Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

straddles the corners of the Mount Antero, St. Elmo, Maysville, and Garfield
quadrangles.
The most complete and detailed previous coverage of the geology and structure of
the Maysville quadrangle area was included as the southwest quadrant of the geologic
map of the Poncha Springs 15-minute quadrangle by Scott and others (1975). This map
also included some information from Kouther (1969), Knepper (1974), and Limbach
(1975). It provided excellent descriptions of many of the main rock units and accurately
depicted the main elements of the geology and structure of the quadrangle. It was
invaluable during the course of our investigations.
The Maysville area is included in the 1:250,000-scale regional geologic
compilation of the Montrose 1° x 2° quadrangle (Tweto and others, 1976). Other
important regional studies that are not part of quadrangle maps include the structure and
tectonic studies by Perry (1971) and Knepper (1974 and 1976). Knepper’s treatment of
the complex structure was a major step in developing a comprehensive structural and
tectonic framework for the region. Additional studies by Russell (1950), Kouther (1969),
Dippold (1999), Xu (2001), and Denesha (2003) provide descriptions of lithologies and
structure of parts of the Maysville quadrangle and adjacent Mount Ouray and Salida West
quadrangles.
The Proterozoic rocks in the southwest quadrant of the Maysville quadrangle are
part of the regional Gunnison-Salida belt of Early Proterozoic supracrustal rocks
(Bickford and Boardman, 1984 and Bickford and others, 1989). Boardman (1976 and
1986), Bickford and Boardman (1984), and Boardman and Condie (1986) provided
detailed descriptions of the supracrustal rocks near Salida. Boardman (1986) described a
13,000 ft thick northern section and a 5,600 ft thick southern section, both with similar
stratigraphic and lithologic character. The sections consist of interlayered metavolcanic
and metasedimentary rocks and abundant gabbro-diabase sheets. The volcanic rocks
represent a bimodal mafic-felsic volcanic suite. Mafic volcanic rocks (tholeiitic basalt to
basaltic andesite) are predominantly volcaniclastic and include massive breccia units,
sedimentary sequences with calcareous matrix, laminated volcaniclastics, and minor
flows and pillow breccias (Bickford and Boardman, 1984). The felsic volcanic rocks are
also dominantly volcaniclastic with dacite to rhyolite compositions. The difference
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