Desiccation of mud in the temperate intertidal zone: studies from the Severn Estuary and eastern England

Abstract

Field studies, including the close temporal monitoring of selected sites, allow desiccation cracks in the hypertidal subzone to be classified on the basis of (i) degree of desiccation, (ii) overall shape of individual cracks, (iii) shape of crack margins, (iv) the angle at which cracks join and the number of sides to a desiccation pillar, and (v) the degree and kind of preferred orientation of cracks. In the Severn Estuary and The Wash, and on the north Norfolk coast, these properties are controlled by the thickness, lithology and degree of stratification of the exposed muds, and by the history of drying as determined by positional, tidal and climatic factors. The cracks are initiated at such defects as bird's footprints, plant stems, and pebbles and shells, and grow slowly along mainly orthogonal paths under the influence of principal stresses that change in orientation with the spread of the fractures they themselves have caused. Hypertidal desiccation cracks commonly open and fill more than once; filling may be accomplished by either a repetition of the same material or a succession of different ones (for example, mud, sand, mud clasts, bivalve shells). The closely monitored sites reveal that hypertidal cracks develop chiefly during the late spring and summer on time scales of hours to months, but that significant fracturing may also occur during the winter, when periods of dry windy weather coincide with relatively weak tides. Because of the nature of the controls, the degree of desiccation and the type and pattern of the cracks varies stratigraphically within the hypertidal zone in a systematic manner which is similar in all the areas studied. Late- and terminal-stage cracks are most prevalent in the upper hypertidal subzone, whereas early-stage forms predominate in the lower subzone. Non-orthogonal patterns predominate in the upper subzone, where various factors promote the destratification of the sediment. Orthogonal fractures are best developed in the middle subzone, where thick muds can be accumulated but where exposure can be lengthy. As an assemblage, and taking into account their stratigraphical variation, temperate-zone hypertidal desiccation cracks appear to be distinct from the associations developed in other marginal and continental environments.