Variability of Incised-Valley Fills Along the French Atlantic Coast During the Pleistocene and Holocene

Eric Chaumillon¹ and J. N. Proust^2
1 Universite de La Rochelle, Pole Sciences, 17042 La Rochelle, Cedex 01, France
² Universite de Rennes1, 35042 Rennes Cedex, France

This contribution aimed to present the main characteristics of a set of modern incised-valleys emplaced in a low accommodation setting in the inner shelf of the Bay of Biscay, along the French Atlantic coast. This study is a synthesis of results obtained from the interpretation of more than 6000 km of high resolution seismic profiles, ground-truthed by vibrocore sampling. To the north of the Bay of Biscay, the valleys are emplaced seaward of an uplifted and tilted area made up of old hercynian magmatic and metamorphic basement rocks. The drowned valleys are connected to small rivers (Odet, Blavet and Vilaine) and to a major one: the Loire River. To the south, the valleys are emplaced in a low subsidence area, composed of Mesozoic sedimentary rocks. The drowned valleys are connected to small (Charente and Lay-Sèvre) and major rivers (Dordogne and Garonne). In the coastal area, tidal range increases northward while wave amplitude decreases. Valley fills usually consist of a single fifth-order sequence emplaced during the last sea-level cycle. Fluvial sediments, at the base of the valley fills, are scarce and the bulk of the valley fills consists of a fining upward transgressive succession. Despite moderate neo-tectonic and seismic activity, the orientation and the morphology of the valleys appear to be structurally controlled. But, in addition to tectonic and sea level forcings, the valley fills seems to be controlled both by local hydrodynamic conditions and marine sediment supply that also depends from the nature of the underlying bedrock.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005