Influence of fluvial processes on the Quaternary geologic framework of the continental shelf, North Carolina, USA

Digital, single-channel, high-resolution seismic reflection profiles were acquired from the insular continental shelf of North Carolina, USA along a data grid extending from Oregon Inlet northward 48 kin to Duck, North Carolina and from the nearshore zone seaward approximately 28 km (total surveyed area= 1334 km(2)). These data were processed and interpreted to delineate principal reflecting horizons and develop a three-dimensional seismic stratigraphic framework for the continental shelf that was compared to stratigraphic data from the shoreward back-barrier (estuarine) and barrier island system. Six principal reflecting horizons (designated R-0 through R-5) were present within the upper 60 in of the shelf stratigraphic succession. Three-dimensional mapping of reflector R-1 demonstrated its origin from fluvial incision of the continental shelf during an episode (or episodes) of lowered sea-level. Fluvial processes during development of reflector R, were responsible for extensive reworking and re-deposition of sediment throughout most of the northern half of the study area. Five seismic stratigraphic units (designated S-1 through S-5) were tentatively correlated with depositional sequences previously identified from the North Carolina back-barrier (estuarine) and barrier island system. These five stratigraphic units span the Quaternary Period (S-1 = early Holocene; S-2 = 51-78 ka; S-3 = 330-530 ka; S-4= 1.1-1.8 Ma; S-5 =earliest Pleistocene). Unit S-1 is composed of fine-grained fluvial/ estuarine sediment that back-filled incised streams during early Holocene sea-level rise. The four other stratigraphic units (S-2-S-5) display tabular depositional geometries, low total relief, and thicken toward the east-southeast as their basal reflectors dip gently between 0.41 in km(-1) (0.02degrees) and 0.54 in km(-1) (0.03degrees). Knowledge of the three-dimensional subsurface stratigraphic architecture of the continental shelf enhances understanding of the development of shelf depositional successions and provides a framework for development of better Quaternary sea-level data, especially offshore North Carolina where such data are sparse. (C) 2002 Elsevier Science B.V. All rights reserved.