The base of the Cenomanian Stage is defined at 36 meters below the top of the Marnes Bleues Formation on the south side of Mont Risou, east of Rosans, Haute-Alpes, France. This level coincides with the first appearance of the planktonic foraminifer Rotalipora globotruncanoides Sigal, 1948, and can be placed in the context of a series of secondary marker levels based on nannofossils, planktonic foraminifera, ammonites and an ornate δ13C curve.
The GSSP of the Cenomanian Stage is located on the western flanks of Mont Risou (1183m), in NE-SW trending gullies in badlands, 3.15 km east of the center of the town Rosans, Haute-Alpes, France, around a point 5° 30′ 43″E; 44° 23′ 33″N (Lambert II Zone coordinates 852.725; 1937.625) , on the French Série Bleue 1:25,000 Sheet 3239 Ouest, Rosans (Gale et al. 1996).
The boundary lies 36m below the top of a thick sequence of constantly eroding marls, the Marnes Bleues of French workers, but can be located in the field in relation to the first limestone that defines the base of the overlying, unnamed unit of limestone-marl alternations.
The GSSP succession in the top 136m of the Marnes Bleues Formation is of marls with varying carbonate and organic carbon content. Levels with higher carbonate content weather out as slightly more resistant levels. Levels of higher organic carbon content are frequently laminated, and are again slightly more resisting to weathering. One such level is the Niveau Breistroffer at 124-135m below the first limestone that defines the base of the overlying, unnamed unit of limestone-marl alternations.
The base of the Cenomanian corresponds to the first occurrence of the planktonic foraminifer Rotalipora globotruncanoides.
Four foraminfer species, Rotalipora appenninica, Rotalipora gandolfii (FO at -40m), Rotalipora globotruncanoides (FO at -36m), and Rotalipora tehamensis (FO at -48m) coexist from level -36m to -19m and form a subzonal association that is a useful indicator for the base of the Cenomanian.
The major faunal change in the ammonite fauna occurs between -30 and -32m, with the disappearance of typical Albian taxa at -32m (Lechites gaudini, Stoliczkaia clavigera) and the appearance of typical Cenomanian taxa at -30m (Neostlingoceras oberlini, Mantelliceras mantelli, etc.).
First occurrence of Calculites anfractus at -40m.
The GSSP preserves an excellent δ13C and δ18O record that provides an auxiliary marker.
Planktonic foraminifer FAD Rotalipora globotruncanoides.
Spline fit of Ar-Ar ages and ammonite zones, plus monitor standard correction. Then cycle stratigraphy to place foraminifer datum relative to ammonite zonation.
Kennedy, W. J., Gale, A. S., Lees, J. A., and Caron, M., 2004. The Global Boundary Stratotype Section and Point (GSSP) for the base of the Cenomanian Stage, Mont Risou, Hautes-Alpes, France. Episodes 27/1, p. 21-32.