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.