Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Cree Creek. The map pattern of the BR zone suggests a north-south oriented fault that
merges with the BR zone along the southwesternmost part of the Shavano fault (plate 1).
However, the north and south extension of the north-south fault is concealed by
Quaternary till/gravel. No evidence exists that this fault extends north to the North Fork
or south across the South Arkansas River. Thus, the continuation of the Shavano fault and
the Salida-Maysville fault beyond the west edge of the quadrangle is uncertain. The field
relations suggest these two faults may have moved nearly simultaneously and neither one
continue on the strike projection with significant offset.
Another major question involves the relationship and relative ages of the Shavano
and Willow Creek faults bounding the western margins of the Upper Arkansas graben
and South Arkansas graben, respectively. The Willow Creek fault has a northeasttrending
bend at its north end. The BR zone and the contact with the Dry Union
Formation along this segment of the Willow Creek fault are parallel to the Shavano fault.
The Willow Creek fault appears truncated by the Salida–Maysville fault. If the Shavano
fault and the Salida-Maysville fault moved at the same time, it would suggest that the
Willow Creek fault is older. The interrelationships of the rift-bounding faults, the
grabens, and the rift-shoulder uplift horst blocks are extremely complicated and the
interpretation will require additional detailed studies in the surrounding areas, including
the Mount Antero, Garfield, Mount Ouray, and Poncha Pass quadrangles.
QUATERNARY DEFORMATION
Geologic evidence suggests that some faults in the quadrangle have been active in
Quaternary time. The most active fault is the Shavano fault zone, which lies at the base of
the Sawatch Range and Mount Shavano in the northwest quarter of the map area (plate
1). Fault scarps (thick lines with hachures on the map) in the Shavano fault zone define a
broad fault zone composed of 3 to 4 parallel, southeast-dipping normal faults in a zone
0.5-1.1 mi wide that displace Quaternary glacial and alluvial deposits. An additional stepfault
crosses the range-front piedmont 2.2-2.5 mi from the range front. The bestpreserved
faults scarps are generally on those fault strands closest to the range front, such
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