Marine Geology 266 (2009) 232–243

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Marine Geology

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Shallow stratigraphy and complex transgressive ravinement on the New Jerseymiddle and outer continental shelf

Sylvia Nordfjord a,b, John A. Goff c,⁎, James A. Austin Jr. c, Laurie Schuur Duncan d

a StatoilHydro, Forusbeen 50, 4035 Stavanger, Norwayb University of Stavanger, 4036 Stavanger, Norwayc Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, J.J. Pickle Research Campus, Bldg. 196 (ROC),10100 Burnet Road (R2200), Austin, TX 78758, USAd Potato Hill Science Writing, 1911 Calle Caliche, Austin, TX 78733, USA

⁎ Corresponding author. Tel.: +1 512 471 0476.E-mail address: goff@utig.ig.utexas.edu (J.A. Goff).

0025-3227/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.margeo.2009.08.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 March 2009Received in revised form 13 August 2009Accepted 30 August 2009Available online 11 September 2009

Communicated by J.T. Wells

Keywords:New Jersey shelftransgressive ravinementHolocenePleistoceneshelf sediment wedgechirp seismic

The ravinement surface is a key marker for examining the effects of the most recent sea-level transgressionon development of the latest Pleistocene–Holocene stratigraphic record of the New Jersey middle and outercontinental shelf. This surface is marked by a moderate-amplitude reflector, identified as the “T” horizon inchirp seismic data, which caps filled incised valleys formed during or soon after the Last Glacial Maximum(LGM) and eustatic lowstand, and also serves as the base of a Holocene sand sheet. The “T” horizon is clearlyerosional; we interpret it as marking landward migration of the shore zone across the coastal plain duringthe earliest-mid Holocene. We use chirp seismic surveys acquired in 1999 and 2001 to map thegeomorphology of the “T” horizon over a ~100 by 60 km region. Shallow, bathymetric contour-paralleldepressions, ~10 km wide and ~8 m deep, are present in the horizon at ~50–60 m and ~70 m water depths;we interpret them as preserved lagoonal/back-barrier topography; portions of filled tidal channels occur inthese depressions. Perhaps these depressions represent pauses in the retreat of the shoreline, either as aresult of eustatic fluctuations or slope steepening, allowing for formation of back-barrier/lagoonal featuressubstantive enough to resist complete destruction by subsequent ravinement. Alternatively, they may belocations of local flattening in the antecedent topography that forced rapid drowning and preservation ofback-barrier morphology. These paleo-estuarine/back-barrier features may also be more susceptible toerosion during transgression, so the presence of these depressions might have focused marine erosion insome way, perhaps by promoting near-bottom turbulence during storm events.Seismic stratigraphy and sediment cores show that the mid-shelf wedge (MSW) is composed of clay-richlatest Pleistocene and sand-rich Holocene components. Latest Pleistocene sediments of the MSW arecomplex, representing both erosional and depositional processes. In part, the seaward boundary of this partof the MSW is defined by the mid-shelf scarp (MSS), an erosion product of transgressive ravinement~11.4 ka. The Pleistocene MSW appears to predate post-LGM glacial lake collapses to the northwest, whichhave been suggested as sources for subaerial, lobate deposition north of the Hudson Shelf Valley (HSV). Theprimary origin of this part of the MSW may instead be subaqueous deltaic formation associated with fluvialdrainage, perhaps part of the ancestral Hudson River system, prior to the LGM. In contrast, the Holocene,sandy part of the MSW must have been deposited in a marine setting, because this deposition postdatestransgressive ravinement. The Holocene MSW may be derived from Hudson river sediments, or fromcontinuing shallow submarine erosion following landward shore-zone passage. The preexisting PleistoceneMSW provided accommodation and a source of reworked sediment for this deposit at its toe. Published sea-level curves confirm that Holocene deposits postdate known glacial lake collapses behind terminal morainesto the north. Erosion on this margin continues and, as a result, the maximum flooding surface has not yetformed.

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1. Introduction

Transgressive deposits on continental shelves are of interest inboth applied and academic researches (Cattaneo and Steel, 2003). Forexample, transgressive sands are commonly more mature bothtexturally and mineralogically than regressive sands, and therefore

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often make excellent hydrocarbon reservoirs (e.g., Snedden andDalrymple, 1999; Posamentier, 2002). Transgressive sands are alsoextensivelymined as aggregates for beach nourishment and industrialapplication (e.g., see the European Marine Sand and Gravel Groupwebsite, http://www.ciria.org.uk/emsagg/). Because these depositsexhibit variable stratigraphic expression, including backsteppingshorelines and deltas, transgressive lags and offlapping shelf wedges(Swift and Thorne, 1991; Posamentier and Allen, 1993; Galloway andHobday, 1996; Gulick et al., 2005), they are also relevant within thecontext of evolving sequence stratigraphic concepts for continentalshelf evolution (Swift and Thorne, 1991; Posamentier, 2002).Transgressive sediments are sometimes difficult to study, becausethey are typically thin and discontinuous over generally flat coastalplains and exposed shelves, usually as a result of marine erosionaccompanying landward migration of the shoreline. Furthermore,they may also be difficult to sample because of their tendency to becoarse-grained, except by vibracoring, and then to only limited depthsbelow the seafloor (e.g., Buck et al., 1999).

Although highstand conditions exist today on most continentalmargins globally, few high-resolution seismic surveys with profiledensity appropriate to the purpose are available to resolve thecomplex depositional geometries of patchy Holocene transgressivedeposits on continental shelves. The continental shelf off New Jersey(Fig. 1), whose shallow stratigraphy has been studied extensively formore than 30 years (e.g., Emery and Uchupi, 1984; Knebel and Spiker,1977; Swift et al., 1980; Milliman et al., 1990; Davies et al., 1992;Austin et al., 1996; Goff et al., 1999; Duncan et al., 2000; Fulthorpe andAustin, 2004; Carey et al., 2005; Gulick et al., 2005; Nordfjord et al.,

Fig. 1. Bathymetry of the Mid-Atlantic Bight, showing study area, data distribution and physioindicates swath bathymetric coverage (Goff et al., 1999). Pink shaded area indicates dense (52006). Orange lines indicate additional chirp track lines outside this area, collected both durfrom the north, colored by depth and contoured in meters. Bathymetry data are adapted frogov/mgg/coastal/coastal.html). Location of Ocean Drilling Program (ODP) Site 1073 (Austin

2005), is a prominent exception. New Jersey shelf studies haveprovided insights into spatial and temporal variations of latestPleistocene–Holocene sediment deposition, erosion and transportcaused by variations in both supply and accommodation. Interpreta-tions based on very high-resolution seismic data of the shallow sub-seafloor, coupled with samples of transgressive deposits of the lastsea-level cycle, are most informative on this shelf because of detailedknowledge of eustatic sea-level behavior during the last ~50 ky(Wright et al., 2009).

The New Jersey continental shelf is ~120 km wide, exhibits low-relief physiography, is very gently sloping (b0.1°), tectonicallyquiescent, and is experiencing a long-term, low sediment-supplyregime. Although this margin is experiencing the effects of glacialisostacy (Peltier, 1998), the effects on the latest Pleistocene–Holocenesea-level curve are minimal (Wright et al., 2009). In general,sediment-supply and hydrodynamic regime control the formationand evolution of internal geometry of surficial shelf depositionalsequences over decades to millennia, even during time periods thatare primarily driven by sea-level oscillations (Thorne and Swift,1991). Sediment starvation on this shelf, coupledwith high-frequencyeustatic fluctuations associated with glacial events, have createdlaterally variable lowstand and transgressive deposits bounded bycomposite erosional surfaces that can only be mapped with three-dimensional, high-resolution seismic control. The New Jersey shelfhas been the site of both modern, high-resolution seismic dataacquisition at appropriate profile densities and enough precisely-positioned sediment cores to provide some ground-truth for thoseimages.

graphic features referred to in the text. Locations of Figs. 2–4 are shown. Yellow outline0–200 m track spacing) chirp seismic coverage collected in 2001 (Nordfjord et al., 2005,ing the 2001 survey and in 1999 (Duncan, 2001). Bathymetry is artificially illuminatedm the National Geophysical Data Center's coastal relief model (http://www.ngdc.noaa.et al., 1998) is also identified.

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Our objective in this paper is to discuss the stratigraphic recordand related key markers in the shallow sedimentary record of themiddle–outer New Jersey shelf that we have identified as part of ourcontinuing investigation of the interplay between erosional anddepositional processes during the Holocene transgression. We haveused a suite of ultra-high resolution (1–4 and 1–15 kHz; ~10–20 cmvertical resolution) chirp seismic reflection data profiles and swathbathymetry data (Fig. 1) to map an important seismic horizon, whichwe interpret as the Holocene transgressive ravinement surface, over awide area. The complex morphology of this ravinement surface,previously identified as the “T” horizon by Duncan et al. (2000) withrespect to underlying latest Pleistocene deposits and the overlyingHolocene sand sheet, demonstrates how “T” influences both deposi-tional architecture and the locus of marine (i.e., post-transgression)erosion during the latest sea-level rise. We also use our seismic resultsto make some observations about the interplay among eustacy,oceanography, climate and tectonic processes as predictors of coastalevolution during such a transgression.

2. Background

2.1. Quaternary stratigraphy

Three primary reflecting horizons can be observed seismicallywithin the surficial, latest Pleistocene–Holocene stratigraphic succes-sion of the middle and outer New Jersey shelf southwest of the HSV(Figs. 1 and 2): the “R” horizon (hereafter termed “R”), the “Channels”horizon (termed “Channels”), which generally incises “R”, and the “T”horizon (termed “T”), which truncates all older horizons and formsthe base of the surficial sand sheet (Davies et al., 1992; Duncan et al.,2000; Duncan, 2001; Goff et al., 2005; Nordfjord et al., 2005, 2006).“R” is a regionally recognized, generally high-amplitude reflection

Fig. 2. Uninterpreted (top) and interpreted (bottom) chirp seismic profile, oriented nor“Channels” and “T” horizons; latest Pleistocene clays between “R” and “T”; filled fluvial/estusands atop the “T” horizon; a local low in “T” coincident with an erosional swale in the seafloone of two strike-oriented, shallow depressions in “T” that we interpret as preferential ero

that forms the base of three seismically recognized sedimentarydeposits of latest Pleistocene–Holocene age: the mid-shelf wedge, theouter shelf wedges (Knebel et al., 1979; Milliman et al., 1990; Gulicket al., 2005), and a complex thin veneer of latest Pleistocenesediments between the two wedges (Fig. 2; Gulick et al., 2005). Theorigin of “R” is uncertain, but it is likely a composite product oferosion. Sediments immediately underlying “R” on the outer shelf are~40 ky old, while sediments overlying “R” are ~30 ky old (Gulick et al.,2005)’ “R” is clearly older than the LGM (~18 ka).

The outer shelf wedge (OSW) consists of a series of offlapping stratathat have been deposited on “R,” with a prograding geometry down-lapping towards the modern/LGM shelf edge. Milliman et al. (1990)originally recognized and mapped this feature, and thought it might betheproduct of a glacially-induced jökulhaupchanneled seaward throughan ancestral Hudson River system. More recently, this OSW has beenpresumed to be a pre-LGM deltaic deposit formed during the regression(Fig. 1; Davies et al., 1992; Duncan et al., 2000; Gulick et al., 2005).

TheMSW is a lobate deposit bounded seaward by the MSS (Thieleret al., 2007) (Fig. 1). Originally thought to be an erosional fossilshoreline (Veatch and Smith, 1939; Dillon and Oldale, 1978; Swiftet al., 1980), the scarp has since been re-interpreted as a depositionalconstruct of the mid-shelf wedge (Uchupi et al., 2001; Duncan, 2001).Uchupi et al. (2001) have suggested that the mid-shelf wedge, andadditional presumed depositional lobes north of the HSV (Fig. 1),were deposited on the exposed continental margin by glacial outburstfloods that occurred ~20–14 ka. In contrast, Duncan (2001) used chirpseismic data to demonstrate that at least the upper part of the wedgesouthwest of the HSV (Fig. 1) formed in a submarine environmentafter the glacial outburst floods had subsided.

“Channels” (Fig. 2) is the basal surface of a series of incised-valleysystems that eroded both the outer shelf wedge and, in places, “R”(Davies et al., 1992; Austin et al., 1996; Duncan et al., 2000; Nordfjord

th–south, illustrating the primary stratigraphic elements discussed in the text: “R”,arine channels eroding the Pleistocene clays and older sediments below “R”; Holoceneor; and older and younger incisions that predate “T” but postdate “Channels”. This low ission of back-barrier deposits in the study area. Location is shown in Fig. 1.

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et al., 2005, 2006). Channel-fill sediments with calendar ages of~14.5 ka (Buck et al., 1999; Alexander et al., 2003; calibrated fromAMS 14C-dates of 12.3±0.45 ka using Fairbanks et al., 2005) indicatethat “Channels”was likely carved during the lowstand associatedwiththe LGM and then was filled by first fluvial and then estuarinesediments as the shoreline migrated across this region during theensuing Holocene transgression (Nordfjord et al., 2005, 2006).

“T” (Fig. 2) caps and truncates channel-fill strata, truncates“Channels” (Duncan et al., 2000; Goff et al., 2005; Nordfjord et al.,2006), and forms the base of the Holocene surficial sand sheet (Goffet al., 1999). Based upon those seismic stratigraphic observations,Duncan et al. (2000) have interpreted “T” as the transgressiveravinement associated with Holocene sea-level rise. Samples fromareas where “T” outcrops at the seafloor reveal abundant gravel and/orshell hash, which has been interpreted as an erosional lag, presumablyassociated with passage of the shoreface during transgression (Goffet al., 2005). Wave action and currents in the surf zone that erode thesubstrate during the formation of such a ravinement act as winnowingagents, resulting in accumulation of such lag deposits (Nummedal andSwift, 1987; Galloway and Hobday, 1996; Posamentier, 2002). The ageof “T” landward of the ~90 m isobath is bracketed by the channel-fillcalendar age of ~14.5 ka (Buck et al., 1999) and theHolocene sand sheetcalendar age of b11.5 ka (Knebel and Spiker, 1977; Alexander et al.,2003; calibrated from 14C dates b10 ka using Fairbanks et al., 2005).

2.2. Seafloor morphology

The latest Pleistocene-to-Holocene transgression has erased mostgeologic evidence of subaerial exposure of the seafloor developedduring the last recessional sea-level cycle; in the process, this latesttransgression has molded the modern seascape of the Mid-AtlanticBight (Swift et al., 1980; Goff et al., 1999; Duncan et al., 2000). Oblique(to regional bathymetric contours, Fig. 1)-trending sand ridges, avariably thick veneer of well-sorted sand, and a recognizable assem-blage of marine fauna on the New Jersey shelf are typical of modernsiliciclastic, sediment-starved shelfal surfaces around the world(Anderson et al., 1996; Berne et al., 1998; Goff et al., 1999; Tessonet al., 2000). TheNew Jersey surficial sand sheetwas deposited upon theinterpreted ravinement surface, “T,” in response to redistribution andwinnowing processes associated with the latest transgression (Fig. 2;Goff et al., 1999; Duncan et al., 2000). This sheet, composed largely ofoblique sand ridges (Fig. 1), originally formed at the shoreface and hassince beenmodified at increasingwater depths by continued reworking(Swift et al., 1972; Snedden and Dalrymple, 1999; Goff et al., 1999).Analyses of swath bathymetry and backscatter, grab samples and chirpseismic data from the outer New Jersey shelf all suggest that sand ridgesin N50 m water depths are moribund (Goff et al., 1999). However,localized erosion, resulting in the formation of ribbon-floored swalesand scour pits, has also modified the surficial sand sheet since itsdeposition, particularly in these outer shelf water depths (Goff et al.,1999, 2005).

Goff et al. (2005) have documented highstandmarine erosion on theouterNew Jersey shelf. The timingof sucherosion is constrainedbothbytruncation at the seafloor of the transgressive ravinement surface, “T”(Fig. 2), and truncation of now moribund sand ridges along erosionalswales oriented parallel to the primary direction ofmodernbottomflow(approximately southwest), but oblique to the strike of the ridges(approximately northeast-southwest). Goff et al. (2005) have inferredthat this erosion postdates passage of the shoreface, including theevolution of bedforms that evolved in the near-shore environmentduring passage of the shoreface. Depths of erosion reported from theirstudy range from a fewmeters to N10 m. Goff et al. (2005) have furthersuggested, based on mixed angular, rounded sand grains and micro-fossils in seafloor samples, that modification of the seabed at greaterwater depths has continued to take place through bottom current-driven erosion. The continuing erosive/non-depositional nature of the

New Jersey shelf implies that a seismic stratigraphic maximum floodingsurface has not yet been established.

3. Data and methods

Amultibeambathymetry and backscatter surveywas conducted onthemiddle and outer New Jersey shelf in 1996 (Fig. 1; Goff et al., 1999).This survey employed a 95 kHz, Simrad EM1000 multibeam systemmounted on the CHS Creed. Excluding artifacts introduced by watercolumn refraction and navigational uncertainty, each 2.5° beam has abathymetric footprint of ~5% of water depth, and vertical resolution isb0.5% of water depth. These data have been combined with archivedata available from the National Geophysical Data Center to create aregional bathymetric map that shows the seafloor features of interestin this study (Fig. 1).

Ultra-high resolution (1–4 and 1–15 kHz), deep-towed (~10–12 moff the seafloor), chirp seismic reflection profiles used for this studywere acquired on the New Jersey shelf in 1999 and 2001.Most of thesedata (N4000 km) were collected in 2001 aboard the R/V Endeavor(cruise EN359) over a broad region of the middle and outer shelfsouthwest of the HSV and Hudson Canyon (Fig. 1). Track line spacingvaried from 50 m to 1 km. The chirp system, built by Florida AtlanticUniversity, was capable of producing seismic images with a verticalresolution of ~10 cm, a lateral trace spacing of ~0.5 m, and couldinsonify surficial strata to sub-seafloor depths of up to ~30 m. The1999 data were collected aboard the R/V Onrust, using a commercial-ly-available 1–15 kHz Edgetech® system. Approximately 400 km ofdip-oriented sub-bottom profiles were collected during this survey(Fig. 1). This Edgetech system has a maximum theoretical verticalresolution of ~25 cm and a horizontal resolution of ~5 m, with sub-seafloor penetration depths up to ~30 m.

Samples and sedimentary age constraints, though sparse, areavailable from a variety of sources. Archival sediment cores and agedates can be found in a number of published sources (Knebel and Spiker,1977; Knebel et al., 1979; Buck et al., 1999). In 2001, grab samples andshort cores were collected on the R/V Cape Henlopen, within themultibeam survey area (Goff et al., 2004; Fig. 1). These provide geologicinformation on surface and near-surface sediments. In 2002, rotarycores were collected using the Automatic Heave Compensated-800system aboard the R/V Knorr (Nielson et al., 2003). Three sites weresampled to a maximum penetration of ~13 mbsf. Sedimentary and ageconstraints for these cores are summarized by Alexander et al. (2003)and Nordfjord et al. (2005).

Using the chirp data, we have interpreted the principal shallowreflectors known as “R”, “Channels”, and “T” throughout the survey area(Fig. 2). “R” is usually distinguished as a high-amplitude, generallycontinuous reflector. “Channels” outlines discontinuous, U- and V-shaped incisions that frequently incise the “R” horizon. Although“Channels” appears discontinuous in cross-section (Fig. 2), regionalmaps show continuous, dendritic systems interpreted as fluvial (Davieset al., 1992;Duncanet al., 2000;Nordfjord et al., 2005). “T” is interpretedby truncation of underlying reflectors, particularly “Channels”, and bysubtle onlap above. Based upon profile interpretation, we havegenerated isopach maps for sediments between “R” and “T” (Fig. 3)and for the surficial sand sheet above “T” (Fig. 4).

4. Observations

4.1. Seismic observations of strike-oriented lows in “T” on the outer shelf

Between the mid-shelf scarp (~50 m water depth, Fig. 1) andpreviously identified Franklin scarp (~100 m water depth; Fig. 1), “T”has anundulatinggeometry that is generally sub-parallel tobathymetrictrends of the modern seafloor. However, along the ~70 m bathymetriccontour and along the base of the MSS (~50–60 m water depth), wemap two broad (~10 km), shallow (~8 m), strike-oriented lows in “T”

Fig. 3. Isopach map of latest Pleistocene sediments above “R” and below “T” in our study area (Fig. 1). Thickness conversion assumes a sound velocity of 1500 m/s. Note the thicknesschanges associated with the deposition lobe northeast of the Hudson Shelf Valley (see Fig. 1) and themid-shelf scarp. Locations of the younger and older incisions are shown (see thetext), as are locations of Figs. 2 and 5–8.

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(Fig. 2). An oblique cross-section of the 70-m trough (Fig. 2) shows thatthis local low in “T” corresponds in part to an erosional swale in theseafloor (Fig. 4; Goff et al., 2005), which is in turn coincident with abroad low in “R” (Fig. 2).

Seaward of the ~70 mtrough in “T”,we see twofilled incisionsbelow“T” that truncate “Channels” (Fig. 2). This geometry implies that theincisions postdate estuarine channel fills with calendar age ~14.5±0.5 ka (Buck et al., 1999; Alexander et al., 2003; calibrated from AMS14C-dates of 12.3±0.45 ka using Fairbanks et al., 2005), and predatepassage of the surf zone indicated by “T.” One of these filled incisionscuts into the fill of the other, sowe use the terms “younger” and “older”,respectively, to identify them. Fig. 5 shows an enlarged, dip-orientedcrossing of these incisions, illustrating their internal seismic facies. Theyounger incision is roughlyV-shaped in cross-section; itsfill consists of ahigh-amplitude, chaotic basal facies overlain by lower amplitude,parallel-continuous U-shaped reflectors. The fill of the older incision,which is much broader, is seismically transparent to weakly laminated.In plan view (Fig. 3), the older incision parallels margin strike and is~12 km long and 2–3 kmwide. The younger incision generally parallelsthe older incision but is not as well defined in map view (Fig. 3). Theseincisions and their fills differ from the underlying “Channels” incisions,which are generally box-shaped, and contain fills consisting of basalchaotic reflectors separated from an overlying transparent seismic unitby a flat-lying to undulating contact (e.g., Fig. 5; Nordfjord et al, 2005).

Figs. 6 and 7 show a second low in “T” at the base of the MSS. Fig. 6displays a dip profile that crosses the MSS south of its bend toward

the southwest (Figs. 1, 3, 4). As observed at the ~70-m low in “T”(Figs. 2 and 5), this low in “T” coincides with a broad swale in theseafloor at 50–60 m, and with a broad underlying low in “R” (Figs. 6and 7). Chirp profiles also show a broad, older incision and anarrower, younger incision beneath “T” and above “Channels” withinthis low in “T” (Fig. 6). These features are remarkably similar to thoseat the ~70 m low (Fig. 5), in that the younger post-“Channels”incisions in Fig. 6 are both V-shaped, and a seismic facies within oneof them is characterized by parallel discontinuous, U-shapedreflectors. The older incision fill is characterized by a moretransparent, weakly laminated facies (Figs. 2, 5 and 6); just seawardof the MSS, some of these fill reflectors dip landward toward the scarp(Fig. 6). “Channels” is not observed immediately seaward of the MSS,but the stratigraphic relationship between “Channels” and theseincisions can be deduced from the relationship between the outerincisions, “T” and “Channels” where it appears farther seaward(Duncan, 2001). Available data suggest that these incisions are sub-parallel to the MSS and that they have similar orientations to theincisions at the 70 m low in “T” (Fig. 2). We see no evidence of filledincisions beneath “T” and above “Channels” in seismic profiles northof the bend in the mid-shelf scarp, where a broad erosional swale inthe seafloor follows its base (Figs. 4, 7). Nonetheless, the data aresufficient to confirm that the 50–60 m low in “T” observed to thesouthwest predates swale-forming erosion affecting the seafloorseaward of this portion of the MSS (Fig. 7). There is also someevidence of a depression in “R” beneath the low in “T” (Fig. 6).

Fig. 4. Isopach map of Holocene sediments above “T” and below the seafloor in our study area (Fig. 1). Thickness conversion assumes a sound velocity of 1500 m/s. Note the thicknesschange associated with the MSS. The Holocene mid-shelf wedge is shown as are the younger and older incisions (see the text), erosional swales in the modern seafloor and thelocations of Figs. 2 and 5–8.

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4.2. Seismic observations of the mid-shelf scarp and wedge

At the seafloor, the MSW, which is bounded seaward by the MSS,appears as a lobate feature southwest of the HSV in ~30–50 m waterdepths (Figs. 1 and 3). TheMSW can be divided into two parts: a lowerpart bounded below by “R” and above by “Channels” and “T” (Figs. 3and 7), and an upper part bounded below by “T” and above by theseafloor (Figs. 4 and 8). A ~10–20 m topographic step in “T” is alsoobserved beneath the MSS, with slopes ranging from almost 2° in the

Fig. 5. Northwest–southeast oriented chirp seismic profile illustrating stratigraphic relationyounger post-“Channels” incision, and “T” horizon. Location shown in Figs. 3 and 4.

central part of theMSS (Fig. 7) to ~0.12° near its southern edge (Fig. 8;Duncan, 2001). Because the two parts of the MSW are separated bythe interpreted transgressive ravinement, we identify the lower partas “Pleistocene” (Figs. 3 and 7), and the upper part “Holocene” (Fig. 4;see also Carey et al., 2005). The Pleistocene portion of the MSWreaches thicknesses of up to 14 m; the step in “T” forms its seawardedge (Fig. 3). The seismic facies of this lower portion of the MSWconsists of transparent to weakly laminated, sub-horizontal reflec-tions (Figs. 6 and 8).

ships and seismic facies associated with “Channels”, the older post-“Channels” incision,

Fig. 6. Dip-oriented chirp seismic profile (top) and stratigraphic interpretation (bottom) located just seaward of the toe of the mid-shelf scarp, south of its bend (see Figs. 3 and 4 forlocation). This section illustrates the shallow local low in “T” at the base of the mid-shelf scarp and coincident older and younger post-“Channels” incisions.

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We observe the Holocene portion of the MSW south of the bend inthe MSS, and near the HSV to the north (Fig. 4). In these locations, theHolocene sand sheet covers the step in “T” (Fig. 7) while generallymatching the shape of the underlying Pleistocene portion of the MSW,reaching a thickness of up to 15 m (Figs. 4 and 8). Another depositional

Fig. 7. Uninterpreted (top) and interpreted (bottom) chirp seismic profile, oriented W–E, crillustrates: “R” and “T”; the thickening of the Pleistocene part of theMSW; presumed fluvial inPleistocene section; and a shallow low in “T” at the base of the mid-shelf scarp that is coin

lobeof similar thickness is observed north of theHSV (Fig. 4).Numerous,weak, seaward-dipping reflectors characterize the Holocene portion ofthe MSW (Fig. 8). Moribund sand ridges and erosional swalescharacterize its seafloor surface (Goff et al., 2005; Fig. 8). Along thecentral part of the MSS, Holocene sands drape “T”, except where

ossing the mid-shelf scarp north of its bend (see Figs. 3 and 4 for location). This sectioncisions defined by “Channels” incising the Pleistocene units; Holocene sands draped thecident with an erosional swale in the seafloor.

Fig. 8. Dip-oriented chirp seismic profile (top) and stratigraphic interpretation (bottom) located landward of the mid-shelf scarp and the step in “T”, and southwest of its bend in thescarp (see Figs. 3 and 4 for location). This section illustrates the internal seaward-dipping reflectors that are observed within the Holocene component of the MSW.

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truncated by the recent erosional swale at thebase of theMSS (Goff et al.,2005; Fig. 7).

Sediment cores from the MSW landward of the MSS by Knebelet al. (1979) contained an upper, Holocene-age sand unit with 14Cdates b8 kyr (calibrated calendar ages b9 kyr; Fairbanks et al., 2005)and a lower, muddy unit with 14C dates N28 ka (calibrated calendarages N33 ka; Fairbanks et al., 2005). Although Knebel et al. (1979) didnot observe “T” on their lower-resolution seismic data, we assumethat the boundary between their Holocene sands and Pleistocenemuds represents our interpreted transgressive ravinement, and thatthe muds in turn represent what we have defined as the PleistoceneMSW.

5. Discussion

5.1. Topographic lows in “T”

We interpret the two broad, strike-oriented troughs in “T”, one at~70 m water depth (Fig. 2) and the other in ~50–60 m water depths(Figs. 6 and 7), as preserved remnants of back-barrier morphology.Similar seismic evidence was used by Sanders and Kumar (1975) toargue for the presence of a relict, back-barrier morphology in ~24 mwater depths off Fire Island, New York. We consider two possibleexplanations for such preservation during transgressive ravinement,which might be expected to erode such features completely duringshoreface retreat (Swift, 1968). The first is that these features representpauses in the retreat of the shoreline during the last transgression. Suchpauses might have provided time for barrier systems and relateddeposits to develop thatwere substantial enoughnot to be erased by thesubsequent passage of the shoreface. The ~50–60 mtrough is situated atthe base of the step in “T” at the MSS (Figs. 7 and 8); this paleo-topographic feature could have slowed shoreline retreat without anychange in the regional rate in sea-level rise.While there is noPleistocenescarp associated with the 70-m trough in “T”, that trough is also

associated with two generations of partially preserved, filled post-“Channels” incisions (Figs. 2 and 5), leading us to suggest that both ofthese troughs may represent short-duration pauses in Holocene sea-level rise as yet undocumented in regional sea-level curves (Wrightet al., 2009).

An alternative explanation for the presence of preserved barrier/lagoonal morphology is suggested by the coincidence of a depressionor flattening of “R” below each of the “T” troughs (Figs. 2, 6). Suchchanges in “R” relief suggest antecedent topography. Storms et al.(2008) have recently modeled the formation and preservation ofbarrier systems during transgression for the Adriatic shelf, findingthat a flattening in antecedent topography will enhance thepreservation of back-barrier morphology by forcing a rapid retreatof the shoreline once the barrier is breached. A similar phenomenonmay have occurred in places on the New Jersey shelf.

Within both of these troughs (Figs. 2, 5 and 6), stacked (older andyounger) incisions are observed that, at least in one location (Figs. 2, 5),clearly postdate filled incisions defined by “Channels”, but predate “T”.Seismic coverage of the older incision at the 70-m low shows that it is a~12-km long, strike-oriented topographic low (Fig. 4), suggesting to usthat it is also a preserved back-barrier feature, perhaps a lagoonalremnant. Our hypothesis is supported by the wedge-shaped, transpar-ent facies filling this incision,which resembles a tidal-inlet fill facies of atruncated shore-zone system(Fig. 5; Duke, 1990;GallowayandHobday,1996; Morton et al., 2000). We also interpret the younger incisions atboth locations as tidal inlets that developed during a later transgressivestage, when the shoreface system was backstepping (Duncan, 2001;Nordfjord et al., 2005, 2006; Figs. 3–5). The asymmetric seismicgeometries and lateral accretion characteristics of the fill units withinthese features are similar to previously studied tidal inlets preserved inGulf of Mexico stratigraphic sections (Tye and Moslow, 1993). Thepossible presence of such back-barrier features coincident with the “T”troughs is consistent with our first hypothesis, that pauses in shorefaceretreat allowed formation of large barrier systems and well-developed

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lagoonal systemsbehind them,but does not preclude thepossibility thatthese troughs developed instead within easily erodible (estuarine)sediments ponded in preexisting “R” depressions.

5.2. Pleistocene and Holocene mid-shelf wedge deposits

The ~10–20 m step in the “T” horizon coincident with the MSS(Figs. 7 and 8) is in places of the seaward edge of the PleistoceneMSW(Fig. 3). Where this occurs (Fig. 7), the edge of the MSW is defined by“T” and appears erosional. Elsewhere, the Pleistocene part of theMSWresembles a depositional lobe (Fig. 3). Using the New Jersey sea-levelcurve of Wright et al. (2009; Fig. 9A), we speculate that the origin ofthe Pleistocene MSW can be traced to deposition during marineisotope stage (MIS) 3A at ~35 ka, whichwas the last time prior to LGMthat sea level was high enough to drown this part of the outer shelf(Fig. 3). Such a brief drowning episode is necessary for deposition ofthe fine-grained marine sediments making up this part of the MSW.The MIS 3A age is consistent with the N33 ka (calibrated) radiocarbondates from MSW sediment derived by Knebel et al. (1979). They haveproposed that these sediments were derived from the paleo-HudsonRiver. Our interpretation is also similar to hypotheses put forward byCarey et al. (1998, 2005) and Sheridan et al. (2000), who have arguedthat higher sedimentation rates during MIS 3, along with very lowshelf gradients (~0.1°), favored preservation of deposits associated

Fig. 9. A. Sea-level curve for the New Jersey shelf (Wright et al., 2009), indicating timing andtiming of the formation of the Pleistocene part of the MSW. B. Evolutionary model for sedimedates provided are calendar ages calibrated (Fairbanks et al., 2005) from radiocarbon dates

with small sea-level changes. Similar mid-shelf MIS 3 deposits havebeen mapped by Lobo et al. (2002) in the Gulf of Cadíz.

The Holocene portion of the MSW was deposited on top of thePleistocene MSW (Fig. 8), directly upon the interpreted transgressiveravinement surface, “T” (Fig. 7). Therefore, the Holocene MSW musthave been deposited in shallow water subsequent to the advance ofthe shoreline across the toe of the Pleistocene MSW, at a calendar ageof ~11.4 kyr (based on the New Jersey sea-level curve of Wright et al.,2009; Fig. 9A). Shell samples recovered in cores from the HoloceneMSW exhibit calendar ages ~8.9 kyr (calibrated from 14C ages of~8 kyr) and younger (Knebel and Spiker, 1977; Knebel et al., 1979). Itsseaward prograding seismic character (Fig. 8) suggests an offlappingtransgressive wedge (Swift and Thorne, 1991). The location of theHolocene MSW is likely determined by the accommodation providedby increased dip at the seaward flank of the Pleistocene MSW (Fig. 3);the presence of that accommodation obviates any need to resort tosea-level changes to explain the presence and preservation of theHolocene MSW (Fernández-Salas et al., 2003).

We interpret the Holocene MSW as a subaqueous depositcomposed of eroded shoreface material and/or glacial lake sedimentsexcavated by ice from the Hudson/Champlain valley, then transportedto the shelf and deposited in shallow water there by the post-glacialHudson River. Isostatic uplift of drained glacial lake beds in theHudson River valley following glacial retreat, and downcutting by the

approximate sea levels of the 6 phases of the evolutionary model (B.), and the probablent formation and preservation during the last transgression. See text for discussion. Age(Knebel et al., 1979; Buck et al., 1999; Alexander et al., 2003; Gulick et al., 2005).

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newly established Hudson River would have created a high sedimentyield to the New Jersey shelf in the period immediately followingtransgression of the study corridor. The Hudson River probablyoccupied the proximal HSV (Figs. 1, 3,4), preexisting from earlier lake-collapse flood events, and carried sediment to a river mouth near thepresent ~40 m isobath at the time in question, ~11.4 ka. Afterbreaching of the lakes, sediment transported offshore by the HudsonRiver must have been composed both of lacustrine clays and sandsand gravels derived from glacial erosion of the Hudson Valley andupstream (McHugh and Olson, 2002). This sedimentation could havetaken place over a short period of time, because estuarine conditionsdeveloped ~10 ka (Weiss, 1974).

North of the HSV (Fig. 4), the stratigraphic interpretation alsoincludes a substantial Holocene deposit above the “T” horizon as partof the depositional lobe. This may be another example of post-LGMdeposition of glacial lake sediments during the transgression. “R”could not be mapped seismically in this area, so the existence of aPleistocene MSW beneath the Holocene deposit cannot be confirmed.

Uchupi et al. (2001) have hypothesized that the MSW, and at leasttwo other depositional lobes proximal to but north of themodern HSV(Fig. 1), represent subaerial deposits associated with massive out-flows, or jökulhaupses, from breached meltwater lakes positionedlandward of what is now the Long Island terminal moraine. Millimanet al. (1990) have also called upon this mechanism to explain theorigin of the OSW. Thieler et al. (2007) cite numerous sources tosuggest that meltwaters exited the Hudson River in a singlecatastrophic event, upon the breaching of dams near what is nowthe Narrows (Fig. 1), somewhere between ~14 ka and ~12.3 ka, andmost likely ~13.35 ka. However, available stratigraphic and radiocar-bon age constraints for the MSW noted above are inconsistent withthe Uchupi et al. (2001) hypothesis; the dates established by Thieleret al. (2007) are instead within the age gap between the Pleistoceneand Holocene components of the MSW. “Channels”-incisions withinthe MSW (Fig. 7) also suggest that it is older than 14.5 kyr, theestimated age of the fill of those incisions (Buck et al., 1999). McHughand Olson (2002) have proposed instead that the bulk of presumedjökulhaup sediments transported via the HSV were deposited on theHudson Apron slope, not on the shelf. A core recovered from the slopeduring Ocean Drilling Program (ODP) Leg 174A at Site 1073 (Fig. 1)contained ~30 m of fine-grained sediment, including abundant woodfragments, with radiocarbon ages between 27.9 and 14.4 kyr (Austinet al., 1998; McHugh and Olson, 2002). Large-scale, transverse-to-flow bedforms buried at the proximal end of the HSV and braidedchannels containing smaller transverse bedforms at its distal end(Thieler et al., 2007), suggest that the HSV was incised during high-energy, fluvial flow events on the exposed continental shelf.Furthermore, the HSV ends at the head of the Hudson Canyon, at~80 m water depth (Fig. 1). The New Jersey sea-level curve (Fig. 9A)suggests that the shoreline sat at ~80 m water depth ~13.5 ka, veryclose to the time that Thieler et al. (2007) have suggested for thecatastrophic flood event. Therefore, these flood waters appear to havebypassed at least this part of the New Jersey shelf during the latestPleistocene. If true, then the additional depositional lobes north of theHSV (Fig. 4) likely formed, as we hypothesize for the Pleistoceneportion of the MSW, as subaqueous deltaic deposits of the latestPleistocene Hudson River system.

5.3. Shelf current reworking of mid–outer shelf sediments

Following deposition of the Holocene part of the MSW and sea-level transgression across the middle shelf ~11.5–10 ka, sea level hascontinued to rise to the Present. The hydrodynamic regime of thisregion has also changed; wave-current activity has decreased and therelative importance of advective bottom currents has increased(McLennen, 1973). The transgressive sand sheet, initially formednearshore and composed of sand ridge and swale geomorphology

(Swift and Field, 1981), is now being reworked by those currents.Wave- and tide-dominated shelves worldwide are similar in thisrespect; regardless of sediment supply during the LGM, preexistingshoreface retreat-derived deposits have been reworked into shelfsand ridges, and sand has been widely distributed to blanketunderlying lowstand sedimentary features (Swift and Field, 1981;Dalrymple and Hoogendoorn, 1997; Berne et al., 1998). Thesesediments continue to be reworked by shelf currents to the Present.

Truncation of stratal horizons at the seafloor, along with thepresence of mixed angular and rounded grains and microfossils,provide direct evidence for recent erosion in the marine setting,following shoreface transgression across this region (Figs. 2, 3 and 7;Goff et al., 2005). Two areas have undergone significant marineerosion (Fig. 4): (1) seaward of the MSS in 50–60 m water depths(Figs. 7) and (2) north (landward) of interpreted back-barrierincisions located in ~70 m water depth (Fig. 2). Both of theseerosional swales coincide with depressions in both “T” and “R”,suggesting that such depressions may represent long-term foci oflocalized erosion. If, as we have hypothesized above, these lows in “T”represent remnant back-barrier morphology, then subsequent ero-sion in these areasmay be a result of lagoonal/back-barrier sediments,for example the filled incisions interpreted as tidal channels (Figs. 2and 5), being more susceptible to erosion. Alternatively, depressionsin “R” and “T” may focus bottom currents capable of causing theobserved erosion. Erosion of Holocene and older sediments appearsfocused in the region north of the bend in the MSS (Figs. 4, 7), wherehigher erosional potential of known, modern SW-directed bottomwater flows would be expected. Conversely, bottom erosion at thebase of the MSS is largely absent south of the bend (Figs. 4 and 8),where such flows would be expected to be less intense.

6. Evolution model for the transgressive ravinement

We present a model for the transgressive ravinement of the NewJersey middle and outer shelf to explain available seismic and geologicobservations (Fig. 9B), tied to the most recent sea-level/age curve forthis shelf (Wright et al., 2009; Fig. 9A) and to the specific stratigraphicand morphological features discussed above. Phase 1 of our modeloccurs at or near the LGM, ~18 ka, when multiple, dendritic sets offluvial incisions formed when the entire shelf was exposed (Fig. 9B —

Phase 1; Davies et al., 1992; Duncan et al., 2000; Nordfjord et al., 2005).By this time, both the OSW and the Pleistocene part of the MSW hadevidently formed based on available radiocarbon age evidence (Knebelet al., 1979; Gulick et al., 2005). During late lowstand/early transgres-sion, shoreface retreat began to occur and the base of the shorefacemigrated landward (Swift, 1968). “Channels” incisions were floodedand filled and eroded remnants of these incised-valley deposits werepreserved below “T” (Fig. 9B — Phase 2; Nummedal and Swift, 1987;Nordfjord et al., 2005). All preexisting deposits were truncated bywaveaction, creating the ravinement surface, “T” (Duncan et al., 2000; Goffet al., 2005; Nordfjord et al., 2006).

By the beginning of Phase 2 (Fig. 9B), sea level had risen to ~−70 m;all known fluvial networks on the mid- and outer shelf have beenconverted into estuaries, which are then infilled with the continued risein ambient sea level and the approach of the landward-retreatingshoreline (Nordfjord et al., 2006). Preservation of back-barrier tidalincisions at ~70 mwater depth, and later at 50–60 mwater depth, maybe due either to (1) pauses in shoreface retreat, which allowedformation of a substantive estuarine/back-barrier system that couldnot be completely destroyed by subsequent transgressive ravinement;or (2) antecedent topography, for example an observed preexistingdepressions now observed in “R” (Fig. 2), allowing the shoreline toretreat rapidly once the barrier was breached and allowing for thepreservation of the back-barrier morphology.

Due to remnant physiography of the latest Pleistocene part of theMSW, landward shoreface retreat slowed down during formation of

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the MSS during Phase 3 (Fig. 9B), at ~11.4 ka when sea level was at~−50 m. This may have focused ravinement erosion along theseaward edge of the MSW, which in turn steepened its slope wherethat slope is coincident with the MSS (Fig. 7). Ravinement of thePleistocene MSW forms the foundation for the MSS (Fig. 6).

During Phase 4 (Fig. 9B), sea level finally rose across the MSS, andthe Holocene part of the transgressive, onlapping MSW wasconstructed from coarse-grained sediment supplied by the HudsonRiver and/or eroded, winnowed shoreface sediments deposited in thenear-shore zone. Powerful storm flows swept southward across thisdepositional lobe during Phase 5 (Fig. 9B), which consequentlyremoved sediments from parts of the MSW and truncated its base(Swift and Freeland, 1978). As the water column continued to deepenand the shoreline continued to recede, shelf sand ridges evolved(Swift and Thorne, 1991; Goff et al., 1999; Snedden et al., 1999).During Phase 6 (Fig. 9B), successive erosion of outer shelf sedimentsand modification of the seafloor has continued to take place throughbottom current-driven erosion, perhaps by undercutting of surface-armored seabeds (Goff et al., 2005) or by preferential erosion withinpreexisting topographic lows (Fig. 7).

We speculate that topography associated with preexisting incisedvalleys and depositional lobes may have been important factors ininfluencing the ravinement trajectory and the generation of paleo-barriers, which in turn controlled the preservation potential oflowstand to early transgressive deposits (Figs. 3 and 4). Paralic fluvialand estuarine deposits are preserved only in fragmented fashionbelow “T”, the ravinement surface (Nordfjord et al., 2006); this isgenerally the case with low-gradient shelf settings that undergo astepwise retreat of the coast, thereby allowing barrier drowning andcannibalization during intervals of accelerated relative sea-level rise.The transgressive ravinement surface dominates New Jersey seafloorbathymetry today, but its morphology is complex and includes ridgeand swale features which have been modified by deeper watererosion (Goff et al., 2005). The lack of deposition and continuederosion of the New Jersey mid- and outer shelf implies that amaximum flooding surface has not yet been established and will,when it is, represent a complex stratigraphic response to a sea-leveltransgression that has been underway for ~18 ky and continues today.

7. Conclusions

The transgressive ravinement on the New Jersey middle and outershelf is not a simple, seaward-inclined ramp. Rather, its morphology isinfluenced by both the deposition and erosion of overlying, post-transgressive strata and imperfect preservation of underlying, mainlylowstand and early transgressive strata. The most significantmorphological features characterizing the transgressive ravinementsurface are two strike-oriented troughs at ~50–60 m and ~70 mwaterdepths, and a ~10–20 m step coincident with a mid-shelf scarp. Weoffer two competing hypothesizes for the troughs. They formedeither: (1) during pauses in shoreface retreat at ~12.8 ka and ~11.4 kathat allowed for formation of back-barrier/lagoonal features, includ-ing tidal channels now filled, that were substantive enough to resistcomplete destruction by subsequent ravinement; or (2) by rapidshoreline retreat across antecedent flattening of topography, whichallowed for partial preservation of otherwise easy to erode back-barrier/lagoonal features. If the former hypothesis is true, then wesuggest that the ~70 m trough was a result of a real, if momentary,pause in known eustatic sea-level rise during the earliest Holocene. Incontrast, the ~50–60 m trough may be the result of a locally-forcedpause in shoreface retreat caused by preexisting topography of anadjacent Pleistocene part of the MSW. Recent seafloor erosion is alsocoincident with depressions in the transgressive ravinement. Lagoon-al facies depositedwithin these depressions, and occasionally partiallypreserved, may be more susceptible to this later erosion. Another,related possibility is that residual bathymetric expression of these

depressions may have served to focus bottom flows that cause sucherosion. The erosive action of such flows continues to the Present.

The modern MSS, which has been called a type locality of a paleo-shoreline (Veatch and Smith, 1939; Dillon and Oldale, 1978; Swiftet al., 1980), alsomarks the seaward edge of what is, in other locationsaway from the MSS, a depositional structure, the MSW. The wedge isin part a Pleistocene deposit N33 kyr old that has been significantlymodified by transgressive ravinement and shelf currents during theHolocene transgression. The Pleistocene MSW was probably depos-ited during MIS 3A, the last time before LGM that sea level was highenough to flood this part of the shelf. Sediments forming thePleistocene MSW may have derived from the paleo-Hudson River,which transited the shelf in latest Pleistocene times south of themodern HSV (Knebel et al., 1979). Our study does not support thehypothesis put forward Uchupi et al. (2001) that the MSW is part of amulti-lobe subaerial deposit associated with massive meltwateroutflows from breached glacial lakes between ~17 and 12 kyr,because the basic structure of the MSW defined by its Pleistocenepart predates those flood events.

The toe of the Pleistocene MSW provided accommodation for theformation of the smaller Holocene component of the MSW, as atopographic step in “T” at ~40–50 m water depth that served toconcentrate river- and shoreface-derived sediment into a transgres-sive offlapping shelf wedge during ensuing sea-level rise. This wedge-shaped unit properly belongs in the transgressive systems tract,because it lies beneath the maximum flooding surface, which has notyet been established on the New Jersey shelf.

Acknowledgements

Continuing analysis and interpretation of seismic and core datafrom the New Jersey margin has been conducted as part of the Officeof Naval Research's STRATAFORM and Geoclutter initiatives, specif-ically under the auspices of grants N00014-00-1-0844 and N00014-04-1-0038 to Austin and Goff. Additional support has been receivedfrom the Jackson School of Geosciences, Exxon-Mobil and Statoil.Chirp seismic data were collected aboard R/V Endeavor in 2001, andwe thank the science party and crew of that vessel, as well as thoseaboard the CHS Creed (multibeam bathymetry), R/V Cape Henlopen(grab samples) and R/V Knorr (AHC-800 cores). We appreciate helpfuldiscussions with B. Christensen (Adelphi University) and C. Fulthorpe(UTIG), and constructive reviews on an earlier draft by C. McHugh(Queens College) and A. Cattaneo (IFREMER). We also thank S. Schockand his associates at Florida Atlantic University for their collaborationon collection of the chirp data, and Steffen Saustrup (UTIG) forprocessing those data. UTIG Contribution #2080.

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