SUBDIVISION OF MANCOS SHALE FOR GEOLOGIC MAPPING IN WESTERN COLORADO

In 2006, for the STATEMAP Program, Colorado Geological Survey (CGS) began geologic mapping in areas of western Colorado that contain Mancos Shale. The shale is of interest because it floors the valleys where many towns are built. Because of its weak character, the shale and its derived soils are associated with swelling soil and rock, collapsing soil, and landslide hazards.

One of the goals of the mapping was to subdivide the Mancos into recognizable, mappable subunits based on color, composition, and biostratigraphy. This resulted in a distinct improvement over older geologic maps, which portrayed it as a single, thick unit. Another aim was to create a stratigraphic framework that would allow the Mancos subunits to be mapped throughout western Colorado.

Our subdivision relied on several sources. Foremost was the detailed biostratigraphic study of the Mancos Shale type location by Leckie, Kirkland, and Elder in the late 1990’s. By the 2000’s, other biostratigraphic studies had been done in Colorado by Bill Cobban and others from the USGS. Utah nomenclature was not used because so many subunit names changed across the border. Also, we found that the subunits in western Colorado were more physically similar to age-equivalent strata in east-central Colorado.

In practice, we considered the Mancos Shale to be a “formation,” and the subunits to be “members.” The lower part overlies the Dakota Formation and Mowry Shale (often undivided). From the bottom up, it consists of the Graneros and Hartland, Bridge Creek and Fairport, Blue Hill, Montezuma Valley, Juana Lopez, Montezuma Valley, Fort Hays and Smoky Hill, Prairie Canyon, and Sharon Springs Members. The uppermost subunit, the Mancos Shale, Upper Part, is interspersed between sandstone tongues of the Mesaverde Group, which are named according to local nomenclature.

To date, the CGS has conducted detailed Mancos Shale mapping in twenty-four 1:24,000-scale quadrangles in western Colorado. The maps have been used in geohazard, groundwater, paleontological, and horticultural studies. Fossil mollusk collections are made during the projects and donated to the USGS for their Western Interior Seaway collection.