Geologic Map of the Silt Quadrangle, Garfield County, Colorado

To update and reinterpret earlier geologic mapping, and to provide sufficient
geologic information for land-use decisions.

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.

Map political location: Garfield County, Colorado
Compilation scale: 1:24,000
Geology mapped in 1992 to 1996.
Compilation completed March 1997.