## 2 Study Area  
Mero River drainage basin (A Coruna, Galicia) (Figure 1) is a small network (350 km\({}^{2}\)) that originates in the Ria de Coruna. This rai, like the rest of the Galician Rias, are very particular formations formed as a result of the flooding by the sea of the lower areas of the valley during the last Holocene transgression [4].  
Mero River basin mainly drains metasediments [9] from Ordes Complex, although the westernmost sector drains Hercynian granitoids (Figure 1). The Ordes Complex is the largest of the complexes in NW Iberia [10]. Its central part is occupied by extensive outcrops of the so-called upper units, up to 12 km thick. One of the units that make up the Ordes Complex is the Betanzos Unit, a sequence with a flysch appearance, composed mainly of metamorphic rocks, whose protooliths present characteristics of turbiditic greywakes, conglomerates, phyllites and calco-silicates, varying to black metapelites and quartzites in the lower part of the unit. The detailed study of these turbidite sequences suggests that their sedimentation took place in a deep submarine fan context, observing sedimentary facies that typically occur in distribution channels and lobes commonly associated with turbidite systems. The metagrauvakic sequences of the Ordes Complex are classified as immature feldspathic sandstones from a first sedimentary cycle and present low-grade metamorphism. In them, the grains are mainly formed by quartz, plagioclase and rock fragments. They present angular or subangular textures with poor classification within a shale matrix. These metagreywakes occupy the upper structural levels and are particularly well exposed along the shoreline, where they outcrop with a shale appearance. A detailed geochemical study of the metagreywakes of the Ordes Complex confirms their immature character, as well as their origin from a felsic igneous source with an affinity for an upper continental crust [10]. The maximum age of sedimentation calculated for the graywakes of the Ordes Series (c. 510-530 Ma) and the age of magmatism in the upper units (c. 500-520 Ma), as well as the presence of the abundant diabase dikes that intrude and cut the series, with age calculated for its formation of circa 510 Ma.  
According to previous studies [4], the current disposition of the Mero River drainage basin over these materials is assumed to be less than 5 Ma ago, at which time the Tertiary endorhei basins, present at the head of the valley, collapsed and drained their waters towards the Atlantic, conditioned by tectonic processes related to the Alpine orogen [4]. In this sense, no neotectonic differences can be observed within the basin [4]. At present, sedimentary deposits from this period are not found in the basin. Only small outcrops of alluvial deposits, probably Pleistocene up to +200 m above present sea level (apsl), are preserved. Among them are the sediments studied here, represented by undated younger alluvial deposits of variable thicknesses between 3 m and 20 m, formed by quartzite gravels (>80%) with sand and silt lenses [4; 11]. For this work were sampled sand lenses (Figure 2) of the best-preserved levels at heights between +2 m to +60 m above present sea level (apsl), some of them located very close to the coastline (in Figure 1 are represented relative heights). These alluvial deposits would represent the last episode of the dismantling of the previous rocky materials and the formation of the fluvial valley that gave rise to the Ria de Coruna. The calculation of their age would extend the geological knowledge and the evolution of the coast in the study area within the age limit of the dating technique used here.  
Figure 2: Alluvial deposits in the Mero basin. Outcrops: Mero-1 (**a**), Mero-3 (**b**) and Mero-2 (**c**).