DEFINING THE ORIGIN AND GEOLOGY OF AN ESTUARINE SAND RIDGE FIELD, PAMLICO SOUND, NC, USA

The Outer Banks of North Carolina have undergone significant geomorphic change, exhibiting varying degrees of barrier island continuity during 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. The modern 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 at approximately 4 to 5 m below sea level in the eastern Pamlico Sound basin, suggesting different hydrodynamic conditions at some time in the past. The sand ridge field extends up to 10 km into the basin, and is oriented perpendicular to the barrier islands. Previous paleoenvironmental work suggests the sand ridges were deposited under high salinity conditions. Defining the mechanism of formation of this sand ridge field will assist in the understanding of the geological evolution of the region. Ten vibracores and 72 km of chirp sub-bottom profiler data have been acquired in order to characterize this system. To understand the geologic history and origin of this sand ridge field, the cored sediments are being analyzed for sedimentology, foraminiferal assemblages, bulk magnetic susceptibility, and geochronology. Chirp seismic data are also being examined to understand the dimensions of the unit that includes the sand ridges. Vibracores reveal that the sand ridge sediments generally lack mud, and have a greater average grain size farther into Pamlico Sound (i.e., distal to the barrier islands) suggesting that the barrier islands are not the source of the sand. Possible explanations for the ridges include: 1) tidal deposits during a phase of greater tidal influence; 2) major storm deposits; 3) reworked Pleistocene interfluves.