A Summary of Late Cretaceous Ammonites and Molluscan Zones, Eastern San Juan Basin, Rio Arriba County, New Mexico

Field: Earth and Planetary Sciences (Physical Sciences)

Upper Cretaceous strata exposed on the eastern side of the San Juan Basin in Rio Arriba County, New Mexico, contain ammonites and other marine molluscan fossils that date from the Turonian, Coniacian and Campanian stages.The stratigraphic units are Greenhorn Limestone, Carlile, Juana Lopez, El Vado Sandstone and Satan members of the Mancos Shale and Lewis Shale.The Mytiloides mytiloides inoceramid zone in the Greenhorn Limestone indicates the ammonite zone of Mammites nodosoides and thus a latest early Turonian age.The Collignoniceras woollgari regulare Subzone of the C. woollgari Zone and the Prionocyclus hyatti Zone are present in the Carlile Member.These zones represent the middle Turonian.The Prionocyclus macombi Zone is present just below the lower calcarenite of the Juana Lopez Member, and the Scaphites warreni and S. ferronensis zones occur in the upper calcarenite of the Juana Lopez.The Coilopoceras colleti Subzone of the P. macombi Zone and the Inoceramus dimidius inoceramid zone are also present in the Juana Lopez.The P. macombi, S. warreni and S. ferronensis zones indicate the lower part of the upper Turonian.The inoceramid Magadiceramus complicatus (Heine) indicates the Scaphites depressus Zone and thus a late Coniacian age for the top of the El Vado Sandstone in the Llaves area.Fossils representative of the Santonian stage have not been collected from the study area.The lowermost Campanian Scaphites leei III Zone is present in the upper Satan Tongue near Gallina.The lower Campanian Scaphites hippocrepis I Zone is present in the uppermost Satan Tongue near Llaves.Ammonites and inoceramids collected from the upper part of the Lewis Shale in the eastern San Juan Basin are middle to late Campanian in age.The ammonite zones include the lowermost middle Campanian Baculites obtusus Zone, the lower middle Campanian B. mclearni, B. asperiformis and Baculites sp.(smooth species) zones, the upper middle Campanian B. perplexus and B. gregoryensis zones, possibly the upper middle Campanian B. reduncus Zone, the uppermost middle Campanian B. scotti Zone, the lowermost upper Campanian Didymoceras nebrascense Zone, the lower upper Campanian Exiteloceras jenneyi and D. cheyennense zones and possibly the middle upper Campanian B. compressus Zone.Twelve ammonite zones were identified from the Lewis Shale, and a total of 21 ammonite zones are present or indicated in Upper Cretaceous strata in the eastern San Juan Basin in Rio Arriba County.We report Placenticeras pseudoplacenta Hyatt from the Prionocyclus hyatti Zone and Baculites yokoyamai Tokunaga and Shimizu from the Scaphites warreni and S. ferronensis zones.P. pseudoplacenta and B. yokoyamai have not previously been reported from Rio Arriba County.From the Satan Tongue, we report the nautiloid Eutrephoceras alcesense Reeside and the ammonites Menabites (Delawarella) vanuxemi (Morton), P. syrtale (Morton), P. placenta (DeKay), Glyptoxoceras, B. aquilaensis Reeside, Scaphites (Scaphites) hippocrepis (DeKay) I Cobban and Haresiceras (Haresiceras) montanaense (Reeside).All, except the latter species, have not previously been reported from the eastern San Juan Basin in Rio Arriba County.