ORGANIC-MATTER SEDIMENTATION IN THE NORTH EAST OF THE PARIS BASIN - CONSEQUENCES ON THE DEPOSITION OF THE LOWER TOARCIAN BLACK SHALES

A borehole realised by the Agence Nationale pour la Gestion des Dechet s Radioactifs (ANDRA), near the village of Montcornet (Aisne), in the Paris basin, allowed to core about 1 km of Jurassic series. The organi c matter (OM) associated to the drilled series has been characterised by various well-proven methods: Rock-Eval pyrolysis, palynofacies stud y and saturated hydrocarbon analysis. The series received variable but notable amounts of inputs of OM of terrestrial origin during all the Jurassic. These inputs probably originated from the Ardennes Massif, n ear the study area. The conditions of preservation of these inputs, be ing poor during the Early Jurassic, improved markedly at the beginning of the Domerian and then apparently remained rather constant, at leas t up to the Callovian. During three distinct and rather well limited p eriods, important amounts of algae-derived OM added to the background of terrestrial OM. The most important and the best known of these epis odes corresponds to the black shales of the base of the Lower Toarcian , lateral equivalent of the well-known 'Schistes Carton'. These result s suggest that, in the Paris basin, the deposition of OM-rich sediment s occurred during episodes of high planktonic production happening whe n medium conditions were favourable to OM preservation. The quantitati ve and qualitative fluctuations of the OM all along the studied series accompany and describe three of the large transgression-regression cy cles recently evidenced in the Paris Basin (Guillocheau, 1991): Late C arnian - Toarcian, Aalenian-Bathonian and Callovo-oxfordian. One episo de of accumulation of planktonic OM appears to have occurred in each o f these cycles, in the neighbourhood of transition transgression-regre ssion In the present state of knowledge, only the confinement behind s ills, in a structured basin, seems capable of explaining the triggerin g of episodes of planktonic production. In the Paris Basin, these cond itions of confinement appear to have mainly resulted from a notable st ructuration of the platforms and of the adjacent troughs due to: (1) s ubsidence during the tectonic cycles and (2) to distensive tectonic ac tivity during tectono-eustatic cycles.