GEOMORPHIC LINKAGES BETWEEN TRANSGRESSING SIMPLE BARRIER ISLANDS AND BACK-BARRIER FLATS IN RESPONSE TO CLIMATE CHANGE, NORTH CAROLINA OUTER BANKS

Barrier island segments within North Carolina’s Outer Banks are divided into two geomorphic types: simple and complex based upon the sediment supply, physical dynamics, and evolutionary history of the islands. Simple island segments are sediment poor resulting in low and narrow barriers dominated by storm-driven interaction between inlet flood-tide deltas (FTDs) and over-wash ramps (OWRs). About 75% of the 280 km-long Outer Banks are simple barrier islands with all geomorphic elements having formed within the last 500 years. Sediment-rich complex barrier islands are high and wide due to substantial sediment input during their evolutionary history. They are characterized by a modern beach that has welded onto an older barrier island segment. The older segments are dominated by multiple sets of beach ridges and back-barrier dune fields that date from about 3,000 years ago to the present. Since most complex islands have little potential for the formation of inlets and/or oceanic overwash, salt influence is minimized allowing for development of extensive maritime forests.

The dominance of transgressive simple barrier islands on the Outer Banks has resulted in the deposition of a broad (1.5 to 8 km wide) and shallow (<1 m deep) back-barrier shoal system known as Hatteras Flats. This shoal system consists of submerged paleo-FTDs and associated tidal channels that are being buried along the barrier island side by OWRs of the transgressing islands. OWR-dominated barrier islands slope from the island berm downward to the back-barrier estuarine shoreline. Lower portions of the OWR grade from the supra-tidal zone dominated by microbial mats and/or interior marshes, to the inter-tidal zone dominated by salt marshes. The shore-parallel, back-barrier platform marshes are split by tidal channels to form the classic molar-tooth structures. The back-barrier platform marshes transition into the Hatteras Flats where the sub-tidal estuarine zone is bound by microbial mats and the shallow submarine estuarine zone is dominated by submerged aquatic vegetation. Thus, the FTD-dominated Hatteras Flats form a stabilized shallow base that is the future foundation upon which the sub-aerial portion of the barrier island will migrate onto as sea-level rise and ocean shoreline recession continues.