Ammonites of the Bathonian-Callovian of the Boulonnais: biodiversity, biostratigraphy and biogeography

Les ammonites du Bathonien-Callovien du Boulonnais : biodiversite´, biostratigraphie et bioge´ographie

The ammonite fauna of the outcropping Bathonian-Callovian of the Boulonnais (Northern France) is described in detail for the first time. In the middle and Late Bathonian, the fauna is very restricted, reduced to some Perisphinctidae (Procerites) and Clydoniceratidae (Clydoniceras, Delecticeras). The abundance and the biodiversity of assemblages increase during the Callovian. These faunas are typically Subboreal, marked by Gowericeratinae (Kepplerites, Sigaloceras), Kosmoceratinae (Kosmoceras) and Proplanulitinae (Proplanulites), associated with Boreal taxa such as Cadoceratinae (Cadoceras) and Cardioceratidae (Quenstedtoceras), and Submediterranean taxa, Pseudoperisphinctinae (Homoeoplanulites, Poculisphinctes), Peltoceratinae (Pseudopeltoceras) and Macrocephalitinae (Macrocephalites). Representing the only Jurassic onshore outcrops, between the English and Normandy coasts, and the Ardennes area, the interest of these faunas for correlations between the Western and Eastern European (Russian) platforms is emphasized. Palaeobiogeographically, particularly concerning the southward migration/dispersion of the Boreal taxa, the role played by the transgressions/regressions and the resulting paleobathymetric and palaeoclimatic implications is discussed. [Copyright &y& Elsevier]

La faune d'ammonites des se´ries affleurantes du Bathonien-Callovien du Boulonnais (Nord de la France) est de´crite en de´tail pour la premie`re fois. Au Bathonien moyen et supe´rieur les spe´cimens sont rares, re´duits a` quelques Perisphinctidae (Procerites) et Clydoniceratidae (Clydoniceras, Delecticeras). L'abondance et la diversite´ des associations augmentent au Callovien ou` elles ont un net cachet subbore´al marque´ par la pre´sence des Gowericeratinae (Kepplerites, Sigaloceras), Kosmoceratinae (Kosmoceras) et Proplanulitinae (Proplanulites), associe´s a` des taxons bore´aux, Cadoceratinae (Cadoceras) et Cardioceratidae (Quenstedtoceras) et subme´diterrane´ens, Pseudoperisphinctinae (Homoeoplanulites, Poculisphinctes), Peltoceratinae (Pseudopeltoceras) et Macrocephalitinae (Macrocephalites). Seul jalon entre les affleurements des coˆtes normandes et anglaises d'une part et la bordure ardennaise d'autre part, l'inte´reˆt de ces faunes est de´montre´ pour les corre´lations entre les plates-formes de l'Europe de l'Ouest et celles de l'Europe de l'Est (Plate-forme russe). Du point de vue pale´obioge´ographique et plus particulie`rement concernant la migration/dispersion vers le sud des taxons bore´aux, le jeu des transgressions/re´gressions et les possibles contraintes pale´obathyme´triques et pale´oclimatiques intervenantes sont discute´es. [Copyright 2003 Elsevier]

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