Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

granodiorite.
The outer part of the range front structural zone consists of a continuous zone of
silicified Proterozoic fault breccia with much coarser clasts than the BR zone rock chips.
The fault breccia occurs as an at least 150-foot wide, concentrated float zone and as small
blocks on a mine dump at a caved shaft. The breccia has angular clasts of Proterozoic
foliated granodiorite (Xgdf) up to a couple of inches in size, in a silicified matrix.
Outboard of the Proterozoic fault breccia to the southeast, a zone of Tertiary rhyolite
porphyry (Trp) and aphyric rhyolite (Tr) dikes occurs as discontinuous outcrop, subcrop,
and float along the range front. The rhyolite dikes are from 20 to 30 ft thick and are
locally brecciated and silicified. The rhyolite dikes and silicified breccias do not occur as
a component of the BR zone, nor are Dry Union Formation conglomerate clasts
indicated.
The continuity of the BR zone and rhyolite dikes to the southwest is uncertain due
to lack of exposure. The range front fault zone is concealed for 6,000 ft by the Quaternary
tills that came out of the north and south forks of Squaw Creek. Farther southwest the
range front structural zone is very poorly exposed and much of it is covered with coarse
Proterozoic rock colluvium. Some rhyolite porphyry on the lower adit dump of the Blank
mine suggests that rhyolite dikes exist along the range front structure in this area. Further
southwest in the steep gulch north-northeast of the Angle of Shavano Campground, a
subcropping BR zone with finely comminuted Proterozoic gneiss is exposed in a small
erosional window in Quaternary till. Between the North Fork and the Cree Creek area the
continuity and width of the BR zones is uncertain due to abundant colluvial float. A large
irregular BR zone wraps around the small hill between Lost Creek and Cree Creek near
the southernmost part of the Shavano fault. These observations suggest there may be a
northeast-trending continuous BR zone associated with the Shavano fault zone, which
extends for over 30,000 ft. across the northwest part of the quadrangle.
The age of the BR zones in the Maysville quadrangle is not well understood and
the brecciation and shattering of the rocks could range from the Proterozoic to recent.
However, the brittle nature of the deformation does not support an Early or Middle
Proterozoic age for the faulting. This leaves a potential age ranging from the Late
Paleozoic to the Neogene. As described above, the BR zones in the southwest part of the
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