Grain-Size Components As Markers of Origin and Depositional Processes In the Coastal Zone of the Golfe Du Lion (Mediterranean Sea, France)
Résumé
A study of beach and nearshore sands between the mouth of the River Aude and Cap d'Agde (northwestern Mediterranean Sea) shows the existence of four sediment types (ST): ST1 to ST4, with ST2 seemingly deriving from ST1, revealed by grain-size modal analysis and confirmed by a deconvolution program (LogNormal Deconvolution of Grain Size). Grain-size skewness is related to a much more persistent multimodality than graphic evidence can show. Multimodality results from mixtures of generally three STs and/or variants (the latter defining monotype grain-size distributions). Monotype mixtures are unequally distributed in morphodynamic zones, very abundant on the lower shoreface, significantly present on the berm and inner bar, but rare in the most dynamic zones (i.e., the collision zone and outer bar). On the berm and lower shoreface, modal values of the three sedimentary variants retain a close grain-size relationship. On the berm, where only bedload is present in the swash, monotype grain-size distribution trends towards a lognormal model. This trend can also be observed on the lower shoreface, where there are numerous monotype grain-size distributions (GSDs), possibly indicating a dominant process.
When compared with ST1-dominated grain-size distributions of the outer bar zone, the distribution of the derived ST2 on the lower shoreface has an added fine tail derived from the coarse-skewed outer-bar sands. In addition, positive skewness of the lower-shoreface sands could indicate input of fine particles from deeper zones under the influence of shoaling waves and upwelling.
Morphodynamic zones are approximately characterized by the distribution of grain-size indices. A change in the direction of the skewness sign can be observed between the shoaling zone and more energetic zones, from the breaker zone up to the berm. Finally, a more detailed description of the spatial properties of grain-size distributions could, first, help explore possible mechanisms underlying sediment sorting in the coastal zone and, second, provide a set of indices that would be of use in paleogeographic reconstructions and organization of basin borders, while improving discrimination and classification of environments.