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Le Lagerstätte de La Voulte-sur-Rhône (France, Callovien) : paléoenvironnement, biodiversité et taphonomie

Abstract : The La Voulte Lagerstätte (Ardèche, France) is characterized by a diverse and exceptionally well-preserved fauna that dates back to the Lower Callovian (Middle Jurassic, ca 160 Myr). It is located along the western margin of the South East Basin that belongs to the Tethyan Ocean. During the Jurassic, this margin was characterized by a complex submarine palaeotopography of tilted blocks generated by a series of faults inherited from the Hercynian tectonics. The submarine relief was steep and corresponded to a major break in the marine bottoms. The sedimentation was dominated by marls and calcareous mudstones suggesting a low energy and deep-water depositional environment. These deposits may be affected by local sedimentary slides (slumps) originating from the fault-controlled escarpment and thus confirming the strong tectonic control. These sediments are rich in fossils whose composition and quality of preservation differ in the two main localities studied here: the Chénier Ravine and the Mines Ravine where the La Voulte Lagerstätte crops out. The fossils from the Chénier Ravine consist exclusively of biomineralized skeletons that occur in alternations of marls and marly limestones. The fauna is dominated by siliceous sponges (Hexactinellids: 11 species, Lithistids: 6 species) and stalked crinoids (Cyrtocrinids: 7 species). Palaeoecological comparisons with Recent representatives of the two groups indicate a deep-water palaeoenvironmental setting probably located in the bathyal zone. The fauna from the La Voulte Lagerstätte contains both numerous sideritic nodules with frequently 3D-preserved crustaceans and marly horizons rich in more or less flattened soft- bodied fossils. The updated faunal inventory recognizes about 60 species distributed within arthropods, coleoid cephalopods, marine worms, echinoderms, vertebrates, bivalves and brachiopods. The major components of the fauna are the arthropods (50% of the species), the coleoid cephalopods (10%) and the echinoderms (10%). Among the arthropods, four groups dominate in numerical abundance: the thylacocephalans (33% of the specimens), and the Solenoceridae (23%), the Coleiidae (16%) and the Penaeidae (9%) crustaceans. The composition and some anatomical features (e.g. bioluminescent organs, hypertrophy of eyes) of these arthropods suggest a relatively deep-water environment characterized by dim-light conditions. The coleoid cephalopods (cirrate octopods, vampyromorph squids, giant teuthoid squids) and other organisms described here for the first time (sea spiders, sea stars), have Recent analogues that all live in deep water niches always exceeding 200 m. The fauna as a whole bears the characteristic features (composition, adaptive features) of the present-day bathyal fauna. The La Voulte Lagerstätte may be one of the rare Jurassic Lagerstätten, if not the unique, to have fossilized a deep marine fauna. It brings key-information and original data on the biodiversity of an assumed bathyal environment during the Middle Jurassic. The presence of both thick iron deposits and unusual sedimentary minerals (e.g. galena, sphalerite, barite, celestite) may indicate hydrothermal processes (vents, cold seeps?), in possible relation with the activity of the La Voulte fault. The fauna from the Lagerstätte, by its composition, preservation and remarkable in situ concentration of benthic organisms shows some resemblances with Recent faunal communities living in the vicinity of hydrothermal vents. The main difference with these present-day hydrothermal communities is the absence of characteristic sessile benthos (ex: giant bivalves, tubeworms) but the originality and the remarkable abundance of the crustaceans reinforce the hydrothermal hypothesis. The La Voulte Lagerstätte, exceptional on several accounts, might be considered as a very particular deep-sea oasis possibly linked to marine hydrothermal site.
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Sylvain Charbonnier. Le Lagerstätte de La Voulte-sur-Rhône (France, Callovien) : paléoenvironnement, biodiversité et taphonomie. Stratigraphie. Université Claude Bernard Lyon 1, 2007. Français. ⟨tel-01891013⟩

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