Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

graben and the Upper Arkansas Valley graben. It extends from Villa Grove in the upper
San Luis Valley to the Salida area and is bounded on the north by the west-northwesttrending
Salida-Maysville fault (fig. 39). Such an interpretation is favored because the
symmetry of the rift shoulder uplifts does not abruptly change but is more of a transition
across this broad 20 mi wide area. On the west side of the rift, the Proterozoic rocks in
the core of the Sawatch Range extend southward (partially under Tertiary volcanic rock
cover) to Mineral Hot Springs where these rocks terminate at the north end of the San
Luis Valley. On the east side of the rift, the Proterozoic rocks in the core of the Sangre de
Cristo Range continue northward to Poncha Springs, where they terminate at the south
end of the Upper Arkansas Valley. Thus the asymmetric rift shoulder uplifts overlap for
about a 20 mile wide area. Therefore a more inclusive name for this complex zone is the
Poncha Springs-Villa Grove accommodation zone (see fig. 5).
Observations presented in this report suggest two and possibly four or more
volcanic ash beds are present in the South Arkansas graben sequence (Td2). Additional
volcanic ash beds have been reported in the Upper Arkansas graben sequence (Td). Table
4 provides a compilation of all Miocene and Pliocene volcanic ash localities that have
been identified in the southern part of the Upper Arkansas graben and the South Arkansas
graben. Nine localities of volcanic ash are identified in the Dry Union Formation (Td and
Td2) and three localities of volcanic ash are in Quaternary deposits (fig. 39). Multiple ash
beds are reported at four of the Dry Union Formation ash localities. Basic characteristics
of the ashes including thickness, presence and character of glass, mineralogy of primary
crystal fragments, and degree of contamination may provide information that could be
used to correlate the different localities. More advanced techniques including
tephrochronology and glass and mineral chemistry may help to confirm correlations and
ultimately provide absolute ages on specific volcanic ashes. Consequently, the volcanic
ash beds in the Dry Union Formation are suggested to be important time-stratigraphic
markers that will be useful in recognizing and separating the different stratigraphic
sequences in the different Dry Union Formation structural domains and providing
absolute age constraints on inception of rift-related faulting and sedimentation.
171