Quantitative variations in the Valanginian ammonite fauna of the Vocontian Basin (southeastern France) within limestone-marl cycles and within parasequence sets

The ammonite assemblages between beds and interbeds are examined. This quantitative analysis was carried out on limestone-marl alternations in the Valanginian Vergol section (Drôme), located in the Vocontian Basin (southeast France). There is a strong faunal contrast between beds and interbeds. The calcareous beds are characterized by high relative abundances of Neocomitids and Olcostephanids, whereas the marls are marked by a high relative frequency of Phylloceratids. Significant modifications of the composition of ammonite spectra are also observed from the base to the top of some parasequence sets. riations in bathymetry and trophic drifts linked to the climatic fluctuations may have controlled the ammonite assemblages. The increase in bathymetry in marls was thus probably beneficial to Phylloceratids, which are a deep-water group. The high planktonic production in limestones could explain the great relative abundance of Neocomitids and Olcostephanids, which lived in the upper part of the water column.