Published on August 20, 2014
15 • 2014 Upper Yampa River State of the Watershed Report General Geology The UYRW is predominantly underlain by: 1) Precambrian metamorphic and intrusive igneous rocks in the eastern one-third of the watershed (Gore and Park Ranges), 2) sedimentary rocks of the Cretaceous age and sedimentary, intrusive, and volcanic igneous rocks of the Tertiary age including the Yampa Coal eld, and 3) unconsolidated surface deposits in the valley oors derived from river alluvium, landslides, and glacial drift which primarily occurred on the toe slopes of the highest southern and western mountains (Figure 13) (Nancy J. Bauch, 2012). Major coal deposits in the Yampa coal eld occur in the Upper Cretaceous Williams Fork and the Iles formations located in the northwestern part of the UYRW. These formations plus other sedimentary rock formations such as the Mancos shale contain soluble major ions and trace elements. These have caused elevated concentrations and, in isolated settings, exceedances of water-quality standards due to natural causes (Nancy J. Bauch, 2012). More than 150 geo-thermal mineral springs are located at or near the intersection of regional 12,000-15,000 feet deep, high-angle faults cross cut by local faults through the Dakota Sandstone. These warm springs may have slightly elevated concentrations of a few minerals but have not triggered any recorded exceedances to date.