Quantitative biochronology (unitary associations) of the Cenomanian ammonite faunas from southeastern France

On the basis of the recent data set published by Thomel [1992], the ammonoi d biochronology of the Cenomanian Stage of southeastern France is reevaluat ed. A data set comprising 138 ammonite species from 40 sections is processe d with the unitary association method [Guex, 1977, 1991]. A logical success ion of analyses optimizes these data and stresses the major inconsistencies in the original documentation. It leads to the construction of a sequence of 27 unitary associations which are grouped into 7 laterally reproducible association zones. Application of this revised zonation highlights lateral facies changes even in the distal part of the basin as well as some diachro nism at the species level (inclusive of some index species). It also sugges ts a sequence of progressively dampened pulsations in the rates of faunal t urnovers during the early and middle Cenomanian, until equilibrium was reac hed during the late Cenomanian to the early Turonian, without any apparent change at the Cenomanian-Turonian boundary. This trend was also accompanied by significant changes in species diversity, with an abrupt doubling durin g the late early Cenomanian and a protracted decrease from the late middle Cenomanian onwards. Finally, this zonation is compared to the current synth etic scales of southeastern France and of the Anglo-Paris Basin.