BEYOND STORMS AND SEA LEVEL: THE IMPRINTS OF ANTECEDENT GEOLOGY AND SEDIMENT SUPPLY ON THE EVOLUTION OF THE VIRGINIA BARRIER ISLANDS

Whereas storms and relative sea-level rise are two primary drivers of barrier-island evolution over multiple spatial and temporal scales, factors such as antecedent geology and geomorphic feedbacks provide secondary controls on the rates of island migration, erosion, and progradation. This study uses stratigraphic, geomorphic, and chronologic data, combined with morphodynamic modelling, from Cedar and Parramore islands (Virginia, USA) to explore the roles of antecedent substrate and sediment fluxes on barrier-island evolution during the late Holocene, and on their future trajectories. Fifty-five sediment cores (7–25 m long) collected along four shore-perpendicular and two shore-parallel transects provide stratigraphic data and field-derived inputs to a cross-shore coupled barrier-marsh-lagoon morphodynamic model. Pleistocene substrate underlying the modern barrier system ranges from stiff blue-green clay, to interbedded clayey fine sand (4.1­–3.4 phi), to coarse sand (2.5–1.6 phi) and pebbles, with most units capped by organic-rich marsh or upland soil deposited during Holocene transgression. Parts of both Cedar and Parramore islands are situated above, and likely “pinned” by, sandy Pleistocene highs, interpreted as barrier islands formed during a former highstand. These features controlled backbarrier accommodation during landward island migration and, during periods of island breaching, acted as a sediment source. Sandy Holocene backbarrier deposits such as washover and flood-tidal delta deposits and relict (350–1000 years ago) 1.5–3 m high foredune ridges behind both islands provide a secondary sand reservoir, available for excavation by waves as the islands continue to erode and migrate. Model simulations indicate a likely period of 1000 years of slowed island migration from the combined effects of antecedent slope and enhanced sediment fluxes associated with substrate erosion. In addition, fluctuations in longshore sediment supply associated with the growth of updrift sediment reservoirs, which influenced past behaviors of Parramore Island, will continue to play a central role in island stability. Together, these data emphasize the role of sediment derived from both longshore and substrate sources in barrier-island response to the forcings from storms and sea-level rise.