"La Grande Vasière" mid-shelf mud belt : Holocene sedimentary structure, natural and anthropogenic impacts

The Quaternary sequence of the Armorican continental shelf in the Bay of Biscay is setting down above one regional erosion surface, which has been produced by successive transgressions that occurred during the last deglaciations. The few meters of the sequence covers the Holocene period and it is composed by two clean and sandy units at the base followed by thin (few decimetres) clayed sand unit and mud (few millimetres) unit in the top. The two upper units form the improperly called "La Grande Vasière" area over 8,000 km2. The settlement of these sedimentary units occurred since 8,000 C14 years BP, previously to other coastal mud fields setting of the shelf. Historical grain size data show that the mud field has lost part of the fine content during the last 30 years. "La Grande Vasière" evolves at the rhythm of the cycles of deposition and remobilization. Fluvial supplies and tide currents are the major control factors of the deposition. Bioturbation enhances the homogenization of the surficial sediments. Waves and anthropogenic actions are the main factors for the remobilization of the fine particles. The action of both the storms and the bottom trawls are tentatively quantified.