upper Minersville Tuff after locations in the Needles Range. Cook (1965) described 5 members of the Needles Range Formation, the lower two of which correlate with the Wah Wah Springs and Minersville Tuff Members at the type locality.

In hand sample the crystal-vitric tuffs of the Needles Range Formation contains an unusually high percentage of biotite which occasionally forms books as much as 6 mm across and 3 mm wide (Cook, 1965). The lowermost member of the Needles Range is a pink to gray, slightly to strongly welded, crystal-rich tuff with euhedral biotite books and flakes, and locally shows a basal black vitrophere up to 50 feet in thickness. Phenocrysts form about 40 percent of the member and are composed on average of about 55 to 65 percent potassium feldspar, 8 to 15 percent biotite, 5 to 13 percent hornblende, 10 to 20 percent quartz, 1 to 3 percent clinopyroxene, and 3 percent orthopyroxene (Ekren and others, 1977). Cook (1965) correlated member 1 with the Wah Wah Springs Member of Mackin (1960).

The overlying member 2 is a grayish orange to purple, crystal to crystal-vitric tuff from 30 to 380 feet thick. About 40 to 50 percent of this member is made up of phenocrysts composed of 60 percent potassium feldspar, 15 to 25 percent hornblende, 9 to 13 percent biotite, 5 percent quartz, 1 to 3 percent clinopyroxene and up to 2 percent orthopyroxene (Ekren and others, 1977). Cook (1965) correlated member 2 with the Minersville Tuff of Mackin (1960).

Member 3 is 60 to 820 feet of pink to gray, nonwelded to moderately welded tuff composed of about 50 percent phenocrysts. The member contains a basal black vitrophere, or local flow breccia and lahar material. Both Member 3 and the overlying Member 4 are composed of about 35 percent phenocrysts which on average, are 57 to 62 percent potassium feldspar, 18 percent quartz, 10 to 14 percent biotite, 7 percent hornblende, 3 percent alkali feldspar, and 3 percent orthopyroxene (Ekren and others, 1977). Member 4 is also a nonwelded, white to pink, crystal-lithic tuff, and is locally overlain by member 5 which is a slightly to highly welded, gray to orange-pink, vitric-crystal tuff with a phenocryst content of about 10 percent (Cook, 1965).

A small exposure of the Needles Range Formation is present along the southern margin of Little Smoky Valley in the Pritchards Station Quadrangle (Dixon and others, 1972). The formation here is a single cooling unit correlative with the Minersville Tuff Member, and is composed of about 50 feet of dark-gray to brown, quartz latitic, vitric, shard-rich, densely welded tuff. Phenocrysts compose about 20 to 35 percent of the unit with 50 to 75 percent potassium feldspar, 15 to 30 percent hornblende, 5 to 10 percent biotite, 0 to 5 percent quartz, 2 to 5 percent clinopyroxene, and 0 to 3 percent opaques (Dixon and others, 1972).

The Needles Range Formation is about 500 feet thick in the southern Snake Range (Whitebread, 1969) and thins to about 100 to 125 feet in the Egan Range (Cook, 1965; Hose and Blake, 1976). The thickest section measured in the study area is 1,360 feet in the Fortification Range, and 1,055 feet were measured in the Seaman Range (Cook, 1965). Cook (1965) estimated the average thickness to be about 617 feet based upon 21 measured sections in Nevada and western Utah. It is only 50 feet thick in the Pritchards Station Quadrangle in the Park Range (Dixon and others, 1972).

The Needles Range Formation is the most widely distributed ash-flow tuff sheet in the Great Basin (Gromme and others, 1972; Mackin, 1960). It is exposed in several ranges in southwestern Utah including the Confusion, Needles and Wah Wah Ranges. In the study area it is present in the Limestone Hills, southern Snake, Schell Creek, southern Egan, White Pine, northern Grant and northern Pancake and Park Ranges, Fortification, Wilson Creek, Seaman, Golden Gate, Quinn Canyon, and Fairview Ranges.