Recent and modern marine erosion on the New Jersey outer shelf

Recent and modern marine erosion on the New Jersey outer shelf

John A. Goff a,T, James A. Austin Jr.a, Sean Gulicka, Sylvia Nordfjorda,Beth Christensenb, Christopher Sommerfieldc, Hilary Olsona, Clark Alexanderd

aInstitute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin,

4412 Spicewood Springs Rd., Austin, TX 78759, USAbDepartment of Geology, Georgia State University, 340 Kell Hall, Atlanta, GA 30303, USA

cCollege of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USAdSkidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USA

Received 15 September 2004; received in revised form 11 February 2005; accepted 16 February 2005

Abstract

Recent chirp seismic reflection data combined with multibeam bathymetry, backscatter, and analysis of grab samples and

short cores provide evidence of significant recent erosion on the outer New Jersey shelf. The timing of erosion is constrained by

two factors: (1) truncation at the seafloor of what is interpreted to be the transgressive ravinement surface at the base of the

surficial sand sheet, and (2) truncation of apparently moribund sand ridges along erosional swales oriented parallel to the

primary direction of modern bottom flow and oblique to the strike of the sand ridges. These observations place the erosion in a

marine setting, post-dating the passage of the shoreface ravinement and the evolution of sand ridges that form initially in the

near shore environment. Also truncated by marine erosion are shallowly buried, fluvial channel systems, formed during the Last

Glacial Maximum and filled during the transgression, and a regional reflector bRQ that is N ~ 40 kyr. Depths of erosion range

from a few meters to N 10 m. The seafloor within eroded areas is often marked by bribbonQ morphology, seen primarily in the

backscatter data as areas of alternating high and low backscatter elongated in the direction of primary bottom flow. Ribbons are

more occasionally observed in the bathymetry; where observed, crests exhibit low backscatter and troughs exhibit high

backscatter. Sampling reveals that the high backscatter areas of the ribbons consist of a trimodal admixture of mud, sand and

shell hash, with a bimodal distribution of abraded and unabraded sand grains and microfauna. The shell hash is interpreted to be

an erosional lag, while the muds and unabraded grains are, in this non-depositional environment, evidence of recent erosion at

the seafloor of previously undisturbed strata. The lower-backscatter areas of the ribbon morphology were found to be a well-

sorted medium sand unit only a few 10’s of cm thick overlying the shelly/muddy/sandy material. Concentrations of well-

rounded gravels and cobbles were also found in eroded areas with very high backscatter, and at least one of these appears to be

derived from the base of an eroded fluvial channel. Seafloor reworking over the transgressive evolution of the shelf appears to

have switched from sand ridge evolution, which is documented to ~ 40 m water depth, to more strictly erosional modification at

greater water depths. We suggest that this change may be related to the reduction with water depth in the effectiveness of

sediment resuspension by waves. Resuspension is a critical factor in the grain size sorting during transport by bottom currents

0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.margeo.2005.02.015

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

E-mail address: [email protected] (J.A. Goff).

Marine Geology 216 (2005) 275–296

www.elsevier.com/locate/margeo

http://www.ngdc.noaa.gov/mgg/coastal/coastal.html

over large bedforms like sand ridges. Otherwise, we speculate, displacement of sand by unidirectional currents will erode the

seafloor.

D 2005 Elsevier B.V. All rights reserved.

Keywords: chirp sonar; grain size analysis; bathymetry; backscatter; sand ridges; ribbons

1. Introduction

The New Jersey continental shelf (Fig. 1) has been

studied extensively since the 1960s. It has evolved

into a premier natural laboratory both for investigating

Neogene–Quaternary stratigraphy as a function of

base-level changes, and for studying seabed evolution

in a sediment-starved setting during the most recent

Fig. 1. Bathymetric map of the mid- and outer New Jersey shelf, with contours in meters and artificial illumination from the North. Inset shows

location within the Mid-Atlantic Bight of the eastern United States. Thin-dashed lines indicate deep-towed chirp seismic profiles collected in

2001 and 2002 aboard the R/V Endeavor. Thick-dashed line indicates the location of the bBarnegat CorridorQ studies of Duncan et al. (2000)

and Duncan (2001). Bathymetry is derived from merged NGDC coastal relief model (http://www.ngdc.noaa.gov/mgg/coastal/coastal.html) and

1996 multibeam data (Goff et al., 1999). Locations of seismic images in Fig. 10a and b are shown.

J.A. Goff et al. / Marine Geology 216 (2005) 275–296276

http://www.ngdc.noaa.gov/mgg/coastal/coastal.html

transgression. Seafloor processes active today create

deposits that provide modern analogs for stratigraphy

observed in seismic records and outcrops. For

example, transgressive sand sheets are thought to be

a source for sand bodies that serve as reservoir rocks

for hydrocarbons (e.g., Ainsworth et al., 1999;

Posamentier, 2002). The modern middle and outer

shelf off New Jersey can be thought of as a major

seismic horizon in formation and a principal boundary

in the eventual sequence stratigraphy, waiting only for

the next eustatic sea-level fall to seal its morphology

beneath a new prograding sediment accumulation.

This paper presents the results of a detailed

examination of shelfal erosion that has occurred in

the present transgressive marine setting. Our study

area is located on the New Jersey shelf between the

Mid-shelf (at ~ 50 m water depth) and Franklin (at

~ 100 m water depth) scarps (Fig. 1). Analysis is

based on an extensive new chirp seismic reflection

data set (Fig. 1) combined with analyses of grab

samples and short cores (Fig. 2), in addition to the

multibeam bathymetry and backscatter data (Fig. 2)

collected earlier. The new data have been collected as

part of the Office of Naval Research’s bGeoclutterQ

program, an effort to investigate acoustic returns

recorded on active sonar systems and interpreted as

potential targets (e.g., submarines), but which in fact

arise from the natural environment. Although marine

erosion has been observed on the middle shelf, in

conjunction with continued sand ridge evolution (e.g.,

Stubblefield and Swift, 1976; Snedden et al., 1999),

on the outer shelf we have identified both localized

a

Fig. 2. Multibeam bathymetry (a) and backscatter (b) data collected in 1996 aboard the CHS Creed (Goff et al., 1999). Contours on both plots in

meters. Artificial illumination of bathymetry in (a) is from the North. Lighter shades in (b) indicate higher backscatter. Filled circles indicate

locations of grab samples collected in 2001 aboard the R/V Cape Henlopen. Numbers indicate station identifications for samples referred to in

Figs. 5–7. Boxes indicate areas covered by Figs. 3a,b and 4.

J.A. Goff et al. / Marine Geology 216 (2005) 275–296 277

and widespread erosion that evidently postdates sand

ridge construction and evolution. These results have

important implications for efforts to model the

formation of shelf strata (e.g., Syvitski and Hutton,

2001), and also lead to questions regarding the

preservability of a transgressive sand sheet in the

stratigraphic record (e.g., Posamentier, 2002).

2. Background: sand ridge evolution

The modern seafloor of the New Jersey shelf has,

as its most prominent constituent, sand ridges, which

represent the primary seabed morphology along most

of the Atlantic continental shelf of the United States

(e.g., Swift et al., 1972). Sand ridges are ~ 1–12 m

tall, spaced ~ 1–5 km apart and are ~ 2–20 km long

(e.g., Swift and Field, 1981; Figueiredo et al., 1981;

Stubblefield et al., 1984; Goff et al., 1999). They form

as shoreface-attached bedforms, oriented oblique to

the shoreline, with angles of ~ 10–608 toward the

dominant along-shore current direction (e.g., McKin-

ney et al., 1974; Swift and Field, 1981; Figueiredo et

al., 1981; Snedden et al., 1994).

A number of hypotheses have been proposed for

the formation of sand ridges along the shoreface (e.g.,

Swift and Field, 1981; McBride and Moslow, 1991;

Boczar-Karakiewicz and Bona, 1986; Huthnance,

1982; Trowbridge, 1995), but thus far without

definitive resolution (see Goff et al., 1999 for a

review). Better understood, however, is the observa-

tion that, once formed, sand ridges become detached

from the adjacent shoreface as sea level rises and

continue to evolve in form, size and grain size

distribution (e.g., Stubblefield and Swift, 1976; Swift

and Field, 1981; Figueiredo et al., 1981; Snedden et

al., 1999). In general, as water depth increases, sand

ridges increase in height, width and area, become

b

Fig. 2 (continued).


more asymmetric (with steeper down-current flanks),

and exhibit lower contrast in grain size between their

up-current and down-current flanks with respect to the

primary coastal flow. These evolutionary processes

have been examined in detail during intensive coring

and 3.5 kHz seismic programs on the New Jersey

shelf that penetrated these ridges in 4–40 m water

depths (e.g., Stubblefield and Swift, 1976; Rine et al.,

1991; Snedden et al., 1999). These studies document

ridge evolution by way of dune-forming processes,

i.e., erosion on the up-current flank, and deposition on

the down-current flank. Subsequent enlargement of

sand ridges in deeper waters is presumably accom-

plished through merging of smaller ridges and

excavation of underlying Pleistocene sands. These

studies confirm that swales between ridges are

erosional features, with excavation up to several

meters below the Holocene sand sheet.

The detailed morphology of sand ridges on the

middle and outer New Jersey shelf has been examined

by Goff et al. (1999), based on multibeam bathymetry

and backscatter data. In water depths less than ~ 40–

50 m, sand ridges exhibit a backscatter asymmetry;

higher backscatter, presumably indicative of coarser

grain sizes, occurs on up-current flanks. Such a grain

size asymmetry would be expected for active dune-

form features, with coarser material preferentially on

eroding flanks. This pattern has also been observed in

samples collected on sand ridges in nearshore as well

as mid-shelf settings (e.g., Swift and Field, 1981;

Stubblefield and Swift, 1981). Up-current flanks also

tend to be less steep and exhibit secondary transverse

bedforms, a feature seen commonly in modern sand

ridge systems (e.g., off Sable Island, as documented

by Dalrymple and Hoogendoorn, 1997). As with the

earlier studies referenced above, Goff et al. (1999)

have concluded that sand ridges in mid-shelf water

depths are still actively evolving.

At water depths N 50 m, however, Goff et al.

(1999) have observed that the asymmetry in back-

scatter between up-current and down-current sand

ridge flanks disappears, replaced by a pattern of

greater backscatter (larger grain sizes?) along crests,

and lower backscatter in swales. Such a pattern has

been predicted by Swift and Field (1981) to be

indicative of a moribund sand ridge, based upon their

expectation that crests would gradually become more

winnowed of fine-grained sediments relative to

swales. Furthermore, sand ridges in deeper waters of

the New Jersey shelf display no evidence of either a

consistent sense of asymmetry in their flanks or

secondary transverse bedforms (Goff et al., 1999).

Kenyon et al. (1981) have also noted, in observations

of sand ridges in the North Sea, that sand waves cease

to be present on the backs of sand ridges in 50–60 m

water depths; they have inferred from these observa-

tions that the ridges are moribund at these depths.

Goff et al. (1999) have speculated that these obser-

vations are indicative of a fundamental change in the

evolution of sand ridges with increasing water depth:

in this deep-water stage, sand ridge surfaces become

armored through the winnowing process against

continued dune-form evolution.

3. Seafloor morphology and bottom

characterization

3.1. Multibeam bathymetry and backscatter

A multibeam bathymetry and backscatter survey

was conducted in 1996 on the middle and outer New

Jersey shelf (Figs. 2 and 3; Goff et al., 1999) as part of

the Office of Naval Research’s STRATAFORM

program (Nittrouer, 1999). This survey employed a

95 kHz, Simrad EM1000 multibeam system mounted

on the CHS Creed. Because incidence angles of this

hull-mounted system are typicallyN308, and because

seafloor slopes are generally much less than 18, the

resultant backscatter map is predominantly a response

to variations in surface roughness and/or sedimentary

properties (Jackson et al., 1986), rather than to

bedform slope (Goff et al., 1999).

Two fabrics dominate the seafloor morphology

between ~ 50 m and 100 m water depths: ~ ENE–

WSW trending sand ridges and ~ NE–SW trending

striations, identified as flow-parallel bribbonsQ by Goff

et al. (1999) using the definition of McLean (1981)

(Fig. 3). The dominant current direction is toward the

SW, with peak velocities of N 40 cm/s measured

during several moderate winter storms (McClennen,

1973; Butman et al., 1979). The two seafloor fabrics

do not coexist spatially; ribbons inhabit low-lying

regions between and around clusters of smooth-sided

ridges. A similar relationship was observed by

Reynaud et al. (1999) in a survey of a large (~ 30 m


high, ~ 5 km wide, ~ 60 km long) sand bank at ~ 140–

170 m water depth west of the English Channel. The

crests of the sand bank appear moribund, with a

coarse grained, high backscattering lag and no active

bedform formation, whereas the lower flanks of the

bank are finer grained, and exhibit abundant dunes

and sand ribbons that appear to be modern responses

to the strong tidal currents that sweep the area.

The low-lying areas of the outer New Jersey shelf

can sometimes be identified as large swales that are,

like the ribbons that floor them, oriented NE–SW,

oblique to the trend of the ridge crests. The ribbon-

Fig. 3. Enlarged portions of multibeam bathymetry (a) and backscatter (b) illustrating several of the primary morphological features discussed in

the text. Locations are shown in Fig. 2. Contours on both plots in meters. Artificial illumination of bathymetry in (a) is from the North. Lighter

shades in (b) indicate higher backscatter. Locations of seismic images in Figs. 9, 11, 12 and 13b are shown in (a).


floored swale identified in the NW corner of Fig. 3a

appears to truncate the sand ridge cluster on its SE

flank. Each type of feature has a characteristic

backscatter response. As noted above, sand ridges

tend to have higher backscatter along their crests (the

sand ridge in the NW corner of Fig. 3a,b is a clear

example), in contrast to sand ridges in shallower

water, where up-current flanks tend to exhibit higher

backscatter (Goff et al., 1999). Ribbons are less

consistent in their response, but crests tend to display

lower backscatter than valleys (Goff et al., 1999).

Clusters of elongated pits ~ 0.5–1.5 km wide, ~ 1–3

km long and up to 10 m deep have been mapped in the

SW part of the multibeam survey (Fig. 4; Goff et al.,

1999). Like the ribbons, these are oriented ~ NE–SW,

along the SW-directed mean current flow direction

(McClennen, 1973; Vincent et al., 1981). The pits are

asymmetric, with steeper slopes on their down-current

ends. These pits represent the strongest evidence,

based on morphology alone, for recent erosion. Goff et

al. (1999) have noted a morphological similarity to

erosional blow-out pits in subaerial desert settings

(e.g., Cooke and Warren, 1973; Thomas, 1989). In

such arid settings, a window in otherwise erosion-

resistant ground vegetation can lead to the formation of

an erosive pit in underlying unconsolidated sands.

These pits migrate downwind by progressively under-

cutting the vegetative cover. This analogy would be apt

in the submarine setting if the subsurface strata are

more easily eroded than seafloor sediment, the latter

perhaps armored as a result of winnowing.

A series of elongated sand-wave fields are present

in the SE corner of Fig. 4, along the top of the

Franklin Scarp. These features, discussed earlier by

Goff et al. (1999), are indicative of high bottom

velocities of ~1 m/s (e.g., Kenyon, 1970; Ashley,

1990). Essentially identical features have been inves-

tigated by Knebel and Folger (1976) near the head of

Wilmington Canyon, ~ 100 km to the SW. Both sets

of features exhibit morphologic and grain size

asymmetries consistent with the SW-directed flow

direction. Knebel and Folger (1976) have inferred that

these sand waves are modern features.

3.2. Grab samples

Nearly 100 grab samples were collected aboard the

R/V Cape Henlopen in 2001 (Fig. 2), then analyzed

Fig. 4. Enlargement of SW portion of multibeam data in a region of elongated scour pits (circled). Location is shown in Fig. 2a. Contours are in

meters, and artificial illumination is from the North. Ribbons, ridges and some outer-shelf sand waves (discussed in Goff et al., 1999) are also

identified. The locations of Core 77 (which sampled clay clasts) and the seismic image in Fig. 14 are also shown.


by Goff et al. (2004) for grain size distribution. In

correlating measures of the resultant grain size histo-

grams to backscatter intensity, they conclude that the

backscatter response of the seafloor (Fig. 2b) is

dominated by the coarse-grained fraction (grain sizes

larger than ~ 4 mm) over mean grain size. The bright

backscatter areas associated with ribbon morphology

(Fig. 3b) are dominated by shell hash (Fig. 5a), while

a few particularly bright backscatter areas (Fig. 3b)

are mostly gravel (Fig. 5b), with rounded cobbles in

some cases N 5 cm in diameter. Grain size histograms

from both high and low backscatter regions of the

ribbon morphology are shown in Fig. 6. The brighter

backscatter sediments are very poorly sorted, in fact

trimodal in their distribution, with significant percen-

tages of both mud and coarse material in addition to

predominant medium sands. Due to the lack of

modern sedimentation on the shelf (e.g., Swift et al.,

1972), the mud in these samples must have been

derived from previously undisturbed strata at these

locations. The lower backscatter sediments are, in

contrast, composed of well-sorted, medium sands,

with negligible amounts of fine or coarse material.

Eighteen hydraulically dampened (slow insertion)

cores, 24�70 cm in length, were also collected aboard

the Cape Henlopen cruise in 2001. The surficial shell

hash prevented core penetration in some areas. One

core (Station 18; Fig. 2) penetrated 23 cm through the

lower backscatter, medium sand to a muddy, sandy

shell hash layer below. Assuming similar stratigraphy

throughout the ribbon morphology, we infer from this

evidence that this layer of shell hash exists broadly

throughout the ribboned regions, either exposed at the

seafloor or covered in places by a thin layer of

medium sand organized in a flow-parallel trend.

Grain size histograms from sediments gathered at

higher and lower backscatter regions of sand ridge

Fig. 5. Photographs of coarse sieve (N4 mm) material sampled at stations 20 (a) and 90 (b). Note the presence of whole shells, shell hash, and

gravel with occasional cobbles. See Fig. 2 for location.

Fig. 6. Grain-size histograms for Stations 17 and 20, representing,

respectively, low and high backscatter regions of ribbon-type

seafloor morphology (Figs. 2 and 3). Values shown include the

overall mean grain size and the percentages of both fine (b63 Am)

and coarse (N4000 Am) fractions. The high backscatter regions are

trimodal, with significant portions of coarse material (shell hash and

gravel) and mud along with medium sand. The low backscatter

regions are well-sorted medium sand. The two samples display

negligible differences in mean grain size.


morphology are presented in Fig. 7. In these areas,

both the higher and lower backscatter sediment

distributions are unimodal. Compared to the lower

backscatter sediments, the higher backscatter sedi-

ments are deficient in grain sizes b 375 Am and

enriched in grain sizes N 500 Am; this observation is

consistent with Goff et al. (1999) speculation that the

high backscatter crests of these outer-shelf sand ridges

are winnowed relative to intervening swales.

Grab samples from both ribbon and sand ridge

environments (Fig. 2) have also been examined for

foraminiferal and mineralogical content. The ribbon-

dominated environments are characterized by a

bimodal benthic foraminiferal population: an inner

shelf (~ 0–50 m water depth), abraded and probably

relict population, and a middle shelf (~ 50–100 m

water depth) population with better preservation,

probably in situ. The preservation of these samples

is variable; replacement by pyrite occurs and is an

indicator of diagenesis. The mineralogy of samples

within the ribbon environments also reflects at least

two populations of quartz grains: (1) angular and clear

and (2) rounded, frosted and often iron-stained. The

second population is indicative of grains that have

been reworked, either at the present-day seafloor or

possibly within glaciers and then transported to the

shelf as ice-rafted debris. The first population,

however, provides evidence of a fresh source, i.e.,

material derived from previously unreworked sedi-

ments in strata being eroded at the seafloor.

Foraminiferal populations in the sand ridge sam-

ples are either absent or very rare and abraded; where

they occur, they indicate a relict, inner shelf environ-

ment. The mineralogy of the sand ridges is dominated

by subrounded quartz grains with lithic fragments.

Hence, unlike the mixed grains in the ribbon

morphology, grains of the sand ridge sediments at

the seafloor have all been reworked.

4. The shallow stratigraphy of the New Jersey shelf

The interpretation of the New Jersey shelf shallow

stratigraphy (Fig. 8) provides a primary constraint on

the timing of erosion and the nature of material being

excavated for seafloor features. Our understanding of

this stratigraphy has evolved considerably over the

last several decades as more samples have been

obtained and more data have been collected with

newer and higher resolution acoustic imaging sys-

tems. More complete descriptions and references are

available from Duncan et al. (2000), Duncan (2001),

and Gulick et al. (2005).

The bRQ horizon (Fig. 8) is a regionally recog-

nized, high-amplitude reflection that forms the base

of the outer shelf wedge. The origin of this surface is

uncertain, but its age (~ 40 kyr) is well constrained

to predate the latest sea level regression. The outer

shelf wedge is a prograding series of offlapping

strata deposited on the bRQ horizon as a regressive

deposit. The bChannelsQ horizon (bCQ, Fig. 8) marks

an incised valley system that erodes into both the

outer shelf wedge and bR.Q Fill ages of ~ 12.5 kyr

indicate that these channels were active during the

latest lowstand and then filled as the shoreline

migrated through this region. The bTQ horizon (Fig.

8) is an intermittent reflection that, in its most

consistent manifestation, caps channel-fill strata and

occasionally truncates the bChannelsQ horizon. Dun-

can et al. (2000) interpret bTQ as the transgressive

ravinement surface associated with Holocene sea-

level rise. This horizon represents a critical chro-

nostratigraphic boundary: the passage from shoreface

to marine sedimentary environment.

Other than the sediments sequestered in channel

fills, the surficial sand sheet, ranging from ~ 0 to 20 m

Fig. 7. Grain-size histograms for stations 85 and 86, representing,

respectively, high and low-backscatter regions within the sand

ridge-type seafloor morphology (Figs. 2 and 3). Values shown

include the overall mean grain size and the percentages of both fine

(b63 Am) and coarse (N4000 Am) fractions. Compared to the

samples in Fig. 6, these samples are unimodal, though skewed, and

well-differentiated by their mean grain sizes.


thick, represents the only Holocene deposition on the

New Jersey shelf. Transferred from the shoreface to

the inner shelf during transgression, these sands are

organized primarily into oblique sand ridges. Net

sediment input to the middle and outer shelf is

negligible at present (e.g., Swift et al., 1972);

processes active on the shelf primarily involve

continued reworking and erosion of the sand sheet,

driven by storm waves and bottom currents (e.g.,

Swift et al., 1972).

The seaward edge of the mid-shelf wedge (Fig. 8;

Knebel et al., 1979; Milliman et al., 1990) forms a

prominent topographic feature on the New Jersey

shelf: the Mid-Shelf Scarp (MSS; Fig. 1), or bshoreQ

(e.g., Swift et al., 1980), sometimes referred to as the

Tiger Scarp (e.g., Knebel et al., 1979) or Fortune

bshoreQ (e.g., Dillon and Oldale, 1978). Thought for

decades to represent a fossil shoreface associated

with a sea-level stillstand (e.g., Veatch and Smith,

1939; Emery and Uchupi, 1972; Dillon and Oldale,

1978; Swift et al., 1980), high-resolution seismic

data has since shown it to be depositional in origin

(Knebel et al., 1979; Milliman et al., 1990; Duncan,

2001). Uchupi et al. (2001) have speculated that the

mid-shelf wedge could be a subaerial deposit

associated with massive outflows from breached

glacial lakes along the Hudson Valley that were

known to have occurred, between ~ 19 and 12 kyr,

during glacial retreat. However, using high resolution

chirp data collected along the Barnegat Corridor

where it crosses the scarp (Figs. 1 and 2), Duncan

(2001) ascertained that at least the outer part of the

wedge consists of a prograding sequence that lies

above the bTQ horizon (the bHoloceneQ mid-shelf

wedge in Fig. 8), and should therefore have been

deposited in the transgressive marine environment:

perhaps ~ 9–10 kyr or later based on eustatic curves,

after the postulated lake collapses. The inner part of

the mid-shelf wedge also includes a substantial

thickening of the Pleistocene sediments between

bRQ and bTQ (the bPleistoceneQ mid-shelf wedge in

Fig. 8; Duncan, 2001). Dates for these muddy

sediments (Fig. 8) are derived from Knebel et al.

(1979).

5. Observations related to marine erosion in chirp

seismic data

The seismic data considered here comprise 2-D

ultra-high resolution chirp profiles collected over a

broad region of the middle and outer New Jersey

shelf south of the Hudson Shelf Valley and Canyon

(Fig. 1). Track line spacing varied from 50 m to 1

km. Shallowly buried incised-valley systems, which

were considered as possible sources of acoustic

clutter, were the primary targets of the survey

(Nordfjord et al., 2005). The data were collected

using a deep-towed chirp sonar aboard the R/V

Endeavor cruise EN359 in August–September, 2001.

Additional data in the southern part of the survey

area (Fig. 1) were collected with the same system

aboard Endeavor during cruise EN370 in May, 2002.

This system is well suited for high-resolution

imaging of the shallow, complex late Quaternary

stratigraphy of the New Jersey shelf. The source

emitted swept-frequency signals of 1–4 kHz and 1–

15 kHz. The 1–4 kHz data employed in this study

Fig. 8. Schematic illustration of the primary stratigraphic units and horizons discussed in the text, as interpreted by Duncan et al. (2000) and

Duncan (2001) in the context of radiometric ages available at that time. The profile is based on data along the Barnegat Corridor (Fig. 1),

spanning ~50 km; vertical exaggeration is very high.


have vertical resolution of ~ 20 cm, with a penetration

in places of N 30 m below the sea floor.

Schlumberger’s Geoquest IESXR software was

utilized in the interpretation of seismic profiles and

mapping horizons. In this manuscript, we focus

specifically on seismic evidence for marine erosion

of the outer New Jersey shelf, the most recent stage in

its evolution.

5.1. Outcropping horizons

The truncation of strata at the seafloor provides

direct evidence for erosion. Truncation of the bTQ

horizon in particular indicates erosion in the marine

setting, after the shoreface migrated across the region.

Fig. 9 displays a chirp seismic cross-section through

the eroded flank of a sand ridge, where both the bTQ

and bRQ horizons outcrop at the seafloor. This profile is

adjacent to the Barnegat Corridor profile (Fig. 1)

presented by Duncan et al. (2000), along which the

truncation of these horizons was first documented. In

Fig. 9, we observe the truncation of bRQ by bChannelsQ

incisions, and the capping of those incisions by the bTQ

horizon. (The seismic signal becomes increasingly

attenuated under the sand ridge.) The bTQ and bRQ

horizons outcrop along the NW flank of the sand

ridge, at a ~ 4–5 km-wide, ribbon-floored swale

oriented NE–SW (Fig. 3a). As noted earlier, this

swale is oblique to the more ENE–WSW trend of the

sand ridges. A minimum of ~ 4 m of marine erosion,

the distance between the projected outcrop of bTQ

and the floor of the swale, is documented (Fig. 9).

Steepening of the seafloor occurs between the

outcropping bTQand bRQ horizons, whereas the seafloor

flattens below the bRQ horizon (Fig. 9). This geometry

may be related to the different material properties of the

units bounded by these horizons. Samples taken from

between bTQ and bRQ are mostly fine-grained muds/

clays (Knebel and Spiker, 1977; Buck et al., 1999;

Nordfjord et al., 2002), whereas above bTQ lies the

surficial sand sheet of medium to coarse grains, and

below bRQ are coarse grained and nearly lithified

sediments (Davies et al., 1992, Nordfjord et al.,

2002). If the clays between bTQ and bRQ are not

overconsolidated, they will be more easily eroded than

the sands above and below.Where a more erodable unit

outcrops from below a resistant unit, the seafloor will

have a higher gradient because it is easily excavated

downwards while being constrained from eroding

laterally by presence of the overlying unit. The lithified

coarse sands below bRQ would also clearly impede

further excavation, so the swale will instead grow

laterally by preferential erosion of the finer-grained

unit and undercutting of the resistant sand sheet above.

Another ~ 8–9-km-wide erosional swale is present

seaward of the MSS, north of a sharp bend in its strike

(Fig. 1). Seismic cross-sections across the scarp,

forming the NW flank of the swale, and across the

edge of a sand ridge complex forming the SE flank,

are displayed in Fig. 10. Again, we recognize the three

Fig. 9. Deep-towed chirp seismic image (1–4 kHz) across the SW flank of an erosional swale, which truncates a sand ridge complex. See Fig. 3

for location. Seismic horizons bRQ, bTQ and bChannelsQ (C) are identified. Minimum depth of marine erosion (~4 m) is noted as the difference

between maximum swale depth and projection of bTQ. The steepening of the seafloor between the bTQ and bRQ outcrops is also noted. Depth

conversion is based on water velocity (1500 m/s).


primary horizons that define the shallow stratigraphy:

bRQ, bChannels,Q which is above and incises bR,Q and

bTQ, which truncates incision flanks and fill. Likewise,

both bTQ and bRQ outcrop at the flanks of the swale,

and the seafloor steepens between the bTQ and bRQ

outcrops, leveling out where material below bRQ is

eroded (Fig. 10). From the geometry, we again infer

that the unit between bRQ and bTQ is more easily

eroded than the capping sands or the sediment below

bRQ. The thickness of the unit between bRQ and bTQ is

greater than in Fig. 9; consequently, the minimum

demonstrable marine erosion is higher: ~ 12 m at the

MSS (Fig. 10a) and ~ 6 m at the conjugate flank of the

erosional swale (Fig. 10b).

These seismic observations at the MSS contrast

with those of Duncan (2001), who ascertained that

the scarp is, at another location, the seaward edge of

a transgressive depositional wedge. Duncan (2001)

seismic crossing of the scarp, along the Barnegat

Corridor, is south of the sharp bend in the scarp

(Fig. 1). The MSS abruptly shoals southward by

~10 m at the bend, and no longer forms the NW

flank of the erosional swale identified in Fig. 10.

The swale instead continues along its southwesterly

trend for nearly 10 km south of the bend. This

morphology suggests that the bend represents an

important boundary along the scarp: primarily ero-

sional to the north, and constructional to the south.

Erosional truncation of strata at the seafloor is not

always confined to NE–SW trending swales. In water

depths more than ~ 75 m, truncations are observed

over broad areas, with no acoustic or grab sample

evidence of the transgressive sand sheet. Fig. 11a

presents a seismic section demonstrating seafloor

Fig. 10. Deep-towed chirp seismic image across the NW (a) and SE (b) flanks of a large erosional swale. See Fig. 1 for location. Seismic

horizons bRQ, bTQ and bChannelsQ (C) are identified. Minimum depths of marine erosion (~12 m on the NE flank; ~6 m on the SE flank) are

noted as the difference between maximum swale depth and projection of bTQ. The steepening of the seafloor between the bTQ and bRQ outcrops is

also noted. Depth conversion is based on water velocity (1500 m/s).


truncation of the bChannelsQ horizon, dipping strata

within the outer shelf wedge, and the basal bRQ

horizon. The bTQ horizon outcrops at the NW end of

the section; the surficial sand sheet is otherwise absent

throughout this area with up to ~ 7 m of indicated

minimum marine erosion. We also find that ribbon

morphology (Figs. 2 and 11b) is only present where

the sand sheet is absent; we can therefore use

backscatter data to infer sand sheet absence where

we have no other seismic constraints.

There is no evident correlation between bathymetry

and the alternations of high and low backscatter (Fig.

11b) that define the ribbons here. This observation is

not surprising given the core evidence. As noted

above, the lower backscatter portions of ribbons in the

area covered by Fig. 11 represent a thin (~ 20–30 cm)

layer of medium sand overlying a surficial shell hash

layer that accounts for the high backscatter; this

thickness represents a small contribution to the ~ 1–2

m undulations evident on the profile. However, some

ribbons do exhibit clear bathymetric expressions (Fig.

3a; Goff et al., 1999); these ribbons tend to be more

linear than those that exhibit only a backscatter

signature (see the following section).

5.2. Channel remnants

Erosion of channel-fill strata has resulted in both

bathymetric and backscatter expressions of channels

fills at the seafloor in a few locations. The gravel

mound identified in Fig. 3 is one prominent example.

The mound was imaged by a hull-mounted chirp

system on the R/V Endeavor. These data are not

nearly of the same quality as the towed system, but

they nevertheless reveal primary structure beneath the

mound (Fig. 12): a prominent undulating reflector that

outcrops at the seafloor on either side of the mound.

The depth of this basal reflector is similar to the bases

Fig. 11. (a) Deep-towed chirp seismic image across the landward edge of the outer shelf wedge, which outcrops at or near the seafloor. See Fig.

3 for location. Seismic horizons bRQ and bChannelsQ (C) are identified. Minimum depth of marine erosion (~7 m) is noted as the difference

between maximum swale depth and projection of bTQ. Depth conversion based on water velocity (1500 m/s). (b) Filtered backscatter (solid) and

residual multibeam bathymetry (dashed) values sampled along the same profile as (a), which is within ribbon-type seafloor morphology,

identified by the alternations between high and low backscatter. Bathymetry profile was detrended with a linear regression. No evident

correspondence can be found in this example between backscatter variations associated with ribbon morphology and bathymetric variations.


of presumed fluvial channels mapped elsewhere in the

survey area (Duncan et al., 2000; Nordfjord et al.,

2005). Subhorizontal reflectors lie above this reflector

within the mound, suggestive of fill strata within

incisions elsewhere; two of these reflectors are also

truncated by the seafloor. Rounded cobbles sampled

from this gravel-prone mound (Fig. 5b) are compat-

ible with the type of lithology expected from the

thalweg of a fluvial channel. All of this evidence

suggests that this structure is the eroded remnant of a

fluvial channel, 2–2.5 km wide, contemporary with

and similar in scale to channels mapped just to the

north by Nordfjord et al. (2005). The mound itself

appears to comprise the uppermost two observed

strata of the interpreted exposed fill sequence (Fig.

12); the mound stands above the surrounding seafloor.

The NE slope, facing into the primary SW-directed

current direction (McClennen, 1973; Vincent et al.,

1981), is steeper, suggesting erosive undercutting of a

resistant upper stratum. The bTQ horizon is not evident

in Fig, 12, presumably because it has been eroded

away.

A small set of ~ 4-km-long ribbon structures are

observed in the NE corner of Fig. 3. They have both

bathymetric and backscatter expressions: low back-

scatter along the crests, and high backscatter in the

troughs. They are located just seaward of a sand ridge

complex, and are very limited in their spatial extent.

The topographic highs of the ribbons are asymmetric

in that they taper to the NE, into the primary current.

The seismic data (Fig. 13) reveal that these ribbons are

essentially carved out of the fill strata of a partially

eroded channel N 2 km wide. Beneath the sand ridge

complex, the channel is undisturbed (Fig. 13a),

revealing the bTQ horizon. The channel becomes

progressively eroded seaward of the sand ridge

complex, with a projected minimum of ~ 7 m of

material removed at the eastern edge of Fig. 13b.

Fig. 12. (a) Hull-mounted chirp seismic image crossing a mound where rounded gravel and cobbles have been sampled (Fig. 5). See Fig. 3 for

location. Depth conversion is based on water velocity (1500 m/s). A sketch interpretation of the observed seismic horizons is provided below the

seismic image, revealing a broad, undulatory reflection with flat-lying horizons above it. We interpret this reflection as the base of a wide,

eroded channel. (b) Filtered backscatter values sampled along the same profile as (a). Vertical lines indicate the boundaries of the high

backscatter anomaly associated with the gravel, and demonstrate that the gravel mound is coincident with the upper strata of the eroded fill units.


Fig. 13. Deep-towed chirp seismic image across (a) uneroded and (b) eroded portions of a major trunk channel (Nordfjord et al., 2005). See Fig.

3 for location. Seismic horizons bTQ and bChannelsQ (C) are identified. Minimum depth of marine erosion (~7 m) is noted as the difference

between maximum swale depth and projection of bTQ. Depth conversion is based on water velocity (1500 m/s). (c) Filtered backscatter (solid)

and multibeam bathymetry (dashed) values sampled along the same profile as (b), which is within one of a few regions of the survey where

ribbon-type morphology, identified by the alternations between high and low backscatter, correlates fairly well to bathymetric variations. As

noted with vertical dashed lines, the lowest backscatter anomalies correlate with ribbon crests (marked with connecting vertical lines), likely

indicating that the crests are finer-grained/depositional. See text for additional discussion.


5.3. Erosion pits

The deep-towed chirp seismic data reveal that the

erosion pits (Figs. 4 and 14) are excavated out of the

laminated strata of the outer shelf wedge, often

creating depressions N 10 m deep. The bChannelsQ

horizon and fill strata are also truncated within some

of the pits. Grab samples within the pits are clay rich.

Around the pits, a surficial sand sheet is not evident

from the seismic data; no bTQ horizon is observed and

the buried channels are truncated by the seafloor. The

seafloor is instead largely covered by ribbon mor-

phology (Fig. 4), alternating between sediment

distributions similar to those shown in Fig. 6.

Abundant coarse material is present at or near the

seafloor. Core 77, taken just south of an erosion pit

field (Fig. 4), penetrated through a ~ 10-cm-thick

surface layer of medium sands and shells to sample

clay clasts mixed with medium sand. Such a

stratigraphy would support our hypothesis for the

formation of these pits as downstream-progressing

features which continue to undermine a surficial

armoring layer. Clay clasts are known to be indicative

of vigorous erosion in the presence of sand, which in

this core may be a remnant of the transgressive

ravinement. Alternatively, Fulthorpe and Austin

(2004) suggest that the vigorous erosion indicated

by Core 77 occurred before the transgression, a result

of possible massive outflows during glacial lake

collapse ~ 14–19 kyr.

6. Discussion

6.1. Timing and extent of marine erosion

Our analysis indicates a genetic relationship

between marine erosion and morphologic structures

oriented along the primary direction of bottom

current flow: large swales that cut across the sand

ridge trend, asymmetric erosional pits and ribbons.

The swales and pits represent direct excavations of

seabed sediment. The large swales are (where

mapped with high resolution swath data) also floored

by ribbon morphology. Our primary stratigraphic

constraint on the timing of this erosion comes from

the truncation of the transgressive ravinement surface

(bTQ) at the seafloor (Figs. 9–11): the erosion

therefore post-dates the passage of the shoreline

and the formation of the Holocene sand sheet. By

this constraint alone, it is possible that the erosion

occurred in the near-shore environment as, for

example, within the swales of near-shore sand ridges,

which are observed in some cases to excavate below

the ravinement (e.g., Snedden et al., 1994). However,

the scale (N5 km wide) and orientation (contour

parallel) of the erosional swales on the outer New

Jersey shelf have no known analogy in the near-

shore, inner shelf or middle shelf environments,

which are dominated by sand ridge morphology

trending oblique to the contours, with swale widths

generally less than ~ 2 km. Nor have asymmetric

Fig. 14. Deep-towed chirp seismic image across some of the erosional scour pits, within the Outer Shelf Wedge. The deepest pit shown here is

over 10 m deep. See Fig. 4 for location. Seismic horizons bRQ and bChannelsQ (C) are identified.


erosion pits been observed in those depths. While

sand ridges are known to be active and evolving up

to ~ 40 m water depth (e.g., Stubblefield and Swift,

1976; Rine et al., 1991; Snedden et al., 1999;

Snedden and Dalrymple, 1999; Goff et al., 1999),

those on the outer New Jersey shelf are apparently

moribund (as evidenced by the pattern of greater

grain sizes at their crests, and lack of secondary

bedforms on their flanks; Goff et al., 1999);

furthermore, sand ridges are truncated in some cases

by the large erosional swales. We infer, therefore,

that such swale erosion post-dates the evolution of

sand ridges. By these morphological arguments, we

argue that the swales and pits are largely products of

water depths at or near present levels, although it is

possible that such marine erosion initiated at

locations that had been previously eroded.

Ribbons with clear bathymetric expression, such as

those shown in Fig. 13b,c, may also be derived by

excavation. However, most of the ribbons observed in

the survey area are constructional in nature, produced

by a thin, patchy layer of medium sand overlying a

shell hash layer with mud and sand. The overlying

sand may have been derived from the underlying

shelly/muddy/sand layer. Evidence from grab sample

suggests that the shelly/muddy/sand layer is a direct

product of erosion. The shells themselves are presum-

ably a lag, the immovable detritus of meters of material

removed from the seafloor at these locations. Much of

the shell hash may, in fact, have been concentrated at

the ravinement surface itself (e.g., Posamentier, 2002)

prior to marine erosion. Because of the lack of modern

sediment input to the New Jersey shelf (e.g., Swift et

al., 1972), the muds that are incorporated in this layer

must be derived from the underlying Pleistocene strata

being eroded. The bimodal distribution of abraded and

fresh sand grains indicates excavation of previously

undisturbed sediment mixed with reworked sand. Both

observations are strong indicators that erosion in these

locations is ongoing; otherwise, the fines would be

winnowed and the grains abraded rapidly. These

ribbons, while constructional, can therefore be con-

sidered indirect indicators of modern erosion. We also

assume that gravel patches (Figs. 2a and 3) represent

erosional lags.

Based on the seismic data and inferences from

morphology, we have generated an interpretive map

indicating areas of significant marine erosion on the

New Jersey outer shelf, between the Franklin and

Mid-Shelf scarps (Fig. 15). At minimum, several to

~ 10 m of sediment have been removed by this

erosion, and perhaps more, depending on the preex-

isting thickness of the transgressive sand sheet in

these areas.

6.2. Transition from sand ridge evolution to marine

erosion

The evolution of sand ridge morphology docu-

mented throughout the inner and middle shelf (e.g.,

Stubblefield and Swift, 1976; Swift and Field, 1981;

Figueiredo et al., 1981; Snedden et al., 1994, 1999;

Snedden and Dalrymple, 1999) comes to an end on

the outer shelf. In water depths N 50 m, sand ridges

appear morphologically moribund, perhaps shielded

against further sediment transport (i.e., reworking) by

the coarse armor of winnowed sands. However, as

noted above, the seafloor shows evidence of recent

and modern marine erosion over broad areas of the

outer shelf, in some cases demonstrably cutting sand

ridges (Fig. 9).

That the outer shelf is experiencing erosion today

is perhaps not surprising, given earlier oceano-

graphic observations. Peak bottom current velocities

of over 40 cm/s have been measured in this region

(McClennen, 1973; Butman et al., 1979), with mean

flows during storm events in excess of 30 cm/s

(Butman et al., 1979). Although these velocities are

not theoretically considered adequate to initiate

movement of sand grains coarser than ~ 200 Am

(e.g., Miller et al., 1977), Butman et al. (1979) note

in their discussion that orbital wave motion and

bottom roughness will increase bottom stress,

enhancing the ability of unidirectional currents to

move sand grains. Butman et al. (1979) have

photographed several episodes of sediment move-

ment and resuspension on the New Jersey outer

shelf associated with peak current and/or wave

activity. These observations document that, under

typical winter conditions, sediment transport at the

depths considered here is frequent. In addition,

elongated sand wave fields toward the shelf edge

(Fig. 4; Knebel and Folger, 1976) provide morpho-

logical evidence of strong current velocities.

An important outstanding question that needs to be

addressed is: why do sand ridges cease to evolve in


water depths N 50 m, despite evidence of frequent

transport of sands in those water depths? One

explanation may be that without sufficient replenish-

ment by new sediment from the continent, the

evolution of sand ridges during a transgression will

lead to a gradual coarsening of the unit through

winnowing, and thus towards an ever-increasing

resistance to reworking. Moribund ridges may simply

be the oldest. However, we do not observe any clear

difference in mean grain sizes from ridges sampled in

this survey and those sampled in shallower water by

Goff et al. (2000).

The second speculation is that storm waves, in

addition to unidirectional currents, are in some way

essential for the evolution of sand ridges. The

influence of waves on bottom water velocities

generally decreases with water depth, in terms of the

percentage of time that waves will generate sufficient

velocity to move sand grains (McClennen, 1973).

Thus, waves will have a decreasing effect on sand

ridges as a function of depth. Butman et al. (1979)

have noted large differences in the movement of

sediments during high current events, with or without

waves: large wave-induced flow results in significant

resuspension of sediments, which are then moved

rapidly by currents, whereas unidirectional currents

without waves move sediment primarily through

bedform migration. Butman et al. (1979) conclude

Fig. 15. Interpreted regions of marine erosion on the outer New Jersey shelf (hachured). Minimum amount of marine erosion is identified at

those locations where we can confidently do so. See text for discussion.


that wave-induced bottom currents result in much

larger bottom stresses compared to unidirectional

flow, a conclusion that is consistent with modeling

predictions (e.g., Komar, 1976). Resuspension may be

critical to the dune-form evolution of the sand ridge,

allowing grains, while they are transported along the

primary flow direction, to be sorted in relation both to

their fall velocities and with respect to differences in

bottom shear stress associated with lee and stoss

flanks of the ridge. Unidirectional flows without wave

resuspension will, in contrast, simply move sediment

along the bottom, a process that does not allow for

sorting over the large scales associated with sand

ridges. Therefore, water depth, rather than age, would

be the determining factor in shutting off the evolution

of sand ridges. We suspect that both factors, pro-

gressive armoring and increasing water depth, lead to

the shutdown of sand ridge evolution as transgression

progresses.

7. Conclusions

In this paper, we have documented widespread

marine and likely modern erosion on the outer shelf

off New Jersey, through analysis of chirp seismic

and grab sample data, as well as seafloor morphol-

ogy. The chirp seismic data provide a key strati-

graphic constraint by defining the presence and

lateral extent of the bTQ horizon, interpreted by

Duncan et al. (2000) as the transgressive ravinement

surface, which serves as the base of the transgressive

sand sheet. The sand sheet is emplaced on the

ravinement surface after the shoreline migrates

landward during a sea-level rise, and is organized

seaward of the shoreface into oblique sand ridges

that continue to evolve as sea level continues to rise

(e.g., Swift et al., 1972).

Truncation of the bTQ horizon at the seafloor

indicates that erosion has occurred in the trans-

gressive marine setting, after migration of the

shoreface across this region. A number of earlier

studies have observed basal reflector truncations

within sand ridge swales in middle shelf settings,

and such erosion is therefore presumed to be

associated with the continued evolution of these

features (e.g., Stubblefield and Swift, 1976; Rine et

al., 1991; Snedden et al., 1999). However, our data

have revealed unexpected marine erosion located

within large swales oriented ~ NE–SW, along the

SW-directed primary current flow (McClennen,

1973; Vincent et al., 1981) and cross-cutting the

oblique-to-flow sand ridge orientation on the outer

shelf. This erosion is therefore not associated with

sand ridge development. Since the geomorphology of

outer shelf sand ridges (i.e., higher grain sizes at

crests, indicative of winnowing, and lack of secon-

dary transverse bedforms on flanks) likely indicates

that they are moribund, the marine erosion we

observe on the outer shelf appears to post-date these

sand ridges.

Other important stratigraphic indicators of recent

erosion included exposures at the seafloor of: (1)

buried fluvial channels (formed at or near the LGM)

and fills (emplaced during the transgression), (2) outer

shelf wedge laminated strata (deposited ~ 40–27 kyr),

and (3) the regional bRQ horizon (formed in part by

erosion of material older than ~ 40 kyr). We estimate

minimum depths of marine erosion ranging from a

few meters to N 10 m.

Flow-parallel ribbon morphology appears to be

genetically linked to eroded surfaces. Where the

ribbons have both bathymetric and backscatter

expressions, their morphology appears to represent

an excavation of seabed material. However, much of

the ribboned areas have clear expression only in the

backscatter data; these features have been found to

represent alternations between a shelly, muddy sand

(yielding high backscatter) and a thin layer (~ 20–30

cm) of well-sorted medium sand (yielding low

backscatter) drifted atop the shelly, muddy sand.

The shell hash presumably represents an erosional

lag, in part perhaps a remnant of the transgressive

ravinement surface; whereas the presence of mud

and unabraded mineral grains indicates modern

excavation of Pleistocene sediments stratigraphically

older than the transgressive sand sheet. Rounded

gravels are also observed in other areas of the

seafloor, some notably from near the base of eroded

fluvial channels, but one large patch broadly

distributed (Fig. 2b). These are all probable erosional

lags.

An important question immediately arises from

the previous conclusion: why do sand ridges become

moribund on the outer shelf, when there is ample

evidence of sufficient modern bottom currents and


wave forcing to move and erode sands (e.g.,

McClennen, 1973; Butman et al., 1979)? One

speculation we raise centers on progressive sand

ridge evolution, suggesting that continued reworking

with time eventually winnows the sand ridges to the

point where they are armored against further grain

movement. However, there is no clear difference in

grain sizes between middle (Goff et al., 2000) and

outer shelf sand ridges sampled here to justify that

claim. Alternatively, water depth may play a central

role in reducing the influence of storm waves, which

create far larger bottom stresses than unidirectional

currents (e.g., Komar, 1976) and are much more

likely to suspend sediment grains (Butman et al.,

1979). We suggest that suspension of sediments is

critical to the progressive sorting of grain sizes

during sediment transport by currents and, ultimately,

the continued evolution of sand ridges in deeper

water.

Acknowledgements

The authors thank the science parties and crews

aboard the CHS Creed, RV Cape Henlopen and RV

Endeavor cruises. We particularly thank S. Schock

and his associates at Florida Atlantic University for

their collaboration on collection of the chirp seismic

data, and Steffen Saustrup of UTIG for his work in

processing the data. We owe a great debt of gratitude

to ONR program managers who have seen the

Geoclutter program through, including Joe Kravitz,

Jeff Simmen and Tom Drake. Comments by Craig

Fulthorpe of UTIG, and reviews by Don Swift of Old

Dominion University, an anonymous reviewer and the

Marine Geology editor on earlier drafts helped to

improve the text. This work was funded by ONR

under grants N00014-0-1-0844 and N00014-4-1-

0038. UTIG contribution #1756.

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Recent and modern marine erosion on the New Jersey outer shelf

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