Upper Cretaceous marine strata in the Little Hatchet Mountains, southwestern New Mexico

Upper Cretaceous marine strata with agediagnostic middle Cenomanian bivalve and ammonite fossils are preserved in the Little Hatchet Mountains of southwestern New Mexico. These strata, long assigned to the upper part of the Lower Cretaceous Mojado Formation, are approximately 100 m (328 ft) thick and are mostly dark gray shale with a few thin interbeds of limestone and sandstone and some limestone septarian concretions. We assign these marine strata to the Mancos Shale; they are unconformably overlain by nonmarine sandstone at the base of the Upper Cretaceous (Campanian) Ringbone Formation. Fossil localities in the Mancos Shale that are 11‐30 m (36‐98 ft) below the Ringbone base yield the following bivalve and ammonite taxa: Ostrea beloiti Logan, Inoceramus arvanus Stephenson, Inoceramus prefragilis Stephenson, cf. Acanthoceras sp., cf. Tarrantoceras sp., Desmoceras sp., Hamites cf. H. simplex d’Orbigny, and Turrilites acutus acutus Cobban and Scott. These