Mass mortality or exceptional fossilization ? The case of the early and middle Toarcian fossiliferous beds from the Digne-Les-Bains area (southeastern France)

The latest Domerian to late Toarcian sedimentary series (from −190 Ma up to −180 Ma) from the « Réserve Géologique de Haute-Provence » (southeastern France) yields two kinds of remarkable fossiliferous beds. The greatest interest of the early Toarcian type is the occurrence of ichtyosaur remains (at least in six sites) among many other fossils such as ammonites, belemnites, bivalves, wood. The middle Toarcian type is specifically rich in ammonites and nautiluses. Litho- and biostratigraphical, palaeontological, sedimentological and geochemical analyses allow us to determine whether these fossiliferous beds are the results of mass mortalities, linked or not to biological crisis, or of exceptional fossilizations of organisms after normal mortality. The early Toarcian accumulations of fossils have been accurately dated from the middle part of the Serpentinum zone, Exaratum sub-zone pro-parte and Falciferum sub-zone pro-parte, strangewaysi and falciferum horizons. Thus, they were not connected to the so called Toarcian biological crisis which occurred previously (during the Semicelatum sub-zone). Organisms likely died according to a normal mortality rate during a time span that lasted around 700 to 800 k.y. All organisms appear to have been removed from their life environments and buried within siliciclastic sediments. Organisms and sediments were trapped owing to the creation of hemigrabens in an extensional tectonic regime, as evidenced by synsedimentary normal faults. Burying and good preservation, i. e. exceptional fossilization, were favoured by the hypoxic or anoxic conditions which prevailed at the sea floor, specifically during these times. High organic carbon content (up to 2.49 per cent) in the fossiliferous silty-quartzose marls proves hypoxia if not anoxia. All these facts were linked to a deepening and transgressive systems tract which succeeded a relative sea-level lowstand at the Pliensbachian-Toarcian boundary. The marine transgression probably reworked and shifted basinwards the siliciclastic sediments and wood remains which formed previously on the exposed land. Thus, continental remains were mixed with the marine fossils. Small scale hummocky cross stratifications in the terrigenous deposits show episodic occurrence of high hydrodynamics events. Such events, likely storms and associated currents, may have provoked the accumulations of dead marine organisms. The middle Toarcian accumulations of cephalopods are dated from the upper part of t