Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

The Wall Mountain Tuff is the most widely distributed Tertiary ash flow unit of
the Central Colorado volcanic field and was considered to be the oldest ash flow unit
(Epis and Chapin, 1974). Chapin and Lowell (1979) suggested that the Wall Mountain
ignimbrite traveled at least 87 mi, and remnants of the tuff indicate the sheet covered at
least 6,460 sq mi. On the basis of a compilation of Wall Mountain Tuff occurrences
(Tweto, 1979b), the distribution pattern of the Wall Mountain Tuff is a northeast-trending
zone that is 87 mi long and up to 50 mi wide and that extends from the southern Upper
Arkansas Valley, across the Front Range Uplift, to the Castle Rock area. The distribution
pattern generally points back to the area between the Mount Aetna cauldron and the
Bonanza caldera. Shannon (1988) suggested a preliminary correlation between the Wall
Mountain Tuff and a postulated caldera structure that was associated with the Mount
Princeton pluton. This tentative correlation was based on the spatial relation of the Wall
Mountain Tuff and the Mount Princeton pluton, on the similarity of preliminary
published and unpublished age determinations on the two units (average age 36.6 Ma),
and suggested mineralogical and chemical links between the Wall Mountain Tuff and the
heterogeneous roof zone of the Mount Princeton pluton. New observations at Cochetopa
dome (volcanic subsidence structure) indicates the presence of Wall Mountain tuff,
indicating that some of the ignimbrite flows traveled west-southwest off the Sawatch
uplift and into the northern part of the San Juan volcanic field (Lipman, 2004, pers. com.;
Lipman, 2007).
Most summaries of the Cenozoic magmatic history of Colorado indicate the end
of the Middle Tertiary magmatic pulse at 25 to 26 m.y. (Tweto, 1975; Mutschler and
others, 1987; and Cunningham and others, 1994). However, the beginning of the Late
Tertiary magmatic pulse is variously interpreted to be 25 Ma (Mutschler and others,
1987) or 15 Ma (Tweto, 1975; Cunningham and others, 1994), suggesting a 1 to 10 m.y.
gap between the two magmatic pulses. At about 25 to 30 m.y. ago the calc-alkaline
magmatism peaked and the composition of the magmatism shifted to more silicic
compositions as Rio Grande rift faulting began (Lipman, 1981).
The highly chemically evolved Climax-type porphyry-molybdenum systems
formed between 33 and 25 m.y. ago in the area of the intersection of the Rio Grande rift
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