Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

We use fractional map units on the geologic map. For Quaternary deposits, if the
surface map unit is thinner than 5 ft and/or discontinuous we map a “fractional” map unit,
shown by a map unit abbreviation that lists the upper (surficial) deposit in the numerator
and the underlying (bedrock) deposit or unit in the denominator (for example, Qco/Td or
Qc/Xgdf). This usage has been adopted by the Colorado Geological Survey on recent
1:24,000 scale geologic maps.
The concept of fractional lithologic units is expanded to bedrock units for the
Maysville quadrangle specifically to assist in characterizing complexities of the
Proterozoic terrane. Extremely poor exposure and extensive mixing of rock float hamper
the interpretation of the various Proterozoic units. Limited outcrop suggest that a lot of
the mixed lithologies are related to both interlayering of the lithologies and structural
complexities. For fractional bedrock units the predominant lithology is in the numerator
and successively less abundant lithologies (but greater than 25 percent) are in the
denominator (for example, Xmfs/YXgp and Xag/Xcs/Xbfg). Fractional units are also
used for showing the distribution of mappable broken rock zones which are structurally
superimposed on the complex Proterozoic lithologies (for example, BR/Xag and
BR/Xgdf).
ACKNOWLEDGMENTS
This mapping project was funded jointly by the Colorado Geological Survey and
U.S. Geological Survey through the STATEMAP program of the National Cooperative
Geologic Mapping Program under agreement No. 05HQAG0064.
The senior author thanks the Colorado Geological Survey for additional support
related to seven whole-rock chemical analyses, twelve geochemical analyses, one
40 Ar/ 39 Ar age determination, 18 thin sections, and 6 polished sections that were used for
this study. These additional studies and data significantly improve the quality of the
report.
The authors thank the many private land owners on the Maysville quadrangle who
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