Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

size. The largest block observed is about 20 ft in size.
Throughout the South Arkansas graben, the conglomerate clast compositions are
predominantly Proterozoic lithologies including amphibolite gneiss (Xag), granite and
pegmatite (YXgp) and mafic amphibolite and meta-gabbro. The larger blocks are usually
granite and pegmatite (YXgp). However, conglomerate clast compositions that are related
to stratigraphic level vary significantly east-west across the graben. The conglomerates
contain abundant (10 to 40 percent) clasts of Paleozoic sedimentary rocks and Tertiary
volcanic rocks in the Pass Creek area in the east part of the graben. In contrast, Tertiary
volcanic rocks are completely lacking and Paleozoic sedimentary rocks are rare in the
west part of the graben.
The South Arkansas graben sequence is preserved in a 9,000 ft by 18,000 ft
rectangular area in the southeast quadrant of the Maysville quadrangle (plate 1). It is in
fault contact (poorly exposed) with the Proterozoic basement along the Willow Creek
fault in the west and in fault contact (concealed) with Quaternary gravels along the
Salida-Maysville fault. The South Arkansas graben sequence is unconsolidated to weakly
indurated and is poorly exposed. Minor weak carbonate cement is locally developed and
calache is locally common as boulder-clast coatings and thin horizons in the sediments.
Localized thin discontinuous layers of gypsum- rich grit and some thin iron oxide rich
horizons may represent paleosols (). The best exposures of the South Arkansas graben
sequence are in a few road cuts along the Green and Willow Creek roads and in the low
hills with narrow incised gulches, on the south side of the South Arkansas River in the
southeast part of the quadrangle.
Limited exposures of the Dry Union Formation in the west part of the South
Arkansas graben indicate a relatively consistent north-south strike and shallow (13° to
19°) dip to the west. Better exposures in the eastern part of the South Arkansas graben
show more variable bedding orientations. Overall the beds strike north-south to northeast
with shallow to moderate (15° to 38°) dips to the northwest. Locally the bedding is
oriented north-northwest with similar dips to the southwest. In the area of the third
Paleozoic rock landslide sheet (Td2ls) in the NW 1 / 4, Section 12, T49N, R7E the bedding
is predominantly east-west to northeast with shallow to moderate (14° to 49°) dips north.
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