A mechanism for the formation of overconsolidated horizons within estuarine floodplain alluvium: implications for the interpretation of Holocene sea-level curves

The stratigraphic sequence held in accumulating estuarine floodplain alluvium provides a detailed record of past sea-level movement. However, contrasting Holocene sea-level curves have been derived for two basins in southern Britain. The sea-level curve of the Thames Estuary Basin describes up to five possible regressive cycles superimposed onto a long-term transgressive trend. In the Severn Estuary no regressive cycles have bran, as yet, identified. These differences in interpretation are, in part, due to the identification of 'overconsolidated' horizons, found within Essex alluvium, which have been argued to represent prolonged periods of desiccation. No such horizons have been found in the Severn Estuary. By examining the sedimentological and geotechnical characteristics of a number of active, land-claimed and regenerated saltmarshes from these regions, this paper identifies the mechanism for the formation of overconsolidated horizons to be geochemically controlled and regionally variable. The presence of suitable quantities of detrital calcium carbonate is important in preventing the formation of these horizons. The carbonate-deficient alluvium in the Thames Estuary region appears to be particularly sensitive to the effects of a lowered saline water table, inducing the deflocculation of clays and formation of dense horizons. By contrast, the sediments in the carbonate-bearing alluvium of the Severn Estuary are insensitive to pore-water salinity changes so an absence of overconsolidated horizons in the floodplain alluvium doss not preclude past sealevel falls.