Influence of inherited geologic framework on barrier shoreface morphology and dynamics

Passive margin coastlines with limited sand supplies, such as much of the U.S. Atlantic margin, are significantly influenced by the geologic framework of older stratigraphic units that occur beneath and seaward of the shoreface. Many U.S. east coast barrier islands are perched barriers in which the underlying, pre-modern sediments determine the morphology of the shoreface and strongly influence modern beach dynamics and composition. Perched barriers consist of varible layers of beach sand on top of older, eroding stratigraphic units with highly variable compositions and geometries. Along many parts of the coastal system, stratigraphically-controlled bathymetric features on the inner shelf modify waves and currents and thereby effect patterns of sediment erosion, transport, and deposition on the adjacent shoreface. It is essential to understand this geologic framework before attempting to model the large-scale behavior of these types of coastal systems. In North Carolina, most shoreline features are controlled by the pre-Holocene stratigraphic framework of the shoreface; the beaches are perched on top of pre-existing Pleistocene, Tertiary, and Cretaceous sediments. The surficial geology of the coastal zone is subdivided into two distinct provinces resulting in different stratigraphic controls of the shoreface. North of Cape Lookout the geological framework consists of a Quaternary sequence that fills a regional depositional basin called the Albemarle Embayment. The coastal zone south of Cape Lookout is dominated by Tertiary and Cretaceous units that crop out across the coastal plain and continental shelf, with very thin Quaternary units only locally preserved. Superimposed upon this regional stratigraphy is an ancient drainage system resulting in a series of fluvial valleys filled with younger coastal sediments separated by large interfluve areas of older stratigraphic units. This results in a coastal system in which the shoreface is either nonheadland or headland dominated, respectively. Headland dominated shorefaces are further divided into subaerial and submarine categories. Nonheadland dominated shorefaces are further divided into those influenced primarily by transgressive or regressive processes, or channel-dominated depositional processes (i.e., inlet migration or stream valley fill). Examples of each of these six types of shorefaces are presented to demonstrate the control that the geologic framework exhibits on shoreface morphologies and processes.