Ostracodes de la limite frasnien-famennien en environnement anoxique (La Serre, Montagne Noire, France)

Abstract

At La Serre, the trench C exposes, close to the Frasnian-Famennian boundary, dark and bituminous limestones and shales related to a hemipelagic sedimentation in dysoxic or anoxic bottom water conditions (on the whole). The organic matter produced probably by microalgae, have subsisted in an euxinic trough of the outer shelf. From bed to bed, the carbonate content fluctuates between 7 and 96 per cent and the total organic carbon (COT) between 0.1 and 4.1 per cent. The thermal maturation stage appears to be high. Twelve meters of rock have been sampled in the late Frasnian and the early Famennian of the La Serre trench C and 34 ostracod species have been recognized, the majority of which are figured herein. Paraparchitacea and Myodocopina (Entomozoacea and Cypridinacea) characterize this ostracod fauna and the specific diversity is relatively low. The presence of Bairdiacea in one bed is explained by temporary better oxygenated bottom water conditions. The ostracod assemblages confirm a generally low or nil palaeo-oxygenation and show that the % COT of samples is not always related to this palaeo-oxygenation. Entomozoids thrive in dysoxic waters, but they leave when anoxia takes place. The most resisting benthic ostracods belong principally to the Paraparchitacea. The recovery from the Frasnian-Famennian event was delayed by the anoxia in the lowermost part of the Famennian at La Serre. Then another ostracod assemblage, characterized by some platycopids, returns. We compare these results with those of contemporaneous better oxygenated environments of Coumiac, 25 km apart. It allows to emphasize the contrast between these ostracod faunas.