PALEOECOLOGY OF BENTHIC ASSOCIATIONS IN SALINITY-CONTROLLED MARGINAL MARINE ENVIRONMENTS - EXAMPLES FROM THE LOWER BATHONIAN (JURASSIC) OF THE CAUSSES (SOUTHERN FRANCE)

Lower Bathonian sediments of the Causses, southern France, largely rep resent very shallow low energy carbonates deposited on an extensive ca rbonate platform. Environments range from distal oolite shoals to shal low lagoons, inter- to supratidal mudflats, brackish bays, coastal swa mps and freshwater ponds. Associated benthic macrofauna occurs often i n high abundance, but low diversity and is dominated by bivalves and g astropods. Stenohaline elements are rare to absent. The fauna can be g rouped in 13 associations, some of them monospecific, and several asse mblages. Some of the associations are clearly substrate-controlled, es pecially by grain size and substrate consistency. However, the main co ntrolling factor appears to have been salinity which fluctuated from z ero (freshwater) to more than 40 parts per thousand (hypersaline). An analysis integrating sedimentological data, microfauna, microflora, an d ecological data of the macrobenthos made it possible to assign the b enthic associations to various salinity regimes. Accordingly, the Nari copsina matheroni association is restricted to freshwater; the Eomiodo n 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 buckman i, Bakevellia waltoni, and Bakevellia waltoni-Protocardia buckmani ass ociations lived in brachyhaline environments. Only two associations, t he Pholadomya lirata and the Ceratomya striata associations, occurred in normal marine water. The opportunistic Myrene ene raristriata appar ently managed to thrive also in hypersaline environments as is shown b y its close association with pseudomorphs after autigenic gypsum and w ith algal layers. The Lower Bathonian macrofauna of southern France re sembles that of other Middle Jurassic salinity-controlled benthic faun as from Scotland, Northern Africa, Western India, and Tibet except for its scarcity of mytilid bivalves and lack of corbulids.