This report is based on lithostratigraphic and biostratigraphic data derived from investigations of Upper Cretaceous strata in Delta, Garfield, Mesa, and Montrose Counties in west-central Colorado. The data were obtained by personnel of the U.S. Geological Survey in the years 1955 through 2004, during studies of the Dakota Sandstone and the overlying Mancos Shale. Described herein are exposed strata of marine origin in the uppermost Dakota and the lower part of the Mancos (Cenomanian, Turonian, and Coniacian stages) and related and associated collections of molluscan fossils.

The stratigraphic nomenclature used in this report for the lower part of the Mancos follows that of Molenaar and others (2002), who divided the lower Mancos into six members, from oldest to youngest, the Graneros, Bridge Creek Limestone, Blue Hill, Juana Lopez, Montezuma Valley, and Niobrara. The strata consist mainly of shale but can include chalk, calcarenite, siltstone, sandstone, and bentonite; commonly, they enclose a variety of concretions. In outcrops, the members are distinguished essentially by differences in their content of calcium carbonate supplemented by the identity of constituent fossils.

Thicknesses of members from the measured outcrops vary in the region, which probably reflects lateral changes in facies and possibly the effects of truncation at disconformities in the stratigraphic sequence. Disconformities might mark the bases of the Juana Lopez and the Niobrara Members. Furthermore, the members at several places are poorly exposed, and dips used for the measurements may be incorrect. The Graneros Member, measured at five localities, ranges in thickness from 35 ft at the Red Rock section to 85 ft at the Peach Valley section. The Bridge Creek Limestone is 42 ft thick near Mack, 50 ft thick near Uncompahgre, and 140 ft thick at the Red Rock section. All of the Blue Hill Member was described at five of the selected outcrops where it ranges in thickness from about 100 ft near Olathe to 217 ft near Uncompahgre. The Juana Lopez is as much as 120 ft thick near Mack, but at five other outcrops to the southeast it is 40 to 86 ft thick. The Montezuma Valley Member is as much as 115 ft thick near Mack and is 52 ft thick at Alkali Creek and 100 ft thick near Olathe.

Reference Cited

• Molenaar, C.M., Cobban, W.A., Merewether, E.A., Pillmore, C.L., Wolfe, D.G., and Holbrook, J.M., 2002, Regional stratigraphic cross sections of Cretaceous rocks from east-central Arizona to the Oklahoma Panhandle: U.S. Geological Survey Miscellaneous Field Studies Map 2382.