Importations et remaniements de thanatofaunes dans les sables de la plate-forme profonde des approches occidentales de la Manche

Abstract

Macro- and micro-faunas are reported from six gravity cores collected at a mean depth of 150 m in the surficial deposits of the Kaiser sand bank in the Southern Western Channel Approaches. These are bivalves (mainly), gastropods, echinids, crustaceans and foraminifers. These fauna present a very rich association of numerous, well preserved species from various ecological settings. Such an association is commonly reported from the continental shelf of the last transgressive cycle. In addition, the sediment comprises two imported faunal associations. The first one corresponds to thanatocaenoses of Pliocene/lowermost Pleistocene and Weichselian age, reworked respectively from the underlying celtic incised valley fills and the Celtic Sand Banks. The absence of Quaternary fauna prior to the Weichselian suggests that the introduction of Pliocene sources occurred during the last climatic cycle. The second stage of importation took place after the last transgression in the Western Channel, and corresponds to the enrichment of the sediment in coastal fauna derived from western Brittany by the the predominant ebb tidal current. The variations of taxonomic diversity from core to core suggest a sediment transport around the bank that matches the one deduced from the study of tidal bedforms. At the present day, the wave action is highlighted by accumulation of species of the same shape, size and density inside storm beds. However, the survival within these beds of very small, juvenile shells indicate that each storm reworking was short in duration.