Stratigraphic framework and correlation of a principal reference section of the Mancos Shale (Upper Cretaceous), Mesa Verde, Colorado
— R. Mark Leckie, James I. Kirkland, and William P. Elder

A principal reference section is proposed for the Mancos Shale in its type area of southwestern Colorado (northern rim of the San Juan Basin). The section is located at the north end of Mesa Verde National Park, where the exposures were trenched and the rocks described with centimeter-scale resolution through the 682 m formation. The Mancos was thoroughly examined for fossils and a molluscan biostratigraphy is presented herein. In addition, bulk samples were collected throughout and a detailed gamma-ray survey conducted, thereby permitting correlation with subsurface geophysical logs. The Mancos Shale spans the upper Cenomanian through basal Campanian Stages at the Mesa Verde principal reference section, and is bounded by the Dakota Sandstone below and the Point Lookout Sandstone above. Eight members are recognized, two of which are new. Stratigraphic nomenclature of the Colorado Front Range can be applied in this region of the Colorado Plateau. The upper Cenomanian Graneros Member overlies the Dakota Sandstone and is 24.1 m thick. Numerous thin beds of light-gray limestone and marlstone characterize the lower to middle Turonian Bridge Creek Limestone Member, which is 13.8 m thick at Mesa Verde. The Fairport, Blue Hill, and Juana Lopez Members are 28.2, 75.5 m, and 42.6 m thick, respectively. Several ledges or benches of well-cemented fossiliferous calcarenite mark the middle and upper parts of the upper Turonian Juana Lopez Member. The 16.2 m-thick upper Turonian Montezuma Valley Member is named herein; it correlates with the upper part of the D-Cross Tongue of the Mancos Shale in the southern and southwestern parts of the San Juan Basin. At Mesa Verde, a major regional disconformity separates uppermost Turonian rocks of the Prionocyclus quadratus ammonite zone from upper middle Coniacian rocks of the Scaphites ventricosus Zone. The "Carlile-Niobrara" disconformity is represented by a sharp shale-on-shale contact and disseminated glauconite and fine quartz sand in the basal meters of the overlying Smoky Hill Member. The Smoky Hill Member (middle Coniacian-middle Santonian) is 88.6 m thick, the upper 23 m of which forms a prominent gray bench of calcareous shale. This unit correlates with the upper part of the lower shale, lower limestone and middle shale units of the Smoky Hill Member, Niobrara Formation, near Pueblo, Colorado. Capping the Mancos Shale section is the 393.0 mthick upper Santonian-basal Campanian Cortez Member, named herein. Six informal units representing two coarsening upwards sequences are recognized within the Cortez Member. The lowest of these informal units (289.0-335.7 m) is gradational with the underlying Smoky Hill Member and correlates with the uppermost part of the middle shale unit and the middle chalk unit of the Smoky Hill Member, Niobrara Formation. A middle sandy unit (389.6-451.7 m) may partially correlate with the Emery Sandstone of central Utah. The uppermost part of the Cortez Member contains sandy mudstone and thicker sandstone beds, and is gradational with the overlying Point Lookout Sandstone through a 22 m interval (660-682 m). The principal reference section of the Mancos Shale is an important lithostratigraphic and biostratigraphic tie point between the thick siliciclastic-dominated sequences deposited on the western margin of the Western Interior Sea, and the thinner carbonate-rich central and eastern sequences.