Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

field are the largest remnants of the middle Tertiary volcanic field (fig. 3; Steven, 1975).
The Central Colorado volcanic field covers approximately 1,500 sq mi and is one
of the larger remnants of the Southern Rocky Mountains middle Tertiary volcanic field
(Steven and Epis, 1968; Steven, 1975; McIntosh and Chapin, 2004). The center of the
Central Colorado volcanic field is approximately 40 mi east-northeast of Maysville (fig.
3). From about 37 to 28 Ma, the Central Colorado volcanic field was developed upon the
late Eocene erosion surface (Epis and Chapin, 1974). The largely andesitic volcanic field
represents the remnants of concurrently erupted volcanic units from two sources: (1)
dominantly andesitic volcanic rocks derived from within the present area of the field (for
example, Guffy center and Buffalo Peaks center) and (2) dominantly silicic ash-flow tuffs
erupted from outside the field (Epis and Chapin, 1974). Five ash flow sheets have been
identified: the Wall Mountain, Badger Creek, East Gulch, Thorn Ranch and Gribbles
Park tuffs (Epis and Chapin, 1968 and 1974). No caldera sources have been identified on
the east side of the Upper Arkansas Valley graben, and thus the sources of all the tuff
units were suggested to be west of their present outcrop areas (Epis and Chapin, 1974).
During the earliest phase of the Middle Tertiary magmatic pulse, a linear zone of
caldera and cauldron subsidence structures was initiated along the crest of the Laramide
Sawatch uplift (fig. 4). The calderas and cauldrons include the 36 Ma Bonanza caldera
(Varga and Smith, 1984), the 34.4 Ma Mount Aetna cauldron (Shannon and others,
1987a; and Shannon, 1988), and the 34 Ma Grizzly Peak cauldron (Fridrich and Mahood,
1984; and Fridrich, 1986). A potential fourth eroded caldera is postulated to be related to
the 36.6 Ma Mount Princeton pluton (Shannon, 1988). More recent work based on
40 Ar/ 39 Ar dating has refined the ages for these caldera-forming events and recognized
additional outflow tuffs and caldera sources (McIntosh and Chapin, 2004). The 37.5 Ma
tuff of Triad Ridge is a newly recognized ash-flow tuff in the Trout Creek paleovalley,
and the proposed 33.7 Ma Marshall Creek caldera occurs about five miles west-northwest
of the Bonanza caldera (fig. 4). The new age determinations indicate the Sawatch Range
calderas and the remnants of intracaldera and outflow tuffs have a 4.6 m.y. age span from
37.5 to 32.9 Ma.
Important elements (ring dikes, andesitic and quartz latitic volcanic rocks, and
volcanic breccias) of a deeply eroded volcanic structure in the Southern Sawatch Range
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