INVESTIGATING BARRIER ISLAND EVOLUTION THROUGH STRATIGRAPHIC ANALYSIS OF VIBRACORES FROM ASSATEAGUE AND CHINCOTEAGUE ISLANDS, VA

Centered between the Delaware and Chesapeake Bays, Chincoteague (CI) and Assateague (AI) form a duplexed barrier island system along the Delmarva Peninsula. These islands are unique as they are the only example of a duplexed system in the Mid-Atlantic Bight. As such standard models of barrier island development may not be useful in projecting future change on these islands.

Here we present analysis of 14 vibracores (10 from AI, 1 from CI, and 3 from Chincoteague Bay (CB)) that record the duplexing of AI and CI. We identified 6 different stratigraphic units on the basis of sediment composition, grain size, and % organic matter. We also extracted material for 9 radiocarbon dates. Our data provide evidence for AI migration and inlet breaching. The core extracted from CI consisted of medium-grained quartz sands that fine upward into silts (dated to 1459 AD) and eventually bay muds. These low-energy deposits were topped by modern salt marsh sediment as AI accreted southward during duplexing. On AI (just E of the N tip of CI) cores were extracted from the recurved spit at the N edge of a large paleoinlet. Nautical charts show this to be near the S end of AI prior to the late-1700s. These cores are characterized by organic mica-rich bay bottom sandy silts and contain shells dated at 1266-1865 AD. These are overlain by bedded medium-grained quartz sands and indicate inlet closure/migration followed by overwash. Evidence for storm overwash or island breaching is also found in the CB cores, which show coarse, massive sands (1856 AD) deposited over bay mud. Subsequent island stabilization is indicated by organic-rich salt marsh facies at the top of the cores.

The dating of environmental changes recorded in these cores provides constraints for development of a model for the growth of AI as it elongated nearly 17 km to the S to complete the modern duplexed barrier system. Duplexing of CI initiated prior to the late 1700s as sediment supply increased dramatically. In the last 8 decades alone, south AI has elongated nearly 6 km at the expense of northern AI near Ocean City, MD. With the extension of Fishing Point, AI appears to be preparing to duplex Wallops Island to the S. Evidence from the modern extension of the island suggests that land use change around the industrial revolution may have contributed to the initiation of duplexing.