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PANCAKE SUMMIT TUFF

Type Section Information

Gromme and others (1972) designated the Pancake Summit Tuff for exposures originally considered part of the Windous Butte Tuff by Cook (1965), in the northern Pancake Range about 2 km southwest of Pancake Summit.

Geologic Age

The Pancake Summit Tuff is Oligocene in age with several K-Ar ages averaging 32 Ma (Gromme and others, 1972; McKee, 1976b). In the Toquima Range, the Pancake Summit Tuff is the basal Tertiary unit. The tuff rests regionally on a variety of formations mostly of Cenozoic and Paleozoic age. In western White Pine County it overlies the Stone Cabin Formation and is overlain by the Windous Butte Tuff (Hose and Blake, 1976).

General Lithology

The Pancake Summit Tuff is a light gray to pink crystal-rich rhyolite tuff with about 30 percent smoky quartz, sanidine, and plagioclase phenocrysts in equal amounts, and small amounts of biotite and small rock fragments in a groundmass of devitrified shards (Gromme and others, 1972; McKee, 1976). Most of the tuff is hard and densely welded, with basal portions locally weakly to non welded and soft in outcrop. In hand sample the tuff commonly appears massive and structureless, but shows a pronounced eutaxitic fabric in most thin sections (McKee, 1976). Gas cavities are locally present but not diagnostic of the unit.

Hose and Blake (1976) suggest that the Pancake Summit Tuff can be broken into two members. The lower member is crystal-poor and grades upward from a non-welded light-brown tuff into dark-gray to black vitrophyre. The upper member is crystal-rich, pink to gray tuff that is rich in large crystals of smoky quartz, feldspar, biotite, and minor hornblende.

In most places the Pancake Summit Tuff cannot be distinguished from the Windous Butte Tuff on the basis of field mapping and modal chemical analyses (McKee, 1976b). The use of detailed regional measurement of the remanent magnetic direction shows a difference of about 20 degrees in magnetic inclination and 30 degrees in declination between the two tuffs (Gromme and others, 1972).

Average Thickness

The Pancake Summit Tuff varies considerably in thickness both as a result of deposition on an irregular erosional surface, and erosion of the tuff before emplacement of later ash-flow tuff sheets (McKee, 1976b). The unit is 600 feet thick on the east side of the northern Toquima Range but thins to zero both to the north and south, in less than 4 miles. It is well exposed in the northern Pancake Range where it is about 450 feet thick (Hose and Blake, 1976). Regionally the tuff is on average 530 feet thick, and reaches a maximum thickness of about 860 feet (Gromme and others, 1972).

Areal Distribution

The Pancake Summit Tuff is a widely distributed ash-flow tuff sheet with an estimated maximum lateral extent of about 120 km, an areal extent of 5,400 square kilometers, and an estimated volume of 750 cubic kilometers (Gromme and others, 1972). It is present in the northern Toquima, Monitor, Hot Creek, Pancake, White Pine, and Fish Creek Ranges.


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Last modified: 09/12/06