Investigating the Initiation of Duplexing of the Mid-Atlantic's Chincoteague-Assateague, Virginia Barrier Island System using Vibracore and GPR Records

Barrier islands (BI) like Chincoteague (CI) and Assateague (AI), located between the Delaware and Chesapeake Bays, are important, dynamic coastal features. They are also extremely vulnerable to short-term and long-term effects of storms and SLR. AI and CI are unique in that they are the only example of overlapping (duplexed) BIs on the eastern seaboard of the U.S. They are also heavily impacted by hard and soft shoreline engineering projects including those at Ocean City, MD and Wallops Island.

The goal of our research is to use sediment core and geophysical data to study environmental change and island migration on AI and CI and contribute to development of refined models for the formation and evolution of this duplexed BI system. Here we reconstruct a >300 yr history of a duplexed section of AI using sedimentological analysis of a vibracore, LOI, AMS 14C dating, paleomaps, ground-penetrating radar (GPR), and magnetic susceptibility analysis.

The 2.3 m long AI vibracore was collected from the modern salt marsh, at the eastern edge of Chincoteague Bay. Two GPR transects were run perpendicular to the long-axis of the island over the core site. The core consists of basal beach sand facies that transition up into sandy tidal channel muds, and then fine bay muds. The muds have a 14C age at 1.45 m of 534 (±22) BP, 1772 cal. AD, indicating inlet closure by this time. The bay muds are topped by bedded quartz sand and storm deposits from high energy, wave-influenced conditions interpreted to be a return to a beach environment. In this unit, the GPR data show a number of anomalies interpreted as channels, dune structures, and migrating bars with downlap. These data combined with historic maps of AI from 1700 and 1859 confirm inlet closure as well as significant BI rollover. At 27 cm depth the core is topped by organic-rich, salt marsh facies deposited as the island accreted eastward and stabilized after 1859.

Our study indicates that duplexing of CI initiated prior to the late 1700s as sediment supply increased dramatically. In the last 8 decades, south AI has elongated nearly 6 km at the expense of north AI near Ocean City, MD. AI is approaching overlap with Wallops Island. Evidence from the modern extension of the island suggests that land use change around the industrial revolution may have contributed to the initiation of duplexin