Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

The clay beds are spatially associated with two horizons of Paleozoic landslide
sheets (Td2ls) and two volcanic ash beds (Td2v) in the lower part of the Dry Union
Formation (Td2) in the eastern part of the South Arkansas graben (plate 1). They are
exposed over a zone about 3,300 ft wide with a suggested true thickness of about 1,800
ft. The clay beds are concentrated in two main sub-parallel zones that trend about N20°E.
The eastern zone is about 1,200 ft wide with an estimated true thickness of about 650 ft.
It is exposed for about 3,000 ft along strike but it is not certain if it crosses Pass Creek
and extends past the south boundary of the quadrangle. The western zone is about 800 ft
wide with an estimated true thickness of about 500 ft (150 m). It extends for about 4,000
ft along strike and becomes difficult to trace in the boulder-covered steeper slopes to the
south. Clay beds may have a significant component of volcanic ash.
Td2v Volcanic ash (middle to upper Miocene) – Two volcanic ash beds are mapped
in the South Arkansas graben sequence in the southeast corner of the Maysville
quadrangle. The first volcanic ash bed locality was located by Van Alstine (1974) in the
SW¼ , NW¼ , Section 7, T.49N., R.8E. (locality 4; see Structural Geology section). A
second volcanic ash bed found during this study is present about 1,800 ft west of the first
locality in the NE¼ , Section 12, T.49N., R.7E. (locality 4A, see Structural Geology
section).
The volcanic ash is white to very light gray, very fine-grained, and ranges from
massive to strongly fissile (fig.15). Most of the ash layers are clean with little or no
evidence of detrital contamination. Hand samples indicate the lack of discernable biotite
or other mafic minerals. One thin section of volcanic ash from Section 7 was examined
for this study. The ash exhibits vitroclastic texture and consists of about 98 percent of
elongated fragments of glass bubble walls (fig. 16). Most of the fragments are tabular
with only a small percentage showing flaring ends. This suggests that the glass vesicles
were relatively large and possibly highly flattened. The elongated bubble walls are
strongly aligned parallel to faint bedding laminations. The volcanic ash exhibits minor
fine-scale bedding laminations produced by variations in the size of the bubble wall
fragments. Crystal fragments contribute about 1 to 2 percent of the ash, including
remnant biotite grains that are completely altered to clay, and minor hornblende,
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