INVESTIGATING OVERWASH AND HISTORIC BARRIER ISLAND INLET FORMATION THROUGH STRATIGRAPHIC ANALYSIS OF VIBRACORES FROM CHINCOTEAGUE BAY, VA

Sedimentary records from barrier islands and back bays document coastal evolution and contain a valuable history of sea level change and the timing/location of island breaching events. Chincoteague Bay (CB) is a shallow (1-3 m depth) back bay located between the Delmarva mainland coast and the unique double barrier island system which includes Assateague and Chincoteague. In 2014 and 2015 we collected five vibracores from central and eastern CB and on Wildcat Point (WCP), a salt marsh on the northern tip of Chincoteague Island.

The sediments in the bay cores consist primarily of fine bay muds and silts which overlie massive quartz sands, all with <10% organic matter. The sands form a sharp contact with the overlying silt/clay. A burrow in the sand layer, infilled with bay mud not present in the core, suggests these transitions are erosional disconformities. Finer-grained silty sand forms the modern sediment-water interface in all but one of the bay cores. Recurved paleoshorelines on Assateague Island near our core transect suggest that sand deposits extending over 6 km into the bay may be related to a former inlet channel. Historic maps from 1855 indicate the presence of temporary inlets in this region. A radiocarbon date extracted at 31 cm depth within a 10-cm layer of non-bedded quartz sand (overwash) returns a median age of 1856 AD. Truncation of paleoshorelines on Chincoteague south of WCP suggest that the modern salt marsh was a flood tidal delta which welded onto the island. The core extracted from WCP resembles cores from Greenbackville Salt Marsh on the mainland side of CB. Peats and organic-rich clays overlie quartz sands. Further analysis of the 2015 cores will include loss on ignition, radiocarbon dating, macro and micro-fossil identification, and grain size analysis, along with additional core sampling on Assateague Island and WCP. The data will allow us to better model the formation and migration of these barrier islands and provide valuable information to coastal planners on the vulnerability of Assateague Island in light of global sea level rise and increased storm intensity.