Reworking of muddy intertidal sediments in the severn estuary, Southwestern U.K.—A preliminary survey

Abstract

The estuary is a large, macrotidal system that faces the prevailing winds. Consequently, it is swept by vigorous tidal currents and is somewhat stormy. The muddy intertidal sediments, ranging in age from the early Flandrian to the present day, are predisposed to reworking chiefly because of (a) the presence of silt and sand laminae, which promotes splitting along the bedding, and (b) the development during the spring and summer of deep, ramifying drying cracks in response to seasonal changes in the tidal and weather regimes. The chief mechanisms and processes of reworking are various kinds of mass movement after failure, wave impact and current shear, and scouring by water-born tools. The chief expressions of reworking are: (1) mud cliffs associated with either an apron or strew of fallen masses of sediment; (2) rip-up clasts; (3) extensive scoured surfaces; (4) ridges and furrows of various scales parallel with tidal currents; (5) ridges and furrows at right-angles to shore shaped by incident waves; and (6) sediment sheets and trains of bedforms composed of either mud clasts (granule- to pebble-sized) or siderite-clay mineral concretions (sand- to granule-sized). Reworking in the estuary occurs on tidal, seasonal and longer time scales. Since the mid Flandrian, when peats with mature trees accumulated throughout the area, the Severn Estuary has retreated up the Severn Vale, the muddy sediments being extensively reworked in the process. During this retreat, the minimum annual turnover of fine sediment through reworking may have amounted to between 7 and 70% of the annual fluvial supply.