Sur les Foraminifères benthiques de la base du Burdigalien dans les « Sables à Mactres » de Martillac (Gironde, SO France)

Northern Aquitaine exhibits highly fossiliferous type deposits of the Lower Burdigalian. They mainly consist of shallow crags of the Léognan and Saucats area. The levels of the earliest basis of the stage, little studied till now, are described in a new section, situated in the vicinity of the stratotype at Martillac (Gironde). They are fine blue-grey carbonated sands with rather abundant Bivalves, called “sables à Mactres”. Foraminifera are well represented (more than a hundred species) and generally in a good state of preservation. The whitish silty and sandy upper level yields a somewhat impoverished foraminiferal fauna. All taxa indicate a shallow infralittoral to near-shore environment, rather a quiet one with a normal salinity; porcelaneous and smaller hyaline foraminifera living in a silty and muddy substrate are common. Many taxa are thermophil; the existence of a tropical climate is confirmed by the associated macrofauna. The malacofauna evokes a sheltered bay submitted to tides (cf. Lesport and Cahuzac, 2005). From the biostratigraphic point of view, only the species Miogypsina globulina is present amongst the Miogypsinidae, it shows an early evolutionary stage and only sometimes exhibits a rudimentary second nepionic spiral. Therefore, the attribution to the Earliest Burdigalian is strengthened. The distribution of a few other species is precised; for example, Bolivina pseudoplicata (fide Hasegawa et al., 1990), already present in the Upper Burdigalian and in the Langhian of Aquitaine, is so observed in the lowermost Burdigalian stage, Elphidiella subnodosa is still existing in this level and the abundance of Neorotalia with supplementary equatorial chambers can be noticed, usually more frequent in the Chattian. So, these deposits corresponding to the beginning of the transgression in the type area are better characterized. These new data will allow to make comparisons with other, neighbouring formations of the same age.