Stanley R. Riggs, William J. Cleary and Stephen W. SnyderFigure 2. Map of the North <strong>Carolina</strong> co<strong>as</strong>tal zone showing the major paleofluvial valleys and <strong>as</strong>sociated interfluve headland features.NORTH CAROLINA COASTAL SYSTEMStructural SettingThe shallow geology of the North <strong>Carolina</strong> co<strong>as</strong>tal zonecan be subdivided into the geologically distinct northern andsouthern provinces. North of Cape Lookout (Fig. 1), theco<strong>as</strong>tal zone is characterized by a thick Quaternary sequence(50 to 70 m) that fills a regional depositional b<strong>as</strong>in parallel toAlbemarle Sound and called the Albemarle Embayment(Ward and Strickland, 1985). South of the Cape LookoutHigh (Fig. 1), the co<strong>as</strong>tal zone is dominated by Tertiary andCretaceous units. The older and more lithified, offlappingstratigraphic sequences wrap around the <strong>Carolina</strong> PlatformHigh, a major b<strong>as</strong>ement structural feature that occurs southof Cape Fear, and crop out across much of the continentalshelf in Onslow and Long Bays (Snyder, 1982; Riggs et al.,1990). These Tertiary and Cretaceous stratigraphic units,along with local, remnant Quaternary sediment units, form ab<strong>as</strong>al platform with variable topography upon which many ofthe modern barriers in the southern province are perched.Influence Upon Beach SedimentsAncient sediment deposits have been vibracored undermany shoreface sands along the entire Atlantic and Gulfco<strong>as</strong>t. Marsh peats, tidal flat muds, fluvial sands and gravels,bay-fill sands and muds, flood- tide delta sands, and inlet-fillsands and gravels commonly occur below a thin veneer ofmodern shoreface sands that are generally
MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTH CAROLINAFigure 3. Oblique aerial photograph looking southwest fromKure Beach (A) to Fort Fisher (B), across the Onslow subaerialheadland (C), and to the Cape Fear River estuary (D). Thisphoto also shows the following features: (1) Pleistocenecoquina sandstone outcrop in the surf zone; (2) man-made rockrevetments to slow the rates of shoreline recession along thewave-cut cliff of Pleistocene friable humate quartzose sands;and (3) rapidly retreating shoreline <strong>as</strong>sociated with the channel-dominatedvalley fill shoreface.characteristics of barrier island beach sands is strong evidencethat relict sediments are being eroded from the shoreface(Moorefield, 1978; Pearson, 1979; Crowson, 1980;Pearson and Riggs, 1981; Cleary and Hosier, 1990). Supportfor the conclusion that relict and residual sediments areactively being eroded from the shoreface and deposited onthe beach includes the following.1. Sections of beach between Nags Head and the Virginialine (Fig. 1) contain abnormally high concentrations ofquartz and lithocl<strong>as</strong>t gravel, which w<strong>as</strong>" mined for constructionaggregate during historical times. These beach gravelsoccur in are<strong>as</strong> where seismic data demonstrate the -presenceof paleofluvial channels p<strong>as</strong>sing beneath the barrier andcropping out on the adjacent continental shelf (Riggs andO'Connor, 1974; Riggs, 1979; Eames, 1983; Riggs et al.,1992).2. The extinct fossil oyster Cr<strong>as</strong>sostrea gigantisima, and<strong>as</strong>sociated Oligocene rock lithocl<strong>as</strong>ts, occur in great abundanceon Onslow Beach and Topsail Island after storms(Crowson, 1980; Cleary and Hosier, 1987). The eroded gravelsare derived from the bioerosion of Oligocene hard bottomscarps that crop out on the inner shelf. These gravels are subsequentlytransported up the shoreface during high-energystorms and left on the beach in the same f<strong>as</strong>hion <strong>as</strong> heavyminerals at the top of the sw<strong>as</strong>h zone of a storm beach.3. Overw<strong>as</strong>h terraces on M<strong>as</strong>onboro Island contain abundantcobble-size coquina cl<strong>as</strong>ts and mollusk shells derived fromhardbottoms exposed on the adjacent inner shelf. Also, muchof the coarse- grained component of the beach sediment canbe attributed to the onshore transport of reworked and palimpsestsediments that mantle these hardbottoms. Stormreworking of the thin shoreface sediment cover and thedegraded character of underlying rock units appear to contributesignificant amounts of coarse material to the adjacentbeaches (Cleary et al., 1992, 1993).4. Black-stained oysters and other estuarine fossils are thedominant shell on many North <strong>Carolina</strong> beaches. Theseshells always produce pre- modern, Holocene ages whendated by carbon- 14 techniques (Pilkey et al., 1969; Wehmiller,1993).5. Mixed <strong>as</strong>semblages of Pleistocene age marine shells occurin great abundance on many of the North <strong>Carolina</strong> beachesanalyzed by amino-acid racemization dating techniques(Wehmiller, 1993).Subaerial Headland ShorefacesFort Fisher to Kure BeachNorth <strong>Carolina</strong>'s only subaerial headlands occur oneither side of the Cape Fear River estuary and include portionsof Yaupon and Long Beaches (Griffin et al., 1977) tothe south, <strong>as</strong> well <strong>as</strong> the shoreline between Fort and KureBeach to the north. In the latter area, an extensive erodingsubaerial headland intersects the co<strong>as</strong>tal zone without a barrierisland-estuarine system (Fig. 2). The co<strong>as</strong>tal system consistsof a wave-cut platform incised into Oligocene throughPleistocene units of the mainland peninsula with a thin beachperched on top of the irregular geometry of the Pleistoceneunits (DuBar et al., 1974; Moorefield, 1978; Meisburger,1979; Cleary and Hosier, 1979; Snyder et al. 1994).Figure 3 shows the dramatic relationship between threedifferent geologic framework situations in the Fort Fisherarea and geometry of the shoreline and upper shoreface. Erosionresistant, lithified and cross-bedded coquina sandstoneforms a headland in the shoreline north of Fort Fisher (Fig.3). Friable humate and iron-cemented Pleistocene sandstone(Fig. 4) forms a 2 m high wave-cut cliff and terrace thatfronts the shoreline immediately south of the headland andseaward of the Civil War Fort Fisher. South of Fort Fisher isa nonheadland segment characterized by a channel-dominated,valley-fill shoreface (Fig. 3) underlain by 10 m ofmuddy estuarine sediments (Swain and Cleary, 1992). Theshape and evolution of the three different co<strong>as</strong>tal compartmentsaround Fort Fisher is clearly related to the presenceand lithology of the outcropping and underlying Pleistocenegeologic framework.Moorefield (1978) mapped beach outcrops of Pleistocenecoquina north of Fort Fisher and their seaward extensionson the inner shelf. Our ongoing studies clearly showthat coquina and its <strong>as</strong>sociated lithologies form a series ofwidespread, irregular, bathymetrically high hardbottom featureswith >3 m of relief. This karstic mosaic includes oneextensive hardbottom area known <strong>as</strong> Sheephead Rock thatlies in 9 m of water with pedestal-like hardbottom featuresrising to within 2.5 m of the ocean surface. Diver observa-33
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