MAPPING MEMBERS OF THE MANCOS SHALE IN COLORADO

The Mancos Shale in Western Colorado is a ubiquitous formation (as is the coeval Pierre Shale of Eastern Colorado) that was deposited within the Late Cretaceous Western Interior Seaway. The Mancos Shale formation name is recognized west of the Front Range basement rocks and within the intermontane structural basins of Central Colorado. This poorly indurated claystone can be a source rock and a tight reservoir for mineral fuels at depth. It is also problematic near the surface for its propensity for slope instability, expansive clay mineralogy, and where rich in sulfates and total dissolved solids can degrade surface water quality where irrigation practices cause deep percolation in many of the agricultural valleys that are underlain by the shale in Western Colorado. Earlier quadrangle mapping did not separate the individual members of the Mancos Shale. However, using seminal lithostratigraphic and biostratigraphic work in Western Colorado, the Colorado Geological Survey geologists have been focused on mapping the individual members of the Mancos Shale since the mid-2000s. Thirty (30) 1:24,000-scale STATEMAP quadrangles have been completed or are in progress in Mancos Shale terrain that include mapping programs at: 1) the Mesa Verde NP escarpment in SW Colorado, 2) the Uncompahgre River valley to the confluence with the Gunnison River and its watershed below the southern flank of Grand Mesa and the Black Canyon of the Gunnison uplift, 3) the Colorado River through the Grand Valley from Grand Junction to near the Utah border, 4) the White River valley above the Grand Hogback water gap near Meeker, and 5) within the Axial Uplift in NW Colorado within the Yampa River watershed. Lithologic marker beds, superposition, and index fossils have been used to map the individual members, and member identification has been useful in tracing and determining fault throw, as well as the variable susceptibilities for swelling soil, instability, and water degradation.