Holocene environmental development of the Bilbao estuary, northern Spain: sequence stratigraphy and foraminiferal interpretation

The foraminiferal assemblages recorded in the sedimentary successions of nine boreholes drilled in the Bilbao estuary have been analyzed. Palaeoenvironmental interpretation of these and other previously studied assemblages defines the Bilbao estuary sedimentary infill as a depositional sequence within a fourth-order eustatic cycle (Holocene). This sedimentary sequence comprises a wide range of foraminiferal assemblage zones (FAZ) that are organized into three systems tracts. Each systems tract is composed by a distinct suite of FAZs separated by continuous stratigraphic surfaces. During Lateglacial low sea-level conditions, sedimentation was represented by fluviatile gravels and coarse sands barren of foraminifera (lowstand systems tract). During marine transgression (transgressive systems tract, 8500–3000 cal years BP), great volumes of mainly near-marine sediments were deposited in the lower estuary, alternation of brackish and near-marine sediments were accumulated in the middle estuary, and brackish materials sedimented in the upper estuary. The highstand systems tract deposited during the upper Holocene (3000 cal years BP-19th century human reclamation) represented brackish intertidal and supratidal conditions as the sedimentary infill was taking place under stabilized sea-level. Comparison of these results with other sedimentary sequences from different coastal areas in the Bay of Biscay indicates that, following postglacial sea-level rise, modern estuaries in the region began to develop around 8500 cal years BP. A final transgressive event was dated at around 3000 cal years BP when sea level reached approximately its present position after a second upward-shallowing sequence commenced.