RAISING THE MANCOS SHALE TO A GROUP: JUSTIFICATIONS AND METHODOLOGIES

The Utah Geological Survey (UGS) is raising the Late Cretaceous Mancos Shale from a formation to a group. The group is formally subdivided into seven members in the southwestern Colorado type area with five members traced across east-central Utah. Nearly all stratigraphic subdivisions of the Mancos Group transform along the continuous outcrop from central Utah into western Colorado, creating ambiguous and conflicting map units and terminology, with some intervals bearing formal names, others informal names, and, in some intervals, no name at all. The goal of this new work is to provide an updated stratigraphic nomenclature for the Mancos Group across its western exposures. The UGS has reexamined the upper and lower stratigraphic boundaries of these units, delineating their areal extent and stratigraphic relationships across the entire region. This project incorporated historical stratigraphic data from previously published research, theses, dissertations, and other unpublished records which were field checked to accurately locate marker horizons and index fossil localities within logged sections. Critical stratigraphic surfaces and marker beds were identified and tied to recorded fossil localities with reference to their distribution on the surface and subsurface. New stratigraphic units were defined, described, and recorded accompanying gamma profiles to insure their utility in surface and subsurface stratigraphic correlations. To limit cross state line discrepancies, we suggest that the presence of marker limestone units should be a major criterion for shifting terminology from that of the Colorado Plateau to that of the Colorado Front Range.