Coastal sediments of the southeastern shores of the Ría de Arosa (Galicia, NW Spain)

The beaches of the southeastern shore of the Ría de Arosa are in general small and enclosed between rocky capes; especially those along the inner ria are not subjected to strong wave action. Nonetheless, the beach sands are often coarse-grained, being partially derived from the coarse-grained intrusive granites exposed in low cliffs along the major part of the shore. Generally, the sands along the low-water mark are finer than the better-sorted sands along the high-water mark, since the backwash is not able to transport the coarse grains. In some cases, however, granules derived from weathering granite are left behind at the low-water mark because the waves are too weak to carry them higher up on the beach. In protected embayments tidal flats originated whose sediments are much finer grained than those of the beaches, although they are relatively coarse-grained as compared with tidal-flat sediments in general. At some places there is a supply of pebbles deriving from colluvium and from raised beaches. The heavy minerals partially reflect the composition of the basement rocks exposed in low cliffs (epidote association, tourmaline association). The hornblende association is supplied by the two main rivers and is found near their mouths, but the hornblende content rapidly decreases due to weathering. The metamorphic association, occupying the largest area, may to some extent have been supplied by colluvium (derived from xenoliths in the granite), but a greater proportion was probably supplied by rivers (Ulla and small rivers on the opposite shore), which deposited sediments on the ria bottom (as shown by seismic profiles), from where it was washed ashore during the late-glacial rise of the sea level. The quartz/feldspar ratios are highest where the beaches are most exposed.