Regional seismic stratigraphy and controls on the ... - Sakai@UD

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Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA David J. Mallinson a,⁎, Stephen J. Culver a, Stanley R. Riggs a, E. Robert Thieler b,1, David Foster b,1, John Wehmiller c,2, Kathleen M. Farrell d,3, Jessica Pierson e,4 a

Department of Geology, East Carolina University, Greenville, NC 27858, United States U.S. Geological Survey, Coastal and Marine Geology Program, 384 Woods Hole Road, Woods Hole, MA 02543, United States Department of Geology, University of Delaware, Newark, DE 19716, United States d North Carolina Geological Survey, Raleigh Field Office and Core Repository, MSC 1620, Raleigh, NC 26699-1620, United States e Department of Geological Sciences, West Virginia University, 98 Beechurst Avenue, 330 Brook Hall, Morgantown, WV 26506, United States b c

a r t i c l e

i n f o

Article history: Received 11 July 2008 Received in revised form 1 October 2009 Accepted 6 October 2009 Available online xxxx Communicated by J.T. Wells Keywords: seismic stratigraphy sequence stratigraphy Quaternary coastal stratigraphy antecedent topography

a b s t r a c t Seismic and core data, combined with amino acid racemization and strontium-isotope age data, enable the definition of the Quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of North Carolina, U.S.A. Seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets, and enable an understanding of the evolution of this margin. Data reveal the progressive eastward progradation and aggradation of the Quaternary shelf. The early Pleistocene inner shelf occurs at a depth of ca. 20–40 m beneath the western part of the modern estuarine system (Pamlico Sound). A mid- to outer shelf lowstand terrace (also early Pleistocene) with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract, occurs at a deeper level (ca. 45–70 m) beneath the modern barrier island system (the Outer Banks) and northern Pamlico Sound. Seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle Pleistocene shelf. Middle to late Pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units, and contain numerous filled fluvial valleys that are incised into the early and middle Pleistocene deposits. The stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea-level fluctuations during the middle to late Pleistocene, expressed as falling stage, lowstand, transgressive and highstand systems tracts. Thick stratigraphic sequences occur within the middle Pleistocene section, suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north. Furthermore, the antecedent topography plays a significant role in the evolution of the geomorphology and stratigraphy of this marginal system. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The response of continental margin depositional processes and the resulting stratigraphic framework to variations in the amplitude and frequency of sea-level fluctuations continues to be an area of intensive active research (e.g., Wheeler, 1958; Sloss, 1963; Miall, 1986; Vail et al., 1987; Haq et al., 1987; Posamentier and Vail, 1988; Posamentier ⁎ Corresponding author. Tel.: +1 252 328 1344. E-mail addresses: [email protected], [email protected] (D.J. Mallinson), [email protected] (S.J. Culver), [email protected] (S.R. Riggs), [email protected] (E.R. Thieler), [email protected] (D. Foster), [email protected] (J. Wehmiller), [email protected] (K.M. Farrell), [email protected] (J. Pierson). 1 Tel.: +1 508 457 2350. 2 Tel.: +1 302 831 2926. 3 Tel.: +1 919 733 7353x29. 4 Tel.: +1 304 293 5603.

et al., 1992; Nittrouer and Kravitz, 1996; Nittrouer, 1999). Understanding continental margin stratigraphy is important because these areas provide an extensive temporal and spatial record of processes that have occurred during their formation, and, as such, provide insight into the frequency and magnitude of the various forcing factors (i.e., eustatic sea-level fluctuations, glacio-hydro-isostasy, tectonics, subsidence, sedimentation patterns, etc.). The corollary to this is if a high-resolution record of forcing factors (e.g., sea-level fluctuations) is known, and can be correlated to the stratigraphic record, then a fundamental understanding of the controls on stratal development may be derived. Such a record is best available for the late Cenozoic, and is best developed for the Quaternary (Lu and Fulthorpe, 2004). The Quaternary Period (ca. 2.6 Ma–0) represents a time of dramatic climate and sea-level change. The early Pleistocene Epoch (ca. 2.6 Ma to 0.78 Ma) was characterized by climate and sea-level

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Please cite this article as: Mallinson, D.J., et al., Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA, Marine Geology (2009), doi:10.1016/j.margeo.2009.10.007

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changes with a periodicity of ca. 41 ky, most likely driven by variations in obliquity (Imbrie et al., 1993). Between ca. 1 Ma, and 0.8 Ma, climate and sea-level cyclicity transitioned to a 100 ky periodicity. Understanding this Pleistocene transition (Raymo et al., 1997) is fundamental to a variety of research areas, especially climate dynamics, sea-level change, and the evolution of continental margins. Few investigations exist of continental margin evolution in response to Quaternary sea-level change along the U.S. Atlantic margin (Riggs et al., 1995; Foyle and Oertel, 1997; Boss et al., 2002; Carey et al., 2005; Mallinson et al., 2005). An expanded record of Quaternary sea-level fluctuations is preserved in the Albemarle Embayment of northeastern North Carolina (Riggs et al., 1992; Boss et al., 2002; Mallinson et al., 2005) (Fig. 1). We are striving to understand the Quaternary record as it relates to the evolution of this expansive coastal system. Sea-level change is clearly a fundamental control on the stratigraphic architecture of this margin (as it is on virtually all margins). An additional significant control is the inherited geologic framework; i.e., the control of the antecedent geology on successively younger depositional environments and sequences (Riggs et al., 1995). The significance of the inherited geologic framework, as it interacts with sea-level cyclicity (e.g., large-scale morphodynamic processes) and contributes to the evolution of stratal geometries, modern coastal geomorphology and behavior, is poorly understood. Riggs et al. (1995) illustrated the role of the inherited shallow geologic framework in governing coastal erosion rates, depositional patterns, and general geomorphology on local scales (1 to 10 km) in coastal North Carolina. On greater spatial scales (10 to N100 km), there is speculation that coastal patterns (e.g., cuspate forelands, drainage patterns, estuary locations and dimensions, etc.) on the U.S. Atlantic coast are associated with large-scale and deep-seated structures, including the crystalline basement high of the Mid-Carolina Platform High (MCPH) (Klitgord et al., 1988; Gohn, 1988) and the Cape Lookout High (CLH), a depositional high composed of Oligocene through Pliocene sedimentary rocks (Snyder et al., 1982; Popenoe, 1985; Snyder et al., 1990; Popenoe, 1990) (Fig. 1). Other workers have shown that modern hydrodynamic processes are very important to the origin and maintenance of cuspate forelands (McNinch and Wells, 1999; McNinch and Leuttich, 2000; Ashton and Murray, 2006). This investigation offers new insight into the Quaternary evolution of the passive margin system in the Cape Hatteras region, and how it responded to sea-level change and antecedent topography. The purpose of this manuscript is to establish the large-scale Quaternary framework and antecedent controls from which more detailed sequence stratigraphic interpretations may be made to elucidate the relative contributions of eustasy, isostasy, paleoceanographic, and depositional/erosional processes to the evolution of this passive margin.

2. Regional setting The regional stratigraphic framework of the southeast U.S. Atlantic margin (South Carolina to Virginia) is dominated by the Mid-Carolina Platform High (a.k.a., the Cape Fear Arch), a broad mid-platform high that consists of shallow (b1 km) Paleozoic crystalline basement rock that extends from approximately Cape Romain, South Carolina, to Cape Lookout, North Carolina (Snyder et al., 1982) (Fig. 1). The position of this high has controlled the location of adjacent depositional basins, and the resulting structural framework of overlying late Mesozoic, and early Tertiary sedimentary deposits (Brown et al., 1972; Klitgord et al., 1988; Gohn, 1988). The Cape Lookout High is a late Tertiary depositional high situated on the northern flank of the MCPH (Fig. 1). Popenoe (1985, 1990) proposed that the core of this feature formed during the Oligocene as a sediment drift on the north side of the Suwannee Current which flowed through the Gulf Trough, a seaway separating siliciclastic

deposits to the north (in central and north Georgia) and the carbonate environments of the Florida Platform to the south. Popenoe (1990) suggested that the CLH occurs beneath Cape Hatteras, with the crest of the high oriented northeast–southwest, parallel to the modern barrier islands south of Cape Hatteras (Fig. 1). Snyder et al. (1982, 1990) interpreted the CLH as being more restricted to the Cape Lookout area, with a more east–west orientation. Boundary–current interactions with the margin also produced numerous erosional surfaces, and controlled the stratigraphic framework of the Miocene section on the continental shelf between Cape Fear and Cape Lookout (Snyder et al., 1982; Riggs et al., 1985; Snyder et al., 1990). On the North Carolina margin, the thickest Quaternary sedimentary deposits occur within the Albemarle Embayment (Ward and Strickland, 1985; Popenoe, 1985; Mallinson et al., 2005), a basin constrained by the Norfolk Arch to the north, and the Cape Lookout High to the south (Brown et al., 1972; Gohn, 1988) (Fig. 1). The Quaternary section attains a maximum thickness of approximately 85–90 m beneath northern Pamlico Sound (Fig. 2), and thins southward to approximately 20 m at the southern extent of the Sound (Mallinson et al., 2004). Geophysical data reveal at least 18 Quaternary seismic sequences within the Albemarle Embayment (Riggs et al., 1992; Mallinson et al., 2005). These seismic sequences are bounded by medium- to high amplitude seismic reflections, indicating unconformities and (in some cases) marine flooding surfaces, and so constitute true depositional sequences, as well as parasequences (Van Wagoner et al., 1988; Catuneanu et al., 2009). Quaternary units in the Dare Headland area (Fig. 2A) generally dip eastward and range in thickness from 5 m to 10 m (O'Conner et al., 1973; Riggs and O'Conner, 1974; Riggs et al., 1992; Mallinson et al., 2005; Parham et al., 2007). Amino acid racemization data yielded four aminozones comprising seven depositional sequences to a depth of ca. 30 m, and ranging in age from Holocene to ca. 1.8 Ma (Riggs et al., 1992). Lithologically-defined depositional sequences contained multiple lithofacies that represent many different coastal environments. In the same general area (Croatan Sound; Fig. 2A), Parham et al. (2007) defined 13 Quaternary depositional sequences, to a depth of 60 m, composed of fluvial, estuarine, barrier island and open shelf facies bounded by subaerial unconformities. Presently, the coastal system in this area exhibits a broad expanse (N100 km) of paralic environments (coastal swamps and marshes, estuaries, shoals, and barrier islands) with an average seaward gradient of ca. 0.01°. The base of the modern shoreface, seaward of the barrier islands (the Outer Banks) occurs at a depth of ca. 15 m to 20 m below present sea level. The present continental shelf consists of reworked surficial siliciclastic sands modified by shelf currents and wave energy to form shelf sand ridges (Swift et al., 1978), interspersed with outcrops of Pleistocene calcareous sandstone and estuarine mud. The shelf break occurs at a depth of ca. 50 m. The western boundary of the Gulf Stream occurs at the shelf break ca. 20 km east of Cape Hatteras (Fig. 1). The Gulf Stream in this area flows at a velocity of ca. 1 m s− 1 and influences sediment transport on the outer shelf, slope, and rise, producing hemipelagic current drift deposits (Stanley et al., 1981). 3. Materials and methods Approximately 3000 km of high-resolution single-channel seismic data were acquired in the estuaries of the northeastern NC coastal system and offshore to 10 km (Fig. 2B) using a GeoPulse and Huntec boomer seismic source and an ITI hydrophone streamer, and were recorded with Triton ISIS acquisition software. All data were integrated with differential GPS for navigation. Seismic data were processed (bandpass filtered) using Sioseis and Promax software. Processed seismic lines were digitized and interpreted using The Kingdom Suite (TKS) v.7 software (copyright, Seismic Micro-Technology, Inc.). Digitized


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Fig. 1. Map showing the location, morphology, and deep structural features of the southern mid-Atlantic coastal system. Elevation data are from the Coastal Relief Model (http:// www.ngdc.noaa.gov/mgg/coastal/coastal.html). Structural controls are based upon Snyder et al. (1982), and Popenoe (1985). MCPH = Mid-Carolina Platform High; CLH = Cape Lookout High.

horizons were gridded using TKS, to produce surface structure maps. All depths were calculated using an acoustic velocity of 1600 m s− 1 for all sediments, which is a reasonable average based on correlations of core logs and seismic data. Twenty-eight core holes were drilled by the NC Coastal Geology Cooperative (NCCGC; involving East Carolina University, U.S.G.S., N.C. G.S., University of Delaware, Virginia Institute of Marine Sciences, and the University of Pennsylvania) on the Outer Banks barrier island system and mainland (Fig. 2B) using a rotasonic drill rig. Cores were

logged for lithology and subsampled for grain-size (Folk, 1980), fossils, radiocarbon age analyses, amino acid racemization analyses (AAR), and Sr-isotope analyses (87Sr/86Sr), and correlated with the offshore seismic data. Foraminifera were picked from the N63 µm size fraction and concentrated using sodium polytungstate (Munsterman and Kerstholt, 1996). Species were identified by comparison to the published literature on modern benthic foraminifera of the U.S. Atlantic coast (e.g., Culver and Buzas, 1980, and included references) and confirmed by comparison with type material lodged in the


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Fig. 2. A). Map showing the general bathymetry of the study area. Depth is in meters below mean sea level (m bsl). Data were compiled using the Coastal Relief Model (Divins and Metzger, 2006). B). Map showing core locations (black dots) and seismic data transects (black lines) used in this investigation. Cores referred to in this paper are labeled. The inset map also shows the location of seismic profiles presented.

Cushman Collection, Smithsonian Institution, Washington, D.C., and The Natural History Museum, London. Robust specimens of the mollusk Mercenaria sp. (not necessarily whole valves) were sampled and analyzed for the extent of racemization of amino acids (AAR) using gas chromatographic methods outlined in Wehmiller and Miller (2000). These methods yield D/L values for at least six different amino acids. Multiple aminozones, or simple clusters of D/L values, are recognized in the MLD-OBX dataset, and definition of these zones follows that of Riggs et al. (1992) (Wehmiller et al., 2006, 2007). The original tri-partite zonation (AZ-2, AZ-3, and AZ-4, late, middle, and early Pleistocene) is now further divided, with designations “plus” (+) or “minus” (−) if the observed D/L value(s) differ by more than 10% from the D/L value of the aminozone(s) first reported by Riggs et al. (1992). Age estimates based on these aminozones are based on comparisons with local radiometric age control (Wehmiller et al., 2004), paired AAR and 14C analysis of single shells (Wehmiller et al., 2007), and optional kinetic models for racemization (e.g., York et al., 1989; Wehmiller et al., 2007, in press). Strontium isotopes (87Sr/86Sr) were measured on foraminifera from cores at the University of Florida laboratory, using SRM-987 as a standard. The 87Sr/86Sr ratio was compared to the Quaternary 87Sr/86Sr curve of Farrell et al. (1995) after a correction of + 0.000012 was applied to account for the difference in the measured values of SRM-987 (Table 2). The timescales of Berggren et al. (1985) and Shackleton et al. (1995) are used to calculate ages. Throughout this manuscript we use the sequence stratigraphic terminology of Catuneanu et al. (2009). Regionally continuous seismic

horizons recognized in this study include subaerial unconformities (SU) with associated incised paleo-fluvial valleys, marine ravinement surfaces (MRS) which are relatively planar surfaces truncating older strata and form by wave erosion along a marine shoreline, bay ravinement surfaces (BRS) which are associated with ravinement of a bay shoreface and occur at the base of estuarine deposits, flooding surfaces (FS) which represent a rapid landward shift in sedimentation, and a regressive surface of marine erosion (RSME) related to submarine erosion of shelf deposits at the seaward limit of regression. Sequence boundaries (SB) are expressed as unconformities (either SU, MRS, or BRS) that are distinguishable in seismic data, as they are evidenced by erosion of underlying strata and the occurrence of paleo-valleys with associated cut-and-fill facies (Riggs et al., 1992; Boss et al., 2002; Mallinson et al., 2005). Flooding surfaces do not exhibit channelization and cut-and-fill facies in seismic data, but are evidenced by an abrupt shift in facies creating an acoustic reflector, that may occur in response to a rapid relative sea-level rise or decrease in sediment supply. Flooding surfaces represent the bounding surfaces for parasequences (Van Wagoner et al., 1988; Catuneanu et al., 2009). 4. Results 4.1. Quaternary stratigraphic framework Several prominent, medium- to high amplitude, regionally continuous acoustic reflections occur within the Quaternary section throughout the area (Figs. 3–5) and define the boundaries for six



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Fig. 3. Line interpretations with corresponding high-resolution single-channel seismic data from the eastern (A–A′) Pamlico Sound, illustrating the complex stratigraphy within the study area. Also labeled are the approximate extents of the Southern Pamlico Valley (SPV), the Central Pamlico Shelf (CPS) and the Northern Pamlico Terrace (NPT). The scales indicate two-way travel time (twtt — in milliseconds) and meters below sea level (m bsl) based on an estimated seismic velocity of 1600 m/s (calculated by correlations to core data).

regionally continuous seismic stratigraphic units (SSU) (Figs. 6 and 7; Table 1). These reflections were found to be illustrative of the overall Quaternary evolution of this system. Characteristics of these reflections and the associated intervening SSUs are presented in Table 1 and discussed below in ascending stratigraphic order, beginning with the basal Quaternary reflection (Q0). The SSUs contain other reflections corresponding to parasequence boundaries, and locally discontinuous (eroded) depositional sequences, particularly within valley-fill complexes (Riggs et al., 1992). However, seismic grid density was insufficient and erosion of the sequences too complex to define them on any significant spatial scale. 4.1.1. Reflection Q0 A high amplitude, regionally continuous reflection was defined that serves as acoustic basement throughout the majority of the study area, except in the far south where the reflection shoals to within ca. 15 m below mean sea level (m bsl), and deeper reflections are seen. This basal reflection is designated Q0 (Figs. 3–5). Q0 defines an embayment with the deepest area (ca. 85 m bsl) beneath northern Pamlico Sound (Fig. 5). The Q0 horizon beneath Pamlico Sound dips generally to the northeast, but defines a subtle arch with the axis trending northeast. The associated reflector (i.e., the physical stratigraphic surface) was penetrated by core MLD02 (at 20 m bsl) on Cedar Island (Fig. 5), and core OBX14 on Ocracoke Island (at 36 m bsl) (Fig. 2B), where a moldic non-reefal limestone, containing abundant echinoderms and mollusks was recovered. The presence of the scallops, Chesapecten

madisonius and Carolina pecten, indicates a Pliocene age (Blackwelder, 1981; L. Ward and P. Weaver, personal communications, 2005). Overlying sediments contain Pleistocene foraminiferal assemblages, thus, Q0 represents the Pliocene/Pleistocene boundary. The occurrence of the Pliocene/Pleistocene boundary at these depths within the Albemarle Embayment is also confirmed by foraminiferal assemblages described by Zarra (1989) from test wells in the Pamlico and Albemarle Sounds. 4.1.2. SSU I Seismic Stratigraphic Unit I (Figs. 6 and 7; Table 1) is bounded by reflections Q0 and Q10, occurs from a minimum depth of ca. 42 m bsl to a maximum depth of ca. 80 m bsl and ranges from 5 to 22 m in thickness. The thickest section is located in the northwest Pamlico Sound. Sediments associated with this unit were recovered in MLD01 and MLD05 (Fig. 7), and are very fine- to coarse-grained sands, and muddy sands, characterized by moderate species richness (16 to 28 species per sample) open shelf foraminiferal assemblages. Dominant taxa are Elphidium excavatum, Hanzawaia strattoni, Quinqueloculina seminula, Epistominella sandiegoensis, Rosalina floridana, Nonionella sp., Buliminella elegantissima, Ammonia parkinsoniana, and Textularia cf. T. gramen. Assemblages contain 1% to 4% planktonic foraminifera. An AAR age analysis of a Mercenaria shell at the top of this unit places it in Amino Zone 4+, suggesting an age of ca. 1840 ka to 1220 ka (Wehmiller et al., 2007, in press) (Fig. 7; Table 2). Sr-isotope data from MLD04-124.3 yield a corrected value of 0.709115 ± 0.000013, indicating a maximum age of ca. 2400 ka (Table 2).


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Fig. 4. Line interpretations with corresponding high-resolution single-channel seismic data from north-central (B–B′; L13f1) and southern (C–C′; L40f1) Pamlico Sound, illustrating the position and character of reflections presented in this study. The scales indicate two-way travel time (twtt — in milliseconds) and meters below sea level (m bsl) based on an estimated seismic velocity of 1600 m/s (calculated by correlations to core data).

4.1.3. Reflection Q10 Q10 is a prominent medium to high amplitude reflection that is sub-parallel to Q0 (Figs. 5–7), and occurs with a closely associated, sometimes coplanar, transgressive marine ravinement surface (apparent across valley-fill) and an overlying marine flooding surface defined as reflection Q10b (Fig. 6). Q10 exhibits a gradual eastward dip from a depth of ca. 50 m in the western Pamlico Sound, to ca. 80 m in eastern Pamlico Sound, mimicking Q0 in dip angle and direction (Fig. 5). 4.1.4. SSU II Seismic Stratigraphic Unit II is constrained by reflections Q10 and Q20, and occurs from ca. 30 to 65 m bsl and ranges from 5 to 20 m in thickness. Sediments included in this unit were recovered in MLD01 and MLD05 (Figs. 6 and 7), and consist of a basal sand with bioclastic gravel lag deposit occurring between reflections Q10 and Q10b, overlain by quartzose sandy muds, with moderately diverse open shelf foraminiferal assemblages containing 2% to 7% planktonic foraminifera, and dominated by Elphidium excavatum, Epistominella sandiegoensis, Quin-

queloculina seminula, and Buliminella elegantissima. The quartzose sandy mud unit is overlain by interbedded quartz sandy muds and muddy sands, with low to moderately diverse open inner shelf foraminiferal assemblages. Planktonic specimens are generally rare or absent and assemblages are strongly dominated by E. excavatum. Amino acid racemization analyses on Mercenaria shell material from OBX11 and MLD01 place this unit in AZ4+, suggesting an age of ca. 1840 ka to 1220 ka (Fig. 7; Table 2), an early Pleistocene age. Sr-isotope data from MLD01-142.7 yield a corrected value of 0.709141 ± 0.000011, indicating a maximum age of ca. 1630 ka (Table 2). 4.1.5. Reflection Q20 Q20 is a moderate to high amplitude semi-continuous hummocky to chaotic reflection that defines a high in the western basin, and a low (an outer shelf lowstand terrace) in the eastern basin (Fig. 5), and occurs with a closely associated, sometimes coplanar, transgressive marine ravinement surface and marine flooding surface defined as reflection Q20b. The relief across these areas is ca. 36 m. These features are designated the Central Pamlico Shelf (CPS) and



the Northern Pamlico Terrace (NPT), respectively (Fig. 5A, Q20 panel). Across the CPS, the reflection defines numerous valley cutand-fill patterns. Across the NPT, the reflection occurs at the base of

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an overall hummocky to chaotic seismic facies of SSU III, and exhibits some small-scale channelization suggesting intermittent subaerial exposure.

Fig. 5. A) Structure contour maps of Quaternary reflections discussed in the manuscript. Depths are in meters below mean sea level (m bsl) and are based upon core correlations, and a conversion of two-way travel time to depth using an acoustic velocity of 1600 m/s. CPS = Central Pamlico Shelf; NPT = Northern Pamlico Terrace; SPV = Southern Pamlico Valley. B) Seismic data from line L51f1 (2003) showing the high amplitude continuous reflection designated Q0, and the correlation to the top of the limestone unit recovered in MLD02. G, S, M refer to relative abundance of gravel, sand, and mud, in the cores, respectively. C) Structure contour map of the Last Glacial Maximum (LGM) boundary, based upon digitization and gridding of the Q99 reflection. Depths are in meters below mean sea level (m bsl).


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Fig. 5 (continued).

4.1.6. SSU III Seismic Stratigraphic Unit III is bounded by reflections Q20 and Q30, occurs from ca. 20 to 60 m bsl and ranges from 0 to 25 m in thickness (Figs. 6 and 7). The seismic stratigraphy of this section is characterized by oblique to sigmoidal clinoforms (N4° dip) that indicate a regressive parasequence set prograding east-northeast onto the Northern Pamlico Terrace (Figs. 3A–A′, and 8). The clinoforms' top terminations are cut by Q30. The relief represented by these clinoforms is approximately 20 m. Sediments associated with this parasequence set were recovered in cores OBX15, OBX16, and OBX17 and are moderately-to well-sorted medium to coarse quartz sand and gravel, containing (in OBX-17) assemblages of inner shelf foraminifera, dominated by Elphidium excavatum, but including Ammonia parkinsoniana, Asterigerina carinata, Textularia cf. T. gramen, Hanzawaia strattoni, Nonionella atlantica, and Quinqueloculina

seminula as subsidiary species. Amino acid racemization analyses on Mercenaria shell material from OBX11 and MLD01 (Fig. 2B) place this unit in AZ4 suggesting an age of ca. 1450 ka to 960 ka (Table 2). Srisotope data from MLD01-73.8 yield a corrected value of 0.709176 ±0.000015, indicating a maximum age of ca. 1070 ka, a late early Pleistocene age (Table 2). A set of thin parasequences onlaps the clinoform deposits in eastern Pamlico Sound. The section comprising these parasequences is approximately 18 to 20 m thick (Fig. 7). The parasequence boundaries define north–south trending, elongate mounded deposits or ridges (Fig. 3B–B′ profile). These ridges are up to 10 m in vertical relief, ca. 2 km across the minor axis, and ten or more km along the long axis. Troughs occur between ridges, and are filled from the east. Seismic data exhibit medium-amplitude, horizontal to draped horizons, lacking incisions.



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Fig. 6. A) Seismic data (top panel) and interpretation (bottom panel) of l43f1 in western Pamlico Sound (see inset map for location) illustrating the seismic horizons in this area. B) Core log, foraminiferal data, and amino zones from MLD01. C) Expanded view of the seismic data from the outlined section in A, with interpretations of seismic stratigraphic units (SSU), systems tracts, and surfaces. TST = transgressive systems tract; HST = highstand systems tract; FS = flooding surface; MRS = marine ravinement surface; SU = subaerial unconformity.


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Fig. 6 (continued).

However, some parasequence boundaries reveal a trochoidal pattern indicative of sand waves (Posamentier, 2002) with a wavelength of approximately 180 m and a height of approximately 2 m, on the crest of larger ridges. Sediments associated with the ridges were recovered in MLD05 from 57 mbsl to 41 mbsl, and are generally very fine micaceous quartz sand with high abundance (up to ca. 5,000,000 specimens per 100 g dry weight of sediment) and high diversity (ca. 20 species) foraminiferal assemblages (generally dominated by the almost ubiquitous Elphidium excavatum and/or Ammonia parkinsoniana but with a considerable number of small specimens of Bolivina spp., Cassidulina spp., Epistominella spp., Rosalina spp., Buliminella elegantissima, Fursenkoina fusiformis and Trifarina angulosa) and up to 5% planktonic foraminifera. The section fines upward, and foraminiferal assemblages suggest a deepening-upward trend (inner shelf at the base, mid-shelf at the top). 4.1.7. Reflection Q30 Above Q20, Q30 is one of the most prominent and continuous reflections within the Quaternary section (Figs. 3, 4, 6). Q30 arises from a subaerial unconformity that truncates and incises into SSU III, and a closely associated, sometimes coplanar, transgressive marine ravinement surface and marine flooding surface defined as reflection Q30b. Q30 and Q30b can be traced throughout much of Pamlico Sound, except in the far south where it is cut by overlying sequence boundaries associated with multiple generations of valley cut–andfill. This surface exhibits approximately 30 m of relief. Digitization and gridding of Q30 reveal the subdued maintenance of the Northern Pamlico Terrace and Central Pamlico Shelf (Fig. 5). The Central Pamlico Shelf occurs beneath the modern mainland, and extends southeast beneath Pamlico Sound, Hatteras Island and northern Ocracoke Island. The east flank of this feature trends NNW beneath Pamlico Sound, and bends to the SW, parallel to the modern shoreline, west of Cape Hatteras. Beneath Ocracoke Island (Fig. 2A), the Central Pamlico Shelf is dissected by a broad paleo-fluvial valley

containing multiple cut-and-fill facies, defining the Southern Pamlico Valley (SPV; Fig. 5A). 4.1.8. SSU IV Seismic Stratigraphic Unit IV is constrained by reflections Q30 and Q50 and ranges from 5 to 25 m in thickness (Figs. 6 and 7). The thickest occurrence includes valley-fill sediments within the SPV, and multiple thin (b5 m) parasequences in the Northern Pamlico Terrace area. AAR analyses yield D/L ratios corresponding to AZ3.5 to AZ4 indicating an early to middle Pleistocene age (ca. 1450 ka to 620 ka; Wehmiller et al., in press) (Fig. 7; Table 2). Sr-isotope data yield values of 0.709157 ± 0.000015 to 0.709202 ± 0.000013 indicating a maximum age of ca. 1400 ka to 550 ka (Table 2). In MLD01, SSU IV sediments are characterized by low diversity open inner shelf foraminiferal assemblages, dominated by Elphidium excavatum. In OBX11, the lower part of SSU V is barren of foraminifera and a fluvial environment is indicated by the gravelly sand lithology. Fluvial sediments are unconformably overlain by finer-grained sediments characterized by moderately high diversity foraminifera assemblages with 2 to 6% planktonic foraminifers. Elphidium excavatum dominates but Ammonia parkinsoniana and Rosalina floridana are also abundant. The lower part of this unit also contains taxa that are often found in reduced oxygen environments (e.g., Buliminella elegantissima, Bolivina lowmani). Similar assemblages occur in the lower part of SSU V in OBX17, where planktonics comprise up to 12% of assemblages. 4.1.9. Reflection Q50 Q50 is a high amplitude continuous reflection that dips gently eastward (Figs. 3–5). This horizon occurs above several incised valley cut-and-fill facies in northern and southern Pamlico Sound (the SPV). Valley complexes were not included with this horizon as several cutand-fill episodes occur, and it cannot be determined which valley is associated with Q50. We classify this surface as a marine ravinement



surface. Based on its shallow nature, and AAR ages of 80 ka to 100 ka of overlying sediments in cores, this surface most likely represents the MIS 6 to 5 (Termination 2) transgressive ravinement surface. 4.1.10. SSU V Seismic Stratigraphic Unit V is constrained by reflections Q50 and Q99, and ranges from ca. 6 to 16 m in thickness (Figs. 6 and 7), forming a wedge that thins to the west. This unit contains several reflections. AAR analyses yield D/L ratios corresponding to AZ2 to 3-, suggesting a late Pleistocene age of 220 ka to 80 ka (Tables 1 and 2). Strontium-isotope data from OBX11-48.0 indicate a maximum age of ca. 750 ka (Table 2). In OBX15, medium to coarse sands with shell gravel contain moderate to high diversity (21 to 27 taxa) open shelf assemblages dominated by Elphidium excavatum, but with Cibicides lobatulus, Rosalina sp., Hanzawaia strattoni, Textularia cf. T. gramen, and Quinqueloculina seminula as important subsidiary taxa.

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the paleo-drainage pattern associated with the last glacial maximum (LGM). 4.1.12. SSU VI Seismic Stratigraphic Unit VI is constrained by Q99 and Q100 (the modern sediment/water interface), and ranges from ca. 0 to 35 m in thickness. The thickest occurrence includes the valley-fill associated with the paleo-fluvial valleys (Mallinson et al., 2005), and the modern barrier island system and the Hatteras Flats (Fig. 2). AAR analyses yield D/L ratios corresponding to the Holocene, and are corroborated by additional radiocarbon and OSL ages from other investigations (Sager and Riggs, 1998; Mallinson et al., 2005; Culver et al., 2007; Mallinson et al., 2008) (Table 1). Assemblages from the medium-grain sands of this unit in OBX11, OBX14, OBX15, OBX16, and OBX17 are generally of low abundance and low diversity, dominated by Elphidium excavatum. 5. Discussion

4.1.11. Reflection Q99 Q99 is a medium to high amplitude semi-continuous reflection immediately underlying the modern sediment–water interface (Fig. 5C). Digitization and gridding of this surface defines the paleofluvial valleys associated with the Roanoke, Pamlico/Tar, and Neuse Rivers. Additionally, a paleo-fluvial drainage is defined beneath the modern Pamlico Sound (Pamlico Creek on Fig. 5C). This surface reveals

5.1. Early Pleistocene stratigraphic evolution The earliest Pleistocene unit, SSU I, fills a structural low in the northwest Pamlico Sound, on the northwest flank of the subtle arch defined by Q0 (Fig. 5A). SSU II is expressed as a wedge of sediment that built the shelf vertically and eastward, and was subsequently eroded in

Fig. 7. A) Seismic data from L16f1 in northwest Pamlico Sound (see C for location of A–A′), B) Interpretation of L16f1, showing major and minor reflections, and the shelf sand ridge (SSR) horizon. C) Structure contour map (in m bsl) of the shelf sand ridge horizon shown in B. Note the N–S orientation of the ridges. D) Expanded view of the seismic data area outlined in B, with correlations to the lithologic log of core MLD05 (the most proximal core), and including foraminiferal data and aminozones. There is a 7 km separation between the seismic data and MLD05, so correlations are approximate.


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Fig. 7 (continued).

central Pamlico Sound. Foraminiferal assemblages and sediments associated with SSU II (in MLD01; Fig. 6) suggest a mid- to outer shelf origin. During the deposition of SSU III (Fig. 6), stratal preservation was greatest on the shallow shelf in the western portion of the study area (the Central Pamlico Shelf), while the outer shelf terrace (the Northern Pamlico Terrace) did not preserve significant sediment.

The structure contour maps (Fig. 5) of the early Pleistocene reflections and seismic stratigraphic units SSU I–II indicate general eastward filling into the Albemarle Embayment, suggesting a sediment point source (or sources) to the west. The coarse nature of the SSU I sediments (see MLD01 and MLD05, Figs. 6 and 7), and open shelf foraminiferal assemblages suggest that this unit is a lowstand shallow



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Table 1 Reflection and seismic stratigraphic unit (SSU) ages and attributes; CPS = Central Pamlico Shelf; NPT = Northern Pamlico Terrace; MIS = Marine Isotope Stage. Reflection

SSU

Age

Q100

Characteristics

Interpretation

Holocene

Modern sed/water interface Thin unit, except within fluvial valleys and beneath Outer Banks and Hatteras Flats. Much gas attenuation in soft sediments.

VI

Holocene Late Pleistocene

Fluvial incision, soil processes evident in cores.

LGM subaerial unconformity (sequence boundary).

V

Late Pleistocene MIS 5

Multiple incised fluvial valleys evident. Horizontally-bedded estuarine and shallow shelf sediments.

Multiple depositional sequences with extensive estuarine, shelf, and paleo-fluvial valley-fill

Late Pleistocene

Gentle eastward dip; minimal relief.

Transgressive ravinement surface associated with MIS 6/5 sea-level rise

IV

Early to middle Pleistocene; MIS 7–13?

Thin (b 5 m) horizontally-bedded sequences with fluvial channel fill. Abundant shelf foraminifera (inner to outer shelf; planktonics and low O2 assemblages).

Multiple eroded depositional sequences. Multiple incised valleys.

Early to middle Pleistocene

High amplitude continuous; incised valleys

Subaerial unconformity.

III

Early Pleistocene

Oblique clinoforms downlapping onto Q20 on edge of CPS. Slightly mounded to horizontally-bedded units in NPT area. Large shelf sand bodies.

Multiple parasequences. FSST and TST. Coastal to shelf deposits adjacent to CPS; shelf mound deposition in NPT.

Early Pleistocene

Moderate amplitude, semi-continuous, chaotic,

Subaerial to shallow marine unconformity; defines lowstand shelf

Early Pleistocene

Shelf sands and muds; some low O2 foram assemblages; high accumulation in west and filling of deep terrace area

Multiple depositional sequences exhibiting LST, TST and HST

Early Pleistocene

Medium-amplitude continuous reflection. NE dipping. Minor shallow channels

Unconformity–minimal subaerial exposure

Early Pleistocene

Uniform open shelf muddy fine sand to sandy mud; some low O2 foram assemblages; high accumulation in west and filling of deep terrace area

Outer shelf

Plio/Pleist boundary

Erosional upper contact on Pliocene non-reefal l.s.

Submarine unconformity

Q99

Q50

Q30

Q20 II Q10 I

Q0

shelf sand sheet. SSU II appears to be dominated by highstand deposits. Erosion of SSU II produced the NPT, and may be the result of deep shoreface ravinement (a regressive ravinement surface; Catuneanu et al., 2009) that truncated sequences as this area was reoccupied several times during the early Pleistocene. Sediments within cores MLD01 and MLD05 reveal coarse to very coarse shelf sands corresponding to the base of SSU III, bounded by Q20 and Q20b, indicating a lowstand to transgressive systems tract. 5.2. Early to middle Pleistocene stratigraphic evolution Reflection Q20 occurs at the base of SSU III and represents a probable bay ravinement surface in MLD05 where the surface underlies estuarine muds. This unconformity and the overlying SSU III are constrained to the late early Pleistocene. A marine ravinement surface separates the

Holocene estuarine, barrier island, and shallow open shelf sediments

estuarine muds from overlying inner shelf sands. Inner shelf sands are bounded by the flooding surface, Q20b. Foraminiferal assemblages associated with the clinoform parasequence set are typical of the shallow inner shelf to shoreface of North Carolina today (e.g., Grossman and Benson, 1967; Schnitker, 1971; Workman, 1981). Assemblage changes suggest an inner shelf to shoreface shoaling depositional system; species diversity in general decreases, as do the number of specimens per given volume of sediment (Fig. 8A). Therefore, we interpret the regressive package as a prograding paleoshoreline, possibly a mainland-attached strandplain (possibly wave-dominated deltaic) beach system, or paleo-cape with the clinoforms indicating the successive positions of the shoreface. The relationship of this early to middle Pleistocene paleoshoreline to overlying highstand deposits of SSU IV, as well as the present depth and relative seaward position, suggest that this is a lowstand shoreline.

Table 2 Strontium isotope data and corresponding aminozones (AZ), estimated AAR ages, and seismic stratigraphic units (SSU). Sample

Downcore

Lab

(m) OBX11-48.0 OBX11-59.3 MLD01-43.1 MLD01-45.8 OBX14-82.0 OBX16-96.6 OBX15-101.1 OBX15-105.7 OBX17-139.7 MLD01-73.8 MLD01-142.7 MLD05-238 MLD04-124.3

14.6 18.1 13.1 14.0 25.0 29.5 30.8 32.2 42.6 22.5 43.5 72.6 37.9

UF UF UF UF UF UF UF UF UF UF UF UF UF

Corrected Sr 87/86

Error +/−

0.709191 0.709201 0.709209 0.709202 0.709179 0.709168 0.709182 0.709195 0.709157 0.709176 0.709141 0.709149 0.709115

0.000013 0.000011 0.000011 0.000013 0.000014 0.000013 0.000013 0.000013 0.000015 0.000015 0.000011 0.000011 0.000013

Corr. max

Corr. min.

Min–max age (ka)

(ka)

0.709204 0.709212 0.709220 0.709215 0.709193 0.709181 0.709195 0.709208 0.709172 0.709191 0.709152 0.709160 0.709128

0.709178 0.709190 0.709198 0.709189 0.709165 0.709155 0.709169 0.709182 0.709142 0.709161 0.709130 0.709138 0.709102

0–700 0–450 0–200 0–500 0–900 0–1100 0–880 0–630 0–1300 0–1000 500–1500 100–1350 900–2250

0–750 0–500 0–200 0–550 0–1000 0–1170 0–930 0–700 0–1400 0–1070 500–1630 200–1500 1000–2400

1

Min–max age

AZ

AAR age estimate

3− 3− 2+ to 3.5] 3.5 NA 4− 4 4 4 4 4+ 4+ 4+

150–220 150–220 125–770 620–770 NA 660–980 960–1450 960–1450 960-1450 960–1450 1220–1840 1220–1840 1220–1840

2

SSU

(ka) V V IV IV IV IV IV IV IV III II I/II I

1

Using Berggren et al. (1985) age model. Using Shackleton et al. (1995) age model. Strontium age ranges (Min–max) are estimated using the corrected 87Sr/86Sr value for each sample, and comparison to data from ODP Site 758 (Farrell et al., 1995) including the entire 2 s.d. error envelope, and utilizing the Berggren et al. (1985) and Shackleton et al. (1995) time scales. The AAR age estimates are based upon a non-linear model. 2


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Fig. 8. A) High-resolution single-channel seismic data from L1F2 (see inset map for location), illustrating the prograding clinoforms indicating a regressive shoreface associated with SSU III. Depositional environments in OBX17 are based on foraminiferal assemblages. The scales indicate two-way travel time (twtt) and meters below sea level (m bsl) based on an estimated seismic velocity of 1600 m/s (calculated by correlations to core data). B) Seismic data with interpretation from L1F19_03 showing the interpretation of the falling stage systems tract (FSST). NR = normal regression, FR = forced regression.



The geometry of SSU III suggests that the clinoform parasequence set represents a falling stage systems tract (FSST) associated with a forced regression, followed by a normal regression (Fig. 8B). The geometry of SSU III is similar to a lowstand shoreline deposit in the DeSoto Canyon area of the Gulf of Mexico (Posamentier, 2004; Catuneanu, 2006, 2009), and conforms well to the sequence stratigraphic models of Plint (1988), Posamentier and Allen (1993) and Willis and Wittenberg (2000). Basinward of this coastal system, the onlapping thin parasequences, containing inner to outer shelf foraminiferal assemblages, indicate healing-phase deposits associated with a transgressive systems tract (TST) (Posamentier and Allen, 1993; Willis and Wittenberg, 2000) (Fig. 8B). The boundary between the two is a conformable surface across which the FSST grades upward into a TST recovered in MLD05, representing inner shelf to outer shelf deposits, including the elongate mounded sediment bodies (Fig. 7A–C). These mounded deposits match the description of shallow-water bottom current sands (Viana et al., 1998) or shelf sand ridges (Swift et al., 1978) that form as a result of the action of strong geostrophic currents and associated eddies or filaments,

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combined with storm-induced shelf currents and tidal currents. The presence of the sand ridge deposits is notable as they are not commonly recognized in subsurface data, although they are a common occurrence on modern shelves (Posamentier, 2002). In MLD05, SSU III is a deepening-upward unit from ca. 57 m bsl to ca. 41 m bsl, as indicated by an upward decrease in sediment grain-size accompanied by an increase in planktonic foraminifera diversity. Collectively Q20 and SSU III present a scenario of lowstand subaerial exposure with relative sea-level (RSL) below ca. 60 m bsl (depths uncorrected for isostasy or compaction), creating a regressive surface of marine erosion (RSME-Plint, 1988; Catuneanu, 2006) (Fig. 8), followed by RSL rise and ravinement to b40 m bsl, followed by RSL fall to ca. 50 m bsl with development of a FSST, followed by a rapid rise and formation of the TST with the shelf sand ridges. The top of this TST is truncated by a subaerial unconformity (Q30) and ravinement surface, so no highstand systems tract (HST) is preserved in SSU III. During the deposition of SSU III, significant sedimentation occurred in the east, on the NPT, suggesting a nearby fluvial source.

Fig. 9. Oblique view of the study area (from the southwest) showing the relief associated with reflections Q30 (middle Pleistocene), Q99 (late Pleistocene; LGM surface), and Q100 (the modern bathymetric surface). Features are indicated that represent antecedent topographic controls. Note the different depth scale for the Q100 surface (to emphasize the shallow features), as opposed to the other two surfaces.


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We hypothesize that the clinoform parasequence set represents a strand plain beach associated with a wave-dominated delta system that continued to feed sediment to the shelf during rising sea-level. Numerous valley cut-and-fill facies are evident in the area, and any one of them may have been the fluvial sediment source. Q30 is a subaerial unconformity exhibiting significant fluvial valley incision in the southern Pamlico Sound, creating the Southern Pamlico Valley. This unconformity is indicated by an abrupt contact between outer shelf sand of SSU III and overlying estuarine muds of SSU IV in MLD05, or fluvial sand in OBX11. However, in more landward areas above the CPS (OBX14, OBX15, OBX16, OBX17), the unconformity is marked by an abrupt upward change from shoreface sands to midshelf muddy sands and represents a marine flooding surface, possibly the maximum flooding surface. This flooding surface is labeled Q30b, and is evident as a medium- to high amplitude discontinuous reflection (Figs. 6 and 7). The intervening lowstand to transgressive systems tracts of SSU IV, consisting of fluvial, estuarine, and shallow shelf facies, ranges from 0 to 6 m in thickness. Immediately above Q30b in OBX17 and OBX15, are 2 m of open shelf muddy sand and sandy mud with diverse foraminiferal assemblages characterized by abundant small specimens. Elphidium excavatum is abundant, but assemblages are dominated by Rosalina spp., with Bolivina lowmani, Textularia cf. T. gramen, Hanzawaia strattoni, and Quinqueloculina spp. as important subsidiary species. Assemblages also contain several percent planktonic specimens and, based on comparisons with modern distributions of foraminifera on the mid-Atlantic shelf (Murray, 1969; Schnitker, 1971; Poag et al., 1980; Culver and Snedden, 1996), an open mid-shelf environment is indicated. 5.3. Middle to late Pleistocene and Holocene stratigraphic evolution Numerous fluvial valleys exhibiting cross-cutting relationships, indicate that the Central Pamlico Shelf was subaerially exposed and eroded numerous times during the middle to late Pleistocene (SSU IV, V, and VI) as the northern hemisphere ice volume grew (during glacials) (Imbrie et al., 1993), and lowstand shorelines shifted to lower elevations beyond the seaward limit of our data. SSU IV and SSU V occur as sedimentary wedges, thinning in a landward direction. Fill within the SPV consists of at least four crosscutting valley-fill sequences, indicating that these seismic units are composite in nature, and represent an expanded record of middle to late Pleistocene sea-level fluctuations. The updip components of these seismic stratigraphic units are exposed as geomorphic features (stranded barrier islands, beach ridges and scarps) on the modern coastal plain. The most prominent is the Suffolk Shoreline, formed during MIS 5 (Mallinson et al., 2008), which is correlative to SSU V. SSU VI consists of a lowstand systems tract characterized by fluvial channel fill, overlain by the transgressive systems tract consisting of estuarine, barrier island, and shoreface facies, deposited since the last glacial maximum during rising sea-level, and including the modern system. The evolution of this Holocene system has been described by Sager and Riggs (1998), Mallinson et al. (2005) and Culver et al. (2007, 2008). 5.4. The role of antecedent topography Although a thorough quantitative assessment of antecedent topographic controls is beyond the scope of this manuscript, certain geomorphic and stratigraphic characteristics appear to be controlled by, or inherited from, the geologic framework (Fig. 9). The initial morphology (Q0 horizon) is most likely a Pliocene expression of the underlying Cape Lookout High based on the location and orientation of the subtle arch axis. North- to northwest-prograding clinoforms are evident in the Pliocene section (below Q0), shown in Figs. 1, 4 and 6. Q0 defines the embayment that produced much of the accommodation

space for Pleistocene deposition, resulting in the preservation of the multiple depositional sequences in the northern Pamlico Sound area. Major fluvial incision occurred during the early to middle Pleistocene (Q20 and Q30), resulting in the formation of the SPV, providing a low that dominated late Pleistocene drainage. The LGM paleo-drainage pattern of Pamlico Creek indicated by Q99 occurs within the paleo-drainage of the SPV (Fig. 9). It appears that the LGM and pre-LGM paleo-drainage patterns were inherited from the SPV. The entire valley has shifted southward through the middle to late Pleistocene, in response to erosion of the south bank, and filling of the northern flank of the valley. An additional possible middle Pleistocene antecedent control may exist as the inflection in the modern coastline at Cape Hatteras roughly coincides with an inflection in the structure of the CPS, and the occurrence of the lowstand paleoshoreline deposits in SSU III, although more quantitative analysis is needed to understand this relationship (Fig. 9). Other antecedent effects are evident and related to the LGM surface, as indicated by reflection Q99 (Fig. 9). Clearly, the modern trunk estuaries occur within the flooded paleo-valleys of the Roanoke, Pamlico/Tar, and Neuse Rivers. Ocracoke Inlet occurs within the fluvial paleo-valley of Pamlico Creek, possibly accounting for the relative stability and longevity of this inlet during historical times (Stick, 1958). Likewise, the location of a recurring historically active inlet (Swash Inlet) through north Core Banks (Figs. 2A, 9) corresponds precisely with the location of the Neuse/Tar fluvial paleo-valley. A major shoal (Bluff Shoal) and the widest portion of Portsmouth Island (Figs. 2A, 9) occur on the Neuse River and Pamlico/Tar River interstream divide. Additionally, a major erosional hotspot presently occurs where the paleo-Roanoke River valley passes beneath the northern Outer Banks (Fig. 9). In addition to changes in patterns of deposition and erosion imparted by sea-level cyclicity and antecedent geology, the progressive shoaling of the regional embayment is a major factor controlling the attributes of sequences seen in the data. Core data reveal that the entire system in the Northern Pamlico Terrace shoals, from mid- to outer shelf deposits in the early Quaternary, to coastal deposits in the late Quaternary, as each depositional sequence was deposited, filling the accommodation space. Associated with this shoaling is an increase in the number and scale (vertical and horizontal) of incised paleovalleys associated with the middle to late Quaternary sequences and likely related to the magnitude of sea-level lowering. The primary effect of shoaling is apparent in the thinning and amalgamation of depositional sequences up-section, and the concomitant increase in the occurrence of fluvial, estuarine and barrier island deposits (Riggs et al., 1992, 1995; Mallinson et al., 2005; Parham et al., 2007). 6. Conclusions Our data reveal that the stratigraphic framework and the modern coastal system morphology of the Cape Hatteras region is the result of the interaction of varying magnitudes of relative sea-level change, the antecedent topography, variable fluvial input, and coastal oceanographic processes. The resulting seismic stratigraphic framework consists of multiple individual and composite seismic units bounded by subaerial unconformities and/or associated ravinement surfaces. Multiple flooding surfaces bounding parasequence sets occur in some thick units and define well developed systems tracts dominated by shelf deposits. However, eventual filling of the accommodation space, and an increase in the magnitude of sea-level change resulted in the preservation of thin middle to late Pleistocene seismic units, consisting of composite depositional sequences dominated by fluvial, estuarine and inner shelf facies. The expanded record occurs only in paleo-fluvial valley-fill. A significant component of the lower Pleistocene section is a lowstand paleoshoreline. The stratigraphic components of this paleoshoreline are similar to those described by Plint (1988) and Catuneanu et al. (2009). The absence of lowstand paleoshoreline deposits and the increase in the number and dimensions of incised



fluvial valleys in the middle to upper Pleistocene section are indicative of the seaward shift in the lowstand shoreline. This shift is commensurate with the middle Pleistocene expansion of northern hemisphere ice sheets, causing significant base-level lowering during glacial episodes. The Quaternary evolution of this expansive continental shelf system is a function of the inherited topography beginning with the Pliocene surface. Early Pleistocene deposits mimic the strike of the underlying Pliocene surface, and are limited in extent by sediment transport processes associated with the distribution of sediment point sources, and shelf currents. Shelf currents associated with wave and tidal energy modified shelf sediments during early Pleistocene lowstands, and deposited a lowstand strandplain paleoshoreline or paleo-cape, possibly associated with a wave-dominated delta, beneath Cape Hatteras and central Pamlico Sound. With the increase in amplitude of sea-level fluctuations, and progressive shelf progradation and aggradation during the middle to late Pleistocene, fluvial activity became a more significant factor in the character of sequences, resulting in the development of numerous incised valleys which still influence the modern drainage patterns, as well as barrier island dynamics. Acknowledgments The authors would like to acknowledge the contributions of Scott Snyder and Dorothea V. Ames of the ECU Department of Geological Sciences, and Erika Hammar-Klose, David Nichols, and Barry Irwin of the United States Geological Survey. We also acknowledge the constructive reviews of Wylie Poag, David Twitchell, and Walter Barnhardt of the USGS. This work was funded by the United States Geological Survey pursuant to cooperative agreement 02ERAG0044. Seismic MicroTechnology, Inc. also provided support in the form of a software grant. References Ashton, A.D., Murray, A.B., 2006. High-angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes. Journal of Geophysical Research 111, F04011. doi:10.1029/2005JF000422. Berggren, W.A., Kent, D.V., van Couvering, J.A., 1985. The Neogene, Part 2: Neogene geochronology and chronostratigraphy. In: Snelling, N.J. (Ed.), The chronology of the geological record: Geological Society of London Memoir, vol. 10, pp. 211–260. Blackwelder, B.W., 1981. Late Cenozoic Stages and Molluscan Zones of the U.S Middle Atlantic Coastal Plain. Paleontology Society Memoir 12, 1–34. Boss, S.K., Hoffman, C.W., Cooper, B., 2002. Influence of fluvial processes on the Quaternary geologic framework of the continental shelf, North Carolina, USA. Marine Geology 183, 45–65. Brown, P.M., Miller, J.A., Swain, F.M., 1972. Structural and stratigraphic framework, and spatial distribution of permeability of Atlantic coastal plain, North Carolina to New York. U.S. Geological Survey Professional Paper, vol. 796. 79 pp. Carey, J.S., Sheridan, R.E., Ashley, G.M., Uptegrove, J., 2005. Glacially influenced late Pleistocene stratigraphy of a passive margin: New Jersey's record of the North American Ice Sheet. Marine Geology 218, 155–173. Catuneanu, O., 2006. Principles of Sequence Stratigraphy. Elsevier, Amsterdam. 375 pp. Catuneanu, O., Abreau, V., Bhattacharya, J.P., Blum, M.D., Dalrymple, R.W., Eriksson, P.G., Fielding, C., Fisher, W., Galloway, W., Gibling, M., Giles, K., Holbrook, J., Jordon, R., Kendall, C., Macurda, B., Martinson, O., Miall, A., Neal, J., Nummedal, D., Pomar, L., Posamentier, H., Pratt, B., Sarg, J., Shanley, K., Steel, R., Strasser, A., Tucker, M., Winker, C., 2009. Towards the standardization of sequence stratigraphy. Earth Science Reviews 92, 1–33. Culver, S.J., Buzas, M.A., 1980. Distribution of recent benthic foraminifera off the North American Atlantic coast. Smithsonian Contributions to the Marine Sciences 6, 1–512. Culver, S.J., Snedden, J.W., 1996. Foraminifera and their implications for the formation of New Jersey shelf sand ridges. Palaios 11, 161–175. Culver, S., Grand Pre, C., Mallinson, D., Riggs, S., Corbett, D., Foley, J., Hale, M., Ricardo, J., Rosenberger, J., Smith, C.G., Smith, C.W., Snyder, S., Twamley, D., Farrell, K., Horton, B., 2007. Late Holocene Barrier Island Collapse: Outer Banks, North Carolina, U.S.A. Sedimentary Record 5, 4–8. Culver, S., Farrell, K., Mallinson, D., Horton, B., Willard, D., Theiler, E., Riggs, S., Snyder, S., Wehmiller, J., Bernhardt, C., Hillier, C., 2008. Micropaleontologic record of late Pliocene and Quaternary paleoenvironments in the northern Albemarle embayment, North Carolina, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 264, 54–77. Divins, D.L., Metzger, D., 2006. NGDC Coastal Relief Model. http://www.ngdc.noaa.gov/ mgg/coastal/coastal.html. 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