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

Abstract

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.