Late Flandrian shoreline oscillations in the Severn Estuary: a geomorphological and stratigraphical reconnaissance

Abstract

The Seven Estuary on the west coast of Britain is large, macrotidal and well mixed, receiving fine sediment from many sources. Within the last few thousand years, at least four discrete lithostratigraphic units, predominantly of sandy to silty clay, have accumulated along the shores of the estuary in the upper intertidal zone. The three youngest are continuing to be deposited, each beneath a distinctive geomorphic surface reached by a proportion of the tides. These surfaces form a stair-like succession on the salt marshes and high mud flats, the most elevated and outermost of the surfaces overlying the formation that, of the three, began to accumulate earliest. We here describe and name these linked geomorphic and lithostratigraphic features, and outline on a reconnaissance basis their distribution within the estuary. The (upper) Wentlooge Formation consists of pale green estuarine silty clays that began to accumulate 2500-3000 years ago and ceased to form in the Roman period or soon after. Reclamation during the Roman period isolated the Wentlooge Surface on large areas of tidal wetland in the lower estuary. The soil that developed on this surface is recognized as the Wentlooge palaeosol in those places where post-Roman breaching of the Roman sea defences led to a resumption of tidal sedimentation. The Rumney Surface is the most elevated of the geomorphic surfaces on the salt marshes of the estuary. It overlies thick largely pink sandy to silty clays, termed the Rumney Formation, that began to form at times ranging from the early mediaeval to the early modern periods. Mediaeval reclamation of wetlands led to the isolation of the Oldbury Surface during the early stages in the accumulation of the Rumney beds. Locally, the breaching of the mediaeval seabanks led to renewed tidal sedimentation on the Oldbury Surface. Wherever the Wentlooge and Rumney formations are seen in contact, the base of the latter so far proves to be sharp and erosional. Locally, the Rumney Formation is found to abut against and smother a low embayed cliff cut into the upper Wentlooge beds. The Awre Surface forms the intermediate level on the salt marshes and overlies pink to grey sandy to silty clays (Awre Formation) that bank against a low cliff and gently shelving platform cut into older deposits. This unit began to accumulate probably in the 19th century. The lowest step on the salt marshes is formed by the Northwick Surface, underlain by the grey sandy to silty clays of the Northwick Formation. Like the Rumney and Awre beds, the Northwick Formation abuts against a cliff and shelving platform eroded into older sediments, chiefly the Rumney and Aware formations. The erosion surface at the base of the Northwick Formation dates from the 19th century and the beds themselves from the early 20th century.