GEOPHYSICAL AND STRATIGRAPHIC INVESTIGATION OF ASSATEAGUE ISLAND, VIRGINIA: UNRAVELING THE EVOLUTION OF THE ONLY MID-ATLANTIC DUPLEXED BARRIER ISLAND SYSTEM

Barrier Islands are dynamic coastal features that contain important habitats and directly protect the mainland and human-built assets from coastal hazards. Assateague and Chincoteague Islands (AI, CI), like adjacent barrier islands of the Mid-Atlantic, are subject to severe coastal erosion and barrier island migration. However, unlike all other barrier islands in the Mid-Atlantic Bight (MA to NC) these form a unique, duplexed island system that is not well understood. Ongoing research is aimed at resolving the evolution of these islands so that more robust models can be developed. Here we present analysis of two Ground Penetrating Radar (GPR) surveys (2016 & 2017) which have been correlated with 9, 1-3 m-long sediment cores collected on AI. These data, further constrained by radiocarbon dates, help to establish the timing of morphologic change.

The 2016 survey imaged the subsurface of the northern portion of North Wash Flats (NWF), a large paleoinlet, and the recurved spit of an accretion mound, just east of the northern tip of CI. Nautical charts show that this was the southern end of AI prior to the late-1700s. It is therefore a critical location to investigate how and when AI began to duplex CI. Based on core stratigraphy we interpret the structures in our radargram data to be the result of a multi-stage process of inlet migration. Patterns in GPR anomalies are interpreted as cross-bedded sands and migrating ripple structures. NW to SE downlapping clinoforms suggest overwash fans helped stabilize the NWF area. We interpret several narrow cut and fill channel features as tidal channels likely formed during intermittent breach events.

The 2017 GPR survey, ~0.75-2.25 km N of NWF, was located within a forested strand plain. At this location, low linear dunes and swales are clearly visible. Pronounced cross-bedding visible in the radargrams is indicative of dunes forming linear beach ridges. Located on the NE side of the AI-CI inlet, these ridges formed prior to the extension of AI in front of CI.

Investigating the stratigraphy and geomorphology of AI will allow us to contribute to models of duplexed barrier island formation.These models will ultimately provide valuable information to help constrain future forecasts for AI and CI as sea level rise and increased storm intensity continue to impact the region.