Review of calcareous nannofossil events in the Late Tithonian-Early Berriasian time interval: implications for the definition of the base of the Cretaceous

Calcareous nannofossils have been proved to be a powerful biostratigraphic tool for dating and for interregional correlations of Jurassic to Recent marine sequences. The latest Jurassic-earliest Cretaceous time interval was crucial for calcareous nannoplankton that experienced a major speciation episode, with the appearance of new taxa that persisted through the rest of the Mesozoic. The dramatic change in calcareous nannofloras, becoming rapidly dominated by highly calcified nannoconids across the J/K boundary interval provides the opportunity to achieve high-resolution biostratigraphic data, amplifying the possibility of dating and correlating. We collected new data and critically revised published nannofossil biostratigraphies for the Tithonian-Berriasian interval to evaluate reproducibility and variability of individual biohorizons with the aim of determining primary, secondary and local events. We estimated the age of nannofossil events relative to magnetostratigraphy, calpionellid and ammonite zonations in the interval spanning magnetochrons (CM) CM20 to CM17. Our work intends to contribute to the Berriasian Working Group effort aimed at the improvement of a self-reinforcing stratigraphic matrix useful for the future choice of the GSSP of the base of the Cretaceous. Our database comprises sites from different paleogeographic settings (Europe, America, Atlantic Ocean). We critically evaluated sampling rates, nannofossil preservation and abundance, and taxonomy applied by individual specialists. We privileged calibrations against magnetostratigraphy to highlight reproducibility of single nannofossil events against magnetochrons, and more specifically their time variability within CM19 and CM18. We also evaluated nannofossil events relative to ammonite and calpionellid biostratigraphies. The main results of our critical revision, point out a sequence of several first occurrences (FO) and that some nannofossil events are more reproducible and reliable than others. We confirm the nannofossil potential as a stratigraphic tool for high-resolution dating and long-ranging correlations, a tool able to overcome paleoprovincialism often shown by ammonites. The majority of the events characterizing the J/K boundary interval are the FOs of some Nannoconus species, specifically: N. globulus minor and N. erbae correlating with the topmost part of CM20N and lowermost part of CM19R, respectively. The FOs of N. globulus globulus and N. wintereri correlate with the middle part of CM19N, and the FOs of N. steinmannii minor and N. kamptneri minor correlate with the lowermost part of CM18R. Other events in the J/K boundary interval include the FOs of Rhagodiscus asper, Cretarhabdus surirellus, Cruciellipsis cuvillieri, Hexalithus geometricus, and Cretarhabdus octofenestratus. The appearance and diversification of nannoconids are accompanied by a major increase in their total abundance. Consequently, the J/K boundary interval is also characterized by distinctive changes in “nannofacies” that are reproducible in the Tethys and Atlantic Oceans. Such assemblage changes further corroborate the nannofossil biostratigraphic characterization of the J/K boundary interval and might be used as additional marker levels. Based on new and literature data, we propose the FO of N. steinmannii minor as the most robust and globally recognized event close to the J/K boundary, as documented in all available nannofossil biozonations of the past four decades. This bioevent correlates with the basal portion of CM18R. If ammonite or calpionellid biohorizons are taken to define the base of the Cretaceous, the FOs of N. globulus globulus, H. geometricus, N. wintereri and C. cuvillieri approximate the base of both the Calpionella (B) Zone and ammonite jacobi-grandis Zone in the middle part of CM19N.