NEW MAPPING OF THE MANCOS SHALE IN WESTERN COLORADO
The Colorado Geological Survey (CGS) conducts detailed mapping of the Mancos Shale in western Colorado as part of the STATEMAP national cooperative geologic mapping program. We use federal and state matching funds to produce 1:24,000-scale geologic maps. The STATEMAP program contributed eighty-nine new quadrangles in Colorado since 1993.
Our western Colorado mapping focuses on population-growth areas in the Uncompahgre and Gunnison River valleys. Collaborating with the USGS, we delineated several members of the Mancos Shale based on physical characteristics and biostratigraphy, identified using field observations and invertebrate fossil collections.
The primary mapping units, from the basal contact upward, include the Graneros, Bridge Creek, Blue Hill, Juana Lopez, Montezuma Valley, Smoky Hill, Prairie Canyon, Sharon Springs, Lujane Point (informal, offshore equivalent of the Castlegate Sandstone); and Buck Tongue Members. We correlate these units with reference sections in the Mesa Verde, Book Cliffs, and Front Range (Pueblo) areas.
The creation of a mappable framework of Mancos Shale subunits is of potential use for oil and gas exploration, selenium abatement studies, and geologic-hazard investigations. An example is the Prairie Canyon Member section north of Delta, which contains previously undescribed sand- and organic-shale-filled submarine channels. In another example, our field observations reveal that selenium is concentrated within coarser-grained units such as the Juana Lopez Member. Selenium dissolution from dark shale units and its associated transport to and precipitation within coarser reservoir facies may merit further study by environmental geologists and geochemists.
CGS geologic maps are available as CD-ROM publications. Each CD contains a geologic map, correlation of map units, oblique view of the map, cross sections, and illustrated authors’ notes.