Southeastern Section - 58th Annual Meeting (12-13 March 2009)

Paper No. 2
Presentation Time: 8:50 AM


RIGGS, Stanley R. and AMES, Dorothea V., Department of Geological Sciences, East Carolina University, Greenville, NC 27858,

The Onlsow Bay coastal compartment (OBCC) extends from Cape Lookout to Cape Fear, NC and is structurally controlled by the basement Carolina Platform High. The OBCC is dominated by Tertiary units producing a wide, shallow, rock-floored continental shelf. The Quaternary record is minimal with thin Holocene barrier island deposits perched on top of and in front of older units of the shallow shelf and steep mainland land mass. Three barrier island types characterize the OBCC: 1) sand-rich, complex islands (Shackelford and Bogue banks); 2) sand-poor, simple islands (Topsail, Figure 8, Wrightsville, Masonboro islands); and 3) strandplain beaches perched on headlands (Onslow, Carolina, Kure beaches).

Sediment sources available to build and maintain the modern barrier island system include: 1) shoreface and inner shelf stratigraphic units dominated by Tertiary rock units; 2) paleo-river valley fill and deltaic deposits dominated by small coastal plain streams; 3) modern inlets and their ebb-tide and flood-tide deltas; and 4) cape shoal structures (Cape Lookout and Frying Pan shoals). The first two sources are inconsequential to the regional sediment budget. Sediment dynamics of modern inlets are highly modified and controversial involving sand mining, stabilization, and channel re-alignment and enlargement. At regional scales, the cape shoal deposits were important for the initial formation and long-term maintenance of the OBCC barrier islands. Human modification of most inlets over the past century (dredging larger and deeper navigational channels, discharging dredged sediments onto the continental shelf, mining ebb-tide deltas, and stabilizing with jetties) has short-circuited the regional-scale sediment dynamics of cape shoal sands. Consequently, ocean shorelines on all OBCC barrier islands are severely receding with most island communities wanting nourishment sand to hold the line in a rising sea level. In spite of vast sand supplies in the cape shoals, they are not fully understood and probably are environmentally problematic and economically unavailable for future beach nourishment.