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 (Drome), located in the Vocontian Ba sin (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 m arked by a high relative frequency of Phylloceratids. Significant modi fications of the composition of ammonite spectra are also observed fro m the base to the top of some parasequence sets. The variations in bat hymetry and trophic drifts linked to the climatic fluctuations may hav e controlled the ammonite assemblages. The increase in bathymetry in m arls was thus probably beneficial to Phylloceratids, which are a deepw ater 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. (C) 1997 Elsevier Scienc e B.V.