TOWARDS A STANDARD AMMONITE ZONATION FOR THE APTIAN (LOWER CRETACEOUS) OF NORTHERN MEXICO

Several ammonite biozonations for the Aptian series of northern Mexico have been established during the last decades (Cantu, 1963; Young, 1969; Contreras-Montero, 1977). However, remained the uncertainty concerning the exact correlation between these Mexican Aptian biozonations and those established for different areas of the Tethyan realm. One of the main problems for broad regional correlations stemmed from apparent taxonomic provinciality during this stage. Furthermore, these biozonal schemes have been helpful for general stratigraphic use, and served worthy purposes, but difficulties arise because their biozones did not include well defined biostratigraphic horizons (FAD´s and LAD´s) of the nominal taxa.

The present work improves the existing ammonite biozonation for the Aptian of northern Mexico by considering constraints associated with a discontinuous spatial and temporal record of the different taxa. Detailed stratigraphic data on about thirty index taxa identified in sections from Durango and Nuevo Leon States in northern Mexico, allow further refinement of biochronological schemes presented by previous authors.

Four ammonite biozones are herein proposed: Dufrenoyia justinae Taxon-Range-Zone for the uppermost lower Aptian, Burckhardtites nazasensis/Rhytidoplites robertsi Interval Zone for the middle Aptian, and Cheloniceras inconstans Taxon-Range-Zone and Hypacanthoplites leanzae Taxon-Range-Zone, both for the upper Aptian.

This new biozonation is intended for regional use in northern Mexico because of recognized endemism of the characteristic taxa of the western Tethyan realm. In addition, because of the intermittent character of the stratigraphic occurrence of the ammonite species in the sections studied, the FAD´s and LAD´s of the taxa are here considered strictly as stratigraphic events rather than evolutionary phenomena. Thus, the absolute boundaries of the proposed biozones may be closely approximate, but they may differ from one area to another.

Further work is intended to obtain a standard ammonite biozonal scheme that can be correlated with ammonite zonations elsewhere.