Pg. C60-C62. Piapi Canyon Formation of Oak Spring Group. Rocks were formerly assigned to upper part of Oak Spring Formation (revised). Overlies Indian Trail Formation (new) of Oak Spring Group in northern part of Nevada Test Site. Overlies Paleozoic rocks halfway between Oak Spring Butte (north) and Skull Mountain (south) (see cross section). Tongues into andesitic rocks of Wahmonie Flat. Is the youngest unit exposed as far south as Wahmonie Flat where andesitic rocks of Wahmonie Flat are at the surface. Underlies basalt of Skull Mountain at Skull Mountain. Includes following members of Hinrichs and Orkild (1961, USGS Prof. Paper 424-D) in ascending order: Survey Butte, Stockade Wash, Topopah Spring, Tiva Canyon, and Rainier Mesa Members (all formerly assigned to Oak Spring Formation). All members, except Survey Butte, are multiple-flow simple or compound cooling units of rhyolitic welded and nonwelded ash-flow tuff and associated ash-fall tuff. Survey Butte is an ash-fall tuff reworked by water and possibly wind, and minor amount of nonwelded ash-flow tuff. Source probably Timber Mountain caldera. Camel bones in basal part. Age is believed to be early Pliocene or younger.
Type locality: in vicinity of Piapi Canyon, about 2 mi northwest of [Shoshone Peak (Lat. 36 deg. 56 min. 37 sec. N., Long. 116 deg. 15 min. 39 sec. W.), Topopah Spring 7.5-min quadrangle], Nevada Test Site, Nye Co., southern NV (Great Basin province). Named from Piapi Canyon.
[US geologic names lexicon (USGS Bull. 1350, p. 574-575) and GNU records (USGS DDS-6; Denver GNULEX) state type locality is 2 mi northwest of Shoshone Mesa. Shoshone Mesa is in Lander Co., northeastern NV.]

Source: US geologic names lexicon (USGS Bull. 1350, p. 574-575); GNU records (USGS DDS-6; Denver GNULEX).

A previously undescribed nonwelded to densely welded ash-flow tuff sheet is named Yucca Mountain Member of Piapi Canyon Formation. The new member is one of five named members of the Piapi Canyon. Yucca Mountain Member is present in the southwest part of the Nevada Test Site, Topopah Spring 15 min quad, Nye Co, NV in the Great Basin province where it overlies unnamed bedded tuff and underlies Tiva Canyon Member, upper member of Piapi Canyon. A 13 m.y. date has been obtained by K-Ar method from the Piapi Canyon.

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

Raised in stratigraphic rank from Piapi Canyon Formation, upper formation of Oak Spring Group, to Piapi Canyon Group. Name Oak Spring abandoned. Piapi Canyon Group is divided into the newly named Miocene? and Pliocene Paintbrush Tuff (at base) and the Pliocene Timber Mountain Tuff (at top). These two new formations can be recognized as part of the group in the Frenchman Flat and Yucca Mountain areas, Nevada Test Site, Nye Co, NV in the Great Basin province, the areas to which Piapi Canyon is restricted. Paintbrush includes unnamed bedded tuffs and four named members, the Stockade Wash and Topopah Spring Members (formerly members of the Piapi Canyon Formation), the newly named Pah Canyon and Yucca Mountain Members, and the Tiva Canyon Member (formerly a member of Piapi Canyon). Yucca Mountain can be recognized in the western and northern parts of the Test Site only. The Timber Mountain is divided into (ascending): Rainier Mesa Member, tuff of Cat Canyon, tuff of Transvaal, and Ammonia Tanks Member. The tuffs of Cat Canyon and of Transvaal can be recognized only in the northern and western areas. Overlies unassociated local informal units. Underlies basalt of Skull Mountain at Skull Mountain. Stratigraphic chart. Cross section.

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

K-Ar determinations on samples from members of Paintbrush Tuff range from 13.2 to 12.4 Ma; on samples from members of Timber Mountain range is from 12.1 to 10.4 Ma. Sketch map shows localities for samples.

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

Stockade Wash Member (of Paintbrush Tuff) raised in rank to Stockade Wash Tuff. Stockade Wash Tuff and tuffs of Area 20 removed from Paintbrush Tuff and Piapi Canyon Group. Tuff of Pinyon Pass added to Paintbrush Tuff. Ammonia Tanks Member (of Timber Mountain Tuff) revised to include tuffs of Cat Canyon and Transvaal within its lower part. Locally tuffs of Crooked Canyon and Buttonhook Wash added to upper part of Timber Mountain Tuff. Stratigraphic relations shown in correlation chart. Areal extent of members shown on sketch maps. [Based on DNAG 1983 Geologic Time Scale, K-Ar ages of Piapi Canyon Group would be Miocene.]

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