Les faunes de Scléractiniaires dans les faluns du Miocène moyen d'Atlantique-Est (bassins de la Loire et d'Aquitaine) : paléobiogéographie et évolution climatique

Abstract

The Aquitaine and Loire basins show in the Middle Miocene numerous marine littoral deposits, often shelly or crag-type. Scleractinia are present, with a distribution and an abundance very variable according to the outcrops. A detailed study of all the available material, recently or historically cropped, allowed to draw up an accurate faunal inventory and to compare the distribution of taxa between the two basins. Out of 90 taxa in total, 62 are known in Aquitaine and 48 in Loire. In the Aquitaine Langhian, when is noted an obvious reduction of hermatypic taxa in comparison with the rich regional Burdigalian faunas, the association includes 44 species (25 reef-building ones). In the Serravallian, the reef-building taxa become significantly scarce and are residual (9 taxa out of 33 in Aquitaine, present in levels belonging to the lower sequence of the stage, and located in sheltered internal parts of the south-Aquitaine and central-Aquitaine gulfs). In the Loire basin, the corals are quite diversified in the Pontilevian facies (19 reef-building taxa out of 48), and much scarcer in the Lublean and Savignean facies. Globally, the Scleractinia are few abundant everywhere. If in the two basins hermatypic taxa persisted in the Langhian, indicating subreefal-type facies, varied factors have impeded permanently the settlement of reefs. In comparison with the Burdigalian, the thermic deterioration gradient, evidenced since the Chattian on the northeastern Atlantic frontage, had an important influence, and the Langhian waters were only subtropical. Other factors acted, at least locally, as the hydrodynamics, the bathymetry, the kind of substratum, the salinity pro parte. Moreover, a latitudinal gradient between the two basins is evidenced by the global species richness and by the ratio of hermatypicity, created here and defined as the fraction of hermatypic taxa reported to ahermatypic ones from a same basin. This ratio can be used at generic level or specific one as well. In the Serravallian, when the diversity was everywhere obviously lower, the influence of the climatic gradient went on, together with other unfavourable conditions (often abundant detritic supplies, high hydrodynamics, spatial biocompetition). Diverse biogeographic and paleogeographic data are also reported. A vast East-Atlantic coralline bioprovince, settled as early as the Chattian with a dispersion center located in Aquitaine, was still active in the Middle Miocene, when large transgressions favoured the faunal exchanges; its history was to be completed at the end of this period.