THE FILLING OF THE RIA DE GERNIKA (BAY OF BISCAY) SINCE THE LATE HOLOCENE

A study of the recent sedimentation of the ria de Gernika (Bay of Bisc ay) is undertaken to characterise different steps of its Holocene fill ing. Sedimentological and micropalaeontological analysis of six cores sampled in the ria, completed with absolute dating (C-14, and based up on Pb-210 exc.), allow us to identify three phases in the filling of t he ria that are Limited by two transgressive shifts (3500 and 2500 yr. B.P.) already described in other areas of the Bay of Biscay. In the f irst phase, lithologies as well as benthic foraminifera and ostracode assemblages are typical of coastal marine palaeo-environment. During t he second phase the euryhaline waters progessively occupy the central area of the estuary. Here the presence of allochthonous microfauna, pa rticularly of planktonic foraminifera, indicates an increase of exchan ges with marine waters that can reach the inner areas of the estuarine system. The last phase (2500 yr. B.P. to the present), which correspo nds to the recent and active sedimentation, is marked by an impoverish ment of the microfaunas, the filling of some channels and the increase of salt marshes. The estimated sedimentation rate for the upper decim etres of the cores (0.73-1.29 cm . yr(-1)) shows faster sedimentary fi lling of the ria with anthropic action evidenced by the presence of me tallic contaminants and hypohaline microfauna. (C) Elsevier, Paris.