Evidence for three successive upper Barremian–lower Aptian rudist faunas in the Urgonian-type deposits of southeastern France and their stratigraphic value

Abstract

The stratigraphic reappraisal of the Urgonian-type carbonate platform series of southeastern France, based on new ammonite discoveries, supports a precise dating of the Brouzet-les-Alès, Orgon and Rustrel upper Barremian–lower Aptian rudist faunas. The Brouzet-les-Alès fauna consists of an assemblage dominated by Homopleura and Agriopleura species spanning the Toxancyloceras vandenheckii Zone and the two lower subzones of the Gerhardtia sartousiana Zone (Gerhardtia sartousiana and Gerhardtia provincialis subzones). The Orgon fauna is characterised by an impoverished requieniid–monopleurid assemblage (missing Agriopleura) restricted to the upper part of the Gerhardtia sartousiana Zone (= Hemihoplites feraudianus Subzone). The Rustrel fauna records the outbreak of caprinid rudists, ascribed to the Martelites sarasini Subzone of the Martelites sarasini Zone, though some its key taxa locally extend into the lower Aptian. The definition of these rudist faunas is based on the combined First Occurrence (FO) or Last Occurrence (LO) of cardinal species and the co-occurrence of subordinate taxa. Two main faunal changes are recognized, viz. the regional disappearance of Agriopleura (collapse of the Bouzet-lès-Alès fauna) and the beginning of the family Caprinidae (Rustrel fauna). Both faunal changes have a widespread records along the western European Tethys margins and coincide with large-scale environmental perturbations.