L'Amelie beach is located near the mouth of the Gironde estuary (south-western Atlantic coast of France). It has been greatly eroded by recent storms, especially in the winter of 2013-2014. Erosion of the dune front and beach has uncovered a set of estuarine and aeolian sediment deposits containing numerous archaeological remains. A campaign of topographic surveys was undertaken with DGPS and TLS equipment during the springs of 2014 and 2015 along the 2 km length of L'Amelie beach in order to reconstruct the elevation and stratigraphy of these deposits. Sedimentological analyses (grain size, micro-faunal) were performed to better constrain the depositional environment. Archaeological remains and several radiocarbon dates obtained from sediment samples collected in the field were used to propose a consistent chronological framework for this sedimentary sequence. Ten main lithofacies were distinguished from the lower part of the beach to the top of the dune. The base of the sedimentary sequence is composed of Pleistocene deposits dating from MIS 9 (unit 1a) to MIS 2 (unit 1b) (Bosq et al., 2019). The Holocene sedimentary infilling began around 5000 BC with an accumulation of coarse marine sands, which suggests the presence of a tidal inlet. On both sides of this tidal channel, archaeological remains dating from the early Neolithic period up to the Bronze Age testify to a human occupation of the coastal area. From ca. 5000 to ca. 3500 cal. a BP, a characteristic deposit of intertidal mudflats indicates the transformation of the tidal inlet into estuarine marshes protected by a coastal dune barrier. Between ca. 3500 and ca. 3000 cal. a BP, the dune barrier records a phase of erosion resulting in the exposure of the marsh to stronger hydrodynamic conditions. From ca. 2800 to 1650 cal. a BP, a second phase of estuarine sedimentation is recognized. This period is characterized by significant human occupation as indicated by the presence of numerous archaeological remains of the Iron Age and Gallo-Roman periods, associated with the exploitation of a saltwater-to-brackish environment. From 1650 to 1250 cal. a BP, the estuarine salt marsh is gradually overrun by northward migrating coastal-dune fields. After 1250 cal. a BP, the marsh is completely covered by aeolian sands. These results are consistent with previously published data and allow us to detail the local sequence of palaeogeographic changes of the north-Medoc peninsula. Using the 'sea-level index points' method (Hijma et al., 2015), the analysis of radiocarbon dated levels also provides new relative sea level records for this part of the French Atlantic coast. Finally, the phases of coastal dune activity/stability recognized along L'Amelie beach are synchronous with those in the wider Aquitaine coastal region, suggesting a regional driving factor. We propose that the shared sedimentary dynamics are the consequence of the onshore migration of intertidal and subtidal sand banks distributed at the mouth of the Gironde.
1.Univ Bretagne Occidentale, Inst Univ Europeen Mer, Lab LETG UMR CNRS 6554, Pl Nicolas Copernic, FR-29280 Plouzane, France 2.Univ Bordeaux Montaigne, Lab AUSONIUS UMR CNRS 5607, Maison Barcheol 8, FR-33607 Pessac, France 3.Univ Paris Sorbonne, Inst Geog, Lab PRODIG UMR CNRS 8586, 191 Rue St Jacques, FR-75005 Paris, France 4.Univ Bordeaux, Lab EPOC UMR CNRS 5805, Allee Geoffroy St Hilaire,CS 50023, FR-33615 Pessac, France 5.Univ Basque Country, UPV EHU, Dept Estratig & Paleontol, Esp Bilbao, Spain 6.Univ Bordeaux, Lab PACEA UMR CNRS 5199, Allee Geoffroy St Hilaire,CS 50023, FR-33615 Pessac, France 7.INRAP, 140 Ave Marechal Leclerc, FR-33130 Begles, France
Recommended Citation:
Stephan, Pierre,Verdin, Florence,Arnaud-Fassetta, Gilles,et al. HOLOCENE COASTAL CHANGES ALONG THE GIRONDE ESTUARY (SW FRANCE): NEW INSIGHTS FROM THE NORTH MEDOC PENINSULA BEACH/DUNE SYSTEM[J]. QUATERNAIRE,2019-01-01,30(1):47-75