Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

pegmatites) and others appear to be abrupt (presence of felsic gneisses, muscovite- and
sillimanite-bearing gneisses, muscovite schist and amphibolite agglomerate) and may be
related to the Salida-Maysville fault.
The Proterozoic rocks are lithologically and structurally complex and poorly
exposed in the Maysville quadrangle. Most outcrops and in-place float are along the
crests of main ridges and spur ridges. Mapping on the tree-covered sides of ridges and
valley floors is hampered by lack of outcrop and significant down slope mixing of
Proterozoic units. Consequently, mapping of most of the southwest quadrant Proterozoic
terrane was conducted by tracking the estimated float proportions of various Proterozoic
units. The mapping of key metamorphic units (for example muscovite-bearing units like
Xmsg and Xmc) showed that they are discontinuous and are locally abruptly truncated.
The truncation of lithologic units occurs along linear zones that are interpreted to be
faults (see Structural Geology section).
Some lithologic-structural domains are lithologically simple, consisting of
predominantly one rock type. Other lithologic-structural domains are composed of two or
more lithologic units that form a mappable coherent pattern. Most of the lithologicstructural
domains are composed of multiple lithologic units that are mixed and display
no coherent patterns. A system of fractional bedrock units is used to describe many of the
lithologically complex domains in the Proterozoic terrane. The first lithology is the
predominant lithologic unit in the domain and additional lithologies exceeding about 25
percent are successively listed in decreasing order of abundance.
Many of the lithologic-structural domains display coherent structural orientations
that are truncated or change across the bounding faults. The Proterozoic terrane in the
southwest part of the Maysville quadrangle was initially a complex sequence of various
lithologic units that have been further complicated and disrupted by a complex pattern of
one or more faulting, and possible folding, events. No younging direction criterion could
be established during this study and the relative ages of the various Proterozoic units are
not known. Following are descriptions of the Proterozoic units arranged according to two
general criterion: from more felsic compositions to more mafic compositions and from
probable sedimentary rock protoliths to probable igneous rock protoliths.
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