Anatomy of a shore face ridge system, False Cape, Virginia

Abstract

The Middle Atlantic Shelf of North America is a broad sand plain, characterized by a subdued ridge and swale topography. Some ridges extend into or merge with the shore face. Three such ridges at False Cape, Virginia, trending southwest, have second order ridges on their flanks. Systematic asymmetry of secondary ridge crests away from major troughs, toward major ridges, indicates that the ridges are maintained by coast-parallel currents forming a pattern of helicoidal flow cells. Median diameters decrease to the southwest along the ridge system. In addition, grain size varies with topography. Troughs tend to be floored with a primary lag deposit of coarse, pebbly sand. Wave-winnowed crests consist of a secondary lag deposit of well-sorted, medium- to fine-grained sand. Flanks are fine- to very fine-grained sand winnowed out of the crests. Vibracores and pinger probe records reveal a three-fold stratigraphy. A basal unit consists of clayey fine-grained sand of probable Late Sangamon-Early Wisconsin age. An intermediate unit of relatively watery mud has yielded a radiocarbon date of 25,700 ± 800. The uppermost unit is the modern sand sheet into which the ridges are molded. The innermost ridge and trough appear to be actively forming in response to southtrending coastal “currents,” and the shelf tide during tropical and extra-tropical storms. The ridges appear to be moving westward, but it is not clear whether their longterm movement is more or less rapid than the rate at which the beach is retrograding. In any case, Bruun coastal retreat appears to be occurring, with sea floor aggradation at the expense of the adjacent shore face. In this manner the retreating coast is generating a shelf “relict” sand blanket with a ridge and swale topography. Coastal sediment movement has a southward component for fine sand through the ridge system, and may have, for coarser sand, a return north along the beach, by littoral drift. The ridge system may be moving south in response to headward erosion of the troughs.