Etude des sédiments meubles de la vasière de la gironde et des régions avoisinantes

Only fifteen or so years ago, the geology and sedimentology of the Gulf of Gascony was not at all well known. Dredging was rare and the only systematic readings of the nature of the sea beds were given by the Hydrographic Service's probes. It is based on this information that DELESSE (1872) made the lithologic map of the French seas, revised and supplemented by THOULET in 1904. In 1955, BERTHOIS published the first summary research. He used dredgings from various campaigns, Pourquoi-Pas?, Quentin-Roosevelt, President Theodore Tissier, and resumed the previous research work of DANGEARD (1928), FURNESTIN (1937) and BERTHOIS (1946). He classified the collected sediments according to the shape of their cumulative curve, their average diameter and created a map of their distribution. In 1959, he supplemented his study through new dredgings and core borings. In 1966, 1967 and especially in 1969, LAPIERRE, thanks to systematic samples extended to the entire Gulf, showed that the sediments of the shelf are not entities characterised by a single method of deposit but are the result of a succession of very different dynamics. Through an original method of studying heavy minerals, he highlighted the direction and source of intakes and established a chronology of the sedimentary stock's build-up. GLEMAREC (1969), in his research work on benthic populations of the Great Mudflat, provides interesting particulars: he points out in particular the presence of rocky massifs buried under a coarse covering and separating the Great Mudflat from the red sands of the edge of the shelf. PINOT (1966), HINSCHBERGER, SAINT-RÉQUIER and TOULEMONT (1967) and VANNEY (1965, 1968) give detailed maps of certain coastal zones or zones located at the edge of the shelf. The research work that we introduce here is within the scope of a detailed mapping of the trawlable sea beds of the Atlantic continental shelf between the parallels of 45°20 and 46°20 N. Three dredging campaigns have been conducted up to the present on board La Pélagia (October 1967, March and October 1968). Figure 1 shows the extent of the area covered. It indicated moreover the names used by fishermen to designate certain grounds: we will be referring to them in the course of our descriptions. This first publication is solely concerned with a more or less rectangular sector that includes the Gironde mudflat: it extends from 45°20' to 46°00' N; to the east, its border follows the coast of the Isle of Oléron, approaches Coubre, then runs parallel to the meridian of 1°13' W; to the west, our study stops at the meridian of 2"33' W between 45°40' and 46"00' N and that of 2°20' W between 45"20' and 45"40' N. Within this perimeter, 290 dredgings were done; 283 of them brought back sediments.