Modern Siliciclastic/Bioclastic Sands of Mesotidal Embayed Beaches of the Galician and Cantabrian Coasts (NW Spain, Iberian Peninsula)

The embayed beaches of northern Spain’s cliffed, wave-dominated coast contain mixtures of siliciclastic sands, supplied mainly by rivers and coastal erosion, and carbonate bio-clastic sands derived from coastal organisms. The proportion of carbonate material, expressed as a percentage of CaCO3, is a reliable indicator of the provenance and transport patterns of sediments. This parameter frequently yields more precise information than grain-size analysis. It is well established that the carbonate content typically diminishes in the direction of sediment transport from the original source. At a regional scale, sediment transport along the Cantabrian coast and much of Galicia is predominantly west-to-east, driven by prevailing northwest and west-northwest wave conditions. While large rivers supply abundant siliciclastic sediments, estuaries, rias, mudflats, and marshes contribute nutrients that enhance marine productivity and carbonate production. In the coastal regions of Galicia, the phenomenon of coastal upwelling has been shown to increase biological productivity, thereby fostering the development of carbonate-rich sediments. The skeletal assemblage is classified as bimol-bivalve dominated. Consequently, elevated carbonate percentages are indicative of regions influenced by nutrient-rich upwelling and productive coastal ecosystems. Across approximately 750 km of the Galician and Cantabrian coastline, variations in bioclastic carbonate content help define coastal compartments, identify sediment sources, and infer longshore transport pathways. In the context of the continental shelf, the interplay of opposing currents generated by northern swells, in conjunction with the influence of the rias, exerts a pronounced regulatory effect on the dispersal of sand and the dynamics of the coastline.