Stratigraphic framework and correlation of a principal reference section of the Mancos Shale (Upper Cretaceous), Mesa Verde, Colorado

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. 163 INTRODUCTION The Mancos Shale is a thick, widespread Upper Cretaceous formation of the Colorado Plateau region, where the unit consists predominantly of marine shales and mudstones (Fig. 1). Subordinate lithologies include thin beds of limestone, calcarenite, bentonite, concretions, dolomitic mudstone and sandstone. The Mancos is bounded below by the transgressive Dakota Sandstone and above by regressive deposits of the Mesa Verde Group, among others, including the Tres Hermanos Formation, Gallup Sandstone, Crevasse Canyon Formation, Toreva Sandstone, Straight Cliffs Formation, Blackhawk Formation and Point Lookout Sandstone (Dyman et al. , 1993; Elder and Kirkland, 1993a, b ). In the northern part of the San Juan Basin, the Mancos Shale records nearly continuous marine deposition from late Cenomanian to earliest Campanian time. This paper summarizes the lithostratigraphy and molluscan biostratigraphy of the Mancos Shale in the type area, southwestern Colorado (northern rim of the San Juan Basin). Because an adequately defined or described type section had not been designated previously, we are establishing a principal reference section for the Mancos Shale. The Mancos Shale was named by Cross (in Cross and Purington, 1899) for "typical exposures" that occur in the Mancos River Valley between the LaPlata Mountains and Mesa Verde near Mancos, Colorado (Fig. 2). He estimated a thickness of 1200 ft in the type area and furnished a brief description. While acknowledging that fossils of the Mancos Shale are characteristic of several distinct formations east of the Front Range, Cross concluded that subdivision of the Mancos west of the Front Range is not practical. Cross et al. ( 1899, p. 4) noted that "the whole formation is well exposed along the north face of the Mesa Verde near Mancos" and included (fig. 3) a photograph of Mancos Shale slopes along the northern edge of Mesa Verde, where today the national park is located (Fig. 3). Later investigators measured and described the nearly continuous exposures of Mancos Shale along the northern face of Mesa Verde National Park, below the sandstone cliffs that form the prominent erosional headland known as Point Lookout. The first detailed study of the Mancos Shale in the type area was that of Pike ( 1947), who described the lithology and collected fossils from a section below Point Lookout. He recognized five faunal zones within the 668 m section. Wanek (1959) measured 609 m of Mancos Shale but excluded the transitional interval with the overlying Point Lookout Sandstone. We measured a total Mancos thickness of 682 m in the section exposed below Point Lookout and recognize eight members, two of which are new (Fig. 4; Table l ). We propose the Mesa Verde section as the principal reference section for the Mancos Shale because (1) this section lies within the type area as defined by Cross (in Cross and Purington, 1899), (2) this is the only place in or near the type area where a complete Mancos Shale section, from the Dakota Sandstone below to the Point Lookout Sandstone above, can be measured and described, and (3) we were able to trench the entire Mancos section so as to expose fresh rock, describe the section with centimeterscale detail, conduct a thorough macrofossil survey, and collect closely spaced gamma-ray data that permit correlation of the outcrop section with nearby subsurface geophysical Jogs through the Mancos Shale.