EVALUATION OF LATE PLEISTOCENE TO HOLOCENE SEDIMENTATION IN GREENBACKVILLE, CHINCOTEAGUE BAY, VA: IMPLICATIONS FOR SEA LEVEL CHANGE AND BARRIER ISLAND EVOLUTION
The lowest sediments (Unit 1) contain organic-rich sandy silt and clay, likely estuarine deposits. With ages ranging from ~46 to 28 ka cal BP, these sediments were deposited during the Marine Isotope Stage 3 (MIS-3) highstand. Unit 1 eroded during the sea level lowstand of MIS-2, the last glacial maximum (LGM), thus forming a discontinuity. Sedimentation resumed post-LGM and Unit 2 was deposited during SLR as a massive-bedded medium to fine quartz sand with lenses of sandy silt. The upper portion of Unit 2 dates to 1841 yr cal BP. Unit 3 contains blue-grey mud and overlies Unit 2 seaward (~230 m) of the modern coastal plain. The sand-mud contact dates to ~1000 yr cal BP. Landward of Unit 3 muds, organic-rich clays and peat overlay Unit 2 sands, and these appear to onlap upslope to the west.
The coarse sediments of Unit 2 indicate deposition in a high-energy coastal environment. This suggests that the Chincoteague and Assateague barrier islands had not yet completely formed prior to ~1800 yr cal BP. Regional sea level curves suggest the rate of SLR slowed before 2000 yr cal BP, and allowed barrier island development several km to the east of their current locations. By ~1000 yr cal BP a substantial decrease in energy regime is noted by deposition of Unit 3. As bay muds accumulated, tidal flats developed the platform on which the modern marsh (Unit 4) grew outward into the bay. The modern marsh edge is experiencing substantial erosion rates on the order of meters/year.