The intertidal sanding up of the Seine-Maritime coast (Normandy, France): Sedimentological and geochemical approaches

Abstract

International audienceThe Seine-Maritime coastline (France) is a macro-tidal environment (8 m tidal range), developingalong an epicontinental sea, the English Channel. The SW-NE coast is opened to westerlyatmospheric flows, generating occasionally wind sea with energetic waves (Hs: 4.65 m decennialreturn). High chalk cliffs and a wide marine erosion platform partially hidden on its upper part by aflint pebble beach, characterise this 130 km long coast.Observations since the end of the 1990’s show a recent and massive sanding up of the marineerosion platform. This raises the question of the origin of the sandy fraction and the sedimentarydynamics on the intertidal area.We present herein an innovative method that combine grain-size and geochemical analysis inorder to highlight sand sources and transport direction along these rocky coast.Sixteen beaches were sampled during low tide and fair-weather conditions. At each site, 3 sampleswere collected along the cross-shore beach profile (from the pebbly upper beach to the low tidelimit).Grain-size results show that for all sites, medium to coarse-grained sands dominate in the upperbeach (mode 315-400μm) while fine sands dominate in the middle and low foreshore (mode160-250μm). A decrease in grain-size is thus evidenced from the upper beach to the low foreshore.The geographical variability of the sand composition and consequently sources was determinedon the basis of geochemical data. In order to avoid the granulometric effect on the data, X-Rayfluorescence analysis (xSORT, SPECTRO AMETEK) were performed on the two major grain-sizemodes of each sample. Eighteen calibrated chemical elements (Si, S, K, Ca, Ti, V, Mn, Fe, Ni, Ga, As,Br, Rb, Sr, Y, Pb, Th and U) were measured at each station. Statistical processing performed step bystep on the data allows to gradually reduce the number of significant geochemical parameters.Finally, 4 major elements (Si, Ca, Sr, K) as well as the ratio Sr/Ca have been considered as the bestproxies of sample discrimination and potential source.The first results indicate a longshore gradient of Si and Ca, especially for the finest sands 60-200μm). From SW to NE, i.e. in the direction of the littoral drift, and whatever the positionacross the beach profile, there are an enrichment in Si (sands are more siliciclastic) and animpoverishment in Ca.This gradient highlights differentiated longshore sediment transport and sorting, in relationprobably with sediment sources (siliclastic sands vs bioclastics sands)