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TUFF OF WILLIAMS RIDGE AND MOREY PEAK

Type Section Information

The Tuff of Williams Ridge and Morey Peak was named by Ekren and others (1973) for exposures at Williams Ridge in T.8 N., R. 52 E. in the Lunar Crater Quadrangle of the Pancake Range. Excellent exposures are also present in the Morey Peak area of the Hot Creek Range.

Geologic Age

The Tuff of Williams Ridge and Morey Peak is Oligocene in age. K-Ar determinations on biotite give a radiometric age of 31.4 +/- 1.0 Ma (Ekren and others, 1973).

General Lithology

The Tuff of Williams Ridge and Morey Peak is composed of three or more compound cooling units of dark gray, brown, orange, and purple, partially to densely welded, rhyodacitic to quartz latitic welded tuff (Quinlivan and others, 1974). In some exposures only 2 of the cooling units are present (Ekren and others, 1972; Ekren and others, 1973). This tuff is characterized by large and abundant, 5 to 6 mm books of biotite and crystals of amethyst quartz. Phenocrysts commonly form 25 to 55 percent of the unit with 42 to 63 percent potassium feldspar, 13 to 30 percent quartz, 2 to 29 percent alkali feldspar, 3 to 12 percent biotite, 0 to 13 percent mafic pseudomorphs, 1 to 7 percent hornblende, 2 percent orthopyroxene and a trace of clinopyroxene (Quinlivan and others, 1974).

The upper portion of the unit in several localities, including Williams Ridge, is a vent breccia with plagioclase crystals up to 1 cm long, hornblende up to 1.5 cm long, and quartz greater than 1 cm in length (Quinlivan and others, 1974). Also present in the tuff are local thin vitrophyres and bedded tuff with xenoliths of volcanic rocks up to 3 feet across, and pumice fragments as much as 8 inches across. Lithic inclusions of Paleozoic and Tertiary rocks are locally abundant and range from 1 inch to 100 feet across. Landslide masses of Paleozoic carbonate rocks up to 2 miles long are present in the Pancake Range (Quinlivan and others, 1974).

At Morey Peak the tuff forms a massif, at least 3,000 feet thick, which is propylitized, and contains irregular clots of crystals several centimeters across which may represent completely devitrified and altered pumice (Kleinhampl and Ziony, 1985). The tuff shows well-developed sheeting joints that suggest resurgence.

In the northern Reveille Range, the tuff is one or more compound cooling units of densely welded and intensely altered tuff with extreme variations in phenocryst mineralogy, size, and amount (Ekren, Rogers, and Dixon, 1973). A basal conglomerate from 5 to 30 feet thick contains well-rounded cobbles and boulders of Paleozoic carbonate and quartzite in a biotite-rich matrix.

Average Thickness

The Tuff of Williams Ridge filled a large caldron, the Williams Ridge caldron, which probably formed in response to eruption of the Windous Butte Tuff. At Williams Ridge the tuff apparently exceeds 6,500 feet in thickness based upon both outcrop and drill hole data (Ekren and others, 1973). 1,000 feet of the tuff are exposed in the northern portion of the Wall Quadrangle, and 3,900 feet are present in the Portuguese Mountain Quadrangle (Ekren and others, 1972; Quinlivan and others, 1974). The unit thins to the east in the Pancake Range. It is about 500 feet thick in the northern Reveille Range (Ekren, Rogers, and Dixon, 1973).

Areal Distribution

The Tuff of Williams Ridge and Morey Peak is exposed across the Pancake, Hot Creek and Reveille Ranges, and the Squaw Hills.


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Last modified: 09/12/06