Sediment distribution pattern in the Rias Baixas (NW Spain): main facies and hydrodynamic dependence

The analysis of over seven hundred samples of sediments have made it possible to define a detailed distribution of seabed surface sediment properties like grain size, carbonate and organic matter content of the rías of Vigo, Pontevedra and Arousa (NW Spain). Furthermore, the comparison of these three rías allows establishing a general facies distribution pattern for a ría and interpreting the hydrodynamic processes, characterized the marine and fluvial prevalence. In a simplified way, three main facies domains are characterized that delimit the area of influence of the different hydrodynamical phenomena: (a) siliciclastic sand with gravel, mainly associated with the river mouths, areas of important fluvial influence; (b) siliciclastic mud with high organic matter content, located in the inner and in the deeper areas of the rías, reflecting low wave action; and (c) biogenic sands with low organic matter content located at the margins of the external ría, where the presence of gravel may occasionally occur associated to the more wave exposed areas. The wide range of wave energy conditions along the rías determines a distinct pattern distribution of facies. Moreover, the comparison of the granulometric characteristics between sediment samples of a ría and a estuary shows that the former is a more energetic sedimentary environment.