HOLOCENE SEA LEVEL VARIATIONS IN THE BAY OF QUIBERON (SOUTH BRITTANY, FRANCE): ARCHAEOLOGICAL AND SEDIMENTOLOGICAL TRACERS

摘要

A precise chronology of the sea level rise occurring since the Last Glacial Maximum is difficult to establish. Geological evidences are generally located on the coastal fringe, a privileged interface for ocean/continent interactions. Two complementary approaches, the archaeological one and the sedimentological one, have been used in order to describe the different steps of sea level rise in the Bay of Quiberon (south of Brittany, France). Numerous megalithic sites have been discovered on the eastern coast of Bay of Quiberon and in the Morbihan Gulf. These man-made rows of standing stones, erected 6,500 years ago, extend below the present sea level. In order to detect the extension of these subtidal megaliths, lateral and interferometric sonar and seismic surveys have been conducted and provided a high resolution bathymetry together with sonar imageries of this area. Thus, Neolithic constructions such as megaliths were identified below the current sea level. Their extension limit corresponds to the Neolithic palaeo-shoreline situated around -5.5 m below the NGF zero. The sedimentological approach, combining seismic profiles with a long core, gives details on the frequency of high energetic events like storms. The end of the Carnacean phenomenon may be related to the time interval characterised by stormy climatic events combined to sea level rise increase, around 4,300 yr BC.