Coastal Erosion and Transgressive Stratigraphy

Abstract

Unconformities of transgressive sequences commonly occur within the basal beds of the sequences, not below them. Such unconformities, called ravinements, separate basal marsh, lagoon, estuarine, and beach deposits from overlying marine sands. Study of the ravinements in the Bay of Fundy shows that they are cut mainly by surf action along a narrow zone at the foot of the shore face. Spits in the Bay of Fundy advance shoreward before the ravinement, leaving no trace of their passage, except where the supply of shingle is so great that the spits are stabilized. Sea level shifts then result in the abandonment of an old barrier and the formation of a new. The Cretaceous Peedee and Black Creek Formations of the Carolina Coastal Plain are separated by a well-developed ravinement; this ancient example makes it possible to observe the effects of ravinement when operating over a considerable period of time. Sporadic lenses of littoral sand at the Black Creek-Peedee interface indicate that the shoreline advanced in the stepwise manner characteristic of many transgressive shorelines. Many explanations have been offered for this phenomenon; probably no one of them has a monopoly on the truth. It is suggested that the ravinement process may contribute to stepwise migration. Shore face erosion may feed a barrier until the shore face is mantled in its own lag; the barrier is then overstepped and a new shoreline is formed. It has recently been shown that erosion by inlet migration is an important coastal process. If the barriers are retreating landward, an especially severe form of ravinement results.