Cambios paleogeográficos durante el Holoceno en las marismas de Forua, Reserva de la Biosfera de Urdaibai

Abstract

The sedimentology and micropalaeontological content of a core located in the Forua marsh of the Nature Reserve of the Urdaibai (Soutern Bay of Biscay) have been analysed in order to characterise the modem Holocene evolution of this marsh. Based upon lithology and benthic Foraminifera assemblages, five units have been identified in this core. The older one represents a subtidal, estuarine palaeoenvironment with marine influences, dated of 4640 years BP, that is, at the end of a transgressive phase initiated at about 5000 y BP, coincident to the Atlantic-Subboreal transition. During the second phase the sea retreated from the Urdaibai and a marsh settled in Forua at around 4500 y BP (end of Neolithic). The filling of this marsh is evidenced by changes of micropalaeontologic assemblages. A third sedimentologic phase is characterised in this area by a brackish marsh that is temporarily flooded with waters from the marine shelf. This event could be related to the transgressive pulsation occurred during the Subboreal-Subatlantic transition (3500-2500 y BP; early-mid Bronze Age). The sedimentary filling of the marsh was completed during the fourth episode, as evidenced by the exclusive occurrence of agglutinated species of Foraminifera typical of upper, vegetated marsh settlements. Calcareous microfauna disappear at the top of this core as a consequence of the total emergence of this area that became a polder used for culturing up to 7 960. The occurrence of few specimens of J. macrescens in the last centimetres of the core evidences the occasional flooding of this area during equinoccial tides