Episodes de sedimentation riche en matiere organique dans les marnes bleues d'age aptien et albien de la partie pelagique du bassin vocontien

Abstract

A synthetic section is proposed for the Aptian-Albian blue marls of the Vocontian basin (SE France), in the western part of the Alpine Tethys sea. In spite of its homogeneous aspect, the section presents a cyclic sedimentation throughout. Rock-Eval pyrolysis data giving total organic carbon and type of organic matter, allows us to sketch the vertical distribution of organic matter. Three major episodes of orgamic rich sedimentation, underlined by some scarce calcareo-phosphatic nodule girdles, are pointed out: (1) Bedoulian (pro parte) at the very base of the formation, (2) Lower and Middle Albian, and (3) Vraconian (lower part). The good preservation of marine organic materials in these sediments is a consequence of anoxia generated by a strong stratification of the water mass. These episodes are not strictly confined to the Vocontian basin, bur are also known in pelagic environments of the Alpine sea and Tethyan domains. They are clearly linked to transgressive pulses. The mechanism that may be involved spilled over into basins of slightly salted waters generated on vast continental platforms like the model proposed by Busson [1984] for genesis of mid-Cretaceous black shales of the northern and central Atlantic.