Morpho-chronology of coastal dunes in Médoc. A new interpretation of Holocene dunes in Southwestern France

Abstract

Previous work on the coastal dunes in the Médoc region was carried out by the BRGM [Marionnaud, J.M., 1972. Carte géologique de la France (1/50000), Feuille St-Vivien-de-Médoc-Soulac-sur-Mer (729–730), Orléans: BRGM Notice explicative par J. Dubreuilh, J.M. Marionnaud (1973), 45 pp.; Dubreuilh, J., Marionnaud, J.M., 1973. Carte géologique de la France (1/50000), Feuille Lesparre-Médoc - Le Junca (753–754), Orléans: BRGM Notice explicative par J. Dubreuilh, J.M. Marionnaud, P. Andreieff (1973), 47 pp.]. They divided the dune systems into four chronological generations based on their morphology and comprising: (i) isolated barchans (>5100 yrs BP), (ii) parabolic dunes (5100–3000 yrs BP), (iii) localized areas of barchan dunes (2300–200 yrs BP), and (iv) present-day littoral dunes. This paper describes the dune morphology and presents a revised chronology for the Médoc dune systems based on historical records and a number of dated palaeosols. This paper confirms the existence of two generations (Buffault, P., 1942. Histoire des dunes maritimes de la Gascogne. Imprimerie Delmas, Bordeaux, 446 pp.) which are constituted by six different dune forms. The first of these generations is termed primary and consists of coalescent compound climbing parabolic dunes naturally fixed by vegetation (possibly mobile around 5000–3500 B.P. or 3000–2300 BP). The primary dunes were invaded by a second system named modern, which consists of simple barchans, isolated compound barchanoid ridges, a field of coalescent compound barchanoid ridges, simple parabolic dunes and compound digitate parabolic dunes; all of which are believed to have been active between 1500 and 200 BP. The highest degree of dune development occurs in the south of the area where the maximum expanse of sand deposits occurs on the continental shelf. In this region, the sequence of dunes inland from the coast consists of the littoral foredune, compound digitate parabolic dunes, simple barchans or isolated compound barchanoid ridges, and coalescent compound barchanoid ridges of increasing height and linearity further landwards. The barchanoid forms are interpreted as the amalgamation of landward migrating parabolic forms which arose as blowouts in the littoral foredunes. The large heights of the barchans can be explained by favourable conditions of sand accumulation due to the presence of the slacks (locally called `lèdes'), the Hourtin lake, and most importantly, the vegetated primary dune system, which curtailed landward sand transport past this point. To the south of the study area, inland of the coalescent compound barchanoid ridges belonging to the modern dune system, is a field of older coalescent compound climbing parabolic dunes of lower height belonging to the primary system.