INVESTIGATING THE DEVELOPMENT OF A BAY PROMONTORY SALT MARSH IN GREENBACKVILLE, VA THROUGH VIBRACORING

As sea level rose during the Holocene, a system of barrier islands, bays, and estuaries developed along the Mid-Atlantic coastline. Salt marches formed in the brackish mixing zones adjacent to the mainland. These marshes commonly form in embayments, but in Chincoteague Bay many have formed points extending into the bay. Despite their prominence on the shoreline, the origin and geologic history of these promontory marshes remain enigmatic. We are investigating the formation of bay promontory salt marshes by interpreting the stratigraphy of sediment cores collected by vibracore in Greenbackville, VA. In 2014, we collected a series of cores, ~2-3 m-long, at 13 sites from the modern salt marsh and at three sites extending across Chincoteague Bay. We classified core stratigraphy into three units, each representing different depositional environments, by comparing sediment composition, grain size, color, and percent organic matter. Unit 1, the deepest unit, consists of quartz sand (0-2 phi) and sandy silt, which we interpret to be a coastal plain deposit. Unit 2 is composed of silt and clay-sized particles suggesting a marine back bay environment. This unit is subdivided into moderate (10-25%) and low (<10%) organic content and is comparable to the fine-grained sediments sampled from the bay cores. Unit 3, the uppermost unit, is composed of an organic-rich clay and peat mixture deposited in the modern salt marsh. Cores VC 6 and 13, ~30-120 m from the present shoreline, contain all three units. Core stratigraphy suggests that a local transgression occurred due to sea level rise or coastal plain subsidence, depositing bay sediment over the seaward portion of the modern coastal plain. The most distal cores, VC 3 and 9, located 250-400 m from the modern shoreline, lack the bay sediments of Unit 2, suggesting a shoreward limit to the transgression. Post-transgression, the marsh began to prograde into the bay, possibly due to an increase in sediment runoff and/or lessened wave energy after the establishment of a barrier island. Our preliminary results show that the coastal plain accreted at least 170 m. Future work will include radiocarbon dating to determine sedimentation rates and establish age control. Our ultimate goal is to provide a model for formation of bay promontory salt marshes in Chincoteague and other bays in the Mid-Atlantic.