The Late Albian to Early Cenomanian succession at Mont Risou near Rosans (Drôme, SE France): An integrated study (ammonites, inoceramids, planktonic foraminifera, nannofossils, oxygen and carbon isotopes)
Abstract The Second International Symposium on Cretaceous stage boundaries held in Brussels from September 8-14 1995 reached a preliminary agreement on the definition of the base of the Cenomanian stage as the first occurrence of the planktonic foraminiferan Rotalipora globotruncanoides Sigal, with a section at Mont Risou near Rosans, Drome, SE France as the global reference section, Sigal, with a section at Mont Risou near Rosans, Drome, SE France as the global reference section, where the first occurrence of R. globotruncanoides lies 36 m below the top of the Marnes Bleues as defined by a zero datum limestone at the base of the succeeding unit. The succession yields ammonites, inoceramid bivalves, nannofossils and planktonic foraminifera that provide a series of marker levels across the boundary, while an ornate δ 1 3 C curve provides a geochemical marker that can also be recognized in NE England and Italy. Proxies for R. globotruncanoides are: the first appearance of Mantelliceras mantelli and other ammonites of the mantelli Zone at -30 m, the first occurrence of the nannofossil Calculites anfractus , the first occurrence of the planktonic foraminiferan Rotalipora gandolfii and the last occurrence of the planktonic foraminiferan Rotalipora ticinensis at -40 m, and the last occurrence of the nannofossil Staurolithites glabra at -12 m. Ammonite, inoceramid, nannofossil and planktonic foraminiferan occurrences are documented, a systematic account of the ammonites, and full range data and nomenclature of the nannofossils provided. Previous alternative definitions of, and boundary stratotypes proposed for the base of the Cenomanian are critically reviewed and compared with the Risou section.