The stratigraphic significance of reworked Quaternary microfossils in the Holocene of the Bilbao estuary (northern Spain)

Abstract

Two boreholes have been analyzed micropalaeontologically (foraminifera and pollen) in order to understand the Quaternary palaeoenvironmental evolution of the Barakaldo abandoned meander area in the Bilbao estuary. Based on the marine microfossil record, two different environments have been distinguished in these borehole sections: a thin, brackish, intertidal environment containing Pleistocene reworked estuarine bioclasts, followed by a terrestrial environment of probable Holocene age. The pollen record shows only one zone throughout the studied section dominated by temperate trees and probably deposited between 8500-6000 years BP. These microfossil results are in good agreement with previous geomorphological work that suggested, firstly, limited fluviatile erosion of this area by the Pleistocene Nervion river during sealevel lowstands and, secondly, the capture of this former meander before the Holocene sealevel highstand. As a consequence, this area contains a weak marine transgressive signal and the first evidence of relict pre-Holocene marine materials in the estuaries of the southern Bay of Biscay.