Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

The western zone of brecciated Paleozoic landslide sheets-blocks found during
this study is about 4,000 ft northwest of the central zone. The western zone is about 800
ft long and oriented about N25°E. Slabs of brecciated carbonate dip about 32° northwest.
The western zone of landslide sheets and blocks is spatially associated with the
stratigraphic central part of the western clay bed zone. Van Alstine (1970) calculated that
the eastern zone is about 700 ft and the central zone is about 2,600 ft above the base of
the Dry Union Formation at the east edge of the South Arkansas graben. A cross section
for this study (section B-B’-B’’, plate 3) indicates the eastern horizon is a minimum of
600 ft, the central horizon is a minimum of 2,000 ft, and the western horizon is a
minimum of 3,500 ft above the lowest exposed horizon of the Dry Union Formation.
A major remaining problem is determining if the three landslide sheet and block
horizons represent three separate landslide events or if a single major event formed one
horizon that has been subsequently disrupted and repeated by block faulting. A fault
duplication model requires that the inferred faults must have had up to the west offset,
which is not compatible with the location of the landslide sheets in the east part of the
half graben where faulting should have down to the west offsets. The relationship of the
detached landslide sheets and volcanic ash beds suggests a complex lake-bed sequence
that potentially recorded three catastrophic landslide events and at least two and possibly
four volcanic ash eruptions. Determining the distribution and ages of the volcanic ashes
may help constrain models for the emplacement of landslide sheets.
TERTIARY IGNEOUS ROCKS – A wide variety of Tertiary igneous rocks cover an
estimated 3 to 4 percent of the total surface area and ten percent of the exposed bedrock
area of the Maysville quadrangle. They are almost exclusively in the northwest quadrant
of the quadrangle except for possible minor Tertiary dikes (Ta) in the southwest quadrant.
These igneous rocks include portions of large intrusions and stocks and abundant dikes.
No igneous rocks related to the Laramide magmatic pulse have been recognized in the
Maysville quadrangle.
Strictly speaking, all of the Tertiary igneous rocks are related to the Middle
Tertiary magmatic pulse; most are associated with the Mount Princeton pluton and the
Mount Aetna cauldron magmatic events. Igneous rocks related to the subsequent
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