Cenozoic mass-transport facies and their ... - Semantic Scholar

Marine Geology 184 (2002) 295^334 www.elsevier.com/locate/margeo

Cenozoic mass-transport facies and their correlation with relative sea-level change, New Jersey continental margin Cecilia M.G. McHugh a;c; , John E. Damuth b , Gregory S. Mountain c a

b

School of Earth and Environmental Sciences, Queens College, City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, USA Earth Resource and Environmental Center and Department of Geology, University of Texas at Arlington, P.O. Box 19049, Arlington, TX 76019, USA c Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Received 1 September 2000; accepted 6 August 2001

Abstract Mass-transport deposits reveal something of the timing, source areas and depositional processes that contributed to the evolution of the New Jersey continental margin. Many of the mass-transport deposits rest upon prominent stratal surfaces and sequence boundaries permitting evaluation of the relationship between mass wasting and eustatic change. Five distinct mass-transport facies representative of intercanyon regions of the slope, canyons and continental rise settings are recognized in the Ocean Drilling Program Leg 150 Sites (902^906). These mass-transport deposits consist predominantly of muddy slumps and debris flows, and to a lesser extent sandy mass flows and gravity-related flows. The styles of soft-sediment deformation, mineralogy, and biostratigraphy of these mass-transport deposits provide new information on the mass-wasting history of the continental margin. At intercanyon sites beneath the continental slope (Sites 902^904), mud with disseminated sand occurs mainly at sequence boundaries and related stratal surfaces in upper Oligocene to upper Miocene sections. In contrast, muddy and sandy debris flows and slumps occur at sequence boundaries, stratal surfaces and within sequences in upper Miocene through Pleistocene sections. The type and preservation of mass-transport facies (10^15% of the total sediment recovered) on the continental slope through time, is associated with changes in sediment progradation during the Miocene, which led to a change in the morphology and gradient of the slope throughout the late Neogene. Mass-wasting facies are best preserved (36% of the total sediment) in the canyon-fill deposits recovered from Site 906. The fill of modern Berkeley Canyon is composed of debris flows, whereas the fill of a buried middle Miocene canyon consists of clast supported slumps, debris flows, and turbidites, which document an early episode of canyon excavation and infilling. Apparently the middle Miocene canyon cutting event occurred very rapidly (V2.5 Ma) and can be correlated to a prominent glacioeustatic sea-level lowering event (13.5 < 0.5 Ma). Approximately 30 m of debris flows and slumps accumulated at Site 905 on the continental rise during the middle Miocene. Lithologies and benthic foraminifer assemblages show that this material was derived from the continental slope. Downslope transport was again significant during the early Pleistocene at Site 905 when 215 m of slumps and debris flows accumulated. The lithology and age of the clasts and matrix suggest that these deposits resulted from episodes of canyon excavation deep into the lithified rocks of the adjacent continental slope. > 2002 Elsevier Science B.V. All rights reserved.

* Corresponding author. E-mail address: [email protected] (C.M.G. McHugh).

0025-3227 / 02 / $ ^ see front matter > 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 4 0 - 7

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C.M.G. McHugh et al. / Marine Geology 184 (2002) 295^334

Keywords: New Jersey; mass wasting; sea-level change; glacioeustacy; seismic stratigraphy; sequence boundaries

1. Introduction The US Atlantic continental margin o¡shore New Jersey is a dynamic system where episodes of sediment accumulation have been interrupted by intervals of widespread erosion (Schlee et al., 1979; Robb et al., 1981; Schlee, 1981; Poag, 1985; Embley and Jacobi, 1986; Laine et al., 1986; Farre and Ryan, 1987; Poag and Mountain, 1987; Poag and Watts, 1987). The eastern US continental margin is a ‘sediment-starved’ margin and consequently the modern continental slope is predominantly an erosional setting incised by large submarine canyons with complex networks of tributaries (Farre et al., 1983; Farre and Ryan, 1987; Pratson et al., 1994). Intercanyon regions are also deeply excavated by gullies, amphitheater-shaped detachment surfaces, and grooved terrain (Robb et al., 1983; Farre and Ryan, 1987; Pratson et al., 1994; Mountain et al., 1996). Buried erosional surfaces with older submarine canyons have also been identi¢ed in seismic-re£ection pro¢les and form major unconformities, which have been correlated landward to sequence boundaries beneath the shelf (Mountain and Tucholke, 1985; Mountain, 1987; Miller et al., 1987, 1990, 1996a,b; Poag and Mountain, 1987; Greenlee et al., 1992; Mountain et al., 1994; Fulthorpe and Austin, 1998). The continental rise extends from the base of the continental slope and its surface is dissected by channels and littered with displaced blocks (Poag and Mountain, 1987; Farre and Ryan, 1987; Mountain, 1987; Schlee and Robb, 1991). Analyses of regional seismic-re£ection pro¢les indicate that the sediments of the rise were deposited by both downslope and parallel-to-slope depositional processes (Mountain and Tucholke, 1985; Mountain, 1987; McMaster et al., 1989; Locker and Laine, 1992). Although much is presently known about continental margins, especially the US Atlantic margin, uncertainty continues as to the causal mechanisms for sediment failure and transport (Dingle, 1977; Embley and Jacobi, 1977). Vail and co-workers proposed a conceptual

sea-level model based on seismic-stratigraphic analysis to explain the processes responsible for the erosion, distribution, and accumulation of sediment on continental margins (Vail et al., 1977, 1991; Haq et al., 1987; Posamentier et al., 1988; Posamentier and Vail, 1988; Vail, 1987; van Wagoner et al., 1990; Mitchum et al., 1977, 1993). In this model, accelerated sea-level lowering exposes the continental shelf and permits intensi¢ed erosion and sediment transport to the slope and rise. This intensive erosion on the shelf and upper slope leads to submarine-canyon incision and the development of prominent regional unconformities, which form sequence boundaries. Although the Vail/Exxon conceptual model is generally accepted, questions as to the timing of erosional and depositional events in response to sea-level cycles remain largely untested (Posamentier et al., 1988; Christie-Blick et al., 1990; Reynolds et al., 1991; Christie-Blick, 1991). The New Jersey sea-level transect was designed to investigate the e¡ects of global sea-level changes on a passive continental margin (Mountain et al., 1994, 1996). The Ocean Drilling Program (ODP) drilled a transect of ¢ve sites (902^ 906), four on the slope and one on the upper continental rise during ODP Leg 150 (Fig. 1A,B). These long, continuously cored sites correlated to multifold seismic data across the margin provide an excellent opportunity to examine the indirect e¡ects of relative sea-level changes on deep-sea sedimentation patterns. We conducted detailed studies on the styles of soft-sediment deformation, mineralogy, and biostratigraphy of the mass-transport and related gravity-controlled deposits recovered in Leg 150 cores from the Oligocene to present. The purpose of the present study is to address: (1) the source areas, transport mechanisms, and processes of mass wasting, (2) the relative contribution of downslope transport of sediment to the development of the continental rise, and (3) the timing of mass-wasting events in relation to sequence boundaries identi¢ed by seismic-stratigraphic analysis.

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2. Mass-transport facies of the New Jersey margin 2.1. Deep-sea gravity-controlled processes Deep-marine depositional processes are brie£y discussed here to clarify terminology used in this report. Sediment failures generate a spectrum of gravity-driven processes whose end members are generally identi¢ed as slides, slumps, debris £ows, and turbidity currents (Fig. 2). Various classi¢cations of these processes and their resulting deposits have been proposed and in this paper we follow the terminology originally proposed by Varnes (1958), Dingle (1977) and Embley and Jacobi (1977), with slight modi¢cation by Shanmugam et al. (1994, 1995). A slide is a mass or block that moves downslope on a planar glide plane or shear surface and shows essentially no internal deformation; whereas, a slump is a block that moves downslope on a concave-up glide plane or shear surface and undergoes rotational movements causing minimal to substantial internal deformation (Dingle, 1977; Embley and Jacobi, 1977; Fig. 2). Slides represent translational movements, whereas slumps represent rotational movements along shear surfaces. Di¡erentiation of slumps and slides based solely on seismic data is often impossible because the presence, type and/or degree of internal deformation is undetectable. However, when long, continuous cores through these deposits are available, slumps and slides can sometimes be di¡erentiated. Although cores alone prohibit recognition of the types of glide planes, slump deposits can often be distinguished from slide deposits by the presence of plastic deformation features including: (1) soft-sediment folds, (2) contorted, discordant and truncated strata, (3) steeply dipping layers (up to 60‡) with various orientations, (4) basal (primary) glide planes and internal (secondary) glide planes, (5) abrupt changes in lithology or fabric, and (6) clastic injections (Helwig, 1970; Woodcock, 1976, 1979; Jacobi, 1976; Gawthorpe and Clemmey, 1985; Martinsen, 1989; Shanmugam et al., 1994, 1995). When a slide block(s) moves down slope, it may disintegrate into numerous smaller blocks of various sizes through brittle deformation and become

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a debris slide (Fig. 2). If a debris slide or slump block continues to move downslope, mass disaggregation can result in mixing with water and transform these deposits into debris £ows, in which sediment is transported as an incoherent, viscous mass via plastic (laminar) £ow (Fig. 2). In debris £ows, inter-granular movements predominate over shear-surface movements. Debris£ow facies are identi¢ed by the following criteria: (1) units with sharp upper and lower contacts, (2) £oating or rafted mud and/or lithic clasts, (3) a planar clast fabric, (4) inverse grading of clasts, (5) basal shear zone, and (6) moderate to high matrix content (i.e. matrix supported) (Johnson, 1970; Fisher, 1971; Hampton, 1972; Middleton and Hampton, 1973; Jacobi, 1976; Shanmugam and Benedict, 1978; Embley, 1980; Shanmugam et al., 1995). Debris slides may be interbedded with debris £ows and di¡erentiation between the two is not always clear. We identify debris slides by the presence of brittle deformation features including (1) angular clasts, which commonly form clay breccias and conglomerates, (2) faults and microfaults, which commonly bound angular clasts, (3) extensive fracturing, and (4) matrix content very low to absent (i.e. clast supported). If £uid content continues to increase as a debris £ow moves downslope, the laminar or plastic turbidity £ow may evolve into turbulent £ow and form a turbidity current (Fig. 2). Debate about what constitutes true turbidity-current deposits and how they can be di¡erentiated from deposits of plastic £ows, such as debris £ows, has recently been renewed (see Shanmugam, 1996 and references therein). In this paper, we follow the de¢nition of turbidity currents in which sediment is supported by £uid turbulence (Middleton and Hampton, 1973). Deposits of turbidity currents (i.e. turbidites) are recognized by: (1) normal size grading, (2) sharp basal contacts, (3) gradational upper contacts, and (4) Bouma divisions (Bouma, 1962; Middleton and Hampton, 1973). 2.2. Study methods for mass-transport deposits We sampled and analyzed both the matrix material and the enclosed mud clasts of the mass-transport deposits recovered from all Leg

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Fig. 1. (A) Location of the New Jersey continental margin MCS pro¢les and Legs 150 (902^906), 150X, 174A, 174X drill sites. (B) SeaBeam bathymetry of the New Jersey upper and middle slopes in the vicinity of Sites 902^904 and 906 showing major submarine canyon systems, DSDP Site 612, Cost B3 well, and Ewing 9009 seismic lines 1005 and 1027 (dashed lines).

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150 sites to study sediment provenance. The mineralogic composition of the mud clasts within mass-transport deposits was studied by X-ray diffraction analyses (bulk and clay mineralogy), calcium carbonate content, and petrographic thin sections. The ages of the clasts were determined from calcareous nannofossils and the paleobathymetry from benthic foraminifers. These data and analyses are present and discussed in detail by McHugh et al. (1996). 2.3. Mass-transport facies We identi¢ed the types of mass-transport deposits of the New Jersey continental margin based on the styles of soft-sediment deformation, texture of the sediment, and composition and age of the clasts following the classi¢cation in Fig. 2. Based on shipboard description of the Leg 150 cores and analysis of the lithology and ages of clasts and matrices of the mass-transport deposits, we (McHugh et al., 1996) previously recognized nine separate sedimentary facies within the

299

mass-transport deposits. For the present study we have done a detailed reevaluation of these nine facies and reclassi¢ed them into ¢ve sedimentary facies (Table 2) using a slightly modi¢ed version of the classi¢cation scheme for deep-water facies proposed by Pickering et al. (1986). 2.3.1. Facies 1: Medium- to thick-bedded disorganized sand Facies 1 is composed of massive or structureless to chaotic sands that contain mud and/or lithic clasts. The sands can be ¢ne- to coarse-grained. This facies is observed in only one V20-m-thick interval on the upper slope (338^358 mbsf, Hole 903A). The sands are poorly sorted, gray to brown in color and the clasts range from coarsesand to gravel size (Fig. 3). One signi¢cant aspect is that the matrix of this bed is late Pliocene and middle Pleistocene in age, whereas the clasts within this matrix range in age from Paleocene to middle Pleistocene. Facies 1 is interpreted as a sandy mass £ow such as a sandy debris £ow. This interpretation

Table 1 Correlations of seismic re£ections and sequence boundaries at Sites 902, 903, and 904 on the New Jersey continental slope, ODP Leg 150 (modi¢ed after Miller et al., 1998; Mountain et al., 1994) Name

Age

b

18

p1 p2b p3b p4b p5b p6b m0.3b m0.5b;c m1a;c m1.5b m2a;c m3a;c m4a m5a m5.2a m5.4a m5.6a m6a o1a;c e1 a b c

Series

N O stage 7/6 transition N18 O stage 9/8 transition N18 O stage 10 hiatus; stage 11/10 transition N18 O stage12.4/12.3 transition N18 O stage 13/12 transition disconf. separating upper Plio. and stage 15 near base of C4n C4Ar? Sr-isotope-based age ^ 10.5^11.3 Ma C5r? Sr-isotope-based age ^ on interpolation of sedimentary rate 12.5^12.6 Ma Sr-isotope-based age on interpolation of sedimentary rate ^ 13.6 Ma Sr-isotope-based age on interpolation of sedimentary rate ^ 14.8 Ma Sr-isotope-based age 16.3^18.0 Ma Sr-isotope-based age 18.2 Ma Sr-isotope-based age 18.4^20.6 Ma Sr-isotope-based age V22 Ma Sr-isotope-based age 23.8 < 0.2 Ma Sr-isotope-based age above ^ NP19^20/NP23 below ^ 30.2^36.7 Ma unconformity NP16/NP19^20

Estimated age (Miller et al., 1996b). Tentative age and seismic correlation (Mountain et al., 1994). Shelf-slope age and seismic correlation (Fulthorpe and Austin, 1998).

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mid. Pleisto. mid. Pleisto. mid. Pleisto. mid. Pleisto. mid. Pleisto. up. Plio./mid. Pleisto. mid. upper Mio. mid. upper Mio. upper mid. Mio. upper mid. Mio. upper mid. Mio. upper mid. Mio. middle mid. Mio. lower/mid Mio. bound. upper lower Mio upper lower Mio lower Miocene Olig./Mio. bound. mid. Olig./upp. Eocene mid./up. Eo not reached

300


Table 2 Classi¢cation of sediment facies of the mass-transport deposits recovered on Leg 150 Facies

Description

Sedimentary process ^ environment of deposition

1

Medium- to thick-bedded disorganized sand. Massive or structureless to chaotic sands with mud and lithic clasts of variables ages and lithologies. Matrix is ¢ne to coarse sand, poorly sorted, ranging from late Miocene to Pleistocene in age. ‘Floating’ lithic clasts are up to pebble size; mud clasts are up to 4 cm. Mud clasts range from Paleocene to middle Pleistocene in age. Thick-bedded organized sand. Medium-sized sand with normally graded mud clasts in lower 1 m. Mud clasts are 6 0.5 to s 4 cm diameter. Sand, above, is mainly medium size with scattered quartz granules and shell fragments and shows slight normal grading. Laminated to thin-bedded organized silt and sand. Silt to ¢ne sand beds with mainly parallel strata, rare cross-strata and grading. Beds are up to 5 cm thick. Interbedded muds have rare thin intervals of soft-sediment deformation. Medium- to thick-bedded mud with disseminated silt and sand. Silt to medium-sized sand are mainly disseminated in mud. Discrete sand laminae and beds also present. Quartz is predominant in Pleistocene, whereas glauconite is predominant in Oligocene and lower Miocene. Beds are up to 8 m thick. Matrix-supported chaotic silty clay. Clasts are of the same age and lithology as the surrounding matrix. Soft-sediment deformation features include: folds, dipping, discordant or truncated beds and layers. Mud clasts are up to 8 cm diameter and commonly deformed. Sand- to cobble-size lithic fragments are less common. Pleistocene mud clasts chlorite-rich; Miocene mud clasts kaolinite-rich. Matrix-supported slightly deformed to undeformed silty clay. Clasts are of variable lithologies. Mud clasts are 6 1 to s 50 cm in diameter and are sub-rounded. Lithic clasts are rarely up to 6 cm in diameter. Pebble-size shells and bioclasts common. Glauconitic sands near base. Matrix-supported chaotic silty clays. Clasts are of variable ages and lithologies than the surrounding matrix. Contorted, discordant dipping beds and folds. Clast ages are Eocene to Pleistocene. Carbonate-rich clasts are common and generally deformed. Mud clasts are up to s 10 cm in diameter. Sand- to pebble-size lithic clasts are less common. Clast-supported very chaotic muds which are predominantly clay conglomerates and breccias. Clasts are of the same lithology as the matrix. Highly deformed silty clays composed predominantly of various sized ( 6 1 to s 50 cm diameter) mud clasts, which form clast-supported clay conglomerates and breccias. Both clasts and matrix are commonly deformed and show both soft-sediment deformation (folds, £owage, clastic injections) and brittle deformation (microfaults, which commonly bound clasts, and extensive fracturing). Clast-supported very chaotic muds which are predominantly clay conglomerates and breccias. Clasts are of variable lithologies and ages than that of the matrix. The major di¡erence between Facies 5d and 5e is that clasts in Facies 5e are of variable lithologies and ages.

Sandy debris £ows in upper slope.

2

3

4

5a

5b

5c

5d

5e

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Turbidity current and/or debris £ow in upper slope. (Has a⁄nities with both types of £ow.)

Turbidity currents and or bottom currents in canyons; intervals of soft-sediment deformation are localized slumps and debris £ows. Turbidity currents, related gravity-controlled £ows and sandy mass £ows in intercanyon slope and rise environments.

Muddy slumps and debris £ows formed in an intercanyon slope environment.

Muddy debris £ows. Form ¢ll in modern Berkeley Canyon.

Muddy slumps and debris £ows on continental rise.

Slumps and debris £ows (possibly a transitional phase from one to the other) in a canyon environment. Some clasts appear to represent deformed matrix material, which contains smaller clasts and suggests two or more cycles of mass-wasting events.

Slumps and debris £ows (possibly a transitional phase from one to the other) in the continental rise possibly a channel. Some clasts appear to represent deformed matrix material, which contains smaller clasts and suggests two or more cycles of mass-wasting events.


301

Fig. 2. Schematic diagram showing end-member types of gravity-driven depositional processes that transport sediment into the deep sea (see text for discussion).

is supported by the randomly distributed suspended or ‘£oating’ clay and lithic clasts in the sandy matrix, which indicate plastic £ow. The fact that Paleocene- to Pleistocene-age clasts have been incorporated into the Pleistocene matrix indicates deep erosion. Paleocene clasts have not been found anywhere else in slope or rise mass-wasting deposits, and Paleocene strata are deeply buried beneath the shelf (Poag and Mountain, 1987). Cretaceous through Eocene £uvial and marine deposits dip basinward along the middle Atlantic Coastal Plain and £uvial terraces and alluvial sheets of late Miocene to Pleistocene age unconformably overlie these deposits (Pazzaglia, 1993). However, deep erosion into the coastal plain has exposed Paleocene, Eocene and Miocene deposits (Pazzaglia, 1993). A possible source for Facies 1 deposits is therefore the coastal plain where the Paleocene and Eocene have been exposed. These sediments were probably transported through several episodes of mass wasting by £uvial and delta systems that crossed the shelf during sea-level low stands. 2.3.2. Facies 2: Thick-bedded organized sand Facies 2 occurs only in one unusual 2.8-m-thick bed on the upper slope (from 144.5 to 142 mbsf, Hole 903B, Fig. 4). This bed consists of normally

graded medium-sized, well-sorted quartz sand, which contains sub-rounded to well-rounded reddish-gray mud clasts in the lower 1 m. The mud clasts are normally graded as well (Fig. 4). The overlying sands in the upper portion of the bed are gray in color and rarely contain scattered quartz granules, shell fragments, and mica. Incomplete recovery above the unit suggests that perhaps the sand bed was much thicker. The age of the matrix is middle Pleistocene and the clasts are ‘barren’ (McHugh et al., 1996). The normally graded mud clasts of Facies 2 are unique among the other intervals of Leg 150 cores that contain mud clasts, and this grading clearly suggests that deposition may have been from a £uidal turbidity current, rather than a plastic debris £ow. However, the generally medium sand of the upper part of the deposit does contain scattered ‘£oating’ coarse sand grains and granules, which suggests plastic mass £ow. Clearly the transporting medium had to be viscous enough to suspend the clasts; however, the medium also had to be £uid enough to allow the clasts to settle by gravity. In the depositional sequence proposed by Hampton (1972), the transition between a debris £ow and a turbidity current involves extensive dilution of the debris-£ow material and a decrease in the density from 2.0 to 1.1 g/cm.

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302


The clasts of Facies 2 were probably formed as part of a debris £ow and were deposited during the transition from debris £ow to turbidity current. 2.3.3. Facies 3: Laminated to thin-bedded organized silt and sand Facies 3 occurs mainly in the canyon (Site 906 at 421^376 mbsf) and consists of laminae and thin beds of silts and ¢ne-grained sands interbedded in silty clay (Figs. 5 and 6). These silts and sands appear to form one 45-m thick thinning-upward and ¢ning-upward unit. The sands contain parallel strati¢cation, rare cross-strati¢cation and grading. The ¢ne-grained sands contain well-sorted quartz, mica, and abundant wood fragments. The beds toward the base of the interval have sharp, scoured bases, and possibly ripple crosslamination. This unit also contains rare intervals of soft-sediment deformation with isoclinal folds, dipping contacts, and sub-rounded clay clasts that represent localized mass-transport events of slumping and debris £ows (Fig. 6). Facies 3 is interpreted as the deposit of multiple turbidity currents £owing down a canyon (e.g. Normark et al., 1997). The ¢ning- and thinningupward cycle may suggest a shoreward shift of the source area. Possibly, when the canyon was initially formed, turbidity currents totally bypassed this portion of the canyon and deposited sediment further downslope. As the source retreated shoreward, or the strength of the £ows diminished, turbidite deposition commenced at this location in the canyon. Because the sands have abundant quartz and plant matter, the source for the turbidity £ows was terrigenous, possibly a delta. 2.3.4. Facies 4: Medium- to thick-bedded mud with disseminated silt and sand Medium- to ¢ne-grained sands disseminated throughout silty clay constitute Facies 4 (Fig. 7). These sands also occur as discrete laminae (Fig. 7E) and beds up to 8 m thick (on the continental rise). Disseminated sands could have been deposited by sand-rich £ows supported by a clay matrix in which the £ow rheology was plastic rather than

£uidal (Fig. 7A,B). The association between mass £ows and sands is evident (Fig. 7C,D) in the sedimentary deposits in which sands of Facies 4 occur with mass £ows of Facies 5a (see below). The sand beds could have been deposited by turbidity currents and related gravity £ows (Fig. 7E). 2.3.5. Facies 5: Deformed or chaotic mud with dipping, contorted, folded, faulted and/or truncated strata; and mud and/or lithic clasts Facies 5 has been subdivided into ¢ve discrete subfacies (5a^5e) as a result of the variety of structural styles and compositions that are present. In general, the subdivisions are based on weather the mass-wasting deposits are matrixor clast-supported. Additionally, the subdivisions are based on the composition and age of the clasts that are contained within the matrix that permit to interpret the origin and transport mechanisms of these deposits. 2.3.5.1. Facies 5a: Matrix supported chaotic silty clay with intervals of mud clasts that are the same age as the surrounding matrix Facies 5a is characterized by deformed and contorted silty clay with intervals containing mud clasts, which are matrix-supported (Fig. 8). A de¢ning characteristic of Facies 5a is that the mud clasts are the same age as their surrounding matrix (McHugh et al., 1996). The matrix of Facies 5a is primarily composed of silty clays that have variegated colors ranging from brownish gray to greenish gray. Thin, deformed silt and ¢ne-sand laminae and thin beds are rarely present. Soft-sediment deformation features are common in the matrix and include: (1) recumbent, chevron, and isoclinal folds, (2) low- to high-angle dipping beds and laminae with various orientations, and (3) discordant or truncated beds (Fig. 8). Quartz, glauconite, mica, shells, and wood fragments constitute the coarse-grained components that are randomly disseminated throughout the matrix and have, in some instances, been incorporated into the clasts. Mud clasts are rare in the Pleistocene, but where present, commonly are sub-rounded with elliptical to elongated shapes (Fig. 8). These clasts

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commonly exhibit £owage or deformation and clast boundaries are generally not well-de¢ned. In contrast, mud clasts in the Miocene section are more indurated, less deformed, and exhibit well-de¢ned boundaries (Fig. 8). Intervals of Facies 5a that display soft-sediment deformation features, such as contorted or folded beds, are interpreted as slumps. The less deformed intervals of Facies 5a, which contain abundant mud clasts are interpreted as muddy debris £ows. The slumps were apparently initiated while the sediment was soft, leading to extensive mixing, remolding, and partial destruction of initial depositional structures (Fig. 8). At this stage the plastic deformation of layers could lead to the formation of mud clasts suspended in a muddy matrix and transformation into debris £ows (Fig. 8). Failure of slightly more indurated sediment may have initiated the Miocene slumps, some of which were transformed into debris £ows containing angular clasts. In summary, the Pleistocene sediments of Facies 5a are mainly muddy slumps, whereas the Miocene sediments are mainly muddy debris £ows. These age and lithologic characteristics of the slumps and debris £ows (i.e. matrix and enclosed clasts of equivalent age and similar clay mineralogy and carbonate content) indicate that these deposits represent failures from localized sources that did not entrain allochthonous material during their transport and deposition. Detachment of sediment from the upper slope is a possible source for the Pleistocene slumps. Such localized slope detachments are a common type of failure observed in the Quaternary sediments seaward of New Jersey and Delaware (McGregor and Bennett, 1977, 1979, 1981; Malaho¡ et al., 1980; Farre and Ryan, 1987). Similar localized slope failures and detachments were also probably the source for the Miocene debris £ows of Facies 5a. However, the paleo shelf break (clinoform in£ection point) was farther inland during the Miocene (Fulthorpe and Austin, 1998). This may have allowed the slides and slumps to move downslope for longer distances resulting in the mixing of the sediment mass with water, its disaggregation into a plastic £ow, and formation of clasts (Fig. 2).

303

2.3.5.2. Facies 5b: Matrix-supported, lightly deformed to undeformed silty clay with intervals of mud and lithic clasts that are of variable lithologies Facies 5b consists of a single 50-m-thick unit in the canyon (Site 906, 55^5 mbsf) composed predominantly of slightly deformed silty clay with clay and lithic clasts, which are matrix-supported (Fig. 9). A de¢ning characteristic of Facies 5b is that the clasts have variable lithologies, which are di¡erent from the surrounding matrix and that the clasts are undeformed and very large (Fig. 9A; McHugh et al., 1996). Facies 5b is interpreted as a series of muddy debris £ows that ¢lled Berkeley Canyon (Figs. 1 and 9). The mixing of lithologies in the clasts and matrix of Facies 5b suggests that these masstransport deposits are derived from transport and erosion in a canyon setting. The age of the clasts could not be determined biostratigraphically because the clasts are barren. The occurrence of large and undeformed clay clasts suggests that some of the material found in Facies 5b is locally derived from the canyon walls. 2.3.5.3. Facies 5c: Matrix-supported chaotic silty clay with intervals of mud clasts of variable ages that are di¡erent than the age of the surrounding matrix Facies 5c is characterized by a silty-clay matrix with well-developed soft-sediment deformation features and clasts of diverse lithologies, ages, and degrees of induration (Fig. 10). The clasts are matrix-supported. This facies was recovered at the continental rise (Site 905 at 20^220 mbsf). Deformation features include contorted, discordant, and dipping beds of variegated colors ranging from light to dark grays, green to greenish grays and brown to brownish grays. Isoclinal and recumbent folds, as well as di¡use features, are common (Fig. 10). Facies 5c di¡ers from Facies 5b in that 5c contains white Eocene carbonate clasts, shows a much higher degree of soft-sediment deformation, contains some clasts that have apparently been involved in more than one episode of transport, and shows a much greater range in clast age, size, and mineralogy (Fig. 10). The clasts in the Pleistocene mass-transport deposits are middle Eocene, late Pliocene, and

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304


Fig. 3. Examples of Facies 1: medium- to thick-bedded disorganized sand. Chaotic sands that contain clay and lithic clasts of Paleocene to Pleistocene age. Scale bar = 5 cm. (A) Coarse-grained sands, granules, and clay clasts (Interval 150-903A-39-2, 25^45 cm). (B) Clay clasts of diverse ages ‘£oating’ in the sandy matrix; note clast rims contain ¢ne- to coarse-grained sands (Interval 150-903A-39-3, 85^110 cm).

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305

Fig. 4. Examples of Facies 2: thick-bedded organized sand. Normally graded with mud clasts. Scale bar = 5 cm. (A) Sands containing ‘£oating’ granules (Interval 150-903B-16-6, 110^130 cm). Normally graded sub-rounded to well-rounded clay clasts in a coarse sand matrix: Interval 150-903B-16-7, 20^40 cm (B), Interval 150-903B-16-7, 40^60 cm (C).

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306


Fig. 5. Examples of Facies 3: laminated to thin-bedded organized sand. Scale bar = 5 cm. (A) Rare thin sand laminae characterize upper part of the unit (1 cm thick at 74 to 73 cm); silt and clay laminae predominate (Interval 150-906A-41-2, 50^80 cm). (B) Sand beds have sharp scoured bases and contain abundant plant matter (Interval 150-906A-45-1, 40^60 cm). (C) Laminated sands, silts, and silty clays. Towards the base of the unit, sand beds are 2^5 cm thick (4 cm thick at 2^5 cm; Interval 150-906A45-2, 0^30 cm).

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307

Fig. 6. Examples of Facies 3: thin intervals of soft-sediment deformation. Scale bar = 5 cm. (A) Low to high angle dipping beds and clasts with £owage features (Interval 150-906A-40-3, 30^50 cm). (B) Recumbent, isoclinal fold, and sub-rounded mud clasts (Interval 150-906A-40-3, 100^120 cm).

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308


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309

Fig. 7. Examples of Facies 4: medium- to thick-bedded sandy mud with disseminated silts and sands. Scale bar = 5 cm. (A) A major erosional surface separates upper Eocene biosiliceous chalks from upper Oligocene terrigenous silty clays at Site 902D. Disseminated glauconite is abundant in upper Oligocene sediment (Interval 150-902D-74-1, 64^84 cm). (B) Miocene glauconitic sands are extensively burrowed (Interval 150-903A-47-CC, 60-88 cm). (C) Glauconite-rich disseminated sands containing granules and supported in a clay matrix (Interval 150-904A-12-1, 60^85 cm). (D) The association between mass £ows and sands is evident in Pleistocene deposits (Interval 150-903A-32-1, 30^60). (E) Discrete laminae rarely occur in the Pleistocene (Interval 150-902B-1-6, 65^95 cm).

Pleistocene in age (McHugh et al., 1996). The age of the matrix in these deposits is consistently early Pleistocene. Facies 5c is interpreted as muddy slumps and debris £ows that were transported through the lower slope or a canyon where these lithologies are exposed (McHugh et al., 1993). Based on the soft-sediment folds, injections, and other structures, two or more episodes of mass-transport may be represented by some of the Facies 5c deposits. 2.3.5.4. Facies 5d: Clast-supported very chaotic muds which are predominantly clay conglomerates and breccias The lithology of the clasts is the same as that of the matrix. Facies 5d is 57 m thick and occurs in a canyon (Site 906 from 421 to 478 mbsf). This facies is characterized by intervals of predominantly clast-supported conglomerates and breccias, which range in thickness from V1 to 6 m and are bounded by sharp, angular-dipping contacts (Fig. 11A). Soft-sediment deformation features, which include isoclinal folds, £ow structures, and deformed clasts, are common in some intervals, whereas other intervals display more brittle deformation, including microfaults and extensive fracturing of the clasts. Zones of injected silty clay with smaller-sized clasts are also present (Fig. 11B). The clasts can be very large (25 cm) and are commonly bound by angular, dipping microfaults (Fig. 11C). Facies 5d is mainly composed of clast-supported deposits that appear to be the product of disaggregation of slump and/or slide blocks to form debris slides as they moved downslope (Fig. 2). The occurrence of both matrix- and clast-supported intervals suggest that these deposits may represent the transitional phase from slumps/debris slides into debris £ows. Some large

clasts appear to be composed of matrix material containing smaller clasts. The occurrence of smaller clasts within larger clasts suggests that at least some of the clasts in this deposit represent two or more cycles of mass-wasting events. Thus, the breccia and conglomerate deposits of Facies 5d are apparently debris eroded from the slope as a result of multiple mass-wasting events. Microfaults, as well as clastic injections, suggest that these deposits were overpressured. High pore pressures could have resulted from rapid loading of the sediment by subsequent mass-transport deposits. A possible sequence of events for the deposition of Facies 5d is that a slope failure occurred, later episodes of mass-transport were directed by gravity into the deep scar left by the original failure and led to continued excavation. Some of the mass-transport deposits derived from the upper slope and/or the canyon walls were preserved at Site 906. 2.3.5.5. Facies 5e: clast-supported very chaotic muds which are predominantly clay conglomerates and breccias The lithology and age of the mud clasts is different from that of the matrix. Facies 5e comprises a 15-m thick (665^680 mbsf) interval of clast-supported conglomerate at Site 905 on the continental rise. Most clasts are angular to subangular, but sub-rounded to rounded clasts also occur (Fig. 12). Sizes range from 6 1 to s 50 cm. Clasts represent a variety of lithologies, exhibit a wide variety of colors and are commonly bounded by steeply dipping microfaults (Fig. 12B,C). Ages of clasts range from middle Eocene to middle Miocene (McHugh et al., 1996). Some clasts are composed of gray sandy clay, which shows evidence of soft-sediment deformation and which contains smaller clasts within them (Fig. 12D,E). These features suggest that the sandy clay clasts

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Fig. 8. Examples of Facies 5a: Pleistocene chaotic silty clay with clasts that are of the same age as the surrounding matrix. Scale bar = 5 cm. (A) Matrix with isoclinal folds and rare sub-rounded mud clasts; clast boundaries are commonly not well de¢ned (Interval 150-902A-2-4, 0^50 cm). (B) Contorted sandy-clay matrix with steeply dipping and folded sand laminae (Interval 150902A-2-4, 100^150 cm). (C) Contorted silty clays with isoclinal folds (Interval 150-902D-2-3, 110^144 cm). (D) Clasts in Miocene silty clays are more indurated and less deformed than in the Pleistocene. Scale bar = 5 cm. Clasts of various sizes, but of similar age and lithology, also gently dipping beds (Interval 150-903C-16-4, 50^70 cm).

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311

Fig. 9. Examples of Facies 5b: slightly deformed to undeformed silty clays. Scale bar = 5 cm. (A) Large (7 cm diameter) lithic fragment in ¢ne-grained matrix (Interval 150-906A-2-5, 40^65 cm). (B) Sharp dipping contact separates two presumably large clasts (Interval 150-906A-5-4, 120^140 cm).

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312


Fig. 10. Examples of Facies 5c: contorted silty clays with clasts of diverse ages, lithologies, sizes, and degrees of induration. Scale bar = 5 cm. (A) Isoclinal folds, £owage features and deformed Eocene chalk clast (Interval 150-905A-21-2, 42^80 cm). (B) Deformed clasts of various ages and lithologies (Interval 150-905A-9-4, 65^100 cm). (C) Large middle Eocene clasts in lower Pleistocene matrix (Interval 150-905A-9-5, 0^40 cm). (D) Flowage features, isoclinal folds, and large middle Eocene chalk clast (Interval 150-905A-3-2, 0^25 cm).

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313

Fig. 11. Examples of Facies 5d: chaotic, clast-supported breccias and conglomerates. Scale bar = 5 cm. (A) Angular-dipping contact separate breccias from dark brown silty clays. Note the light gray breccia clast contains soft-sediment deformation features and angular dark brown silty clay clasts (Interval 150-906A-45-3, 120^150 cm). (B) Microfaults and zones of injected material with smaller clasts. This occurrence of clasts within the breccia clast indicates more than one episode of mass wasting (Interval 150-906A-48-3, 75^100 cm). (C) Angular and fractured clasts bounded by angular-dipping microfaults (Interval 150-906A-46-3, 70^90 cm).

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314


Fig. 12. Examples of Facies 5e: very chaotic clast-supported conglomerates with clasts of diverse ages and lithologies. Scale bar = 5 cm. (A) Clasts of diverse ages (middle Eocene, Oligocene and Miocene), sizes, lithologies, and degrees of induration (Interval 150-905A-73-2, 100^120 cm). (B) Steeply dipping faults and angular contacts are common (Interval 150-905A-74-6, 30^60 cm). (C) Abrupt contact between large clast and underlying debris £ow (Interval 150-905A-73-6, 130^150 cm). (D) Deformed clasts within clasts indicate that some clasts have been involved in at least three episodes of deformation (note clast between 40 and 46 cm) (Interval 150-905A-75-3, 35^60 cm). (E) Chalk clasts within (27 to 30 cm) and interbedded (17 to 21 cm) with dark brown mud indicate two and three episodes of deformation (Interval 150-905A-75-4, 15^45 cm).

were originally derived from the matrix material of an older mass-transport deposit, hence, some of the material in Facies 5e appear to have been involved in two or more episodes of mass transport. Benthic foraminifer assemblages contained in

clasts indicate that the benthics could have been deposited in any water depths between neiritic and lower bathyal (McHugh et al., 1996). The predominant faunal assemblages indicate that deposition was in the middle to lower slope (600^ 2000 m). However, some faunal components in-

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Fig. 13. Line drawing interpretation of dip oriented MCS pro¢le Ew9009, Line 1005 across the outer continental shelf and slope (Mountain et al., 1994). See Fig. 1B for location of pro¢le and Table 1 for explanation of seismic re£ections. Bold numbers indicate locations of facies types described in this paper and their correlation to seismic re£ections.


315

dicate more than one episode of transport from the neiritic zone ( 6 200 m), whereas others are similar to the depauperate Miocene assemblages from the upper slope (450 m). The clast-supported nature of this facies at Site 905 plus both brittle (e.g. faults) and plastic deformation (e.g. folds) suggest that this facies appears to be the product of disaggregation of larger slide and/or slump blocks that evolved into debris slides, possibly as transitional phase from slump or slide into debris £ow (Fig. 2). The occurrence of smaller clasts within larger clasts indicate two, or in some cases, three separate episodes of masswasting events. Smaller clasts in some of these intervals may have been deposited initially by debris £ows. Benthic foraminifer assemblages indicate that these deposits could have been transported from the upper to the middle slope and then to the continental rise or from the middle and lower slopes to the continental rise. In summary, Facies 1, 2, 4, and 5a are predominantly found in intercanyon regions of the slope. Facies 3 occurs in a canyon setting and rarely in intercanyon regions or the upper continental rise. Facies 5b and 5d are characteristic of slope canyons and show an evolution in the degree of internal deformation and clast character down canyon. Facies 5c and 5e occur beneath the continental rise and are characterized by an even greater degree of deformation and clast diversity than the slope facies.

3. Depositional environments of mass-transport and their correlation with stratal surfaces and sequence boundaries 3.1. Background Core and wireline log results from ODP Leg 150 have been correlated to multi-channel seismic (MCS) pro¢les that pass through each of the drill sites (Mountain et al., 1994; Miller et al., 1996a,b; Figs. 13 and 14). Stratal unconformities were matched to seismic re£ections and traced across much of the Ew9009 seismic grid extending from the middle continental shelf to the uppermost continental rise (Fig. 1A,B). Several re£ec-

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tions beneath the shelf were interpreted as sequence boundaries (Miller et al., 1996a,b; Fulthorpe and Austin, 1998; Table 1). Geochronology at all Leg 150 sites was derived by integrating Sr isotopic stratigraphy and magnetostratigraphy with planktonic foraminiferal, nannofossil, dinocyst and diatom biostratigraphy (Mountain et al., 1994; Miller et al., 1996a,b). These ages provided correlations to the N18 O record that were independently derived from deep-sea benthic foraminiferal studies (Miller et al., 1996a,b, 1998). At many levels throughout the Oligocene to middle Miocene Leg 150 slope sections, re£ectors that were correlated to potential sequence boundaries matched depressions on the N18 O curve, further indicating that these surfaces formed during times of glacioeustatic lowerings. Site 905 recovered middle Miocene to lower Pleistocene sediment from the continental rise approximately 34.5 km from the base of the slope (Fig. 1A). None of the mappable seismic re£ections identi¢ed on the slope at Sites 902^904 and 906 (Figs. 13 and 14) can be traced beneath the continental rise to Site 905. Within the interval drilled at Site 905 re£ections merlin and blue are roughly late middle Miocene and mid-Pliocene, respectively; (Mountain and Tucholke, 1985). The Leg 150 Shipboard Party de¢ned two other re£ections, brown and yellow, near the Pliocene^ Pleistocene boundary, and pre-merlin middle Miocene, respectively (Mountain et al., 1994). The nature and timing of mass-wasting events and their implications for the erosional history of the New Jersey margin, can be interpreted from the ¢ve sedimentary facies described above. In general, these sedimentary facies represent three major environments of deposition : (1) intercanyon regions of the slope, (2) canyons, and (3) upper continental rise. In many cases, these sedimentary facies also appear to correlate with stratal surfaces and sequence boundaries interpreted on the seismic data across the slope sites 903, 902, 904 and 906 (Figs. 13 and 14). This suggests a possible relationship between deposition of mass-transport deposits and eustatic sea-level changes (Table 1 and 3; Figs. 13 and 14). However, this relation does not always hold because mass-transport deposits are also present within

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Fig. 14. Line drawing interpretation of strike oriented MCS pro¢le Ew9009, Line 1027 along the continental slope (Mountain et al., 1994). See Fig. 1B for location of pro¢le and Table 1 for explanation of seismic re£ection. Bold numbers indicate locations of facies types described in this paper and their correlation to seismic re£ections.

316

Table 3 Correlation of mass-transport sediment facies (Table 2) with seismic re£ections (Table 1) at Leg 150 drill sites Sequence

Intercanyon upper slope Site 903 (mbsf)

83^142

p3^p2 p4^p3

189^147 190^252

p5^p4 p6^p5

252^274 274^359

m0.3^p6

359^405

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m0.5^m0.3 405^520

m0.7^m0.5 m1^m0.7 ?^604.5 m1.5^m1 604.5^624.5 m2^m1.5 can-1^m2 can-2^can-1 m3^m2 m4^m3 m5^m4 m5.2^m5 m5.4^m5.2 m5.6^m5.4 m6^can-2 m6^m5.6 o1^m6 e1^o1 e2^e1 Total depth

(mbsf)

5a-4-2 (119^121; 125^142) 5a and 4 (209^210; 221^231; 241^247) 5a and 4 (269^274) 4 and 1 (298^307; 326^327; 337^353) 4 and 1 (353^359; 369^392; 403^404) 4 (421^422; 424^429; 440^459; 487^493; 500^520) ^ 5a and 4 (578^581; 602^604.5) 5a and 4 (622^624.5)

694^808 808^850 850^900 900^940 940^960

5a and 4 (731^733) ^ 4 (899^900) ^ ^

?^1010 1010^1064

4 (977^978) 4 (1060^1064)

?^1215 1149.7

^

Intercanyon middle slope Site 902 (mbsf)

Facies and depth

0^23 23^58

5a (12^22) 5a (44^47)

84^58 84^122

5a (98^106; 110^122)

?^152.5

4 (132^135; 151^152.5)

208^290

4 (250^256; 260^262; 278^290)

?^407

(mbsf)

Intercanyon lower slope Site 904 (mbsf)

Facies and depth

Canyon Site 906

Facies and depth

(mbsf)

(mbsf)

(mbsf)

?

5b (0^55)

?^115

4 (112^115)

115^158 158^279.2 279.2^384 384^421.1

3 (149^157) 4 (278.5^279) 3^5a (373^384) 5d and 3 (384^ 421)

56^81 81^97

4 (71^74) 5a (95^98)

?^106 106^180.3

5a and 4 (104^106) 4 (136^39; 164^168; 179^80)

4 (394.5^395; 402^404)

?^220

3 (216^221)

407^450 450^493 493^523

^ 4 (453^465) 4 (511^512; 522^523)

220^235

4 (225^231)

?^272

4 (240^272)

540^568 568^595

4 (564^568) ^

595^620 620^681

4 (609^612) 4 (680^681)

?^316 316^341 341^417 417^526 576.7

4 (279^316) 4 (316^341)


p0 p1^p0 p2^p1

Facies and depth

421^478.2 5d^4 (421^478.2)

740.1

478.2^555 4 (500^506)

602.4

See Figs. 15^18. 317

318


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319

Fig. 15. Site 903 lithology showing facies types at, and in between, seismic surfaces and sequence boundaries (see Table 1). Facies 1, 2, 4, and 5a frequently correlate with surfaces in the upper Miocene and Pleistocene. However, Facies 1, 4, and 5a also occur in between surfaces. The correlation between Facies 4 and 5a and sequence boundaries is minor from the upper Oligocene to the upper Miocene. Facies 5a predominates during the Pleistocene. In contrast, Facies 4 dominates in the Oligocene and Miocene.

sequences and in this case local conditions of the margin, such as changes in the slope morphology and gradient, appear to control their deposition. On the continental rise the sedimentary facies of Site 905 are related to seismic re£ections of Mountain and Tucholke (1985). 3.2. Intercanyon regions of the slope 3.2.1. Upper slope Gravity-controlled mass-wasting deposits are best preserved, thickest and most abundant in intercanyon areas beneath the modern upper slope (Site 903, 1150 mbsf; Fig. 15). Facies 1, 2, 4, and 5a are present; however, these deposits form only about V13% of the total sediment section recovered at Site 903. The majority of the sediment consists of hemipelagic siliciclastics and pelagic biosiliceous chalks. We correlated mass-wasting facies to sequence boundaries and stratal surfaces (Table 3, Figs. 13 and 15) and found a strong correlation of mass-transport deposits to seismic re£ections and sequence boundaries. However, mass-transport deposits also occur within the sequences and in between stratal surfaces, especially in the upper Miocene and Pleistocene (Fig. 15). 3.2.2. Upper middle slope The preservation of mass-transport deposits of Facies 4 and 5a represent only a small percentage (V11%) of the total sediment recovered on the middle slope, Site 902 (736 mbsf). The remaining sediment is composed of predominantly hemipelagic siliciclastics and pelagic biosiliceous chalks. Mass-transport deposits of Facies 5a predominate in the Pleistocene section (126 m thick) and the bases of some of these deposits correlate with sequence boundaries. As in the upper slope, the correlation between mass-transport deposits of Facies 5a and 4 with seismic surfaces is not as strong for the Pleistocene in the middle slope

(Fig. 16). Overall there are less mass-wasting facies preserved in the middle slope than in the upper slope. Sands of Facies 4 are the only type of mass wasting that was preserved at the middle slope from the Oligocene to the Pleistocene where it shows a strong correlation to sequence boundaries (Figs. 14 and 16, and Table 3). 3.2.3. Lower slope Mass-transport deposits of Facies 3, 4 and 5a constitute a minor component (V15%) of the total sediment (570 m) recovered at 904. The correlation between mass-wasting facies and sequence boundaries is not strong for the Pleistocene. In contrast, there is a strong correlation between Facies 4, 3 and sequence boundaries for the Oligocene and Miocene (Figs. 14 and 17; Table 3). In summary, correlation of mass-transport facies with seismic surfaces and sequence boundaries is strong for the upper slope during the Pleistocene and for the middle and lower slope for the Oligocene and Miocene. However, mass-transport facies also occur in between sequences especially in the Pleistocene and late Miocene. These observations suggest that there is a correlation between erosion and mass-transport deposition to seismic surfaces and sequence boundaries; however, preservation of mass-wasting deposits was in£uenced by the morphology and gradient of the existing paleoslope at the time of deformation. Steepening of the slope in response to sediment progradation in the late Miocene and Pleistocene (Fulthorpe and Austin, 1998) may have led to upper slope failure and to the preservation of some detached sediment in the slope. In contrast, the more gentle gradients of the Oligocene and lower and middle Miocene may have resulted in increased deposition and preservation of mass-transport deposits on the middle to lower paleoslope (Figs. 13 and 14).

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320


3.3. Canyon regions of the slope Submarine-canyon ¢lls contain the thickest preserved sections of mass-transport deposits of all slope sites (36% of the total sediment recovered at Site 906). The mass-wasting facies contained in the lower section recovered from Site 906 document a sequence of events interpreted as the initial excavation and subsequent in¢lling of a now buried middle Miocene slope canyon. The entire episode of canyon excavation and in¢lling during the Miocene took place extremely rapidly during a period of V2.5 Myr as a result of a major relative sea-level lowering at 13.5 < 0.5 Ma (Mountain et al., 1996). The facies in the upper portion of the section document the in¢lling of the modern Berkeley Canyon (Figs. 1B, 14 and 18). The Miocene canyon-¢ll is 199 m thick and is composed of Facies 5d and 3 sediment (Figs. 14 and 18). Detailed studies of Facies 5d deposits indicate that the clast- and matrix-supported conglomerates and breccias have not been transported for long distances. This indicates that Facies 5d deposits originated nearby, possibly from the canyon walls. Microfaults, as well as clastic injections, suggest that the deposits were overpressured, possibly as a result of rapid loading of the sediment by subsequent mass-transport deposits. Facies 5d is interpreted as the initial stages of the slope-canyon excavation. The overlying interval of the Miocene canyon ¢ll is composed of laminated sands and silts of Facies 3 (Fig. 18). These depositional events are interpreted as episodes of canyon in¢lling, ¢rst by turbidity currents and then by pelagic and hemipelagic clays. A possible sequence of events is that canyon excavation, represented by Facies 5d, was accentuated during a relative sea-level fall that lead to intensi¢ed erosion and sediment transport. Some of the material eroded during this episode of canyon excavation was preserved on the slope (Facies 3 and 5d), whereas other material was transported basinward. As relative sea-level rose, the sediment source moved farther shorewards or the turbidity £ows diminished in intensity and turbidites were preserved in the canyon. Approximately 306 m of middle to upper Miocene hemipelagic siliciclastic sediment separates

the buried Miocene canyon ¢ll from the £oor and base of the ¢ll of modern Berkeley Canyon (Figs. 1B and 14). The sediment is predominantly in situ and sands of Facies 3 and 4 apparently correlate to sequence boundaries m1 (115 mbsf) and m1.5 (158 mbsf) (Fig. 18; Mountain et al., 1994; Miller et al., 1996a,b; Fulthorpe and Austin, 1998). The topmost unit at Site 906 is 55 m thick and constitutes the ¢ll of modern Berkeley Canyon. The sediment is Pleistocene in age and dominated by a series of muddy debris £ows of Facies 5b. The dominant ¢ne-grained character of Facies 5b debris £ows suggests that the terrestrial source for these deposits was not close by, and that canyon in¢lling must have occurred during a sea-level high-stand or as a result of the source moving farther north or south. This canyon-¢ll sequence could not be correlated to the shelf or other slope sites (Mountain et al., 1994). In summary, at Site 906 there is a strong correlation between the occurrence of mass-transport deposits and sequence boundaries associated with middle Miocene canyon excavation and in¢lling. These correlations lend support to the Vail/Exxon sequence stratigraphic model in which accelerated relative sea-level fall leads to exposure and intensi¢ed erosion of the shelf and slope and to canyon incision (Vail et al., 1977, 1991; Haq et al., 1987; Posamentier et al., 1988; Posamentier and Vail, 1988; Vail, 1987; van Wagoner et al., 1990). 3.4. Continental rise Approximately 30% (260 m) of the 910 m of sediment recovered at Site 905 is mass-transport deposits of Facies 4, 5a, 5c, and 5e (Fig. 19). The most extensive downslope transport of sediment to the upper rise occurred during the early Pleistocene when deposits 215 m thick were emplaced. Older sediments appear to have been deposited mainly by normal hemipelagic sedimentation; however, mass-transport deposits also occur in the Miocene section (Fig. 19). The oldest mass-transport unit is composed of middle Miocene slump and debris-£ow deposits (Facies 5c and 5e, e.g. Figs. 10 and 12) and comprises the interval from 655 to 680 mbsf. This

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321

Fig. 16. Site 902 lithology showing facies types at, and in between, seismic surfaces and sequence boundaries (Table 1). Some correlation occurs between Facies 4 and 5a and seismic surfaces in the Pleistocene; but Facies 4 and 5a also occur within surfaces. Correlation between Facies 4 and sequence boundaries is strong for the Oligocene and Miocene.

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322


Fig. 17. Site 904 lithology showing facies types at, and in between, seismic surfaces and sequence boundaries. Facies preservation is poor for the Pleistocene. In contrast, Facies 3, 4 and 5a correlate with all seismic re£ectors and sequence boundaries identi¢ed from the Oligocene and Miocene (Table 1).

deposit apparently rests upon an erosional unconformity marked by re£ector yellow (Mountain and Tucholke, 1985; Mountain et al., 1994; Fig. 19) and is the result of canyon cutting events upslope as evidenced by the variable ages and mineralogy of the clasts. A middle Miocene slump/debris-£ow deposit composed of Facies 5a occurs from 618 to 620 mbsf (Figs. 8 and 19). The clasts in this deposit are uniformly middle Miocene in age (McHugh et al., 1996). This deposit is interpreted to have resulted from localized slope failure and detachment in an intercanyon area of the slope because of the uniform age of the clasts and matrix. Other localized slope detachments of late Miocene age are also present between 553^567 and 568^573 mbsf.

Re£ector merlin, a major regional seismic re£ection, occurs between the top of the middle Miocene deposit and the lower portion of the oldest upper Miocene deposit (Mountain and Tucholke, 1985; Mountain et al., 1994; Fig. 19). The Pleistocene section at Site 905 is composed mainly of mass-transport deposits of Facies 4, 5a and 5c (Fig. 19). The matrix in these deposits is consistently early Pleistocene in age. The two lowermost Pleistocene mass-transport units (80^98 and 106^215 mbsf) are characterized by muddy slumps and debris £ows of Facies 5c (Fig. 10) and contain clasts of variable ages, including middle and late Eocene, Pliocene and Pleistocene (McHugh et al., 1996). The interval from 106 to 215 mbsf may actually contain multiple events

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323

Fig. 18. Site 906 lithology showing facies types at, and in between, seismic surfaces and sequence boundaries. Seismic re£ection correlation to facies type was only possible in the intervals of hemipelagic deposition (in situ sediment) where a strong correlation occurs between Facies 3 and 4 and sequence boundaries (Table 1). Facies 3, 4, and 5d are associated with the Miocene canyon excavation event. Facies 5b represents the ¢ll of modern Berkeley Canyon. There is no correlation of seismic re£ectors to sequence boundaries in Berkeley and middle Miocene canyon ¢lls.

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324


Fig. 19. Correlation of lithology, Facies 4, 5a, 5c, and 5e and prominent seismic re£ections at Site 905. Re£ectors brown, merlin, and yellow represent regional erosional unconformities in many parts of the western North Atlantic (Mountain and Tucholke, 1985; Mountain et al., 1994).

because some of the mass-transport deposits are separated by a 6-m thick interval of silty clay and by a 5-m thick sand layer. The interval from 80 to 98 mbsf is separated from the deposits above and below by an 8-m thick sand interval of Facies 4

and an 8-m thick silty clay interval, respectively. These Pleistocene slump/debris-£ow deposits as well as the 50 m above (interval from 20 to 72 mbsf) are interpreted to have resulted from canyon-cutting events on the slope because of the

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325

Fig. 20. Schematic history of erosion and mass-transport deposition on the New Jersey continental margin based on a limited number of core sites. (A) Late Oligocene and Early Miocene. (B) Early Miocene to late middle Miocene. (C) Late Miocene to Pleistocene. (D) Pleistocene to Recent.

multiple ages and variable lithology of the clasts within them. The base of Pleistocene section in Site 905 coincides with the prominent regional re£ector brown (Mountain et al., 1994). Benthic foraminifer assemblages recovered from clasts in the slumps and debris £ows of Site 905 indicate that these mass-wasting events could have been initiated anywhere between 6 200 and 2000 m water depths (McHugh et al., 1996). Gradients on the modern continental slope and rise are approximately 4‡ and 1‡, respectively, but Miocene paleoslopes were apparently even less steep (Fulthorpe and Austin, 1998). Transport distances from the upper paleoslope to Site 905 on the continental rise were probably a maximum of 75 km and a minimum of 35 km from the base of the paleoslope.

Deformed clasts contained within larger clasts and other deformation features suggest that some of these mass-transport deposits record at least two or three separate episodes of transport and deformation. This suggests entrainment of relict slumps and debris £ows during subsequent mass-wasting events. This is not surprising because sampling of blocks that commonly lie scattered on the £oors and walls of the New Jersey continental-slope canyons revealed that the blocks are relic Miocene mass-transport deposits (Ryan, Miller and McHugh, unpublished data from Alvin dives). Such deposits could have easily been entrained by subsequent mass-wasting events. Benthic foraminifer assemblages also indicate that some of these deposits were transported more than once.

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Fig. 21. Perspective image constructed from SeaBeam bathymetry showing extent of erosion on the modern continental slope and rise o¡-shore from New Jersey (courtesy of W. Haxby, Lamont-Doherty Earth Observatory). The image is approximately 90 km long along the strike of the slope (300 to 2500 m of water depth) which is deeply incised by canyons and gullies. The continental rise forms a smooth apron beyond the base of the slope (2500 to 2700 m).

4. History of mass-transport on the New Jersey margin Variability in the characteristics and preservation of the mass-transport facies described above can be related to an increase in the volume of sediment transported to the New Jersey margin and to changes in slope morphology. Sediment supply to the margin increased in response to tectonic uplift and erosion in the hinterland, which indirectly controlled the location and migration of sediment sources (Poag and Sevon, 1989; Pazza-

glia, 1993), climate cooling (Poag, 1992; Steckler et al., 1999), and eustatic lowerings (Miller et al., 1996a,b). Glacioeustacy appears to have been a major control for sediment transport and erosion on the New Jersey margin because the greatest changes in the morphology of the slope occurred from the late Miocene to Pleistocene when climate cooled. During the Oligocene and Miocene the margin had a gently dipping steplike ramp geometry. The shelf and upper slope evolved as a result of increasing seaward sediment progradation to a £at shelf with a well-de¢ned shelfbreak and a

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steep slope (4^8‡) in the late Miocene and Pleistocene (Fulthorpe and Austin, 1998; Steckler et al., 1999). The complexity and volume of masstransport can also be related to slope progradation and steepening from the late Miocene to the Pleistocene because mass-wasting facies are more abundant and complex at that time. Most importantly, mass-wasting facies are present in between sequence boundaries that suggest other controls than glacioeustacy. Detailed analyses of the sediment facies and their correlation to sequence boundaries and stratal surfaces permit the reconstruction of a history of mass wasting within the context of the evolution of the slope. Although these analyses are limited to the Leg 150 Sites, prior observations from an Atlantis II coring program (McHugh et al., 1993) and Deep Sea Drilling Project Sites from Leg 95 (Poag and Mountain, 1987) have also contributed to these interpretations.

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middle Miocene (13.5 < 0.5 Ma) a canyon cutting episode resulted in the deposition of 114 m of clast-supported slumps, debris £ows, and turbidites of Facies 5d and 3 at Site 906. Sediment progradation in the late middle Miocene (V11.0^8.0 Ma) lead to development of a concave slope morphology, which began to resemble the modern slope (Fulthorpe and Austin, 1998). The ¢rst evidence of slope detachments in the late middle Miocene are the debris £ows of Facies 5a at Site 903 (Fig. 20B). These slope failures as well as canyon cutting events are related to sediment progradation and to a steepening of the slope. Mass-transport during the late middle Miocene also extended to the continental rise where approximately 30 m of clast- and matrix-supported slumps and debris £ows (Facies 5c and 5e) were deposited at Site 905 and the surrounding area (Fig. 20B). 4.3. Late Miocene to Pleistocene

4.1. Late Oligocene to early Miocene During the late Oligocene a ramp-type margin existed with a pronounced shelf break that was much farther landward than the Leg 150 slope sites (Fulthorpe and Austin, 1998). The Leg 150 sites penetrated the gently dipping paleoslope with gradients of 6 1‡. Glauconite-rich, disseminated sands of Facies 4 are the only mass-wasting facies recovered from these deposits. The sands were probably deposited by uncon¢ned gravity £ows that were transported along these gradual slopes by currents. There is no record of mass-transport deposits associated with slope detachments and/or canyon-cutting events (Fig. 20A). 4.2. Early Miocene to late middle Miocene Slope sedimentation during the early Miocene to middle Miocene in the vicinity of the Leg 150 slope sites was characterized by hemipelagic clays and rare mass-transport deposits, which are preserved as disseminated sands of Facies 4 (Fig. 20B). The existing slope was gentle, arcuate to convex in shape, and had gradients of approximately 1.5^2.0‡; submarine canyons were rare (Fulthorpe and Austin, 1998). Towards the late

In£ux of terrigenous sediment led to continued seaward progradation of the depositional shelf break in the upper Miocene slope. This led to a marked increase in the accumulation of disseminated sands of Facies 4 (Fig. 20C). The in£ux of the terrigenous component is manifested in a change in the composition of sands from glauconite-rich to quartz-rich. Sandy debris £ows (Facies 1), possibly derived from a £uvial source, also indicate the increased in£ux of terrigenous sediment. The diverse age and lithology of the clasts contained in the sandy debris £ows, suggests that these deposits resulted from deep erosion of the coastal plain where older lithologies are exposed. Sediment transport to the continental rise also occurred around the late Miocene (Fig. 20C). An unconformity that correlates with re£ector merlin occurs between the upper middle and upper Miocene mass-wasting deposits and represents a regional unconformity in many parts of the western North Atlantic (Mountain and Tucholke, 1985). This indicates that the middle and upper Miocene episodes of mass-wasting were not isolated events, but rather were common occurrences for the western North Atlantic. However, the relatively thin

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Fig. 22. (A) GLORIA side-scan sonar mosaic showing variation in backscatter patterns on the New Jersey continental slope and rise (EEZ-SCAN 87 Scienti¢c Sta¡, 1991). (B) Interpretation of GLORIA backscatter patterns. High backscatter trails (lighter tones) extend from slope canyons Baltimore, Wilmington, Spencer, Lindenkhol, Carteret, Berkeley, Toms, Hendrickson, and Mey to the continental rise Baltimore-Mey Canyons ‘gather area’ and converge into Wilmington Valley.

intervals (V18 m total thickness) of late Miocene mass-transport deposits at Site 905 suggests that these deposits did not contribute signi¢cantly to the growth of the upper rise, at least near Site 905.

4.4. Pleistocene to Recent The volume (215 m thick) and complexity of mass-transport facies is greatest in the Pleistocene where nearly all facies types (1, 2, 4, 5a, 5b, and

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5c) are present beneath the modern slope and rise (Fig. 20D). Large-scale mass wasting is evidenced on the multibeam bathymetry images of the margin, which reveal a submarine terrain incised by canyons, gullies, and detachment surfaces (Fig. 21). Echograms (3.5 kHz) across the general region of the Leg 150 slope sites revealed a ‘blocky’ sediment slide complex, which suggests that mass-

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transport deposits are widespread beneath the slope (Vassallo et al., 1983). The acoustic backscatter of GLORIA imagery from the New Jersey continental margin reveals high-backscatter patterns that continue from submarine-canyon heads downslope towards the continental rise (Schlee and Robb, 1991; Fig. 22). Piston cores taken to ‘ground-truth’ the GLORIA

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330


backscatter and deep-towed side-scan sonar records of the region, reveal that debris £ow deposits occur beneath high-backscatter zones. Therefore, these high back-scatter patterns on the GLORIA images most likely mark the paths of mass-transport deposits from canyons to the continental rise. These patterns show that the major canyon systems which have contributed sediment to the continental rise in the vicinity of Site 905 are Carteret, Berkeley, and Tom Canyon systems. Re£ector brown, which coincides with the base of the Pleistocene slumps and debris £ows, has been traced for tens of kilometers to the levee of Hudson Canyon (Mountain et al., 1994). The chaotic character of the overlying seismic unit can also be traced along strike to the Hudson Canyon levee (Mountain et al., 1994). This suggests that slope failures and canyon excavation events were prevalent along the western Atlantic continental margin during this Pleistocene interval. Erosion and downslope transport of sediment during the Pleistocene appear to have been much more extensive and contributed more to the growth of the upper continental rise than did Miocene episodes of mass wasting. As in the western North Atlantic, studies of turbidite deposition on the Madeira abyssal plain also show an increase in sediment transport during the Miocene and into the Pleistocene in response to global changes in climate and sea-level. The deposition of these turbidites on the Madeira abyssal plain appears to correlate to the rising and falling of sea-level not to low stands of sealevel as predicted by the seismic stratigraphic model. As in the New Jersey margin, turbidite deposition is not only linked to eustacy but also to such localized conditions as sediment instability of the continental slope (Weaver and Kuijpers, 1993; Weaver et al., 1992; Weaver et al., 1998). Results of these two very detailed studies of sediment transport in the Madeira abyssal plain and New Jersey continental margin indicate that eustacy is a signi¢cant driver in sediment transport to the basin. However, common to both margins sedimentation patterns is the extent to which localized circumstances such as the margin’s stability and sediment supply control the availability of sediment.

5. Conclusions Five distinct mass-transport facies are recognized in ODP Leg 150 drill sites (902^906) on the New Jersey continental slope and rise. These mass-transport deposits consist predominantly of muddy slumps and debris £ows, and to a lesser extent sandy mass £ows and gravity-related £ows. Many of these mass-transport deposits apparently rest upon prominent stratal surfaces and sequence boundaries on the continental slope and rise and these correlations suggest that extensive mass wasting is associated with the formation of major erosional unconformities, which, in turn, appear to correlate with sea-level lowerings. However, this relationship of mass-transport deposits to seismic re£ections and sequence boundaries is complex. In the Pleistocene and upper Miocene sections, most mass-wasting deposits rest directly upon sequence boundaries and stratal surfaces at locations that represent the upper to middle paleo continental slopes. However, this correlation does not consistently occur on the lower paleo slopes. In contrast, in the upper Oligocene to the upper Miocene section, the occurrence of mass-transport deposits at sequence boundaries is greatest on middle to lower paleoslopes, but is less frequent on upper paleoslopes. These observations suggest that lowering of sea-level leads to slope erosion, to generation of mass-transport deposits, and to the formation of sequence boundaries. However, preservation of mass-wasting deposits was signi¢cantly in£uenced by the morphology and gradient of the existing paleoslope. Steepening of the slope during the late Miocene to the Pleistocene in response to more rapid seaward sediment progradation may have led to increased sediment failure and movement of failed sediment to the continental rise. Some of the detached sediment was preserved on the upper continental slope, but most (V215 m) was transported downslope and deposited on the continental rise. In contrast, the more gentle gradients of the Oligocene to middle Miocene paleoslopes may have allowed increased deposition and preservation of mass-transport deposits on the more distal middle to lower continental slope. The correlation between mass-transport deposits

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and sequence boundaries is further complicated because mass-transport deposits also occur within sequences, especially in the upper Miocene and Pleistocene sections. These observations suggest that local processes (i.e. sediment supply, slope morphology) may be directly related to mass wasting and erosion of continental margins and that these localized conditions may be more signi¢cant that globally modulated sea-level change. The causal mechanism for canyon incision and subsequent in¢lling of the middle Miocene canyon at Site 906 can be linked to a rapid sea-level fall (type 1 unconformity) at 13.5 < 0.5 Ma followed by sea-level rise, and thus would appear to be in concert with the Vail/Exxon conceptual sea-level model. Downslope transport contributed to the growth of the continental rise (V30% of the total sediment recovered). Sediment transport to the upper rise was much more extensive during the Pleistocene than during the middle and upper Miocene, and may be directly related to the evolution of the slope (i.e. steeper gradients, more erosion). Slides and debris £ows were transported from the slope to the continental rise across distances of at least 75 km.

Acknowledgements We are grateful to K.G. Miller, P. Blum, and the Leg 150 Shipboard Scientific Party for making this study possible. We thank the captain, officers, and the crew of the R/V Joides Resolution for their efforts during the collection of the cores. We thank William B.F. Ryan and Mohammed El Tabak for their contributions to this study. We thank Dee Breger, Pat Malone, Lenny Canone, and John Walsh for their support in sediment analyses. We are thankful to Neil Kenyon, Lincoln Pratson, and John Farre. Their comments contributed to the improvement of the manuscript. This study was supported by JOI/USSAC# 7-70137 and PSC-CUNY# 6-22850 ; 6-12660 ; 6-69234. This is Lamont-Doherty Earth Observatory Contribution No. 6224.

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