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Source discrimination of fine-grained deposits occurring on marine beaches: The Calvados beaches (eastern Bay of the Seine, France)


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Titre de la revue : Estuarine, Coastal and Shelf Sciences Volume : 72 N° : 1-2 Pagination : 138-154 Date de publication : 01/03/2007

Résumé

In tide-dominated systems, fine-grained deposits occur in intertidal areas of inner estuaries as loci of convergence of fluvial and marine material. Even in the vicinity of estuaries, mud deposits are rare on open sandy beaches. This study focuses on temporary occurrences of muddy sediments on marine beaches along the Calvados coastline in Normandy, adjacent to the macrotidal Seine estuary (France). Clay mineralogy, major-minor-trace elements and radionuclides were used as particulate markers to determine the provenance of the mud deposits. The fine fraction, defined here as particles smaller than 50 μm, was analysed, in surficial muddy sediments on seven beaches between the Seine estuary and the Orne river mouth, sampled between February 2002 and June 2003. The deposits were compared to earlier Holocene relict deposits, which crop out on some beaches and on the shoreface, and to Mesozoic marls and limestones, which have detached from the coastal cliffs. The use of the three types of particulate markers revealed no significant seasonal or geographical variations between the beaches. The muddy deposits were made up of the same sedimentary pool of particles. The clay mineralogical and geochemical characteristics of these mud deposits enabled modern (i.e. present-day and earlier Holocene fine silts and clays) to be distinguished from ancient (i.e. Mesozoic) sediments. The use of radionuclides (60Co and 137Cs) confirmed the marine influence in the modern deposits, with a decreasing gradient along the Calvados beaches towards the east. The presence of several specific major-minor-trace elements (e.g. Ti, P) revealed that some of the fine material originated in the Seine estuary. Despite the occurrence of easily erodable rocks (i.e. clays, marls and limestones) in the coastal cliffs, this source represented a limited supply, which is only of local significance. Small rivers, such as the Orne, Dives and Touques which discharge in the study area act as secondary suppliers of fine-grained material to this system. In spite of the exposed coastal setting where strong tidal currents and waves occur, the beaches have become a distal part of the Seine estuarine system and form a temporary sink for strongly mixed fine material mainly of riverine (Seine River) and open marine (Bay of the Seine and the Central English Channel) origins. This is consistent with the infilling of the estuary, the reduction in accommodation space in its inner part, and the offshore shifting of the depocentre of the mud.