SANTONIAN AMMONITES AND OTHER MOLLUSCS FROM THE DALTON SANDSTONE MEMBER OF THE CREVASSE CANYON FORMATION NEAR CABEZON, SANDOVAL COUNTY, NEW MEXICO

Ammonites and other molluscs were collected from the Dalton Sandstone Member of the Crevasse Canyon Formation near Cabezon, New Mexico. These fossils come from the lower part of the merged Dalton-Hosta interval, ~ 13 m above the Dalton base, in a calcareous sandstone bed intercalated with carbonaceous shale. The fauna includes the ammonites Placenticeras syrtale, Baculites codyensis, Glyptoxoceras sp., Clioscaphites sp., gastropods including Gyrodes conradi and cf. Rostellites sp., and the bivalves Anomia subquadrata, Ostrea sp., Trigonarca obliqua, a cardiid, Platyceramus cycloides and others. In 1917, W. T. Lee reported on a rather similar fauna near Cabezon. Thus, he listed a molluscan fauna from several hundred feet below the top of the Mancos Shale. However, he did not report Glyptoxoceras or Clioscaphites from this interval. His list included the ammonites Placenticeras sancarlosense, Stantonoceras pseudocostatum and Baculites anceps var. obtusus. P. sancarlosense and S. pseudocostatum have both been synonymized with P. syrtale. B. anceps var. obtusus was synonymized with Baculites obtusus. But, B. obtusus is the lowest middle Campanian Zone taxon, which is too young of an age for the Dalton Sandstone Member.

P. cycloides appears just above the base of the lower Santonian and ranges through the Santonian and possibly higher. P. syrtale has been reported from the upper part of the Mancos Shale in the upper Rio Grande Valley. P. syrtale, B. codyensis and Glyptoxoceras also occur in the Satan Tongue of the Mancos Shale near La Ventana. P. syrtale ranges from the lower Santonian to the lower Campanian. B. codyensis ranges from the middle Coniacian to the middle Santonian. B. codyensis and Glyptoxoceras sp. have also been reported from the upper shale unit of the Smoky Hill Member of the Niobrara Formation in the Raton Basin. The Clioscaphites specimen collected from the Dalton near Cabezon is either C. saxitonianus or C. vermiformis. C. saxitonianus and C. vermiformis are the lowest Santonian and middle Santonian zone taxa, respectively. Also, the upper Coniacian taxa Protexanites bourgeoisianus and Magadiceramus complicatus occur stratigraphically lower in the El Vado Sandstone in the Cabezon area. The biostratigraphic data thus indicate a Santonian age for the Dalton Sandstone near Cabezon.