Sedimentary evolution of a bedrock-conditioned incised valley since the Last Glacial Maximum: the Ría de Arousa (NW Spain)

Coastal sedimentary environments are dynamic systems in continuous change responding to different temporal and spatial scales. Their sedimentary record offers invaluable data to unveil the effect of different drivers, such as relative sea-level rise, on their evolution.

The Ría de Arousa is located on the Atlantic coast of Galicia (NW Spain) and represents the largest of the so-called “Galician Rias”, with a total area of 230 km2. It corresponds to a mesotidal tide-dominated incised valley characterized by a complex physiography with numerous smaller bays, islands and peninsulas.

The identification of elements of sedimentary architecture was used to study the sedimentary evolution of this incised valley since the Last Glacial Maximum (ca 20 kyr BP to present). This approach was based on the combined analysis of seismic and sedimentary facies, complemented with radiocarbon, geochemical and pollen data.

During the lowstand of the Last Glacial Maximum, a river basin occupied the deep axial valley whose physiography was controlled by the rocky basement morphology and the presence of preserved older sedimentary units. The postglacial transgression changed the base level of rivers, flooding the valley and leading to the formation of an estuary. Facies distribution during this phase (Late Pleistocene) was characterized by large tidal sandbanks and sandflats in the outer area and a bayhead delta at the river mouths. As the transgression proceeded, during the Early Holocene, the system evolved into a tide-dominated estuary. Tidal sandbanks and sandflats occupied large extensions in the axis of the valley, flanked by mudflats. The presence of a small group of islands in the middle area of the incised valley gave way to the existence of an ancient strait during most of the postglacial transgression (Late Pleistocene and Holocene), modulating the relative influence of hydrodynamic conditions and probably leading to tidal currents amplification due to the local morphological narrowing. These structural highs favored the formation of a rock-bounded tidal inlet in the middle of the valley, characterized by scarce deposition and erosional processes.

During the Middle and the Late Holocene, most of the incised valley became drowned, and wave influence increased. A wave ravinement surface is identified, which was developed around 8 cal kyr BP coeval with the initiation of large storm fans associated with rocky barriers.

Finally, a maximum flooding surface is recognized at ca 5 cal kyr BP while the slow rise of sea level forced river mouths to retreat to its present position and marine processes became dominant in the basin.