Geologic Setting

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Field Research Faciltiy, Duck NC - note sand bar pattersn

The Outer Banks form the seaward margin of the Coastal Plain province. West of the sounds, the coastal plain is low-lying and covered by extensive swamps and lakes. The main topographic features are a series of north-south trending terraces which rise in a stepwise manner westward and mark former shorelines corresponding to higher sea level stands during the Pleistocene (Meisburger and Judge, 1989).

The eastern half of the coastal plain is underlain by Quaternary sediments that fill a depositional basin known as the Albemarle Embayment and unconformably overlie late Tertiary sediments (Meisburger and Judge, 1989; Riggs et al., 1995). Superimposed on this regional stratigraphy is an ancient drainage system resulting in series of fluvial valleys filled with younger sediment separated by interfluve areas of older stratigraphic units (Riggs et al., 1995). In northeastern North Carolina, Riggs et al. (1992) documented portions of as many as 18 Quaternary sea-level highstands within 60 m of these Quaternary deposits. Quaternary sea-level fluctuations have produced an extremely complex sediment record reflecting migration of depositional regimes and associated erosional events (Riggs et al., 1992).During lowered sea level of glacial periods, fluvial sediments were distributed across the continental shelf, and evidence of extensive fluvial channeling remains (Rice et al., 1998).Fluvial sand and gravel deposits remain in cored sections of channel deposits (Riggs et al., 1992), and fluvial and estuarine sediments remain in backfilled paleochannels (Rice et al., 1998).

Field and Duane (1976) presented evidence that most barrier islands in the mid-Atlantic region formed seaward of the present coast during the Holocene transgression and migrated to their present position in response to rising sea level. Thus, the northern Outer Banks barrier system is perched on underlying pre-modern sediments. Offshore contours are relatively straight to 13 m depth with some irregularities adjacent to the research pier. One or two nearshore sandbars are usually present (Lippmann and Holman, 1990).The shoreface is covered by a sand sheet (Schwartz et al., 1997) which thins to less than 1 m at about 11-12 m depth (Rice et al., 1998). At approximately 18 m water depth, the bathymetry portrays significant (> 3 m) variability and is accompanied by an increase in the number of paleofluvial channels that crop out on the seafloor (Rice et al., 1998). Older sediments are exposed on the inner shelf as bathymetric highs and influence modern shoreface dynamics and composition (Cox et al., 1995; Riggs et al., 1995).

Sediments become finer offshore to 13 m depth (Schwartz et al., 1997) and are well-sorted fine to very fine sands (0.21 to 0.07 mm or 2.3 to 3.8 f).Sediments consist primarily of quartz sand, with a secondary component of rock-fragment and shell gravel (Meisburger and Judge, 1989).Five nonopaque heavy minerals (garnet, staurolite, epidote, amphiboles, and tourmaline) occur with regularity and with frequency of 2 % or higher (Meisburger and Judge, 1989). Mica, an easily eroded and transported mineral, and is often associated with sediments of finer grain size. Glauconite pellets are common in most sediment samples but are probably detrital grains and do not form in situ (Meisburger and Judge, 1989). The dominant foraminifera in all samples are Elphidium excavatum (Terquem).

Tides are semi-diurnal and have a mean range of approximately 1 m. Average annual significant wave height is 1.0 + 0.6 m (1980-1991) with a mean peak spectral period of 8.3 + 2.6 s (Leffler et al., 1993). Extratropical northeasters are the most common significant storms with increased incidence from October to March. Tropical storms and hurricanes can occur from July to October but are not as common.

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The content for this web page was taken from the Dissertation Storm Sedimentation on the Surf Zone and Inner Continental Shelfof Rebecca Lenel Beavers in the Department of Geology of Duke University

This web site was created by Doug Call (Contract Student, University of Virginia) on July 31, 2001.