HOLOCENE BACKBARRIER DEVELOPMENT IN RESPONSE TO SEA-LEVEL RISE, ANTECEDENT TOPOGRAPHY, AND BARRIER GEOMORPHIC CHANGE: PARRAMORE AND CEDAR ISLANDS, VIRGINIA’S EASTERN SHORE

The Virginia barrier islands face the highest rate of sea-level rise on the US East Coast (~5 mm/yr) and receive limited sediment from updrift coasts. System-wide rates of shoreline retreat have averaged > 5 m/yr since ca. 1850 AD, driven by a combination of erosion and barrier migration, together threatening backbarrier marshes. This study seeks to refine our knowledge of this system through an investigation of backbarrier stratigraphic records from one island experiencing rapid shore-parallel retreat (Cedar Island) and a second (Parramore Island) that is undergoing erosion, but is not migrating. Four cross-shore transects of 9-m long vibracores from the backbarrier marshes and tidal flats of both islands provide insights into the antecedent topography over which these barriers transgress and the depositional history of the system. The base of the backbarrier sedimentary sequence is composed of a spatially heterogenous silty-clay to silty-sand. Behind northern Parramore, the top of this unit is located 5-8 m below the surface and it generally coarsens from east to west; the finer constituent is entirely absent behind southern Parramore. Basal backbarrier deposits behind Cedar Island are more complex: sand (6-8 m deep) dominates proximal to the barrier, whereas the central backbarrier is mud-dominated throughout the upper 9 m. Shallow stratigraphy is more homogenous throughout the system: rounded gravel is common 2-m deep proximal to the mainland, while elsewhere basal backbarrier units are consistently overlain by silt and clay units of variable thickness and topped by a modern marsh unit of < 1.5 m thick. A single radiocarbon date of 360 ± 103 years BP from the basal section of the marsh behind Parramore Island confirms the young age of this marsh. These stratigraphic and radiocarbon data confirm earlier studies that indicate a shift over the late Holocene – possibly driven by barrier island migration – from a high-energy backbarrier environment in which only sand could be deposited to a more quiescent period, allowing for the development of extensive marsh. Moreover, this spatially heterogeneous stratigraphy highlights the potential role of antecedent topography in dictating the rate of this migration, as well as the sediment types excavated, and possibly recycled into the backbarrier, through shoreface erosion.