Ecologie des pseudobiohermes des Terres Noires jurassiques liés à paléo-sources sous-marines. Le site oxfordien de Beauvoisin (Drôme, Bassin du Sud-Est, France)

Abstract

Pseudobioherms are fossiliferous calcareous bodies well exposed within Jurassic calcareous shales of SE France. Their genesis is unknown. Oxfordian psedobioherms of the outcrop of Beauvoisin are studied here. Several kinds of results show that they are probably built at vent of submarine springs in a bathyal environment. Among their related fauna, bivalves are thriving. They show some remarkable analogies (morphology, growth, distribution) with some typical bivalves of recent springs (hydrothermal vents or cold seeps) like Calyptogena. Isotopic composition of their shell exhibits a highly positive δ 13C and emphasizes this resemblance. This suggests a chemosynthetic food supply through a bacterial symbiosis. The whole fauna (microfauna and macrofauna) was inventoried within pseudobioherms and their close and far environment. A comparative paleoecological study has been carried out to evaluate the faunal anomaly shown by a selected pseudobioherm: two contemporaneous sections, one close to this pseudobioherm and one other in a distant area without pseudobioherms, have been studied in detail. All taphonomic disturbances, which are able to alter data, have been carefully studied. The paleobiocoenose suggests a slightly higher environmental stability, because of an additional food supply within an environment usually less favorable. It also suggests some permanency of spring activity. The impact of the supposed springs on the benthic fauna is localized and slightly preserved through fossil fauna. It mainly expresses itself in bivalves and organisms able to eat them (crustacean, cephalopods…). Considering these various observations, and after a critical comparison with characteristics and geological setting of recent submarine springs, a model is suggested to understand the highly original paleo-ecosystem of pseudobioherms.