Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

the Upper Arkansas graben (fig. 5) was named the Salida-Maysville fault by Perry
(1971). This fault extends to the east-southeast off the east edge of the Maysville
quadrangle and merges with a set of faults that define the boundary between the Salida
graben and the Sangre de Cristo horst. Knepper (1976) estimated a minimum of 2,000 to
3,000 ft of offset across this structural boundary.
The syn-rift sedimentary and volcanic deposits preserved in the axial basins of the
Rio Grande rift are recommended to collectively be known as the Santa Fe Group
(Chapin and Cather, 1994). This includes the entire syn-rift basin fill, both volcanic and
sedimentary, ranging in age from late Oligocene to Quaternary, but excluding deposits
that postdate entrenchment of the Rio Grande in early to middle Pleistocene time. The
term Santa Fe Formation has been applied to the older rift fill in the San Luis Valley, as
far north as the Villa Grove area (Scott and others, 1978). All of the older rift fill in the
Wet Mountain Valley is referred to as Santa Fe Formation (Scott and Taylor, 1975) and
has been subdivided into upper and lower Santa Fe Formation (Scott and others, 1978).
The earliest mention of the sedimentary deposits in the Upper Arkansas Valley
was by Hayden (1869), who regarded them as Pliocene lake deposits and referred to them
as the “Arkansas marls.” They were correlated with the “Santa Fe marls.” The earliest
descriptions of the deposits in the Leadville mining district were by Emmons (1886) and
Emmons and others (1927). They described a series of “lake beds” that were only
exposed in mine workings and were thought to have accumulated in a broad glacial lake
that was produced by damming of the Arkansas River at Granite by glacial moraines. The
deposits were described as chiefly interstratified clay and sand with only a few layers of
calcareous material and occasional layers of gravel.
Van Alstine and Lewis (1960) described the probable lower Pliocene sediments
near Salida as consisting of gray, yellow, brown, pink and red interbedded clays, silts,
sands, and gravels. The sediments are poorly consolidated, except for some wellcemented
calcareous lenses of gravel and sand and an argillaceous siltstone, and dip
generally less than 10 degrees in various directions. The mammalian fossils described
indicated an early Pliocene age for the deposits. Tweto (1961) named the Pliocene
sediments the Dry Union Formation for occurrences at the type locality in Dry Union
gulch about 5 mi south of Leadville. He mentioned the presence of minor, interbedded
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