Oligocene Ignimbrites are widespread throughout the study area and include units described individually below. Many units of lesser or unknown extent often lumped and undifferentiated during mapping, have not been described individually. These include the Tuff of Hot Creek Canyon, Tuff of Twin Peaks, Tuff of Kiln Canyon, Tuff of Big Round Valley, Tuff of Cottonwood Canyon, Tuff of Black Rock Summit, Tuff of Crested Wheat Ridge, Tuff of Pot Hole Valley, Tuff of Halligan Mesa, Tuff of Palisade Mesa, Tuff of Slanted Buttes, Tuff of the Needles, and Tuff of Chaos Creek in the Hot Creek, Park and Pancake Ranges (Quinlivan and Rogers, 1974; Quinlivan and others, 1974; Ekren and others, 1973; Ekren, Rogers, and Dixon, 1973; Dixon and others, 1972; Snyder and others, 1972; Ekren and Dixon, 1972), and the Tuff of Cowboy Rest, Tuff of Dry Creek, Tuff of Iron Spring, and Tuff of Bottle Summit in the Simpson Park and Toiyabe Ranges (McKee, 1968a, 1968b; Stewart and McKee, 1978), and Tuff of Stoneberger Canyon and Tuff of Mount Jefferson in the Toquima Range (Kleinhampl and Ziony, 1985; McKee, 1972).

About 200 feet of unnamed rhyolitic welded tuff is present in the central portion of the Carlin-Pinon area and along the eastern crest of the Cortez Mountains where it rests with angular unconformity on the Oligocene Indian Well and Cretaceous Newark Canyon Formations (Smith and Ketner, 1976) K-Ar dating assigns an age of 31.2 Ma to this unit.

To the northwest of the Pinon Range, rhyolitic to dacitic ash-flow tuffs are exposed in the Willow Creek Reservoir and Little Rock Creek Quadrangles west of the Tuscarora Mountains. These tuffs are dominantly pinkish-gray to gray, biotite rhyolite with 10 to 60 percent phenocrysts of of quartz, sanadine, and biotite (Coats, 1985). Individual flows units are as much as 200 feet thick and the total thickness is probably several thousand feet. This tuff sequence has been dated at 32.1 Ma (Coats, 1985).

Farther north in Elko County, rhyolitic and dacitic ash-flow tuffs are exposed in the Mountain City and Wild Horse Quadrangles where petrologic and thickness data are lacking. Although presently undated, these tuffs are described with Oligocene ignimbrites (Coats, 1985).

In the southwestern portion of the Mountain City Quadrangle, pink-gray, pumiceous, biotite rhyodacitic and dacitic welded tuff are exposed (Coats, 1985). Flattened pumice is abundant in the tuffs and has converted to aggregates of sanidine and tridymite. Tridymite also lines the numerous bleached vesicles in the rock. The tuff contains a low percentage of phenocrysts that are dominantly biotite and lesser plagioclase (Coats, 1985). In the Mountain City and northeastern Wild Horse Quadrangles this unit also contains rhyolite flows and tuffs with phenocrysts of plagioclase and in some cases additional quartz, sanidine, and green biotite. Vesicles in the rock contain topaz (Coats, 1985).

To the east in the Mount Velma and adjacent Charleston Reservoir and Marys River Northwest Quadrangles, 300 to 400 feet of dacitic to rhyodacitic tuffs overlie older Eocene tuffs (Coats, 1985). Phenocrysts in these ignimbrites include various combinations of sanidine, plagioclase and hypersthene with local quartz, hornblende, and augite (Coats, 1985).

In White Pine County Hose and Blake (1976) were able to trace rhyolitic and andesitic flows and tuffs to eruptive vents that now form dikes and shallow intrusive bodies related to granitic plutons. Ignimbrites in the Schell Creek and Snake Range as well as those in the Dry Hills area along the Eureka-White Pine County boundary, contain xenoliths of shallowly underlying intrusive rocks up to several inches across, suggesting the subvolcanic guts of these caldera systems are often near the surface.

Individual Oligocene ignimbrites with regional distribution and significance are described below in some detail. They are described from generally youngest to oldest.