THE SEISMIC STRATIGRAPHIC FRAMEWORK OF EASTERN PAMLICO SOUND, NORTH CAROLINA: IMPLICATIONS FOR ANTECEDENT CONTROLS ON COASTAL GEOMORPHOLOGY

Geophysical surveys (single channel seismic and chirp sonar) conducted in the back-barrier estuarine environment of Pamlico Sound in North Carolina allow the regional geologic framework to be mapped in detail and related to the geomorphology of the modern coastal system. Within the study area, a discontinuous basal reflection marks the acoustic basement at ~100 ms (75 to 85 m below sea level) in the north, and rises to the south to a depth of ~50 ms (~37 mbsl) beneath Ocracoke Inlet. Based on correlations to boreholes in the area, this basal reflection is interpreted as the contact between Lower Pliocene and Upper Pliocene to Recent beds. Multiple Quaternary seismic sequences bounded by high amplitude reflections and exhibiting extremely complex incised channel-fill facies occur throughout the area. Based on the abundant incised channel facies, it is clear that the seismic sequence boundaries represent depositional sequence boundaries. At least 15 depositional sequences are evident in the upper 80 ms (~60 m) of data. In northern Pamlico Sound, seismic data reveal depositional sequences that dip gently southward into the Albemarle Embayment. In the central study area (between Salvo and Avon), beds are nearly horizontal. To the south, in the Cape Hatteras area, beds dip northward and are controlled by the presence of an antecedent high that is approximately 30 meters thick and 20-30 km wide. Major subsurface features include an antecedent high beneath Buxton and Hatteras; a large valley-fill complex (40 km wide by 30 m deep) beneath Salvo to Avon; several intermediate-scale (5 km wide by 15 m deep) channel complexes that appear to have been reoccupied over several successive sea-level cycles; and many small incised channels. Ocracoke and Hatteras Inlets appear to occupy fluvial channels incised to depths of at least 30 mbsl. The present location of Oregon Inlet does not appear to occupy a fluvial channel. Data indicate strong ties between the modern coastal geomorphology (cape and shoal structures and inlet locations and stability) and the antecedent geology.