2012-09-06_REVISION - M2008070 (22)

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GEOLOGIC SETTING Physiography and Structure Most of the White River basin is within the extreme northeastern part of the Colorado Plateau physiographic province, although the easternmost part of the basin is within the Southern Rocky Mountains physiographic province. Several major structural features extend into the drainage basin, including the White River uplift, the Grand Hogback monocline, the Piceance structural basin, and the Douglas Creek arch (fig. 2). The Meeker dome is a smaller, though significant, local feature 3 mi east of Meeker (fig. 2). Bedrock Formations Although rocks of Precambrian through Tertiary age are exposed in the White River basin (fig. 2), only Jurassic through Tertiary formations border and underlie the White River in the study area. These rocks include (in ascending order) the Morrison Formation, Dakota Sandstone, Mancos Shale, Mesaverde Group, Wasatch Formation, and Green River Formation (fig. 2). The Morrison Formation, a variegated shale and mudstone of Jurassic age, is exposed within the study area only at the Meeker dome east of Meeker. The Cretaceous Dakota Sandstone, a light -gray to tan quartzose sandstone, under- lies the easternmost part of the study area and also is exposed at the Meeker dome. Underlying Meeker and the rest of the alluvial aquifer in Agency Park is the Mancos Shale of Cretaceous age, a dark -gray marine shale. The Mowry Shale Member of the Mancos Shale, a silver -gray siliceous shale, and the Frontier Sandstone Member of the Mancos Shale, a tan calcareous sandstone, also are exposed southeast of Meeker. West of Meeker, the resistant light - brown to white sandstones and interbedded shales and coal layers of the Cretaceous Mesaverde Group form the Grand Hogback. West of the Grand Hogback, the Tertiary Wasatch Formation, a variegated claystone, siltstone, sandstone, and conglomerate, is exposed. The Tertiary Green River Formation is present farther west and consists of marlstone, oil shale, siltstone, and sandstone. In the western part of the study area the White River again flows over the Wasatch Formation, the Mesaverde Group, and, near Rangely, the Mancos Shale (fig. 2). Alluvial Formations Alluvium deposited by the White River and underlying its flood plain is probably of late Wisconsin age (J. W. Whitney, U.S. Geological Survey, oral commun., 1982). This alluvium is referred to as White River alluvium in this report because it is the most important constituent of the White River allu- vial aquifer. Other unconsolidated deposits in the study area include tribu- tary alluvium, alluvial fans, terrace alluvium, and colluvium. White River alluvium consists principally of silty sand and rounded gravel and cobbles composed of fragments of rocks found east of the study area. These fragments usually are sandstone, quartzite, basalt, and granite. Tributary alluvium is composed of finer - grained material, mostly of local origin, confined to the valleys and mouths of relatively large tributaries 5