The Holocene estuarine sequence at Redwick, Welsh Severn Estuary Levels, UK: the character and role of silts

Abstract

Laser granulometry and the analysis of foraminiferal assemblages have been used to characterize the full sequence of the largely-exposed, c . 10m-thick Wentlooge Formation present on the Welsh coast of the Severn Estuary in southwest Britain. Resting on Pleistocene deposits, it consists of six, unequal, transgressive-regressive cycles composed of silts of estuarine salt-marsh origin that grade up into radiocarbon-dated peats recording highest-intertidal to supratidal organic marshes. Deposition began c . 8200 years ago and was concluded by wetland embanking in either Roman or medieval times. Two cycles record marine transgression across terrestrial surfaces (Pleistocene/raised bog), one registers transgression in the context of continuous intertidal sedimentation, and three express the resumption of deposition on intertidal erosion surfaces which, in one case, is equivalent to a seventh transgressive-regressive cycle developed at a neighbouring locality. The silt units show vertical patterns of grain size (distance from effective sources of silt) and foraminiferal assemblage (height in tidal frame) appropriate to the circumstances of transgression-regression and are largely concordant with a model of salt-marsh morphodynamics in the Severn Estuary. The cycles express the response of the sedimentary surface to regional and local controls that are argued to change in relative importance from one combination to another. That these combinations varied over the Severn Estuary Levels in the mid Holocene — a time of episodic peat formation in southern Britain and mainland northwest Europe generally — is demonstrated by a brief analysis of dated peat occurrences in different parts of the extensive outcrop.