Detailed investigation of sorted bedforms, or “rippled scour depressions,” within the Martha's Vineyard Coastal Observatory, Massachusetts

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Continental Shelf Research 25 (2005) 461–484 www.elsevier.com/locate/csr

Detailed investigation of sorted bedforms, or ‘‘rippled scour depressions,’’ 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 a

University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759, USA b University of New Hampshire Center for Coastal and Ocean Mapping, Durham, NH 03824, USA c Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA d Hawaii Institute of Geophysics and Planetology, Honolulu, HI 96822, USA e University 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 Corresponding author. Tel.: +1 512 471 0476; fax: +1 512 471 0999.

E-mail address: [email protected] (J.A. Goff). 0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.09.019

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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 is often an active sedimentary environment, impacted by the effects of storm-generated waves and flows and longshore currents. It is in this environment that the largest bedforms on the continental shelf are generated. Oblique sand ridges, for example, have been studied for decades in inner-shelf locations all over the world (e.g., Swift and Field, 1981; Parker et al., 1982; Dalrymple and Hoogendoorn, 1997; van de Meene and van Rijn, 2000; Park et al., 2003). Up to kilometers wide, tens of kilometers long and several meters high, sand ridges often form along the shoreface in areas where sand sediment supply is high. Another type of large bedform has also been recognized as ubiquitous in nearshore settings where sediment supply is low: so-called ‘‘rippled scour depressions’’ (RSDs; the term was coined by Cacchione et al., 1984). RSDs are bathymetrically more subtle features than sand ridges, typically witho1 m of relief (Cacchione et al., 1984). They are most clearly identified in sidescan sonar backscatter surveys and, when ground-truthed, appear to be highly elongated patches of rippled, coarse sand/gravel/shell hash, oriented approximately shore perpendicular, and slightly depressed by up to a meter with respect to surrounding fine sands. RSDs are typically tens to hundreds of meters wide and hundreds to thousands of meters long (see Cacchione et al., 1984, and Murray and Thieler, 2004, for comprehensive reviews of prior literature on RSDs).

There is a general consensus among investigators as to how RSDs are able to maintain a clear segregation of coarse and fine sands on the seafloor (Murray and Thieler, 2004). The larger ripples that exist within the coarse sediment patches will lead to strong bottom boundary turbulence, which will in turn inhibit the deposition of fine-grained sands. Thus, fine sand grains that are either transported into or winnowed from an RSD will tend to keep traveling until they reach an area already covered by fine sands. There is less agreement, however, on how RSDs initiate or to what flow regime they respond. Early hypotheses centered on cross-shelf flows, perhaps caused by downwellings during storms (Cacchione et al., 1984). However, recent detailed investigations of RSDs 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 in morphology that suggest they are a response to southwest-directed long-shore currents. Furthermore, sidescan sonar surveys taken before and after a storm event showed significant movement of the RSDs along shelf in the direction of the longshore currents (Murray and Thieler, 2004). The term ‘‘rippled scour depression’’ has itself come under recent scrutiny. The use of ‘‘depression’’ as a morphological descriptor is only partly correct, as noted by Murray and Thieler (2004) in their detailed investigation of RSD morphology on Wrightsville Beach. While the upcurrent side of the RSD (in relation to the alongshore current) is usually depressed relative to the surroundings, the

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downcurrent side is often raised. Schwab et al. (2000) also determined, in their backscatter and bathymetry mapping of RSDs off Long Island, New York, that the high backscatter/coarse sands were coincident with the upcurrent flanks of low amplitude, flow-transverse bedforms. These observations suggest that RSDs and surrounding fine sands are nearly dune-like in their apparent stoss and lee relationships, respectively, to an alongshore current direction. On the other hand, not all observations of RSDs yield the same results. Cacchione et al. (1984), for example, report no coastwise asymmetry in RSDs off Central California, and no obvious asymmetry is evident in RSDs observed by Green et al. (2004) off the coast of New Zealand. There may, therefore, be two classes of features lumped together by the term ‘‘RSD,’’ distinguished by their asymmetry. We report here on a detailed investigation of seabed morphology off the south coast of Martha’s Vineyard, Massachusetts (Fig. 1). As shall be demonstrated in this paper, the features we observe here are akin to those observed off Wrightsville Beach and Long Island. We therefore follow Murray and Thieler’s (2004) suggestion of referring 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.

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bedforms’’ rather than ‘‘rippled scour depressions.’’ 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 to differentiate components of the sorted bedforms. The geology of Martha’s Vineyard is comprised principally of glacio-fluvial outwash sediments seaward of the terminal moraine on the north and east sides of the island (e.g., Uchupi and Oldale, 1994, and references therein), deposited during the last glaciation ca. 23 ka (Balco et al., 2002). These outwash plains are dissected by southward-trending valleys, presumably formed by spring sapping (Uchupi and Oldale, 1994). Our survey area encompasses the Martha’s Vineyard Coastal Observatory (MVCO) operated by Woods Hole Oceanographic Institution. The study was conducted under the auspices of the Office of Naval Research’s (ONR) Mine Burial Prediction Program, which seeks to understand the process of solid object burial by such processes as liquefaction and bedform migration. Our surveys represent a geological reconnaissance of the area, with particular emphasis on understanding the structure and evolution of the sorted bedforms. Data reported here include: (1) acoustic backscatter, collected during three surveys during February 2001, September 2001, and July 2002, (2) swath bathymetry, collected during the September 2001 and July 2002 surveys, as well as 1965 soundings by the National Oceanographic Service, (3) high resolution deep-towed chirp seismic data, (4) sedimentological/textural analysis from vibracores, (5) grain-size analysis from diver push cores across two CSZ boundaries, and (6) grain-size analysis from seafloor grab samples. The chirp data are resolved vertically to o10 cm; to our knowledge these data are among the first successful seismic imaging of the subseafloor structure of RSDs/sorted bedforms. Our study is primarily observational in nature, focusing on the areal morphology and crosssectional structures of the sorted bedforms and their temporal variability. Important new observational constraints are provided by the combined data sets of high resolution bathymetry, backscatter, and subsurface imagery, along with extensive surface

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and subsurface sampling. The MVCO survey area is also notable for the strong variability in the size and shape of sorted bedforms, which provides a unique opportunity for posing and testing hypotheses for their formation. While this paper raises more questions than it answers, future reports by collaborating scientists in the ONR Mine Burial program will greatly progress our understanding of these important features.

2. Data The site survey of the MVCO was focused in the vicinity of the MVCO node (Figs. 2 and 3), which provides cabled access to the shore for powering and monitoring of scientific equipment. Data coverage was also extended over a broader area (Figs. 2b, 3a) to provide regional geologic context for understanding sedimentary activity near the node site. Emphasis was placed on obtaining timeseries mapping results, to investigate the evolution of seafloor features over time scales of months to years. 2.1. Swath mapping An initial sidescan sonar backscatter survey of the MVCO was conducted in February of 2001 by the United States Geologic Survey (USGS), using a towed EdgeTech DF1000 sidescan sonar system operating at 100 kHz (Fig. 2a). The USGS was subsequently funded in part by ONR to survey in September 2001 over a broader area using a polemounted Submetrix interferometric swath mapping sonar system operating at 234 kHz, which collected both backscatter (Fig. 2b) and interferometric bathymetry (Fig. 3a). The weather during this survey was rough, and the data were consequently a bit noisy but still of value both as a regional reconnaissance and for identifying princi-

pal morphological structures associated with the sorted bedforms. A high-resolution multibeam bathymetric survey was conducted in July 2002 by the University of New Hampshire and Science Applications International Corporation (SAIC). An approximately 3  5 km area surrounding the MVCO node was surveyed using a 455 kHz Reson 8125 focused multibeam sonar. In a 250  400 m area immediately surrounding the MVCO node, high resolution data were collected with a line spacing of approximately 4 m; line spacing was relaxed to approximately 12 m to an area of 1 km  1 km around the MVCO node and finally to 25–40 m in the rest of the area. The very narrow beam width of the Reson 8125 (0.51) combined with the high density of data within the 1 km  1 km box support gridding at 25 cm or less. Kinematic GPS navigation was used to provide a navigation precision of less than 10 cm. This will be essential for the comparison with subsequent bathymetric surveys. Here we include the results of the larger area survey presented at a grid resolution of 50 cm (Fig. 3b). Although the Reson 8125 data were not intended for extraction of backscatter values, additional processing at the University of New Hampshire Center for Coastal and Ocean Mapping provided backscatter rendering which, although not representing ideal backscatter data, nevertheless provides us with clear identification of the sorted bedform grain-size boundaries at the time of this survey (Fig. 2c). 2.2. Grab samples Seafloor sediment samples were collected at 89 stations within the MVCO swath survey area (Figs. 2a, 3b) using a Smith–Mcintyre grab sampler (Murdoch and MacKnight, 1994) aboard the R/V Cape Henlopen in August, 2002. Navigation for each grab sample was recorded from

Fig. 2. Backscatter survey maps from (a) February 2001, using an Edgtech DF1000 100 kHz sidescan system, (b) September 2001, using a Submetrix 234 kHz interferometric system, and (c) July 2002, using a Reson 8125 455 kHz focused multibeam system. Higher backscatter is indicated by lighter shades. Bathymetric contours, in meters, are derived from merged NGDC and Reson 8125 data. Squares in (b) indicate grab sample stations (filled squares correspond to filled symbols in Fig. 4) and numbered, filled circles indicated vibracore sites and their station numbers. The larger coarse sand zones, seen as high backscatter areas, are identified as CSZs1-3. Star indicates location of the MVCO node. Locations are given for profile figures and the sites of diver cores presented later.

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Fig. 2. (Continued)

differential GPS, with an accuracy of
5 m. A hand-operated plug corer was used to extract subsamples from the grab for grain-size analysis. Many of the grab samples in the central portion of the survey area (between CSZs 1 and 2; Fig. 2b) were collected at stations collocated with a preliminary set of samples taken the year before aboard the R/V Asterias. These earlier samples included grabs taken from the shoreface, in
6 m of water, which could not safely be duplicated with the Henlopen. Weight percent of fine-grained material, or mud fraction (o63 mm) was determined by wet sieving, and coarse-grained percentages in the 2–4 mm (granules) and 44 mm (pebbles and gravel) bins were obtained by dry sieving. The remainder of the

distribution was estimated through settling tube analysis using a visual accumulation method. Half-f bins were computed from 63 to 500 mm, and 1f bins from 500 to 2000 mm (grain size in mm ¼ 2f). These values were normalized to the total dry sample weight. The settling tube analysis was very precise. As reported by Goff et al. (2004), rms variation on mean sand grain size for this methodology was at most 0.04f over multiple independent runs on split portions of the same sample. Differences between subsamples from the same grab were slightly higher, with an rms differential in mean sand grain size of 0.09f over all grabs. Both the 44 mm and o63 mm portions exhibited rms differential of 3.4%, 1.4% for

Fig. 3. Artificially illuminated bathymetry maps from (a) September 2001, using a Submetrix 234 kHz interferometric system, and (b) July 2002, using a Reson 8125 455 kHz focused multibeam system. Illumination is from the northwest. Bathymetric contours, in meters, are derived from merged NGDC and Reson 8125 data. Thin-dashed lines in (a) indicate chirp seismic track lines. Squares in (a) indicate grab sample stations, and circles indicated vibracore sites. Within the three large CSZs, a transition is noted in (b), with thickdashed lines, between westward dipping bathymetry at shallow depths and eastward dipping bathymetry at greater depths. Star indicates location of the MVCO node. Locations are given for profile 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 3 4 5 6 7 8 9 10 11 12 13 14.1 14.2 14.3 14.4 14.5 15 16.1 16.2 16.3 16.4 16.5 17 18 19.1 19.2 19.3 19.41 19.42 19.5 20.1 20.2 20.3 21.11 21.12 21.2 21.3 22 23 24 25 26 27 28 29

41.344097 41.344080 41.343848 41.343947 41.344093 41.343817 41.344085 41.346908 41.347060 41.347168 41.347247 41.347118 41.336487 41.336568 41.336648 41.336602 41.336472 41.336647 41.336588 41.336723 41.336690 41.336843 41.336512 41.336562 41.336632 41.336685 41.336680 41.336703 41.336720 41.336680 41.336650 41.336842 41.337030 41.337078 41.337025 41.337000 41.336982 41.336570 41.336902 41.329955 41.330243 41.329922 41.330417 41.330067 41.320823 41.321135

70.571907 70.564888 70.560873 70.559443 70.558382 70.557042 70.554962 70.554072 70.556321 70.559192 70.561993 70.567770 70.573280 70.572898 70.572645 70.572400 70.571988 70.566987 70.566158 70.565647 70.565583 70.565478 70.565220 70.563875 70.560550 70.558690 70.558277 70.558117 70.557678 70.557797 70.556980 70.556823 70.556248 70.556067 70.555443 70.555605 70.555255 70.555020 70.553970 70.555643 70.558418 70.561802 70.566075 70.570783 70.569027 70.564390

1.22 1.14 2.92 0.53 2.78 0.27 0.43 2.01 2.08 2.15 2.08 2.08 3.09 3.31 0.76 0.56 0.55 0.73 0.12 0.35 1.02 2.78 2.84 2.89 2.89 0.44 0.80 0.61 0.45 0.53 2.76 2.70 2.78 2.50 0.52 2.08 0.72 0.90 2.47 2.48 0.70 2.76 3.07 0.14 3.19 3.22

0.74 0.70 0.54 0.79 0.56 0.85 1.13 0.62 0.50 0.57 0.54 0.50 0.49 0.69 0.94 0.76 0.72 0.85 1.48 1.09 1.08 0.58 0.60 0.50 0.48 1.02 0.88 0.92 1.20 1.15 0.54 0.57 0.56 0.75 1.03 0.86 0.73 0.74 0.55 0.84 0.94 0.74 0.48 1.26 0.60 0.71

30 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

41.320968 41.313255 41.313228 41.313395 41.308172 41.336557 41.336617 41.336460 41.336525 41.336640 41.336678 41.336540 41.326468 41.326447 41.326530 41.326483 41.326438 41.326482 41.326328 41.336592 41.336560 41.336607 41.336628 41.336682 41.336612 41.336633 41.336597 41.326645 41.326442 41.326480 41.326463 41.326543 41.326530 41.326428 41.326523 41.326420 41.326483 41.326618 41.326502 41.326558 41.326448 41.326610 41.326418 41.326528 41.326425

70.559640 70.553723 70.562308 70.569835 70.570100 70.601913 70.600820 70.599148 70.597743 70.597053 70.596187 70.594823 70.603430 70.602202 70.600675 70.599523 70.598317 70.597083 70.595303 70.536635 70.534927 70.531232 70.528628 70.528183 70.527720 70.527185 70.526395 70.537218 70.533972 70.533007 70.531932 70.531097 70.529365 70.527312 70.525762 70.558987 70.563560 70.567227 70.570720 70.573048 70.574117 70.576688 70.579035 70.582628 70.585258

2.76 2.86 3.19 3.24 3.00 2.08 0.64 1.32 2.11 0.83 1.56 2.28 2.64 1.04 1.70 2.18 1.36 1.62 2.46 2.25 1.03 0.26 0.20 2.22 1.09 0.83 1.92 2.40 1.16 0.63 1.36 1.29 0.47 0.12 2.46 2.53 2.95 3.17 3.12 0.10 0.73 3.00 3.25 3.28 2.38

0.582 0.69 0.66 0.64 1.22 0.58 0.69 0.72 0.65 0.68 0.65 0.78 0.86 0.74 0.77 1.00 0.72 0.80 0.96 0.67 0.64 0.98 1.49 0.56 0.80 0.94 0.51 0.61 0.58 0.68 1.63 0.67 0.64 1.34 0.62 0.80 0.53 0.43 0.56 1.26 0.62 0.80 0.58 0.65 0.95

the 2–4 mm portion. Differences between collocated Henlopen and Asterias grab samples differed by an average of 0.16f. Mean and rms 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 the grab samples, including abundant worm tubes,

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particularly in the finer sediments, occasional large bivalves, hermit crabs and sand dollars. 2.3. Vibracores Cores were collected at 35 stations using a Rossfelder P-5 electric vibracoring system during the same R/V Cape Henlopen cruise in August of 2002. Depth of penetration ranged from o0.5 m to nearly 2 m; such poor recoveries are to be expected for predominantly sandy sediments such as are found within the MVCO. Cores were capped aboard ship and stored at the Woods Hole Core Repository for post-cruise analysis. Onshore the cores were split, photographed, described and sampled. Representative samples were taken from the distinct units within each core. Samples were generally of the order of 100 g dry weight. Grainsize analyses were done by dry sieving the samples at 0.5f intervals between 2.5 and 0, and 1f intervals from 0 to 4. Sediment finer than 63 mm (4f) was negligible, although it is possible that the vibracoring process may have washed out some of the very finest components of the sediment.

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vehicle two Tonpilz-type piston sources with operating bands of 1–5 and 4–16 kHz were driven simultaneously to generate the wideband output pulses. The seabed reflections were received by a horizontal, 1 m long  1 m wide planar hydrophone array. Display and interpretation of chirp data were assisted by Schlumberger GeoFrame software. 2.5. Diver cores Short push cores were collected by divers spanning two transitions between coarse and fine sands (Fig. 2b). Ten cores were recovered at each site at 1 m intervals, as determined by a knotted cord. Core penetration ranged from
15 to
30 cm. The cores were split, photographed and subsampled at a range of depths guided by visual evidence of grain-size boundaries. Grain-size analysis of the subsamples was conducted in the same manner as that of the grab samples. Some mixing of adjacent subsamples may be possible due to imprecision of dividing the core samples and pull-down at the core edges.

2.4. Chirp seismic reflection 3. Sorted bedform morphology A chirp seismic reflection survey was conducted aboard R/V Cape Henlopen for 2 days immediately following the sampling work in August 2002. Track lines were mostly oriented E–W, parallel to the southern Martha’s Vineyard shoreline, with two N–S tie lines (Figs. 2b, 3a). The chirp sonar, designed and fabricated by Florida Atlantic University, measures acoustic reflections at normal incidence to the seabed using a dual pulse technique to produce high-resolution imagery of near surface sediments and lower resolution imagery of deeper sediments. A 40 ms-long FM pulse with a band of 1.5–4 kHz was transmitted to provide imagery of the top 40 m of the seabed with a vertical resolution of 40 cm while a 10 ms-long FM pulse with a band of 1.5–15 kHz was alternately transmitted to yield 10 cm resolution imagery of sediments in the top 10 m. The latter provided the best imagery of sorted bedform stratigraphy, which often included reflections just tens of centimeters below the seafloor. In the sonar

3.1. Areal morphology The backscatter maps of the MVCO area (Fig. 2) are, for the most part, directly translatable into maps of grain-size variability. With few exceptions, grain-size distributions from seafloor samples are unimodal, well-sorted and wellcharacterized by the mean grain size. Fig. 4 displays mean grain size versus backscatter gray scale values derived from the Submetrix survey (Fig. 2b), where a strong positive correlation is observed between the two over most of our samples. The correlation is inverted, however, for fine sands within the central and deeper portions of the survey area (Figs. 2b, 4). The increase of backscatter at finer grain sizes is perhaps best observed in Fig. 2a, where a brightening is observed toward the southwestern corner; mean grain size within this elevated backscatter region among 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 70135.50 W and 701330 W) 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 majority of the survey area, and particularly for the coarser sands sampled, backscatter and mean grain size are positively correlated. The MVCO backscatter maps (Fig. 2) exhibit structures that have previously been defined as ‘‘rippled scour depression’’: areas of coarse sand extending nearly perpendicularly out from the shoreface, tens to hundreds of meters wide and extending kilometers from the shoreface, that are surrounded by fine sands. With very few exceptions (primarily in the western part of the survey area), mean grain sizes in the medium-sand range of
2–2.3f (
200–250 mm) are absent from the sorted bedforms (Table 1), indicating a truly bimodal separation of grain sizes between the coarse and fine sand zones. The shoreface samples (stations 9–13 in Table 1), however, all exhibit mean grain sizes in this range.

Three notably large CSZs are present in the central to eastern portions of the survey area, numbered for identification as CSZs 1–3 (Figs. 2, 3). As Murray and Thieler (2004) noted in their observations off Wrightsville beach, these sorted bedforms display a number of asymmetries that may be associated with large transverse bedforms. As seen particularly on CSZs 2 and 3, as well as on many of the CSZs in the western portion of the survey area, one edge tends to be well-defined, while the other is either more poorly delineated or ‘‘feathered’’ in appearance. Within the CSZs, grain size, as evidenced by the backscatter, tends to be largest toward the sharp-edged side, and decreases systematically away from that edge. This observation will be more clearly demonstrated in profile analyses in the following section. The CSZs also tend to be oriented systematically such that the more well-defined edge is at a slightly acute angle to the shoreface. Many of the CSZs in the central portion of the survey area, including CSZ1, do not exhibit noticeable asymmetry in the backscatter map. The polarity of the grain-size asymmetries across CSZs does not remain consistent throughout the survey area. As seen in the Submetrix backscatter map (Fig. 2b), CSZs at the western edge of the survey area have their sharpedged boundaries on their western side, and trend slightly east of north. The reverse pattern is observed at the eastern edge of the survey area: the sharp-edged boundary is on their eastern side, and trend slightly west of north. The Reson backscatter map (Fig. 2c), which extends to shallower waters than the Submetrix survey, provides evidence that the polarity of backscatter/grain-size asymmetry changes with depth as well as along-shore position. For example, CSZ3, at the easternmost edge of the survey area, exhibits a higher-backscatter eastern edge at depths shallower than
9 m, while CSZ2 exhibits a similar transition at
12 m (again, profile analysis in the following section will better demonstrate this observation). The orientation of CSZs in these areas also change at these approximate depths from being westof-north in deeper water to east-of-north in shallower water.

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The bathymetric expression of the sorted bedforms, as seen on the contour maps superposed on the backscatter (Fig. 2) and shaded relief maps (Fig. 3), is highly variable. Bathymetric asymmetry within the CSZs can be observed by the down-tothe-right or down-to-the-left slant of the contours (Figs. 2,3). CSZs 2 and 3 in the eastern half of the survey area display strong asymmetries, with the more sharply defined, coarser edge typically depressed relative to the surrounding seafloor, while the opposite edge is raised. This observation is identical to that noted for the Wrightsville Beach sorted bedforms (Murray and Thieler, 2004). The bathymetric asymmetry closely follows the changes in grain-size/backscatter asymmetry, switching east–west polarity going from west to east and from deeper to shallower water (Fig. 3b). Within CSZ3, the change in bathymetric polarity occurs at
10 m water depth, while within CSZ2 the polarity changes at
13 m water depth. CSZ1 also exhibits bathymetric asymmetry, although more subdued in comparison. Like CSZs 2 and 3, the bathymetric asymmetry within CSZ1 changes polarity with depth; here the transition is at
14–15 m water depth. Bathymetric expression of the CSZs is not constrained to simple depth asymmetries across their widths. For example, the edges of the CSZ1 are also marked by a distinct moat. Asymmetries and edge moats exist within most of the smaller CSZs in the central and eastern portions of the survey area as well. The bathymetric expressions of the CSZs in the western half of the survey area are quite subdued. 3.2. Cross-sectional morphology In this section we employ profile views of bathymetry, backscatter and chirp seismic data to provide detailed views of the sorted bedform structures within the MVCO. To mitigate the effects of speckle and track-line artifacts on backscatter profiles, we employ the cross-profile filtering methodology of Goff et al. (2000). In this algorithm, values are averaged along cross-lines oriented parallel to structure, as defined by the user, rather than along the profile. This technique emphasizes structures sampled along the profile

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while reducing random effects, such as noise, or data patterns that cut across seafloor structures, such as track-line artifacts. Fig. 5 displays chirp seismic, bathymetric and backscatter profiles across sorted bedform morphology within the central region of the survey (Figs. 2b, 3a). On Fig. 5 and subsequent profile plots, CSZ edges are distinguished between those that have a more abrupt backscatter transition and/or a deeper bathymetry, and those that have a more gradual backscatter transition and/or a shoaler bathymetry. The CSZs in the central region display a complex but generally consistent morphology. Bathymetric steps occur at both CSZ edges, creating a depression of the CSZ in relation to the surrounding bathymetry. Within the CSZ, the bathymetry slopes downward toward the western CSZ boundary, but with a convex-up geometry that creates moat-like depressions at both CSZ edges. Some CSZs have abrupt backscatter transitions on both boundaries (e.g., at
0.8 km on Fig. 5), whereas others display some measure of asymmetry (e.g., at
0.5 km on Fig. 5). In the chirp seismic reflection data, a seismic horizon is observed consistently beneath the fine sands, oftentimes intersecting the seafloor at the bottom of the CSZ moat at the deeper edge. A seismic horizon is also observed, more occasionally but nevertheless frequently, within the CSZs. Where observed, this horizon is often contiguous with the horizon seen below the fine sands at the shoaler CSZ boundary (e.g., at
0.9 km in Fig. 5). Evidence for the possible origin of this horizon will be given below. A buried dendritic channel system was also mapped in the survey area (Fig. 5), which suggests evidence of subaerial erosion prior to the latest sea-level rise. The channel flanks are truncated by the shallow horizontal seismic horizon described above. Fig. 6 displays profile data from sorted bedform morphology in the western sector of the survey area. The polarity of asymmetry in this area is the same as in the central region, but the bathymetric expression is less pronounced. Here, only a small moat-like depression is observed at the sharpedged side of the CSZ. The contrast in grain size between the coarse and fine-grained sediment domains 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. Heavydashed 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 the central region (
500–750 versus
100–200 mm) of the survey area. In addition, the backscatter transitions at the eastern boundaries of the western CSZs are more gradational than observed in the central region. The subsurface stratigraphy is, however, very similar between the western and central regions: a well-defined seismic horizon beneath the fine sands intersecting the seafloor at the bottom of the moat at the deeper/sharp-edged CSZ boundary, and an occasional, usually less distinct seismic horizon within the CSZs that is often seen as contiguous with the fine sand horizon at the more gradual CSZ boundary.

Fig. 7 displays profile data from the eastern sector of the survey, spanning CSZ3 and smaller CSZs to either side. Here, the morphological observations are essentially identical to those noted on Fig. 5, but with asymmetry that is reversed east to west, and stronger evidence of asymmetry in the backscatter, particularly CSZ3. In deeper water (Fig. 8), the bathymetric expression and coarse versus fine grain-size contrast of CSZ3 increases (
400–2500 versus
175–225 mm), and the ‘‘moat and step’’ morphology is replaced by a distinctive dune-like morphology (e.g., Swift and 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 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. (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
4 times smaller, and far less acute angle with the shoreline than ‘‘typical’’ shoreface attached ridges elsewhere 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 an accompanying reversal in the asymmetry evident in the backscatter profile. Beneath the sand ridges shown on Fig. 8, and clearly contiguous with the seismic horizon beneath fine sands to the west, is a seismic horizon that outcrops on the east flanks of the two ridges profiled, indicating that the base of the east flanks of these ridges have been eroded below this horizon. Grab samples gathered above this contact consist of well-sorted, coarse sands, whereas

samples taken below the contact contain significant quantities of gravel. A similar relationship appears likely in Fig. 7, although it is less certain there whether this horizon intersects the seafloor. These observations suggest that gravel content may be a factor in the impedance contrast that forms the seismic horizon observed within the CSZs. 3.3. Core samples Vibracores provide observational constraints on the nature of the chirp seismic horizons in the shallow subsurface. Aside from the buried channels, the primary observed seismic horizons are generally located within a meter of the seafloor, and thus accessible by coring. Three principal units were observed outside the fill units of the buried channels: 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 buried tens of centimeters beneath either fine sands, coarse sands, or both. A gravel/very coarse sand layer was sampled in nearly every core. The fine/ coarse sand transition and the gravel/very coarse sand layer represent the most likely candidates for generating seismic impedance contrasts in the upper meter of sediment. Table 2 displays the depth to the fine/coarse transition, the upper and lower bounds of the gravel/very coarse sand layer, and the depth to the chirp seismic horizons (not including channel fill horizons). The latter are accurate to approximately 70.05 m. The seismic horizon is most consistently and robustly observed beneath the fine sands at the seafloor, so it is reasonable to

assume that the fine/coarse transition may be a significant factor in producing this impedance contrast. However, where a fine/coarse transition is present in the core, the corresponding seismic horizon tends to be deeper ranging from a few centimeters to as much as 30 cm, and more typically
10 cm. Some or all of this discrepancy may be attributed to loss of fine sands at the top of the core during recovery. However, the depth to the shallowest seismic horizon is often more consistent with the depth range of the gravel/very coarse sand layer, including cores where no fine sands are present. Given that this seismic horizon is often seen to be contiguous beneath CSZ boundaries, we surmise that the gravel/very coarse sand layer is primarily responsible for generating this 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 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 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. 2 provide a limited time-series record of the sorted bedforms in the MVCO study area. Fig. 10 displays a close-up view of the Submetrix sidescan data (September 2001) in the central survey area with digitized CSZ boundaries from the earlier DF1000 survey (February 2001) and later Reson survey (July 2002) superposed. Systematic shifts in the CSZ boundaries are observed over this time frame, but with regional variations. CSZ boundaries in the northwestern sector of this map have shifted to the east by as much as 60 m. The shifts seen in the northeastern sector of the map range from
0 to
30 m, also to the east. A variety of

responses are observed in the southern half of the study area. A number of CSZ boundaries show little or no shift over the time span mapped. Others display a westward shift between February and September 2001, and then an eastward shift by July 2002 (e.g. along the edge of CSZ1). One boundary, along the west side of CSZ2, shifted westward during both time periods. The navigational uncertainty in hull-mounted swath maps (Figs 2b, c) is at most 5 m, and perhaps a bit more for the towed instrument (Figs 2a), so these observations should be considered robust. The scale of movement, a few tens of meters, is consistent with Murray and Thieler’s (2004) observations of CSZ migration off Wrightsville Beach. However, unlike their observations, the direction of migration is not always consistent with 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, the eastern edge of CSZ2 (Fig. 10) is the coarsergrained side, which assuming it is a transverse bedform, would indicate that the stoss side faces east into a westward transport direction. Nevertheless, this boundary migrated eastward between the September 2001 and July 2002 surveys. 4.2. Bathymetric mapping Our modern bathymetric time series records are presently limited to the September, 2001 Submetrix survey and July 2002 Reson survey. This comparison is not ideal because of the differences in resolution and noise between the two. However, a comparison of profiles (Fig. 11) indicates that the CSZ ‘‘moats’’ are usually well-resolved features in the noisier Submetrix data. With one exception in Fig. 11, the moats that can be clearly identified in the Submetrix bathymetry are aligned

perfectly with the moats observed in the Reson bathymetry. The one exception, just before 0.8 km distance along the profile, is a shift to the west, which is the opposite direction of the shift seen consistently in the backscatter data (Figs. 10, 11). A long-term bathymetric comparison can also be made by comparing the multibeam data with historical records derived from digital National Ocean Service (NOS) data. The area surveyed by the multibeam maps in Fig. 3 were surveyed in 1965 with single point measurements. Fig. 12 compares two profiles sampled from the interpolated 1965 NOS data with collocated profiles through the swath data. While cautioning against interpreting too much detail in this comparison, it is nevertheless clear that the larger sorted bedform structures throughout the survey were captured at least in part by the 1965 NOS survey, and furthermore that the positions of these features

ARTICLE IN PRESS J.A. Goff et al. / Continental Shelf Research 25 (2005) 461–484 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 2.1–2.4 3.1,3.2,3.3,3.5 3.4 4 5 6 7 8 9 10

— —
0.30 — — — 0.38 — 0.17 0.21 —

0.40c 0.45b 0.50b 0.50b 0.35b 0.35 0.50b 0.45b (weak) 0.40b 0.40 0.45

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

— 0.31 — 0.12 0.26 0.42 — — — 0.13 — 0.33 0.50 0.45 0.52 1.24 0.80

0.41–0.48 None 0.30–0.54 0.30–0.60 0.08–0.25 0.22–0.36 0.38–0.66c 0.28–0.40 0.32–0.55c 0.21–0.27c 0–0.15/ 0.22–0.34/ 0.65–0.68d None 0.35–0.75 None 0.12–0.44 0.26–0.35 0.53–1.62c 0.40–0.43 0.61–0.64 0.10–0.20 0.13–0.33 0.28–0.32 0.48–0.49 0.50–0.60 None 0.52–0.67c None 0.80–0.91c

28

0.96

0.96–1.04

0.40 (weak) 0.35 Noneb 0.20 0.30 0.45 0.45 0.55 0.25 0.15b 0.40 0.45 0.60 0.50b 0.65b 0.90b 0.45/0.95/ 1.30e 1.00/1.30e

a

Assuming 1700 m/s sound speed. First horizon above buried channel. c Bottom of core. d Two gravel/VC layers observed on this core. e Multiple seismic horizons observed at these locations. b

have not changed substantially over the span of nearly four decades. 4.3. Diver push cores at coarse/fine transitions Earlier recognition that the CSZ boundaries are not stable led us to collect a series of 1-m spaced, diver-located push cores across two coarse/fine

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sand transitions (Fig. 2b). The purpose of these cores was to determine if boundary migration leaves a recognizable stratigraphic signature. Unique among seafloor sedimentary samples within the MVCO, the transition cores yielded strongly bimodal sand distributions, with a clear separation at
250 mm between coarse and fine sands (Fig. 13). This indicates significant mixing of the two sediment types at the transition. This mixing cannot, in general, be attributed to any sampling issues, such as pull-down along the core edges or imprecise separation of subsamples. The sample shown in Fig. 13, for example, was selected from the top of the core (eliminating pull-down) and is well separated from any significant fine sand layers farther down in the core (Fig. 14). Fig. 14 displays the relative proportion of coarse versus fine sands down-core in each of the push cores. The finecoarse transition is seen readily as fines overlying coarse sands in increasing thickness away from the CSZ. However, additional, sometimes continuous and other times isolated zones of fine sands are seen farther down-core, indicating both that fine sands have deposited or migrated over coarse sands and likewise that coarse sands have deposited or migrated over fine sands.

5. Discussion 5.1. Sorted bedform morphology The sorted bedforms observed within the MVCO survey area exhibit a broad spectrum of bathymetric relief (from
10 cm to
3 m), grainsize contrast (from
250 mm to 42000 mm) and morphologic form (moats, steps, and dune forms; Fig. 15). These three factors correlate with each other: dune forms occur with the largest bathymetric relief and grain-size contrast (e.g., Figs. 8 and 9), ‘‘step and moat’’ morphology exhibits an intermediate relief and grain-size contrast (e.g., Figs. 5 and 7), and small moats are present at the more well-defined edges of the CSZs with the lowest relief and grain-size contrast (e.g., Fig. 6). All morphologic forms observed display lateral asymmetry in both grain size and bathymetric expression. 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.

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same bedform process, but perhaps in response to differing intensities of flow or somehow modulated by other processes, such as ripple migration forced by wave skewness. An important question is raised by these observations: why does the step-and-moat morphology evolve in some circumstances rather than simple dune-like morphology, and what role is played by current intensity and grain-size contrast? We concur with Schwab et al. (2000) and Murray and Thieler (2004) that the asymmetries exhibited by the sorted bedforms indicate that they are primarily a response to a bedformtransverse, along-shore flow. But if so, then clearly the MVCO must have a very complex flow regime, as evidenced by the changes in the polarity of asymmetry (Fig. 2b), with an eastward flow in the western half and shallower part of the eastern half of the survey area, and a westward flow in deeper water in the eastern half. Complex nearshore current conditions are certainly possible for this area; e.g., an eastward alongshore current meeting a strong tidal current coming around the shoals on the east end of Martha’s Vineyard. However, detailed physical oceanographic data on the current regime across this area are not presently available. Wave forcing may also play an important role, although we hypothesize below that wave-induced migration of sorted bedform morphology is ephemeral, and has no relationship to asymmetry. 5.2. Short-term sorted bedform evolution

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, typically seen in the backscatter maps as the more well defined of the two CSZ edges where that distinction can be made. The proximity of these sorted bedforms sizes, grain-size contrasts and morphologic forms, and their regional similarity of trends and asymmetries, suggest that they are different expressions of the

Our analysis of repeated bathymetric and backscatter maps indicates that, while the coarse/fine sand transitions can move tens of meters over the span of months (Figs. 10, 11), bathymetry appears quite stable over that time frame (Fig. 11), and some of the larger features have remained in essentially the same location over a span of nearly four decades (Fig. 12). Furthermore, the direction of CSZ boundary shifts are not always consistent with the direction expected based on the asymmetries of the sorted bedforms, and over the coarse of three backscatter surveys some of the boundaries are seen to alter their direction of migration (Fig. 10). Finally, our grain-size analysis of diver push cores 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 boundary, both of fine sands advancing over coarse sands and coarse sands advancing over fine sands.

These observations appear to indicate that, whatever alongshore currents may be controlling formation 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 is responding to other, likely more variable factors. The shifts we observe are locally consistent; that is both sides of the CSZ advancing westward or eastward by roughly the same amount. From this we infer that oceanographic conditions, rather than sediment flux (i.e., input or removal of fine sands), are controlling short-term CSZ boundary movements. Wave orbital velocity skewness is perhaps the most likely cause for short-term sediment migration, as evidenced by the fact that sand ripples on the seafloor align perpendicular to wave direction and have been observed to migrate in the direction of wave propagation (Traykovski et al., 1999; Traykovski and Goff, 2003). Although wave records from the MVCO node (http:// mvcodata.whoi.edu/cgi-bin/mvco/mvco.cgi; Figs. 2 and 3) show that the largest waves (up to 3–4 m with 6–8 s periods) are from the SW–SSW, waves from individual storms may transport sediment either eastward or westward, depending on the direction from which waves propagate toward the shore. Observations of transport processes in coarse sand also do not show

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significant amounts of along-shore transport compared to transport associated with wave forced ripple migration (Traykovski et al., 1999). These observations were made in an environment with waves and currents similar to those at the MVCO node. It is not known if currents are substantially stronger in the southeastern portion of the study area, where the largest dune-like morphology is observed. If the bathymetric expression of sorted bedforms is stable on short time scales where the fine/ coarse sand transitions are unstable and oscillatory, then we must ask what the relationship is between the two. The more ephemeral nature of the coarse/fine sand transition suggests that the bathymetric features, such as the moats, are somehow instrumental in anchoring the CSZ boundaries, keeping them from straying too far from the dictates of the morphologic expression of the sorted bedforms. What is the feedback between the moats and the coarse/fine sand transitions? The presence of medium-grained sands on the shoreface presents us with additional, and possibly related questions. The samples collected from the shoreface (stations 9–13 on Table 1) are similar in grain-size distribution, and well sorted. We can reasonably speculate that this narrow range of grain sizes is somehow selected by the ambient wave energy conditions near the shoreface that are insufficient to transport coarse grains onto the shoreface and too high to retain fine grains. We may therefore question whether the bimodality of the grain-size distribution among the sorted bedforms is a result of sediment transport processes associated with these features, or due to the sequestration of interim grain sizes in the shoreface. In other words, sorted bedform morphology may be a consequence rather than a cause of grainsize bimodality. Questions such as these will require additional bedform modeling and physical oceanographic investigations. The possible sources for these sediment types is considered in the following section. 5.3. Basal horizon The shallow seismic horizon lying below both fine and coarse sands is evidently related to a

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gravel/very coarse sand layer of variable thickness distributed throughout the survey area (Fig. 15). This layer truncates the channel flanks, which were created in a fluvial environment, and caps the channel fill, which is presumably estuarine and beach sediments. We interpret this layer as an erosional lag associated with the transgressive ravinement of the seafloor. This interpretation is consistent with that of Duncan et al. (2000) for the channel-capping ‘‘T’’ horizon on the middle New Jersey shelf, and we use the same identification here (Fig. 15). We speculate that the sediments below this layer are glacio-fluvial source sediments from which sediments were eroded in the formation of the sorted bedforms, both coarse and fine sands. To investigate the possible relationship between surface sands and the presumed ‘‘source’’ sand at depth, we computed average grain-size distributions from two populations of samples (Fig. 16): (1) coarse sands from the lower sections of the cores, below the gravel/very coarse sand layer and removed from any channel-type fill, and (2) coarse sands from grab samples in the vicinity of the cores. The deeper coarse sands are more poorly sorted than the surface coarse sands, with greater proportions of gravel and very coarse sands (4500 mm) as well as fine sands (125–250 mm). It is possible that the well-sorted coarse and fine sands at the surface could be derived from such a source, 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 bedform morphology (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 residing above what we have identified as the ravinement surface. For these sands to have been derived from a common source, such sediment should exhibit large fractions of both fine and coarse grains. The same mass-balance problem is also likely true for the medium sands of the shoreface, although we have no constraints on the volume of those sediments. We therefore suggest that most of the fine and medium sands on the seafloor within the MVCO have been transported there from proximal sources, the location of which is uncertain.

6. Conclusions Our examination of sorted bedform morphology within the Martha’s Vineyard Coastal Observatory (MVCO) reveals a spectrum of bathymetric relief (from
10 cm to
3 m), grainsize contrast (from
250 to 42000 mm) and morphologic forms (dune-forms, steps and moats). At the largest bathymetric relief and grain-size contrast, the sorted bedforms exhibit a dune-form morphology, similar to sand ridges but smaller in scale. At intermediate relief and contrast, downward steps mark the transition from fine to coarse sand, along with moats at the CSZ edges. At the smallest relief and contrast, bathymetric expression is marked primarily by a small moat at one edge. Important commonalities are nevertheless observed across this range in sorted bedform morphology. The CSZs tend to be asymmetric, generally (although not always) with a well-defined edge on one side and a poorly defined or ‘‘feathered’’ edge on the other as observed in the backscatter map. Grain size tends to be larger, and the bathymetry deeper toward the more welldefined edge. Also, the CSZs tend to form a slightly acute angle between the shoreline and the more well-defined edge. These similarities, along with proximity, imply a similarity of process across the spectrum of sorted bedform shapes, sizes and grain-size contrasts. The variations may, perhaps, be related to regional variations in flow intensity, or to varying degrees of modulation by other processes, such as wave forcing.

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The asymmetries in sorted bedform morphology lead us to concur with previous authors who concluded that sorted bedforms are primarily a response to transverse, along-shore flows. However, if true, then the flow regime in this setting must be very complex, as evidenced by the change in the polarity of asymmetry that occurs alongshore and with depth. Additional studies of the physical oceanography of the MVCO will be required to test this prediction. Our observations of temporal changes in coarse/ fine sand boundaries and bathymetric features indicate that, while the bathymetry appears stable over both short and long time scales, the coarse/ fine sand boundaries can migrate tens of meters over spans of just months to years. However, several lines of evidence suggest that the coarse/ fine sand boundaries oscillate, rather than continue to migrate in one direction: (1) push cores gathered at coarse/fine sand transitions showed migration of fine sand over coarse sand and coarse sand over fine sand at each site, (2) there is a clear association of bathymetry, which appears stable, with the grain-size pattern, and (3) some of the observed boundary migration directions are not consistent with the long-term mean current forcing conditions implied by the sorted bedform asymmetries. We speculate that the oscillation of CSZ boundaries is caused by wave forcing. Wave transport processes are strong, but they tend to be directionally scattered and thus unlikely to result 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 (o 1 m depth) seismic horizon, observed consistently beneath the fine sands and frequently beneath the coarse sands. Often as well the horizon is seen to be contiguous beneath both seafloor sediment types, specifically at the less well-defined CSZ boundaries. This horizon is likely derived from a variable-thickness layer of gravel/very coarse sand found below surface sands, both fine and coarse, on most of the cores, and in grab samples gathered from seafloor locations eroded below a surface outcrop of this horizon. We speculate that this layer is an erosional lag representing the trans-

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gressive ravinement surface, and forms a boundary between glacio-fluvial host sediments below and mobile, reworked coarse and fine sands above. The latter may be derived by the former through winnowing, although the volume of fine sands may be problematic and could require transport into the study area from another source.

Acknowledgements The authors gratefully acknowledge a large number of scientists who participated in the collection of the varied data sets incorporated into this study: Bill Schwab and Bill Danforth from the USGS, Woods Hole, who collected the first backscatter and bathymetric data; personnel from SAIC who collaborated with UNH in the collection of Reson multibeam data; Steven Schock, Jim Wulf, Gwendoline Quentin, Pierre Beaujean, Csaba Vaczo, and Hernando Nieto from Florida Atlantic University and Hilary Gittings from the Woods Hole Oceanographic Institution, who collected the chirp seismic data; and Barbara Kraft, Eric Jabs, Peter Simpkin, Andy McLeod, and Jarrod Millar from the University of New Hampshire, who participated in the collection of samples and seafloor acoustic measurements. The authors benefited from discussions with Ron Boyd, of the University of Newcastle, Australia, who spent a semester sabbatical at UNH. Reviews by B. Schwab, M. G. Kleinhams, and an anonymous reviewer resulted in important improvements in the manuscript. Funding for this work was provided by the Office of Naval Research under grants N00014-02-1-0206 (JAG), N00014-02-1-0138 (LM), N00014-01-1-0564 (PT), N00014-01-1-0957 (RLE). UTIG contribution 1712. References Balco, G., Stone, J.O.H., Poter, S.C., Caffee, M.W., 2002. Cosmogenic-nuclide ages for New England coastal moraines, Martha’s Vineyard and Cape Cod, Massachusetts, USA. Quaternary Science Reviews 21, 2127–2135. Cacchione, D.A., Grant, W.D., Tate, G.B., 1984. Rippled scour depressions on the inner continental shelf off central

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