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OLIGOCENE BASALT, MINOR BASALTIC ANDESITE, AND ANDESITE

Oligocene alkali olivine basalt flows, basaltic tuff and tuff breccia related to small local cinder cones are exposed in several areas in northern Elko County including the Mount Velma, Owyhee, and Mountain City Quadrangles, and the Jarbidge Mountains area.

In the Jarbidge Mountains area, the Oligocene Seventy Six Basalt is named for exposures of black, fine-grained, porphyritic and vesicular basalt along Seventy Six Creek in the southern Jarbidge Quadrangle (Coats, 1964). These basalt flows and sills vary from less than 6 feet to masses 200 feet thick. From 1 percent to as much as a third of the rock is composed of plagioclase phenocrysts from .5 to 6 inches in length (Coats, 1964, 1985). The basalt has a subophitic texture with pyroxene, olivine, magnetite, illmenite and apatite in a glassy groundmass (Coats, 1985). Correlative tuffs suggest the basalts are from 23 to 26 Ma in the Jarbidge area (Coats and others, 1977). The Seventy Six Basalt is equivalent to the olivine and augite-rich basalt of Panguipah Spring in the Owyhee Quadrangle (Coats, 1971). The basalt of Panguipah Spring is underlain by a considerable thickness of biotitic tuffaceous sandstone and gravel at the base (Coats, 1971, 1985). Basalts correlated with the Seventy Six Basalt are also present in the Mountain City Quadrangle where they are 22.9 +/- 3 Ma (McKee and others, 1976; Coats, 1985).


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Last modified: 09/12/06