Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

oundaries between the basin segments are complex structural zones referred to as
transfer faults (Gibbs, 1984) or accommodation zones (Bosworth and others, 1986).
Tweto (1979a) estimated that subsidence of the Upper Arkansas Valley graben post-dated
29 Ma Oligocene volcanic rocks but may have pre-dated 28 Ma.
Chapin and Cather (1994) suggested that the Rio Grande rift is subdivided by a
series of northeast-trending accommodation zones that developed along pre-existing
transverse structural lineaments related to late Cenozoic clockwise rotation of the
Colorado Plateau. The main accommodation zones include the Socorro, the Embudo, and
the Tijeras accommodation zones. Two less understood accommodation zones include
the Santa Ana accommodation zone, which separates the northern Albuquerque and
Espanola basins in the central part of the rift, and the Villa Grove accommodation zone,
which separates the San Luis and Upper Arkansas basins in the northern part of the rift
(Chapin and Cather, 1994).
The distribution of preserved Miocene-Pliocene sedimentary rocks indicates a
very complex pattern associated with the Upper Arkansas Valley and San Luis Valley
segments of the rift (figs. 4 and 5). The upper portion of the valley fill in the Wet
Mountain Valley, on the east side of the Sangre de Cristo Range, includes up to 1,000 ft
of Miocene-Pliocene sediments and suggests that this basin is an eastern sub-basin of the
San Luis Valley segment of the rift (Scott and Taylor, 1975 and Taylor, 1975). However,
it is not clear if these basins were connected at one time and were divided by later uplift
and tilting of the Sangre de Cristo Range (Taylor, 1975 and Lindsey and others, 1983).
The Pleasant Valley graben is a small northwest-trending sub-basin between the Upper
Arkansas Valley and Wet Mountain Valley basins; this sub-basin also contains Miocene-
Pliocene (rift) fill.
The Upper Arkansas Valley graben extends north for about 64 miles from the
Salida area to just west of Leadville (fig. 4). It has an overall trend of N24°W, is up to 14
mi wide in the Salida-Maysville area, and tapers to as little as about 5 mi wide in the
Leadville area. The structures that bound the west side of the Upper Arkansas Valley
graben are not well exposed. The bounding structures are a complex series of rightstepping,
en echelon faults that define the range front along the north-northwest-trending
segment of the rift from Browns Creek to Pine Creek (Miller, 1999; McCalpin and
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