Palaeoecology of benthic associations in salinity-controlled marginal marine environments: Examples from the Lower Bathonian (Jurassic) of the Causses (southern France)

Abstract

Lower Bathonian sediments of the Causses, southern France, largely represent very shallow low energy carbonates deposited on an extensive carbonate platform. Environments range from distal oolite shoals to shallow lagoons, interto supratidal mudflats, brackish bays, coastal swamps and freshwater ponds. Associated benthic macrofauna occurs often in high abundance, but low diversity and is dominated by bivalves and gastropods. Stenohaline elements are rare to absent. The fauna can be grouped in 13 associations, some of them monospecific, and several assemblages. Some of the associations are clearly substrate-controlled, especially by grain size and substrate consistency. However, the main controlling factor appears to have been salinity which fluctuated from zero (freshwater) to more than 40‰ (hypersaline). An analysis integrating sedimentological data, microfauna, microflora, and ecological data of the macrobenthos made it possible to assign the benthic associations to various salinity regimes. Accordingly, the Naricopsina matheroni association is restricted to freshwater; the Eomiodon angulatus, Myrene raristriata, Neomiodon ruthenensis, and Neomiodon ruthenensis-Placunopsis socialis associations occupied the oligohaline-mesohaline regimes; the Placunopsis socialis association ranged from the upper mesohaline to the lower brachyhaline regime; the Protocardia stricklandi, Protocardia buckmani-“Ostrea” sp., Protocardia buckmani, Bakevellia waltoni, and Bakevellia waltoni-Protocardia buckmani associations lived in brachyhaline environments. Only two associations, the Pholadomya lirata and the Ceratomya striata associations, occurred in normal marine water. The opportunistic Myrene raristriata apparently managed to thrive also in hypersaline environments as is shown by its close association with pseudomorphs after autigenic gypsum and with algal layers. The Lower Bathonian macrofauna of southern France resembles that of other Middle Jurassic salinity-controlled benthic faunas from Scotland, Northern Africa, Western India, and Tibet except for its scarcity of mytilid bivalves and lack of corbulids.