GEOLOGIC MAPPING OF A MUDDY SHOREFACE: OAK ISLAND, NORTH CAROLINA

Oak Island is a 22km long eroding shoreline segment located in the extreme eastern part of Long Bay on the low energy flank of the Cape Fear foreland. This extensively developed reach consists of a Pleistocene subaerial headland segment (Yaupon Beach) and flanking transgressive spits (Caswell and Long Beaches). As part of a regional mapping and sand resource inventory, sidescan sonar data, vibracores, and surface samples were collected in an 80km2 area along the central and western shoreface. Data were integrated and used to evaluate the geologic framework, hardbottom distribution and nature of the modern sediment cover.

The seafloor surface exhibits complex textural variability that is controlled by a combination of bathymetry, hardbottom distribution and relief, sediment supply and the hydraulic regime. Hardbottoms are common, consisting of variably indurated limestone and sandstone, low relief (< 0.75m) scarps and platforms. Rocks forming the hardbottoms can be grouped into three major stratigraphic units; 1) limestone sequences 1 and 2 of the Castle Hayne Formation from the Eocene, 2) Turritellid Limestone from the upper Paleocene or lower Eocene, and 3) siliceous sandstones of the PeeDee Formation and Rocky Point Member from the Cretaceous.

The shoreface sediment package is thin and discontinuous. The turbid and muddy nature of the nearshore water column results in the deposition of fluidized mud and hardbottom mud drapes. The mud content of the underlying sediments is relatively high, testifying to the reworked nature of the sequence. The most likely source of the mud is the Cape Fear River on the eastern margin of the area and its transient westward flowing plume. A secondary source may be Lockwood's Folly Inlet/River to the west. Very coarse sands and gravel-sized clasts with variable amounts of mud are found proximal to hardbottom scarps, the broad bathymetric high off Lockwood's Folly Inlet and the rippled scour depression field dominating the eastern half of the study area.