SEDIMENTOLOGICAL EVIDENCE FOR FORMATION OF A CHINCOTEAGUE BAY PROMONTORY SALT MARSH IN GREENBACKVILLE, VA LINKED TO LOCAL SEA LEVEL FLUCTUATIONS WITHIN THE LAST 3000 YEARS

Rates of sea level rise slowed between 2000 and 3000 cal yrs BP, establishing the modern shoreline position and the system of coevolving barrier islands, bays, and estuaries found in the Mid-Atlantic. Rather than forming in embayments, many salt marshes in Chincoteague Bay formed promontories. Despite their prominence, little is known about the origin and geologic history of these promontory marshes. We present the results from radiocarbon dating and lithological analysis from 19 cores (2 - 3 meters long) collected in 2014 and 2015 from the modern salt marsh in Greenbackville, VA and extending across Chincoteague Bay. Facies identified in these cores include bedded and massive medium to fine-grained quartz sand, dark marine silt/clay, and organic-rich clay and peat. Facies analysis shows that a sandy coastal plain existed in this area prior to 1800 cal yrs BP. Bay muds were deposited on top of the seaward portion (~230 m) of the sandy coastal plain. This indicates that a local transgression occurred caused by sea level rise or coastal plain subsidence of >1 m. A similar sea level transgression has been documented at ~2000 cal yrs BP along the Delaware coast. Dates within the lower portion of the bay mud facies fall within 900-1050 cal yrs BP, however we have no age control for the upper limit of the transgressive interval. Sometime after 900 cal yrs BP, the bay mud facies was covered by increasingly shallower water and salt marsh facies of organic-rich clay and peat. Salt marsh progradation may be attributed to sea level regression or alternatively a local increase in sediment runoff and/or decreased wave energy after the establishment of ancestral barrier islands. Thus the formation of bay promontory salt marshes in Chincoteague Bay and likely others in the Mid-Atlantic is tied to localized sea level change over the last 3000 years. By implication, modern salt marshes in the Delmarva region are extremely sensitive to sea level rise and are vulnerable to erosion and retreat from the combined effects of glacial isostatic subsidence and global sea level transgression.