Nouvelles données biostratigraphiques sur les séries du Bathonien et du Callovien des environs de Digne (Alpes de Haute-Provence)

New biostratigraphical data in the Bathonian and Callovian of Digne area (Alpes-de-Haute-Provence) In the type sections of "Clues de Chabrières" (platform series) a detailed biostratigraphical analysis of the Bathonian-Callovian ammonite faunas of the "Marno-calcaires à Cancellophycus", puts in light numerous discontinuities underlined by ferrugineous levels related with gaps in the fauna ; the most important of it ranges from the upper part of Middle Callovian to the Lower part of Middle Oxfordian. From Upper Bathonian to Middle Callovian a decrease of the rate of sedimentation can be observed. In the "Terres noires" faciès of the Digne tectonic belt (basin series) a similar approach shows a more continuous sedimentation during the whole Callovian when the Middle Bathonian is often lacking and the Upper Bathonian uncomplete. The rate of sedimentation is important with a thick serie in the Middle-Upper Callovian and the Lower Oxfordian. The comparison between these two areas shows a quite similar history during the Upper Bathonian, and a more and more marked différenciation from the lowermost part of the Callovian to the Middle Oxfordian. This différenciation is interpreted as the result of syn-sedimentary tectonic effects related with a discriminatory local subsidence which partly hides eustatic events ; a new paleogeogra-phic scheme is proposed to replace the classical platform/basin interpretation.Dans les coupes de référence des Clues de Chabrières (séries de type plate-forme) l'analyse biostratigraphique détaillée des ammonites des "marno-calcaires à Cancellophycus" d'âge Bathonien-Callovien a mis en évidence de nombreuses discontinuités matérialisées par des niveaux ferruginisés associés à des lacunes fauniques ; la plus importante va du sommet du Callovien moyen à la base de l'Oxfordien moyen. Du Bathonien supérieur au Callovien moyen, on constate une diminution progressive du taux de sédimentation. Dans les faciès "Terres noires" de la nappe de Digne (séries de type bassin), une analyse semblable plaide en faveur d'une sédimentation continue durant tout le Callovien, alors que le Bathonien moyen est souvent absent et le Bathonien supérieur incomplet. Le taux de sédimentation est important avec en particulier une épaisse série d'âge callovien moyen-supérieur à oxfordien inférieur. La comparaison de ces deux domaines montre une histoire assez similaire au Bathonien supérieur alors qu'elle est assez différente du Callovien basai à l'Oxfordien moyen. Cette différenciation est inter¬ prétée comme le résultat des effets d'une tectonique synsédimentaire liée à une subsidence différencielle accusée qui masque en partie les événements eustatiques ; une image paléogéographique sensiblement différente de celle couramment admise (plate-forme/bassin) est proposée.Zany D., Atrops François, Marchand D., Thierry Jacques. Nouvelles données biostratigraphiques sur les séries du Bathonien et du Callovien des environs de Digne (Alpes de Haute-Provence). In: Géologie Méditerranéenne. Tome 17, numéro 1, 1990. pp. 39-53

Ammonoid taphonomy, palaeoenvironments and sequence stratigraphy at the Bajocian/Bathonian boundary on the Bas Auran area (Subalpine Basin, south-eastern France)

Bas Auran, in south-eastern France, is the candidate area for Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage (Middle Jurassic). In the Bas Auran area, upper Bajocian and lower Bathonian deposits are made up of limestone beds alternating with marls that correspond to the ‘Marno-calcaires à Cancellophycus’, below the ‘Terres Noires’ Formation. Taphonomic analyses of the successive ammonoid fossil assemblages provide new and complementary data for biostratigraphic completeness, palaeoenvironmental setting and sequence stratigraphy. Lithologic differentiation between limestone and marl intervals resulted from alternating episodes of carbonate input and starvation. Both lithologic phases may contain evidence for sedimentary and taphonomic reworking, associated with scours, that reflects low rates of sedimentation and stratigraphic condensation. Three successive types of elementary cycles resulted from increasing rates of stratigraphic condensation, sedimentary condensation and substrate stabilization during early Bathonian. The occurrence of reelaborated ammonoids (i.e. exhumed and displaced before their final burial) implies that tractive current flows or winnowing affected the burial of concretionary internal moulds. In the lower Bathonian strata, the dominance of homogeneous concretionary internal moulds of phragmocones, completely filled with sediment, is indicative of low rates of sedimentation and sediment accumulation, respectively associated with low degrees of stratigraphic and sedimentary condensation. However, at the Bajocian/ Bathonian transition, hemipelagic, bed-scale limestone–marl alternations show a maximum value of biostratigraphic completeness and there is no evidence for taphonomic condensation in the ammonoid fossil assemblages. Taphonomic analyses of the successive ammonoid fossil assemblages and taphofacies confirm the development of a deepening phase associated with sedimentary starvation, which characterizes the last episode within the deepening half-cycle of third and second order cycles, in the Bas Auran area of French Subalpine Basin during early Bathonian