NEW CRITERIA FOR DEFINING THE BASE OF THE ALBIAN STAGE

The most recent proposal for establishment of a Global Boundary Stratotype Section and Point (GSSP) for the base of the Albian recommended candidate sections in the Vocontian Basin (southeast France) and suggested the first appearance of the ammonite Leymeriella tardefurcata at the base of the Niveau Paquier of the Marnes Bleues sequence as the primary defining criterion. This proposal was not accepted by the Subcommission on Cretaceous Stratigraphy because no corresponding microfossil or chemostratigraphic event was found associated with the L. tardefurcata datum and the geographic distribution of L. tardefurcata was considered to be too limited. Recent study of exceptionally well preserved planktonic foraminifera across Aptian/Albian boundary intervals (i.e., above the appearance of elliptical forms of the coccolithophore Prediscosphaera columnata) at several DSDP and ODP deep-sea sections in the Atlantic and Indian Ocean resolved major taxonomic and biostratigraphic discrepancies and documented dramatic changes in shell structure, ornamentation, and size at the level of a major species turnover. Detailed quantitative study of planktic foraminiferal assemblages from the Col de Pré-Guittard section, located in the Vocontian Basin, documents this same foraminiferal turnover at the Niveau Kilian organic rich bed. A major negative carbon isotope shift has also been documented at this level in the Pré-Guittard section and at several deep-sea sites in the North and South Atlantic Ocean. We propose that the base of the Albian Stage be placed at Pré-Guittard using globally identifiable criteria defined by the extinction of the planktonic foraminifer Paraticinella eubejaouaensis, which corresponds with a major negative carbon isotope excursion and is just below the first appearance of the planktonic foraminifer Microhedbergella minglobularis. The P. eubejaouaensis extinction occurs within the Hypocanthoplites jacobi Ammonite Zone, is about 42 m below the base of the L. tardefurcata Ammonite Zone, and is within the evolutionary transition from elliptical to circular forms of the nannofossil Prediscosphaera columnata.