Abstract

Grain-size distributions in lower beach and nearshore sands of a segment of the coast of the Golfe du Lion (France) were analyzed by a parametric method and by modal analysis. The results have been considered both in a spatial framework delineating geographically distinct sedimentary compartments and cells, and in a morphodynamic framework separating the dynamic components of the shoreline (berm, collision zone, inner bar, outer bar, lower shoreface).

Modal statistics indicate that a mixture of three dominant components (end members) contributes to the grain-size distribution (GSD) of the sediments: fine sand (Sedimentary Type I: Modal value: 0.195 mm; σI (Φ): 0.35), medium and coarse sand (ST II: Modal value: 0.680 mm; σI (Φ): 0.8) and very coarse sand and gravel (ST III I: Modal value: 2.3 mm; σI (Φ): 1.5). Quasi log-normal one-component GSDs form a minority group and mixtures produce various apparently unimodal sands and bimodal sediments. A large variation of grain-size indices results from mixing. A simulation of mixing of log-normal populations close to the end members shows the great sensitivity of parameters (sorting, skewness, and kurtosis) to small changes in the compositional formula of the mixture.

The procedure followed supplies a good preliminary tool enabling rapid identification and localization of the main sand sources on a regional scale. These components derive from potential sources available in the region, well known from previous studies. ST I originates from the River Rhone and provides the textural base for most of the sediments. ST II and III form a local component originating either from fluvial input (Aude, Orb, and Herault rivers) or from the nearshore reworking of ancient Quaternary sediments and late Holocene beachrocks. Each ST takes on local aspects in relation to the morphological environment, but the regional scale of the study was too large to precisely reflect the local morphodynamic regime. A significant change in the ST I sorting of bar sands is, however, observed, probably due to the net offshore migration reported for the bar system.