Lower Aptian (Lower Cretaceous) ammonites from the basal strata of the La Peña Formation of Nuevo León State, northeast Mexico: biochronostratigraphic implications

Abstract

Detailed paleontological studies of the basal strata of the La Peña Formation from a stratigraphic section of the State of Nuevo León, 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 Dufrenoyia 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 Peña 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 Peña 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 Peña Formation on a regional scale.