Sedimentation de la matiere organique dans le nordest du Bassin de Paris: consequences sur le depot des argilites carbonees du Toarcien inferieur (Organic matter sedimentation in the northeast of the Paris Basin: consequences on the deposition of the lower toarcian black shales)

• Published in: Chemical geology 1996-09, Vol.131 (1-4), p.15-35
• Main Authors: Disnar, J R, Le Strat, P, Farjanel, G, Fikri, A
• Format: Article
• Language: English
• ISSN: 0009-2541
• DOI: 10.1016/0009-2541(96)00021-6

A borehole realised by the Agence Nationale pour la Gestion des Dichets Radioactifs (ANDRA), near the village of Montcomet (Aisne), in the Paris basin, allowed to core about 1 km of Jurassic series. The organic matter (OM) associated to the drilled series has been characterised by various well-proven methods: Rock-Eval pyrolysis, palynofacies study 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, near the study area. The conditions of preservation of these inputs, being poor during the Early Jurassic, improved markedly at the beginning of the Domerian and then apparently remained rather constant, at least up to the Callovian. During three distinct and rather well limited periods, important amounts of algae-derived OM added to the background of terrestrial OM. The most important and the best known of these episodes corresponds to the black shales of the base of the Lower Toarcian, lateral equivalent of the well-known 'Schistes Carton'. These results suggest that, in the Paris basin, the deposition of OM-rich sediments occurred during episodes of high planktonic production happening when medium conditions were favourable to OM preservation. The quantitative and qualitative fluctuations of the OM all along the studied series accompany and describe three of the large transgression-regression cycles recently evidenced in the Paris Basin (Guillocheau, 1991): Late Carnian -- Toarcian, Aalenian-Bathonian and Callovo-oxfordian. One episode of accumulation of planktonic OM appears to have occurred in each of these cycles, in the neighbourhood of transition transgression-regression. In the present state of knowledge, only the confinement behind sills, in a structured basin, seems capable of explaining the triggering of episodes of planktonic production. In the Paris Basin, these conditions of confinement appear to have mainly resulted from a notable structuration of the platforms and of the adjacent troughs due to: (1) subsidence during the tectonic cycles and (2) to distensive tectonic activity during tectono-eustatic cycles.