Introduction Evaluation Prospects











Type Section Information

The Monotony Tuff is named for Monotony Valley and has a type section about 2.5 miles southwest of the Oswald Mine in Secs. 2 and 3, T. 3 S., R. 53 E. (Ekren and others, 1971).

Geologic Age

The Monotony Tuff is Oligocene and has been dated radiometrically at 26 to 27 Ma (Ekren and others, 1977).

General Lithology

The Monotony Tuff is composed of 1 to 3 compound cooling units of rhyodacite to quartz latitic composition. These units commonly contain about 50 percent phenocrysts composed of 15 percent biotite, 45 to 65 percent potassium feldspar, 10 to 17 percent quartz, a trace to 10 percent alkali feldspar, 2 to 4 percent hornblende, 2 to 3 percent opaques, and 3 percent clinopyroxene (Kleinhampl and Ziony, 1985).

In the Pancake Range near Portuguese Mountain, three quartz latite cooling units are present (Quinlivan and others, 1974). The lowest unit is an orange-pink to gray-brown, partially welded and devitrified, phenocryst-rich tuff with abundant biotite and amethyst quartz crystals, that grades downward either into vitrophere or a partially welded tuff with elongate dark-gray glassy pumice blocks up to 4 inches across in a devitrified groundmass. Phenocrysts make up 30 to 38 percent of the lower unit with 56 to 60 percent potassium feldspar, 8 to 16 percent biotite, 8 to 14 percent quartz, 6 to 10 percent alkali feldspar, 2 to 8 percent hornblende, 1 to 2 percent clinopyroxene and 1 to 2 percent opaques (Quinlivan and others, 1974).

The middle unit is a brown densely welded and devitrified tuff which grades downsection into black and brown vitric shard-rich zones and a light-gray nonwelded base. Phenocrysts make up about 22 to 30 percent of the unit with 56 to 62 percent quartz, 14 to 17 percent biotite, 4 to 10 percent quartz, 4 to 10 percent hornblende, 3 to 10 percent alkali feldspar, 1 to 3 percent opaques and a trace to 2 percent clinopyroxene and mafic psuedomorphs. The upper unit is a gray-orange to brown, densely welded and devitrified tuff, with a nonwelded vitric tuff at the base. Phenocrysts compose 41 percent of the rock with 63 percent potassium feldspar, 15 percent quartz, 9 percent biotite, 4 percent alkali feldspar, 3 percent mafic psuedomorphs, 2 percent opaques, and 2 percent hornblende and clinopyroxene.

In the Moores Station Quadrangle the tuff is a simple rhyodacitic to quartz latitic cooling unit which is purplish to brown in color and is partially welded to vitric with thin beds of well-rounded pebble conglomerate locally exposed at the base (Ekren and others, 1973). In the Wall Quadrangle portion of the Pancake Range, the Monotony is composed of an upper and lower cooling unit. The upper unit is a densely welded and mostly devitrified gray-brown rhyodacitic ash-flow tuff. The phenocrysts are 46 to 63 percent potassium feldspar, 10 to 22 percent biotite, 10 to 22 percent quartz, 5 to 12 percent alkali feldspar, 1 to 5 percent clinopyroxene, 1 to 2 percent opaques, and a trace of orthopyroxene (Ekren and others, 1972). The lower unit is the same lithology with only a trace of clinopyroxene and no orthopyroxene and includes a 10-foot bed of well-rounded conglomerate with pebbles, cobbles, and boulders.

In the Reveille Range, the Monotony Tuff has been divided by Ekren, Rogers, and Dixon (1973) into an intracaldera facies called the Tuff of Goblin Knobs that locally reaches thicknesses of 5,000 feet, and an extra caldera facies that is quartz latitic in composition and is the typically described Monotony Tuff.

Average Thickness

Near Portuguese Mountain in the Pancake Range the Monotony Tuff is about 800 feet thick (Quinlivan and others, 1974), it reaches 1,000 feet in thickness in the Moores Station Quadrangle and 1,500 feet in the Wall Quadrangle (Ekren, Rogers, and Dixon, 1973). The Monotony is as much as 3,000 feet thick in the central and southern Hot Creek Range, and up to 5,000 feet thick in the Reveille Range (Quinlivan and Rogers, 1974; Ekren, Rogers and Dixon, 1973; Kleinhampl and Ziony, 1985).

Areal Distribution

The Monotony Tuff is exposed within the Reveille, Pancake, Hot Creek, Fairview and Wilson Creek Ranges.

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Last modified: 09/12/06