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Stanley R. Riggs, William J. Cleary and Stephen W. SnyderBruun (1962) is too simplistic and based upon too manyassumptions. Pilkey et al. (1993) attacked the use of such asimplistic model on the grounds that four basic assumptionsare erroneous: 1) sediment movement is only driven byincoming wave orbitals acting on a sandy shoreface; 2) aquantifiable closure depth exists with no net transport of sedimentto and from the shoreface; 3) the shoreface is sand-richand the underlying or offshore geology do not influence theprofile shape; and 4) if a shoreface is sand-rich, thesmoothed profile described by the equilibrium profile equationmust provide a useful approximation of the real shorefaceshape.The present paper focuses on assumption three of Pilkeyet al. (1993). Our objective is to demonstrate the role of thegeologic framework in determining coastal barrier morphologyand shoreface dynamics for the North Carolina coastalsystem. The conclusions seem self evident, but it has becomeobvious that the concept of shore face profile of equilibriumhas been over simplified, is poorly understood, and is somewhatcontroversial. We believe that it is imperative to incorporatethe geologic framework into all models concerningthe large-scale behavior of any coastal system.Perched Beach SystemsMany beaches do not achieve profiles of equilibriumdue to eustatic sea-level fluctuation, lack of adequate sedimentsupplies, or the variable influence of the geologicframework upon which the beach is superimposed. Obviousexamples of the latter are the active margin coastlines of theU.S. Pacific, which are dominated by wave-cut platformsand associated strandplain beaches. These shoreface profilesare unquestionably dictated by the characteristics of theeroding headlands. In a similar but less dramatic way, passivemargin coastlines with limited sand supplies, such as theU.S. east coast, commonly have barrier islands perched uponpre-modern stratigraphic units that occur beneath and seawardof the shoreface. These stratigraphic units control themorphology of the shoreface and strongly influence modernbeach dynamics, sediment composition, and sediment fluxes(Riggs and O'Connor, 1974; Pearson, 1979; Riggs, 1979;Crowson, 1980).Perched barriers will not develop a profile of equilibrium,as previously defined by Bruun (1962), for two reasons.First, perched barriers consist of thin and variablelayers of surficial beach sands on top of older, eroding, stratigraphicunits with highly variable compositions and geometries.Depending upon the composition and geometry, thisunderlying platform will act as a submarine headland influencingthe shoreface dynamics and resulting profiles. Forexample, if these submarine headlands are composed ofcompact muds, limestones, or sandstones there will be agreater effect upon both the planform of barriers and morphologyof the shoreface and inner shelf than shorefacescomposed of unconsolidated sands and soft muds. Second,along many parts of the coastal system, bathymetric shoalfeatures occur on the inner shelf. These features will modifyincoming wave and current energy affecting the patterns ofsediment erosion, transport, and deposition on the adjacentbeaches.Fisher (1967) described the mid-Atlantic coastal systemas a series of coastal compartments. Each compartment consistedof an eroding mainland beach at the northern end witha barrier spit extending southward and grading into a seriesof barrier islands fronting a major estuarine system. Fisher(in Swift, 1969) interpreted the entire North Carolina barrierisland system as a "southern spit" that formed off a singleeroding headland at Cape Henry, Virginia (Fig. 1). Fisher'sinterpretation was a good first approximation based totallyupon subaerial databases and studies. However, when theunderlying geologic framework is considered, there are awhole series of eroding headlands that occur along the NorthCarolina coast (Fig. 2). These underlying framework unitsgenerally occur either in the shallow subsurface or submergedbelow the shallow coastal waters.Classes of Inherited Geologic FrameworkSix general classes of shoreface systems are recognizedalong the Onslow Bay portion of the North Carolina coastand are based on differences in the geologic framework. Theclassification scheme is based upon the designation of headlandand nonheadland categories (Fig. 2). The headland featuresand associated valley fill segments represent thetopographic or bathymetric features of Pleistocene or olderunits of varying compositions that produce the regional controlsfor the North Carolina coastal system (Riggs, 1979;Snyder, 1982, 1994; Riggs et al., 1990, 1992; Snyder et al.,1994).A. Headland dominated shorefaces are those with morphologicalfeatures that rise above the active ravinement surfaceand are dominantly composed of semi-indurated to indurated,Pleistocene or older sediment units. Two subclassesare recognized.1. Subaerial headland shorefaces are characterized by awavecut cliff and platform that are actively being incised intoPleistocene or older sediments with a perched beach.2. Submarine headland shorefaces are submerged morphologicalfeatures that have been incorporated into the modernshoreface and upon which the barrier-estuarine system isperched. Older sediments crop out on the eroding shorefaceand commonly occur on the inner shelf as bathymetric highsseaward of the modern shoreface and thus modify incomingwaves.B. Non-headland dominated shorefaces are those withoutheadland associations and are dominated by Holocene processesand sediments. Four subclasses are recognized.3. Transgressive shorefaces are composed of compact30