Mammoth Mountain rhyolite of Piedra group of Potosi volcanic series. A single thick rhyolite flow of rather uniform character, at base of Piedra group of Potosi volcanic series in northeastern part of Creede area and in mountains to east and northeast. Has been followed to east as far as Bellows Creek, but not recognized to west, south, or north of Creede Area. Wedges out in upper drainage basin of Nelson Creek. Thickness 0 to 1,000 feet. Overlain by rhyolite tuff. Age is Miocene.
Named from Mammoth Mountain, [northeast of North Creede, in NW/4 sec. 20, T. 42 N., R. 1 E., Lat. 37 deg. 52 min. 46 sec. N., Long. 106 deg. 54 min. 09 sec. W., San Luis Peak 7.5-min quadrangle, Mineral Co.], Creede district, southwestern CO.

Source: US geologic names lexicon (USGS Bull. 896, p. 1275).

Mammoth Mountain Rhyolite. Revised in Creede area, Mineral County, Colorado (San Juan Mountain province) to include Mammoth Mountain Rhyolite as defined by Emmons and Larsen (1923), the overlying soft tuffs of Emmons and Larsen (1923), and all equivalent rocks, which together form a composite ash-flow sheet. Composition of the ash-flow tuffs range from rhyolite to quartz latite. Is a widespread unit recognized as far as 15 miles southeast, 10 miles southwest, and 18 miles south of Creede in the central San Juan Mountains. Is absent along Continental Divide north of Creede where the mountainous terrain stood above the level of accumulation. Is as much as 2,000 feet thick; over most areas is 500 to 1,000 feet thick. As redefined, ranges from a crystal-poor welded ash-flow tuff, with 5 to 10 percent phenocrysts, to a crystal-rich welded ash-flow tuff, with 30 to 60 percent phenocrysts. Plagioclase phenocrysts are predominant; sanidine phenocrysts common in crystal-poor rocks, but absent in most of the crystal-rich phase; biotite is the most prevalent mafic phenocryst. Age is middle and late Tertiary. Thought to be younger than Farmers Creek Rhyolite and older than Wason Park Rhyolite, and of the same general age as the Huerto Formation.

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

Age changed from Oligocene --to-- late Oligocene.

Source: Changes in stratigraphic nomenclature, 1974 (USGS Bull. 1405-A, p. A12-A13).

Mammoth Mountain Tuff. Is of zoned rhyolite to quartz latite composition. Had an estimated volume of 100 to 500 cubic km. Areal extent in southwestern Colorado shown (see fig. 19); present in Hinsdale County, Colorado (San Juan Mountain province). Source unknown. Indirect evidence suggests subsidence took place during eruption of Mammoth Mountain, but postulated caldera has been destroyed by the Creede caldera. Early episodic pyroclastic eruptions became pulsating and nearly continuous. Most of the flows are coherent, dense tuff that shows obscure compound cooling. Accumulated in the moat of the resurgent Bachelor caldera. Wedges out or is absent over top of resurgent core of Bachelor caldera. Remnants are preserved on the Continental Divide south of Creede. Age is late Oligocene (26.7 Ma).

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

Revised to informal status and assigned to Carpenter Ridge Tuff. Restudy of Creede area, Mineral Co, CO, San Juan Mountain province, has shown that previously mapped boundary between Mammoth Mountain and Bachelor Mountain Member of Carpenter Ridge Tuff is an alteration boundary that lacks evidence for major cooling break. Mammoth Mountain is phenocryst-rich silicic dacite in upper part of compositionally zoned Carpenter Ridge ashflow sheet. Older than Wason Park Tuff. Oligocene age (27.35 Ma).

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