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Continental Shelf Research 25 (2005) 461–484

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Detailed investigation of sorted bedforms, or ‘‘rippled scourdepressions,’’ within the Martha’s Vineyard Coastal

Observatory, Massachusetts

John A. Goffa,�, Larry A. Mayerb, Peter Traykovskic, Ilya Buynevichc,Roy Wilkensd, Richard Raymondb, Gerd Glangb, Rob L. Evansc,

Hilary Olsona, Chris Jenkinse

aUniversity of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759, USAbUniversity of New Hampshire Center for Coastal and Ocean Mapping, Durham, NH 03824, USA

cWoods Hole Oceanographic Institution, Woods Hole, MA 02543, USAdHawaii Institute of Geophysics and Planetology, Honolulu, HI 96822, USA

eUniversity of Colorado Institute of Arctic & Alpine Research, Boulder, CO 80309, USA

Received 17 February 2004; received in revised form 13 July 2004; accepted 17 September 2004

Available online 30 December 2004

Abstract

We examine in detail the seafloor and cross-sectional morphology of sorted bedforms (i.e., ‘‘rippled scour

depressions’’) in the Martha’s Vineyard Coastal Observatory (MVCO). Sorted bedforms are seen as alternating bands

of coarse and fine sands oriented nearly perpendicular to the shoreline. The coarse sand zones (CSZs) of the sorted

bedforms are tens to hundreds of meters wide, and extend up to several kilometers from the shoreface. Data considered

here include time series of swath backscatter and bathymetry, high resolution chirp seismic reflection, and grain-size

analyses from grab samples, vibracores and push cores. The sorted bedforms observed within the MVCO survey area

exhibit a broad spectrum of bathymetric relief (from �10 cm to �3 m), grain-size contrast (from �250 to42000 mm)

and morphologic form (moats, steps, and dune forms). All forms observed display lateral asymmetry in both grain size

and bathymetric expression. In general, grain size is largest and bathymetry is deepest toward one side, typically seen in

the backscatter maps as the more well defined of the two CSZ edges where that distinction can be made. These

observations are consistent with earlier studies suggesting that sorted bedforms are a response to a transverse,

alongshore flow. Within the MVCO survey area, the sense of asymmetry changes polarity going from west/shallow

water to east/deeper water, suggesting a complex hydrographic regime.

Our time series data demonstrate variability in the location of the boundaries between coarse and fine sands, with

movements of tens of meters over spans of months, but great stability in the bathymetric features, with little or no

migration seen over the same time span and little detectable movement observed for larger features over a span of

nearly four decades. Furthermore, the direction of migration of the coarse/fine sand boundaries is often at odds with

e front matter r 2004 Elsevier Ltd. All rights reserved.

r.2004.09.019

ng author. Tel.: +1 512 471 0476; fax: +1 512 471 0999.

ss: [email protected] (J.A. Goff).

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J.A. Goff et al. / Continental Shelf Research 25 (2005) 461–484462

expectations based on the asymmetries of the sorted bedforms. We speculate that sorted bedform migration may, in the

short term, be controlled by small-scale ripple migration forced by wave orbital velocity skewness, and in the long term

by alongshore currents.

Beneath the sorted bedforms lies a shallow, horizontal seismic reflector, a few tens of centimeters below the seafloor in the

shallower waters, and41 m in deeper water. This reflector is consistently present below the fine sands and is often observed,

although less defined, beneath the coarse sands. It is often continuous beneath transitions between fine and coarse sands at

the surface. In sediment cores, this reflector appears to correlate to a variable-thickness layer of gravel/very coarse sands that

is frequently present beneath both coarse and fine surface sands. This surface also caps a buried fluvial channel system. We

interpret this horizon as an erosional lag delineating a transgressive ravinement surface and the contact between poorly

sorted glacio-fluvial sediments below and reworked, well- to moderately well-sorted fine and coarse sands above.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Grain size; Backscatter; Bathymetry; Seismic reflection; Inner shelf

1. Introduction

The nearshore zone of the continental shelf isoften an active sedimentary environment, im-pacted by the effects of storm-generated wavesand flows and longshore currents. It is in thisenvironment that the largest bedforms on thecontinental shelf are generated. Oblique sandridges, for example, have been studied for decadesin inner-shelf locations all over the world (e.g.,Swift and Field, 1981; Parker et al., 1982;Dalrymple and Hoogendoorn, 1997; van de Meeneand van Rijn, 2000; Park et al., 2003). Up tokilometers wide, tens of kilometers long andseveral meters high, sand ridges often form alongthe shoreface in areas where sand sediment supplyis high. Another type of large bedform has alsobeen recognized as ubiquitous in nearshore set-tings where sediment supply is low: so-called‘‘rippled scour depressions’’ (RSDs; the term wascoined by Cacchione et al., 1984). RSDs arebathymetrically more subtle features than sandridges, typically witho1 m of relief (Cacchione etal., 1984). They are most clearly identified insidescan sonar backscatter surveys and, whenground-truthed, appear to be highly elongatedpatches of rippled, coarse sand/gravel/shell hash,oriented approximately shore perpendicular, andslightly depressed by up to a meter with respect tosurrounding fine sands. RSDs are typically tens tohundreds of meters wide and hundreds to thou-sands of meters long (see Cacchione et al., 1984,and Murray and Thieler, 2004, for comprehensivereviews of prior literature on RSDs).

There is a general consensus among investiga-tors as to how RSDs are able to maintain a clearsegregation of coarse and fine sands on theseafloor (Murray and Thieler, 2004). The largerripples that exist within the coarse sedimentpatches will lead to strong bottom boundaryturbulence, which will in turn inhibit the deposi-tion of fine-grained sands. Thus, fine sand grainsthat are either transported into or winnowed froman RSD will tend to keep traveling until they reachan area already covered by fine sands. There is lessagreement, however, on how RSDs initiate or towhat flow regime they respond. Early hypothesescentered on cross-shelf flows, perhaps caused bydownwellings during storms (Cacchione et al.,1984). However, recent detailed investigations ofRSDs off Long Island, New York (Schwab et al.,2000) and Wrightsville Beach, North Carolina(Thieler et al., 1995, 2001; Murray and Thieler,2004) found important alongshore asymmetries inmorphology that suggest they are a response tosouthwest-directed long-shore currents. Further-more, sidescan sonar surveys taken before andafter a storm event showed significant movementof the RSDs along shelf in the direction of thelongshore currents (Murray and Thieler, 2004).

The term ‘‘rippled scour depression’’ has itselfcome under recent scrutiny. The use of ‘‘depres-sion’’ as a morphological descriptor is only partlycorrect, as noted by Murray and Thieler (2004) intheir detailed investigation of RSD morphologyon Wrightsville Beach. While the upcurrent side ofthe RSD (in relation to the alongshore current) isusually depressed relative to the surroundings, the

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J.A. Goff et al. / Continental Shelf Research 25 (2005) 461–484 463

downcurrent side is often raised. Schwab et al.(2000) also determined, in their backscatter andbathymetry mapping of RSDs off Long Island,New York, that the high backscatter/coarse sandswere coincident with the upcurrent flanks of lowamplitude, flow-transverse bedforms. These ob-servations suggest that RSDs and surrounding finesands are nearly dune-like in their apparent stossand lee relationships, respectively, to an along-shore current direction. On the other hand, not allobservations of RSDs yield the same results.Cacchione et al. (1984), for example, report nocoastwise asymmetry in RSDs off Central Cali-fornia, and no obvious asymmetry is evident inRSDs observed by Green et al. (2004) off the coastof New Zealand. There may, therefore, be twoclasses of features lumped together by the term‘‘RSD,’’ distinguished by their asymmetry.

We report here on a detailed investigation ofseabed morphology off the south coast ofMartha’s Vineyard, Massachusetts (Fig. 1). Asshall be demonstrated in this paper, the featureswe observe here are akin to those observed offWrightsville Beach and Long Island. We thereforefollow Murray and Thieler’s (2004) suggestion ofreferring to the morphology as a whole as ‘‘sorted

Fig. 1. Location map for the Martha’s Vineyard Coastal

Observatory. Bathymetry, derived from the coastal relief model

of the National Geophysical Data Center (NGDC), is

artificially illuminated from the north, with contours in meters.

bedforms’’ rather than ‘‘rippled scour depres-sions.’’ We also refer to any area of coarse sand,which would formerly be referred to as an RSD,instead simply as a ‘‘coarse sand zone,’’ or CSZ;we employ this generic terminology to help us todifferentiate components of the sorted bedforms.

The geology of Martha’s Vineyard is comprisedprincipally of glacio-fluvial outwash sedimentsseaward of the terminal moraine on the northand east sides of the island (e.g., Uchupi andOldale, 1994, and references therein), depositedduring the last glaciation ca. 23 ka (Balco et al.,2002). These outwash plains are dissected bysouthward-trending valleys, presumably formedby spring sapping (Uchupi and Oldale, 1994). Oursurvey area encompasses the Martha’s VineyardCoastal Observatory (MVCO) operated by WoodsHole Oceanographic Institution. The study wasconducted under the auspices of the Office ofNaval Research’s (ONR) Mine Burial PredictionProgram, which seeks to understand the process ofsolid object burial by such processes as liquefac-tion and bedform migration. Our surveys repre-sent a geological reconnaissance of the area, withparticular emphasis on understanding the struc-ture and evolution of the sorted bedforms. Datareported here include: (1) acoustic backscatter,collected during three surveys during February2001, September 2001, and July 2002, (2) swathbathymetry, collected during the September 2001and July 2002 surveys, as well as 1965 soundingsby the National Oceanographic Service, (3) highresolution deep-towed chirp seismic data, (4)sedimentological/textural analysis from vibra-cores, (5) grain-size analysis from diver push coresacross two CSZ boundaries, and (6) grain-sizeanalysis from seafloor grab samples. The chirpdata are resolved vertically to o10 cm; to ourknowledge these data are among the first success-ful seismic imaging of the subseafloor structure ofRSDs/sorted bedforms.

Our study is primarily observational in nature,focusing on the areal morphology and cross-sectional structures of the sorted bedforms and theirtemporal variability. Important new observationalconstraints are provided by the combined data setsof high resolution bathymetry, backscatter, andsubsurface imagery, along with extensive surface

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and subsurface sampling. The MVCO survey area isalso notable for the strong variability in the size andshape of sorted bedforms, which provides a uniqueopportunity for posing and testing hypotheses fortheir formation. While this paper raises morequestions than it answers, future reports by colla-borating scientists in the ONR Mine Burial programwill greatly progress our understanding of theseimportant features.

2. Data

The site survey of the MVCO was focused in thevicinity of the MVCO node (Figs. 2 and 3), whichprovides cabled access to the shore for poweringand monitoring of scientific equipment. Datacoverage was also extended over a broader area(Figs. 2b, 3a) to provide regional geologic contextfor understanding sedimentary activity near thenode site. Emphasis was placed on obtaining time-series mapping results, to investigate the evolutionof seafloor features over time scales of months toyears.

2.1. Swath mapping

An initial sidescan sonar backscatter survey ofthe MVCO was conducted in February of 2001 bythe United States Geologic Survey (USGS), usinga towed EdgeTech DF1000 sidescan sonar systemoperating at 100 kHz (Fig. 2a). The USGS wassubsequently funded in part by ONR to survey inSeptember 2001 over a broader area using a pole-mounted Submetrix interferometric swath map-ping sonar system operating at 234 kHz, whichcollected both backscatter (Fig. 2b) and interfero-metric bathymetry (Fig. 3a). The weather duringthis survey was rough, and the data were conse-quently a bit noisy but still of value both as aregional reconnaissance and for identifying princi-

Fig. 2. Backscatter survey maps from (a) February 2001, using an E

using a Submetrix 234 kHz interferometric system, and (c) July 2002,

backscatter is indicated by lighter shades. Bathymetric contours, in m

Squares in (b) indicate grab sample stations (filled squares correspond

vibracore sites and their station numbers. The larger coarse sand zone

indicates location of the MVCO node. Locations are given for profile

pal morphological structures associated with thesorted bedforms.

A high-resolution multibeam bathymetric sur-vey was conducted in July 2002 by the Universityof New Hampshire and Science ApplicationsInternational Corporation (SAIC). An approxi-mately 3� 5 km area surrounding the MVCOnode was surveyed using a 455 kHz Reson 8125focused multibeam sonar. In a 250� 400 m areaimmediately surrounding the MVCO node, highresolution data were collected with a line spacingof approximately 4 m; line spacing was relaxed toapproximately 12 m to an area of 1 km � 1 kmaround the MVCO node and finally to 25–40 m inthe rest of the area. The very narrow beam widthof the Reson 8125 (0.51) combined with the highdensity of data within the 1 km � 1 km boxsupport gridding at 25 cm or less. Kinematic GPSnavigation was used to provide a navigationprecision of less than 10 cm. This will be essentialfor the comparison with subsequent bathymetricsurveys. Here we include the results of the largerarea survey presented at a grid resolution of 50 cm(Fig. 3b). Although the Reson 8125 data were notintended for extraction of backscatter values,additional processing at the University of NewHampshire Center for Coastal and Ocean Map-ping provided backscatter rendering which,although not representing ideal backscatter data,nevertheless provides us with clear identification ofthe sorted bedform grain-size boundaries at thetime of this survey (Fig. 2c).

2.2. Grab samples

Seafloor sediment samples were collected at 89stations within the MVCO swath survey area(Figs. 2a, 3b) using a Smith–Mcintyre grabsampler (Murdoch and MacKnight, 1994) aboardthe R/V Cape Henlopen in August, 2002. Naviga-tion for each grab sample was recorded from

dgtech DF1000 100 kHz sidescan system, (b) September 2001,

using a Reson 8125 455 kHz focused multibeam system. Higher

eters, are derived from merged NGDC and Reson 8125 data.

to filled symbols in Fig. 4) and numbered, filled circles indicated

s, seen as high backscatter areas, are identified as CSZs1-3. Star

figures and the sites of diver cores presented later.

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Fig. 2. (Continued)


differential GPS, with an accuracy of �5 m. Ahand-operated plug corer was used to extractsubsamples from the grab for grain-size analysis.Many of the grab samples in the central portion ofthe survey area (between CSZs 1 and 2; Fig. 2b)were collected at stations collocated with apreliminary set of samples taken the year beforeaboard the R/V Asterias. These earlier samplesincluded grabs taken from the shoreface, in �6 mof water, which could not safely be duplicated withthe Henlopen.

Weight percent of fine-grained material, or mudfraction (o63 mm) was determined by wet sieving,and coarse-grained percentages in the 2–4 mm(granules) and 44 mm (pebbles and gravel) binswere obtained by dry sieving. The remainder of the

Fig. 3. Artificially illuminated bathymetry maps from (a) September 2

July 2002, using a Reson 8125 455 kHz focused multibeam system.

meters, are derived from merged NGDC and Reson 8125 data. Thin-d

indicate grab sample stations, and circles indicated vibracore sites. Wit

dashed lines, between westward dipping bathymetry at shallow dep

indicates location of the MVCO node. Locations are given for profile

distribution was estimated through settling tubeanalysis using a visual accumulation method.Half-f bins were computed from 63 to 500 mm,and 1�f bins from 500 to 2000 mm (grain size inmm ¼ 2�f). These values were normalized to thetotal dry sample weight.

The settling tube analysis was very precise. Asreported by Goff et al. (2004), rms variationon mean sand grain size for this methodologywas at most 0.04f over multiple independentruns on split portions of the same sample.Differences between subsamples from the samegrab were slightly higher, with an rms differentialin mean sand grain size of 0.09f over allgrabs. Both the 44 mm and o63 mm portionsexhibited rms differential of 3.4%, 1.4% for

001, using a Submetrix 234 kHz interferometric system, and (b)

Illumination is from the northwest. Bathymetric contours, in

ashed lines in (a) indicate chirp seismic track lines. Squares in (a)

hin the three large CSZs, a transition is noted in (b), with thick-

ths and eastward dipping bathymetry at greater depths. Star

figures presented later.

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Table 1

Mean and rms grain sizes, specified in phi values, derived from surface grab samples

Sta. Latitude Longitude Mean rms Sta. Latitude Longitude Mean rms

1 41.344097 �70.571907 1.22 0.74 30 41.320968 �70.559640 2.76 0.582

3 41.344080 �70.564888 1.14 0.70 32 41.313255 �70.553723 2.86 0.69

4 41.343848 �70.560873 2.92 0.54 33 41.313228 �70.562308 3.19 0.66

5 41.343947 �70.559443 0.53 0.79 34 41.313395 �70.569835 3.24 0.64

6 41.344093 �70.558382 2.78 0.56 35 41.308172 �70.570100 3.00 1.22

7 41.343817 �70.557042 0.27 0.85 36 41.336557 �70.601913 2.08 0.58

8 41.344085 �70.554962 0.43 1.13 37 41.336617 �70.600820 0.64 0.69

9 41.346908 �70.554072 2.01 0.62 38 41.336460 �70.599148 1.32 0.72

10 41.347060 �70.556321 2.08 0.50 39 41.336525 �70.597743 2.11 0.65

11 41.347168 �70.559192 2.15 0.57 40 41.336640 �70.597053 0.83 0.68

12 41.347247 �70.561993 2.08 0.54 41 41.336678 �70.596187 1.56 0.65

13 41.347118 �70.567770 2.08 0.50 42 41.336540 �70.594823 2.28 0.78

14.1 41.336487 �70.573280 3.09 0.49 43 41.326468 �70.603430 2.64 0.86

14.2 41.336568 �70.572898 3.31 0.69 44 41.326447 �70.602202 1.04 0.74

14.3 41.336648 �70.572645 0.76 0.94 45 41.326530 �70.600675 1.70 0.77

14.4 41.336602 �70.572400 0.56 0.76 46 41.326483 �70.599523 2.18 1.00

14.5 41.336472 �70.571988 0.55 0.72 47 41.326438 �70.598317 1.36 0.72

15 41.336647 �70.566987 0.73 0.85 48 41.326482 �70.597083 1.62 0.80

16.1 41.336588 �70.566158 0.12 1.48 49 41.326328 �70.595303 2.46 0.96

16.2 41.336723 �70.565647 0.35 1.09 50 41.336592 �70.536635 2.25 0.67

16.3 41.336690 �70.565583 1.02 1.08 51 41.336560 �70.534927 1.03 0.64

16.4 41.336843 �70.565478 2.78 0.58 52 41.336607 �70.531232 0.26 0.98

16.5 41.336512 �70.565220 2.84 0.60 53 41.336628 �70.528628 �0.20 1.49

17 41.336562 �70.563875 2.89 0.50 54 41.336682 �70.528183 2.22 0.56

18 41.336632 �70.560550 2.89 0.48 55 41.336612 �70.527720 1.09 0.80

19.1 41.336685 �70.558690 0.44 1.02 56 41.336633 �70.527185 0.83 0.94

19.2 41.336680 �70.558277 0.80 0.88 57 41.336597 �70.526395 1.92 0.51

19.3 41.336703 �70.558117 0.61 0.92 58 41.326645 �70.537218 2.40 0.61

19.41 41.336720 �70.557678 0.45 1.20 59 41.326442 �70.533972 1.16 0.58

19.42 41.336680 �70.557797 0.53 1.15 60 41.326480 �70.533007 0.63 0.68

19.5 41.336650 �70.556980 2.76 0.54 61 41.326463 �70.531932 �1.36 1.63

20.1 41.336842 �70.556823 2.70 0.57 62 41.326543 �70.531097 1.29 0.67

20.2 41.337030 �70.556248 2.78 0.56 63 41.326530 �70.529365 0.47 0.64

20.3 41.337078 �70.556067 2.50 0.75 64 41.326428 �70.527312 �0.12 1.34

21.11 41.337025 �70.555443 0.52 1.03 65 41.326523 �70.525762 2.46 0.62

21.12 41.337000 �70.555605 2.08 0.86 66 41.326420 �70.558987 2.53 0.80

21.2 41.336982 �70.555255 0.72 0.73 67 41.326483 �70.563560 2.95 0.53

21.3 41.336570 �70.555020 0.90 0.74 68 41.326618 �70.567227 3.17 0.43

22 41.336902 �70.553970 2.47 0.55 69 41.326502 �70.570720 3.12 0.56

23 41.329955 �70.555643 2.48 0.84 70 41.326558 �70.573048 0.10 1.26

24 41.330243 �70.558418 0.70 0.94 71 41.326448 �70.574117 0.73 0.62

25 41.329922 �70.561802 2.76 0.74 72 41.326610 �70.576688 3.00 0.80

26 41.330417 �70.566075 3.07 0.48 73 41.326418 �70.579035 3.25 0.58

27 41.330067 �70.570783 0.14 1.26 74 41.326528 �70.582628 3.28 0.65

28 41.320823 �70.569027 3.19 0.60 75 41.326425 �70.585258 2.38 0.95

29 41.321135 �70.564390 3.22 0.71


the 2–4 mm portion. Differences between collo-cated Henlopen and Asterias grab samplesdiffered by an average of 0.16f. Mean andrms grain sizes are reported in Table 1, inclu-

ding Asterias samples from shoreface grabs(stations 9–13).

A number of biota were also observed in thegrab samples, including abundant worm tubes,

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particularly in the finer sediments, occasional largebivalves, hermit crabs and sand dollars.

2.3. Vibracores

Cores were collected at 35 stations using aRossfelder P-5 electric vibracoring system duringthe same R/V Cape Henlopen cruise in August of2002. Depth of penetration ranged from o0.5 m tonearly 2 m; such poor recoveries are to be expectedfor predominantly sandy sediments such as arefound within the MVCO. Cores were cappedaboard ship and stored at the Woods Hole CoreRepository for post-cruise analysis. Onshore thecores were split, photographed, described andsampled. Representative samples were taken fromthe distinct units within each core. Samples weregenerally of the order of 100 g dry weight. Grain-size analyses were done by dry sieving the samplesat 0.5f intervals between �2.5 and 0, and 1fintervals from 0 to 4. Sediment finer than 63 mm(4f) was negligible, although it is possible that thevibracoring process may have washed out some ofthe very finest components of the sediment.

2.4. Chirp seismic reflection

A chirp seismic reflection survey was conductedaboard R/V Cape Henlopen for 2 days immedi-ately following the sampling work in August 2002.Track lines were mostly oriented E–W, parallel tothe southern Martha’s Vineyard shoreline, withtwo N–S tie lines (Figs. 2b, 3a). The chirp sonar,designed and fabricated by Florida AtlanticUniversity, measures acoustic reflections at normalincidence to the seabed using a dual pulsetechnique to produce high-resolution imagery ofnear surface sediments and lower resolutionimagery of deeper sediments. A 40 ms-long FMpulse with a band of 1.5–4 kHz was transmitted toprovide imagery of the top 40 m of the seabed witha vertical resolution of 40 cm while a 10 ms-longFM pulse with a band of 1.5–15 kHz was alter-nately transmitted to yield 10 cm resolutionimagery of sediments in the top 10 m. The latterprovided the best imagery of sorted bedformstratigraphy, which often included reflections justtens of centimeters below the seafloor. In the sonar

vehicle two Tonpilz-type piston sources withoperating bands of 1–5 and 4–16 kHz were drivensimultaneously to generate the wideband outputpulses. The seabed reflections were received by ahorizontal, 1 m long � 1 m wide planar hydro-phone array. Display and interpretation of chirpdata were assisted by Schlumberger GeoFramesoftware.

2.5. Diver cores

Short push cores were collected by diversspanning two transitions between coarse and finesands (Fig. 2b). Ten cores were recovered at eachsite at 1 m intervals, as determined by a knottedcord. Core penetration ranged from �15 to�30 cm. The cores were split, photographed andsubsampled at a range of depths guided by visualevidence of grain-size boundaries. Grain-sizeanalysis of the subsamples was conducted in thesame manner as that of the grab samples. Somemixing of adjacent subsamples may be possibledue to imprecision of dividing the core samplesand pull-down at the core edges.

3. Sorted bedform morphology

3.1. Areal morphology

The backscatter maps of the MVCO area (Fig.2) are, for the most part, directly translatable intomaps of grain-size variability. With few excep-tions, grain-size distributions from seafloor sam-ples are unimodal, well-sorted and well-characterized by the mean grain size. Fig. 4displays mean grain size versus backscatter grayscale values derived from the Submetrix survey(Fig. 2b), where a strong positive correlation isobserved between the two over most of oursamples. The correlation is inverted, however, forfine sands within the central and deeper portionsof the survey area (Figs. 2b, 4). The increase ofbackscatter at finer grain sizes is perhaps bestobserved in Fig. 2a, where a brightening isobserved toward the southwestern corner; meangrain size within this elevated backscatter regionamong the fine sands is �100–125 mm, and outside

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Fig. 4. Mean grain size derived from grab samples plotted

versus backscatter values from the September 2001 Submetrix

survey (Fig. 2b). For most of the samples (open circles), a

strong positive correlation (r ¼ þ0:79) is observed between

these two measurements. Filled circles indicate fine sand

samples in the central (between 70135.50W and 701330W) and

deeper (4 13 m) portion of the survey area (filled squares in

Fig. 2b), where an inverse correlation (r ¼ �0:42) is noted

between grain size and backscatter intensity. Both correlations

are different from 0 at499% confidence. Dashed lines assist in

identifying linear trends.


it is �150–200 mm. Nevertheless, over the majorityof the survey area, and particularly for the coarsersands sampled, backscatter and mean grain sizeare positively correlated.

The MVCO backscatter maps (Fig. 2) exhibitstructures that have previously been defined as‘‘rippled scour depression’’: areas of coarse sandextending nearly perpendicularly out from theshoreface, tens to hundreds of meters wide andextending kilometers from the shoreface, that aresurrounded by fine sands. With very few excep-tions (primarily in the western part of the surveyarea), mean grain sizes in the medium-sand rangeof �2–2.3f (�200–250 mm) are absent from thesorted bedforms (Table 1), indicating a trulybimodal separation of grain sizes between thecoarse and fine sand zones. The shoreface samples(stations 9–13 in Table 1), however, all exhibitmean grain sizes in this range.

Three notably large CSZs are present in thecentral to eastern portions of the survey area,numbered for identification as CSZs 1–3 (Figs. 2,3). As Murray and Thieler (2004) noted in theirobservations off Wrightsville beach, these sortedbedforms display a number of asymmetries thatmay be associated with large transverse bedforms.As seen particularly on CSZs 2 and 3, as well as onmany of the CSZs in the western portion of thesurvey area, one edge tends to be well-defined,while the other is either more poorly delineated or‘‘feathered’’ in appearance. Within the CSZs, grainsize, as evidenced by the backscatter, tends to belargest toward the sharp-edged side, and decreasessystematically away from that edge. This observa-tion will be more clearly demonstrated in profileanalyses in the following section. The CSZs alsotend to be oriented systematically such that themore well-defined edge is at a slightly acute angleto the shoreface. Many of the CSZs in the centralportion of the survey area, including CSZ1, do notexhibit noticeable asymmetry in the backscattermap.

The polarity of the grain-size asymmetriesacross CSZs does not remain consistent through-out the survey area. As seen in the Submetrixbackscatter map (Fig. 2b), CSZs at the westernedge of the survey area have their sharp-edged boundaries on their western side, andtrend slightly east of north. The reverse patternis observed at the eastern edge of the surveyarea: the sharp-edged boundary is on theireastern side, and trend slightly west of north.The Reson backscatter map (Fig. 2c), whichextends to shallower waters than the Sub-metrix survey, provides evidence that the polarityof backscatter/grain-size asymmetry changeswith depth as well as along-shore position. Forexample, CSZ3, at the easternmost edge ofthe survey area, exhibits a higher-backscattereastern edge at depths shallower than �9 m,while CSZ2 exhibits a similar transition at �12 m(again, profile analysis in the following sectionwill better demonstrate this observation). Theorientation of CSZs in these areas also changeat these approximate depths from being west-of-north in deeper water to east-of-north inshallower water.

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The bathymetric expression of the sorted bed-forms, as seen on the contour maps superposed onthe backscatter (Fig. 2) and shaded relief maps(Fig. 3), is highly variable. Bathymetric asymmetrywithin the CSZs can be observed by the down-to-the-right or down-to-the-left slant of the contours(Figs. 2,3). CSZs 2 and 3 in the eastern half of thesurvey area display strong asymmetries, with themore sharply defined, coarser edge typicallydepressed relative to the surrounding seafloor,while the opposite edge is raised. This observationis identical to that noted for the Wrightsville Beachsorted bedforms (Murray and Thieler, 2004). Thebathymetric asymmetry closely follows thechanges in grain-size/backscatter asymmetry,switching east–west polarity going from west toeast and from deeper to shallower water (Fig. 3b).Within CSZ3, the change in bathymetric polarityoccurs at �10 m water depth, while within CSZ2the polarity changes at �13 m water depth. CSZ1also exhibits bathymetric asymmetry, althoughmore subdued in comparison. Like CSZs 2 and 3,the bathymetric asymmetry within CSZ1 changespolarity with depth; here the transition is at�14–15 m water depth.

Bathymetric expression of the CSZs is notconstrained to simple depth asymmetries acrosstheir widths. For example, the edges of the CSZ1are also marked by a distinct moat. Asymmetriesand edge moats exist within most of the smallerCSZs in the central and eastern portions of thesurvey area as well. The bathymetric expressionsof the CSZs in the western half of the survey areaare quite subdued.

3.2. Cross-sectional morphology

In this section we employ profile views ofbathymetry, backscatter and chirp seismic datato provide detailed views of the sorted bedformstructures within the MVCO. To mitigate theeffects of speckle and track-line artifacts onbackscatter profiles, we employ the cross-profilefiltering methodology of Goff et al. (2000). In thisalgorithm, values are averaged along cross-linesoriented parallel to structure, as defined by theuser, rather than along the profile. This techniqueemphasizes structures sampled along the profile

while reducing random effects, such as noise, ordata patterns that cut across seafloor structures,such as track-line artifacts.

Fig. 5 displays chirp seismic, bathymetric andbackscatter profiles across sorted bedform mor-phology within the central region of the survey(Figs. 2b, 3a). On Fig. 5 and subsequent profileplots, CSZ edges are distinguished between thosethat have a more abrupt backscatter transitionand/or a deeper bathymetry, and those that have amore gradual backscatter transition and/or ashoaler bathymetry. The CSZs in the centralregion display a complex but generally consistentmorphology. Bathymetric steps occur at both CSZedges, creating a depression of the CSZ in relationto the surrounding bathymetry. Within the CSZ,the bathymetry slopes downward toward thewestern CSZ boundary, but with a convex-upgeometry that creates moat-like depressions atboth CSZ edges. Some CSZs have abrupt back-scatter transitions on both boundaries (e.g., at�0.8 km on Fig. 5), whereas others display somemeasure of asymmetry (e.g., at �0.5 km on Fig. 5).In the chirp seismic reflection data, a seismichorizon is observed consistently beneath the finesands, oftentimes intersecting the seafloor at thebottom of the CSZ moat at the deeper edge. Aseismic horizon is also observed, more occasion-ally but nevertheless frequently, within the CSZs.Where observed, this horizon is often contiguouswith the horizon seen below the fine sands at theshoaler CSZ boundary (e.g., at �0.9 km in Fig. 5).Evidence for the possible origin of this horizon willbe given below. A buried dendritic channel systemwas also mapped in the survey area (Fig. 5), whichsuggests evidence of subaerial erosion prior to thelatest sea-level rise. The channel flanks aretruncated by the shallow horizontal seismichorizon described above.

Fig. 6 displays profile data from sorted bedformmorphology in the western sector of the surveyarea. The polarity of asymmetry in this area is thesame as in the central region, but the bathymetricexpression is less pronounced. Here, only a smallmoat-like depression is observed at the sharp-edged side of the CSZ. The contrast in grain sizebetween the coarse and fine-grained sedimentdomains is also lower in the western region

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Fig. 5. Collocated profile data through the central sector of the survey area (Figs. 2a, 3, and 10). (Top) Chirp seismic profile with

interpretation of shallow horizon: solid where beneath fine sands at surface, dashed where beneath coarse sands. Arrows and values

and arrows indicate approximate locations of grab samples and mean grain sizes in microns, respectively. Percentage, where given,

indicates a significant coarse fraction (4 4 mm) greater than 10%. Grey bars indicate depths to base of fine sands seen in cores at these

locations. (Bottom) Reson 8125 bathymetry and Submetrix backscatter profiles. To minimize speckle and track-line artifacts,

backscatter data were averaged along 150 m-long cross lines oriented parallel to structure and centered on the main profile. Heavy-

dashed horizontal line indicates value used to indicate the transition between bright (coarse sand zone, or CSZ: hachured) and dark

(fine sand) backscatter regions. Solid vertical line indicates the side with the more abrupt backscatter transition and/or deeper

bathymetry; shaded line indicates the side with the more gradual transition and/or shoaler bathymetry. Here, CSZ bathymetry is

characterized by a ‘‘step and moat’’ morphology, dipping toward the western edge of the CSZ.


(�300–600 versus �150–250 mm) than in thecentral region (�500–750 versus�100–200 mm) ofthe survey area. In addition, the backscattertransitions at the eastern boundaries of the westernCSZs are more gradational than observed in thecentral region. The subsurface stratigraphy is,however, very similar between the western andcentral regions: a well-defined seismic horizonbeneath the fine sands intersecting the seafloor atthe bottom of the moat at the deeper/sharp-edgedCSZ boundary, and an occasional, usually lessdistinct seismic horizon within the CSZs that isoften seen as contiguous with the fine sand horizonat the more gradual CSZ boundary.

Fig. 7 displays profile data from the easternsector of the survey, spanning CSZ3 and smallerCSZs to either side. Here, the morphologicalobservations are essentially identical to thosenoted on Fig. 5, but with asymmetry that isreversed east to west, and stronger evidence ofasymmetry in the backscatter, particularly CSZ3.In deeper water (Fig. 8), the bathymetric expres-sion and coarse versus fine grain-size contrast ofCSZ3 increases (�400–2500 versus �175–225 mm),and the ‘‘moat and step’’ morphology is replacedby a distinctive dune-like morphology (e.g., Swiftand Field, 1981), with stoss flank characteristics(coarser grained, lower slope) to the east and lee

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Fig. 6. Collocated profile data through the western sector of the survey area (Figs. 2b, 3a). (Top) Chirp seismic profile with


indicate approximate locations of grab samples and mean grain sizes in microns, respectively. (Bottom) Submetrix bathymetry and

Submetrix backscatter profiles. Here, CSZ bathymetry is characterized primarily by a ‘‘moat’’ at the western edge of the CSZ. See Fig.

5 for further details.


flank characteristics (finer grained, steeper slope)to the west. These features are akin to sand ridges,although with vertical and horizontal scales �4times smaller, and far less acute angle with theshoreline than ‘‘typical’’ shoreface attached ridgeselsewhere along the eastern US (Swift and Field,1981). Profile data across CSZ3 in �8–9 m depth(Fig. 9) also exhibit very dune-like morphology,but with the lee characteristics to the east and anaccompanying reversal in the asymmetry evidentin the backscatter profile.

Beneath the sand ridges shown on Fig. 8, andclearly contiguous with the seismic horizonbeneath fine sands to the west, is a seismic horizonthat outcrops on the east flanks of the two ridgesprofiled, indicating that the base of the east flanksof these ridges have been eroded below thishorizon. Grab samples gathered above this contactconsist of well-sorted, coarse sands, whereas

samples taken below the contact contain signifi-cant quantities of gravel. A similar relationshipappears likely in Fig. 7, although it is less certainthere whether this horizon intersects the seafloor.These observations suggest that gravel contentmay be a factor in the impedance contrast thatforms the seismic horizon observed within theCSZs.

3.3. Core samples

Vibracores provide observational constraints onthe nature of the chirp seismic horizons in theshallow subsurface. Aside from the buried chan-nels, the primary observed seismic horizons aregenerally located within a meter of the seafloor,and thus accessible by coring. Three principal unitswere observed outside the fill units of the buriedchannels: fine sands, coarse sands, and a very

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Fig. 7. Collocated profile data through the west sector of the survey area (Figs. 2b, 3). (Top) Chirp seismic profile with interpretation

of shallow horizon: solid where beneath fine sands at surface, dashed where beneath coarse sands. Arrows and values indicate

approximate locations of grab samples and mean grain sizes in microns, respectively. Percentage, where given, indicates a significant

coarse fraction (4 4 mm) greater than 10%. (Bottom) Reson 8125 bathymetry and Submetrix backscatter profiles. Here, CSZ

bathymetry is characterized by a ‘‘step and moat’’ morphology (compare with Fig. 5), dipping toward the eastern edge of the CSZ. See

Fig. 5 for further details.


coarse to gravelly layer of variable thickness(typically between 5 and 30 cm) usually buriedtens of centimeters beneath either fine sands,coarse sands, or both. A gravel/very coarse sandlayer was sampled in nearly every core. The fine/coarse sand transition and the gravel/very coarsesand layer represent the most likely candidates forgenerating seismic impedance contrasts in theupper meter of sediment.

Table 2 displays the depth to the fine/coarsetransition, the upper and lower bounds of thegravel/very coarse sand layer, and the depth to thechirp seismic horizons (not including channel fillhorizons). The latter are accurate to approxi-mately 70.05 m. The seismic horizon is mostconsistently and robustly observed beneath thefine sands at the seafloor, so it is reasonable to

assume that the fine/coarse transition may be asignificant factor in producing this impedancecontrast. However, where a fine/coarse transitionis present in the core, the corresponding seismichorizon tends to be deeper ranging from a fewcentimeters to as much as 30 cm, and moretypically �10 cm. Some or all of this discrepancymay be attributed to loss of fine sands at the top ofthe core during recovery. However, the depth tothe shallowest seismic horizon is often moreconsistent with the depth range of the gravel/verycoarse sand layer, including cores where no finesands are present. Given that this seismic horizonis often seen to be contiguous beneath CSZboundaries, we surmise that the gravel/very coarsesand layer is primarily responsible for generatingthis impedance contrast.

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Fig. 8. Collocated profile data through the central sector of the survey area (Figs. 2b, 3). (Top) Chirp seismic profile with


indicate approximate locations of grab samples and mean grain sizes in microns, respectively. Percentage, where given, indicates a

significant coarse fraction (4 4 mm) greater than 10%. (Bottom) Reson 8125 bathymetry and Submetrix backscatter profiles. Here,

CSZ bathymetry is characterized by dune-form morphology, with lee characteristics, indicating probable sediment transport direction

to the west. See Fig. 5 for further details.


4. Temporal evolution of sorted bedforms

4.1. Backscatter mapping

The three backscatter maps presented in Fig. 2provide a limited time-series record of the sortedbedforms in the MVCO study area. Fig. 10displays a close-up view of the Submetrix sidescandata (September 2001) in the central survey areawith digitized CSZ boundaries from the earlierDF1000 survey (February 2001) and later Resonsurvey (July 2002) superposed. Systematic shifts inthe CSZ boundaries are observed over this timeframe, but with regional variations. CSZ bound-aries in the northwestern sector of this map haveshifted to the east by as much as 60 m. The shiftsseen in the northeastern sector of the map rangefrom �0 to �30 m, also to the east. A variety of

responses are observed in the southern half of thestudy area. A number of CSZ boundaries showlittle or no shift over the time span mapped. Othersdisplay a westward shift between February andSeptember 2001, and then an eastward shift byJuly 2002 (e.g. along the edge of CSZ1). Oneboundary, along the west side of CSZ2, shiftedwestward during both time periods.

The navigational uncertainty in hull-mountedswath maps (Figs 2b, c) is at most 5 m, andperhaps a bit more for the towed instrument (Figs2a), so these observations should be consideredrobust. The scale of movement, a few tens ofmeters, is consistent with Murray and Thieler’s(2004) observations of CSZ migration off Wrights-ville Beach. However, unlike their observations,the direction of migration is not always consistentwith what might be inferred from the grain-size

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Fig. 9. Collocated profile data through the central sector of the survey area (Figs. 2c, 3). (Top) Chirp seismic profile with interpretation

of shallow horizon: solid where beneath fine sands at surface, dashed where beneath coarse sands. Grab samples were not collected in

this vicinity (Bottom) Reson 8125 bathymetry and backscatter profiles. Here, CSZ bathymetry is characterized by dune-form

morphology, with lee characteristics, indicating probable sediment transport direction to the east. See Fig. 5 for further details.


asymmetry within the CSZs. For example, theeastern edge of CSZ2 (Fig. 10) is the coarser-grained side, which assuming it is a transversebedform, would indicate that the stoss side faceseast into a westward transport direction. Never-theless, this boundary migrated eastward betweenthe September 2001 and July 2002 surveys.

4.2. Bathymetric mapping

Our modern bathymetric time series records arepresently limited to the September, 2001 Subme-trix survey and July 2002 Reson survey. Thiscomparison is not ideal because of the differencesin resolution and noise between the two. How-ever, a comparison of profiles (Fig. 11) indicatesthat the CSZ ‘‘moats’’ are usually well-resolvedfeatures in the noisier Submetrix data. With oneexception in Fig. 11, the moats that can be clearlyidentified in the Submetrix bathymetry are aligned

perfectly with the moats observed in theReson bathymetry. The one exception, justbefore 0.8 km distance along the profile, is a shiftto the west, which is the opposite direction of theshift seen consistently in the backscatter data(Figs. 10, 11).

A long-term bathymetric comparison can alsobe made by comparing the multibeam data withhistorical records derived from digital NationalOcean Service (NOS) data. The area surveyed bythe multibeam maps in Fig. 3 were surveyed in1965 with single point measurements. Fig. 12compares two profiles sampled from the inter-polated 1965 NOS data with collocated profilesthrough the swath data. While cautioning againstinterpreting too much detail in this comparison, itis nevertheless clear that the larger sorted bedformstructures throughout the survey were captured atleast in part by the 1965 NOS survey, andfurthermore that the positions of these features

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Table 2

Recorded depths to the base of the surface fine layer, depth

range for the gravel/very coarse sand (VC), and shallow seismic

horizon at each of the core locations shown in Fig. 2b

Core ID Base of fines

(m)

Gravel/VC

layer (m)

Depth to

horizona

(70.05) (m)

1 — 0.41–0.48 0.40c

2.1–2.4 — None 0.45b

3.1,3.2,3.3,3.5 �0.30 0.30–0.54 0.50b

3.4 — 0.30–0.60 0.50b

4 — 0.08–0.25 0.35b

5 — 0.22–0.36 0.35

6 0.38 0.38–0.66c 0.50b

7 — 0.28–0.40 0.45b (weak)

8 0.17 0.32–0.55c 0.40b

9 0.21 0.21–0.27c 0.40

10 — 0–0.15/

0.22–0.34/

0.65–0.68d

0.45

11 — None 0.40 (weak)

12 0.31 0.35–0.75 0.35

13 — None Noneb

14 0.12 0.12–0.44 0.20

15 0.26 0.26–0.35 0.30

16 0.42 0.53–1.62c 0.45

17 — 0.40–0.43 0.45

18 — 0.61–0.64 0.55

19 — 0.10–0.20 0.25

20 0.13 0.13–0.33 0.15b

21 — 0.28–0.32 0.40

22 0.33 0.48–0.49 0.45

23 0.50 0.50–0.60 0.60

24 0.45 None 0.50b

25 0.52 0.52–0.67c 0.65b

26 1.24 None 0.90b

27 0.80 0.80–0.91c 0.45/0.95/

1.30e

28 0.96 0.96–1.04 1.00/1.30e

aAssuming 1700 m/s sound speed.bFirst horizon above buried channel.cBottom of core.dTwo gravel/VC layers observed on this core.eMultiple seismic horizons observed at these locations.


have not changed substantially over the span ofnearly four decades.

4.3. Diver push cores at coarse/fine transitions

Earlier recognition that the CSZ boundaries arenot stable led us to collect a series of 1-m spaced,diver-located push cores across two coarse/fine

sand transitions (Fig. 2b). The purpose of thesecores was to determine if boundary migrationleaves a recognizable stratigraphic signature.Unique among seafloor sedimentary samples with-in the MVCO, the transition cores yielded stronglybimodal sand distributions, with a clear separationat �250 mm between coarse and fine sands(Fig. 13). This indicates significant mixing of thetwo sediment types at the transition. This mixingcannot, in general, be attributed to any samplingissues, such as pull-down along the core edges orimprecise separation of subsamples. The sampleshown in Fig. 13, for example, was selected fromthe top of the core (eliminating pull-down) and iswell separated from any significant fine sand layersfarther down in the core (Fig. 14). Fig. 14 displaysthe relative proportion of coarse versus fine sandsdown-core in each of the push cores. The fine-coarse transition is seen readily as fines overlyingcoarse sands in increasing thickness away from theCSZ. However, additional, sometimes continuousand other times isolated zones of fine sands areseen farther down-core, indicating both that finesands have deposited or migrated over coarsesands and likewise that coarse sands have depos-ited or migrated over fine sands.

5. Discussion

5.1. Sorted bedform morphology

The sorted bedforms observed within theMVCO survey area exhibit a broad spectrum ofbathymetric relief (from �10 cm to �3 m), grain-size contrast (from �250 mm to 42000 mm) andmorphologic form (moats, steps, and dune forms;Fig. 15). These three factors correlate with eachother: dune forms occur with the largest bathy-metric relief and grain-size contrast (e.g., Figs. 8and 9), ‘‘step and moat’’ morphology exhibits anintermediate relief and grain-size contrast (e.g.,Figs. 5 and 7), and small moats are present at themore well-defined edges of the CSZs with thelowest relief and grain-size contrast (e.g., Fig. 6).All morphologic forms observed display lateralasymmetry in both grain size and bathymetricexpression. In general, grain size is largest and

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Fig. 10. September 2001 Submetrix backscatter data in the central sector of the survey, with digitized coarse/fine sand boundaries from

the February 2001 DF1000 survey (yellow) and July 2002 Reson 8125 survey (light blue) overlain. The location of the profile used for

Figs. 5 and 11 is shown. Higher backscatter is indicated by lighter shades.

Fig. 11. July 2002 Reson 8125 (solid) and September 2001 Submetrix (dashed) bathymetry (top) and backscatter (bottom) profiles

(Figs. 2b,c 3 and 10). Vertical lines (solid for Reson, dashed for Submetrix surveys) mark locations of identifiable features, moats for

the bathymetry and the shoulders of backscatter highs in the backscatter data, which are used to established temporal shifts between

the two surveys. The backscatter features consistently indicate a �10–20 m shift to the east between the earlier Submetrix and later

Reson surveys. In contrast, most of the bathymetric features show no shift.


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(a)

(b)

Fig. 12. Comparison of swath bathymetry profiles with

collocated profiles sampled from interpolated soundings

collected during a 1965 National Oceanographic Service survey.

Locations are shown on Fig. 3. Many of the larger bathymetric

features of sorted bedform morphology are seen to be present

and largely unshifted (within likely error bounds of the earlier

survey) between the two data sets.

Fig. 13. Grain-size histogram derived from one of the diver

core subsamples (see Fig. 2b for location) demonstrating

strongly bimodal distribution.


bathymetry is deepest toward one side, typicallyseen in the backscatter maps as the more welldefined of the two CSZ edges where that distinc-tion can be made.

The proximity of these sorted bedforms sizes,grain-size contrasts and morphologic forms, andtheir regional similarity of trends and asymmetries,suggest that they are different expressions of the

same bedform process, but perhaps in response todiffering intensities of flow or somehow modulatedby other processes, such as ripple migration forcedby wave skewness. An important question is raisedby these observations: why does the step-and-moatmorphology evolve in some circumstances ratherthan simple dune-like morphology, and what roleis played by current intensity and grain-sizecontrast? We concur with Schwab et al. (2000)and Murray and Thieler (2004) that the asymme-tries exhibited by the sorted bedforms indicate thatthey are primarily a response to a bedform-transverse, along-shore flow. But if so, then clearlythe MVCO must have a very complex flow regime,as evidenced by the changes in the polarity ofasymmetry (Fig. 2b), with an eastward flow in thewestern half and shallower part of the eastern halfof the survey area, and a westward flow in deeperwater in the eastern half. Complex nearshorecurrent conditions are certainly possible for thisarea; e.g., an eastward alongshore current meetinga strong tidal current coming around the shoals onthe east end of Martha’s Vineyard. However,detailed physical oceanographic data on thecurrent regime across this area are not presentlyavailable. Wave forcing may also play an impor-tant role, although we hypothesize below thatwave-induced migration of sorted bedform mor-phology is ephemeral, and has no relationship toasymmetry.

5.2. Short-term sorted bedform evolution

Our analysis of repeated bathymetric and back-scatter maps indicates that, while the coarse/finesand transitions can move tens of meters over thespan of months (Figs. 10, 11), bathymetry appearsquite stable over that time frame (Fig. 11), andsome of the larger features have remained inessentially the same location over a span of nearlyfour decades (Fig. 12). Furthermore, the directionof CSZ boundary shifts are not always consistentwith the direction expected based on the asymme-tries of the sorted bedforms, and over the coarse ofthree backscatter surveys some of the boundariesare seen to alter their direction of migration (Fig.10). Finally, our grain-size analysis of diver pushcores collected across two CSZ boundaries in-

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Fig. 14. Results of grain-size analysis on diver cores gathered at the (a) western and (b) eastern edges of CSZ1 (Fig. 2b). Cores are

located at 1 m intervals crossing the transition between fine and coarse sands. After splitting, between 5 and 10 subsamples were

selected from each core for analysis, guided by visual identification of grain-size boundaries. Observations of bimodality in the

distribution lead us to characterize the coarse versus fine sand content of each sample by measuring the total weight percentage greater

than 0.25 mm, a value which clearly separated the two peaks of the distribution. Heavy lines indicate interpreted contiguous grain-size

boundaries.


dicates back-and-forth movement of the bound-ary, both of fine sands advancing over coarsesands and coarse sands advancing over fine sands.

These observations appear to indicate that,whatever alongshore currents may be controllingformation and evolution of sorted bedforms in the

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Fig. 15. Schematic, interpretive representation of the spectrum

of sorted bedform relief, grain-size contrast, morphology (top

panel: moats; middle panel: steps and moats; bottom panel:

dune forms) and internal stratigraphy observed within the

MVCO survey area. Each panel is drawn with the same sense of

asymmetry to facilitate comparison. The ‘‘F/C’’ horizon is the

contact between fine and coarse sands, which was established by

coring (dune-form features were not cored, so the existence of

an F/C horizon beneath them is unknown). The ‘‘T’’ horizon is

the interpreted transgressive ravinement surface, observed as a

seismic horizon in the chirp data, and correlated to a thin layer

of gravel/very coarse sand in the cores.


long term, sediment transport in the short term isresponding to other, likely more variable factors.The shifts we observe are locally consistent; that isboth sides of the CSZ advancing westward oreastward by roughly the same amount. From thiswe infer that oceanographic conditions, ratherthan sediment flux (i.e., input or removal of finesands), are controlling short-term CSZ boundarymovements. Wave orbital velocity skewness isperhaps the most likely cause for short-termsediment migration, as evidenced by the fact thatsand ripples on the seafloor align perpendicular towave direction and have been observed to migratein the direction of wave propagation (Traykovskiet al., 1999; Traykovski and Goff, 2003). Althoughwave records from the MVCO node (http://mvcodata.whoi.edu/cgi-bin/mvco/mvco.cgi; Figs.2 and 3) show that the largest waves (up to3–4 m with 6–8 s periods) are from the SW–SSW,waves from individual storms may transportsediment either eastward or westward, dependingon the direction from which waves propagatetoward the shore. Observations of transportprocesses in coarse sand also do not show

significant amounts of along-shore transportcompared to transport associated with waveforced ripple migration (Traykovski et al., 1999).These observations were made in an environmentwith waves and currents similar to those at theMVCO node. It is not known if currents aresubstantially stronger in the southeastern portionof the study area, where the largest dune-likemorphology is observed.

If the bathymetric expression of sorted bed-forms is stable on short time scales where the fine/coarse sand transitions are unstable and oscilla-tory, then we must ask what the relationship isbetween the two. The more ephemeral nature ofthe coarse/fine sand transition suggests that thebathymetric features, such as the moats, aresomehow instrumental in anchoring the CSZboundaries, keeping them from straying too farfrom the dictates of the morphologic expression ofthe sorted bedforms. What is the feedback betweenthe moats and the coarse/fine sand transitions?

The presence of medium-grained sands on theshoreface presents us with additional, and possiblyrelated questions. The samples collected from theshoreface (stations 9–13 on Table 1) are similar ingrain-size distribution, and well sorted. We canreasonably speculate that this narrow range ofgrain sizes is somehow selected by the ambientwave energy conditions near the shoreface that areinsufficient to transport coarse grains onto theshoreface and too high to retain fine grains. Wemay therefore question whether the bimodality ofthe grain-size distribution among the sorted bed-forms is a result of sediment transport processesassociated with these features, or due to thesequestration of interim grain sizes in the shore-face. In other words, sorted bedform morphologymay be a consequence rather than a cause of grain-size bimodality. Questions such as these willrequire additional bedform modeling and physicaloceanographic investigations. The possible sourcesfor these sediment types is considered in thefollowing section.

5.3. Basal horizon

The shallow seismic horizon lying below bothfine and coarse sands is evidently related to a

http://mvcodata.whoi.edu/cgi-bin/mvco/mvco.cgi

http://mvcodata.whoi.edu/cgi-bin/mvco/mvco.cgi

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gravel/very coarse sand layer of variable thicknessdistributed throughout the survey area (Fig. 15).This layer truncates the channel flanks, which werecreated in a fluvial environment, and caps thechannel fill, which is presumably estuarine andbeach sediments. We interpret this layer as anerosional lag associated with the transgressiveravinement of the seafloor. This interpretation isconsistent with that of Duncan et al. (2000) for thechannel-capping ‘‘T’’ horizon on the middle NewJersey shelf, and we use the same identificationhere (Fig. 15). We speculate that the sedimentsbelow this layer are glacio-fluvial source sedimentsfrom which sediments were eroded in the forma-tion of the sorted bedforms, both coarse andfine sands.

To investigate the possible relationship betweensurface sands and the presumed ‘‘source’’ sand atdepth, we computed average grain-size distribu-tions from two populations of samples (Fig. 16):(1) coarse sands from the lower sections of thecores, below the gravel/very coarse sand layer andremoved from any channel-type fill, and (2) coarsesands from grab samples in the vicinity of thecores. The deeper coarse sands are more poorlysorted than the surface coarse sands, with greaterproportions of gravel and very coarse sands(4500 mm) as well as fine sands (125–250 mm). Itis possible that the well-sorted coarse and finesands at the surface could be derived from such asource, leaving behind a very coarse sand lag layer.However, the portion of fine sand is very small:just a few percent of the ‘‘source’’ mix. In contrast,our seismic interpretation of the sorted bedformmorphology (Figs. 5–9, 15) indicates roughly

Fig. 16. Averaged grain-size histograms of (1) coarse sands

found in the lower sections of vibracores (below the gravel/very

coarse layer, where fully penetrated; 24 samples from 17 cores),

and (2) coarse sands from 22 grab samples in the central region

of the survey area, in the vicinity of the vibracores.

similar volumes of coarse and fine sand residingabove what we have identified as the ravinementsurface. For these sands to have been derived froma common source, such sediment should exhibitlarge fractions of both fine and coarse grains. Thesame mass-balance problem is also likely true forthe medium sands of the shoreface, although wehave no constraints on the volume of thosesediments. We therefore suggest that most of thefine and medium sands on the seafloor within theMVCO have been transported there from prox-imal sources, the location of which is uncertain.

6. Conclusions

Our examination of sorted bedform morphol-ogy within the Martha’s Vineyard Coastal Ob-servatory (MVCO) reveals a spectrum ofbathymetric relief (from �10 cm to �3 m), grain-size contrast (from �250 to 42000 mm) andmorphologic forms (dune-forms, steps and moats).At the largest bathymetric relief and grain-sizecontrast, the sorted bedforms exhibit a dune-formmorphology, similar to sand ridges but smaller inscale. At intermediate relief and contrast, down-ward steps mark the transition from fine to coarsesand, along with moats at the CSZ edges. At thesmallest relief and contrast, bathymetric expres-sion is marked primarily by a small moat at oneedge.

Important commonalities are nevertheless ob-served across this range in sorted bedformmorphology. The CSZs tend to be asymmetric,generally (although not always) with a well-definededge on one side and a poorly defined or‘‘feathered’’ edge on the other as observed in thebackscatter map. Grain size tends to be larger, andthe bathymetry deeper toward the more well-defined edge. Also, the CSZs tend to form aslightly acute angle between the shoreline and themore well-defined edge. These similarities, alongwith proximity, imply a similarity of process acrossthe spectrum of sorted bedform shapes, sizes andgrain-size contrasts. The variations may, perhaps,be related to regional variations in flow intensity,or to varying degrees of modulation by otherprocesses, such as wave forcing.

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The asymmetries in sorted bedform morphologylead us to concur with previous authors whoconcluded that sorted bedforms are primarily aresponse to transverse, along-shore flows. How-ever, if true, then the flow regime in this settingmust be very complex, as evidenced by the changein the polarity of asymmetry that occurs along-shore and with depth. Additional studies of thephysical oceanography of the MVCO will berequired to test this prediction.

Our observations of temporal changes in coarse/fine sand boundaries and bathymetric featuresindicate that, while the bathymetry appears stableover both short and long time scales, the coarse/fine sand boundaries can migrate tens of metersover spans of just months to years. However,several lines of evidence suggest that the coarse/fine sand boundaries oscillate, rather than con-tinue to migrate in one direction: (1) push coresgathered at coarse/fine sand transitions showedmigration of fine sand over coarse sand and coarsesand over fine sand at each site, (2) there is a clearassociation of bathymetry, which appears stable,with the grain-size pattern, and (3) some of theobserved boundary migration directions are notconsistent with the long-term mean current forcingconditions implied by the sorted bedform asym-metries. We speculate that the oscillation of CSZboundaries is caused by wave forcing. Wavetransport processes are strong, but they tend tobe directionally scattered and thus unlikely toresult in large-scale morphological asymmetry.Current forcing may be weaker, but ‘‘irreversible’’in that currents will have a dominant direction,and so result in asymmetric forms.

Beneath the sorted bedforms lies a shallow (o1 m depth) seismic horizon, observed consistentlybeneath the fine sands and frequently beneath thecoarse sands. Often as well the horizon is seen tobe contiguous beneath both seafloor sedimenttypes, specifically at the less well-defined CSZboundaries. This horizon is likely derived from avariable-thickness layer of gravel/very coarse sandfound below surface sands, both fine and coarse,on most of the cores, and in grab samples gatheredfrom seafloor locations eroded below a surfaceoutcrop of this horizon. We speculate that thislayer is an erosional lag representing the trans-

gressive ravinement surface, and forms a boundarybetween glacio-fluvial host sediments below andmobile, reworked coarse and fine sands above. Thelatter may be derived by the former throughwinnowing, although the volume of fine sands maybe problematic and could require transport intothe study area from another source.

Acknowledgements

The authors gratefully acknowledge a largenumber of scientists who participated in thecollection of the varied data sets incorporated intothis study: Bill Schwab and Bill Danforth from theUSGS, Woods Hole, who collected the firstbackscatter and bathymetric data; personnel fromSAIC who collaborated with UNH in the collec-tion of Reson multibeam data; Steven Schock, JimWulf, Gwendoline Quentin, Pierre Beaujean,Csaba Vaczo, and Hernando Nieto from FloridaAtlantic University and Hilary Gittings from theWoods Hole Oceanographic Institution, whocollected the chirp seismic data; and BarbaraKraft, Eric Jabs, Peter Simpkin, Andy McLeod,and Jarrod Millar from the University of NewHampshire, who participated in the collection ofsamples and seafloor acoustic measurements. Theauthors benefited from discussions with RonBoyd, of the University of Newcastle, Australia,who spent a semester sabbatical at UNH. Reviewsby B. Schwab, M. G. Kleinhams, and ananonymous reviewer resulted in important im-provements in the manuscript. Funding for thiswork was provided by the Office of NavalResearch under grants N00014-02-1-0206 (JAG),N00014-02-1-0138 (LM), N00014-01-1-0564 (PT),N00014-01-1-0957 (RLE). UTIG contribution1712.

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