New Mexico Geological Society Annual Spring Meeting — Abstracts

At CebolIita Mesa southeast of Grants in Cibola County (ToN, RlOW), the D-Cross Member of the Mancos Shale is ~52 m thick and is underlain by the Tres Hermanos Formation and overlain by the Gallup Sandstone. The ammonite fauna includes Scaphites ferronensis Cobban, Prionocyclus novimexicanus (Marcou), and Coilopoceras infiatum Cobban and Hook.

S. ferronensis is locally abundant in the D-Cross Member of the Mancos Shale at Cebollita Mesa. Specimens are clustered in small concretions low in the section, ~8 m above the base of the D-Cross.

P. novimexicanus is abundant in the D-Cross Member at Cebollita Mesa and occurs throughout most of the D-Cross Member's thickness, from its base to up to ~9 m below the D-Cross-Gallup contact. Although two inner whorls were collected, most specimens are adult whorls in concretions and are delicate and difficult to extract. P. novimexicanus is characterized by a conspicuous, finely serrate keel, a compressed whorl section (higher than wide) that is subquadrangular, sinuously ribbed, and possesses a single row of ventrolateral tubercles.

C. infiatum is restricted to the lowermost 5-6 m of the D-Cross Member. The adult robust specimens are characterized by a distinctive inflated appearance with a prominently ribbed shell possessing radial swellings. All but one specimen collected at Cebollita Mesa are the robust form of C. infiatum.

The occurrence of the Scaphites ferronensis Zone at Cebollita Mesa indicates correlation of the lower part of the D-Cross there with the upper part of the Juana Lopez Member of the Mancos Shale at the type section and the lowest part of the D-Cross Member of the Mancos Shale at Puertecito. Scaphites whitfieldi Cobban has not been recovered from the D-Cross at Cebollita Mesa, but the middle-upper part of the member there should be in that zone.