New Mexico Geological Society Annual Spring Meeting — Abstracts

The Acanthoceras amphibolum Zone is present in the Paguate Member of the Dakota Formation on the western side of the Rio Puerco northwest of Albuquerque, New Mexico, at NMMNH localities 3263, 4406 -4409, and 4429 -4432. In this area, the Paguate Member is approximately 22 m thick, and invertebrate fossils occur in ripple-laminated and concretionary calcareous sandstones, 15 - 16 m above its base.

A. amphibolum is fairly common in the Paguate Member of the Dakota Formation in the study area. Numerous specimens were collected, including a fairly complete shell with body chamber, several other adult specimens, many partial shells and a few inner whorls. A. amphibolum is characterized by ventrolateral horns on the later whorls and a broadly arched venter with a very low mid-ventral ridge. It is distinguished from Acanthoceras alvaradoense in possessing clavate upper ventrolateral and siphonal tubercles on the early whorls, rather than nodate ones. A lack of constrictions on the early whorls further distinguishes our specimens from A. alvaradoense.

A well preserved bivalve and gastropod fauna is associated with A. amphibolum. This fauna includes Pycnodonte kellumi (Jones), Exogyra trigert (Coquand), Avicula gastrodes? Meek (new record for Paguate), Legumen ellipticum Conrad, Ostrea beloiti Logan, Granocardium enstromi (Bergquist), Idonearca depressa White (new record for Paguate), and Turritella sp. Other than a single specimen of Turrilites acutusamericanus Cobban & Scott, all ammonites collected are A. amphibolum.

The occurrence of the Acanthoceras amphibolum Zone in the Rio Puerco Valley suggests correlation with the Paguate Member of the Dakota Formation northeast of Thoreau, in the Laguna area, and other locations in west-central New Mexico. The Acanthoceras amphibolum Zone is of middle Cenomanian age.