Les ostracodes indicateurs des paléoenvironnements au miocène moyen (serravallien) en aquitaine (sud-ouest de la France)

Abstract

As a part of the study of the Aquitaine Middle Miocene, Serravallian ostracod faunas have been studied from eighty samples, which come from new outcrops located in the northern Aquitaine basin (Gironde), in the gulf of central Aquitaine and in the southern Aquitaine basin : north of the anticlinal of Audignon and Midouze gulf, syncline of Chalosse, gulf of Salies-de-Béarn / Orthez and area of Soustons near the Atlantic coast. About 160 taxa can be listed as a whole. The faunal associations and data provided from lithology and other groups of organisms make it possible to recognize different palaeoenvironments on the continental platform : continental, coastal, coastal with brackish influences, internal and external infralittoral environments. A comparison is made with some deeper synchronous palaeoenvironments from boreholes (Soustons, Frouas) located near the Atlantic coast. The evolution of this fauna compared with the previous periods in the Aquitaine basin is also sketched out, with a few palaeoclimatic data. The Serravallian fauna is less diversified than the Lower Miocene one, but seems a bit richer than the Langhian one. Both Langhian and Serravallian stages have numerous ostracods in common (about 100 taxa); about ten taxa first appear in the Aquitaine Serravallian. During the Serravallian, the thermophile pattern of the association is still noticeable, but less marked than in the Burdigalian. Several data indicate a probable cooling of the littoral waters, at least during the upper sequence of the Serravallian Stage. Strong affinities can be noted with the Mid Miocene ostracofauna from the Tage basin in Portugal.