ORIGIN, EVOLUTION, AND DEMISE OF THE DOUBLE BARRIER ISLAND SYSTEM OF NORTHEASTERN NORTH CAROLINA’S OUTER BANKS

Hatteras Flats is a broad, shallow-shoal system in eastern Pamlico Sound, NC. It is perched on top of a major Pleistocene interstream divide separating the now flooded Pamlico Creek paleodrainage basin from its offshore counterpart. Hatteras Flats ranges from 1 to 5 km in width and contains two barrier island systems. The larger-scale Outer Banks barrier island system formed on the eastern flank of Hatteras Flats in response to oceanic processes, while the smaller-scale Pamlico barrier island system formed on the western flank in response to estuarine dynamics of Pamlico Sound. A shallow central basin runs irregularly and discontinuously down the middle of Hatteras Flats with <1 to 2 m water depths. Deeper channels (2 to 3 m) flow out of the central basin and often consist of multiple tributary or distributary channels related to either the pre-flooding drainage systems or subsequent inlets and associated flood-tide deltas. These now drowned submarine channels still control water flow onto and off Hatteras Flats.

During lower stages of Holocene sea-level, the flooding shoreline occurred around the subaerial ridge of the interstream divide as a strandplain beach. This shoreline eroded wave-cut platforms and scarps into the Pleistocene sediment units, producing sand for the associated beaches. As rising sea level overtopped the interstream divide during the late Holocene, two barrier island systems formed along the eastern and western perimeters of the broad ridge. During the last several hundred years of sea-level rise, most of the smaller-scale, sediment-poor Pamlico barrier islands have systematically collapsed into a broad system of intertidal and subtidal shoals. Cedar Island has adequate sediment sources and is the only surviving and healthy island of the Pamlico barrier system. Bird and Gull islands are in their final stages of collapse. The smaller-scale Pamlico barrier system serves as a model for the future evolution of the larger-scale Outer Banks barrier system, which already contains island segments in similar stages of collapse.