Holocene environmental change in the inner Severn estuary,UK: an example of the response of estuarine sedimentation to relative sea-level change

Abstract

Holocene sediments of the wetlands around the inner Severn estuary, UK, demonstrate a consistent stratigraphy of a single woody peat layer, up to at least 4.5 m thick, capped with a grey, silty clay. The peat directly overlies sand and gravel or bedrock. This differs from the multiple peat beds and sands and gravels described from the outer estuary. The contact between the peat and clay is sharp, and the altitude of that contact is around 5 m ODN throughout the area. Fifty-eight boreholes at three principal study sites suggest that the contact decreases in altitude inland, not seawards. Palaeoecology of the sediments around the contact shows a decline in alder and rise in sedge prior to the clay deposition. There are no salt-marsh indicators, and diatoms of the clay layer, although scarce, suggest nutrient-rich, fresh or only very slightly brackish conditions for the environment of clay deposition. The change from alder fen to sedge fen, and then to standing fresh water, indicates a rising water table. The silty clay does not show a fining upwards, nor away from the river, and the sediment source is thought to be estuarine, not fluvial. A rising relative sea level eventually allowed sediment-charged tidal waters to penetrate inland across former fenland. It is proposed that the inner Severn estuary acted as a series of basins, and the expansion of the tide into each basin led to a decrease in tide height. Flooding began after 3110 BP, and was later further inland. Each basin may have been flooded fairly suddenly, accounting for the sharp contact. The topographic controls make correlation with regional sea-level curves difficult. Above the silty clay there are distinct overbank deposits, suggesting that the phase of standing, tidal water was directly succeeded by fluvial sedimentation. Reclamation of the wetlands took place during this last phase.