Estratigrafía y Sedimentología del relleno detrítico del estuario del Río Urumea (Donostia-San Sebastián, España)

Resumen

Two different depositional phases can be identified in the Urumea estuarine infill. They were chronologically separated by a long period without sedimentological evidence. The first one, Pleistocene in age (> 43500 BP), is composed of a marsh facies overlaid by fluvio-estuarine facies. This could indicate that the sea level at the time was similar or slightly lower than the present parameter. The later information rules out the coldest stages of the Pleistocene as their origin. On top of the uppermost Pleistocene layer, a Holocene sequence developed which extends from 7040 ± 30 BP (7565–7390 cal. BP) to modern times. In this part of the infill, 3 transgressive pulses can be identified, separated or followed by phases of sea level stabilization or even regression. The first pulse began before 7040 ± 30 BP (7565– 7390 cal. BP) and extended to 5200 ± 30 BP (5995–5910 cal. BP). The first regressive pulse took place around the last date, placing the shoreline a few meters below its present position. After 5200 ± 30 BP (5995–5910 cal. BP), a fast sea level rise can be recorded, followed by a stabilisation phase first. Also, a minor regression happened afterwards, which was used by local communities to build two ramps of unknown utility. This regressive pulse was dated 4710 ± 30 BP (4945–4650 cal. BP). A new transgressive pulse took place after 3000 BP extending through a gentle sea level rise to 1240 ± 30 BP (1190–1065 cal. BP). From this date, sea level stabilisation (and maybe a new transgression in historical times) favoured the total infilling of the estuary, which, together with the development of a tombolo on the left bank, induced the formation of a semi-enclosed estuary which can still be observed nowadays.