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

Abstract

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 km 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 m 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 1 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 m km −1 (0.02°) and 0.54 m km −1 (0.03°). 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.