Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

METHODS
The present study focuses on the geologic mapping in the Maysville 7.5-minute
quadrangle at a scale of 1:24,000. The geologic map (plate 1), the Correlation of Map
Units (plate 2), and three geologic cross sections and quadrangle oblique view (plate 3)
accompany this report. Field work in the Maysville quadrangle was undertaken during
the summer of 2005 and some field checking and follow-up geologic mapping was
carried out during the summer of 2006. Bedrock mapping was completed by James R.
Shannon (Colorado School of Mines). Bedrock rock was mapped on an enlarged U.S.
Geological Survey topographic base at a scale of 1:12,000 and later compiled on to
1:24,000-scale base maps. Field control was maintained with a handheld Garmin
GPSmap 60C. Position accuracy is estimated to be 12 to 60 ft, at best. Bedrock contacts
were digitized in 2-D with ESRI ArcView at the Colorado Geological Survey. Specific
locations of sites discussed in this report are given in UTM coordinates (Datum NAD27,
zone 13, in meters). Appendix I contains the Maysville quadrangle point-file data base
with location information and structure data. A number of geologic units, including
Proterozoic dikes (Xmd) and lithologic units (Xq and Xag), and Tertiary dikes (Ta, Trp,
Tr, Tcf, and Tqlp) and lithologic units (Td2v and Td2ls) are shown on the geologic map
(plate 1) and geologic cross sections (plate 3) with exaggerated thickness.
The classification of igneous rocks described in this report is based on the
International Union of Geological Sciences (IUGS) Subcommission on the Systematics
of Igneous Rocks (Streckeisen, 1973, 1976 and 1979). Where whole-rock chemical
analyses are available the rocks are chemically classified using the R 1 R 2 -diagrams of De
la Roche and others (1980). The mean composition of igneous rock families (from De la
Roche and others, 1980) are used for whole-rock chemical comparisons. The formal
names Mount Aetna Quartz Monzonite, Mount Pomeroy Quartz Monzonite and Mount
Princeton Quartz Monzonite will not be used in this report because they are not classified
as quartz monzonite with current IUGS nomenclature and chemical classifications of De
la Roche and others (1980). Plagioclase compositions were approximated optically using
the Michel-Levy (1877) statistical method.
Surficial deposits were mapped by James P. McCalpin (GEO-HAZ Consulting)
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