Mass mortality or exceptional fossilization ? The case of the early and middle Toarcian fossiliferous beds from the Digne-Les-Bains area (southeastern France)
Authors: Floquet Marc , Cecca Fabrizio , Mestre Matthieu , Macchioni Francesco , Guiomar Myette , Baudin François , Durlet Christophe , Alméras Yves 
Published: June 2005

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 the Bifrons zone, Bifrons sub-zone pro-parte, lusitanicum and bifrons horizons. They are contained within 3 to 4 calcareous beds, 60 up to 85 centimetres thick in the whole. Since the corresponding deposition time span lasted around 850 k.y. up to 1 m.y., the sedimentation rate was very weak : about 1 centimetre or less each 1 k.y. Subsequently, these accumulations are regarded firstly as faunal condensations. The weak sedimentation rate is considered as linked to a major deepening and flooding event which led to sedimentary starvation in the involved Vocontian basin. Moreover, this phenomenon was probably favoured by a well known crisis of the carbonate production at that time. High values of manganese at the base of these condensed limestones are useful in correlating lithostratigraphic units of the same age in the entire basin. Previous studies gave rise to similar interpretations of these units, i. e. records of major flooding and wide connection to the oceanic domain. In addition, the taphonomic study indicates that many fossils were reworked due to episodes of high hydrodynamism, as indicated by erosion of internal molds of ammonites, and in addition to bioturbation. So, the accumulations may come also from faunal concentrations.

Finally, this study shows that the inventory of the local and general conditions that govern fossilization must be done before interpreting all exceptional and widespread fossiliferous beds in terms of mass mortality or extinction.