Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

elations, age constraints from fossils (Van Alstine, 1974), and tentative
tephrochronological age determinations on volcanic ash beds (Xu, 2001; Xu and others,
2001; Hubbard, and others, 2001) suggest that the South Arkansas graben sequence (Td2)
is middle Miocene near the base and is older than the Upper Arkansas graben sequence
(Td) with indicated late Miocene, Pliocene, or upper Pliocene ages.
Td Upper Arkansas graben sequence (Miocene and Pliocene) – Consists of
white, pinkish-white, orangeish-tan, and light-greenish-gray, very fine-grained siltstone,
minor fine-grained sandstone, with minor, thin, discontinuous pebble conglomerate to
boulder conglomerate lenses (fig. 13). The siltstone and conglomerate are generally very
poorly indurated to unconsolidated. Bedding varies from massive siltstone sequences to
moderately well-bedded siltstone-conglomerate sequences. Bedding is generally on the
order of inches to feet. The pebble to boulder conglomerate beds are relatively thin
discontinuous lenses that range up to about 5 ft thick and up to hundreds of feet long.
Minor thin beds of gritty siltstone to sandstone and pebble conglomerate are
moderately indurated and are more resistant to weathering. Very poorly to moderately
indurated beds are normally cemented with calcium carbonate. The more indurated zones
are controlled by bedding horizons and no cementation is visibly controlled by structures.
The Dry Union Formation (Td) in the Maysville quadrangle is preserved in the
southwestern corner of the Upper Arkansas graben. The sequence is preserved between
the northeast-trending Shavano fault zone on the northwest and the west-northwesttrending
Salida-Maysville fault on the southwest; the sequence forms a large trapezoidal
area in the west-central part and northeast quadrant of the Maysville quadrangle (plate 1).
It is present as a series of narrow to broad bands that form a semi-radiating pattern that
converge, or point back to, the northeast-trending Shavano fault zone at the range front in
the area of the North Fork and Squaw Creek.
The best exposures of the Upper Arkansas graben sequence (Td) are in a northtrending
zone of rugged badlands topography that is present in the western part of the
Salida West quadrangle and that extends along the northeastern edge of the Maysville
quadrangle. Most areas of the unit are adjacent to and beneath thin layers of glacial
outwash and Quaternary gravel that cap semi-flat ridge piedmont surfaces developed on
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