Ammonite biostratigraphy of two stratigraphic sections of the La Pena Formation (Aptian, Lower Cretaceous) in Nuevo Leon State, northeast Mexico

The ammonite record of two stratigraphic sections, La Boca Canyon and La Huasteca Canyon, of the La Pena Formation in Nuevo Leon State, northeast Mexico, was studied in this work. In Mexico, ammonite provincialism during the Aptian prevents the use of European zonations. We followed previous works where we proposed the critical need to define an independent ammonite zonation for the Central Atlantic Province. The current study constitutes an advance in the development of an Aptian ammonite zonation for Mexico. The methodology employed is the usual in this kind of contribution, but two additional methodologies must be highlighted, a semi-quantitative biostratigraphy and the use of 3D models in showing the more relevant specimens. These methodologies are not new, but are uncommon in the contributions of the majority of the Lower Cretaceous ammonite workers, and can be an interesting example to follow. Three ammonite zones and one subzone were recognized: Dufrenoyia justinae interval Zone, Caseyella aguilerae interval Zone, Kazanskyella minima interval Zone and Dufrenoyia scotti/ Burckhardtites nazasensis concurrent range Subzone. In addition, taxonomic contributions in some species of the genera Cheloniceras, Epicheloniceras and Kazanskyella have been made. The lower/upper Aptian transition in Mexico and Europe is discussed. Several options of study have been proposed in order to better characterize the lower/upper Aptian transition in Europe and its correlation with Mexico. This work aims at stabilizing the Mexican zonation and its applicability/reproducibility for the whole Central Atlantic Province. (C) 2021 Elsevier Ltd. All rights reserved.