Report:  Defining the Spatial and Temporal Characteristics of Paleo-Inlet Channels beneath the Outer Banks of North Carolina.

 

Introduction:

This investigation expanded our understanding of the shallow (<10 m) geology and age of units beneath the Outer Banks of North Carolina, from Oregon Inlet to Hatteras Inlet, and along Ocracoke Island.  The data from this investigation have enhanced the understanding of the relationship between the geology, island history, island characteristics (geomorphology) and the potential for coastal erosion (hazards potential). The data have contributed to our understanding of the potential future evolution of these islands and hazards potential (e.g. future inlet formation, breaching, island collapse, changes in erosion rates, etc.), which is vital for responsible management, planning, and maintenance of infrastructure.

 

Ground penetrating radar data, seismic data, and cores were acquired within the study area.  Data derived from cores include sediment grain size statistics, fossil assemblages, and age.  Ages were acquired on quartz sand sediments using a technique known as optically stimulated luminescence (OSL), which provides the time that has elapsed since burial of the quartz sand.  Using these data, we are able understand the ages of portions of the islands, the environments that existed in certain regions and how the island has changed through time. 

 

Results:

Of particular interest are the location of paleo-inlet channels (old historic inlets, and pre-historic or undocumented inlets) and peat horizons beneath the islands.   The locations of these types of deposits are important in understanding the history and erosive potential of these regions, as inlet sands are more easily eroded than peat horizons, and may explain why inlets commonly open where older inlet channel sediments occur.  The locations of paleo-inlets and other sedimentary units (overwash, flood tide delta, peat) are shown in geologic cross-section format (Plate 1) and map format (Plate 2a).  Paleo-inlet channel morphologies are indicative of either migrating or non-migrating inlets.  A notable outcome of this project is the recognition that the widest portions of the islands occur where major inlet channels occur, indicating the importance of inlets in providing sand to the back-barrier system (as flood tide deltas), which provide for increased island volume.  Narrow regions are underlain by distal flood-tide delta deposits (typically) and represent much older portions of the islands that “need” an inlet in order to maintain island width and elevation.

 

Plate 2b illustrates the age of different portions of the islands, based upon a compilation of data from this and other studies (Smith, 2004; Ricardo, 2005; Twamley, 2006; Rosenberger, 2006; Smith 2006).  OSL and radiocarbon dating of inlet channel sediments, and adjacent units, indicate that much of the island framework is less than 500 years old.  Samples from Oregon Inlet support the historical record of the opening of this inlet.  The oldest inlet channel sediments dated are from Rodanthe (ca. 800 y BP), northern Salvo (ca. 700 y BP) and Kinnakeet (ca. 1200 y BP).  There is a clear clustering of ages between ca. 550 y BP and 275 y BP, indicating significant migrating inlet activity at Salvo, Kinnakeet, and Avon.  This time period corresponds to the Little Ice Age, and suggests that strong nor’easters and increased wave energy in the North Atlantic may have contributed to the formation and maintenance of these large migrating inlets.     

 

Based upon these data, it would be the recommendation of the P.I.s that coastal management policies be implemented that recognize the importance of new inlets in maintaining island volume and elevation.  New inlets should not necessarily be filled immediately, but should be allowed to function for a period of time, which will enable substantial sand transport to the back-barrier system.  Sand supply to the back-barrier is vital to maintaining island width, and elevation, and providing a shallow platform for island migration in the face of rising sea level and storm activity.  Data also suggest a link between climate variability and the activity and characteristics of inlets.