Geologic Map of the Silt Quadrangle, Garfield County, Colorado

New 1:24,000-scale geologic mapping in the Silt 7.5' quadrangle,
in support of the USGS Western Colorado I-70 Corridor Cooperative
Geologic Mapping Project, provides new interpretations of the
stratigraphy, structure, and geologic hazards in the area of the
southwest flank of the White River uplift, the Grand Hogback, and
the eastern Piceance Basin.
The Wasatch Formation was subdivided into three formal members,
the Shire, Molina, and Atwell Gulch Members. Also a sandstone
unit within the Shire Member was broken out. The Mesaverde Group
consists of the upper Williams Fork Formation and the lower Iles
Formation. Members for the Iles Formation consist of the Rollins
Sandstone, the Cozzette Sandstone, and the Corcoran Sandstone
Members. The Cozzette and Corcoran Sandstone Members were mapped
as a combined unit. Only the upper part of the Upper Member of
the Mancos Shale is exposed in the quadrangle.
From the southwestern corner of the map area toward the
northwest, the unfaulted early Eocene to Paleocene Wasatch
Formation and underlying Mesaverde Group gradually increase in
dip to form the Grand Hogback monocline that reaches 45-75 degree
dips to the southwest (section A-A'). The shallow west-
northwest-trending Rifle syncline separates the northern part of
the quadrangle from the southern part along the Colorado River.
Geologic hazards in the map area include erosion, expansive
soils, and flooding. Erosion includes mass wasting, gullying,
and piping. Mass wasting involves any rock or surficial material
that moves downslope under the influence of gravity, such as
landslides, debris flows, or rock falls, and is generally more
prevalent on steeper slopes. Locally, where the Grand Hogback is
dipping greater than 60 degrees and the Wasatch Formation has
been eroded, leaving sandstone slabs of the Mesa Verde Group
unsupported over vertical distances as great as 500 m, the upper
part of the unit has collapsed in landslides, probably by a
process of beam-buckle failure. In the source area of these
landslides strata are overturned and dip shallowly to the
northeast. Landslide deposits now armor Pleistocene pediment
surfaces and extend at least 1 km into Cactus Valley. Gullying
and piping generally occur on more gentle slopes. Expansive
soils and expansive bedrock are those unconsolidated materials or
rocks that swell when wet and shrink when dry. Most floods are
restricted to low-lying areas.
Several gas-producing wells extract methane from coals from the
upper part of the Iles Formation.