AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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CAETANO TUFF

Type Section Information

The Caetano Tuff was named by Gilluly and Masursky (1965) for welded tuffs and associated water-lain tuff and conglomerate which cover a large portion of the northernmost Toiyabe Range, north of the Wenban Fault and south of Cortez Canyon, west of Cortez. The name is taken from the Caetano Ranch.

Geologic Age

The Caetano Tuff is Oligocene in age (Gilluly and Masursky, 1965; Wells and others, 1971; Stewart and McKee, 1977). K-Ar ages range between 30.6-33.5 Ma in the Toiyabe and Shoshone Ranges (Stewart and McKee, 1977). It unconformably overlies the Valmy Formation in the Northern Toiyabe Range (Gilluly and Masursky, 1965).

General Lithology

The great thickness, amount of interbedded volcaniclastic material, and variation in lithic debris led Stewart and McKee (1977) to suggest that the Caetano Tuff is a composite ash-flow tuff sheet of at least two ash flow tuffs separated by complete cooling breaks.

The dominant rock type is a gray, reddish, or purple, crystal-rich, quartz-latite welded tuff that locally contains fragments of intermediate composition volcanic rocks up to several inches in diameter (Gilluly and Masursky, 1965; Stewart and McKee, 1977). Phenocrysts of smoky quartz, and sanidine and lesser amounts of biotite and plagioclase 1 to 3 mm in diameter, make up as much as 40 percent of the rock. The groundmass is devitrified glass that often shows eutaxitic texture. Locally (as in the Shoshone Range), biotite and plagioclase in the tuff are highly altered to chlorite, clay and limonite.

Gilluly and Masursky (1965) describe minor andesitic volcanics near the base of the Caetano Tuff in the northern Toiyabe Range. These andesites are greenish or purplish gray with phenocrysts of plagioclase, hornblende, or pyroxene in a fine-grained groundmass.

Average Thickness

Gilluly and Gates (1965) estimated the total thickness of the Caetano Tuff at 8,000 feet based on "the assumption that the formation is almost completely duplicated by normal faulting", along the Crescent fault. Individual exposures in the Shoshone and southern Toiyabe Ranges are commonly about 500 feet in thickness (Stewart and McKee, 1977).

Areal Distribution

The Caetano Tuff is exposed at Battle Mountain, in the Shoshone and Toiyabe Ranges, and west of the study area in the northern Fish Creek Mountains (Stewart and McKee, 1977). Masursky (1960) suggested that the Caetano Tuff accumulated in a very large (50 mile long and 10 mile wide) east-west volcano-tectonic depression within the central Toiyabe-Shoshone Range area, which probably formed by collapse following extrusion of the tuff.


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