ORIGIN AND DEPOSITIONAL PROCESSES OF COASTAL SANDS REVEALED BY GRAIN-SIZE ANALYSIS (GOLFE DU LION, MEDITERRANEAN SEA, FRANCE)

Abstract:

The study focused on the lower beach and nearshore zone of the Golfe du Lion (northwestern Mediterranean Sea -France). We looked at grain-size distribution in the berm, collision zone, inner and outer bars, lower shoreface applying parametric methods, grain-size modal analysis and a deconvolution program (LNDoGS).

Results indicate a mixture of three recognizable sedimentary types (ST1 – ST3 – ST4) and explain grain-size distribution (GSD) of the sands in both multimodal and apparently unimodal GSDs. ST1 flows from the River Rhone, ST3 and 4 are local components originating from either fluvial input or nearshore reworking of Quaternary sediments.

A fourth component (ST2), derived in situ from ST1, is found far from the overwhelming footprint of the Rhone river sands, westward in the lower shoreface.

The mixing process entails significant variations in grain-size indices and skewness appears as a very sensitive and meaningful parameter. Skewness and bimodality, generated in all zones, are two aspects of the mixing process, depending on the relative proportions of STs.

Using LNDoGS makes deciphering the STs a lot easier. It also reveals that some GSDs can result from a combination of 3 sedimentary variants (SVs) belonging to the same ST (monotype grain-size distributions). Monotype mixtures are very abundant on the lower shoreface, significantly present on the berm and inner bar but rare in the most dynamic zones. On the berm they tend towards a lognormal model for only bedload is present in the swash. This trend towards a lognormal model can also be observed on the lower shoreface, possibly indicating the occurrence of suspension as a dominant process.

Thus, each segment of the nearshore - foreshore profile can be characterized using coarseness, skewness sign, the presence of locally generated ST2, abundance of bimodalities, occurrence of SVs. Finally, a more detailed description of the spatial properties of grain-size distributions could help explore possible mechanisms underlying sediment sorting in the coastal zone and provide a useful set of indices for paleogeographic reconstructions.