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.
