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Address : 1Université de Bretagne Occidentale, UFR Sciences et Techniques, France; 2Museum d’Histoire naturelle de Nice, France;3Université de Provence, France; 4Christian-Albrechts-Universitنt, Germany
Celebration date : 29 Monday September 2008
Author name : Granier, Bruno1; Moullade, Michel2; Ropolo, Pierre3; Tronchetti, Guy3; Kuhnt, Wolfgang4

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  • Article title : The Aptian stage: Input from the historical stratotype
    Article type : Paleontology and Stratigraphy
    Location : International Geological Congress,oslo 2008

    Fulltext :

    For several centuries the type locality of a geologic stage was a little like that of the type species in relation to the taxon it characterizes. Because of deficiencies and gaps in some stratotypes, it was proposed for a time that they be replaced by "hypo-" or "para- stratotypes" chosen in more favorable localities. The failure of this attempt then led stratigraphers to focus on the concept of a stratotype for stage boundaries ("GSSP"). So currently their work is concentrated on finding and cataloguing miracle sections, not without a certain competetiveness from which nationalistic patriotism is not always excluded.
    It appears that this approach has heightened the importance of the concept of stratigraphic boundaries, although in this domain, as in many others, nature is rather "gradualistic"; so the original stratotypes are reduced to the rank of historical curiosities, to the detriment of knowledge about the content of the stages.
    Precisely for the Aptian, perhaps exemplary in this respect, the reactivation of our research on a multidisciplinary basis during the last decade shows, however, that certain "old" stratotypes still contain many hidden possibilities. In fact, the successions at Cassis-La Bédoule and at Apt perhaps constitute the only places in the world where most of the Aptian stage is represented in continuous, expanded sedimentary successions without hiatus. These complete successions allowed detailed study of rich macro- and micro- fossil assemblages as well as geochemical analyses (stable isotopes and trace elements). As a whole, these works, nearing completion, yielded the following main results:
    • Barremian-Aptian boundary defined by the FAD of the ammonite genus Deshayesites;
    • homophyletic zonation of the Bedoulian (=Lower Aptian) using the evolution of representatives of the Deshayesitidae;
    • homophyletic zonation of the Gargasian (=middle Aptian) using the evolution of representatives of the Cheloniceratidae;
    • improvement of correlations between the Tethyan and Boreal Aptian zonations;
    • refinement and more precise calibration of micropaleontologic zonations (planktonic foraminifera, calcareous nannofossils) for the Bedoulian and Gargasian;
    • discovery of a triple micropaleontologic datum (benthic and planktonic foraminifera, ostracodes) that is coincident with the Bedoulian-Gargasian boundary as it is defined on the basis of ammonite distribution;
    • the combination of biostratigraphy with cyclostratigraphy of marl-limestone rhythms and isotope stratigraphy allows an estimate of the duration of the Gargasian, which is closer to the time-scale proposed by Fiet et al. (2006): 3 Ma rather than that of Ogg et al. (2004): 6 Ma;
    • defining precisely the position of the global anoxic event AOE1a (پgGoguelپh or "Selli") within the middle part of the upper Bedoulian.

     
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