Late Flandrian (Holocene) tidal palaeochannels, Gwent Levels (Severn Estuary), SW Britain: character, evolution and relation to shore

The exceptionally well-exposed Flandrian sediments on the Welsh shores of the Severn Estuary reveal tidal palaeochannel networks uniquely tied to vertically stacked peat–silt couplets. Field relationships show that channels multiplied and were enlarged as each peat (organogenic marsh) was rapidly transgressed, replaced by, and buried beneath silt (mineralogenic marsh). The networks infilled and shrank as the silt marsh later gave way regressively to the next peat marsh. Simulation and hydraulic models show that this cycle depended on an increase followed by a decrease in the tidal prism imposed on the marsh during a sea-level fluctuation. Coupled patterns of channel width, downstream orientation and coarseness of fill suggest that the many networks tied to the final Flandrian peat–silt couplet (Cycle III) ranged inland from a marsh-edge lying of the order of 2 km seaward from the modern coast.