GEOMORPHOLOGY AND EVOLUTION OF THE SMITH ISLAND COMPLEX, SOUTHEASTERN NC

The Smith Island Complex is located adjacent to the sub-aqueous Frying Pan Shoals and the associated subaerial cape system, Cape Fear, of southeastern North Carolina, at the southernmost point of Onslow Bay. The complex consists of a series of east-west trending relict beach ridges (including Bluff Island, Middle Island, and Bald Head Island) that are separated by wide salt marshes and tidal creeks. The geomorphology of the Smith Island Complex is controlled by a combination of sea level, wave/tidal energy, and sediment supply. Variations in these factors have led to the deposition of distinct lithofacies within the complex that represent shoreface, eolian, estuarine, marsh, and tidal channel environments. This study examines stratigraphic relationships between these lithofacies in nine vibracores collected from off-ridge marsh sites around the island complex and shallow cores from the beach ridges. The core data is supplemented by topographic and LiDAR (light detection and ranging) elevation control as well as age dates from limited radiocarbon and OSL measurements; these results were correlated with other studies of Late Holocene coastal evolution in this region.

The general geomorphology of the Smith Island Complex is characterized by the southward progradation of beach ridges over the last >4,000 years. The beach ridges were likely built during periods of relative sea level stability while the inter-ridge marsh areas are likely associated with barrier island spit accretion farther to the south. Radiocarbon ages from intertidal and sub-tidal environments allow an evaluation of the rates of salt marsh accretion in response to sea level rise. Accretion rates range between 1.02 mm/yr (3,400 years before present) and 2.94 mm/yr (870 years before present). These results are similar to accumulation rates from other studies of the North Carolina coast.