OVERWASH DEPOSITS ON HATTERAS AND OCRACOKE ISLANDS, NC, CAPE HATTERAS NATIONAL SEASHORE – A SEDIMENTARY RECORD OF TROPICAL AND EXTRATROPICAL STORMS ALONG THE ATLANTIC COAST AND VARIATION WITH SHORELINE ORIENTATION

Overwash depositional events north and south of Cape Hatteras, N.C. are strongly influenced by differences in shoreline orientation and storm type. The coastline south of Cape Hatteras faces southeast and is mainly impacted by tropical storms. In contrast, the east-facing coastline north of the Cape is impacted mainly by extratropical storms.

Analysis of vertical aerial photographs taken after Hurricane Isabel (Sept. 2003) reveals a broader extent of overwash south of Cape Hatteras versus north. For every 1 km of coast along Ocracoke Island, 1.1 x 10⁵ m² of overwash was deposited, whereas only about 4.0 x 10⁴ m² of overwash was deposited per km of coast north of the Cape. Additionally, topographical surveys reveal that Isabel-induced overwash fans along Ocracoke Island are as much as 4x greater in area (17,000-19,000 m²) than some of the largest Isabel overwash fans north of Cape Hatteras (~5,000 m²).

Eight sediment cores from two fans along Ocracoke Island suggest that less frequent, but larger-magnitude overwash events characterize this area. The deposits reveal a 35-50 cm thick uppermost layer of medium-grained sand deposited by overwash during Hurricane Isabel, which directly overlies an organic-rich mud-silt layer interpreted as the product of prolonged marsh deposition. By comparison, ten sediment cores from three overwash fans north of Cape Hatteras show a complex stratigraphy characterized by numerous 10-30 cm thick, very coarse to fine-grained sand units. The abundance of individual sedimentation units, combined with the lack of marsh deposits, points to a pattern of frequent, small-scale overwash deposition.

GPR data shows a detailed layering of subsurface sediment within overwash fans north of Cape Hatteras. Buried channels, some of which have been infilled, are seen in transverse profiles and lagoonward onlap is observed in a longitudinal profile. In contrast, profiles south of the Cape show a lack of reflective layers that are attributed to the highly conductive marshy layer close to the surface which limits radar penetration depth. This knowledge of depositional variations relative to storm type and shoreline orientation will hopefully be beneficial for future conservation, long-term hazard management, and ecological management of the national seashore.