Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

(YXgp) typically make coarser rock float and small knobby outcrops in contrast to the
gneissic rocks, which are present as fine (less than ½ inch) chips. The field relations
suggest that the fine-grained gneisses behaved more brittlely and were preferentially
shattered in comparison to the medium to very coarse-grained Berthoud-type intrusive
bodies (YXgp).
The field relations and distribution of the BR zones are complex and unclear. In
the Maysville quadrangle, four mappable main BR zones range from about 100 ft to
1,200 ft wide (plate 1). Three of the mappable BR zones are present in the southwest
quadrant, and numerous smaller zones range from tens of feet up to about 100 ft wide but
are too narrow or discontinuous to show at 1:24,000 scale. These smaller zones occur
along many of the lithologic-structural domain boundary faults and support that these
zones are a common feature of the major fault zones in the Proterozoic rocks. Two of the
mappable BR zones are oriented northwest, a similar orientation to many of the domain
boundaries. The westernmost BR zone is about 7,000-ft long, extends off the west edge
of the map, and pinches out near the fault terminus in Willow Creek. The second
northwest-trending BR zone is about 8,500 ft long and extends from the South Arkansas
River to north of Willow Creek where it intersects the third main BR zone. This
northwest-trending BR zone skirts by the southwest edge of a large Paleozoic outlier of
Ordovician Manitou limestone, about 7,000 ft west-southwest of Maysville. The
limestone/dolomite is not broken and shattered along this contact.
The third main BR zone is a north-south-trending, irregular zone along the
western bounding structural zone of the South Arkansas graben, the Willow Creek fault.
This zone is the longest and most irregular, extending from near the South Arkansas
River southward about 14,000 ft across Green Creek and beyond the south edge of the
quadrangle. The relationship of the BR zone to the four Ordovician limestone/dolomite
occurrences along the west edge of the South Arkansas graben is problematic. The
Ordovician limestone/dolomite bodies are interpreted to be structural-erosional remnants
of Paleozoic sedimentary rock and thus are considered to be part of the basement rocks
making up the west edge of the South Arkansas graben. The limestone/dolomite bodies
are localized in highly irregular jogs in the BR zone and adjacent Willow Creek fault.
The northernmost Paleozoic body consists of locally brecciated and shattered Manitou
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