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High-resolution bathymetry of the Alderney Race and its geological and sedimentological description (Raz Blanchard, northwest France)

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posted on 11.09.2019 by Lucille Furgerot, Yohann Poprawski, Marc Violet, Emmanuel Poizot, Pascal Bailly du Bois, Mehdi Morillon, Yann Mear

We present a high-resolution 1:15,000 bathymetric map (Main map) of Alderney Race located offshore of northwestern France, with the strongest currents in Europe. We use this map, underwater video transects and Shipek grabs to improve geological maps previously published. We distinguished Proterozoic crystalline rocks, Paleozoic and Cretaceous sedimentary rocks on the present-day sea floor. Some structures as faults and folds are also mapped. We identified a Quaternary cover made of pebbles, boulders and blocks interpreted as corestones resulting in differential erosion and alteration of the substratum. This cover is commonly encrusted by fixed fauna, such as bryozoans and barnacles. Finally, we describe the present-day mobile sediment cover characterized by sand patches and pebble dune fields (up to 10 m in height). Our videos show the presence of mobile fine-grained sediment patches under the resolution of our map lying between the cobble and pebble cover. We summarize our interpretations on a non-exhaustive geological-sedimentary map.

Funding

This work was supported by the HYD2M project [ANR-10-IEED-0006-07] funded by the program called ‘Investissements d’avenir’ for the building of France Energies Marines, the Oceanquest project from Hydroquest and the department council of Manche; Agence Nationale de la Recherche.

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