Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Formation is part of the Santa Fe Group (Chapin and Cather, 1994).
Dry Union Formation– The Dry Union Formation in the Maysville quadrangle includes
two separate sequences that represent different stratigraphic horizons: (1) the Upper
Arkansas graben sequence (Td); and (2) the South Arkansas graben sequence (Td2). The
two sequences are structurally separated by the west-northwest-trending Salida-Maysville
fault. Significant differences between the two sequences include the overall character of
the sediments and the presence of very fine-grained clay beds (Td2c) and detached
landslide sheets (Td2ls) in the South Arkansas graben sequence.
In the Nathrop quadrangle, Van Alstine and Lewis (1960) and Van Alstine (1974)
reported two fossil localities that indicated an early Pliocene age (equivalent to lower
Ogallala of Nebraska) for the part of the Dry Union exposed along the eastern part of the
Upper Arkansas graben. In contrast, fossils reported by Powers (1935) near Salida (in the
Salida graben) led Van Alstine and Lewis (1960) to suggest that the Dry Union
Formation there may be slightly younger in age (equivalent to upper Ogallala). Both of
these localities are on the eastern side of the graben, and the ages may support that the
youngest preserved Dry Union Formation (Td) is in the Salida sub-graben. Xu (2001) and
Xu and others (2001) conducted tephrochronological studies on two of the volcanic ash
layers southwest of Salida. They suggest the source for a white biotite-bearing dacitic ash
is the Basin and Range. A second ash, a gray vitric tuff, has compositions typical of the
8.5 to 10.5 Ma Twin Falls volcanic field. The tentative late Miocene age for the volcanic
ash layers are a little older than the Pliocene or upper Pliocene age indicated by fossils
from the same general locality.
The South Arkansas graben sequence (Td2) has a slightly older age indicated by
mammal fossils and preliminary age determinations on volcanic ash beds. Van Alstine
(1974) described fossil horse teeth that indicate a late Miocene age (U.S.G.S. vertebrate
fossil locality D750) from the Maysville quadrangle (from Elephant Rock area). Hubbard
and others (2001) reported ages, based on tephrochronology, of two volcanic ash layers
from near Little Cochetopa Creek in the Maysville quadrangle. The two ash layers
yielded ages of 13.3 +/- 0.2 and 14.2 +/- 1.4 Ma, suggesting a middle Miocene age that is
slightly older than the late Miocene age based on mammal fossils. Thus, structural
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