Lower aptian (Lower Cretaceous) ammonites from the basal strata of the La Pena Formation of Nuevo Leon state, northeast Mexico: biochronostratigraphic implications

Detailed paleontological studies of the basal strata of the La Pena Formation from a stratigraphic section of the State of Nuevo Leon, northeastern Mexico, reveal a rich ammonite assemblage representative of the local biozone indicative of the uppermost level of the early Aptian corresponding to the Dufrenoyia justinae Taxon Range Zone. The Lower Aptian assignment of this Mexican biozone is corroborated by its distinctive ammonite assemblage characteristic of both uppermost early Aptian standard ammonite zones of the stage; namely the Dufrenovia furcata Zone, of the tripartite Mediterranean standard scheme, and its equivalent Tropaeum bowerbankii Zone of the northeastern Europe bipartite scheme. Thus, in Mexico, the shift of deposition from a shallow carbonate platform locally represented by facies of the Cupido Formation, to the deeper facies of the La Pena Formation took place sometime during the late early Aptian and not in the late Aptian as previously assigned. To avoid diachronic interpretations of the Cupido-La Pena formational contact, the ammonites analyzed in this study for biochronostratigraphic purposes, were compared to other coeval assemblages reported from the basal strata of the La Pena Formation on a regional scale.