Named as a member (1 of 8) of Oak Spring formation for Tiva Canyon, northwest side of Shoshone Mesa, Nevada Test Site, Nye Co, NV, Great Basin province. No type locality designated. Shown on cross section as above Topopah Spring member (new), beneath Rainier Mesa member (new), and tonguing into Survey Butte member (new), all members of Oak Spring. Lower part: 0-250 ft thick gray and yellow, fine- to lapilli-bedded, medium- to very thick-bedded tuff; locally thin beds contain more than 20 percent rock fragments up to 3 cm long; phenocrysts of quartz and feldspar 10-20 percent, biotite is less than 5 percent, fragments of gray welded tuff less than 5 percent; thickest sections on south flank of French Peak; extends south to Lookout Peak and east to Nye Co line; absent in west part of Test Site. Forms ledgy slopes; lower contact unconformable. Upper part: 0-550 ft thick gray, purple, brown, densely welded tuff that weathers platy; forms steep cliffs; has local zones of lithophysae, spherulites, and porous finely crystalline partially welded tuff; white and pink nonwelded shards and pumice in basal part; locally cavernous; progressively less welded eastward, phenocrysts make up less than 5 percent of basal part, 10 to 25 percent in middle and upper part; phenocrysts chiefly quartz and potassium feldspar; mafic minerals less than 10 percent. Of Miocene? or younger age.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Reassigned from being one of the members of the Oak Spring Formation to member (1 of 5) of Piapi Canyon Formation (new), upper formation of Oak Spring Group (rank raised). Shown on cross section as tonguing into Survey Butte Member (also reassigned from Oak Spring Formation to Piapi Canyon) and as above Topopah Spring Member and below Rainier Mesa Member. Topopah Spring and Rainier Mesa reassigned from Oak Spring Formation to Piapi Canyon Formation of Oak Spring Group. Is recognized on cross section only in Piapi Canyon area, Nevada Test Site, Nye Co, NV in Great Basin province. Is a multiple-flow simple or compound cooling unit of rhyolitic welded and nonwelded ash-flow tuff and associated ash-fall tuff. Thought to have been derived from Timber Mountain area to west. Of early Pliocene or younger age.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Petrographic examination of Tiva Canyon Member (Piapi Canyon Formation) at Jackass Divide, Four Corners Butte, Yucca Pass, Massachusetts Mountain, French Peak, and Scarp Canyon shows that this unit has characteristics of lowest cooling unit of Rainier Mesa Member, and therefore these strata belong to Rainier Mesa Member (Piapi Canyon Formation).

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Lower contact of Tiva Canyon revised in that it overlies the newly named, previously undescribed nonwelded to densely welded ash-flow tuff sheet, Yucca Mountain Member. The new member underlies Tiva Canyon in the southwest part of the Nevada Test Site, Topopah Spring 15 min quad, Nye Co, NV in the Great Basin province. A 13 m.y. date has been obtained by K-Ar method from the Piapi Canyon.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Is the upper member of five named members reassigned from Piapi Canyon Formation to Paintbrush Tuff (new) of Piapi Canyon Group (rank raised). Overlies Yucca Mountain Member of Paintbrush. Underlies on Frenchman Flat and in the north and west parts of the Nevada Test Site, Nye Co, NV in the Great Basin province, Rainier Mesa Member, basal member of Timber Mountain Tuff (new), upper formation of Piapi Canyon Group. Is separated from the Rainier Mesa in the Yucca Flat area by unnamed bedded tuff of Paintbrush. Nomenclature chart. Cross section. An age of Miocene? and Pliocene assigned to Paintbrush (and hence all its members). Name Oak Spring as a formation or as a group abandoned.

Source: GNU records (USGS DDS-6; Denver GNULEX).

K-Ar determinations on biotite from Tiva Canyon Member of Paintbrush Tuff gave age of 12.4 Ma. Locations of samples shown on sketch map.

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Is one of four named members of the Paintbrush Tuff. Overlies Yucca Mountain Member of Paintbrush. Underlies unnamed bedded tuff of Paintbrush. Distribution map; present around north, east, and south sides of Timber Mountain caldera, and at Sleeping Butte; extends across southern Nye Co, NV into eastern Inyo Co, CA in the Great Basin province. Stratigraphic chart. Consists of a compositionally zoned extensive ash-flow sheet and the gradationally overlying upper thick quartz latite called the tuff of Chocolate Mountain. Is a compound cooling unit, predominantly gray to brown devitrified densely welded ash-flow tuff with minor brown and gray nonwelded to partially welded tuff at its base and top. Lower part is crystal-poor sanidine- and hornblende-bearing high silica rhyolite tuff. Grades upward into a middle crystal-poor rhyolite with biotite and an upper crystal-rich quartz-latitic tuff. Is locally flow-laminated and has flow folds inside the cauldron. Has a welded caprock vitrophyre. Overlying tuff of Chocolate Canyon in the cauldron distinguished by increase of dark-gray phenocryst-rich collapsed pumice lenticles of quartz latite. Tuff of Chocolate Mountain can be divided into subunits. Erupted from the Claim Canyon cauldron. Has a maximum thickness of 200 m. Total volume may be in excess of 1,000 cubic km. Pliocene age. Modal analyses.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Unit age in report area [Bare Mountain, Nye Co, NV] is middle Miocene based on revised age for Paintbrush Tuff, locally.

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Within southwest Nevada volcanic field, Nye Co, NV, Great Basin province, units previously of formation rank are raised to group rank to allow for combining petrographically, geochemically, and temporally related lava flows and nonwelded tuffs with the principal correlative welded ash-flow tuff sheets (formerly members of the previous formation-rank tuffs). The ash-flow sheets are raised to formation rank and designated as "Tuff." Tiva Canyon is uppermost of four formally formations of Paintbrush Group (rank raised). [Paintbrush also consists of informal units not discussed in this report.] Older than Rainier Mesa Tuff of Timber Mountain Group (rank raised for both units); younger than Yucca Mountain Tuff (rank raised) of Paintbrush. Nomenclature on table 1. Tiva Canyon was derived from Claim Canyon caldera. 40Ar/39Ar dates on an average of 3 sanidine samples yielded an age of 12.7 +/-0.03 Ma (Miocene).

Source: GNU records (USGS DDS-6; Denver GNULEX).