Named as the upper member of the newly named Crater Flat Tuff for Prow Pass at the north end of Yucca Mountain, Nye Co, NV in the Great Basin province. Consists of 15.2 m of devitrified welded tuff at type. Is 50 m thick at Crater Flat Tuff type, and is partly glassy and slightly welded there. Near its eastern and southeastern edges, the member is 15 m thick, nonwelded and zeolitized. Is gray or pink where devitrified, and brown where glassy. Is petrologically the most unique of the silicic tuffs in the region in that it contains orthopyroxene as the dominant mafic phenocryst. The orthopyroxene is preserved only in the upper and lower parts at the type. Underlies all constructional lavas of the Wahmonie-Salyer volcanic center, and the rhyolite lavas of Calico Hills. Distribution map. Shown as present from Crater Flat east across Yucca Mountain, Jackass Flats, Skull Mountain, all in Nye Co. Modal and silica range diagram. Miocene age. Has normal remanent magnetism. Erupted from Sleeping Butte caldera.

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

Mapped as upper member of Crater Flat Tuff in northwest part of map area near Prow Pass and in subsurface in cross section, Nye Co, NV in the Great Basin province. Overlies Bullfrog Member of Crater Flat; underlies Salyer Formation. Considered to be ash-flow tuff, a compound cooling unit divisible into three subunits. Lower subunit is partly to moderately welded, gray, orange, pink, devitrified, contains quartz, potassium feldspar, plagioclase, hornblende and sparse red-brown mudstone and brown-gray volcanic lithic inclusions. Middle subunit is partly welded, red, gray, devitrified, has more red-brown mudstone lithic inclusions than other subunits. Upper subunit is partly to moderately welded, gray and red-orange, devitrified, contains quartz, potassium feldspar, plagioclase, hornblende, biotite, and pyroxene, has some red-brown mudstone and brown volcanic lithic inclusions. Ranges from 60+/-m to 166 m thick. Normal magnetic polarity. Miocene age.

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

Is one of three members of Crater Flat Tuff (ascending): Tram (new), Bullfrog, and Prow Pass. Is similar lithologically to Bullfrog; both contain 10-20 percent phenocrysts, and highly resorbed quartz phenocrysts. Distinguished mainly by its mudstone fragments, presence of orthopyroxene phenocrysts, and scarcity of biotite and hornblende. Rare pilotaxitic lava xenoliths also found in Prow Pass. Distribution and thickness map; present in south-central Nye Co, NV in the Great Basin province. Ranges from 0 to 256 ft thick. Age is bracketed by average K-Ar ages of 13.5 m.y. on Bullfrog and 13.4 m.y. on tuffs of Calico Hills. Geologic map; stratigraphic chart.

Source: GNU records (USGS DDS-6; Denver 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." Prow Pass is one of three formally named formations of Crater Flat Group (rank raised). [Crater Flat also consists of informal units not discussed in this report.] Younger than Bullfrog Tuff (rank raised) of Crater Flat; older than Wahmonie Formation. Nomenclature shown on table 1. Miocene age.

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