LATE HOLOCENE EVOLUTION OF A SAND SINK ON THE NORTHERN VIRGINIA COAST: IMPLICATIONS FOR REGIONAL SEDIMENT TRANSPORT

Abstract

Globally, beach- and foredune- ridge plains may archive coastal evolutionary processes, autogenic shoreline behavior, sea-level change, human coastal impacts, changing allogenic climate forcings, and time-varying sediment fluxes. The focus of this study is on the latter record. We pair coastal mapping (e.g., stratigraphic architecture, sedimentology, and morphology) and shoreline geochronology of beach and foredune ridges on Assateague Island, Chincoteague Island, and northernmost Wallops Island (ACW barriers) to reconstruct the late-Holocene evolution of a coastal sediment sink on the Virginia coast. The ACW barriers trapped 216 million cubic meters of sand through the growth of beach and foredune ridges over the last ~2000 years. From 360 to 190 years ago, sand sequestration in updrift flood-tidal deltas reduced sediment fluxes to the ACW barriers, halting island progradation and resulting in the formation of an 8+ m tall foredune ridge on Assateague Island. Progradation/elongation of southern Assateague Island resumed ~190 years ago and has since trapped an of annual average of 681,000 cubic meters of sand. Fluxes of sand into northern Wallops and Assateague islands are at least 60% of estimated regional longshore transport rates. We propose that the development of the ACW barrier system longshore sediment trap and associated wave refraction are two important drivers of downdrift barrier island change on the Virginia coast. This study emphasizes the important controls of tidal inlet sand sequestration and sediment trapping through barrier island progradation on longshore sediment fluxes along barrier coasts.