Holocene transgression recorded by sand composition in the mesotidal Galician coastline (NW Spain)

Abstract

This study confirms several inferences regarding Holocene coastal dynamics and climate through a petrographic modal analysis of 60 Holocene sand samples recovered in seven sites along the NW coast of the Iberian Peninsula. Fluvial sand can be discriminated from more mature intertidal and aeolian sand according to texture and composition. Fluvial sand contains soil products and coastal sand has significant bioclasts. Quartzofeldspathic sand appears in the western area (produced by the erosion of granite and granitoid), and quartzolithic sand occurs in the eastern area (produced by the erosion of metasediment). Changes in sand composition during Holocene deposition are manifested by an increase in modern carbonate clasts (MC) correlated with the Holocene transgression. Episodes of faster sea-level rise and subsequent erosion of surrounding cliffs are indicated by the preservation of high proportions of feldspar in intertidal sand. In contrast, fluvial sand is characterized by greater quartz enrichment. These inferences were confirmed by petrographic indices (carbonate clasts/total clasts, MC/T; total feldspars/monocrystalline quartz, F/Qm; and plagioclase/total feldspars, P/F). The different maturity of intertidal and aeolian sands is revealed by their variable quartz contents, despite similar proportions of plagioclase and K-feldspar. This suggests mechanical abrasion as the main factor controlling maturity. In contrast, fluvial sand shows depleted plagioclase contents as the result of inland weathering processes. Intertidal, beach and aeolian sands are essentially the products of the erosion of coastal cliffs and head deposits, with only the scarce contribution of fluvial drainages. The long-distance transport of Galician coastal sands is discarded based on the close relationship between their composition and that of local sand sources. Our findings indicate that short-distance transport of sediments from the west closed off coastal wetlands and occluded estuarine mouths during the Holocene transgression by deposition on sediment-trap zones along the irregularly shaped Galician coast.