Influence of inherited geologic framework on barrier shoreface morphology and dynamics

Marine Geology 126 (1995) 213-234

Influence of inherited geologic framework on barrier shoreface morphology and dynamics

Stanley R. Riggs a, William J. Cleary b, Stephen W. Snyder ’

a Department of Geology, East Carolina University, Greenville, NC 27858, USA b Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, NC28403, USA

’ Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC27695, USA

Received 15 February 1994; revision accepted 23 November 1994

Abstract

Passive margin coastlines with limited sand supplies, such as much of the U.S. Atlantic margin, are significantly influenced by the geologic framework of older stratigraphic units that occur beneath and seaward of the shoreface. Many U.S. east coast barrier islands are perched barriers in which the underlying, pre-modern sediments determine the morphology of the shoreface and strongly influence modern beach dynamics and composition. Perched barriers consist of varible layers of beach sand on top of older, eroding stratigraphic units with highly variable compositions and geometries. Along many parts of the coastal system, stratigraphically-controlled bathymetric features on the inner shelf modify waves and currents and thereby effect patterns of sediment erosion, transport, and deposition on the adjacent shoreface. It is essential to understand this geologic framework before attempting to model the large-scale behavior of these types of coastal systems.

In North Carolina, most shoreline features are controlled by the pre-Holocene stratigraphic framework of the shoreface; the beaches are perched on top of pre-existing Pleistocene, Tertiary, and Cretaceous sediments. The surficial geology of the coastal zone is subdivided into two distinct provinces resulting in different stratigraphic controls of the shoreface. North of Cape Lookout the geological framework consists of a Quaternary sequence that fills a regional depositional basin called the Albemarle Embayment. The coastal zone south of Cape Lookout is dominated by Tertiary and Cretaceous units that crop out across the coastal plain and continental shelf, with very thin Quaternary units only locally preserved. Superimposed upon this regional stratigraphy is an ancient drainage system resulting in a series of fluvial valleys filled with younger coastal sediments separated by large interlluve areas of older stratigraphic units. This results in a coastal system in which the shoreface is either nonheadland or headland dominated, respectively. Headland dominated shorefaces are further divided into subaerial and submarine categories. Nonheadland dominated shorefaces are further divided into those influenced primarily by transgressive or regressive processes, or channel-dominated depositional processes (i.e., inlet migration or stream valley fill). Examples of each of these six types of shorefaces are presented to demonstrate the control that the geologic framework exhibits on shoreface morphologies and processes.

1. Introduction

1.1. Shorefacepro3le of equilibrium

Fenneman ( 1902) originally defined a shoreface proJile of equilibrium as a profile that “the water would ultimately impart, if allowed to carry its

work to completion.” Recently, many other workers have expanded upon the definition of the shoreface profile of equilibrium including the following: Schwartz (1982): “a long-term profile of ocean bed produced by a particular wave climate and type of coastal sediment”; Dean (1983): “an idealization of conditions which occur in nature

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214 S. R Riggs et al. JMarine Geology 126 (1995) 213-234

for particular sediment characteristics and steady wave conditions”; and Larson (1991): “a beach of specific grain size, if exposed to constant forcing conditions, normally assumed to be short-period breaking waves, will develop a profile shape that displays no net change in time.”

Bruun (1962) developed a simple model to char- acterize the profile of equilibrium that is assumed to exist for all shorefaces, and which has become the basis for most models in the design of coastal engineering projects, However, Kriebel et al. ( 1991) argued that “a beach profile in true equilibrium never exists in nature because nearshore water levels, waves, and currents are constantly changing.”

Pilkey et al. (1993) believe that there are many “shoreface profiles of equilibrium” and the model as developed by Bruun (1962) is too simplistic and based upon too many assumptions. Pilkey et al. (1993) attacked the use of such a simplistic model on the grounds that four basic assumptions are erroneous: ( 1) sediment movement is only driven by incoming wave orbitals acting on a sandy shoreface; (2) a quantifiable closure depth exists with no net transport of sediment to and from the shoreface; (3) the shoreface is sand-rich and the underlying or offshore geology do not influence the profile shape; and (4) if a shoreface is sand- rich, the smoothed profile described by the equilibrium profile equation must provide a useful approximation of the real shoreface shape.

The present paper focuses on assumption three of Pilkey et al. (1993). Our objective is to demonstrate the role of the geologic framework in deter- mining coastal barrier morphology and shoreface dynamics for the North Carolina coastal system. The conclusions seem self evident, but it has become obvious that the concept of shoveface projile of equilibrium has been over simplified, is poorly understood, and is somewhat controversial. We believe that it is imperative to incorporate the geologic framework into all models concerning the large-scale behavior of any coastal system.

1.2, Perched beach systems

Many beaches do not achieve profiles of equilibrium due to eustatic sea-level fluctuation, lack of

adequate sediment supplies, or the variable influence of the geologic framework upon which the beach is superimposed. Obvious examples of the latter are the active margin coastlines of the U.S. Pacific which are dominated by wave-cut platforms and associated strandplain beaches. These shoreface profiles are unquestionably dictated by the characteristics of the eroding headlands. In a similar but less dramatic way, passive margin coastlines with limited sand supplies, such as the U.S. east coast, commonly have barrier islands perched upon pre-modern stratigraphic units that occur beneath and seaward of the shoreface. These stratigraphic units control the morphology of the shoreface and strongly influence modern beach 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 variable layers of surficial beach sands on top of older, eroding, stratigraphic units with highly variable compositions and geometries. Depending upon the composition and geometry, this underlying platform will act as a submarine headland influencing the shoreface dynamics and resulting profiles. For example, if these submarine headlands are composed of compact muds, lime- stones, or sandstones there will be a greater effect upon both the planform of barriers and morphology of the shoreface and inner shelf than shorefaces composed of unconsolidated sands and soft muds. Second, along many parts of the coastal system, bathymetric shoal features occur on the inner shelf. These features will modify incoming wave and current energy effecting the patterns of sediment erosion, transport, and deposition on the adjacent beaches.

Fisher (1967) described the mid-Atlantic coastal system as a series of coastal compartments. Each compartment consisted of an eroding mainland beach at the northern end with a barrier spit extending southward and grading into a series of barrier islands fronting a major estuarine system. Fisher (in Swift, 1969) interpreted the entire North Carolina barrier island system as a “southern spit” that formed off a single eroding headland at Cape Henry, Virginia (Fig. 1). Fisher’s interpretation

S.R Riggs et al./Marine Geology 126 (199.5) 213-234 215

Virainia

NORTH CAROLINA

COASTAL PLAIN - 36’00

Scale 0 40 kildmelers I

Wilson

Wilmington

North Carolin?

, Morehead

Onslow Bay

Atlantic Ocean

Fig. 1. Map of the North Carolina coastal zone showing geographic features mentioned in the text, along with the location of the four coastal segments discussed in the text (boxed areas).

was a good first approximation based totally upon subaerial data bases and studies. However, when the underlying geologic framework is considered, there are a whole series of eroding headlands that occur along the North Carolina coast (Fig. 2). These underlying framework units generally occur either in the shallow subsurface or submerged below the shallow coastal waters.

1.3. Classes of inherited geologic framework

Six general classes of shoreface systems are (A) Headland dominated shorefaces are those recognized along the North Carolina coast and with morphological features that rise above the

are based on differences in the geologic framework. The classification scheme is based upon the desig- nation of headland and nonheadland categories (Fig. 2). The headland features and associated valley fill segments represent the topographic or bathymetric features of Pleistocene or older units of varying compositions that produce the regional controls for the North Carolina coastal system (Riggs and O’Connor, 1974; Riggs, 1979; Snyder, 1982, 1993; Riggs et al., 1990, 1992; Snyder et al., 1994).

216 S. R. Riggs et al.IMarine Geology 126 (1995) 213-234

- 36Ooo

- 35%0

I> ,” CAPE HWRY SUBAEIUAL HEADLAND

CURRITUCK

ROANOKEM5EMARLE VALLEY PILL

NORTH CAROLINA COASTAL SYSTEM

VER VALLEY FILL

Fig. 2. Map of the North Carolina coastal zone showing the major paleofluvial valleys and associated interfluve headland features.

active ravinement surface and are dominantly com- (B) Non-headland dominated shorefaces are those posed of semi-indurated to indurated, Pleistocene without headland associations and are dominated or older sediment units. Two subclasses are by Holocene processes and sediments. Four sub- recognized. classes are recognized.

( 1) Subaerial headland shorefaces are characterized by a wavecut cliff and platform that are actively being incised into Pleistocene or older sediments with a perched beach.

(2) Submarine headland shorefaces are submerged morphological features that have been incorporated into the modern shoreface and upon which the barrier-estuarine system is perched. Older sediments crop out on the eroding shoreface and commonly occur on the inner shelf as bathymetric highs seaward of the modern shoreface and thus modify incoming waves.

(3) Transgressive shorefaces are composed of compact Holocene peat and mud that extend from the estuaries, under the barrier sands, and crop out within the surf zone and upper shoreface. The shoreface is often characterized by an irregular geometry with discontinuous, highly scarped dune ridges and abundant washover fans on the barrier island.

(4) Regressive shorefaces are composed of unconsolidated Holocene sands and occur along barrier island stretches with adequate sediment supplies. They often form in association with head-

S. R. Riggs et al.IMarine Geology 126 (I 995) 213-234 217

lands, cape structures, and inlets and exhibit con- cave progradational geometry with accretionary beach ridges on the barrier island.

(5) Channel-dominated inlet-fill shorefaces are composed of unconsolidatedHolocene sand and gravel sediments that have back-filled old inlet systems. They have limited longshore extent and form on barrier islands in response to inlet systems that actively open, migrate, and close.

(6) Channel-dominated valley-Jill shorefaces are sections along barrier islands that have historically been occupied by paleofluvial drainage systems and are underlain by thick accumulations of fluvial-estuarine channel-fill sediments in response to deglaciation and Holocene sea-level rise.

The basic structural and stratigraphic characteristics of any coastal complex significantly influence the resulting barrier island morphology and shoreface dynamics, and therefore prevent the concept of one equilibrium profile from being realized. We present examples of perched beaches associated with each of the six different shoreface classes defined above (Figs. 1 and 2). Although all examples are from North Carolina, we do not attempt to address and classify the entire North Carolina ocean shoreline.

2. North Carolina coastal system

2.1. Structural setting

The shallow geology of the North Carolina coastal zone can be subdivided into the geologi- cally distinct northern and southern provinces. North of Cape Lookout (Fig. l), the coastal zone is characterized by a thick Quaternary sequence (50-70 m) that fills a regional depositional basin parallel to Albemarle Sound and called the Albemarle Embayment (Ward and Strickland, 1985). Seismic data (Popenoe and Ward, 1983; Popenoe, 1985) suggest that the Quaternary section has filled the last remnants of the Aurora Embayment, a pre-Miocene depositional basin northwest of the Cape Lookout High. This Oligocene paleotopographic high was the precur- sor that separated North Carolina into two depositional embayments (Snyder, 1982; Riggs et al.,

1990). The Pleistocene section within the northern province represents a complex record of multiple cycles of coastal deposition and erosion in response to numerous glacial-eustatic sea-level cycles (Riggs et al., 1992). During each glacial episode, fluvial channels severely dissected previously deposited coastal systems. The subsequent transgression sequentially backfilled the valleys with fluvial and estuarine sediments and then produced a ravinement surface that migrated landward. Shoreface erosion truncated large portions of previously deposited coastal sediments. Present sea level has produced a modern sequence of coastal sediments deposited unconformably over the eroded remnants of these Pleistocene sequences. Thus, the modern barrier island system is stacked on top of numerous highly dissected, partially preserved lithostratigraphic units with irregular, erosional geometries and composed of sediments ranging from compact peat and mud to indurated sands and gravels.

South of the Cape Lookout High (Fig. l), the coastal zone is dominated by Tertiary and Cretaceous units. The older and more lithified, offlapping stratigraphic sequences wrap around the Carolina Platform High, a major basement structural feature that occurs south of Cape Fear, and crop out across much of the continental shelf 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 a basal platform with variable topography upon which many of the modern barriers in the southern province are perched.

2.2. Influence upon beach sediments

Ancient sediment deposits have been vibracored under many shoreface sands along the Atlantic coast (Hoyt and Henry, 1967; Kraft, 1969; Riggs and O’Connor, 1974; Hayes, 1976; Griffin et al,, 1977; Kraft and John, 1979; Pearson, 1979; Rampino and Sanders, 1981). Marsh peats, tidal flat muds, fluvial sands and gravels, bay-fill sands and muds, flood-tide delta sands, and inlet-fill sands and gravels occur below a thin veneer of modern shoreface sands that are generally < 1 m thick. Such coastal areas are characterized

218 S. R. Riggs et al./Marine Geology 126 (1995) 213-234

by a seaward thinning and fining veneer of modern shoreface sands resting disconformably on Pleistocene or older strata. The modern sand veneer is ephemeral and easily removed from the shoreface during storms, exposing the older underlying strata on the shoreface to erosion (Pearson, 1979; Niedoroda et al., 1985). Thus, the erosional response and post-storm shape of the shoreface profile is at least partly controlled by degree of consolidation of the underlying sediments.

During storms, ancient strata cropping out on the shoreface also provide an immediate source of ‘new’ sediment to the modern beach system, a process called shoreface bypassing by Swift ( 1976). In North Carolina, the general grain size characteristics of barrier island beach sands is strong evidence that relict sediments are being eroded from the shoreface (Moorefield, 1978; Pearson, 1979; Crowson, 1980; Pearson and Riggs, 1981; Cleary and Hosier, 1990). Support for the conclusion that relict and residual sediments are actively being eroded from the shoreface and deposited on the beach includes the following.

( 1) Sections of beach between Nags Head and the Virginia line (Fig. 1) contain abnormally high concentrations of quartz and lithoclast gravel, which was mined for construction aggregate during historical times. These beach gravels occur in areas where seismic data demonstrate the presence of paleofluvial channels passing beneath the barrier and cropping out on the adjacent continental shelf (Riggs and O’Connor, 1974; Riggs, 1979; Eames, 1983; Riggs et al., 1992).

(2) The extinct fossil oyster Crassostrea gigantis- ima, and associated Oligocene rock lithoclasts, occur in great abundance on Onslow Beach and Topsail Island after storms (Crowson, 1980; Cleary and Hosier, 1987). The eroded gravels are derived from the bioerosion of Oligocene hardbottom scarps that crop out on the inner shelf. These gravels are subsequently transported up the shoreface during high energy storms and left on the beach in the same fashion as heavy minerals at the top of the swash zone of a storm beach.

(3) Overwash terraces on Masonboro Island contain abundant cobble-size coquina clasts and mollusk shells derived from hardbottoms exposed on the adjacent inner shelf. Also, much of the

coarse-grained component of the beach sediment can be attributed to the onshore transport of reworked and palimpsest sediments that mantle these hardbottoms. Storm reworking of the thin shoreface sediment cover and the degraded character of underlying rock units appear to contribute significant amounts of coarse material to the adjacent beaches (Cleary et al., 1992, 1993).

(4) Black-stained oysters and other estuarine fossils are the dominant shell on many North Carolina beaches. These shells always produce pre-modern, Holocene ages when dated by 14C techniques (Pilkey et al., 1969; Wehmiller, 1993).

(5) Mixed assemblages of Pleistocene age marine shells occur in great abundance on many of the North Carolina beaches analyzed by amino-acid racemization dating techniques (Wehmiller, 1993).

2.3. Subaerial headland shorefaces

Fort Fisher to Kure Beach In this area, an extensive eroding subaerial head-

land intersects the coastal zone without a barrier island-estuarine system (Fig. 2). The coastal system consists of a wave-cut platform incised into Oligocene through Pleistocene units of the mainland peninsula with a thin beach perched on top of the irregular geometry of the Pleistocene units (DuBar et al., 1974; Moorefield, 1978; Meisburger, 1979; Cleary and Hosier, 1986; Snyder et al., 1994).

Fig. 3 shows the dramatic relationship between three different geologic framework situations in the Fort Fisher area and geometry of the shoreline and upper shoreface. Erosion resistant, lithified and crossbedded coquina sandstone forms a headland in the shoreline north of Fort Fisher (Fig. 4). Friable humate and iron-cemented Pleistocene sandstone (Figs. 5 and 6) forms a 2 m high wave- cut cliff and terrace that fronts the shoreline imme- diately south of the headland and seaward of the Civil War Fort Fisher (Figs. 3 and 4). South of Fort Fisher is a nonheadland segment characterized by a channel-dominated, valley-fill shoreface (Fig. 3) underlain by 10 m of muddy estuarine sediments (Swain and Cleary, 1992). The shape and evolution of the three different coastal compartments around Fort Fisher is clearly related to

S.R. Riggs et aL/Marine Geology 126 ( 1995) 213-234 219

Fig 3. Oblique aerial photograph looking north from Fort Fisher (A) to Kure Beach (B) where the Onslow subaerial headland (C) intersects the Atlantic Ocean. This photo also shows three coastal segments: (I) a subaerial headland formed by the outcropping Pleistocene coquina sandstone; (2) eroding wave-cut cliff of Pleistocene friable humate quartzose sands; and (3) the rapidly retreating shoreline associated with the channel-dominated valley-fill shoreface.

Fig. 4. Oblique aerial photograph looking southwest from Kure Beach (A) to Fort Fisher (II), across the Onslow subaerial headland (C), and to the Cape Fear River estuary (0). This photo also shows the following features: (I) Pleistocene coquina sandstone outcrop in the surf zone; (2) man-made rock revetments to slow the rates of shoreline recession along the wave-cut cliff of Pleistocene friable humate quartzose sands; and (3) rapidly retreating shoreline associated with the channel-dominated valley-fill shoreface.

the presence and lithology of the outcropping and underlying Pleistocene geologic framework.

Moorefield (1978) mapped beach outcrops of Pleistocene coquina north of Fort Fisher and their seaward extensions on the inner shelf (Fig. 7). Our ongoing studies clearly show that coquina and its associated lithologies form a series of widespread, irregular, bathymetrically high hardbottom fea-

Fig. 5. Photograph looking north at Fort Fisher (A) and the Pleistocene coquina subaerial headland (B). This photo also shows the: (C) Pleistocene friable humate- and iron-cemented sandstone that forms a 2 m high wave-cut cliff; (0) rock revetment built to protect the rapidly receeding shoreline and the Civil War Fort Fisher; and (I?) the strandplain beach. Photo was taken in 1977 and the shoreline had already receeded behind the rock revetment; today the shoreline has receeded an additional 20 to 25 m.

Fig. 6. Closeup photograph of the wave-cut cliff of Pleistocene friable humate- and iron-cemented sandstone (A) and associated strandplain beach (B) at Fort Fisher. Photo location is point C on Fig. 5.

tures with > 3 m of relief (Fig. 7). This karstic mosaic includes one extensive hardbottom area known as Sheephead Rock that lies in 9 m of water with pedestal-like hardbottom features rising to within 2.5 m of the ocean surface. Diver observations and cores suggest that the sediment cover is both patchy and very thin across much of this region and in many areas is totally lacking.

The extensive series of coquina outcrops on the inner shelf act as barriers that could significantly affect the refraction of wave energy, as well as

220 S. R. Riggs et aLlMarine Geology 126 (1995) 213-234

Modem kdlments PleistocelW units

q Sand DepositIon f wavscut Cliff b)luhore) Ledge wfsh0re)

w NetDdfi coqlha

. oamw-l Exposures)

Fig. 7. Map of the subaerial headland in the Fort Fisher area showing (1) three different coastal compartments each with a different underlying geologic framework and resulting shoreline morphology; (2) the near-shore bathymetry (in m below MSL); (3) location of Pleistocene coquina sandstone outcrops in the shoreface and southward across the inner shelf; and (4) the surface sediment distribution on the inner shelf. Figure is modified from Moorefield (1978).

the movement of sand across this shoreface. Moorefield (1978) believes that sand from both the rapidly eroding beach at Fort Fisher and littoral drift, are transported seaward of the rock barrier during storms and prevented from return- ing to the beach during subsequent low energy periods (Fig. 7). The result of this process is a net sediment deficiency in which the rapidly retreating bluff shoreline is consuming the historic Fort.

Subaerial headlands are also located along the shoreline south of the Cape Fear River estuary, including portions of Yaupon and Long Beaches (Griffin et al., 1977). Another area is north of Currituck Sound and Back Bay of Virginia where the Cape Henry headland dominates the coastal system.

2.4. Submarine headland shorefaces and bathymetric highs

Kill Devil Hills to Oregon Inlet The barrier island system from Kill Devil Hills

to Cape Hatteras (Fig. 1) is migrating up onto the interfluve peninsula between Albemarle Sound and Pamlico River (Fig. 2). The Dare Headland controls the morphology of northern Pamlico Sound, occurs in the subsurface below this portion of the Outer Banks, and forms extensive bathymetric shoals with Pleistocene cores on the inner shelf. These erosional remnants control (1) changes in orientation of the barrier islands; (2) occurrence of minor cape structures on the barrier beach; and (3) a series of offshore ridge and shoal structures. High-resolution seismic and sonar data, as well as drill records demonstrate that these latter features are oriented at oblique angles to the barrier, have significant relief above the present ravinement surface; and contain Quaternary outcrops that produce scarped hardbottom features (Snyder, 1993).

Riggs et al. (1992) present a series of interpretive cross-sections of the Nags Head area based upon deep core holes down the axis of the barrier island (Riggs and O’Connor, 1974; Eames, 1983), shallow core holes down the beach face (Pearson, 1979) and a dense network of high-resolution seismic traces (Pearson, 1979; Riggs, 1979; Eames, 1983) in consort with a series of Quaternary age assignments based upon amino-acid racemization of fossil mollusks (York, 1990). Pearson’s (1979) shore perpendicular profiles, which extended from the shoreline to about 5 km offshore, contained two major sediment units: a modern, thin shoreface sediment blanket over in situ relict sediments dominated by fluvial and back-barrier estuarine sediments deposited during previous sea-level events. The relict units ultimately cropped out on the inner shelf. The contact between the modern sand sheet and underlying relict units is erosional and constitutes a major reflector in high-resolution seismic data. Based upon diving observations and presence of relict sediments in the thin surface sands, Pearson (1979) concluded that the “modern sand sheet is periodically stripped away during extreme high energy periods, exposing the relict units which may experience erosion. By this mecha-

S. R Riggs et al./Marine Geology 126 (1995) 213-234 221

nism, relict sediments could be eroded and intro- duced into the modern sediment regime.”

Shoreline recession rates, and consequently the resulting shoreface profiles, will be quite different for shorefaces underlain by compact estuarine muds as compared to inlet-fill sands. This relationship is readily apparent when the average long- term shoreline erosion data for the region between Kill Devil Hills and Oregon Inlet (Benton et al., 1993) are compared with the high-resolution seismic and drill hole data of Pearson ( 1979), Eames (1983), and Riggs et al. (1992). Areas with the most rapid rates of shoreline recession generally occur in areas of old inlet and channel fill structures dominated by sand sediments.

Oregon Inlet to Cape Hatteras The Oregon Inlet to Cape Hatteras segment of

the Outer Banks (Fig. 1) is perched on a major ravinement surface cut into the nose of the Dare Headland which defines the basic geometry of this portion of the Outer Banks (Fig. 2). It is characterized by the following four coastal features. ( 1) A major change in barrier island orientation occurs at Rodanthe. (2) A series of bathymetric highs occur on the inner shelf associated with each of several minor capes and intersect the lower beach- face at acute angles. These include Wimble Shoals which extend from Rodanthe to Salvo and Kinnakeet Shoals which extend from Kinnakeet to Avon (Fig. 8). (3) In Pamlico Sound south of Rodanthe, the backside of the barrier island is characterized by the Hatteras Flats, a broad and very shallow platform bounded on the west by a vertical scarp up to 3 m high (Fig. 8). (4) Minor cape structures occur on the barrier beach at the towns of Rodanthe and Avon with rapidly receding beach segments occurring between the minor cape structures (Figs. 8 and 9).

The change in shoreline characteristics from Rodanthe to Cape Hatteras may be controlled by gently dipping Pleistocene sediments in the shallow subsurface along the southern edge of the Albemarle Basin. Pierce and Colquhoun (1970) also recognized the uniqueness of this portion of the coast. Based upon 20 auger holes (9-26 m deep) along the barrier islands between Oregon Inlet and Cape Hatteras, they interpreted this

section of barrier as an “eroded and modified primary barrier”. They differentiate “primary barriers” from “secondary barriers” on the basis of the substrata upon which the barrier occurs. Primary barriers are built upon pre-existing sediments that form paleotopographic highs and have been exposed to weathering; whereas, secondary barriers build out over contemporaneous marine sediments as prograding shoals or spits (Pierce and Colquhoun, 1970).

Pierce and Colquhoun (1970) described a Pleistocene unit with a distinct soil profile on top that occurred at about 11.4 m below MSL just south of Oregon Inlet and systematically rose southward to 8.6 m below MSL at Rodanthe and 5.1 m below MSL at Avon. They are overlain by sediment units labeled “lagoon” and “barrier beach” facies of Pierce and Colquhoun (1970) which also have a soil profile on top. Both of these sediment sequences could represent different Pleistocene depositional events separated by weathering profiles (Fig. 10). At Rodanthe, these older units are overlain by a thin (up to 3 m) Holocene back-barrier marsh peat and estuarine sand unit that crops out in the surf zone and the modern barrier island sand sheet.

Wimble and Kinnakeet Shoals are a series of ridges that are oriented NNE-SSW at about 25” to 30” angles to the barrier. These offshore Pleistocene hardbottom features have up to 6 m of relief and rise up to between 7 and 9 m below sea level. These shoals have not been sampled directly, however, high-resolution seismic and side scan sonar traces over them demonstrate that they are scarped hardbottoms with terminated reflectors. Commercial fishermen work these rocks for reef-fish species and commonly obtain pieces of Pleistocene, carbonate-cemented sandstones in their nets when fishing around the flanks of the shoals. All of this suggests that these are relict erosional features and not constructive depositional sand bars as inferred by Swift et al. (1973).

Nearshore topographically high features such as Wimble and Kinnakeet Shoals should have dramatic impacts upon the energy regime affecting the adjacent beaches through wave refraction and wave setup. Minor cape features occur at the towns of Rodanthe and Avon (Fig. 9) and separate

222 S. R. Riggs et al.lMarine Geology 126 (1995) 213-234

Average Long Average Long Term Erosion Term Accretior

Fig. 8. Map of the Outer Banks from Rodanthe to Avon showing (1) bathymetry (in m below MSL) for the inner shelf and eastern portion of Pamlico Sound; (2) inner-shelf shoal structures that occur between Rodanthe and Salvo and between Kinnekeet and Avon; (3) Hatteras Flats, a shallow back-barrier platform bounded on the west by a vertical scarp; (4) minor cape features landward of the shoals with broad beaches and narrow recessional beaches between the minor capes; (5) differences in the slope of the shoreface profiles at the point where the shoals attach to the barrier compared to the cuspate portions of the barriers with deeper bathymetry between the shoals; (6) plot of the average long-term erosion and accretion rates along this stretch of the barrier (Benton et al., 1993); and (7) location of section A-A’. Map is from N.O.A.A. Nautical Chart 11555, Wimble Shoals to Ocracoke Inlet.

beach segments characterized by rapidly receding shorelines. Bathymetric charts suggest fairly steep and deep shoreface profiles occur directly off the minor cape features with the actual morphology probably being controlled by outcropping units in the shoreface. Whereas, in the adjacent receding portions of the beach, shoreface profiles are relatively broad and shallow. Long-term average annual erosion rates (Benton et al., 1993) correlate well with the offshore bathymetry and shoal structures (Fig. 8). Sections of the beach that occur

opposite the major portion of the shoal structure are dominated by shoreline accretion and development of minor cape features, whereas, large embayed sections of beach that are dominated by shoreline recession, occur both in the updrift and downdrift reaches.

The Hatteras Flats occur on the west side of the barrier islands between Oregon Inlet and Avon. This broad and shallow platform is between 4 and 6 km wide and considerably < 1 m deep. From Oregon Inlet south to Rodanthe, the flats slope

S. R. Riggs et al. IMarine Geology I26 (1995) 213-234 223

Fig. 9. Oblique aerial photograph looking south from the towns of Rodanthe (A) to Salvo (E), Kinnekeet (C), Avon (D), and showing the location of the minor capes where the shoals attach to the barrier (B and C), coves between the minor capes with rapidly receeding shorelines (E), and the Hatteras Flats with Gull Island at the outer edge of the flats (3’).

gradually into Pamlico Sound. However, beginning near Rodanthe and continuing southward to Avon, the western side of these flats is characterized by a vertical scarp that drops from 0.5 m or less into 3 to 4 m water depth. This vertical bank is probably underlain by pre-Modern sediments (Fig. 10) that are lithologically different than the overwash sands or inlet flood tide delta sands that occur north of Rodanthe. These pre-Modern sediments have been semi-indurated during a post-depositional weathering episode.

High-resolution seismic data within Pamlico

Sound demonstrates that the Quaternary sediments rise and thin southward out of the Albemarle Embayment and onto the Cape Lookout High. These seismic profiles display major Quaternary reflectors that rise from depth beneath Roanoke Island to outcrop in the area extending from Rodanthe southward to Avon. The structural orientation of semi-indurated to indurated units cropping out in the Rodanthe to Avon area forms the interstream divide of the Dare Headland (Fig. 2) and readily explains the geologic control of the large-scale behavior of this coastal segment.

Onslow Beach to Topsail Island The New River Inlet coastal area (Fig. 1) is a

submarine headland which forms a small seaward bulge in the coastline of central Onslow Bay (Fig. 2). This shoreline protrusion is produced by the Oligocene Silverdale Formation, an indurated moldic limestone and calcareous-cemented quartz sandstone unit. The Silverdale Formation crops out at or slightly below sea level in the mouth of the New River estuary. It occurs extensively on dredge spoil islands of the Intracoastal Waterway behind Topsail Island and Onslow Beach, and forms a series of bathymetric ridges on the inner shelf on either side of New River Inlet (Crowson, 1980). Crowson mapped these prominent submarine rock features as a series of ridges that occur

MSL

3

6

c fs E

0 2 4 L I ,

Auger Hole (Pierce a Cdquham 1970)

kilometers

Fig. 10. A W-E schematic geologic cross-section of the perched Outer Banks at Rodanthe. This figure shows the bathymetry and it’s relationship to an interpretation of the underlying geologic framework. Location of section A-A’ is presented on Fig. 8.

224 S. R Riggs et al.lA4arine Geology 126 (199.5) 213-234

seaward of the lower shoreface, have steep landward-facing scarps with smooth surfaces that dip gently away from the beach, and have up to 5 m of relief above the surrounding ravinement surface (Fig. 11). The ridges rise locally to about 5 m below MSL, higher than the elevation of the lower shoreface, which is probably high enough to cause major refraction of storm waves and currents and possibly affect the patterns of erosion and deposition on the adjacent beaches.

The ridges are oriented at acute angles to the beach and intersect the shoreface on Topsail Island and Onslow Beach (Fig. 12). Core drilling by Cleary and Hosier (1987) demonstrated that the rock ridges continue under Onslow Beach and into the back-barrier estuarine system (Figs. 13 and 14). Similar limestone ridges pass beneath Topsail Island and into the back-barrier estuarine system (Clark et al., 1986) where the rock structures

appear to be related to the occurrence and orientation of Pleistocene barrier island systems (Fig. 12).

The Oligocene submarine headland appears to subdivide these two barriers into coastal compartments that have different orientations and shoreface dynamics. Figs. 15 and 16 show the changes in shoreline geometry that coincides with the intersection of the Oligocene rock ridges. The northern segment of Onslow Beach is characterized by a cuspate shoreline geometry with wide beaches, a recurved accretionary beach ridge, a nearly continuous high dune ridge, and shoreline accretion rates that average 2 m/yr (Cleary and Hosier, 1987). In contrast, the southern segment is characterized by a narrow shoreface with abundant rock gravel on the beach, a single discontinuous scarped foredune ridge, presence of major washover terraces, and current erosion rates up to 6 m/yr.

Crowson (1980) believes that active bioerosion

Fig. 11. Schematic diagram looking southwest along the hardbottom scarps of the Oligocene Silverdale Formation that crop out on

the inner continental shelf off of Topsail Island. The scarps are parallel to and face the beach, up to 5 m high with major overhangs,

and dip gently seaward. Extensive bioerosion and wave processes produce “new” gravel sediment which is transported directly to

the beach during storms. The sandy limestone crops out at sea level in the estuary behind the barrier; however, it is not known how

the barrier is perched on top of this rock unit. Figure is modified from Crowson (1980).

225 S. R Riggs et al. JMarine Geology 126 (1995) 213-234

SUBMIARINE HEADLAND

Fig, 12. Map showing the outcrop pattern of the Oligocene rock scarps on the inner shelf formed by the Onslow submarine headland. In addition, the map shows the lowstand channel cut by the New River and the area of intersection where the topographically high rock features intersect the barrier islands. Notice the similarity of orientation patterns of both the estuaries and older Pleistocene beach ridges.

of the rock scarps represent a major source and supply of ‘new sediment’ to the adjacent beaches (Fig. 11). Abundant gravel, up to boulder-size grains, is derived from the rock scarps and lower shoreface and delivered to the beach during storms where it is rapidly broken down to sand-sized grains in the surf zone.

2.5. Non-headland dominated shorefaces

Transgressive shorefaces In North Carolina, a significantly large portion

of the barrier islands are underlain by estuarine peat and mud deposits (Fig. 17) often containing in situ tree stumps. These fossil estuarine units were overrun by barrier island systems as they migrated upward and westward in response to the general Holocene transgression. During winter storms, much beach sand is transported off the beach and stored in bars, exposing the semi- indurated, back-barrier sediments that now occur in the surf zone (Fig. 10).

Deposits of estuarine peat and clay crop out in the surf zone along major portions of the North

Carolina Outer Banks from Corolla to Southern Shores, Nags Head to Cape Hatteras, and from Portsmouth to Cape Lookout (Fig. 1). Pierce and Colquhoun (1970) found that these peats typically were continuous from just below low-tide level on the ocean beach into the present-day back-barrier marsh. Peat and clay occurred extensively along the entire eroding south shoreline of Oregon Inlet. Recent construction of the Oregon Inlet jetty and associated beach nourishment projects have tem- porarily buried most of these outcrops.

At the northeast end of Topsail Island, a 0.5 m thick peat crops out periodically in the surf zone and can be traced laterally around New River Inlet to a modern back-barrier salt marsh. Underlying the peat is a compact gray clay of unknown thickness. Storm erosion produces large boulders (up to 0.7 m) of peat and clay, along with Oligocene rock fragments from the offshore scarps; these gravels represent a significant input of ‘new’ post-storm beach sediment.

Discontinuous zones of 1200 to 1700 year old peat have been mapped along major portions of both Masonboro Island and Long Beach (Fig. 1).

226 S. R. Riggs et aL/Marine Geology 126 (1995) 213-234

M ,Wathered Surface

Fig. 13. Geologic cross-section along Onslow Beach based upon continuous split-spoon drill holes located in Figure 14. This section shows two Oligocene rock ridges that rise to approximately 5 m below sea level and pass under Onslow Beach.

At Long Beach the outcrop belt extends approximately 8 km along the low tide beach. Closely spaced cores show that these peats extend,,beneath the modern barriers and into the seaward fringes of the barrier-impounded estuaries (Griffin et al., 1977; Cleary and Hosier, 1986).

Thus, we estimate that about 50% of the North Carolina beaches are underlain by older estuarine peat and clay sediments. Obviously, these sediments have very different compositions, densities, cohesiveness, and resistance to erosion and transport than normal beach sands. Consequently, their occurrence in the shoreface will affect the beach width, shoreface profile, and rates of shoreline recession.

Regressive shorefaces Only local and relatively small segments of the

North Carolina shoreline are presently characterized by regressive shoreface conditions. These areas generally occur on the flanks of headlands and represent temporary episodes of coastal progradation that usually alternate with episodes of longer-term truncation as the headland recedes. However, during episodes of regression, these shorefaces are relatively stable, are characterized by progradational geometries, beach ridge accretion, dune ridge development, and have the poten- tial for approximating the idealized “profile of equilibrium”. Hatteras Island, southwest of Cape Hatteras, as well as portions of Portsmouth Island,

S. R. Riggs et al/Marine Geology 126 (1995) 213-234 227

WASffOMR TERRACES’ 6%

Fig. 14. Map of Onslow Beach showing the (1) location of drill holes used in Fig. 13, (2) location (middle arrow) of coastal inversion that takes place where the Oligocene rock ridges intersect the island, and (3) plot of the average annual rates of shoreline recession from Benton et al. (1993) along Onslow Beach. The southern portion of the island is experiencing severe shoreline recession whereas the northern portion is experiencing accretion and dune ridge development, The area of most severe erosion adjacent to New River Inlet is largely a direct response to inlet modification.

Fig. 15. Oblique aerial photograph looking northeast from New River Inlet to Onslow Beach (A). The approximate location and orientation of the submerged Oligocene rock ridges are indicated on the photo (E); notice the major change in orientation of Onslow Beach northeast of the intersection of these ridges with the barrier island (C).

Cape Lookout, Shackelford Banks, and Bald Head Island are examples of this type of system (Fig. 1).

Shoreface regression also takes place on a small scale on beaches adjacent to some inlets. This process depends upon and will develop only if

Fig. 16. Oblique aerial photograph looking southwest from the southern end of Onslow Beach (A), across New River Inlet (B), and to Topsail Island (C). The approximate location and orientation of the submerged Oligocene rock ridges are indicated on the photo (D); notice the major change in orientation of Topsail Island southwest of the intersection of these ridges with the barrier island (E).

there is an adequate sand supply. The result is the progradation of the shoreface adjacent to the inlet to produce the classic ‘drumstick’ barrier of Hayes ( 1976). The drumstick portion of the barrier

228 S. R Riggs et al.IMarine Geology 126 (1995) 213-234

Fig. 17. Oblique aerial photograph of a transgressive shoreface system with outcrops of Holocene peat (A) in the surf zone. These peats have 14C ages that range between 1200 and 1700 yrs B.P. Photo was taken in 1978 looking north at Long Beach, North Carolina.

is characterized by wide beaches, continuous dune ridges, multiple recurved accretionary beach ridges, and shoreline accretion. The northeastern end of Onslow Beach (Fig. 9), adjacent to Browns Inlet, displays these characteristics (Cleary and Hosier, 1987). Similar inlet influenced barrier segments include Sunset Beach west of Tubbs Inlet and Figure Eight Island downdrift of Rich Inlet (Fig. 1).

Channel-dominated inlet-fill shorefaces Fisher (1962) mapped the spatial and temporal

distribution of historic inlets along the North Carolina coast north of Cape Lookout from aerial photographs. An estimation based upon Fisher’s map suggests that about 50% of this coastal system has been occupied by inlets during the historic past with >78% having been occupied by inlets based upon the presence of old flood-tide deltas landward of the barriers. These segments of the barriers are underlain by thick accumulations of inlet fill sands and gravels (Clear-y and Hosier, 1979a; Eames 1983; Hine and Snyder, 1985; Riggs et al., 1992).

Fig. 18 is a geologic cross-section down the axis of the northern Outer Banks showing the inlet fill channel facies for two major inlets that dominate the shallower sediment package to depths of approximately 10 m below MSL. The southern inlet is the modern Oregon Inlet which opened in about 1846 and migrated at least 4 km southward

to its present location. The northern inlet facies was produced by Roanoke Inlet, which was active when the colonists arrived in 1585 and migrated at least 4 km southward before it finally closed in about 1817. As the shoreface recedes in response to ongoing transgression, the upper portion underlain by inlet fill, readily erodes and produces relatively steep slopes (Fig. 19). Recession of the shoreface in old inlet fill will also result in an abundant supply of ‘new sand and gravel sediment’ of appropriate size and composition for the adjacent beaches.

The area south of Cape Lookout (Fig. 1) represents a very different coastal compartment dominated by the Onslow submarine headland (Fig. 2). Thirteen modern inlets occur within the coastal compartment between Cape Lookout and Cape Fear as compared to only 4 active inlets in the two coastal compartments north of Cape Lookout. The eight modern inlets along the 76 km between Onslow Beach and Carolina Beach in the southern portion of Onslow Bay are highly migratory (Cleary and Hosier, 1979a). They conclude that over 70% of the barrier island length south of New River Inlet has inlet channel-dominated shorefaces as a result of inlets migrating along the island during the last several centuries. For example, most of the coastal area of Shell, Figure Eight, and Coke Islands have historically been occupied by inlets (Fig. 20). The result is a shoreface system that is underlain by channel sands and gravels that formed in response to a series of rapidly migrating Holocene inlets.

Channel-dominated valley-Jill shorefaces Portions of barrier islands that are within

the valleys of major Piedmont drainage systems (Fig. 2) have shorefaces that are characterized by complex sediment sequences deposited in ancient paleofluvial valleys (Pearson, 1979; Hine and Snyder, 1985; Riggs et al., 1992; Snyder et al., 1994). These drainage systems have repeatedly incised themselves into their valleys and associated valley fill during each Pleistocene glacial episode when sea level occupied lowstand positions. During the subsequent transgression, these large channel complexes were systematically back-filled with a new vertical succession of fluvial and estua-

S. R. Riggs et al./Marine Geology 126 (1995) 213-234 229

20

25

Kll Hawk Kill Devil Hills Nags Head

North South

Fig. 18. Geologic cross section down the center of the present barrier island, extending from the north side of Kitty Hawk Bay, southward through Kill Devil Hills, Nags Head, and to the south side of Oregon Inlet. This section shows the (1) inlet-filled channels of modern Oregon Inlet and historic Roanoke Inlet; (2) Holocene valley-filled channel of the prehistoric Roanoke River/ Albemarle estuarine system; (3) portions of at least three stacked Quatemary depositional sequences (DS-3 and DS-6 are Pleistocene in age; DS-7 is Holocene in age); and (4) major sediment facies of each depositional sequence. Figure is modified from Riggs et al. (1992).

AVALON PIER PROFWE

NAOS HEAD PIER PROFILE

5 ENNETTES PIER PROFILE

0 metwe

V.E. S4 X 0 1 km

I

Fig. 19. Plot of shoreline profiles on channel-dominated shoreface systems in Nags Head and Kill Devil Hills, North Carolina (modified from Pearson, 1979). Jennettes pier profile is in Nags Head and is an inlet-fill shoreface produced by historic Roanoke Inlet; it is underlain by inlet sands and gravels. Nags Head pier profile is in Nags Head and is a transgressive shoreface; the upper portion of the shoreface is underlain by estuarine peats and muds. Avalon pier profile is in Kitty Hawk and is a valley-fill shoreface produced by the Roanoke River/Albemarle estuarine system; the lower portion of the shoreface is underlain by dense estuarine muddy fine sands.

230 S. R. Riggs et aL/Marime Geology 126 (I 995) 213-234

ATLANTIC OCEAN

Fig. 20. Map showing the position and movement of historic inlet zones on Shell, Figure Eight, and Coke Islands, North Carolina. Notice that the majority of these barriers have been impacted by historic inlets; vibracoring on the islands demonstrates that 100% of these islands consist of inlet channel backfill sediments. Figure is modified from Hosier and Cleary (1979) and Brooks and Cleary (1989).

rine sediments. The upper estuarine sediments generally are composed of fine-grained, muddy sediments that can be fairly cohesive.

A major portion of the barrier island in the Kitty Hawk area (Fig. 1) has prehistorically been dominated by the paleofluvial drainage of the Roanoke River/Albemarle Sound estuarine system (Riggs et al., 1992). Fig. 18 outlines the location and general geometry of the large Roanoke- Albemarle valley fill. Pearson (1979) mapped a relatively less steep profile down the shoreface in the middle of this valley fill as compared to similar profiles on the shoreface dominated by transgressive shorefaces and by the historic Roanoke Inlet channel fill (Fig. 19). Pearson’s high-resolution seismic profiles and associated vibracores down the shoreface also demonstrate that muddy estuarine sediments crop out at the surface or underlie ~0.5 m of modern shoreface sand from the offshore bar seaward to 22.5 m below MSL.

As the shoreface recedes in response to ongoing transgression, the portion that is underlain by estuarine valley fill, erodes slowly and produces relatively shallower slopes (Fig. 19). Also, erosion of the valley fill releases fine-grained sediments

that are easily suspended and transported offshore and do not supply the adjacent beaches. Consequently, beaches such as those in the Kitty Hawk area are characterized by relatively higher rates of shoreline recession (Benton et al., 1993) than adjacent areas such as Southern Shores to the north and Kill Devil Hills to the south as demonstrated in Fig. 21.

3. Conclusions

Along continental margins with limited sand supplies, such as the U.S. Atlantic coast, the shoreface is not an infinitely thick pile of sand. Rather, it is a thin, variable, and temporal accumu- lation of sand superimposed or perched upon a pre-existing and highly-dissected geologic framework. Holocene sea-level rise has produced a modern transgressive barrier island, estuarine, and fluvial sequence of coastal sediments that are being deposited unconformably over irregularly preserved remnants of pre-existing stratigraphic sequences consisting of many sediment and rock units of variable ages, origins, and compositions.

S. R Riggs et al. JMarine Geology 126 (1995) 213-234 231

Fig. 21. Oblique aerial photograph looking south from Southern Shores (A) to Kitty Hawk (B), Kill Devil Hills (C), and Kitty Hawk Bay (0). Notice the prominent change in the shoreline (E) that occurs approximately at the southern boundary of the Roanoke River/Albemarle estuarine system valley fill complex. The major erosional notch in the shoreline (F) is dominated by valley fill sediments, whereas the more stable portion to the south (G) is predominantly a transgressive shoreface.

It is the complex variability in this underlying geologic framework, in consort with the physical dynamics of each specific coastal system, that ultimately determines the (1) 3-D shoreface morphology, (2) composition and texture of beach sediments, and (3) shoreline recession rates.

Based upon the pre-Modern geologic framework, there are six general categories of shoreface systems that occur along the North Carolina coast. Headlands are morphological features that rise above the active ravinement surface and are composed of semi-indurated to indurated, Pleistocene or older sediment units. ( 1) Subaerial headlands are characterized by the active incisement of a wavecut platform and cliff into Pleistocene or older stratigraphic units with an associated perched beach. (2) Submarine headlands are submerged morphological features composed of Pleistocene or older stratigraphic units that have been incorporated into the modern shoreface and upon which the barrier-estuarine system is perched. Ancient sediments crop out on the eroding shoreface and commonly occur on the inner shelf as bathymetric highs seaward of the modern shoreface and thus, modify incoming waves and currents.

Nonheadland shorefaces are dominated by Holocene processes and sediments and can be

divided into four general subclasses. (3) Trans- gressive shorefaces are composed of compact peat and mud that extend from the modern estuaries, under the barrier sands, and crop out within the surf zone and upper shoreface. The steep shoreface is often characterized by an irregular geometry and the beach is dominated by discontinuous and highly scarped dune ridges with abundant washover fans. (4) Regressive shorefaces are composed of unconsolidated sand and occur along barrier segments that have adequate sediment supplies and are often associated with inlets, headlands, and cape structures. The beaches are dominated by progradational geometries with accretionary beach ridges. Channel-dominated shorefaces consist of two distinctive types of systems. (5) Inlet-fill shorefaces are composed of unconsolidated sand and gravel sediments with clinoform infill geometry that forms in response to actively migrating inlets. (6) Valley-fill shore-

faces have historically been occupied by large paleofluvial drainage systems and are underlain by thick accumulations of fluvial-estuarine channel- fill sediments in response to deglaciation and Holocene sea-level rise. Valley-fill sediments are typically composed of fine-grained sediment and therefore will erode differently than inlet-fill sands and gravels. The sediments derived from valley-fill erosion are often not compatible with the dynamics of the adjacent beach systems. Consequently, valley-fill shorefaces are often characterized by lower slopes and sediment deficient beaches that are actively eroding.

Thus, the basic structural, stratigraphic, and geomorphic characteristics of the pre-barrier land surface interacts in a complex way with modern coastal processes to determine the barrier beach morphology and shoreface dynamics. Each barrier beach and shoreface are total products of their geologic heritage; the signature of their geologic history controls and intluences the present morphology, shoreface dynamics, and rates of shoreline recession. Consequently, the concept of a common equilibrium profile for all shorefaces is neither realistic nor adequate when considering detailed processes along any given coastal segment. It is imperative that society learn to live with and manage our complex shorelines. In order to do

232 S.R. Riggs et al./Marine Geology 126 (1995) 213-234

this, we must understand the detailed geologic framework underlying the shoreface and the inner shelf, as well as the physical dynamics operating within and upon regional segments of the shoreface system. Then we can realistically begin to model the large-scale behavior of coastal systems.

Acknowledgements

This research is in part a product of research supported by the following grants: NOAA- University of North Carolina Sea Grant College Program (1970-1974) to SRR; N.C. Office of Marine Affairs (19781979) to SRR; National Science Foundation OCE-8 110907, OCE-8 118 164, and OCE-8342777 (1981-1984) to SRR; U.S. Marine Corps at Camp Lejeune, N.C. (1985-1987) to WJC; NOAA-National Undersea Research Center/University of North Carolina at Wilmington (1992-1993) to WJC and SRR. Appreciation is expressed to the many students of the Geology Departments at East Carolina University and University of North Carolina at Wilmington who have helped develop the level of understanding of North Carolina coastal systems that lead to this paper. This paper is a contribution to International Geological Correlation Program (IGCP 274) “Quaternary Coastal Evolution”, and to IGBP Land-Ocean Interaction in the Coastal Zone core project (LOICZ).

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Influence of inherited geologic framework on barrier shoreface morphology and dynamics

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