GEOLOGY AND ORIGIN OF SAND RIDGES IN A BACK-BARRIER ESTUARY, PAMLICO SOUND, NC

The Outer Banks barrier islands of North Carolina have undergone significant geomorphic change, exhibiting varying degrees of continuity throughout the late Holocene. These changes affect environmental conditions (salinity, tidal and wave energy, currents, etc.) in Pamlico Sound, the estuarine system behind the Outer Banks, south of Roanoke Island. The current estuarine system is characterized by minimal tidal energy (tidal range of approximately 10 cm), and limited exchange with the marine environment through three inlets, resulting in the accumulation of organic-rich muds in the basin, containing mid- to high-salinity estuarine foraminifera. However, a sand ridge field occurs in eastern Pamlico Sound basin at 3.5 m to 6 m below sea level, suggesting more energetic hydrodynamic conditions in the past. The sand ridge field consists of 9 ridges oriented perpendicular to the barrier islands, and extending up to 10 km into the basin. The two southeastern ridges, which are approximately 7 km in length, are characterized using eight vibracores, two gravity cores, and 58 km of chirp sub-bottom profiler data. To understand the geologic history and origin of this sand ridge field, the cored sediments have been analyzed for sedimentology, foraminiferal assemblages, bulk magnetic susceptibility, and geochronology. Chirp seismic data have been examined to understand the dimensions and stratigraphy of the stratigraphic unit that includes sand ridge sediments. Vibracores reveal that the sand ridge sediments are coarse to fine quartz sands that generally lack mud. Average ridge sand grain size is greater farther into Pamlico Sound (i.e., distal to the barrier islands) suggesting that the barrier islands are not the source of the sand. Foraminiferal assemblages are typical of mid- to high- salinity estuarine environments and are dominated by Elphidium excavatum. Radiocarbon ages suggest that the sand ridges formed no earlier than approximately 2000 yrs BP. Possible explanations for the ridges include: 1) tidal deposits during a phase of greater tidal influence; 2) major storm deposits; 3) reworked Pleistocene interfluves.