Upper Cretaceous marine strata in the Little Hatchet Mountains, southwestern New Mexico
Lucas, S.G.; Lawton, T.F.
New Mexico Geology ust; 27(3): 63-69
Upper Cretaceous marine strata with age-diagnostic 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 sand-stone and some limestone septarian concretions. We assign these marine strata to the Mancos Shale; they are unconformably over-lain by nonmarine sandstone at the base of the Upper Cretaceous (Campanian) Ring-bone 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 prof agilis Stephenson, cf. Acanthoceras sp., cf. Tarrantoceras sp., Desmoceras sp., Hamites cf. H. simplex d'Orbigny, and Turrilites acutus acutus Cobban and Scott. These fossils indicate the ammonite zone of Acanthoceras amphibolum Morrow and thus a middle Cenomanian age. In the Little Hatchet Mountains, marine strata of the Mancos Shale overlie a thick succession of quartzarenite strata of the Mojado Formation that contain hummocky cross-lamination and a few marine bivalves. The Mancos was deposited in a shelfal setting below storm wave base and thus records post-Mojado transgression across the axis of the former Bisbee rift basin. The presence of shelfal deposits and Western Interior inoceramids in the New Mexico bootheel implies that the former highland of the Mogollon-Burro uplift was completely submerged by the middle Cenomanian transgression. These biostratigraphic data support previous interpretations, based on post-Cenomanian sandstone composition and fluvial sediment dispersal, that transgression of the Cenomanian seaway took place from the northeast, across the trend of the former Mogollon-Burro uplift.