Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

South Arkansas graben, in the Droz Creek area, and on the east side of Marshall Pass
(Kouther, 1969; Perry, 1971 and Dippold, 1999). The zone of outliers continues to the
area just south of the Bonanza caldera where it is truncated and concealed by the San
Luis Valley graben (Perry, 1971 and Tweto and others, 1976). Thus, the Paleozoic rock
outliers occur in a N40°W-trending belt that is at least 50 miles long and about 12 miles
wide. The Maysville quadrangle is along the east margin of this belt (fig. 4).
Or Paleozoic sedimentary rock remnants and fault slices (Ordovician and
Mississippian) – There are six Paleozoic rock bodies in the Proterozoic terrane in the
southwest part of the Maysville quadrangle. The three largest are interpreted to be
remnants of erosional or structural outliers that were folded or downfaulted into
Proterozoic rocks. Two smaller bodies are thin slices of Paleozoic rocks along faults and
the sixth occurrence is interpreted to be an intensely brecciated fault slice.
Of the six Paleozoic remnants in the Maysville quadrangle, only one shows
intense brecciation. It occurs as a small mass (about 250 ft long) exposed in a window
through the Quaternary gravels along the South Arkansas River on the east edge of
Maysville. The relations of the Paleozoic remnant to Proterozoic basement rocks about
1,000 ft southwest and to the closest Dry Union Formation (Td) about 800 to 1,000 ft
north are not evident. This locality is close to the intersection of two major, concealed
fault zones: (1) the roughly north-trending Willow Creek fault, and (2) the westnorthwest-trending
Maysville-Salida fault.
The Paleozoic mass is composed of medium-gray, very fine-grained, massive to
finely bedded limestone and cherty limestone and is probably Manitou Formation ().
The limestone is cut by irregular zones of brecciated limestone and also by a fault with
N16°E and 75°NW to vertical orientation. Localized zones of intense shattering are
similar to shattering in the Paleozoic landslide sheets (Td2ls) in the South Arkansas
graben. Quaternary Bull Lake outwash deposits lap onto the Paleozoic mass and contain
subrounded blocks of limestone up to 8 ft in diameter.
Three other Paleozoic remnants occur along the Willow Creek fault bounding the
west edge of the South Arkansas graben at approximately 2,700 ft south-southwest, 6,000
ft southwest and 11,600 ft south of Maysville. The Paleozoic remnant about 2,700 ft
110