HOLOCENE ENVIRONMENTAL-CHANGE IN THE INNER SEVERN ESTUARY, UK - AN EXAMPLE OF THE RESPONSE OF ESTUARINE SEDIMENTATION TO RELATIVE SEA-LEVELCHANGE

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 3.5 m thick, capped with a grey, silty clay. The pear d irectly overlies sand and gravel or bedrock. This differs from the mul tiple pear beds and sands and gravels described from the outer estuary . The contact between the pear and clay is sharp, and the altitude of that contact is around 5 m ODN throughout the area. Fifty-right boreho les at three principal study sites suggest that the contact decreases in altitude inland, nor seawards. Palaeoecology of the sediments aroun d 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 th e clay layer, although scarce, suggest nutrient-rich, fresh or only ve ry slightly brackish conditions for the environment of clay deposition . The change from alder fen ro sedge fen, and then to standing fresh w ater, indicates a rising water table. The silty clay does not show a f ining upwards, nor away from the river, and the sediment source is tho ught to be estuarine, not fluvial. A rising relative sea level eventua lly allowed sediment-charged tidal waters to penetrate inland across f ormer 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 i o a decrease in tide height. Flooding began after 3110 BP, and was lat er further inland. Each basin may have been hooded fairly suddenly, ac counting for the sharp contact. The topographic controls make correlat ion with regional sea-level curves difficult. Above the silty clay the re are distinct overbank deposits, suggesting that the phase of standi ng, tidal water was directly succeeded by fluvial sedimentation. Recla mation of the wetlands took place during this last phase.