Evolution of a shoal retreat massif, North Carolina Shelf: Inferences from areal geology

Abstract

The Albemarle Shelf Valley Complex on the northern North Carolina Shelf is a leveed shelf-floor channel trending seaward from the modern Albemarle estuary. Vibracores and seismic profiles indicate that the levees have a twofold internal structure. Levee cores consist of fine sand interbedded with mud lenses. The cores are mantled with a generally coarser, cleaner sand. The levees are interpreted as shoal retreat massifs; deposits formed as the littoral-drift depositional centers on either side of the ancestral Albemarle estuary retreated landward during the Holocene transgression. The shelf-valley complex underwent systematic changes in bottom morphology and sediment character as the reversing tidal jet of the estuary mouth was replaced by the intermittent south-trending storm flows of the open shelf. A pattern of ridges and troughs with 10 m of relief was incised into north sides of the massifs flanking the shelf valley. The coarser surficial sand into which the bedform pattern has been impressed appears to be a lag deposit formed by the winnowing of the massifs during storm flow. On ridge crests this surficial sand sheet has been molded into sand waves up to 3 m high. The relative orientations of sand waves, ridges, and the shoreline indicate that the structure of the velocity field during peak flows must be quite complex. Ridges converge southward with the shoreline at angles of 15–20°, while the sand waves atop them make angles of about 75° with the shoreline; the sand waves are thus neither orthogonal with respect to the shoreline nor with respect to the ridges. Sand-wave orientation indicates southward bottom flow with an offshore component. Existing hydrodynamical theory does not adequately explain this pattern. Extensive hydraulic measurements of careful design will be required so we can interpret the bedform pattern and its role in the coastal sand budget.