Recent and modern marine erosion on the New Jersey outer shelf

Transcription

1 Marine Geology 216 (2005) Recent and modern marine erosion on the New Jersey outer shelf John A. Goff a, T, James A. Austin Jr. a, Sean Gulick a, Sylvia Nordfjord a, Beth Christensen b, Christopher Sommerfield c, Hilary Olson a, Clark Alexander d a Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Rd., Austin, TX 78759, USA b Department of Geology, Georgia State University, 340 Kell Hall, Atlanta, GA 30303, USA c College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA d Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USA Received 15 September 2004; received in revised form 11 February 2005; accepted 16 February 2005 Abstract Recent chirp seismic reflection data combined with multibeam bathymetry, backscatter, and analysis of grab samples and short cores provide evidence of significant recent erosion on the outer New Jersey shelf. The timing of erosion is constrained by two factors: (1) truncation at the seafloor of what is interpreted to be the transgressive ravinement surface at the base of the surficial sand sheet, and (2) truncation of apparently moribund sand ridges along erosional swales oriented parallel to the primary direction of modern bottom flow and oblique to the strike of the sand ridges. These observations place the erosion in a marine setting, post-dating the passage of the shoreface ravinement and the evolution of sand ridges that form initially in the near shore environment. Also truncated by marine erosion are shallowly buried, fluvial channel systems, formed during the Last Glacial Maximum and filled during the transgression, and a regional reflector brq that is N ~ 40 kyr. Depths of erosion range from a few meters to N 10 m. The seafloor within eroded areas is often marked by bribbonq morphology, seen primarily in the backscatter data as areas of alternating high and low backscatter elongated in the direction of primary bottom flow. Ribbons are more occasionally observed in the bathymetry; where observed, crests exhibit low backscatter and troughs exhibit high backscatter. Sampling reveals that the high backscatter areas of the ribbons consist of a trimodal admixture of mud, sand and shell hash, with a bimodal distribution of abraded and unabraded sand grains and microfauna. The shell hash is interpreted to be an erosional lag, while the muds and unabraded grains are, in this non-depositional environment, evidence of recent erosion at the seafloor of previously undisturbed strata. The lower-backscatter areas of the ribbon morphology were found to be a wellsorted medium sand unit only a few 10 s of cm thick overlying the shelly/muddy/sandy material. Concentrations of wellrounded gravels and cobbles were also found in eroded areas with very high backscatter, and at least one of these appears to be derived from the base of an eroded fluvial channel. Seafloor reworking over the transgressive evolution of the shelf appears to have switched from sand ridge evolution, which is documented to ~ 40 m water depth, to more strictly erosional modification at greater water depths. We suggest that this change may be related to the reduction with water depth in the effectiveness of sediment resuspension by waves. Resuspension is a critical factor in the grain size sorting during transport by bottom currents T Corresponding author. Tel.: ; fax: address: goff@ig.utexas.edu (J.A. Goff) /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.margeo

2 276 J.A. Goff et al. / Marine Geology 216 (2005) over large bedforms like sand ridges. Otherwise, we speculate, displacement of sand by unidirectional currents will erode the seafloor. D 2005 Elsevier B.V. All rights reserved. Keywords: chirp sonar; grain size analysis; bathymetry; backscatter; sand ridges; ribbons 1. Introduction The New Jersey continental shelf (Fig. 1) has been studied extensively since the 1960s. It has evolved into a premier natural laboratory both for investigating Neogene Quaternary stratigraphy as a function of base-level changes, and for studying seabed evolution in a sediment-starved setting during the most recent Fig. 1. Bathymetric map of the mid- and outer New Jersey shelf, with contours in meters and artificial illumination from the North. Inset shows location within the Mid-Atlantic Bight of the eastern United States. Thin-dashed lines indicate deep-towed chirp seismic profiles collected in 2001 and 2002 aboard the R/V Endeavor. Thick-dashed line indicates the location of the bbarnegat CorridorQ studies of Duncan et al. (2000) and Duncan (2001). Bathymetry is derived from merged NGDC coastal relief model ( and 1996 multibeam data (Goff et al., 1999). Locations of seismic images in Fig. 10a and b are shown.

3 J.A. Goff et al. / Marine Geology 216 (2005) transgression. Seafloor processes active today create deposits that provide modern analogs for stratigraphy observed in seismic records and outcrops. For example, transgressive sand sheets are thought to be a source for sand bodies that serve as reservoir rocks for hydrocarbons (e.g., Ainsworth et al., 1999; Posamentier, 2002). The modern middle and outer shelf off New Jersey can be thought of as a major seismic horizon in formation and a principal boundary in the eventual sequence stratigraphy, waiting only for the next eustatic sea-level fall to seal its morphology beneath a new prograding sediment accumulation. This paper presents the results of a detailed examination of shelfal erosion that has occurred in the present transgressive marine setting. Our study area is located on the New Jersey shelf between the Mid-shelf (at ~ 50 m water depth) and Franklin (at ~ 100 m water depth) scarps (Fig. 1). Analysis is based on an extensive new chirp seismic reflection data set (Fig. 1) combined with analyses of grab samples and short cores (Fig. 2), in addition to the multibeam bathymetry and backscatter data (Fig. 2) collected earlier. The new data have been collected as part of the Office of Naval Research s bgeoclutterq program, an effort to investigate acoustic returns recorded on active sonar systems and interpreted as potential targets (e.g., submarines), but which in fact arise from the natural environment. Although marine erosion has been observed on the middle shelf, in conjunction with continued sand ridge evolution (e.g., Stubblefield and Swift, 1976; Snedden et al., 1999), on the outer shelf we have identified both localized a Fig. 2. Multibeam bathymetry (a) and backscatter (b) data collected in 1996 aboard the CHS Creed (Goff et al., 1999). Contours on both plots in meters. Artificial illumination of bathymetry in (a) is from the North. Lighter shades in (b) indicate higher backscatter. Filled circles indicate locations of grab samples collected in 2001 aboard the R/V Cape Henlopen. Numbers indicate station identifications for samples referred to in Figs Boxes indicate areas covered by Figs. 3a,b and 4.

4 278 J.A. Goff et al. / Marine Geology 216 (2005) b Fig. 2 (continued). and widespread erosion that evidently postdates sand ridge construction and evolution. These results have important implications for efforts to model the formation of shelf strata (e.g., Syvitski and Hutton, 2001), and also lead to questions regarding the preservability of a transgressive sand sheet in the stratigraphic record (e.g., Posamentier, 2002). 2. Background: sand ridge evolution The modern seafloor of the New Jersey shelf has, as its most prominent constituent, sand ridges, which represent the primary seabed morphology along most of the Atlantic continental shelf of the United States (e.g., Swift et al., 1972). Sand ridges are ~ 1 12 m tall, spaced ~ 1 5 km apart and are ~ 2 20 km long (e.g., Swift and Field, 1981; Figueiredo et al., 1981; Stubblefield et al., 1984; Goff et al., 1999). They form as shoreface-attached bedforms, oriented oblique to the shoreline, with angles of ~ toward the dominant along-shore current direction (e.g., McKinney et al., 1974; Swift and Field, 1981; Figueiredo et al., 1981; Snedden et al., 1994). A number of hypotheses have been proposed for the formation of sand ridges along the shoreface (e.g., Swift and Field, 1981; McBride and Moslow, 1991; Boczar-Karakiewicz and Bona, 1986; Huthnance, 1982; Trowbridge, 1995), but thus far without definitive resolution (see Goff et al., 1999 for a review). Better understood, however, is the observation that, once formed, sand ridges become detached from the adjacent shoreface as sea level rises and continue to evolve in form, size and grain size distribution (e.g., Stubblefield and Swift, 1976; Swift and Field, 1981; Figueiredo et al., 1981; Snedden et al., 1999). In general, as water depth increases, sand ridges increase in height, width and area, become

5 J.A. Goff et al. / Marine Geology 216 (2005) more asymmetric (with steeper down-current flanks), and exhibit lower contrast in grain size between their up-current and down-current flanks with respect to the primary coastal flow. These evolutionary processes have been examined in detail during intensive coring and 3.5 khz seismic programs on the New Jersey shelf that penetrated these ridges in 4 40 m water depths (e.g., Stubblefield and Swift, 1976; Rine et al., 1991; Snedden et al., 1999). These studies document ridge evolution by way of dune-forming processes, i.e., erosion on the up-current flank, and deposition on the down-current flank. Subsequent enlargement of sand ridges in deeper waters is presumably accomplished through merging of smaller ridges and excavation of underlying Pleistocene sands. These studies confirm that swales between ridges are erosional features, with excavation up to several meters below the Holocene sand sheet. The detailed morphology of sand ridges on the middle and outer New Jersey shelf has been examined by Goff et al. (1999), based on multibeam bathymetry and backscatter data. In water depths less than ~ m, sand ridges exhibit a backscatter asymmetry; higher backscatter, presumably indicative of coarser grain sizes, occurs on up-current flanks. Such a grain size asymmetry would be expected for active duneform features, with coarser material preferentially on eroding flanks. This pattern has also been observed in samples collected on sand ridges in nearshore as well as mid-shelf settings (e.g., Swift and Field, 1981; Stubblefield and Swift, 1981). Up-current flanks also tend to be less steep and exhibit secondary transverse bedforms, a feature seen commonly in modern sand ridge systems (e.g., off Sable Island, as documented by Dalrymple and Hoogendoorn, 1997). As with the earlier studies referenced above, Goff et al. (1999) have concluded that sand ridges in mid-shelf water depths are still actively evolving. At water depths N 50 m, however, Goff et al. (1999) have observed that the asymmetry in backscatter between up-current and down-current sand ridge flanks disappears, replaced by a pattern of greater backscatter (larger grain sizes?) along crests, and lower backscatter in swales. Such a pattern has been predicted by Swift and Field (1981) to be indicative of a moribund sand ridge, based upon their expectation that crests would gradually become more winnowed of fine-grained sediments relative to swales. Furthermore, sand ridges in deeper waters of the New Jersey shelf display no evidence of either a consistent sense of asymmetry in their flanks or secondary transverse bedforms (Goff et al., 1999). Kenyon et al. (1981) have also noted, in observations of sand ridges in the North Sea, that sand waves cease to be present on the backs of sand ridges in m water depths; they have inferred from these observations that the ridges are moribund at these depths. Goff et al. (1999) have speculated that these observations are indicative of a fundamental change in the evolution of sand ridges with increasing water depth: in this deep-water stage, sand ridge surfaces become armored through the winnowing process against continued dune-form evolution. 3. Seafloor morphology and bottom characterization 3.1. Multibeam bathymetry and backscatter A multibeam bathymetry and backscatter survey was conducted in 1996 on the middle and outer New Jersey shelf (Figs. 2 and 3; Goff et al., 1999) as part of the Office of Naval Research s STRATAFORM program (Nittrouer, 1999). This survey employed a 95 khz, Simrad EM1000 multibeam system mounted on the CHS Creed. Because incidence angles of this hull-mounted system are typicallyn308, and because seafloor slopes are generally much less than 18, the resultant backscatter map is predominantly a response to variations in surface roughness and/or sedimentary properties (Jackson et al., 1986), rather than to bedform slope (Goff et al., 1999). Two fabrics dominate the seafloor morphology between ~ 50 m and 100 m water depths: ~ ENE WSW trending sand ridges and ~ NE SW trending striations, identified as flow-parallel bribbonsq by Goff et al. (1999) using the definition of McLean (1981) (Fig. 3). The dominant current direction is toward the SW, with peak velocities of N 40 cm/s measured during several moderate winter storms (McClennen, 1973; Butman et al., 1979). The two seafloor fabrics do not coexist spatially; ribbons inhabit low-lying regions between and around clusters of smooth-sided ridges. A similar relationship was observed by Reynaud et al. (1999) in a survey of a large (~ 30 m

6 280 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 3. Enlarged portions of multibeam bathymetry (a) and backscatter (b) illustrating several of the primary morphological features discussed in the text. Locations are shown in Fig. 2. Contours on both plots in meters. Artificial illumination of bathymetry in (a) is from the North. Lighter shades in (b) indicate higher backscatter. Locations of seismic images in Figs. 9, 11, 12 and 13b are shown in (a). high, ~ 5 km wide, ~ 60 km long) sand bank at ~ m water depth west of the English Channel. The crests of the sand bank appear moribund, with a coarse grained, high backscattering lag and no active bedform formation, whereas the lower flanks of the bank are finer grained, and exhibit abundant dunes and sand ribbons that appear to be modern responses to the strong tidal currents that sweep the area. The low-lying areas of the outer New Jersey shelf can sometimes be identified as large swales that are, like the ribbons that floor them, oriented NE SW, oblique to the trend of the ridge crests. The ribbon-

7 J.A. Goff et al. / Marine Geology 216 (2005) floored swale identified in the NW corner of Fig. 3a appears to truncate the sand ridge cluster on its SE flank. Each type of feature has a characteristic backscatter response. As noted above, sand ridges tend to have higher backscatter along their crests (the sand ridge in the NW corner of Fig. 3a,b is a clear example), in contrast to sand ridges in shallower water, where up-current flanks tend to exhibit higher backscatter (Goff et al., 1999). Ribbons are less consistent in their response, but crests tend to display lower backscatter than valleys (Goff et al., 1999). Clusters of elongated pits ~ km wide, ~ 1 3 km long and up to 10 m deep have been mapped in the SW part of the multibeam survey (Fig. 4; Goff et al., 1999). Like the ribbons, these are oriented ~ NE SW, along the SW-directed mean current flow direction (McClennen, 1973; Vincent et al., 1981). The pits are asymmetric, with steeper slopes on their down-current ends. These pits represent the strongest evidence, based on morphology alone, for recent erosion. Goff et al. (1999) have noted a morphological similarity to erosional blow-out pits in subaerial desert settings (e.g., Cooke and Warren, 1973; Thomas, 1989). In such arid settings, a window in otherwise erosionresistant ground vegetation can lead to the formation of an erosive pit in underlying unconsolidated sands. These pits migrate downwind by progressively undercutting the vegetative cover. This analogy would be apt in the submarine setting if the subsurface strata are more easily eroded than seafloor sediment, the latter perhaps armored as a result of winnowing. A series of elongated sand-wave fields are present in the SE corner of Fig. 4, along the top of the Franklin Scarp. These features, discussed earlier by Goff et al. (1999), are indicative of high bottom velocities of ~1 m/s (e.g., Kenyon, 1970; Ashley, 1990). Essentially identical features have been investigated by Knebel and Folger (1976) near the head of Wilmington Canyon, ~ 100 km to the SW. Both sets of features exhibit morphologic and grain size asymmetries consistent with the SW-directed flow direction. Knebel and Folger (1976) have inferred that these sand waves are modern features Grab samples Nearly 100 grab samples were collected aboard the R/V Cape Henlopen in 2001 (Fig. 2), then analyzed Fig. 4. Enlargement of SW portion of multibeam data in a region of elongated scour pits (circled). Location is shown in Fig. 2a. Contours are in meters, and artificial illumination is from the North. Ribbons, ridges and some outer-shelf sand waves (discussed in Goff et al., 1999) are also identified. The locations of Core 77 (which sampled clay clasts) and the seismic image in Fig. 14 are also shown.

8 282 J.A. Goff et al. / Marine Geology 216 (2005) by Goff et al. (2004) for grain size distribution. In correlating measures of the resultant grain size histograms to backscatter intensity, they conclude that the backscatter response of the seafloor (Fig. 2b) is dominated by the coarse-grained fraction (grain sizes larger than ~ 4 mm) over mean grain size. The bright backscatter areas associated with ribbon morphology (Fig. 3b) are dominated by shell hash (Fig. 5a), while a few particularly bright backscatter areas (Fig. 3b) are mostly gravel (Fig. 5b), with rounded cobbles in some cases N 5 cm in diameter. Grain size histograms from both high and low backscatter regions of the ribbon morphology are shown in Fig. 6. The brighter backscatter sediments are very poorly sorted, in fact trimodal in their distribution, with significant percentages of both mud and coarse material in addition to predominant medium sands. Due to the lack of modern sedimentation on the shelf (e.g., Swift et al., 1972), the mud in these samples must have been derived from previously undisturbed strata at these locations. The lower backscatter sediments are, in contrast, composed of well-sorted, medium sands, with negligible amounts of fine or coarse material. Eighteen hydraulically dampened (slow insertion) cores, cm in length, were also collected aboard the Cape Henlopen cruise in The surficial shell hash prevented core penetration in some areas. One core (Station 18; Fig. 2) penetrated 23 cm through the lower backscatter, medium sand to a muddy, sandy Fig. 6. Grain-size histograms for Stations 17 and 20, representing, respectively, low and high backscatter regions of ribbon-type seafloor morphology (Figs. 2 and 3). Values shown include the overall mean grain size and the percentages of both fine (b63 Am) and coarse (N4000 Am) fractions. The high backscatter regions are trimodal, with significant portions of coarse material (shell hash and gravel) and mud along with medium sand. The low backscatter regions are well-sorted medium sand. The two samples display negligible differences in mean grain size. shell hash layer below. Assuming similar stratigraphy throughout the ribbon morphology, we infer from this evidence that this layer of shell hash exists broadly throughout the ribboned regions, either exposed at the seafloor or covered in places by a thin layer of medium sand organized in a flow-parallel trend. Grain size histograms from sediments gathered at higher and lower backscatter regions of sand ridge Fig. 5. Photographs of coarse sieve (N4 mm) material sampled at stations 20 (a) and 90 (b). Note the presence of whole shells, shell hash, and gravel with occasional cobbles. See Fig. 2 for location.

9 J.A. Goff et al. / Marine Geology 216 (2005) morphology are presented in Fig. 7. In these areas, both the higher and lower backscatter sediment distributions are unimodal. Compared to the lower backscatter sediments, the higher backscatter sediments are deficient in grain sizes b 375 Am and enriched in grain sizes N 500 Am; this observation is consistent with Goff et al. (1999) speculation that the high backscatter crests of these outer-shelf sand ridges are winnowed relative to intervening swales. Grab samples from both ribbon and sand ridge environments (Fig. 2) have also been examined for foraminiferal and mineralogical content. The ribbondominated environments are characterized by a bimodal benthic foraminiferal population: an inner shelf (~ 0 50 m water depth), abraded and probably relict population, and a middle shelf (~ m water depth) population with better preservation, probably in situ. The preservation of these samples is variable; replacement by pyrite occurs and is an indicator of diagenesis. The mineralogy of samples within the ribbon environments also reflects at least two populations of quartz grains: (1) angular and clear and (2) rounded, frosted and often iron-stained. The second population is indicative of grains that have been reworked, either at the present-day seafloor or possibly within glaciers and then transported to the shelf as ice-rafted debris. The first population, however, provides evidence of a fresh source, i.e., Fig. 7. Grain-size histograms for stations 85 and 86, representing, respectively, high and low-backscatter regions within the sand ridge-type seafloor morphology (Figs. 2 and 3). Values shown include the overall mean grain size and the percentages of both fine (b63 Am) and coarse (N4000 Am) fractions. Compared to the samples in Fig. 6, these samples are unimodal, though skewed, and well-differentiated by their mean grain sizes. material derived from previously unreworked sediments in strata being eroded at the seafloor. Foraminiferal populations in the sand ridge samples are either absent or very rare and abraded; where they occur, they indicate a relict, inner shelf environment. The mineralogy of the sand ridges is dominated by subrounded quartz grains with lithic fragments. Hence, unlike the mixed grains in the ribbon morphology, grains of the sand ridge sediments at the seafloor have all been reworked. 4. The shallow stratigraphy of the New Jersey shelf The interpretation of the New Jersey shelf shallow stratigraphy (Fig. 8) provides a primary constraint on the timing of erosion and the nature of material being excavated for seafloor features. Our understanding of this stratigraphy has evolved considerably over the last several decades as more samples have been obtained and more data have been collected with newer and higher resolution acoustic imaging systems. More complete descriptions and references are available from Duncan et al. (2000), Duncan (2001), and Gulick et al. (2005). The brq horizon (Fig. 8) is a regionally recognized, high-amplitude reflection that forms the base of the outer shelf wedge. The origin of this surface is uncertain, but its age (~ 40 kyr) is well constrained to predate the latest sea level regression. The outer shelf wedge is a prograding series of offlapping strata deposited on the brq horizon as a regressive deposit. The bchannelsq horizon (bcq, Fig. 8) marks an incised valley system that erodes into both the outer shelf wedge and br.q Fill ages of ~ 12.5 kyr indicate that these channels were active during the latest lowstand and then filled as the shoreline migrated through this region. The btq horizon (Fig. 8) is an intermittent reflection that, in its most consistent manifestation, caps channel-fill strata and occasionally truncates the bchannelsq horizon. Duncan et al. (2000) interpret btq as the transgressive ravinement surface associated with Holocene sealevel rise. This horizon represents a critical chronostratigraphic boundary: the passage from shoreface to marine sedimentary environment. Other than the sediments sequestered in channel fills, the surficial sand sheet, ranging from ~ 0 to 20 m

10 284 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 8. Schematic illustration of the primary stratigraphic units and horizons discussed in the text, as interpreted by Duncan et al. (2000) and Duncan (2001) in the context of radiometric ages available at that time. The profile is based on data along the Barnegat Corridor (Fig. 1), spanning ~50 km; vertical exaggeration is very high. thick, represents the only Holocene deposition on the New Jersey shelf. Transferred from the shoreface to the inner shelf during transgression, these sands are organized primarily into oblique sand ridges. Net sediment input to the middle and outer shelf is negligible at present (e.g., Swift et al., 1972); processes active on the shelf primarily involve continued reworking and erosion of the sand sheet, driven by storm waves and bottom currents (e.g., Swift et al., 1972). The seaward edge of the mid-shelf wedge (Fig. 8; Knebel et al., 1979; Milliman et al., 1990) forms a prominent topographic feature on the New Jersey shelf: the Mid-Shelf Scarp (MSS; Fig. 1), or bshoreq (e.g., Swift et al., 1980), sometimes referred to as the Tiger Scarp (e.g., Knebel et al., 1979) or Fortune bshoreq (e.g., Dillon and Oldale, 1978). Thought for decades to represent a fossil shoreface associated with a sea-level stillstand (e.g., Veatch and Smith, 1939; Emery and Uchupi, 1972; Dillon and Oldale, 1978; Swift et al., 1980), high-resolution seismic data has since shown it to be depositional in origin (Knebel et al., 1979; Milliman et al., 1990; Duncan, 2001). Uchupi et al. (2001) have speculated that the mid-shelf wedge could be a subaerial deposit associated with massive outflows from breached glacial lakes along the Hudson Valley that were known to have occurred, between ~ 19 and 12 kyr, during glacial retreat. However, using high resolution chirp data collected along the Barnegat Corridor where it crosses the scarp (Figs. 1 and 2), Duncan (2001) ascertained that at least the outer part of the wedge consists of a prograding sequence that lies above the btq horizon (the bholoceneq mid-shelf wedge in Fig. 8), and should therefore have been deposited in the transgressive marine environment: perhaps ~ 9 10 kyr or later based on eustatic curves, after the postulated lake collapses. The inner part of the mid-shelf wedge also includes a substantial thickening of the Pleistocene sediments between brq and btq (the bpleistoceneq mid-shelf wedge in Fig. 8; Duncan, 2001). Dates for these muddy sediments (Fig. 8) are derived from Knebel et al. (1979). 5. Observations related to marine erosion in chirp seismic data The seismic data considered here comprise 2-D ultra-high resolution chirp profiles collected over a broad region of the middle and outer New Jersey shelf south of the Hudson Shelf Valley and Canyon (Fig. 1). Track line spacing varied from 50 m to 1 km. Shallowly buried incised-valley systems, which were considered as possible sources of acoustic clutter, were the primary targets of the survey (Nordfjord et al., 2005). The data were collected using a deep-towed chirp sonar aboard the R/V Endeavor cruise EN359 in August September, Additional data in the southern part of the survey area (Fig. 1) were collected with the same system aboard Endeavor during cruise EN370 in May, This system is well suited for high-resolution imaging of the shallow, complex late Quaternary stratigraphy of the New Jersey shelf. The source emitted swept-frequency signals of 1 4 khz and 1 15 khz. The 1 4 khz data employed in this study

11 J.A. Goff et al. / Marine Geology 216 (2005) have vertical resolution of ~ 20 cm, with a penetration in places of N 30 m below the sea floor. Schlumberger s Geoquest IESXR software was utilized in the interpretation of seismic profiles and mapping horizons. In this manuscript, we focus specifically on seismic evidence for marine erosion of the outer New Jersey shelf, the most recent stage in its evolution Outcropping horizons The truncation of strata at the seafloor provides direct evidence for erosion. Truncation of the btq horizon in particular indicates erosion in the marine setting, after the shoreface migrated across the region. Fig. 9 displays a chirp seismic cross-section through the eroded flank of a sand ridge, where both the btq and brq horizons outcrop at the seafloor. This profile is adjacent to the Barnegat Corridor profile (Fig. 1) presented by Duncan et al. (2000), along which the truncation of these horizons was first documented. In Fig. 9, we observe the truncation of brq by bchannelsq incisions, and the capping of those incisions by the btq horizon. (The seismic signal becomes increasingly attenuated under the sand ridge.) The btq and brq horizons outcrop along the NW flank of the sand ridge, at a ~ 4 5 km-wide, ribbon-floored swale oriented NE SW (Fig. 3a). As noted earlier, this swale is oblique to the more ENE WSW trend of the sand ridges. A minimum of ~ 4 m of marine erosion, the distance between the projected outcrop of btq and the floor of the swale, is documented (Fig. 9). Steepening of the seafloor occurs between the outcropping btq and brq horizons, whereas the seafloor flattens below the brq horizon (Fig. 9). This geometry may be related to the different material properties of the units bounded by these horizons. Samples taken from between btq and brq are mostly fine-grained muds/ clays (Knebel and Spiker, 1977; Buck et al., 1999; Nordfjord et al., 2002), whereas above btq lies the surficial sand sheet of medium to coarse grains, and below brq are coarse grained and nearly lithified sediments (Davies et al., 1992, Nordfjord et al., 2002). If the clays between btq and brq are not overconsolidated, they will be more easily eroded than the sands above and below. Where a more erodable unit outcrops from below a resistant unit, the seafloor will have a higher gradient because it is easily excavated downwards while being constrained from eroding laterally by presence of the overlying unit. The lithified coarse sands below brq would also clearly impede further excavation, so the swale will instead grow laterally by preferential erosion of the finer-grained unit and undercutting of the resistant sand sheet above. Another ~ 8 9-km-wide erosional swale is present seaward of the MSS, north of a sharp bend in its strike (Fig. 1). Seismic cross-sections across the scarp, forming the NW flank of the swale, and across the edge of a sand ridge complex forming the SE flank, are displayed in Fig. 10. Again, we recognize the three Fig. 9. Deep-towed chirp seismic image (1 4 khz) across the SW flank of an erosional swale, which truncates a sand ridge complex. See Fig. 3 for location. Seismic horizons brq, btq and bchannelsq (C) are identified. Minimum depth of marine erosion (~4 m) is noted as the difference between maximum swale depth and projection of btq. The steepening of the seafloor between the btq and brq outcrops is also noted. Depth conversion is based on water velocity (1500 m/s).

12 286 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 10. Deep-towed chirp seismic image across the NW (a) and SE (b) flanks of a large erosional swale. See Fig. 1 for location. Seismic horizons brq, btq and bchannelsq (C) are identified. Minimum depths of marine erosion (~12 m on the NE flank; ~6 m on the SE flank) are noted as the difference between maximum swale depth and projection of btq. The steepening of the seafloor between the btq and brq outcrops is also noted. Depth conversion is based on water velocity (1500 m/s). primary horizons that define the shallow stratigraphy: brq, bchannels,q which is above and incises br,q and btq, which truncates incision flanks and fill. Likewise, both btq and brq outcrop at the flanks of the swale, and the seafloor steepens between the btq and brq outcrops, leveling out where material below brq is eroded (Fig. 10). From the geometry, we again infer that the unit between brq and btq is more easily eroded than the capping sands or the sediment below brq. The thickness of the unit between brq and btq is greater than in Fig. 9; consequently, the minimum demonstrable marine erosion is higher: ~ 12 m at the MSS (Fig. 10a) and ~ 6 m at the conjugate flank of the erosional swale (Fig. 10b). These seismic observations at the MSS contrast with those of Duncan (2001), who ascertained that the scarp is, at another location, the seaward edge of a transgressive depositional wedge. Duncan (2001) seismic crossing of the scarp, along the Barnegat Corridor, is south of the sharp bend in the scarp (Fig. 1). The MSS abruptly shoals southward by ~10 m at the bend, and no longer forms the NW flank of the erosional swale identified in Fig. 10. The swale instead continues along its southwesterly trend for nearly 10 km south of the bend. This morphology suggests that the bend represents an important boundary along the scarp: primarily erosional to the north, and constructional to the south. Erosional truncation of strata at the seafloor is not always confined to NE SW trending swales. In water depths more than ~ 75 m, truncations are observed over broad areas, with no acoustic or grab sample evidence of the transgressive sand sheet. Fig. 11a presents a seismic section demonstrating seafloor

13 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 11. (a) Deep-towed chirp seismic image across the landward edge of the outer shelf wedge, which outcrops at or near the seafloor. See Fig. 3 for location. Seismic horizons brq and bchannelsq (C) are identified. Minimum depth of marine erosion (~7 m) is noted as the difference between maximum swale depth and projection of btq. Depth conversion based on water velocity (1500 m/s). (b) Filtered backscatter (solid) and residual multibeam bathymetry (dashed) values sampled along the same profile as (a), which is within ribbon-type seafloor morphology, identified by the alternations between high and low backscatter. Bathymetry profile was detrended with a linear regression. No evident correspondence can be found in this example between backscatter variations associated with ribbon morphology and bathymetric variations. truncation of the bchannelsq horizon, dipping strata within the outer shelf wedge, and the basal brq horizon. The btq horizon outcrops at the NW end of the section; the surficial sand sheet is otherwise absent throughout this area with up to ~ 7 m of indicated minimum marine erosion. We also find that ribbon morphology (Figs. 2 and 11b) is only present where the sand sheet is absent; we can therefore use backscatter data to infer sand sheet absence where we have no other seismic constraints. There is no evident correlation between bathymetry and the alternations of high and low backscatter (Fig. 11b) that define the ribbons here. This observation is not surprising given the core evidence. As noted above, the lower backscatter portions of ribbons in the area covered by Fig. 11 represent a thin (~ cm) layer of medium sand overlying a surficial shell hash layer that accounts for the high backscatter; this thickness represents a small contribution to the ~ 1 2 m undulations evident on the profile. However, some ribbons do exhibit clear bathymetric expressions (Fig. 3a; Goff et al., 1999); these ribbons tend to be more linear than those that exhibit only a backscatter signature (see the following section) Channel remnants Erosion of channel-fill strata has resulted in both bathymetric and backscatter expressions of channels fills at the seafloor in a few locations. The gravel mound identified in Fig. 3 is one prominent example. The mound was imaged by a hull-mounted chirp system on the R/V Endeavor. These data are not nearly of the same quality as the towed system, but they nevertheless reveal primary structure beneath the mound (Fig. 12): a prominent undulating reflector that outcrops at the seafloor on either side of the mound. The depth of this basal reflector is similar to the bases

14 288 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 12. (a) Hull-mounted chirp seismic image crossing a mound where rounded gravel and cobbles have been sampled (Fig. 5). See Fig. 3 for location. Depth conversion is based on water velocity (1500 m/s). A sketch interpretation of the observed seismic horizons is provided below the seismic image, revealing a broad, undulatory reflection with flat-lying horizons above it. We interpret this reflection as the base of a wide, eroded channel. (b) Filtered backscatter values sampled along the same profile as (a). Vertical lines indicate the boundaries of the high backscatter anomaly associated with the gravel, and demonstrate that the gravel mound is coincident with the upper strata of the eroded fill units. of presumed fluvial channels mapped elsewhere in the survey area (Duncan et al., 2000; Nordfjord et al., 2005). Subhorizontal reflectors lie above this reflector within the mound, suggestive of fill strata within incisions elsewhere; two of these reflectors are also truncated by the seafloor. Rounded cobbles sampled from this gravel-prone mound (Fig. 5b) are compatible with the type of lithology expected from the thalweg of a fluvial channel. All of this evidence suggests that this structure is the eroded remnant of a fluvial channel, km wide, contemporary with and similar in scale to channels mapped just to the north by Nordfjord et al. (2005). The mound itself appears to comprise the uppermost two observed strata of the interpreted exposed fill sequence (Fig. 12); the mound stands above the surrounding seafloor. The NE slope, facing into the primary SW-directed current direction (McClennen, 1973; Vincent et al., 1981), is steeper, suggesting erosive undercutting of a resistant upper stratum. The btq horizon is not evident in Fig, 12, presumably because it has been eroded away. A small set of ~ 4-km-long ribbon structures are observed in the NE corner of Fig. 3. They have both bathymetric and backscatter expressions: low backscatter along the crests, and high backscatter in the troughs. They are located just seaward of a sand ridge complex, and are very limited in their spatial extent. The topographic highs of the ribbons are asymmetric in that they taper to the NE, into the primary current. The seismic data (Fig. 13) reveal that these ribbons are essentially carved out of the fill strata of a partially eroded channel N 2 km wide. Beneath the sand ridge complex, the channel is undisturbed (Fig. 13a), revealing the btq horizon. The channel becomes progressively eroded seaward of the sand ridge complex, with a projected minimum of ~ 7 m of material removed at the eastern edge of Fig. 13b.

15 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 13. Deep-towed chirp seismic image across (a) uneroded and (b) eroded portions of a major trunk channel (Nordfjord et al., 2005). See Fig. 3 for location. Seismic horizons btq and bchannelsq (C) are identified. Minimum depth of marine erosion (~7 m) is noted as the difference between maximum swale depth and projection of btq. Depth conversion is based on water velocity (1500 m/s). (c) Filtered backscatter (solid) and multibeam bathymetry (dashed) values sampled along the same profile as (b), which is within one of a few regions of the survey where ribbon-type morphology, identified by the alternations between high and low backscatter, correlates fairly well to bathymetric variations. As noted with vertical dashed lines, the lowest backscatter anomalies correlate with ribbon crests (marked with connecting vertical lines), likely indicating that the crests are finer-grained/depositional. See text for additional discussion.

16 290 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 14. Deep-towed chirp seismic image across some of the erosional scour pits, within the Outer Shelf Wedge. The deepest pit shown here is over 10 m deep. See Fig. 4 for location. Seismic horizons brq and bchannelsq (C) are identified Erosion pits The deep-towed chirp seismic data reveal that the erosion pits (Figs. 4 and 14) are excavated out of the laminated strata of the outer shelf wedge, often creating depressions N 10 m deep. The bchannelsq horizon and fill strata are also truncated within some of the pits. Grab samples within the pits are clay rich. Around the pits, a surficial sand sheet is not evident from the seismic data; no btq horizon is observed and the buried channels are truncated by the seafloor. The seafloor is instead largely covered by ribbon morphology (Fig. 4), alternating between sediment distributions similar to those shown in Fig. 6. Abundant coarse material is present at or near the seafloor. Core 77, taken just south of an erosion pit field (Fig. 4), penetrated through a ~ 10-cm-thick surface layer of medium sands and shells to sample clay clasts mixed with medium sand. Such a stratigraphy would support our hypothesis for the formation of these pits as downstream-progressing features which continue to undermine a surficial armoring layer. Clay clasts are known to be indicative of vigorous erosion in the presence of sand, which in this core may be a remnant of the transgressive ravinement. Alternatively, Fulthorpe and Austin (2004) suggest that the vigorous erosion indicated by Core 77 occurred before the transgression, a result of possible massive outflows during glacial lake collapse ~ kyr. 6. Discussion 6.1. Timing and extent of marine erosion Our analysis indicates a genetic relationship between marine erosion and morphologic structures oriented along the primary direction of bottom current flow: large swales that cut across the sand ridge trend, asymmetric erosional pits and ribbons. The swales and pits represent direct excavations of seabed sediment. The large swales are (where mapped with high resolution swath data) also floored by ribbon morphology. Our primary stratigraphic constraint on the timing of this erosion comes from the truncation of the transgressive ravinement surface (btq) at the seafloor (Figs. 9 11): the erosion therefore post-dates the passage of the shoreline and the formation of the Holocene sand sheet. By this constraint alone, it is possible that the erosion occurred in the near-shore environment as, for example, within the swales of near-shore sand ridges, which are observed in some cases to excavate below the ravinement (e.g., Snedden et al., 1994). However, the scale (N5 km wide) and orientation (contour parallel) of the erosional swales on the outer New Jersey shelf have no known analogy in the nearshore, inner shelf or middle shelf environments, which are dominated by sand ridge morphology trending oblique to the contours, with swale widths generally less than ~ 2 km. Nor have asymmetric

17 J.A. Goff et al. / Marine Geology 216 (2005) erosion pits been observed in those depths. While sand ridges are known to be active and evolving up to ~ 40 m water depth (e.g., Stubblefield and Swift, 1976; Rine et al., 1991; Snedden et al., 1999; Snedden and Dalrymple, 1999; Goff et al., 1999), those on the outer New Jersey shelf are apparently moribund (as evidenced by the pattern of greater grain sizes at their crests, and lack of secondary bedforms on their flanks; Goff et al., 1999); furthermore, sand ridges are truncated in some cases by the large erosional swales. We infer, therefore, that such swale erosion post-dates the evolution of sand ridges. By these morphological arguments, we argue that the swales and pits are largely products of water depths at or near present levels, although it is possible that such marine erosion initiated at locations that had been previously eroded. Ribbons with clear bathymetric expression, such as those shown in Fig. 13b,c, may also be derived by excavation. However, most of the ribbons observed in the survey area are constructional in nature, produced by a thin, patchy layer of medium sand overlying a shell hash layer with mud and sand. The overlying sand may have been derived from the underlying shelly/muddy/sand layer. Evidence from grab sample suggests that the shelly/muddy/sand layer is a direct product of erosion. The shells themselves are presumably a lag, the immovable detritus of meters of material removed from the seafloor at these locations. Much of the shell hash may, in fact, have been concentrated at the ravinement surface itself (e.g., Posamentier, 2002) prior to marine erosion. Because of the lack of modern sediment input to the New Jersey shelf (e.g., Swift et al., 1972), the muds that are incorporated in this layer must be derived from the underlying Pleistocene strata being eroded. The bimodal distribution of abraded and fresh sand grains indicates excavation of previously undisturbed sediment mixed with reworked sand. Both observations are strong indicators that erosion in these locations is ongoing; otherwise, the fines would be winnowed and the grains abraded rapidly. These ribbons, while constructional, can therefore be considered indirect indicators of modern erosion. We also assume that gravel patches (Figs. 2a and 3) represent erosional lags. Based on the seismic data and inferences from morphology, we have generated an interpretive map indicating areas of significant marine erosion on the New Jersey outer shelf, between the Franklin and Mid-Shelf scarps (Fig. 15). At minimum, several to ~ 10 m of sediment have been removed by this erosion, and perhaps more, depending on the preexisting thickness of the transgressive sand sheet in these areas Transition from sand ridge evolution to marine erosion The evolution of sand ridge morphology documented throughout the inner and middle shelf (e.g., Stubblefield and Swift, 1976; Swift and Field, 1981; Figueiredo et al., 1981; Snedden et al., 1994, 1999; Snedden and Dalrymple, 1999) comes to an end on the outer shelf. In water depths N 50 m, sand ridges appear morphologically moribund, perhaps shielded against further sediment transport (i.e., reworking) by the coarse armor of winnowed sands. However, as noted above, the seafloor shows evidence of recent and modern marine erosion over broad areas of the outer shelf, in some cases demonstrably cutting sand ridges (Fig. 9). That the outer shelf is experiencing erosion today is perhaps not surprising, given earlier oceanographic observations. Peak bottom current velocities of over 40 cm/s have been measured in this region (McClennen, 1973; Butman et al., 1979), with mean flows during storm events in excess of 30 cm/s (Butman et al., 1979). Although these velocities are not theoretically considered adequate to initiate movement of sand grains coarser than ~ 200 Am (e.g., Miller et al., 1977), Butman et al. (1979) note in their discussion that orbital wave motion and bottom roughness will increase bottom stress, enhancing the ability of unidirectional currents to move sand grains. Butman et al. (1979) have photographed several episodes of sediment movement and resuspension on the New Jersey outer shelf associated with peak current and/or wave activity. These observations document that, under typical winter conditions, sediment transport at the depths considered here is frequent. In addition, elongated sand wave fields toward the shelf edge (Fig. 4; Knebel and Folger, 1976) provide morphological evidence of strong current velocities. An important outstanding question that needs to be addressed is: why do sand ridges cease to evolve in

18 292 J.A. Goff et al. / Marine Geology 216 (2005) Fig. 15. Interpreted regions of marine erosion on the outer New Jersey shelf (hachured). Minimum amount of marine erosion is identified at those locations where we can confidently do so. See text for discussion. water depths N 50 m, despite evidence of frequent transport of sands in those water depths? One explanation may be that without sufficient replenishment by new sediment from the continent, the evolution of sand ridges during a transgression will lead to a gradual coarsening of the unit through winnowing, and thus towards an ever-increasing resistance to reworking. Moribund ridges may simply be the oldest. However, we do not observe any clear difference in mean grain sizes from ridges sampled in this survey and those sampled in shallower water by Goff et al. (2000). The second speculation is that storm waves, in addition to unidirectional currents, are in some way essential for the evolution of sand ridges. The influence of waves on bottom water velocities generally decreases with water depth, in terms of the percentage of time that waves will generate sufficient velocity to move sand grains (McClennen, 1973). Thus, waves will have a decreasing effect on sand ridges as a function of depth. Butman et al. (1979) have noted large differences in the movement of sediments during high current events, with or without waves: large wave-induced flow results in significant resuspension of sediments, which are then moved rapidly by currents, whereas unidirectional currents without waves move sediment primarily through bedform migration. Butman et al. (1979) conclude

19 J.A. Goff et al. / Marine Geology 216 (2005) that wave-induced bottom currents result in much larger bottom stresses compared to unidirectional flow, a conclusion that is consistent with modeling predictions (e.g., Komar, 1976). Resuspension may be critical to the dune-form evolution of the sand ridge, allowing grains, while they are transported along the primary flow direction, to be sorted in relation both to their fall velocities and with respect to differences in bottom shear stress associated with lee and stoss flanks of the ridge. Unidirectional flows without wave resuspension will, in contrast, simply move sediment along the bottom, a process that does not allow for sorting over the large scales associated with sand ridges. Therefore, water depth, rather than age, would be the determining factor in shutting off the evolution of sand ridges. We suspect that both factors, progressive armoring and increasing water depth, lead to the shutdown of sand ridge evolution as transgression progresses. 7. Conclusions In this paper, we have documented widespread marine and likely modern erosion on the outer shelf off New Jersey, through analysis of chirp seismic and grab sample data, as well as seafloor morphology. The chirp seismic data provide a key stratigraphic constraint by defining the presence and lateral extent of the btq horizon, interpreted by Duncan et al. (2000) as the transgressive ravinement surface, which serves as the base of the transgressive sand sheet. The sand sheet is emplaced on the ravinement surface after the shoreline migrates landward during a sea-level rise, and is organized seaward of the shoreface into oblique sand ridges that continue to evolve as sea level continues to rise (e.g., Swift et al., 1972). Truncation of the btq horizon at the seafloor indicates that erosion has occurred in the transgressive marine setting, after migration of the shoreface across this region. A number of earlier studies have observed basal reflector truncations within sand ridge swales in middle shelf settings, and such erosion is therefore presumed to be associated with the continued evolution of these features (e.g., Stubblefield and Swift, 1976; Rine et al., 1991; Snedden et al., 1999). However, our data have revealed unexpected marine erosion located within large swales oriented ~ NE SW, along the SW-directed primary current flow (McClennen, 1973; Vincent et al., 1981) and cross-cutting the oblique-to-flow sand ridge orientation on the outer shelf. This erosion is therefore not associated with sand ridge development. Since the geomorphology of outer shelf sand ridges (i.e., higher grain sizes at crests, indicative of winnowing, and lack of secondary transverse bedforms on flanks) likely indicates that they are moribund, the marine erosion we observe on the outer shelf appears to post-date these sand ridges. Other important stratigraphic indicators of recent erosion included exposures at the seafloor of: (1) buried fluvial channels (formed at or near the LGM) and fills (emplaced during the transgression), (2) outer shelf wedge laminated strata (deposited ~ kyr), and (3) the regional brq horizon (formed in part by erosion of material older than ~ 40 kyr). We estimate minimum depths of marine erosion ranging from a few meters to N 10 m. Flow-parallel ribbon morphology appears to be genetically linked to eroded surfaces. Where the ribbons have both bathymetric and backscatter expressions, their morphology appears to represent an excavation of seabed material. However, much of the ribboned areas have clear expression only in the backscatter data; these features have been found to represent alternations between a shelly, muddy sand (yielding high backscatter) and a thin layer (~ cm) of well-sorted medium sand (yielding low backscatter) drifted atop the shelly, muddy sand. The shell hash presumably represents an erosional lag, in part perhaps a remnant of the transgressive ravinement surface; whereas the presence of mud and unabraded mineral grains indicates modern excavation of Pleistocene sediments stratigraphically older than the transgressive sand sheet. Rounded gravels are also observed in other areas of the seafloor, some notably from near the base of eroded fluvial channels, but one large patch broadly distributed (Fig. 2b). These are all probable erosional lags. An important question immediately arises from the previous conclusion: why do sand ridges become moribund on the outer shelf, when there is ample evidence of sufficient modern bottom currents and