QUATERNARY BARRIER ISLAND STRATA: COMPARISON OF ONSHORE GPR AND OFFSHORE SEISMIC PROFILES NEAR CAPE LOOKOUT, NC

Subsurface profiles were surveyed with ground penetrating radar and seismic reflection along adjacent onshore and nearshore transects, respectively, at Bogue and Shackleford Banks, North Carolina. Overall, 31 km of GPR and 58 km of seismic reflection profiles were analyzed for this study. The shore-parallel seismic profiles reveal several paleochannels buried beneath the shoreface and inner shelf of both barrier islands. Based on correlations to stratigraphies published for nearby cores and boreholes, many of the stacked paleochannels observed off central Bogue Banks are Pleistocene fluvial features. Generally the upper flanks of the channels offshore Bogue Banks are truncated at 14 m below sea level or deeper. Although the morphology of central Bogue Banks indicates the presence of at least one Holocene channel (inlet) bisecting the island, young paleochannels were not clearly imaged in the offshore seismic profiles. Nor did the adjacent GPR profiles reveal obvious paleochannel features. The older and deeper paleochannels offshore Bogue Banks were not observed by GPR because that the 50 and 100 MHz GPR signals typically were attenuated at subsurface depths greater than 7.5 m.

A relict flood tidal delta in the backbarrier of Shackleford Banks suggests that a relatively recent Holocene inlet channel bisected the island. We observed a distinct paleochannel with a NW-SE trending axis in seismic profiles off central Shackleford. Reflections marking the flanks of this channel within the shoreface wedge extend upwards to near the seafloor in water depths of 8-10 m, indicating a probable Late Holocene age. The NC Geological Survey obtained a vibracore at the site of this paleochannel in October 2004, but the core has not yet been opened. The Shackleford GPR profiles appeared to image this channel, as well as other possible paleochannels elsewhere along the island. The results of this study indicate that under certain circumstances, depending on the depth and geometry of buried features, ground-penetrating radar provides a complementary technique for mapping the subsurface stratigraphy of barrier islands, and can guide onshore borehole placement targeting features identified in offshore seismic surveys.