Age of the Cretaceous Menefee Formation, Gallina Hogback, Rio Arriba County, New Mexico

A BSTRACT . —At the Gallina hogback on the eastern edge of the San Juan Basin, the Menefee Formation is ~180 m thick. It consists of trough crossbedded and thinly laminated sandstone beds, between which are mudstone, coal and coaly mudstone beds. Previous age interpretations of the Menefee have been based on the ammonite biostratigraphy of the underlying and overlying strata. An Ar/Ar age of 78.22±0.26 Ma of an ash bed high in the Menefee section indicates a middle Campanian age, but is not consistent with previously published radioisotopically-calibrated ammonite biostratigraphy. Also, we report a pollen assemblage collected from throughout the Menefee section at that Gallina hogback that indicates an age of late Santonian to early Campanian). We conclude that the the Menefee Formation at the Gallina hogback is best assigned an early Campanian age, about 81 Ma, but that uncertainty remains in assigning a precise age until the discrepancies between ammonite biostratigraphy, palynostratigraphy and radioisotopic dating are fully resolved.