L'évolution des peuplements d'ammonites au cours de l'Oxfordien inférieur (Zone à Mariae et Zone à Cordatum) du Jura (Est de la France) [The evolution of ammonite associations during the Early Oxfordian (Mariae and Cordatum zones) in the Jura area (eastern France)]

The study of more than 40 sections in the "Creniceras renggeri marls" of the French Jura Range (Lower Oxfordian) has found: * a precise biochronostratigraphic subdivison of 16 successive populations or associations which appears to be stable over the whole geographic area concerned; * the correlation of these populations with the Lower Oxfordian zones of the GFéJ (French Research Jurassic Group) and with those of southern England (Weymouth) and Poland (Cracow area); * an autecologic and synecologic analysis of Oxfordian ammonite associations in the Jura Mountains; * an interpretation of these faunas in terms of palaeo-depth and sequence stratigraphy units.The main results of the study are the following: * identification of the Costicardia Subzone; * validation of the subdivision of the Mariae Zone established by Fortwengler and Marchand (1994), with a woodhamense horizon above a scarburgense horizon; * division of the woodhamense horizon into two units: the lower one is the woodhamense s.s. unit and the upper one includes abundant specimens of woodhamense var. normandiana Spath; * correlation of tectonic units on either side of the Salins fault and of some characteristics of the ammonite populations; * identification of five steps of changes in depth in the Jura area which may be interpreted as sequence stratigraphic units: a first episode of deepening during the period bounded by the scarburgense and praemartini horizons, an episode of stability bounded by the beginning and the end of the alphacordatum horizon, a slight regressive trend that lasted throughout the duration of the praecordatum horizon, a second episode of stability that persisted from the Bukowskii Subzone to the Costicardia Subzone, and a third deepening trend during the existence of the Cordatum Subzone.