SEISMIC FACIES OF INCISED-VALLEY FILLS, NEW JERSEY ... · Using the new seismic data, we have...

Journal of Sedimentary Research, 2006, v. 76, 1284–1303

Research Article

DOI: 10.2110/jsr.2006.108

SEISMIC FACIES OF INCISED-VALLEY FILLS, NEW JERSEY CONTINENTAL SHELF: IMPLICATIONS FOREROSION AND PRESERVATION PROCESSES ACTING DURING LATEST PLEISTOCENE–

HOLOCENE TRANSGRESSION

S. NORDFJORD,1,2 J.A. GOFF,2 J.A. AUSTIN, JR.,2 AND S.P.S. GULICK2

1University of Texas at Austin, Department of Geological Sciences, John A. and Katherine G. Jackson School of Geosciences, 1 University Station C1100,

Austin, Texas 78712, U.S.A2Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas, 4412 Spicewood Springs Road, Building 600,

Austin, Texas 78759-8500, U.S.A

e-mail: [email protected]

ABSTRACT: Incised-valley fills shallowly buried beneath the New Jersey middle–outer shelf reveal a retrogradational shift offour seismic facies, as observed in 1–4 kHz deep-towed chirp seismic data. These facies, the only preserved stratigraphic recordof the latest Quaternary–Holocene drowning and infilling of fluvial drainage systems developed on this exposed shelf at or nearthe Last Glacial Maximum (LGM), are interpreted as (1) fluvial lag deposits, SF1; (2) estuarine mixed sand and muds, SF2;(3) estuary central bay muds, SF3; and (4) redistributed estuary-mouth sands, SF4. These fills are truncated by a transgressiveravinement, the T horizon, which is in turn overlain by Holocene marine sand deposits. The seismic facies are bounded byreflectors marking either source diastems or unconformities: (1) the Channels horizon is the lowstand fluvial incision surface,(2) B1 is a bay flooding surface, (3) B2 is an intra-estuarine depositional surface, (4) B3 is a tidal ravinement surface, and(5) the T horizon represents erosion at or near the shoreface during Holocene transgression. The Channels horizon is generallypreserved only in valley axes. Elsewhere, this sequence boundary has been modified by surfaces B1 and/or B3. Dip-orientedchanges in the thickness of SF3 and SF4 suggest either a stillstand in the passage of the shoreline, which allowed such spatialvariations, or that local valley shape controlled hydrodynamic conditions for sediment transport and deposition. Narrowervalleys may have promoted tidally dominated, fine-grained deposition within these drowning estuaries, while broader valleysattenuated tidal flow velocities and allowed the filling of the estuary to be dominated by wave and current energy, promotingmore coarse-grained deposition. Our study demonstrates that wave- and tide-dominated estuarine facies can coexist within suchfill strata.

INTRODUCTION

Buried incised-valley systems are common features of Quaternarystratigraphy preserved on many continental shelves (e.g., Foyle andOertel 1997; Duncan et al. 2000; Warren and Bartek 2002; Anderson et al.2004). Formed originally by fluvial incision during shelf exposure, theyprovide evidence of paleo flow conditions during intervals of loweredrelative sea level (Nordfjord et al. 2005). Such valleys are also importantbecause, as coastal depressions, they provide accommodation forlowstand and early transgressive sedimentation in shelf environments(Vail 1987; Van Wagoner et al. 1988; Posamentier and Allen 1993).Incised valleys also protect entrained sediments from removal bysubsequent transgressive erosion (Swift and Thorne 1991). Therefore,remnants of such preserved valley-fill successions are key elements forinferring processes that create, modify, and preserve continental-marginsequence stratigraphy (Posamentier et al. 1988; Thorne 1994).

Conceptual models have been introduced to explain responses of themorphology and general sediment facies distribution of incised-valley fillsto environmental forcing (Dalrymple et al. 1992; Zaitlin et al. 1994). Suchforcing includes fluctuations in sediment supply and attendant physio-graphic changes accompanying drowning as a result of rising sea level.Models of this kind provide a basis for comparison of such drowned

valley systems globally and investigate the process by which incised-valleysystems evolve into estuaries during transgression. They divide drownedvalley fills into: (1) a landward zone, dominated by riverine sedimentation(e.g., bayhead deltas or straight tidal and fluvial channels), (2) a seawardzone, dominated by wave and/or tidal processes (e.g., an estuary mouthcomplex), and (3) an intermediate zone of mixed energy, effectivelya sediment sink experiencing competing marine and non-marineinfluences (e.g., muddy central basin deposits; Masselink and Hughes2003).

An idealized incised-valley fill succession responding to steady sea-levelrise and continuous sediment supply incorporates at least three super-imposed transgressive surfaces within its seaward portion, the estuarinebay (Masselink and Hughes 2003). The bay flooding surface forms byinitial flooding of the fluvial system (Nummedal and Swift 1987) andseparates underlying fluvial deposits from overlying estuarine deposits.Ensuing erosion by tidal currents in coastal inlets or channels of thedeveloped estuary creates a tidal ravinement surface (as defined by Zaitlinet al. 1994), which erodes underlying deposits confined within this part ofthe system (Allen 1991). While this tidal ravinement is forming in or nearthe mouth of the drowning incised valley, a bayhead channel diastem isproduced locally in the landward part of the estuary-mouth complex by

Copyright E 2006, SEPM (Society for Sedimentary Geology) 1527-1404/06/076-1284/$03.00

seaward progradation of a bayhead delta (e.g., Ashley and Sheridan1994). Finally, an extensive wave ravinement surface is created bylandward retreat of the shoreface during transgression (Swift 1968; Roy1994). This erosional event truncates all underlying estuarine deposits. Tocreate such a surface, wave and current erosion must remove both floodtidal-delta deposits and possibly some central-basin facies (Ashley andSheridan 1994). All of these transgressive surfaces are diachronous, andcould become amalgamated within a preserved sequence boundaryassociated with the transgression (McHugh et al. 2004).

Over the last decade, very high-resolution seismic profiling of middleand outer shelf environments (e.g., Reynaud et al. 1999; Duncan et al.2000) has provided new insights into 3D geometries of drowned incised-valley fill deposits in the shallow subsurface. The juxtaposition of seismicfacies characterizing the sedimentary fill of these valleys can beinterpreted to show how these valleys responded geologically to drowningcaused by Holocene transgression. This paper summarizes sequencestratigraphic analyses of dense 2D and pseudo-3D, deep-towed chirpseismic (1–4 kHz and 1–15 kHz) profiles collected over preserved valley-

fill sequences now shallowly buried beneath the middle and outer NewJersey shelf (Fig. 1). These incised valleys, which previously wererecognized using more sparse, lower-frequency seismic data (e.g., Davieset al. 1992; Austin et al. 1996) have been interpreted as riverine systemswhich formed during shelf exposure accompanying the Last GlacialMaximum (LGM), , 22 ka. The subsequent Pleistocene–Holocenetransgression, , 15–12 ka, filled and modified these valleys, formingestuaries (Davies et al. 1992; Duncan et al. 2000; Nordfjord et al.2005).

The New Jersey shelf (Fig. 1) today is quiescent and receives littleterrigenous sediment. As a result, this shelf has become a naturallaboratory for the long-term study of erosion and preservation ofsedimentary strata in accommodation-dominated, marginal-marine en-vironments (Nittrouer and Kravitz 1995). Subsidence of this margin hasbeen very low since the Neogene (Greenlee et al. 1988). Therefore,eustatic changes have been the primary driver of sedimentary processesduring its Quaternary evolution. Although incised-valley fills on the shelfconstitute only a small portion of the deposits of continental margins,

FIG. 1.—Location of deep-towed chirp seismic profiles (solid lines) collected in 2001 aboard R/V Endeavor (EN359), superimposed on NOAA’s bathymetry, mergedwith STRATAFORM swath mapping (Goff et al. 1999), off the New Jersey middle and outer continental shelf. Enlarged view of dendritic incised-valley systems mappedusing the chirp data is shown (right inset; Nordfjord et al. 2005); visual differences between this enlarged and the smaller version (center) result from pixel size andresolution. Locations of figures are indicated on the right inset map, as are locations of important preexisting vibracores (on both maps). Sites 1–3 identify long corescollected by the DOSECC AHC-800 drilling system aboard R/V Knorr (KN168) in fall 2002. Contours are in meters below present sea level. Regional location of the areaof study is identified on a 3-D image (left inset) of the New York Bight region from NOAA’s Ocean Explorer (http://oceanexplorer.noaa.gov/).

SEISMIC FACIES OF INCISED VALLEY-FILLS, NEW JERSEY CONTINENTAL SHELF 1285J S R

they are likely to be the only preserved geological record of the earlystages of the Holocene sea-level rise.

Using the new seismic data, we have described and mapped the 3Ddistribution of seismic facies representing the sedimentary fill of twoshallowly buried, middle–outer shelf incised-valley systems off NewJersey, U.S.A. (Fig. 1). We use these detailed facies maps to compare theNew Jersey systems to existing sequence stratigraphic models of drownedincised-valley fills. Specifically, we compare our distribution of seismicfacies with recognized tripartite zonation models developed for wave- andtide-dominated estuaries (Dalrymple et al. 1992; Allen and Posamentier1993; Zaitlin et al. 1994). Finally, we compare our observed faciesarchitecture to a number of other such systems, both recent and ancient,in order to assess the extent to which our latest Quaternary evolutionarymodel for the New Jersey shelf can be generally applied.

GEOLOGICAL SETTING

General Physiography

The New Jersey outer shelf is part of the east-coast U.S. margin (Fig. 1),a mature, passive continental platform presently characterized by lowsubsidence rates and negligible terrigenous sediment influx (Milliman and

Emery 1968; Greenlee et al. 1988; Duncan et al. 2000). This part of the shelfis 120–150 km wide and slopes very gently (, 0.001u) seaward (Fig. 1).The modern hydrodynamic regime is represented by a tidal range of 1–2 m,mean significant wave height of , 1 m (e.g., Carey et al. 1998), andgenerally southwest-directed ocean currents (e.g., Vincent et al. 1981).Periglacial conditions in a storm-dominated, open marine environmenthave prevailed on this shelf since the LGM (Masselink and Hughes 2003).

Latest Pleistocene–Holocene Stratigraphy of the New Jersey Shelf

The latest Quaternary seismic stratigraphy beneath the New Jerseymiddle–outer continental shelf has been studied using various geophysicalmethods for decades (e.g., Emery and Uchupi 1984). Comprehensivedetails of recent investigations are given in Duncan et al. (2000),Nordfjord et al. (2005), and Gulick et al. (2005). These results providea critical geologic context for the geophysical analyses presented in thispaper. In the following section, we provide a brief summary of thatstratigraphy, from the deepest regionally mapped surface to the seafloor(Fig. 2).

R Horizon, Outer-Shelf Sediment Veneer and Shelf-Edge Wedges.—TheR horizon, recognized regionally as a prominent reflector of varying

FIG. 2.—Schematic map and cross section of principal latest Quaternary seismic stratigraphic horizons and intervening sequences beneath the middle to outer NewJersey shelf (Duncan et al. 2000; Goff et al. 2005; Nordfjord et al. 2005).

1286 S. NORDFJORD ET AL. J S R

amplitude, was originally interpreted as a subaerial exposure surfaceformed during the last lowstand (Milliman et al. 1990; Sheridan et al.2000). Duncan et al. (2000) reinterpreted this interface as a combinedsubaerial and marine unconformity that formed diachronously acrossa sediment-starved shelf. On the basis of sparse age control from availablecores, the R horizon was determined to have formed 47–33 ka (Duncan etal. 2000; Alexander et al. 2003; Gulick et al. 2005). On the outer shelf, thissurface is sequentially overlain by an outer-shelf sediment veneer whichincludes two offlapping sediment accumulations, the outer-shelf anddeep-shelf wedges (Fig. 2; Gulick et al. 2005). These wedges, deposited, 33–22 ka (Alexander et al. 2003), have been interpreted as sequentialdeltaic accumulations which prograded during pre-LGM regression(Gulick et al. 2005).

Channels Horizon (Defining Incised-Valley Systems) and ValleySediment Fills.—Davies et al. (1992) identified and named ‘‘Channels’’originally in reference to a reflecting horizon which appeared to inciseboth the outer-shelf wedge and the R horizon (Fig. 2). Duncan et al.(2000) mapped these incisions over a wide area, . 5000 km2, confirmingtheir dendritic nature, lateral extent, and the complex nature of valley-fillunits. A vibracore taken through , 3 m of these preserved, incised-valleyfill sediments sampled benthic foraminifera characteristic of fluctuatinginner to middle-shelf and marginal-marine depositional environments,implying at least three subtle (, 10 m?) base-level fluctuations within theincised valley during infilling (Buck et al. 1999). These sediments yieldedan AMS 14C age of 12,300 6 450 yr (Lagoe et al. 1997; Buck et al. 1999),consistent with a hypothesis that this valley was incised due to subaerialexposure of this part of the shelf during the last lowstand, , 25–15.7 ka,and then filled during the subsequent Holocene transgression (Davies etal. 1992).

T Horizon, the Transgressive Ravinement Surface.—Duncan et al.(2000) also identified a shallower, widespread, variable-amplitude seismicreflection that truncates the Channels horizon and caps incised-valley fills(Fig. 2). Duncan et al. (2000) interpreted this T horizon as a transgressiveravinement surface formed by progressive shoreface erosion in responseto both waves and currents during the Holocene sea-level rise (Roy 1994).The T horizon is subparallel to the seafloor where observed (Fig. 2).

Post-T Surficial Sand Sheet.—A surficial sand sheet has been depositedupon the ravinement surface, the T horizon, during the Holocene ina landward-migrating shoreface environment (Goff et al. 1999). This unitis composed largely of oblique sand ridges, formed at the shoreface andmodified through mid-shelf water depths by continued reworking duringand since transgression (Fig. 2; Swift et al. 1972; Goff et al. 2005).Analyses of swath bathymetry, backscatter, grab samples, and chirpseismic data all suggest that such sand ridges on the middle–outer NewJersey shelf are for the most part relict; erosion by bottom currents maystill continue to modify this surficial sand sheet (Goff et al. 2005).

METHODOLOGY

Seismic Data

High-resolution, 1–4 kHz and 1–15 kHz seismic reflection records wereobtained using a deep-towed, chirp sonar aboard R/V Endeavor in 2001.The seismic profiles used for this study were collected at profile spacingsranging from 50 to 400 m (typically 200 m) over , 600 km2 (Fig. 1). Thedata provide a vertical resolution of , 10 cm, a horizontal resolution of, 2 m, and image depths up to 30 m sub-seafloor. Processing of thesedata included: (1) time-varying gain to compensate for sphericaldivergence, (2) seafloor smoothing at 75-trace (, 35 m) intervals,(3) predictive deconvolution to reduce short-period multiples and remove

the source signature, (4) a 1–3.5 kHz bandpass filter to the 1–4 kHz datato reduce noise, (5) zeroing returns prior to the seafloor arrival to removeringing in the water column, and (6) numerically shifting each record tothe correct seafloor arrival time, based on measurements of direct-wave(fish–fish), ghost (fish–sea surface–fish) and seafloor reflections, takinginto account the local tidal record (Pulliam et al. 1996; Luhurbudi et al.1998). Ship positions to , 1–2 m accuracies were obtained usinga differential global positioning system (DGPS), and chirp fish locationsrelative to the ship were determined by a short-baseline acousticpositioning system. Because the chirp system was generally towed 10–15 m off the seafloor, ghost reflections generally did not interfere withprimary acoustic returns from the upper 15–20 m sub-seafloor, ourgeologic interval of interest (Fig. 2). Depths (below mean sea level) tomapped acoustic surfaces were estimated assuming an average compres-sional wave velocity of 1500 m/s in water and 1750 m/s in sediments,consistent with direct measurements (Goff et al. 2004).

Seismic Stratigraphic Analysis

Previous work, using multichannel and boomer seismic data collectedon both the U.S. Atlantic (Ashley and Sheridan 1994; Belknap and Kraft1994; Foyle and Oertel 1997) and Gulf of Mexico (Smyth et al 1988;Siringan and Anderson 1993; Thomas and Anderson 1994; Bartek et al.2004) continental shelves, has established some general seismic faciescharacteristics for latest Quaternary–Recent incised-valley fill environ-ments. For example, a chaotic seismic facies suggests fluvial deposits; thisfacies often immediately overlies the sequence boundary, representing thefluvial incision at or near the relevant lowstand of relative sea level.Thomas and Anderson (1994), Foyle and Oertel (1997), and Bartek et al.(2004) all recognize transgressive facies within their incised-valley systemsas fluvial drainages evolving into estuarine systems: muddy central basindeposits, estuary-mouth complexes, and bayhead deltas. These studiesprovide useful analogs for recognizing seismic facies in the coeval NewJersey incised-valley systems.

We have interpreted groups of reflections throughout our study areabased on external form, configuration, continuity, amplitude, andfrequency, in order to define distinct seismic facies (Fig. 3; Mitchum etal. 1977; Mitchum and Vail 1977; Sangree and Widmier 1977). Fourseismic facies units, SF1 through SF4, characterize our imaged incised-valley fills. We then use the mapped spatial associations of SF1–SF4,along with available geological ground truth (Fig. 4), to infer bothdepositional environments and formative processes. To aid this in-terpretation, we mapped five seismic surfaces: the previously recognizedChannels and T horizons (Fig. 2), along with three more we recognizewithin the valley fills, B1 through B3. We used both frequency ranges, 1–4 kHz and 1–15 kHz, of the Chirp seismic data for our seismic mapping.The 1–15 kHz data more clearly imaged seismic facies boundaries, whilethe 1–4 kHz data were more useful for interpreting the internalcharacteristics and geometries of seismic facies (Fig. 3).

Tying Seismic Stratigraphy to Available Lithostratigraphy

To provide ground truth for our seismic mapping, three sites weredrilled in the study area in 2002 (Fig. 1), using the DOSECC AHC-800coring system (Nielson et al. 2003) deployed from R/V Knorr. A total of, 26 m of core was collected, with , 80% average recovery. Un-fortunately, only Site 3 targeted the flank of a filled incised valleyconsidered in this analysis (Fig. 1). That penetration, to , 7.5 m sub-seafloor, sampled valley fill, a valley flank, and the R horizon (Fig. 4). Aseries of measurements of physical properties, including saturated bulkdensity, compressional wave velocity, and magnetic susceptibility, wereobtained on whole-round, 1.5-m-long sections of cored sediments usinga GeoTek multisensor core logger (MST). We used these data to correlatethe seismic stratigraphy with the lithologic succession. Sampling of these


FIG. 3.—Representative, co-located chirp images at two frequencies, 1–15 kHz (top) and 1–4 kHz (bottom). The 1–15 kHz data were useful for interpreting seismicboundaries, like B1–B3, while the 1–4 kHz data, shown in subsequent figures, resolved internal reflection characteristics and geometric details of seismic facies, SF1–SF4,more clearly. See Figure 1 for location.


cores also provided material for grain-size analysis, radiometric dating,and foraminiferal biostratigraphy (Alexander et al. 2005).

To begin to infer depositional environments from the mapped seismicdata, we correlated the seismic reflection character at Site 3 with thelithostratigraphy sampled at that location by generating a syntheticseismogram from sonic and density core logs (Fig. 4; Sheriff 1995;Nordfjord et al. 2005). Where observed and modeled seismic reflectionslook similar, we assumed that the cored lithostratigraphic units represent

the imaged seismic facies succession, both at Site 3 (Fig. 4) and elsewherewithin our mapped area (Fig. 1).

SEISMIC FACIES

We recognize and map four fill units within New Jersey incised valleys(Fig. 3), based on stratigraphic superposition, observed geometries, andthe character of bounding seismic surfaces. To illustrate along-drainage

FIG. 4.—Synthetic seismogram generated from Site 3 MST data, compared to chirp sonar profile crossing Site 3 (Fig. 1; Nordfjord et al. 2005). Columns (from left toright): depths in meters below sea level (mbsl) and seafloor (mbsf), lithologic description of Site 3, MST gamma density log generated from whole-round core sections, animpedance log generated from MST density and P-wave logs, and chirp sonar data bracketing Site 3. We use the seismogram to tie mapped seismic facies units SF1, SF2,and SF3, and intervening boundaries B1 and B2, to lithologic data at the site. Sand occurs from the seafloor to , 2.5 mbsf; this represents the surficial sand sheet abovethe T horizon (the ravinement surface). SF3 likely ties to stiff clays at , 2.5 mbsf, while SF4 correlates with sands overlying these clays but below the T horizon. Wecorrelate SF2 to stiff clays sampled below , 2.5 down to , 3.55 mbsf. A , 10-cm-thick, iron-oxide-rich, sandy layer at , 3.55–3.65 mbsf, which is a prominent featurein the synthetic seismogram, is interpreted as SF1. We correlate an , 15 cm-thick mud layer with abundant sand lenses immediately underlying the iron-oxide layer at, 3.65–3.8 mbsf, as the actual incision surface or valley flank, or seismically as the Channels horizon. Less stiff muds, with some interbedded sand lenses (near the base),occurring from , 3.8 mbsf to , 5.6 mbsf, correlate with the section between the Channels-horizon and the R horizon. The R horizon marks the transition from thesemuds to underlying sands, which extend down to the base of the cored section.


variations within these units, we present upstream and downstream crosssections from both mapped drainage systems (Fig. 1), the larger one tothe northeast (Figs. 5 and 6, respectively) and the smaller one to thesouthwest (Figs. 7 and 8, respectively).

Descriptions of Facies Units

In addition to considering reflector continuity, amplitude, frequency,phase, and spacing, combined with geometry and superposition of seismicsequences in our analysis, we interpreted: (1) acoustically transparentintervals as sediments that are homogeneous in velocity and density, and(2) acoustic laminations as interbedded sediments of contrasting grainsize. Such inferences are supported by preexisting comparisons of sparsecore data with 2D and 3D seismic data from the New Jersey shelf (Fig. 4;Buck et al. 1999; Goff et al. 1999; Duncan et al. 2000; Nordfjord et al.2002; Gulick et al. 2003; Alexander et al. 2005).

Seismic Facies 1 (SF1).—SF1 (Fig. 3) is characterized by chaotic,variable-high amplitude reflections situated near the bases of observedincisions, as defined by the Channels horizon (Figs. 2, 5–8). The base ofthis facies in proximity to the Channels horizon is poorly resolved wherethere is a small acoustic impedance contrast between the eroded base ofthese valleys and incised sediments (Fig. 7).

SF1 seems to exhibit higher-amplitude, more chaotic reflections in thesmaller, southwestern system (Figs. 7, 8) than in the larger, northeasternsystem (Figs. 5, 6). Part of this difference might be ascribed to the factthat sediments into which the southwestern drainage is incised exhibita similar chaotic, high-amplitude acoustic response (Fig. 7).

The isopach map for the northern system shows that SF1 ranges from, 1–7 m in thickness (Fig. 9A). SF1 reaches its maximum thickness,, 7 m, in the middle of the main trunk channel of this system, withsimilar thicknesses in the middle of its two largest tributaries. Elsewhere,SF1 is generally , 4 m thick. In the southwestern system, SF1 iseverywhere , 3–4 m, but with a maximum thickness again reached in itsmain trunk channel (Fig. 9A).

Seismic Facies 2 (SF2).—SF2 (Fig. 4) consists generally of high-amplitude, parallel-continuous reflectors within and along valley axes(Figs. 5–8). Along valley flanks, packets of subhorizontal reflectors gradelaterally into small, acoustically transparent wedges (Figs. 3, 5–6).

Upstream in both systems, SF2 thicknesses are generally , 1 m(Fig. 9B); thickness remains generally constant across valley axes (Figs. 5,7). Along valley flanks, SF2 occurs in irregular patches 2–3 m thick, theresult of the presence of transparent sediment wedges that occur variablyalong these flanks (Figs. 3, 8). Such flank deposits can be observed onlyin the northeastern system; they are more persistent where the main valleyis wide, while other flank deposits are located where major tributariesmerge with this trunk channel (Figs. 3, 9B). Similar SF2 thicknesses occuralong portions of valley centers (Fig 9B). In both systems, seawardthicknesses are more variable, but are generally , 1–2 m (Figs. 6, 8).

Seismic Facies 3 (SF3).—SF3 (Fig. 3) is either acoustically transparentor weakly layered; upstream, low-amplitude, subhorizontal reflectorsonlap valley flanks (Figs. 5, 7). Downstream, higher-amplitude, subhor-izontal reflectors also occasionally occur within this facies (Fig. 6).However, individual reflectors cannot generally be mapped from oneprofile to the next over lateral distances of , 200 m, implying onlylocalized impedance contrasts.

The SF3 isopach map displays thicknesses of up to 6–7 m, withmaxima localized in zones landward of tributary junctions with the mainchannel within both mapped systems (Fig. 9C). SF3 is thickest upstream(Figs. 5, 7), becoming generally , 1 m thick, patchy, or occasionallyabsent downstream (Figs. 6, 8).

Seismic Facies 4 (SF4).—SF4 (Fig. 3) is observed predominantlydownstream on profiles crossing both mapped incised-valley systems(Figs. 6, 8). This facies exhibits variable amplitudes and seismicconfigurations, and includes parallel-continuous reflectors (Fig. 8), wavyreflections (Fig. 3), and small clinoforms (Fig. 8). SF4 is generallytruncated by either the T horizon or the seafloor (Figs. 6, 8).

Where present, SF4 exhibits a thickness variation of , 1 to , 7–8 m(Fig. 9D). Along the axes of the main valleys of both mapped systems,SF4 is generally thicker where SF3 is thinner, and vice versa. SF4 isgenerally absent within smaller tributaries of both systems.

Seismic Boundaries

We have mapped five seismic horizons delineating the boundaries ofincised-valley fills on the New Jersey shelf. Each of these surfaces hasbeen characterized on the basis of its location, truncation style, andassociated facies distributions both above and below (Fig. 3).

Channels Horizon.—This reflector is the seismic surface identified inthis part of the shelf by Davies et al. (1992), Austin et al. (1996), andDuncan et al. (2000) which defines the axes of incised-valleys (Fig. 2);because it defines incisions everywhere, it is generally concave upward(Figs. 5–8, 10A). The depth of the Channels horizon illustrates an, 25 m of elevation change over a distance of , 15 km (Fig. 10A),representing an average seaward dip of 0.1u. Plots of incision depth withdownstream distance for both systems are linear, with no evidence fornickpoints within either main channels or major tributaries (Fig. 11). TheChannels horizon is interpreted most easily where it is overlain by thehigh-amplitude, chaotic SF1 (Fig. 7, top). This horizon is usuallytruncated either by the seafloor or by the T horizon, except along somevalley flanks (Figs. 5, 7).

B1.—Along incised-valley axes, horizon B1 forms the upper boundaryof SF1 and is overlain by SF2 (Figs. 3, 5–8). Horizon B1 appears totruncate the Channels horizon along valley flanks (Fig. 3), and it deepensalong valley flanks downstream (e.g., compare Fig. 5 with Fig. 6).Deepening of B1 is generally associated with thicker SF2 deposits. In bothmapped incised-valley systems, B1 exhibits lateral irregularities that mayrepresent its erosion of the SF1 facies (Figs. 5, 6).

The structure contour map of horizon B1 (Fig. 10B) confirms thatthis surface deepens downstream, and it also indicates its generallywider cross-valley extent than that of the Channels horizon(Fig. 10A), particularly seaward. Seismic evidence that B1 truncates theChannels horizon along some valley flanks (Fig. 3) suggests that B1represents erosion that widened preexisting, partially filled (by SF1)incisions.

B2.—The B2 horizon (Fig. 3) marks the boundary between facies SF2and SF3. B2 is onlapped by weak, subhorizontal reflectors within SF3(Figs. 3, 7). The depth distribution of B2 (Fig. 10C) indicates a narrowercross-valley extent than for B1 (Fig. 10B), demonstrating that B2 iscontained within a depression formed by B1 (Fig. 6). B2 is occasionallyabsent from the downstream part of the southwestern incised-valleysystem, where it is truncated there by the B3 horizon (Fig. 8). Suchtruncation of B2 by B3 is more widespread in this incised-valley system(Figs. 8, 10B).

B3.—We observe the B3 horizon (Fig. 3) only in the downstream partsof our mapped incised-valley systems; the structure contour map of thishorizon shows that it is absent from smaller tributaries of both systems(Fig. 10D). We identify B3 as the boundary between underlying SF3 andoverlying SF4. B3 resembles a series of small incisions localized within thelarger incised valley defined by the Channels horizon and B1 (Fig. 8). In


the northeastern system, B3 is occasionally difficult to distinguish fromhigh-amplitude reflectors within SF4 (Fig. 3).

T horizon.—The T horizon (Fig. 2) caps the valley-fill units (Duncanet al. 2000). We identify this boundary by its truncation of underlyingreflectors and by subtle onlap onto it by overlying reflectors (Figs. 3, 5–7;Goff et al. 2005). We also recognize the T horizon as a faciescontrast upward from horizontal reflectors within SF4 to acousticallytransparent material (Figs. 5–7). On both boomer and chirp data,the T horizon is a reflector of generally moderate but highly variableamplitude (Duncan et al. 2000; Goff et al. 2005; Figs. 5, 7). Itsstructure contour map (Fig. 10E) shows that the T horizon isrelatively planar, except where it has been truncated by the seafloor(Fig. 8). These truncated regions are elongated in a NE–SW direction(Fig. 10E), parallel to the principal modern bottom-current direction(Vincent et al. 1981).

DISCUSSION

Stratigraphic Significance of Distribution of Seismic Facies

We hypothesize that the four mapped seismic facies (SF1–4) making upour incised-valley fill represent distinct sedimentary successions (Figs. 3,4). We illustrate our interpreted facies units and seismic-stratigraphicboundaries in Figure 12, an idealized dip section through the main trunkvalley of the northeastern system (Fig. 1). First, we use results from ourseismic correlation at Site 3 (Fig. 4) to attempt to tie the seismic facies toavailable geologic data. Then, we expand that correlation regionally, withthe aid of two conceptual models that relate such facies distributions toknown sedimentary processes associated with drowned river estuaries(Fig. 13; Dalrymple et al. 1992; Zaitlin et al. 1994).

SF1—Fluvial Lag Deposits.—We correlate SF1 with the thin(, 10 cm), iron-oxide-rich sand layer sampled at Site 3 immediately

FIG. 5.—A seismic cross section (top) and interpretation (bottom) within the upstream portion of the larger, northeastern, filled incised-valley system (see Fig. 1 forlocation), showing seismic facies units SF1–SF3 and stratal boundaries Channels horizon, B1, B2, and T horizon. Facies unit SF4 and boundary B3 do not occur in thispart of the system (compare with Fig. 6).


above the Channels horizon, which forms the upper valley flank (Fig. 4).The presence of iron oxide suggests to us subaerial exposure at this site.Furthermore, both the chaotic, high-amplitude seismic characteristics ofthis facies (e.g., Fig. 3), and its position immediately above the Channelshorizon (e.g., Fig. 7), the regional basal incision surface (Fig. 2), implydeposition under high-energy, riverine conditions. Therefore, we interpretSF1 as a fluvial lag, most likely deposited as these systems were firstincised and developed during the lowstand associated with the LGM(Fig. 13A). Consistently, Goff et al. (2005) interpreted a grab sampletaken at the base of an eroded channel, just south of our seismic coverage,as being of fluvial origin. This sample consisted of abundant roundedgravel and cobbles up to 6 cm in diameter, confirming that these lagdeposits are coarse-grained. Nordfjord et al. (2005), using these samemapped and interpolated incised-channel geometries as a guide, employedempirically derived hydraulic equations for modern rivers and estuaries toestimate paleo-discharges, velocities, and maximum shear stresses in thesepresumed fluvial systems. The resultant estimated fluvial discharges andboundary shear stresses would have been sufficient to transport particlesup to , 1.5 cm in diameter as bed load. The channel-like geometrieswithin which SF1 lags were deposited suggest meandering or braidedfluvial systems flowing to base level (Davies et al. 1992; Austin et al. 1996;Duncan et al. 2000). The lack of evident nickpoints (Fig. 11) suggests thatthese channels systems, in their fluvial phase, reached some sort ofequilibrium in depth adjustment during LGM exposure (as opposed toreaching equilibrium in sinuosity). Local flattening of the northeasterntrunk channel could indicate the earlier presence of a delta at that

location, although if such a delta existed it has been eroded away, in asmuch as there is no seismic evidence for it.

SF2—Estuarine Flank Deposits.—We suggest that SF2 (Fig. 3) wasdeposited as Holocene transgression began to backfill incised fluvialvalleys. During this early stage of transgression, a zone of fluvialaggradation and tidal influence migrated landward with the shoreline asbase level rose (Fig. 12B; Dalrymple et al. 1992; Allen and Posamentier1993; Zaitlin et al. 1994). Salt-marsh and tidal-flat sediments formed theprimary deposits along channel margins (Masselink and Hughes 2003);SF2 correlates with stiff clays at Site 3 which could represent suchdeposits (Fig. 4). Foraminiferal and sedimentological evidence fromavailable vibracores (Buck et al. 1999) and Site 3 (Alexander et al. 2005)suggest initiation of an estuarine depositional sequence in this strati-graphic position. The parallel-continuous reflections composing SF2 maybe related to the fluctuating organic-carbon contents and/or lithificationof such sediments; similar seismic facies characteristics have beenobserved for early-flooding-stage sediments of the Delaware Riverestuary (Fletcher et al. 1990).

Acoustically transparent, weakly layered sediment wedges along theincised-valley flanks within SF2 (Figs. 3, 6) and adjacent to junctions oftributaries with main channels (Fig. 9B) may represent a meandering tidalfacies, or point-bar deposits; similar deposits are found today in the wave-dominated estuary of Raritan River, New Jersey (Fig. 13B; Ashley andRenwick 1983). These flank deposits could also represent remnants ofsmall bayhead deltas or swash bars, which could have developed and been

FIG. 6.—A seismic cross section (top) and interpretation (bottom) within the downstream portion of the larger, northeastern, filled incised-valley system (see Fig. 1 forlocation), showing seismic facies units SF1–SF4 and stratal boundaries Channels horizon, B1–B3, and T horizon.


preserved in association with turbidity maxima near distributary mouthsof a tide-dominated estuary (Masselink and Hughes 2003).

SF3—Central-basin-fill deposits.—Subhorizontal reflections of variableamplitude in association with acoustically transparent zones, acousticevidence within SF3 of a low-energy, passively infilling depositionalenvironment, suggest the presence of central basin muddy deposits

(Dalrymple et al. 1992). Only Site 3 sampled this facies (Fig. 4), whichindeed proved to be stiff clay. Such muds were likely deposited in tranquilconditions during a more advanced stage of the Holocene transgression.Central-basin fluid muds correspond to the estuarine turbidity maximum(Allen 1991), where fresh and salt water mix, resulting in clay flocculationand deposition of what eventually become stiff clays. Thomas andAnderson (1994) have demonstrated the presence of a similar seismic

FIG. 7.— A seismic cross section (top) andinterpretation (bottom) within the upstreamportion of the smaller, southwestern, filledincised-valley system (see Fig. 1 for location),showing seismic facies units SF1–SF3 and stratalboundaries Channels horizon, B1, B2, and Thorizon. Facies unit SF4 and boundary B3 donot occur in this part of the system (comparewith Fig. 8).


facies within the Trinity–Sabine incised valleys on the East Texas shelf;when cored in the Gulf of Mexico, this facies is found to represent stiffclays similar to those which we correlate with SF3 at Site 3 (Fig. 4).

The thickness of SF3 is variable but is in general thicker upstream andthinner downstream in both mapped systems (Figs. 9C, 12). Regionally,thicker SF3 deposits occur just landward of junctions of main channelswith major tributaries (Fig. 9C). Such a variation in SF3 thickness mustbe a function of riverine input and proximity to the shoreline (Allen1991). For example, a combination of low fluvial discharge and higherwave and tidal-current activity near the mouth of the drowning estuarywould shift both the turbidity maximum and associated settling of fine-grained sediments upstream (Allen 1991). Two paleoenvironmentalfactors would lead to such spatial variations in central-basin depositsthrough time: (1) Holocene sea-level rise did not occur at a constant rate,and (2) differences in valley morphology lead to significant variations inlocal depositional processes within the drainage system. Variable sea-levelrise has been corroborated globally (Chappell et al. 1996). Pauses orperhaps even small retreats in the transgression, as suggested for the NewJersey shelf by vibracore evidence (Buck et al. 1999), could producevariable sediment accumulations along the filling incised-valley axes, asobserved (Fig. 9C). The dendritic geometries of these must also havemodulated the sedimentological impact of waves and tidal currents asthese systems migrated landward. For example, we suspect that the widemouths of flooding main trunk valleys downstream of tributary junctionsexperienced greater wave and local current activity during transgression,pushing turbidity maxima into narrower parts of meandering valleysupstream of those junctions, in turn leading to higher depositional rates inthose locations (Masselink and Hughes 2003).

SF4—Estuary-Mouth Complex.—We suggest that SF4 (Figs. 6, 8, 9D)reflects deposition under complex and energetic wave and currentconditions (Fig. 13C), a dynamic set of environments represented byfrequent lateral variations in sedimentary facies, ranging from tidal inletsto washovers, flood-tidal deltas, and/or barrier beaches (Dalrymple et al.1992; Zaitlin et al. 1994; Masselink and Hughes 2003). High-amplitude,subhorizontal to gently dipping reflections (Figs. 3, 6, 8) within SF4 mayrepresent migrating sand waves, linear shoals, or tidal bars, sourced bylongshore drift across the paleo-estuary mouth and/or by reversing tidalflows within a paleo-estuary entrance (e.g., Foyle and Oertel 1997;Masselink and Hughes 2003). The antithetic relationship between thethickness distributions of SF3 vs. SF4 (compare Fig. 9C and D) suggeststhat their depositional regimes are connected; SF4 appears to beoccupying topographic lows in B3 (i.e., accommodation in the top ofSF-3; Fig. 12). The depositional setting seaward of the turbiditymaximum should be influenced primarily by wave (orbital) energy, whichfavors coarse-grained deposition of shoals and bars (Masselink andHughes 2003). In contrast, any variation in depositional conditions thatfavors a static location of the turbidity maximum tends to form a morepaired system, consisting of finer-grained central-basin deposits of theestuary upstream and more coarse-grained, estuary-mouth complexdeposits nearer the shoreline (Fig. 13C).

Siringan and Anderson (1993) sampled the upper part of a flood-tidaldelta from a delta complex off Bolivar Peninsula in Texas. That complexconsists of interlaminated clay and fine sand, which could produce thehigh-amplitude packages of reflectors near the SF3–SF4 boundary(Figs. 3, 8). Marine sands deposited in incisions within estuarine mudhave also been observed within the James River microtidal barrier

FIG. 8.—A seismic cross section (top) and interpretation (bottom) within the downstream portion of the smaller, southwestern, filled incised-valley system (see Fig. 1for location), showing seismic facies units SF1–SF4 and stratal boundaries Channels horizon and B1–B3. SF3 is nearly absent in this cross-section, truncated by B3. The Thorizon is not observed in this part of the system.


estuary, located along the transgressive coast of New South Wales,Australia; Nichol (1991) interprets these as estuary-mouth sediments. Wesuspect that sands lying atop muds just below the T horizon at Site 3(Fig. 4) are also estuary-mouth deposits, and that SF4 thereforerepresents the final stage of drowning and filling of New Jersey incised-valley systems.

Holocene Surficial Sand Sheet.—Much of the modern New Jersey shelfis overlain by a , 1-m-thick, patchy veneer of Holocene shelf sands lyingatop the T horizon (Swift et al. 1980; Goff et al. 1999; Goff et al. 2004;Figs. 3, 4). Sediments lying on the transgressive ravinement surface(Fig. 5) have likely been formed initially as shoreface deposits, then latermodified by current reworking in outer-shelf depths (Goff et al. 1999;Goff et al. 2004; Goff et al. 2005). This marine facies is interpreted as theuppermost part of the transgressive systems tract (Swift and Thorne1991). These sands are largely acoustically transparent or weakly layered(Fig. 3); occasional mounded geometries also occur (Fig. 8). We considerthese deposits to be analogous to the sheet-like geometries of shallowmarine clastics deposited by waves in the Gulf of Mexico (Tye andMoslow 1993).

Valley-Fill Sequence Stratigraphic Framework

The lack of accommodation space on the very slowly subsiding NewJersey middle–outer shelf during the Holocene transgression did not allowsignificant vertical separation of the lowstand surface from subsequentbounding stratigraphic surfaces associated with ensuing inundation(Fig. 12). Within incised paleo-valleys, however, we have been ableto map seismic surfaces separating the valley-fill facies successionspreviously described: fluvial, estuarine, and estuary-mouth deposits(Figs. 12, 13). Within these filled incisions, high-frequency sea-levelevents associated with the last transgression have been preserved (Buck etal. 1999). Our seismic horizons are spatially confined within the paleo-valleys (Fig. 10), so we conclude that they must have been produced bylocalized processes, associated first with fluvial incision and then withfilling and drowning of these mapped systems. These processes includetidal channel migration, inlet scour, and/or wave-base erosion (Fig. 13;Masselink and Hughes 2003). We further suggest that all of the seismichorizons within the valley fill must have been produced in response torelative sea-level changes: fluvial incision during shelf exposure andsubsequent flooding and redeposition associated with the Holocene

FIG. 9.— Isopach maps of seismic facies units: A) SF1, B) SF2, C) SF3, and D) SF4, for both the northeastern and southwestern filled incised-valley systems.


transgression. The hierarchy and geometry of flooding surfaces reflectsthe landward translation of valley-fill facies during sea-level rise (Fig. 12).These boundaries also often represent composite surfaces, where youngerstratal boundaries merge with subjacent horizons in both cross-valley(Figs. 5–8) and dip directions (Fig. 12). One good example of this processis the re-excavation of the lowstand fluvial surface, the Channels horizon,by horizon B1 along valley flanks (Figs. 3, 6, 8). Furthermore, althoughthe spatial relationships between any two superimposed seismic faciesmay appear conformable, for example facies filling the floors of maintrunk channels in both systems (Figs. 6, 8), we believe that minor erosionand/or absolute age discontinuities of varying duration are probablyassociated with all of our seismic boundaries (e.g., Buck et al. 1999).

Channels Horizon—Lowstand Fluvial Incision Surface.—The Channelshorizon (Fig. 2) has long been interpreted as a true sequence boundary,a fluvial incision surface cut at some time during lowered sea level duringor near the LGM (Davies et al. 1992; Duncan et al. 2000). Our mappingof the incised paleo-valleys concurs with the previous work that thesebegan as fluvial systems (Fig. 13A), on the basis of their systematic

incision into underlying latest Pleistocene strata, the presence of chaoticseismic fill (SF1) at their bases (Fig. 12), and their dendritic plan-viewgeometries, with junction angles that are consistent with a riverine origin(Fig. 1; Nordfjord et al. 2005). The chaotic fill (SF1) may be indicative ofnon-marine, coarse-grained lag deposits (Austin et al. 2001; Goff et al.2005). The Channels horizon correlates at Site 3 to a transition from claybelow to an iron-rich sand layer above, indicative of an interval ofsubaerial exposure within a terrestrial (coastal plain) environment(Fig. 4).

B1—Bay Flooding Surface.—We interpret B1 (Fig. 3) as a bay floodingsurface (Nummedal and Swift 1987). Also known as an estuarinetransgressive or tidal flooding surface (Allen and Posamentier 1993),a bay flooding surface is cut within an estuarine setting. B1 is equivalentto the transgressive surface in an idealized sequence-stratigraphic modelof a wave-dominated estuary (Zaitlin et al. 1994). Its leading edge is thelandward limit of tidal influence, and it is defined by the so-called bayline(Fig. 13B; Posamentier and Vail 1988; Anderson et al. 1992). The baylineis cut by wave and/or tidal-current scour, represents the landward limit of

FIG. 10.—Structure contour maps in depth of seismic sequence boundaries: A) Channels horizon, B) B1, C) B2, D) B3 for both mapped incised-valley systems, andE) T horizon, which extends beyond mapped incised-valley system boundaries. The T horizon is absent where it has been truncated by seafloor erosion (next page).


transgressive estuarine deposits, and is also the upstream equivalent of theT horizon (Fig. 12), which develops near or at the shorelines underconditions of higher wave and current energy (Dalrymple et al. 1992;Masselink and Hughes 2003). Development of B1 is confined to coastalembayments. Erosion associated with this surface may modify all parts ofthe preexisting fluvial geomorphology. However, off New Jersey thefluvial incision surface, the Channels horizon, is preserved within the axisof main trunk valleys (Fig. 3). B1 truncates the Channels horizon alongvalley flanks (Figs. 3, 6, 8).

B2—Depositional Surface within Estuarine Fill.—We interpret B2(Fig. 3) as a depositional surface within the estuarine sequence (Figs. 12,13C). At Site 3, this conformable seismic boundary superimposes muddycentral basin deposits of SF3 over equally fine-grained estuarine flankdeposits of SF2. Downstream, B2 is truncated by or merges with B3(Figs. 8, 12).

B3—Tidal Ravinement Surface.—We interpret B3 (Fig. 4) as a tidalravinement surface (Figs. 12, 13C; Zaitlin et al. 1994), formed as a resultof both landward and lateral migration of an estuary-mouth duringcontinuing transgression. B3 is preserved as an erosional surface ofmoderate relief within the paleo-estuary bay (Figs. 6, 8), while alongmargins this surface shoals and is generally truncated by either the Thorizon (Fig. 6) or the modern seafloor (Fig. 8).

B3 is typically associated with overlying flood-tidal-delta complexeswithin wave-dominated estuaries, i.e., SF4 (Fig. 12; Zaitlin et al. 1994).As this surface migrates laterally and landward, it is buried bydownlapping flood-tidal delta sands of SF4 that also prograde landward(Figs. 6, 8, 12, 13C; Colman et al. 1988; Fletcher et al. 1990). Suchdownlapping sands are sourced by littoral drift from the adjacentcoastline and reworked by shore-orthogonal, reversing tidal currentsat the bay mouth (Ludwick 1972; Masselink and Hughes 2003). Wedistinguish B3 from the stratigraphically higher T horizon by larger localseaward gradients (Fig. 12) and by its absence outside our filled incised-valley systems (Figs. 6, 8, 10D).

Allen and Posamentier (1993) recognized a similar tidal ravinementsurface in the Gironde River estuary, France, particularly where inner-estuary deposits have been eroded and overlain by estuary-mouth sands.Lericolais et al. (2001) used chirp sonar data to highlight that tidalravinement surface.

T Horizon—Transgressive Ravinement Surface.—The T horizon(Fig. 4) was first interpreted by Duncan et al. (2000) as the seismicexpression of a transgressive ravinement surface. We believe that the Thorizon was formed by erosional shoreface retreat during rising sea level(Swift 1968; Nummedal and Swift 1987), and therefore represents the firstregional marine flooding event across the submerging continental shelf.This boundary caps both estuarine deposits within the incised-valley fills

FIG. 10.—Continued.


(SF3 and/or SF4, Figs. 5–7) and older, outer-shelf sediment veneeroutside these valleys (Gulick et al. 2005). The T horizon also forms thebase of the Holocene sand sheet (Figs. 3, 12). The T horizon truncatedunderlying incised-valley fills during its formation, but it has also sincebeen truncated in part by latest transgression-highstand erosion (Figs. 8,10E; Goff et al. 2005).

Siringan and Anderson (1993) documented, seismically and with cores,inner-shelf sediments overlain by a ravinement surface marked bya transgressive lag deposit in the Bolivar Road, Gulf of Mexico, tidalinlet, along the East Texas coast. This lag consists of large, mixed shellfragments, calcareous nodules, and other granule-size to pebble-size lithicfragments. Posamentier (2002) recorded similar shell-lag depositsassociated with a transgressive ravinement surface in a study of sandridges offshore Java. At Site 3, the T horizon is not associated with a lagdeposit but instead is correlated with the base of the surficial sands(Fig. 4). However, large areas of the New Jersey shelf which have beeneroded down to or below the T horizon are covered by a shell hash, whichshows up as bright returns on backscatter images (Goff et al. 2004;Goff et al. 2005). Goff et al. (2005) infer that these lag deposits are re-excavated portions of the T horizon (Fig. 10E) which contribute to thelarge impedance contrasts producing observed high-amplitude seafloorreturns.

Incised-Valley Deposition during Wave- Versus Tide-Dominated Conditions

Dalrymple et al. (1992) and Zaitlin et al. (1994) have described thestratigraphic arrangement of facies under wave- and tide-dominatedestuarine conditions. In their models, the estuary is divided into threesegments: (1) an inner, river-dominated zone, (2) a relatively low-energy,central zone where river flow is countered by flood-tidal energy, and(3) an outer, marine zone dominated by waves and/or tides. The wave-dominated estuary contains a barrier beach and a tidal-inlet complex atits mouth, whereas the tide-dominated estuary is fronted by intertidalsand bars that grade to mud flats and peripheral salt marshes up-estuary.The tide-dominated estuary does not exhibit such a pronounced tripartite(i.e., coarse–fine–coarse) distribution of lithofacies. In the wave-domi-nated case, coarse-grained fluvial sediments are deposited at the head ofthe estuary, forming a bayhead delta (Dalrymple et al. 1992). In contrast,tidal energy penetrates further upstream than wave energy in tide-dominated estuaries, and bayhead delta and muddy central-basin depositsare not present in the river-dominated portion of these estuaries. Instead,this inner zone consists of tidal meanders and straighter fluvial and tidalchannels even farther landward (Dalrymple et al. 1992).

These sequence stratigraphic models both assume a constant-energysetting throughout a relative sea-level cycle, and a tidal excursion withinincised valleys during late lowstand–early transgression. Studies ofHolocene systems, coupled with ancient outcrop studies, however,suggest that levels of depositional energy through such a cycle are morecomplicated, in response to coastal bathymetry, shelf width, tidalresonance, and sea-level behavior (Yoshida et al. 2005).

We believe that our SF2 facies corresponds to the upstream zone oftidal flats and salt marsh within the tide-dominated-estuary model; SF2sediments accumulated during an aggradational phase that began as thesevalleys began to backfill. Antecedent fluvial topography on a very gentlydipping shelf created increased accommodation (Fig. 13A), which likelycaused an increase in the tidal prism and therefore promoted tide-dominated settings early in the valley-fill sequence (Cooper 2002). Tideranges on the New Jersey shelf may also have been larger at or near theLGM (Egbert et al. 2004). Localized wedge-shaped accumulations alongvalley flanks, in places associated with merging tributaries (Fig. 9B), mayrepresent small bayhead deltas and/or modified point-bar deposits(Figs. 3, 6). However, our seismic evidence for bayhead deltas is sparse,perhaps a result of continuing fluvial (coarse-grained?) sediment inputduring initial transgression (Boyd and Honig 1992). Our SF2 facies isconsistent in character with the inner segment of wave-dominated,incised-river estuaries (Dalrymple et al. 1992; Zaitlin et al. 1994).

The tripartite zonation of wave-dominated estuaries (Dalrymple et al.1992; Zaitlin et al. 1994) calls for a general pattern of net bed-loadtransport. Fine-grained SF3 sediments accumulated primarily landwardof tributary junctions (Fig. 9C), while coarse-grained SF4 sediments,winnowed by waves and currents, accumulated seaward (Fig. 9D). Wesuggest two possibilities to explain downdip changes in the thickness ofthese two facies units: (1) transgressive migration of the seashore acrossthis region did not occur uniformly, but rather with pauses or eventransitory regressions that affected total accumulation of these units asa function of proximity to the shoreline (e.g., Locker et al. 1996; Buck etal. 1999), or (2) partition between fine- and coarse-grained sedimentswithin valley fills is governed by depositional processes interacting withlocal valley morphology. For example, wave forces could have favoredcoarse-grained deposition in wider portions of these systems seaward oftributary junctions (Fig. 9D). A third possibility is that the boundarybetween these two facies through time is related to location of theturbidity maximum, which in turn is related both to proximity of theshoreline and to preexisting geometries of the fluvial systems beingtransformed by transgression into estuaries. Landward of that maximum,fine-grained estuarine sediments dominated, while marine processes

FIG. 11.—Along-channel thalweg depths for both northeastern and southwest-ern trunk channels. Both are highly linear, with the only exception a possibleflattening trend observed at the seaward end of the northeastern trunk channelprofile. There is no evidence of nickpoints either along these channels or intributary channels.


produced coarse-grained sediment deposits nearer to the estuary mouths(Masselink and Hughes 2003).

Due to their location near the lowstand shoreline (the modern shelfbreak), our mapped incised-valley systems must have been filled anddrowned during early stages of the Holocene transgression. Radiocarbondating of benthic foraminifera from one vibracore sample of an incised-valley flank in our study area constrain the age of this fill to AMS 14C12.3 6 0.45 ka (Fig. 1; Lagoe et al. 1997; Buck et al. 1999). By contrast,valleys that incise the landward edge of other continental shelves, such asthe Gironde estuary, France (Allen and Posamentier 1993) andShoalhaven and Lake Macquarie, Australia (Roy 1994; Umitsu et al.2001), are also subjected to late transgressive–highstand sedimentation.Such sedimentation can produce prograding bayhead deltas (at sea-levelstillstand) and/or increased fluvial sediment input (overwhelming the sea-level rise) (Reynaud et al. 1999). However, because our study area wascompletely submerged by , 12 ka, we assume that a highstand-systems-tract facies, such as a regressional and progradational bayhead delta, waseither never deposited or not preserved. The ravinement surface, or Thorizon, may also have removed some or all of these deposits (Figs. 5, 7,10E).

The widespread presence of estuarine-flank sediments within SF2,specifically salt-marsh and tidal-flat deposits (Fig. 13B), along withlimited evidence for bayhead deltas within this facies (Fig. 9B), suggestthat New Jersey incised-valley systems were initially tide-dominated (e.g.,Fletcher et al. 1992). These estuaries must have experienced a tidal prismlarge enough to maintain tidal currents against both longshore and cross-shore, wave-driven littoral sediment transport (Cooper 2002; Egbert et al.2004). The shape of valley systems being flooded may control the nature

of the facies developed in that estuary, particularly during early infilling(Dalrymple et al. 1992). We speculate that, as a result of the funnel shapeof the valleys that we observe off New Jersey (Fig. 1), flood tides wereprogressively compressed into smaller cross-sectional areas, therebyaccentuating tidal amplitudes and accelerating flood-tidal currents.

As transgression of the New Jersey shelf continued and antecedentfluvial geomorphology was further modified, early tide-dominated valleyslikely became wave-dominated systems (Fletcher et al. 1992; Masselinkand Huges 2003). The shoreline became more proximal to these incisedvalleys, tidal currents dissipated, and estuarine cross-sectional areasincreased behind barrier bars produced and maintained by wave energy.Central-basin muds (SF3) were deposited behind these barriers as waveinfluence increased (Masselink and Hughes 2003). Finally, tidal currentsdistributed sediment along the paleo-shoreface, forming the tidalravinement surface B3 (Fig. 13C; Zaitlin et al. 1994). Finally, astransgression continued, flood-tidal deltas, tidal inlets, and wash-overdeposits near the drowning estuary mouth produced facies SF4(Fig. 13C).

Comparisons with Other Drowned Incised-Valley Systems

East Coast United States.—The Delaware Bay estuary displays faciessimilar to the incised-valley fills described in this paper (Fletcher et al.1992). Flooding of this estuary also occurred during the Holocene, as theshoreline retreated northwest along a path determined by pre-trans-gression topography. Fletcher et al. (1992) suggest that a tidal-wetlandlithofacies was deposited during the early stages of inundation, when tidaldistribution was the primary mode of sediment dispersal. These mainly

FIG. 12.—Distribution of interpreted seismic facies and bounding seismic horizons along a dip section through the trunk channel of the northeastern incised-valleysystem. See Figure 1 for location.


marsh deposits likely correlate with our SF2 (Figs. 3, 5–6). Landwardmigration of an estuarine turbidity-maximum depocenter provided thebulk of fine sediments that now form the coastal Holocene section, ourSF3 facies. Later stages of inundation, producing greater fetch within theopen estuary, increased wave energy and produced overlying coarse-

grained deposits (Fletcher et al. 1992), which we correlate with our SF4facies (Fig. 13C).

Foyle and Oertel (1997) have identified transgressive paleo-valley-fillsuccessions on the Virginia inner shelf. As with our New Jersey systems,these fluvially incised valleys have been modified during subsequent

FIG. 13.—Schematic representations of theevolution of New Jersey incised paleo-valleysystems, including sedimentary facies and strati-graphic boundaries: A) fluvial incisions, withpreserved channel lags; B) aggradational estua-rine system, at the initiation of back-filling;C) passive infilling of the estuary, with developedcentral-basin muds and estuary-mouth com-plexes. Not shown is formation of the trans-gressive ravinement (T horizon), which hasreworked and selectively removed portions ofthese fill deposits. Figure is modified from Allenand Posamentier 1993.


marine transgression, as dendritic riverine drainage basins evolved tobecome estuaries. The fill of these valleys is punctuated by a bay floodingsurface, our B1, and both tidal and transgressive ravinements, our B3surface and the T horizon, respectively. However, Virginia valley fillswere dominated by estuary-mouth deposits of the outer zone (Dalrympleet al. 1992; Zaitlin et al. 1994) overlying a tidal ravinement surfaceexhibiting high relief. Fluvial deposits, our SF1 facies, are only locallypreserved immediately above the fluvial incision surface. In contrast, wehave mapped SF1 throughout our survey area (Fig. 9A). Aggradation oflowstand fluvial deposits may not have occurred within Virginia inner-shelf fluvial valleys during the latest Pleistocene. Instead, higher-relieffluvial channels, incising the emerging shelf during regression andlowstand, may have bypassed fluvial sediments seaward, towards thepaleo-shoreline.

Gulf of Mexico.—Incised valleys mapped offshore Mobile, Alabama(Bartek et al. 2004) appear morphologically and stratigraphically similarto New Jersey incised valleys. Two valleys were cut during streamrejuvenation associated with subaerial exposure during the LGM, thenthrough headward (nickpoint) erosion of coastal streams initiated at theshelf edge. Nordfjord et al. (2005) have called upon a similar formativemechanism for the New Jersey systems. The rapid rate of sea-level fall onboth shelves (Fig. 1; Chappell et al. 1996) resulted in incision of numerousriverine valley systems in both locations; individual drainage systems werenot able to keep pace with the falling sea level (Wood et al. 1993).However, the vertical facies-stacking pattern off Alabama includes a well-developed, prograding bayhead-delta facies, which differs from thestacking patterns preserved off New Jersey (Figs. 12, 13). Mappedincised-valley systems off Alabama are also wider, deeper, and closer tothe shoreline today than our New Jersey systems; they may be connectedwith the Mobile River (Bartek et al. 2004).

One Ancient Analog.—The Pennsylvanian Morrow Formation ineastern Colorado and Kansas is an ancient setting that is similar to ourpreserved, infilled New Jersey paleo-valleys. The Morrow Formation isinterpreted as incised-valley-fill deposits (Bowen and Weimer 2003).Valleys were cut during glacially induced eustatic sea-level fall, thengradually filled with sediment as base level rose during ensuingtransgression.

CONCLUSIONS

We suggest that filled incised-valley systems on the New Jersey shelfdocumented here are the product of the last sea-level cycle, incorporatingboth lowstand and transgressive systems tracts. Mapped valley fillsoccupy fluvial incisions that were produced during shelf exposure near orduring the LGM. Estuaries resulted from drowning of these river valleysduring ensuing sea-level rise; their stratigraphic organization reflectsmigration of depositional environments during transgression, in responseto base-level changes, tides, waves, and open-marine currents. From ourextensive seismic and limited ground truth, we recognize facies that can beinterpreted as fluvial lag deposits (SF1), estuarine mixed sand and muds(SF2), central-bay muds (SF3), and redistributed estuary-mouth sands(SF4). Estuary filling occurred within these valleys as they drownedduring Holocene relative sea-level rise. The resulting transgressive facies isrepresented by central-basin deposits (SF3), flood-tidal delta and tidal-inlet bars, washovers, and barriers (SF4), and bounding surfaces causedby flooding (B1) and ravinement (B3 and the T horizon).

Evidence from the preserved valley-fill stratigraphy of the middle andouter New Jersey shelf suggests a transition from tide-dominated to wave-dominated estuarine sedimentation through time, but this must beconfirmed by future systematic sampling of these facies. New Jerseyincised valleys did not receive significant fluvial sediment supply during

transgression, inasmuch as bayhead deltas are, at best, minimally present.Furthermore, our mapped systems do not tie to major river systemslandward, like the Delaware or Hudson rivers (Fig. 1). Instead, sandswithin and beyond the upper fill units of New Jersey valleys were likelyderived by a combination of longshore transport, headland erosion, andcontinued modification by post-transgressive erosion.

ACKNOWLEDGMENTS

This analysis of data from the New Jersey margin has been conducted aspart of the Office of Naval Research’s Geoclutter initiative, specifically underthe auspices of grants N00014-00-1-0844 and N00014-04-1-0038 to Austinand Goff. Chirp data were collected aboard R/V Endeavor in 2001, and coredata were collected by the DOSECC (Drilling, Observation and Sampling ofthe Earth’s Continental Crust, Inc.) AHC-800 heave-compensated drillingsystem deployed from R/V Knorr in 2002. We thank the crews of both vesselsand the DOSECC drilling personnel. We also appreciate helpful discussionswith C. Sommerfield, C. Alexander, B. Christenson , S. Yoshida, and C.Fulthorpe, and the use of radiometric dates provided by C. Alexander.Reviewers Martin Gee and Neil Mitchell significantly improved themanuscript. UTIG contribution #1819.

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Received 7 June 2005; accepted 11 March 2006.


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