Lower Cretaceous tides revealed by cross-bedding with mud drapes

Abstract

The sand-dominated Folkestone Beds, widely exposed around the Weald of south-east England, are of Upper Aptian to Lower Albian (Lower Cretaceous) age and range in thickness up to 84 m (refs 1–3). Their content of ammonites, bivalves and sponges indicates a marine origin, while their relative coarseness and the incorporation of drifted timber point to a shallow water deposition near land. One striking sedimentary facies is formed by medium to coarse-grained cross-bedded sands in which several foreset and/or bottomset surfaces support mud drapes. Although it was suggested⁴ that this facies may have been formed by the storm-induced movement of sand waves, I now consider that the facies is best explained by the migration of sand waves under the influence of strongly asymmetrical tides, and that the mud drapes are formed at slack water. A model of this process has been described elsewhere⁵ and I now describe measurements of a section which supports my earlier conjecture⁶that the separation of the mud drapes is systematically related to the phase of the spring–neap tidal cycle at the time of deposition. Although criteria for recognizing tidal deposits have long been available it has not previously been possible to analyse a series of ancient sediments with a view to characterizing the tidal regime.