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.为支持美国地质调查局（USGS）西科罗拉多I-70走廊合作地质填图项目，本次针对Silt 7.5'标准图幅开展了全新的1:24000比例尺地质填图工作，为怀特河隆起西南翼、大霍格巴克（Grand Hogback）以及皮斯安斯盆地东部区域的地层、构造与地质灾害提供了全新解译成果。 本次研究将瓦萨奇组（Wasatch Formation）划分为三个正式成员：夏尔成员（Shire Member）、莫利纳成员（Molina Member）与阿特韦尔峡谷成员（Atwell Gulch Member），同时拆分出夏尔成员内部的一套砂岩地层单元。梅萨维德群（Mesaverde Group）由上覆的威廉姆斯福克组（Williams Fork Formation）与下伏的艾尔斯组（Iles Formation）组成；艾尔斯组的成员单元包括罗林斯砂岩成员（Rollins Sandstone Member）、科泽特砂岩成员（Cozzette Sandstone Member）与科克伦砂岩成员（Corcoran Sandstone Member），其中科泽特与科克伦砂岩成员被合并为一个填图单元。曼科斯页岩（Mancos Shale）的上段仅在该图幅内出露。 从图幅西南角向西北方向延展，未受断层改造的始新世至古新世早期瓦萨奇组及其下伏的梅萨维德群倾角逐渐增大，形成大霍格巴克单斜构造，其西南向倾角可达45°至75°（剖面A-A'）。沿科罗拉多河分布的浅西西北向走向来弗尔向斜（Rifle Syncline），将该图幅的南北两部分分隔开来。 本图幅内的地质灾害类型涵盖侵蚀作用、膨胀土与洪水灾害。侵蚀作用包括块体运动、冲沟侵蚀与管涌侵蚀。块体运动指受重力驱动沿坡向下移动的各类岩石或表生沉积物，如滑坡、碎屑流或落石，通常在较陡坡度区域更为发育。局部区域中，当大霍格巴克构造倾角超过60°且瓦萨奇组被侵蚀后，梅萨维德群的砂岩板体失去支撑，在最大垂向距离达500米的范围内发生垮塌，该单元上段大概率以梁屈曲破坏的形式发生滑坡。在这些滑坡的源区，地层发生倒转且倾角较缓，指向东北方向。现今滑坡沉积物覆盖了更新世山麓面，并至少延伸1公里至仙人掌谷（Cactus Valley）。冲沟侵蚀与管涌侵蚀通常发育于坡度较缓的区域。膨胀土与膨胀基岩指遇湿膨胀、遇干收缩的未固结物质或岩石。多数洪水灾害局限于低洼区域。 多口产气井从艾尔斯组上段的煤层中开采甲烷。