New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Cretaceous marine strata with age-diagnostic fossils of bivalves and ammonites are preserved in the hinge of the Howells Well syncline (sec. 15, T28S, R16W) in the Little Hatchet Mountains of southwestern New Mexico. These strata, long assigned to the upper part of the Lower Cretaceous Mojado Formation, are at least 50 m 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 sharply overlain by nonmarine sandstone at the base of the Upper Cretaceous (Campanian) Ringbone Formation. Fossil localities in the Mancos Shale that are 11 to 30 m below the Ringbone base yield the following taxa: Ostrea beloiti Logan, Inoceramus arvanus Stephenson, Acanthoceras, Tarrantoceras, Moremanoceras and Turrilites acutus Passy. These fossils identify the ammonite zone of Acanthoceras amphibolum Morrow, and thus a middle Cenomanian age. These marine strata gradationally overlie a thick succession of quartzarenite strata of the Mojado Formation that contain hummocky cross-lamination and a few bivalves. They were deposited in a shelfal setting below storm wave base and thus record post-Mojado transgression and continuing rapid subsidence along the axis of the former Bisbee rift basin. Paleogeogeographically, the presence of the Mancos Shale in the Little Hatchet Mountains forces a small but significant shift in paleogeographic maps of the middle Cenomanian seaway in the Western Interior. Thus, the seaway (and its southwestern/southern shoreline) can be extended at least 80 km to the SW from the Cooke’s Range, and a similar distance to the southeast from Virden, which are the closest previously known outcrops of middle Cenomanian strata to the northeast and northwest of the Little Hatchet Mountains.