North Carolina inner shelf

ARTICLE IN PRESS Continental Shelf Research 28 (2008) 2428– 2441

Contents lists available at ScienceDirect

Continental Shelf Research journal homepage: www.elsevier.com/locate/csr

Anatomy of a shoreface sand ridge revisited using foraminifera: False Cape Shoals, Virginia/North Carolina inner shelf Marci M. Robinson a,, Randolph A. McBride b a b

US Geological Survey, Eastern Earth Surface Processes, 926A National Center, Reston, VA 20192, USA George Mason University, Department of Atmospheric, Oceanic, and Earth Sciences, Fairfax, VA 22030-4444, USA

a r t i c l e in f o

a b s t r a c t

Article history: Received 12 February 2008 Received in revised form 8 May 2008 Accepted 9 June 2008 Available online 12 June 2008

Certain details regarding the origin and evolution of shelf sand ridges remain elusive. Knowledge of their internal stratigraphy and microfossil distribution is necessary to define the origin and to determine the processes that modify sand ridges. Fourteen vibracores from False Cape Shoal A, a welldeveloped shoreface-attached sand ridge on the Virginia/North Carolina inner continental shelf, were examined to document the internal stratigraphy and benthic foraminiferal assemblages, as well as to reconstruct the depositional environments recorded in down-core sediments. Seven sedimentary and foraminiferal facies correspond to the following stratigraphic units: fossiliferous silt, barren sand, clay to sandy clay, laminated and bioturbated sand, poorly sorted massive sand, fine clean sand, and poorly sorted clay to gravel. The units represent a Pleistocene estuary and shoreface, a Holocene estuary, ebb tidal delta, modern shelf, modern shoreface, and swale fill, respectively. The succession of depositional environments reflects a Pleistocene sea-level highstand and subsequent regression followed by the Holocene transgression in which barrier island/spit systems formed along the Virginia/North Carolina inner shelf 5.2 ka and migrated landward and an ebb tidal delta that was deposited, reworked, and covered by shelf sand. Published by Elsevier Ltd.

Keywords: Shoreface sand ridges Shelf facies Shelf sedimentation Foraminifera Micropaleontology Inner continental shelf

1. Introduction Shelf sand ridges are common features on the US Atlantic continental shelf, often forming ridge and swale topography with persistent, elongate sand bodies creating the topographic highs with older muds exposed in the swales (Duane et al., 1972; Swift et al., 1972b). While shelf sand ridges may be shore-parallel (Stubblefield et al., 1984; Swift et al., 1984), this paper focuses on those sand ridges oriented at oblique angles to the adjacent shoreline. Shore-oblique sand ridges are typically oriented at angles between 101 and 501 to the adjacent shoreline, with an average orientation of 301 (McBride and Moslow, 1991), are usually over 1 km in length, 0.5 km wide with a relief up to 10 m and side slopes that average less than 11 (Duane et al., 1972; Field, 1980; Figueiredo et al., 1981). The distribution of shelf sand ridges has been extensively studied, and their origin and evolution widely debated, since the 1930s, in part due to the occurrence of significant petroleum reserves in some ancient shelf sand shoals, yet important basic questions remain regarding the origin and evolution of these features despite several insightful studies on

 Corresponding author. Tel.: +1703 648 5291; fax: +1703 648 6953.

E-mail addresses: [email protected] (M.M. Robinson), [email protected] (R.A. McBride). 0278-4343/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.csr.2008.06.002

the subject (Swift and Field, 1981; McBride and Moslow, 1991; Snedden and Dalrymple, 1999; Snedden et al, 1999). Early morphologic studies of the mid-Atlantic shelf, based on depth recordings and later bathymetric mapping, defined the distribution, clustering and migration of sand ridges (e.g., Veatch and Smith, 1939; Uchupi, 1968), and surficial sampling documented surface sediment texture (e.g., Sanders, 1962; Swift et al., 1972a). With this information and measurements of current- and wave-generated flows, hydrodynamic models were developed to explain sand-ridge development, maintenance, and migration. Huthnance (1982) devised a simple hydrodynamic model by which a sand ridge forms around an initial bathymetric irregularity large enough to disturb linear water flow under conditions of adequate available sand and active currents. This model, however, neglects to define the nature of the initial irregularity. Knowledge of the internal stratigraphy of and microfossil distribution within sand ridges is necessary to define the initial irregularity and to determine the processes that may subsequently modify the ridges. In this study, microfossils from cores collected from False Cape Shoal A, a well-developed shoreface-attached sand ridge on the inner continental shelf near the Virginia/North Carolina border, were examined in order to document the internal stratigraphy and benthic foraminiferal assemblages, as well as to reconstruct the depositional environments recorded in down-core sediments.

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

2. False cape shoals False Cape Shoals is a system of three shoreface-attached and detached sand ridges located immediately offshore the Virginia/ North Carolina border (Fig. 1). The shoals display a maximum relief of 6.1 m and slopes of p21 (Duane et al., 1972; Swift et al., 1972a). False Cape Shoal A trends northeast at a 161 angle to the shoreline (McBride and Moslow, 1991) and emerges from the shoreface as a single ridge before bifurcating into two subridges to the north. False Cape Shoals were extensively studied by Swift et al. (1972a) who analyzed the bathymetry, surficial sediments and stratigraphy of this shore-oblique three-ridge system. Bathymetric mapping revealed a change in slope of the shoreface at 9 m water depth. False Cape Shoals A and B emerge from the upper, steeper shoreface and continue on the lower, less steep shoreface; Shoal C lies directly on the inner shelf floor which intersects with the shoreface at a water depth of 17 m, and is not attached to the shoreface (Swift et al., 1972a). Surficial sediment samples revealed a relationship between grain size and topography in which ridge crests were covered with fine- to medium-grained sand, the shoreface, ridge flanks and trough margins were covered with fine to very fine sand, and trough axes were floored by pebbly, medium- to coarse-grained sand overlying dense clay (Swift et al., 1972a). The general stratigraphy of False Cape Shoals consists of a basal clayey fine sand unit, an intermediate muddy unit, and an upper sand sheet that forms the ridges (Swift et al., 1972a). No microfossil examinations of False Cape Shoals exist prior to the research presented here. Detailed foraminiferal assemblage data for the modern North Carolina and Virginia continental shelves, however, are provided in Schnitker (1971) and Cronin et al. (1998), respectively. Rine et al. (1991), Snedden et al. (1994) and Culver and Snedden (1996) studied foraminiferal assemblages in New Jersey shelf sand ridges, documenting distinct assemblages found in inner- and mid-shelf ridges, but

2429

these studies lack sufficient microfossil sampling intervals to interpret acute environmental changes recorded in the sand-ridge sediments.

3. Materials and methods 3.1. Laboratory Fourteen vibracores were collected from False Cape Shoal A in June 2001 (Table 1, Fig. 1). Once collected, each vibracore was cut into 1 m sections that were split longitudinally, trimmed, described in detail, and photographed. Half of each core was archived; the other half was sampled for microfossil analysis at approximately 20–25 cm intervals, with additional samples collected immediately below and above sedimentologic contacts and at other locations of stratigraphic interest. Core FCS-01-14, due to its complete and complex stratigraphy, was sampled at 12.5 cm intervals. For microfossil analysis, sediment samples of 10 cm3 were processed using standard procedures in which bulk samples are dried in an oven at p50 1C, weighed, then disaggregated in a beaker with warm tap water and 2 ml of dilute sodium hexametaphosphate solution (5 g/1 L water). The samples were agitated on a shaker for 1 h, washed through a 63 mm sieve to remove clay and silt-sized material, and dried in an oven at p50 1C. In sandy samples, the foraminifera were concentrated by the soap-floating technique described in Harris and Sweet (1989). A split of 300–350 benthic specimens was sought from the 4125 mm size fraction. FCS-01-3 was examined for microfossils at both 4125 and 463 mm size fractions to ensure that smaller species were not overlooked by the choice of a larger size fraction for this study. No additional species were found in the expanded size fraction, though individual species percentages in individual samples did rarely differ from the corresponding 4125 mm

37 Cape Henry VA NC

Virginia Beach

15 Virginia North Carolina

Back Bay

20m

14 10 12 11 13 A 1 7 8 2 6 3 4 5

16° Currituck Sound

False Cape Sho als

B

C

Albemarle Sound

36

10m Roaoken Sondu Croat an Sound

1.5km

Oregon Inlet Pamlico Sound

-76

Rodanthe

-75

Fig. 1. False Cape Shoals A, B and C and core locations on the inner continental shelf of Virginia and North Carolina.

ARTICLE IN PRESS 2430

M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

Table 1 Core localities Core

Latitude

Longitude

Length (cm)

Water depth (m)

FCS-01-1A FCS-01-2 FCS-01-3 FCS-01-4 FCS-01-5 FCS-01-6 FCS-01-7 FCS-01-7Jet FCS-01-8 FCS-01-10 FCS-01-11 FCS-01-12 FCS-01-13 FCS-01-14 FCS-01-15

36135.160 36134.230 36133.580 36133.060 36132.370 36134.000 36135.000 36135.000 36134.610 36134.550 36136.440 36136.350 36136.260 36137.220 36138.040

75151.760 75151.730 75151.570 75151.390 75151.090 75151.380 75151.410 75151.410 75151.080 75151.670 75151.380 75151.030 75150.640 75151.280 75151.430

382 403 496 448 451 325 200 183 456 505 526 328 535 498 315

10.0 9.2 8.2 7.6 8.7 6.1 5.2 5.2 7.6 10.1 8.5 11.3 11.0 8.8 10.7

samples by as much as 26% due to the addition of smaller specimens of Ammonia tepida and Haynesina germanica. Although many samples contained fewer than 300 individuals, and many were barren, a few samples required multiple splitting using a CarpcoTM sample splitter. Specimens were sorted, identified, and glued to a 60 square micropaleontological slide. Specimen identifications were confirmed by comparison with type and figured specimens in the Cushman Collection, National Museum of Natural History, Smithsonian Institution, Washington, DC. Thirty-seven species of benthic foraminifera were identified in the False Cape Shoals cores. Tables showing species counts, number of specimens per sample, and species richness for each sample are archived online. These statistics vary greatly between samples within individual cores. Scanning electron micrographs of chosen species are presented in Plate 1. 3.2.

14

C age estimates

Two mollusks deemed suitable for 14C AMS dating were found in the sand-ridge cores. 14C AMS age estimates were obtained from the National Ocean Science AMS Facility at the Woods Hole Oceanographic Institution, where ages were calculated using 5568 years as the half-life of radiocarbon and reported without reservoir corrections. Results were then calibrated using CALIB Radiocarbon Calibration v.5.1.0 (Stuiver and Reimer, 1993; Stuiver et al., 2005) and the standard 400-year reservoir correction. A Busycon carica eliceans specimen from FCS-01-10 in clay at 302 cm depth was dated at 5195 ybp, and an Arctica islandica specimen from FCS-01-6 in poorly sorted massive sand at 149 cm was dated at 441,000 ybp (Table 2). The Busycon age of 5195 ybp is believed to accurately represent the age of the clay unit as the mollusk was very well preserved and did not appear reworked. The Arctica in FCS-01-6 was broken and less well preserved and is considered to be reworked and isotopically dead as it is stratigraphically above the younger dated specimen.

4. Results 4.1. Sedimentary and foraminiferal facies Seven sedimentary and foraminiferal facies are defined in the False Cape Shoal A cores, based on sediment texture and mineralogy, physical and biogenic sedimentary structures, foraminiferal assemblages, and geographic position of the vibracore on the inner shelf, and correspond to the following units, from

bottom to top: (1) fossiliferous silt, (2) nearly barren sand, (3) fossiliferous clay to sandy clay, (4) fossiliferous laminated and bioturbated sand, (5) fossiliferous poorly sorted massive sand, (6) fossiliferous fine, clean sand, and (7) fossiliferous poorly sorted clay to gravel. Figs. 2–5 graphically represent the position of these facies in the sand-ridge cores, as well as the species richness and relative abundance of diagnostic foraminiferal species of samples within each unit. The lowermost facies, fossiliferous silt (Facies 1), is penetrated only by FCS-01-13 (Fig. 4). This medium gray micaceous silt coarsens upward from mud to very fine sand and contains abundant foraminifera, dominated by Elphidium excavatum but including Buccella frigida, Hanzawaia strattoni, and Pseudopolymorphina novangliae as well as Trochammina inflata, Jadammina macrescens and Miliammina fusca. Three planktic foraminifera were found in the deepest of the three samples, two Turborotalita quinqueloba and one Globigerinita glutinata. This unit is separated from the overlying barren sand by a distinct contact. The nearly barren sand (Facies 2) is light tan to light gray and composed of well-sorted very fine to medium quartz sand, heavy mineral grains, and mica. This unit is represented in seven of the 14 sand-ridge cores (FCS-01-2, -3, -4, -10, -12, -13 and -14), reaching up to 2 m in thickness (Figs. 2–5). No clay or shells or shell fragments are found in this unit. Although nearly barren, four samples from three separate cores contain one to three specimens of either E. excavatum, Elphidium gunteri, Quinqueloculina seminula, or Elphidium poeyanum. In FCS-01-12 and -13, this unit is unconformably overlain by a layer of poorly sorted clay to quartz gravel. Massive dark gray clay to sandy clay (Facies 3), often containing large oyster and Mercenaria shell fragments, is found in eight inner shelf cores (FCS-01-1A, -2, -3, -4, -10, -12, -14 and -15), always overlying the barren sand (Figs. 2–5). Where sandy, the clay exhibits horizontal laminations and bioturbation. Layers of poorly sorted sand to gravel are evident at the top of this unit in FCS-01-4, -12, -13, and -14. The upper contact is often distinct and erosional. This unit contains both barren intervals and some of the most fossiliferous samples, which contain E. excavatum, H. germanica, Haplophragmoides wilberti, Haplophragmoides manilaensis, A. tepida, and T. inflata and rare planktic foraminifera. The maximum thickness of the clay to sandy clay is 335 cm. The sand ridges overlie the previously discussed facies and are composed of two distinct sand units. First, the laminated and bioturbated sand (Facies 4) is characterized by interlaminated to interbedded mud and fine sand. This greenish to medium gray unit is finely laminated and mildly bioturbated, often with wavy mud inclusions that could represent flaser bedding and/or bioturbation. This unit contains E. excavatum, H. germanica, Ammonia parkinsoniana, A. tepida, B. frigida, H. strattoni, Q. seminula, E. gunteri, Elphidium mexicanum, Elphidium sp., H. wilberti, H. manilaensis, T. inflata and planktics and has the highest species richness and some of the most fossiliferous samples of the seven sedimentary facies. This unit is thickest down the length of the ridge axis where it reaches 315 cm and thins in all directions. The laminated and bioturbated sand is found in eight sand-ridge cores (FCS-01-4, -5, -6, -8, -11, -12, -13 and -14) where it is often separated from the overlying poorly sorted massive sand (Facies 5) by a shell layer (as in FCS-01-4) or a mud layer (as in FCS-01-5, 8 and -13). The poorly sorted massive sand (Facies 5) is medium gray in color, composed of very fine quartz sand to gravel, shell fragments, and rock fragments and represents the uppermost unit in all but two cores. No sedimentary or biogenic structures are apparent. The poorly sorted massive sand is characterized by E. excavatum, H. germanica, A. parkinsoniana, B. frigida, Buccella inusitata, H. strattoni, Q. seminula, E. gunteri, and E. mexicanum, a lower

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

1.

2.

4.

3.

6.

7.

10.

14.

5.

8.

9.

12.

11.

15.

2431

13.

16.

17.

Plate 1. Scale bar, 100 mm. (1) Quinqueloculina impressa Reuss, (2) Quinqueloculina jugosa Cushman, (3) Quinqueloculina seminula (Linne´), (4) Guttulina lactea (Walker and Jacob), (5) Pseudopolymorphina novangliae (Cushman), (6) Eponides repandus (Fichtel and Moll), (7) Cibicides lobatulus (Walker and Jacob), (8) Nonionella atlantica Cushman, (9) Nonionella opima Cushman, (10) Hanzawaia strattoni (Applin), (11) Buccella frigida (Cushman), (12) Buccella inusitata Andersen, (13) Elphidium excavatum (Terquem), (14) Elphidium galvestonense Kornfeld, (15) Elphidium gunteri Cole, (16) Elphidium mexicanum Kornfeld, and (17) Elphidium subarcticum Cushman.

Table 2 False Cape Shoal A

14

C AMS age estimates

Sample core and depth (cm)

Mollusk

Water depth (m)

d13C

Conventional age (ybp)

Calibrated age (ybp)

NOSAMS accession no.

FCS-01-6, 149 FCS-01-10, 302

Arctica islandica Busycon carica eliceans

6.1 10.1

0 0

41,6007520 4870740

n/a 5195

OS-57805 OS-57804

species richness, and lower numbers of specimens as compared to the underlying laminated and bioturbated sand. The poorly sorted massive sand ranges in thickness from 0 to 293 cm.

The poorly sorted massive sand (Facies 5) and fine, clean sand (Facies 6) are distinguished by grain size and shell fragment content. The fine, clean sand, found only at FCS-01-2 at the

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

1A

Species Richness 05

Species Richness

2

0

0

0

100

100 depth (cm)

depth (cm)

2432

200

300

5

10

200

300

400

400 20 40 0 0 100 200 20 0 % E. excavatum % Shelf and % Estuary and Total Forams Species Richness Ridge Species Marsh Species 80

60

3

5

0

60

40

20

100

80

% E. excavatum

100

20

0

40

20

0

% Shelf and Ridge Species

60 0

100

200

300

Total Forams

% Estuary and Marsh Species

10

0

Species Richness

4

5

0

10

15

0 100

depth (cm)

depth (cm)

100 200

300

200

300

400 400 500 40

20 0 60 80 100 0 % E. excavatum % Shelf and Ridge Species

20

40

60

0

100

200

Total Forams

% Estuary and Marsh Species

500 80

Species Richness

5

5

0

100

0

20

0

20

% E. excavatum % Shelf and % Estuary and

0

200 400

600

Total Forams

Ridge Species Marsh Species

10

0

depth (cm)

100

Clay to Gravel Fine, Clean Sand

200

Poorly Sorted Sand Lam/Bioturb Sand

300

Clay to Sandy Clay Barren Sand

400

Gravel

40

60

80

% E. excavatum

100 0

20

40

% Shelf and Ridge Species

60

0

20

40

60

% Estuary and Marsh Species

0

100 200 300 400 Total Forams

Fig. 2. Sedimentary facies, species richness, and relative fractional abundance of diagnostic species in non-barren samples of vibracores FCS-01-1A through -5. Shelf and ridge species include A. parkinsoniana, B. frigida, B. inusitata, C. lobatulus, E. galvestonense, E. gunteri, E. mexicanum, E. subarcticum, G. lactea, H. strattoni, N. atlantica, N. opima, P. novangliae, five Quinqueloculina species, Ep. repandus and P. lateralis. Estuarine and marsh species include A. parkinsoniana, A. tepida, Elphidium sp., H. wilberti, H. germanica, J. macrescens, M. fusca, and T. inflata.

surface, is composed of well sorted very fine to medium quartz sand with very fine heavy mineral grains and little to no shell material. The foraminiferal assemblages are similar in species and specimen number to the poorly sorted massive sand. FCS-01-1A and -10, in the swale between the shoreface and the sand ridge, contain a very poorly sorted clay to fine quartz sand (Facies 7) containing sand lenses, clay pebbles, quartz gravel, and coarse shell fragments. This unit is barren in FCS-01-10 but contains E. excavatum, H. germanica, H. manilaensis, and T. inflata in FCS-01-1A. This unit is 158–192 cm thick.

4.2. Stratigraphy Stratigraphic correlation of the 14 sand-ridge cores is based on the sedimentary facies and erosional surfaces described above. Fig. 6 shows the location of the False Cape Shoal A cross-sections. Two north–south cross sections are shown in Fig. 7. A–A0 traces the entire ridge axis, whereas B–B0 parallels the shoreline along the upper shoreface. Two west–east cross sections are shown in Fig. 8. C–C0 crosses the ridge near the middle, and D–D0 crosses the two subridges near the distal end. The sand ridge is composed of

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

2433

Species Richness 7

0

5

100

0

0

7J

depth (cm)

100

200

300

400 0

20

40

60

80

20

0

100

% E. excavatum

40

60

80

100

0

20

40

60

80

25

50

Total Forams

% Estuary and Marsh Species

% Shelf and Ridge Species

Species Richness 0

5

10

Species Richness 051015

8

0

0

100

100

200

200

depth (cm)

depth (cm)

6

300

400

300

400 40

60

0

80

100 0 20 0 20 40 % Shelf and % Estuary and % E. excavatum Ridge Species Marsh Species

150

75 Total Forams

500 10

60

0

Species Richness 0

5

80

0

100

% E. excavatum

10

20

0

40

% Shelf and Ridge Species

20

% Estuary and Marsh Species

0

250

500

Total Forams

depth (cm)

100

Clay to Gravel

200

Poorly Sorted Sand Lam/Bioturb Sand

300

Clay to Sandy Clay Barren Sand 400 Gravel 500 0

20

40

60

% E. excavatum

80

100

0

20

40

60

80

% Shelf and Ridge Species

100

0

20 0

% Estuary and Marsh Species

100

200

300

Total Forams

Fig. 3. Sedimentary facies, species richness, and relative fractional abundance of diagnostic species in non-barren samples of vibracores FCS-01-6 through -10. Shelf and ridge species include A. parkinsoniana, B. frigida, B. inusitata, C. lobatulus, E. galvestonense, E. gunteri, E. mexicanum, E. subarcticum, G. lactea, H. strattoni, N. atlantica, N. opima, P. novangliae, five Quinqueloculina species, Ep. repandus and P. lateralis. Estuarine and marsh species include A. parkinsoniana, A. tepida, Elphidium sp., H. wilberti, H. germanica, J. macrescens, M. fusca, and T. inflata.

the laminated and bioturbated sand and the poorly sorted massive sands. The erosional surface beneath these sand units forms a trough extending from FCS-01-11 shoreward through FCS-01-8 to FCS-01-5. This depression is occupied by the laminated and bioturbated sand (Fig. 8).

fossiliferous silt and barren sand represent a Pleistocene estuary and shoreface, respectively. The remaining facies are interpreted to represent the following Holocene depositional environments: estuary, ebb tidal delta, modern shelf, modern shoreface, and swale fill (Table 3). 5.1. Pleistocene estuary

5. Discussion The seven facies delineated in False Cape Shoal A (fossiliferous silt, barren sand, clay to sandy clay, laminated and bioturbated sand, poorly sorted massive sand, fine, clean sand, and poorly sorted clay to gravel) reveal a succession of depositional environments that is characteristic of the sand ridge. The

The well-sorted fine-grained sediments and lack of shell fragments in the fossiliferous silt are indicative of an estuarine environment. The abundant and diverse foraminiferal assemblage contains a mixture of estuarine, marsh, and shelf species dissimilar to assemblages in other stratigraphic units in this study and also to modern assemblages documented from the

ARTICLE IN PRESS 2434

M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

Species Richness 051015

11 0

depth (cm)

100

200

300

400

500 20

40

60

80

100

0

% E. excavatum

20

0

40

20

40

60

0

100

200

300

400

Total Forams

% Estuary and Marsh Species

% Shelf and Ridge Species

Species Richness

12

0

5

10

0

depth (cm)

100

200

300 40

60

80

100

% E. excavatum

0

20

40

60

0

20

40

60

0

% Estuary and Marsh Species

% Shelf and Ridge Species

100

150

200

Total Forams Species Richness 0

13

50

5

10

15

0

100

Lam/Bioturb Sand

depth (cm)

200 Poorly Sorted Sand

300

Clay to Sandy Clay Barren Sand 400 Fossiliferous Silt Gravel 500

0

20

40

60

% E. excavatum

80

100

0

20

% Shelf and Ridge Species

0

20

40

60

80

% Estuary and Marsh Species

100

0

300

600

Total Forams

Fig. 4. Sedimentary facies, species richness, and relative fractional abundance of diagnostic species in non-barren samples of vibracores FCS-01-11 through -13. Shelf and ridge species include A. parkinsoniana, B. frigida, B. inusitata, C. lobatulus, E. galvestonense, E. gunteri, E. mexicanum, E. subarcticum, G. lactea, H. strattoni, N. atlantica, N. opima, P. novangliae, five Quinqueloculina species, Ep. repandus and P. lateralis. Estuarine and marsh species include A. parkinsoniana, A. tepida, Elphidium sp., H. wilberti, H. germanica, J. macrescens, M. fusca, and T. inflata.

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

2435

Species Richness 0

14

5

10

0

depth (cm)

100

200

300

400

500 0

20

40

60

0

100

80

% E. excavatum

20

40

60

80

0

% Shelf and Ridge Species

20

40

50

0

60

100

Total Forams

% Estuary and Marsh Species

Poorly Sorted Sand Lam/Bioturb Sand Clay to Sandy Clay Barren Sand Gravel el Species Richness 15

5

0

0

10

15

depth (cm)

100

200

300 0

20

40

60

% E. excavatum

80

100

0

20

40

% Shelf and Ridge Species

60

0

20

40

60

80

% Estuary and Marsh Species

100

0

100 150 50 Total Forams

Fig. 5. Sedimentary facies, species richness, and relative fractional abundance of diagnostic species in non-barren samples of vibracores FCS-01-14 and -15. Shelf and ridge species include A. parkinsoniana, B. frigida, B. inusitata, C. lobatulus, E. galvestonense, E. gunteri, E. mexicanum, E. subarcticum, G. lactea, H. strattoni, N. atlantica, N. opima, P. novangliae, five Quinqueloculina species, Ep. repandus and P. lateralis. Estuarine and marsh species include A. parkinsoniana, A. tepida, Elphidium sp., H. wilberti, H. germanica, J. macrescens, M. fusca, and T. inflata.

Outer Banks area (see Robinson and McBride, 2006) in that Glabratellina sagrai and Fursenkoina fusiformis are present, forming up to 8% and 1% of the total assemblage, respectively. Woo et al. (1997) found Glabratellina sp. in the E. excavatum-dominated assemblages from the higher salinity (30–32 ppt) estuaries of the Virginia Eastern Shore where barrier islands are tide-dominated, and inlets are more closely spaced and less migratory than in the study area. G. sagrai is found in the fine, clean sand at the top of FCS-01-2 and in laminated and bioturbated sand and clay throughout the sand ridge. Individual specimens of F. fusiformis are found in the fine, clean sand of FCS-01-2 and the clay of

FCS-01-3. Neither species is found in nearby estuarine cores (Robinson and McBride, 2006). The fossiliferous silt also contains the marsh species T. inflata, J. macrescens, and M. fusca, indicating proximity to or transport from a marsh. The fossiliferous silt is interpreted as a Pleistocene estuary with higher salinities than occur today in Currituck, Albemarle, and Pamlico Sounds (US Department of Commerce, 1985), possibly due to a higher frequency of tidal inlets contributing more water of normal ocean salinity to the estuary. Absolute ages were not attained for this unit and the overlying barren sand because of the lack of carbonate material needed for dating.

ARTICLE IN PRESS 2436

M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

A‘

N 15

14

D

10 12 11 13

D‘

B‘ 1 C

7 8 C‘ 2

6

3

4 A

5 B

10m 1.5 km Fig. 6. Cross-section locations across False Cape Shoal A on the Virginia-North Carolina inner continental shelf for Figs. 7 and 8.

Pleistocene age estimates of 23–39 kybp have been reported from similar depths (17 mbsl) further south along Currituck Spit (Mallinson et al., 2005), and the fossiliferous silt is interpreted as the northern extension of this Pleistocene unit. 5.2. Pleistocene shoreface The barren sand is similar in grain size and mineralogy to the modern fine, clean sand found at the top of FCS-01-2, along the shoreface of the modern Currituck Spit. The rare foraminifera found in the generally barren sand are either E. excavatum, E. gunteri, E. poeyanum, or Q. seminula, species found in the modern core tops of the inner shelf and shoreface cores. Shoreface sands, however, are rarely preserved in the stratigraphic record because they are planed off during the shoreface ravinement process (Niedoroda et al., 1985). More commonly preserved is the lower portion of an overwash deposit or flood tidal delta shoal or inlet fill. A distinct contact separates the barren sand from the underlying clay, supporting any of these three interpretations. However, the barren sand is found in FCS-01-2, -3, -4, -10, -12, -13, and -14. The barren sand is more laterally extensive than

overwash deposits, tidal delta deposits or inlet fill and is up to 2 m in thickness. This barren sand is therefore interpreted as a regressive shoreface deposit. Similar regressive shoreface deposits underlying Holocene estuarine deposits have been identified on the Virginia inner shelf just north of the mouth of the Chesapeake Bay through analysis of high-resolution seismic reflection data (Foyle and Oertel, 1997) and elsewhere along the mid-Atlantic continental shelf (Swift, 1975; Field, 1980; Kraft, 1971a, b; Kraft and Chrzastowski, 1985). Topping the barren sand in FCS-01-12 and -13 and 1 m down in the sand in FCS-01-10 is a very poorly sorted clay to quartz gravel layer (thick black line in Fig. 8). This gravel layer could represent either the shoreface or bay ravinement surface, depending on the presence or absence of barriers at that time. Alternatively, this gravel layer could represent channel deposits, as gravel outcrops along the Outer Banks have been linked to relict fluvial and inlet channels (Browder and McNinch, 2006). The gravel layer appears in cores surrounding the topographic low under FCS-01-11 but is not reached in FCS-01-11 (Fig. 8). 5.3. Holocene estuary The clay to sandy clay unit is typical of a modern backbarrier estuary along the Outer Banks near an inlet. Sand lenses and laminations were likely deposited during overwash events. The foraminiferal assemblage, dominated by E. excavatum, includes other calcareous species (H. germanica, Elphidium sp., and planktics) and agglutinated marsh species (H. wilberti, H. manilaensis, A. tepida, and T. inflata). Therefore, this unit is interpreted as a Holocene estuary similar to those existing along the Outer Banks today (Abbene et al., 2006; McBride and Robinson, 2003; Robinson and McBride, 2006; Vance et al., 2006). The age of this estuarine unit was estimated at 5195 ybp at 13.1 mbsl in FCS-01-10 (Fig. 8). The 5195 ybp age estimate is 500 years older than the oldest previously existing age estimate for estuarine sediments from the northern Outer Banks (Mallinson et al., 2005). The younger age estimate of 4.5 kybp in conjunction with a transition from marine to estuarine sedimentation at 10–13 mbsl has been interpreted to represent the time and depth of establishment of the relatively continuous Outer Banks barrier island chain at the mouth of the Albemarle Sound (Mallinson et al., 2005). In FCS-01-10, the base of the estuarine unit occurs at 13.9 mbsl, indicating an earlier and deeper/more eastward establishment of the northernmost barrier island/spit. Because the modern barrier islands of the northern Outer Banks receive sediment eroded from northern headlands through net littoral drift and accrete downdrift, it follows that the initial barriers formed in the north and grew southward to form the modern barrier island and spit chain. Alternatively, FCS-01-10 may be located in a paleo-fluvial valley in which case this calculation does not accurately represent relative sea-level rise. 5.4. Ebb tidal delta A shoreface ravinement surface separates the estuarine clay from the laminated and bioturbated sand and is marked by poorly sorted clay to gravel deposits in FCS-01-4, -12, -13, and -14 (Figs. 2–5). The laminated and bioturbated sand is interlaminated to interbedded with mud and fine sand as is typical of ebb tidal deltas (Oertel, 1975; Boothroyd, 1985). The wavy and lenticular mud inclusions within this unit are indicative of tidal processes. The foraminiferal assemblage of the laminated and bioturbated sand contains a mixture of shelf, estuarine and marsh foraminiferal species similar to the assemblage Grossman and Benson

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

0

2437

A‘

A 7

6 4

5 mbsl

11

14 15

41,600ybp 10

15 0

2

1

3

4

5

7

6

9

8

10

km 0

B

B‘ Clay to Gravel (7) Fine, Clean Sand (6)

4

5

3

mbsl

Poorly Sorted Sand (5)

2

5

1A

Lam/Bioturb Sand (4) Clay to Sandy Clay (3)

10

Barren Sand (2) Gravel

15 1

0

3

2

4

5

6

km Fig. 7. South to north stratigraphic cross-sections of False Cape Shoal A. A–A0 traces the entire ridge axis. B–B0 parallels the shoreline. The age estimate of 441 kbp in Core 6 is not considered accurate (see text). Numbers in legend refer to Facies designation. See Fig. 6 for core locations; vertical exaggeration: 200  .

0

C‘

C

0

D‘

D

7 Clay to Gravel (7) 8

5

Lam/Bioturb Sand (4)

1A

11 10

Clay to Sandy Clay (3) Barren Sand (2)

10

Fossiliferous Silt (1)

mbsl

mbsl

5

Poorly Sorted Sand (5)

12

10

13

5195ybp

Gravel

15

15 0

1

2

km 20 0

1

2

km Fig. 8. West to east stratigraphic cross-sections of False Cape Shoal A. C–C0 crosses the ridge near the middle. D–D0 crosses the two subridges at the distal end. Numbers in legend refer to Facies designation. See Fig. 6 for core locations.; vertical exaggeration: 200  .

(1967) found on the ebb tidal deltas of Ocracoke and Drum Inlets, NC. Mixing of species from these diverse environments to create a deposit over 3 m thick in places requires an open conduit between the shelf and estuary and continual transportation of marsh and estuarine species to the shelf that can only be accomplished by tidal inlet processes.

Occasionally, backbarrier deposits can be preserved in the transgressive succession (Field and Duane, 1976; Field, 1980). The foraminiferal signature in the laminated and bioturbated sand could be interpreted as a flood tidal delta as well as an ebb tidal delta. Flood tidal delta deposits on the shelf, however, typically occur below the shoreface ravinement surface. The biconvex

ARTICLE IN PRESS 2438

M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

Table 3 Sedimentary facies of False Cape Shoal A Facies

Sedimentology

Paleontology

Specimen count

Species richness

Occurrence

Interpretation

Poorly sorted clay to gravel

Very poorly sorted clay to fine quartz sand, containing sand lenses, clay pebbles, quartz gravel, and coarse shell fragments; medium gray Very fine to medium quartz sand, light tan to light or medium gray; containing very fine heavy mineral grains, no shells or shell fragments Poorly sorted quartz sand, very fine to gravel, medium gray, containing shell fragments and rock fragments

Elphidium excavatum, Haynesina germanica, Haplophragmoides manilaensis, Trochammina inflata

0–120

0–5

Cores 1A, 10

Swale Fill

Elphidium excavatum, Haynesina germanica, Buccella inusitata, Elphidium subarcticum, Quinqueloculina impressa Elphidium excavatum, Haynesina germanica, Ammonia parkinsoniana, Buccella frigida, Hanzawaia strattoni, Quinqueloculina seminula, Elphidium gunteri, Elphidium mexicanum, planktics All species found in both the upper ridge sands and in the clay to sandy clay

45

6

Core 2

Modern Shoreface

0–89

0–8

Cores 1A, 3, 4, 5, 6, 7, 8, 11, 12, 13, 14, 15

Modern Shelf

0–604

0–18

Cores 4, 5, 6, 8, 11, 12, 13, 14

Ebb Tidal Delta

0–672

0–12

Cores 1A, 2, 3, 4, 10, 12, 14, 15

Holocene Estuary

0–3

0–2

Cores 2, 3, 4, 10, 12, 13, 14

Pleistocene Shoreface

100–582

8–11

Core 13

Pleistocene Estuary

Fine, clean sand

Poorly sorted massive sand

Laminated and bioturbated sand Clay to sandy clay

Barren sand

Fossiliferous silt

Well-sorted quartz silt to fine sand, often containing shell fragments, greenish to medium gray; laminated and bioturbated Dark gray sticky clay to sandy clay, often with sand lenses or laminations; some cores contain oyster and clam shells and gravel layers

Quartz sand, very fine to medium, light tan to light gray; containing mica and very fine heavy mineral grains; no shell fragments, no clay Well-sorted very fine quartz sand to clay, medium gray; containing mica, but not containing shells or shell fragments

Elphidium excavatum, Haynesina germanica, Elphidium sp., Haplophragmoides wilberti, Haplophragmoides manilaensis, Ammonia tepida, Trochammina inflata, planktics Where not entirely barren, Elphidium excavatum, Elphidium gunteri, Quinqueloculina seminula, Elphidium poeyanum Elphidium excavatum, Glabratellina sagrai., Haynesina germanica, Haplophragmoides wilberti, Elphidium sp., Fursenkoina fusiformis, planktics

shape of this laminated and bioturbated sand would not have been preserved but instead would have been planed off during transgression if this were a flood tidal delta deposit. This deposit occurs above the shoreface ravinement surface and fills the long narrow depression beneath FCS-01-5, -11, and -8 (Figs. 7 and 8). As tidal inlets migrate laterally with net littoral drift and landward in response to the Holocene transgression, they excavate linear shore-oblique inlet retreat scars on the inner shelf that are backfilled with ebb tidal delta sands (McBride and Moslow, 1991; Foyle and Oertel, 1997). The laminated and bioturbated sand, therefore, is interpreted as an ebb tidal delta deposit occupying the inlet migration scar. 5.5. Modern shelf The poorly sorted massive sand is found at the core top of all but two sand-ridge cores. The range in grain size and the inclusion of rock and shell fragments are typical of eroded, reworked, and/or transported coastal sediment. Along the modern sediment-starved mid-Atlantic coast, shelf sediments are derived from some combination of eroding headlands, reworked shelf deposits and barrier island shoreface retreat (Swift, 1975; Field and Duane, 1976). As an example, the Arctica shell dated at 441 kybp was deposited in the poorly sorted massive sand of FCS-01-6 (Fig. 7) possibly after having been eroded from the Virginia Beach pre-Holocene headland and transported southward. This age estimate is not considered to represent the age of this unit as this Arctica shell is reworked and isotopically dead. Foraminifera living on the inner shelf contribute to the eroded, reworked and transported sediment. Foraminiferal species typical

of the Virginia and North Carolina inner shelf (E. excavatum, A. parkinsoniana, B. frigida, H. strattoni, Q. seminula, E. gunteri, E. mexicanum, and planktics (Schnitker, 1971; Cronin et al., 1998) are found in this sand. The poorly sorted massive sand is interpreted as a surficial sand sheet of the modern inner continental shelf (Stahl et al., 1974; Swift, 1975, 1985) that blankets and incorporates the ebb tidal delta and blankets Holocene estuarine deposits. In a comparison of an inner shelf ridge and a mid-shelf ridge off New Jersey, Rine et al. (1991) found the species Nonionella atlantica and P. novangliae to be restricted to the mid-shelf ridge and interpreted this restriction to support a mid-shelf origin for this ridge. Unlike in the New Jersey study, both species occur in False Cape Shoal A sediments on the inner shelf. Culver and Snedden (1996) examined four New Jersey shelf sand ridges, including a shoreface-attached sand ridge, and determined that surface sands of all ridges reflected their present position on the shelf and that the underlying ridge sands of their shorefaceattached sand ridge were similar to surface sands in their foraminiferal assemblages. In False Cape Shoal A, the foraminiferal assemblages of the underlying sands (ebb tidal delta deposits) are different from the modern shelf sands at the surface. The surface sands are massive and poorly sorted and contain rock and shell fragments. The underlying sands are laminated, bioturbated, and well sorted. The surface sands contain E. excavatum, H. germanica, A. parkinsoniana, B. frigida, H. strattoni, Q. seminula, E. gunteri, E. mexicanum, and planktics. The underlying sands contain these shelf species as well as Elphidium sp., H. wilberti, H. manilaensis, A. tepida, and T. inflata, and, in addition, samples from the underlying sands show increased specimen counts and species richness (Figs. 2–5).

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

5.6. Modern shoreface The fine, clean sand is found only at the core top of FCS-01-2 along the modern shoreface (Fig. 7). This sand is similar to the modern shelf sediment in its foraminiferal assemblage but is better sorted and does not contain shell fragments. Currituck Spit receives sediment eroded from the northern headlands carried south by net littoral drift and deposited as shoreface sands. The shoreface generally does not receive sediment from reworked shelf deposits and is typically better sorted and lacks shell or rock fragments (Swift, 1976). The foraminiferal assemblage found at the shoreface is similar to that of the inner shelf because the environments are adjacent to each other. The fine, clean sand contains a foraminiferal assemblage characterized by E. excavatum, H. germanica, B. inusitata, Elphidium subarcticum, and Quinqueloculina impressa. The fine, clean sand is interpreted to represent the modern shoreface.

5.7. Swale fill The poorly sorted clay to gravel stratigraphic unit ranges in grain size from clay and clay pebbles to shell and quartz gravel. This unit only occurs in the swales between the ridge and the shoreline, in FCS-01-1A and -10, and does not occur in cores containing ebb tidal delta (laminated and bioturbated sand) deposits (Fig. 5). The foraminiferal assemblage is composed of E. excavatum, H. germanica, H. manilaensis, and T. inflata and is similar to the Holocene estuary assemblage, containing both calcareous and agglutinated forms but with fewer specimens and lower species richness. This swale fill assemblage is similar to the swale/tidal inlet-fill assemblage of Peahala Ridge, NJ (Snedden et al., 1994). The swale separating False Cape Shoal A from Currituck Spit is deepened and widened during intense storms in response to shelf storm flow, erosional shoreface retreat, and aggradation of the adjacent shelf sand sheet (Swift et al., 1972a). This unit is interpreted as swale fill because clay clasts eroded from the Holocene estuarine deposits during storms and containing estuarine foraminifera could be redeposited in the swale during calm weather, along with large shell fragments and quartz gravel from the sand ridge, to create this poorly sorted clay to gravel unit. In summary, the sedimentary facies of False Cape Shoal A, in general, represent a typical transgressive succession in which a thin backbarrier mud, capped by a thin coarse lag marking the ravinement surface, is overlain by offshore ebb tidal delta and modern shelf deposits. Finer, cleaner shoreface sands and poorly sorted swale fill occur at the surface between the barrier spit beach along Currituck Spit and the False Cape sand ridge. Pleistocene coastal deposits are found beneath the Holocene transgressive succession. The stratigraphic succession of modern inner shelf sand overlying Holocene estuarine clay overlying Pleistocene coastal and/or fluvial deposits is common to the Maryland shelf (Field, 1980; Toscano et al., 1989) and to previous studies of False Cape Shoals (Swift et al., 1972a) and Peahala Ridge, NJ (Snedden et al., 1994). While Swift et al. (1972a) estimated the age of the estuarine clay underlying the surface sand unit comprising False Cape Shoals to be 20–25 kybp (based on a bulk carbonate sample), they recognized the uncertainty in their dating procedures and interpreted this unit as possible Holocene estuarine clay. The False Cape Shoal A results presented here are similar to results of studies of Peahala Ridge, a shore-oblique, shorefaceattached sand ridge on the New Jersey shelf. Minor differences, however, do exist. Peahala Ridge sand overlies a flat shoreface ravinement surface (Snedden et al., 1994), whereas False Cape

2439

Shoal A ridge sand inhabits a long, linear, shore-oblique topographic low. The ebb tidal delta nucleus of Peahala Ridge is found only at the distal end of the sand ridge (Snedden et al., 1994), whereas this nucleus is broadly represented across False Cape Shoal A. Finally, foraminiferal assemblages in the surficial and core ridge sands are similar in Peahala Ridge (Culver and Snedden, 1996) but are easily distinguished in False Cape Shoal A. These three differences indicate more advanced reworking at and/ or migration of Peahala Ridge than has occurred at False Cape Shoal A.

6. Summary and conclusions Analysis of the sedimentology, stratigraphy and benthic foraminiferal assemblages from 14 sand-ridge cores has revealed seven distinct facies that, in the Holocene part of the record, represent a transgressive succession of depositional environments in which a thin backbarrier mud, capped by a thin coarse lag marking the shoreface ravinement surface, is overlain by offshore deposits. From bottom to top, the depositional environments represent a Pleistocene estuary, Pleistocene regressive shoreface, Holocene estuary, ebb tidal delta, modern shelf, modern shoreface, and swale fill. This succession of depositional environments relates a Pleistocene sea-level highstand and subsequent regression followed by the Holocene transgression in which barrier island/spit systems formed along the VA/NC inner shelf and migrated landward. An ebb tidal delta was deposited, the remaining depositional feature is being reworked by shelf processes, and shelf sand is being deposited on top of the ebb tidal delta core. The age of the Holocene estuarine clay beneath False Cape Shoal A was estimated at 5195 ybp at a depth of 13.1 mbsl. The 5195 ybp age estimate may represent a minimum age of the establishment of the barrier island system at the northernmost segment of the Outer Banks and is 500 years older than the oldest date for estuarine sediments at the mouth of Albemarle Sound. The base of the Holocene estuarine clay beneath False Cape Shoal A occurs at 13.9 mbsl, indicating an earlier and deeper/more eastward establishment of the northernmost barrier island/spit than at the mouth of Albemarle Sound. If so, this provides evidence for an initial northern spit/barrier system slowly accreting southward over a period of 500 years to eventually form the northern part of the Outer Banks barrier island chain. However, it is also possible that the mollusk could have been transported or reworked or that the clay is deeper at the FCS-Core 10 site due to the flooding of a paleo-river channel.

Acknowledgments Funding for this study was provided by the Office of Naval Research (Grants nos. N00014-99-1-0817 and N00014-00-10247), US Department of the Interior Minerals Management Service and Virginia Institute of Marine Science (Grant no. 055800/1249), George Mason University’s Summer Research Funding for Faculty through the Provost Office, George Mason University Department of Environmental Science and Policy Student Research Fellowships, and by the USGS Earth Surface Dynamics Program. 14C AMS results were made possible through the generosity of Thomas Cronin under the NSF Cooperative Agreement, OCE-9807266. Rocı´o Caballero graciously provided valuable technical assistance in the lab. We thank Harry Dowsett, Steve Culver, Thomas Cronin, Richard Diecchio and Sheryl Beach for their thoughtful feedback throughout this study and Lynn Wingard for help in mollusk identification. Reviews by Steve

ARTICLE IN PRESS 2440

M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

Culver and an anonymous reviewer improved the quality of this manuscript.

Appendix A Foraminiferal taxa observed in False Cape Shoals deposits Ammonia parkinsoniana Orbigny (1839) Ammonia tepida Cushman (1926) Buccella frigida Cushman (1922) Buccella hannai Phleger and Parker (1951) Buccella inusitata Andersen (1952) Cibicides lobatulus (Walker and Jacob), in Kanmacher (1798) Elphidium crispum Linne (1758) Elphidium excavatum Terquem (1875) Elphidium galvestonense Kornfeld (1931) Elphidium gunteri Cole (1931) Elphidium macellum von Fichtel and Moll (1803) Elphidium mexicanum Kornfeld (1931) Elphidium poeyanum Orbigny, (1839) Elphidium subarcticum Cushman (1944) Elphidium sp. Eponides repandus von Fichtel and Moll (1798) Fissurina lucida Williamson (1848) Fursenkoina fusiformis Williamson (1858) Glabratellina sagrai Todd and Bro¨nnimann (1957) Guttulina lactea (Walker and Jacob), in Kanmacher (1798) Hanzawaia strattoni Applin et al. (1925) Haplophragmoides manilaensis Andersen (1953) Haplophragmoides wilberti Andersen (1953) Haynesina germanica Ehrenberg (1840) Jadammina macrescens Brady (1870) Miliammina fusca Brady (1870) Nonionella atlantica Cushman (1947) Nonionella opima Cushman (1947) Planulina mera Cushman (1944) Poroeponides lateralis Terquem (1878) Pseudopolymorphina novangliae Cushman (1944) Quinqueloculina compta Cushman (1947) Quinqueloculina impressa Reuss (1851) Quinqueloculina jugosa Cushman (1944) Quinqueloculina lata Terquem (1876) Quinqueloculina seminula Linne (1758) Trochammina inflata Montagu (1808)

Appendix A. Supplementary materials Supplementary data associated with this article can be found in the online version at doi:10.1016/j.csr.2008.06.002.

References Abbene, I.J., Culver, S.J., Corbett, D.R., Buzas, M.A., Tully, L.S., 2006. Distribution of foraminifera in Pamlico Sound, North Carolina, over the past century. Journal of Foraminiferal Research 36, 135–151. Andersen, H.V., 1952. Buccella, a new genus of the rotalid foraminifer. Washington Academy of Science Journal 42, 143–151. Andersen, H.V., 1953. Two new species of Haplophragmoides from the Louisiana coast. Contributions to the Cushman Foundation for Foraminiferal Research 4, 21–22. Applin, E.R., Ellisor, A.E., Kniker, H.T., 1925. Subsurface stratigraphy of the coastal plain of Texas and Louisiana. American Association of Petroleum Geologists Bulletin 9, 79–122. Boothroyd, J.C., 1985. Tidal inlets and tidal deltas. In: Davis, R.A. (Ed.), Coastal Sedimentary Environments. Springer, New York, pp. 445–532. Brady, H.B., 1870. Analysis and descriptions of the foraminifera. In: Brady, G.S., Robertson, D. (Eds.), The Ostracoda and Foraminifera of Tidal Rivers. Part II. Annals and Magazine of Natural History, ser. 4, vol. 6, pp. 273–309. Browder, A., McNinch, J.E., 2006. Linking framework geology of the nearshore: correlation of paleo-channels with shore-oblique sandbars and gravel outcrops. Marine Geology 231, 141–162. Cole, W.S., 1931. The pliocene and pleistocene foraminifera of Florida. Florida State Geological Survey Bulletin 6, 79.

Cronin, T.M., Ishman, S.E., Wagner, R.S., Cutter, G.R., 1998. Environmental Studies Relative to Potential Sand Mining in the Vicinity of Virginia Beach, Virginia. Part 5: Benthic Foraminifera and Ostracoda from Virginia Continental Shelf. US Department of the Interior, Minerals Management Service, College of William and Mary, Virginia Institute of Marine Science, School of Marine Science, 36pp. Culver, S.J., Snedden, J.W., 1996. Foraminiferal implications for the formation of New Jersey shelf sand ridges. Palaios 11, 161–175. Cushman, J.A., 1922. Results of the Hudson Bay Expedition 1920. I. The foraminifera. Canadian Biological Board Contributions to Canadian Biology 9, 135–147. Cushman, J.A., 1926. Recent foraminifera from Puerto Rico. The Carnegie Institution of Washington Publications 344, 73–84. Cushman, J.A., 1944. Foraminifera from the shallow water of the New England Coast. Cushman Laboratory for Foraminiferal Research Special Publication 12, 1–37. Cushman, J.A., 1947. New species and varieties of foraminifera from off the southeastern coast of the United States. Contributions from the Cushman Laboratory for Foraminiferal Research 23, 86–92. Duane, D.B., Field, M.E., Meisburger, E.P., Swift, D.J.P., Williams, S.J., 1972. Linear shoals on the Atlantic inner continental shelf, Florida to Long Island. In: Swift, D.J.P., Duane, D.B., Pilkey, O.H. (Eds.), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and Ross, Stroudsburg, PA, pp. 447–498. Ehrenberg, G.C., 1840. Eine, weitere Erlauterung des Organismus meherer in Berin lebin beobachteter Polythalamien der Nordesee, pp. 18–23. Field, M.E., 1980. Sand bodies on coastal plain shelves: Holocene record of the US Atlantic inner shelf of Maryland. Journal of Sedimentary Petrology 50, 505–528. Field, M.E., Duane, D.B., 1976. Post-Pleistocene history of the United States inner continental shelf: significance to origin of barrier islands. Geological Society of America Bulletin 87, 691–702. Figueiredo, A.G., Swift, D.J.P., Stubblefield, W.L., Clarke, T.L., 1981. Sand ridges on the inner Atlantic shelf of North America: morphometric comparisons with huthnance stability model. Geo-Marine Letters 1, 187–191. Foyle, A.M., Oertel, G.F., 1997. Transgressive systems tract development and incised-valley fills within a Quaternary estuary-shelf system: Virginia inner shelf, USA. Marine Geology 137, 227–249. Grossman, S., Benson, R.H., 1967. Ecology of Rhizopodea and Ostracoda of southern Pamlico Sound region, North Carolina. The University of Kansas Paleontological Contributions, Serial number 44, 90pp. Harris, A.G., Sweet, W.C., 1989. Mechanical and chemical techniques for separating microfossils from rock, sediment and residue matrix. In: Feldmann, R.M., Chapman, R.E., Hannibal, J.T. (Eds.), Paleotechniques, vol. 4. The Paleontological Society, Special Publication, pp. 70–86. Huthnance, J.M., 1982. On one mechanism forming linear sand banks. Estuarine and Marine Coastal Science 14, 79–99. Kanmacher, F. (Ed.), 1798. Adam’s Essays on the Microscope: The Second Edition, with Considerable Additions and Improvements. Dillon and Keating, London, 712pp. Kornfeld, M.M., 1931. Recent littoral foraminifera from Texas and Louisiana. Contributions from the Department of Geology of Stanford University 1, 77–101. Kraft, J.C., 1971a. Sedimentary facies patterns and geologic history of a Holocene marine transgression. Geological Society of America Bulletin 82, 2131–2158. Kraft, J.C., 1971b. The migration of Holocene sedimentary environments in coastal Delaware, North American continental shelf. Quaternaria 14, 23–38. Kraft, J.C., Chrzastowski, M.J., 1985. Coastal stratigraphic sequences. In: Davis, R.A. (Ed.), Coastal Sedimentary Environments. Springer, New York, pp. 625–663. Linne, C., 1758. Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymnis, locis. G. Englemann (Lipsiae), ed. 10, vol. 1, pp. 1–824. Mallinson, D., Riggs, S., Thieler, E.R., Culver, S., Farrell, K., Foster, D.S., Corbett, D.R., Horton, B., Wehmiller, J.F., 2005. Late Neogene and Quaternary evolution of the northern Albemarle Embayment (mid-Atlantic continental margin, USA). Marine Geology 217, 97–117. McBride, R.A., Moslow, T.F., 1991. Origin, evolution and distribution of shoreface sand ridges, Atlantic inner shelf, USA. Marine Geology 97, 57–85. McBride, R.A., Robinson, M.M., 2003. Geomorphic evolution and geology of Old Currituck Inlet and its flood tidal delta, Virginia/North Carolina, USA (Part 1). In: Proceedings of Coastal Sediments ‘03, American Society of Civil Engineers (ASCE), East Meets West Productions, Corpus Christi, TX, 14pp. Montagu, G., 1808. Testacea Brittanica, Supplement. Exeter, England, S. Woolmer, 183pp. Niedoroda, A.W., Swift, D.J.P., Hopkins, T.S., 1985. The shoreface. In: Davis, Jr., R.A. (Ed.), Coastal Sedimentary Environments. Springer, New York, pp. 533–624. Oertel, G.F., 1975. Post-Pleistocene island and inlet adjustment along the Georgia coast. Journal of Sedimentary Geology 45, 150–159. Orbigny, A.d., 1839. Foraminiferes. In: de la Sagra, R. (Ed.), Histoire physique, politique et naturelle de l’Ile de Cuba. A. Bertrand, Paris, p. 224. Phleger, F.B., Parker, F.L., 1951. Ecology of foraminifera, northwest Gulf of Mexico. Part II. Foraminifera species. Memoirs of the Geological Society of America 46, 1–64. Reuss, A.E., 1851. Ueber die fossilen Foraminiferen und Entomostraceen der Septarienthone der Umgegend von Berlin. Zeitschrift der Deutschen Geologischen Gesellschaft, Berlin 3, 49–92. Rine, J.M., Tillman, R.W., Culver, S.J., Swift, D.J.P., 1991. Generation of late Holocene sand ridges on the middle continental shelf of New Jersey, USA—evidence for

ARTICLE IN PRESS M.M. Robinson, R.A. McBride / Continental Shelf Research 28 (2008) 2428–2441

formation in a mid-shelf setting based on comparisons with a nearshore ridge. In: Swift, D.J.P., Oertel, G., Tillman, R.W. (Eds.), Shelf Sand and Sandstone Bodies: Origin, Facies and Distribution. International Association of Sedimentologists, Special Publication, vol. 14, pp. 395–423. Robinson, M.M., McBride, R.A., 2006. Benthic foraminifera from a relict flood tidal delta along the Virginia/North Carolina Outer Banks. Micropaleontology 52, 67–80. Sanders, J.E., 1962. North-south trending submarine ridges composed of coarse sand off False Cape, Virginia. Abstracts, American Association of Petroleum Geologists Bulletin 46, 278. Schnitker, D., 1971. Distribution of foraminifera on the North Carolina continental shelf. Tulane Studies in Geology and Paleontology 8, 169–215. Snedden, J.W., Dalrymple, R.W., 1999. Modern shelf sand ridges: from historical perspective to a unified hydrodynamic and evolutionary model. In: Bergman, K.M., Snedden, J.W. (Eds.), Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation, vol. 64. SEPM, Special Publication, pp. 13–28. Snedden, J.W., Kreisa, R.D., Tillman, R.W., Schweller, W.J., Culver, S.J., Winn, R.D., 1994. Stratigraphy and genesis of a modern shoreface-attached sand ridge, Peahala Ridge, New Jersey. Journal of Sedimentary Research B 64, 560–581. Snedden, J.W., Kreisa, R.D., Tillman, R.W., Culver, S.J., Schweller, W.J., 1999. An expanded model for modern shelf sand ridge genesis and evolution on the New Jersey Atlantic shelf. In: Bergman, K.M., Snedden, J.W. (Eds.), Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation, vol. 64. SEPM, Special Publication, pp. 147–163. Stahl, L., Koczan, J., Swift, D.J.P., 1974. Anatomy of a shoreface-connected sand ridge on the New Jersey shelf: implications for the genesis of the shelf surficial sand sheet. Geology 2, 117–120. Stubblefield, W.L., McGrail, D.W., Kersey, D.G., 1984. Recognition of transgressive and post-transgressive sand ridges on the New Jersey continental shelf. In: Tillman, R.W., Siemers, C.T. (Eds.), Siliclastic Shelf Sediments. Society of Economic Paleontologists and Mineralogists (SEPM), Special Publication 34, Tulsa, OK, pp. 1–23. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215–230. Stuiver, M., Reimer, P.J., Reimer, R.W., 2005. CALIB 5.0. [WWW program and documentation]. Swift, D.J.P., 1975. Barrier island genesis: evidence from the central Atlantic shelf, eastern USA. Sedimentary Geology 14, 1–43. Swift, D.J.P., 1976. Continental shelf sedimentation. In: Stanley, D.J., Swift, D.J.P. (Eds.), Marine Sediment Transport and Environmental Management. Wiley, New York, pp. 311–350. Swift, D.J.P., 1985. Response of the shelf floor to flow. In: Tillman, R.W., Swift, D.J.P., Walker, R.G. (Eds.), Shelf Sands and Sandstone Reservoirs: Tulsa, SEPM (Society for Sedimentary Geology). Short Course Notes 13, pp. 135–241. Swift, D.J.P., Field, M., 1981. Evolution of a classic sand ridge field, Maryland sector, North America inner shelf. Sedimentology 28, 462–482. Swift, D.J.P., Holliday, B., Avignone, N., Shideler, G., 1972a. Anatomy of a shoreface ridge system, False Cape, Virginia. Marine Geology 12, 59–84.

2441

Swift, D.J.P., Kofoed, J.W., Saulsbury, F.P., Sears, P., 1972b. Holocene evolution of the shelf surface, central and southern Atlantic shelf of North America. In: Swift, D.J.P., Duane, D.B., Pilkey, O.H. (Eds.), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson, and Ross, Stroudsburg, PA, pp. 499–573. Swift, D.J.P., McKinney, T.F., Stahl, L., 1984. Recognition of transgressive and posttransgressive sand ridges on the New Jersey continental shelf—discussion. In: Tillman, R.W., Siemers, C.T. (Eds.), Siliclastic Shelf Sediments. Society of Economic Paleontologists and Mineralogists (SEPM), Special Publication 34, Tulsa, OK, pp. 25–36. Terquem, O., 1875. Essai sur le classement des animaux qui vivent sur la plage et dans les environs de Dunquerque. Fasc. 1, Paris, pp. 1–54. Terquem, O., 1876. Essai sur le classement des animaux qui vivant sur la plage et dans les environs de Dunquerque, Pt. 1. Memoires de las Societe Dunquerquoise pour l’Encouragement des Sciences des Lettres et des Arts (1874–1875), vol. 19, pp. 405–457. Terquem, O., 1878. Les foraminife`res et les entomostracea—ostracodes du Plioce`ne Supe´rieur de I’lle de Rhodes. Me´moirs de la Societe´ Ge´ologique de France 3/1 (3), 1–35. Todd, R., Bro¨nnimann, P., 1957. Recent Foraminifera and Thecamoebina from the eastern Gulf of Paria, vol. 3. Cushman Foundation for Foraminiferal Research Special Publication, 43pp. Toscano, M.A., Kerhin, R.T., York, L.L., Cronin, T.M., Williams, S.J., 1989. Quaternary stratigraphy of the inner continental shelf of Maryland. Report of Investigations, Maryland Geological Survey Report 50, 116pp. Uchupi, E., 1968. Atlantic continental shelf and slope of the United States: Physiography. US Geological Survey Professional Paper 529-C, 30pp. US Department of Commerce, 1985. National Estuarine Inventory Data Atlas, Volume 1: Physical and Hydrologic Characteristics. US Department of Commerce, Washington, DC, 111pp. Vance, D.J., Culver, S.J., Corbett, D.R., Buzas, M.A., 2006. Foraminifera in the Albemarle estuarine system, North Carolina: distribution and recent environmental change. Journal of Foraminiferal Research 36, 15–33. Veatch, A.C., Smith, P.A., 1939. Atlantic submarine valleys of the United States and the Congo submarine valley. Special Paper of the Geological Society of America, vol. 7, 101pp. von Fichtel, L., Moll, J.P.C., 1798. Testacea Microscopica Aliaque Minuta ex Generibus Argonauta et Nautilus ad Naturam Delineata et Descripta (Microscopische und andere klein Schalthiere aus den geschlechtern Argonaute und Schiffer). Camesianische Buchandlung, Wien, 123pp. von Fichtel, L., Moll, J.P.C., 1803. Testacea Microscopica Aliaque Minuta ex Generibus Argonauta et Nautilus ad Naturam Delineata et Descripta (Microscopische und andere klein Schalthiere aus den geschlechtern Argonaute und Schiffer), second ed. Camesianische Buchandlung, Wien, 124pp. Williamson, W.C., 1848. On the recent British species of the genus Lagena. The Annals and Magazine of Natural History, Series 2, vol. 1, pp. 1–20. Williamson, W.C., 1858. On Recent Foraminifera of Great Britain. Ray Society (London) Publication, 107pp. Woo, H.J., Culver, S.J., Oertel, G.F., 1997. Benthic foraminiferal communities of a barrier-lagoon system, Virginia, USA. Journal of Coastal Research 13 (4), 1192–1200.