Chapter 15 Sediment Waves and Bedforms

C H A P T E R

1 5

S EDIMENT W AVES AND B EDFORMS R.B. Wynn and D.G. Masson National Oceanography Centre, Southampton (NOCS), Southampton, UK

Contents 15.1. 15.2. 15.3. 15.4. 15.5.

Introduction Location, Morphology and Genesis of Fine-Grained Sediment Waves Location, Morphology and Genesis of Coarse-Grained Sediment Waves Related Large-Scale Features Generated by Bottom Currents Applications to Bottom-Current Reconstruction: A Case Study From the NW UK Acknowledgements

15.1.

289 290 294 296 298 300

INTRODUCTION

Large-scale sediment waves are some of the most distinctive and frequently described depositional features generated by bottom currents, and can cover huge areas of sea floor (>1000 km2). They occur in a wide range of deep-water environments (Figure 15.1) and are highly variable in terms of their morphology, dimensions and sediment composition. Wynn et al. (2000) defined a sediment wave as ‘‘a large-scale (generally tens of metres to a few kilometres wavelength and several metres high), undulating, depositional bedform, generated beneath a current flowing at, or close to, the sea-f loor’’. An overview of deep-water sediment waves, generated by both bottom currents and turbidity currents, can be found in Wynn and Stow (2002b). The present study will build upon that review by incorporating recently published data, and will have the following specific aims: 1. to outline the morphology, genesis, identification and depositional environment of both fine- and coarse-grained sediment waves formed by bottom currents; 2. to describe and illustrate some of the related large-scale features generated by bottom currents, e.g. sand ribbons and erosional furrows; 3. to investigate the application of data obtained from sediment waves and related features to bottom-current reconstruction. Developments in Sedimentology, Volume 60 ISSN 0070-4571, DOI: 10.1016/S0070-4571(08)00215-X

Ó 2008 Elsevier B.V. All rights reserved.

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Figure 15.1 Global bathymetric map showing location of well-documented examples of bottom-current sediment waves (white circles), and location of the case-study area (black rectangle; Figure 15.2). Numbered white circles refer to the following areas: (1) Norwegian Sea ^Atlantic Ocean gateway (e.g. Roberts and Kidd, 1979; Dorn and Werner, 1993; Manley and Caress, 1994; Howe, 1996; Kuijpers et al., 2002; Masson et al., 2002; Wynn et al., 2002a; Howe et al., 2006); (2) Mediterranean Sea ^Atlantic Ocean gateway (Kenyon and Belderson, 1973; Habgood et al., 2003); (3) Central Mediterranean Sea (Marani et al., 1993); (4) Blake ^ Bahama Outer Ridge (Flood, 1994; Flood and Giosan, 2002); (5) Gulf of Mexico (Kenyon et al., 2002); (6) Carnegie Ridge (Lonsdale and Malfait,1974); (7) Argentine Basin (e.g. Flood and Shor,1988; Flood et al.,1993; Manley and Flood,1993a, b;Von Lom-Keil et al., 2002); (8) Antarctic ^Atlantic Ocean gateway (Cunningham and Barker, 1996; Howe et al., 1998); (9) Chatham Ridge (e.g. Lewis and Pantin, 2002). Base bathymetric map obtained from USGS website: http:// walrus.wr.usgs.gov/infobank/gazette/html/bathymetry/gl.html. A multicolour version of this figure is on the enclosed CD-ROM.

Although bottom-current sediment waves are geographically widespread (Figure 15.1), this study will focus only on case studies from the NW UK continental margin (Figures 15.1 and 15.2), as this is a well-studied region with abundant high-quality data.

15.2.

L OCATION, MORPHOLOGY AND G ENESIS OF F INE -G RAINED S EDIMENT W AVES

Fine-grained sediment waves generated by bottom currents are generally found draping the flanks and crests of sediment drifts (Figures 15.3 and 15.4) (Fauge`res et al., 1999), in basinal and lower continental-rise environments (Roberts and Kidd, 1979; Richards et al., 1987; Flood and Shor, 1988; Flood et al., 1993; Marani et al., 1993; Manley and Flood, 1993a, b; Flood, 1994; Manley and Caress, 1994; Cunningham and Barker, 1996; Howe, 1996; Howe et al., 1998; Flood and Giosan, 2002; Lewis and Pantin, 2002; Masson et al., 2002; Von Lom-Kiel et al., 2002; Howe et al., 2006).

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Figure 15.2 Location map of case-study area (for location: see Figure 15.1). Modified from Masson et al. (2002); with permission from Elsevier. Surface and deep-water circulation shown by filled and open arrows, respectively. Grey-shaded arrow illustrates inferred path of intermediate depth water deflected to the west by the Wyville-Thomson Ridge. Contour intervals: 200 m. Locations of Figures 15.3, 15.4, 15.8 and 15.9 are indicated.

Fine-grained sediment waves are dominantly composed of mud, silt and fine sand (often poorly sorted), and may be siliciclastic, volcaniclastic and/or bioclastic in composition (Stow et al., 1998a; Wynn and Stow, 2002b). They typically show features diagnostic of contourite deposition, e.g. poorly developed laminae and intense bioturbation resulting from steady quasi-continuous sedimentation (Stow and Lovell, 1979). Wave dimensions are impressive, with wave crests often >10 km in length, wave heights of up to 50 m (and occasionally 150 m: Flood et al., 1993), and wavelengths ranging from 1 to 10 km (Figures 15.3 and 15.4). Wave dimensions appear to be related to flow velocity and/or sedimentation rate, and often decrease towards the margin of wave fields where bottom-current influence is reduced (Flood and Shor, 1988; Cunningham and Barker, 1996). In plan view, wave crests usually appear straight or slightly sinuous, and bifurcation is rare (Figure 15.4a). This planform geometry is a reflection of the typically unidirectional, steady bottom-current flow responsible for generating the waves.

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NW (a) 880 1 km

SE Sediment waves

Elongate drift Moat

940 m 1000 1060

970

(b)

1 km

Wave-migration direction

1000 m 1030

1060

Figure 15.3 Profiles across the f lank of an elongate drift adjacent to the Hebrides Slope (for location: see Figure 15.2). Images modified from Masson et al. (2002); with permission from Elsevier. (a) Deep-tow boomer profile across a small sediment wave field. (b) Enlarged image showing up-slope wave migration, resulting from asymmetric sediment deposition across the wave crest. Note the reduction in wave-migration rate during deposition of the middle transparent unit, which is interpreted to represent the last glacial lowstand.

Most examples of fine-grained bottom-current sediment waves on f lat basin f loors have wave crests aligned roughly perpendicular to the f low, and wave migration is in an upcurrent direction (i.e. opposite to flow). Waves developed on open slopes typically have wave crests aligned at a low angle (10–50°) to the flow. Wynn and Stow (2002b) concluded that, where the mean bottom-current flow is roughly slope-parallel, examples of oblique waves usually migrate in an up-slope and upcurrent direction (Figures 15.3 and 15.4). The lateral wave-migration rate is largely controlled by the sedimentation rate (Figure 15.3b), with areas of relatively high sedimentation rate displaying faster lateral migration (up to 1.0 m ka 1: Masson et al., 2002). However, it seems likely that flow velocity and/or wave-crest orientation also affect migration rates, with higher flow velocities and higher angles between wave crest and flow direction leading to higher wave-migration rates (Flood, 1988; Blumsack and Weatherly, 1989). Sediment-wave initiation is still a poorly understood process, but is thought to involve irregular deposition from a bottom current passing over a pre-existing sea-floor perturbation, e.g. the crest of a sediment drift (Howe et al., 1998; Von Lom-Keil et al., 2002; Wynn and Stow, 2002b). Various models have been proposed for wave growth and migration once the bedform is established, with the most widely accepted being the lee-wave model introduced by Flood (1988), and later modified by Blumsack and Weatherly (1989) and Hopfauf and Speiss (2001). The lee-wave model suggests that internal lee waves can develop within a weakly stratified bottom current as it passes over a sediment wave, leading to increased flow velocities on the downcurrent (lee) flank. This flow pattern leads to asymmetrical sediment deposition across the wave, with enhanced deposition on

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Y (a)

Sediment core locations

1 km

54 53

X Wavemigration direction

SW 1100 (b) 1 km

NE X

53 54

Y

1150 m 1200 1250

Figure 15.4 TOBI 30 kHz side-scan sonar image (a) and 3.5 kHz profile (b) crossing a sediment-wave field on the flank of a broad sheeted drift in the northern Rockall Trough. For location see Figure 15.2. Comparison between image and profile shows that high backscatter stripes (white) correspond to the lee (downcurrent) wave flanks. Low backscatter stripes (black) correspond to upcurrent wave flanks with thicker accumulations of well-sorted Holocene contourite sand (as confirmed by cores 53 and 54). X^Y locates section of profile crossing the side-scan image. Both images modified from Masson et al. (2002); with permission from Elsevier.

the upcurrent flank, leading to the observed upcurrent migration of the wave (Figures 15.3 and 15.4). The model predicts that flow velocities of approximately 0.09–0.3 m s 1 are required for active migration to occur, with aggradation at lower velocities and localised erosion or non-deposition at higher velocities. A number of studies carried out in the Argentine Basin during Project MUDWAVES successfully tested the lee-wave model against field data. Wave morphology and sediments were measured and used to predict active bottom-current flow direction and velocity, which was tested using long-term current moorings and found to be in general agreement once alongwave flow components were taken into account (Blumsack, 1993; Flood et al., 1993; Manley and Flood, 1993b; Weatherly, 1993). Wynn and Stow (2002b) outlined several criteria for distinguishing between fine-grained sediment waves generated beneath bottom currents and turbidity currents. These included regional setting, wave regularity, sediment type, crest alignment and sequence thickness. Key characteristics of bottom-current sediment waves include the following: (1) environmental location away from turbiditycurrent input, e.g. on contourite-drift flanks, (2) no consistent up- or down-slope

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trend in wave dimensions, (3) contouritic sediments with associated high levels of bioturbation, (4) crests aligned oblique to bathymetric contours (when located on slopes) and (5) no consistent spatial trend in sediment-wave sequence thickness. However, it should be noted that, without integrated datasets comprising both geophysical and sedimentological data, specific identification of the wave-forming process may not always be possible, especially as some sediment wave fields are formed by a combination of both bottom-current and turbidity-current activity (Rebesco et al., 1996; Kenyon et al., 2002). Lee et al. (2002) and Wynn and Stow (2002b) also described how fine-grained sediment waves can be distinguished from soft-sediment deformation features, e.g. creep folds and extensional faults (Kenyon et al., 1978; Hill et al., 1982; Mulder and Cochonat, 1996; O’Leary and Laine, 1996; Lee and Chough, 2001; Lee et al., 2002). Soft-sediment deformation features can normally be identified on the basis of the following: (1) they do not show lateral migration, (2) individual reflectors may be difficult to trace from crest to crest, (3) they show no spatial trends in dimensions and (4) they are often discontinuous in plan view. However, again, there may be examples where multiple processes are operating, involving both soft-sediment deformation and near-bottom currents (Fauge`res et al., 2002c; Cattaneo et al., 2004).

15.3.

L OCATION, MORPHOLOGY AND G ENESIS OF C OARSE -G RAINED S EDIMENT W AVES

Coarse-grained sediment waves generated by bottom currents are relatively common in shelf environments, but in deep water they are scarcer than their fine-grained counterparts. Most published examples are located in areas of enhanced bottom-current flow such as topographic ridges or gateways between basins (Lonsdale and Malfait, 1974; Dorn and Werner, 1993; Kenyon et al., 2002; Kuijpers et al., 2002; Wynn et al., 2002a; Habgood et al., 2003). Coarse-grained sediment waves are dominantly composed of sand-sized sediments, although sampled examples are rare (Wynn et al., 2002a; Habgood et al., 2003). They can occur as linear waves, with crests aligned roughly perpendicular to the flow and spaced a few tens to hundreds of metres apart (Dorn and Werner, 1993; Habgood et al., 2003), or, more commonly, as distinctive barchan dunes up to 200 m wide and a few metres high (Figure 15.5) (Kenyon and Belderson, 1973; Lonsdale and Malfait, 1974; Lonsdale and Speiss, 1977; Kenyon, 1986; Dorn and Werner, 1993; Kenyon et al., 2002; Kuijpers et al., 2002; Wynn et al., 2002a; Habgood et al., 2003). In contrast to their fine-grained counterparts, coarse-grained barchanoid waves migrate in a downcurrent direction, in a similar fashion to subaerial barchans. Sediment is transported across the upcurrent flank and then avalanches down the lee face before moving along the barchan horns towards the tips. Barchanoid wave forms (Figure 15.5) only occur in areas of sparse sediment supply, where flow velocities exceed 0.4 m s 1, and they often sit on a coarse (gravel or sand) substrate (e.g. Kenyon et al., 2002; Wynn et al., 2002a). The bottom-current

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(a) Gently rippled sea-floor (straight-crested ripples) Sinuous/ linguoid ripples Linguoid ripples Sinuous/ linguoid ripples

Linguoid ripples Smooth sea-floor with faint lineations

Not to scale Accumulation of pale sediment

Linguoid ripples

Smooth sea-floor without ripples

Closely spaced ripples arranged in a fan shape

Gently rippled sea-floor with gravel patches

(b) Current flow direction

Not to scale

No current

Figure 15.5 Bottom-current flow field and ripple distribution across coarse-grained sediment waves in an active contourite system. (a) Interpreted distribution of ripple types across a typical barchanoid wave in the Faroe ^Shetland Channel, based on sea-floor video and photographs. For location see Figure 15.8. (b) Interpreted current flow over the dune surface based on ripple distribution. Arrow size schematically represents flow velocity. Both images modified fromWynn et al. (2002a); with permission from Elsevier.

f low f ield across deep-water barchans has been reconstructed by Lonsdale and Malfait (1974) and Wynn et al. (2002a), on the basis of ripple distribution across the barchan surface (Figure 15.5). Linguoid ripples on the upper barchan surface reflect higher flow velocities than the sinuous and straight-crested ripples on the barchan flanks and surrounding sea floor, while an area of smooth sea floor with no ripples is found just beyond the lee face (Figure 15.5). Further details of ripple formation and distribution in contourite systems can be found in Martin-Chivelet et al. (2008).

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15.4.

R ELATED LARGE -SCALE FEATURES G ENERATED BY B OTTOM C URRENTS

Sediment waves form part of a continuum of bottom-current-generated features, with the largest being contourite drifts and sheets (Figure 15.3a) – often on the scale of tens of kilometres – and the smallest being current ripples (Figure 15.5) – usually a few centimetres across. In this section, we will examine those features that are between 1 m and 10 km in size, such as erosional scours, furrows, sand ribbons and comet marks, and that are resolvable with high-resolution geophysical instrumentation. Identification of such features can be important in the reconstruction of bottom-water flow fields, and an example from the NW UK continental margin is discussed later. Identification of certain erosional features, such as scours, is also important in deep-water geohazard assessment. These features are generated beneath currents with velocities higher than 1.0 m s 1, which may be capable of damaging sea-floor infrastructure, including pipelines and telecommunications cables. Large-scale erosional scours may be tens of metres deep and several kilometres across (Figure 15.6), and are only found in areas of topographic constriction where (a) 1 km

Scarp Sand ribbons?

Y Furrows X

Artefacts

970 m 1020

(b) X

Y 1 km Sediment drift ?

Erosional scarp

1070

Figure 15.6 Large-scale erosional features in an active contourite system (images modified from Masson et al., 2004; with permission from Blackwell Publishing) (a) TOBI 30 kHz side-scan sonar image showing part of a large-scale bottom-current scour. High backscatter is white. The image shows an irregular headwall scarp, backscatter banding parallel to the inferred flow direction (sand ribbons?) and limited development of furrows. For location see Figure 15.8. (b) 3.5 kHz profile showing a sediment drift and inferred deposition within the scour, suggesting recent infill. X^Y locates section of profile crossing the side-scan image.

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bottom currents are strongly focused and flow velocities reach 1.0–2.5 m s 1 (Kenyon and Belderson, 1973; Bulat and Long, 2001; Masson et al., 2004). Morphologically, they are very similar to large-scale scours found in environments swept by high-energy turbidity currents, such as channels and channel mouths (e.g. Wynn et al., 2002b). They typically display roughly rectangular or oval planform morphology, with a steep headwall and sidewalls, and a shallower downcurrent opening (Figure 15.6). Erosional furrows, in the form of relatively narrow lineations, often occur within, or adjacent to, scours (Figure 15.6); they may be several kilometres in length, a few tens of metres wide, and a few tens of centimetres deep, and are usually cut into coarse gravel and sand substrates (Masson et al., 2004). Similar features have been described from shelf environments where strong tidal currents sweep across gravel substrates (Belderson et al., 1988); they are believed to occur under flow velocities of 1.0–1.5 m s 1. Furrows cut into fine-grained cohesive sediments have also been described by Flood (1983), and occur under lower flow velocities (<0.3–0.7 m s 1). Sand ribbons are depositional bedforms that also frequently occur in association with the erosional features described above (Figure 15.6). They are up to 500 m wide and several kilometres long, and are believed to form under flow velocities of 0.75–1.5 m s 1 (Dorn and Werner, 1993; Kuijpers et al., 2002). Comet marks are found around obstacles, such as boulders, in areas of strong bottom-current flow (0.6 to more than 1.0 m s 1), and take the form of deflated zones around the upcurrent and lateral margins of the obstacle (Figure 15.7). Shadow zones or sand tails, where sands and gravel are deposited in an elongate ribbon, are found in the lee of the obstacle. The genesis of comet marks was described by Werner et al. (1980). There is some evidence to show that longer sand

Figure 15.7 Well-defined comet mark developed around a large boulder on a sand and fine-gravel sea floor in the Faroe ^Shetland Channel (modified from Masson et al., 2004; with permission from Blackwell Publishing). Black arrow indicates interpreted flow direction. Location is stationWTS13 in Figure 15.8, at a water depth of 1095 m.

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tails behind obstacles are related to a higher velocity flow (Kuijpers et al., 2002). In the Faroe–Shetland Channel, sand tails vary in length from 1 to 700 m (Dorn and Werner, 1993; Kuijpers et al., 2002; Masson et al., 2004).

15.5.

APPLICATIONS TO B OTTOM -C URRENT RECONSTRUCTION: A C ASE STUDY FROM THE NW UK

Recent studies by Kuijpers et al. (2002) and Masson et al. (2004) have used bedforms and related erosional features to map the distribution and strength of bottom-current flow through the Faroe–Shetland and Faroe Bank Channels (Figures 15.8 and 15.9). These channels form a broad conduit that is a major gateway for the flow of deep cold water between the Norwegian Sea and the North Atlantic (Hansen and Osterhus, 2000). Erosional scours, furrows, comet marks, barchan dunes, sand sheets/ribbons and sediment drifts have all been identified and mapped in the study area, using geophysical data combined with sea-floor photographs and sediment cores (Figure 15.8). Published data relating to bedform type and flow velocity (Table 15.1) were then used to reconstruct the bottom-current flow through this gateway (Figure 15.9).

Figure 15.8 Summary interpretation of bottom-current features in the Faroe ^Shetland and Faroe Bank Channel, based on side-scan sonar images and 3.5 kHz profiles (modified from Masson et al., 2004; with permission from Blackwell Publishing). Symbols showing surficial sediment classification are based on core/dredge samples and/or sea-floor photographs. FSC = Faroe ^Shetland Channel; FBC = Faroe Bank Channel; WTR =Wyville-Thomson Ridge. For location: see Figure 15.2. A multicolour version of this figure is on the enclosed CD-ROM.

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Figure 15.9 Reconstructed bottom-current flow field through the Faroe ^Shetland and Faroe Bank Channel based on the distribution of bedforms and related bottom-current-generated features shown in Figure 15.8 (modified from Masson et al., 2004; with permission from Blackwell Publishing). For location: see Figure 15.2. Red arrows indicate currents related to northeastward transport of near-surface water masses; blue arrows relate to southwestward transport of Norwegian Sea Deep Water (NSDW); orange arrows show recirculation of NSDW (and intermediate water masses?) at the southern and eastern margins of the Faroe Bank and Faroe ^Shetland Channels, respectively. Black dots show the locations of sea-floor photography stations. A multicolour version of this figure is on the enclosed CD-ROM.

The results obtained were in broad agreement with direct measurements obtained from short-term current meter deployments (Hansen and Osterhus, 2000), but with the advantage that they represent longer timescales and are more spatially extensive. Masson et al. (2004) discussed in detail the timescales at which the described features may be operating, and concluded that larger features, such as fine-grained sediment waves and drifts, may require thousands to a few million years to form, whereas small-scale features such as current ripples may only record the last significant event, e.g. a benthic storm. It is thought that most bedforms and related features represent the peak current flowing across an area. In the Faroe–Shetland and Faroe Bank Channels, the fresh appearance of most observed features (Figures 15.5–15.7) was provided as evidence for active bottom currents at the present-day. The high-velocity core of southwest-directed Norwegian Sea Deep Water (NSDW) travels across a sill at the northern edge of the Faroe–Shetland Channel before being directed by Coriolis forces along the western channel margin at water depths of 800–1200 m (Figure 15.9). In this area, strong bottom currents produce numerous erosional scours, scarps and furrows, indicating flow velocities in excess of 1.0–1.5 m s 1 (Kuijpers et al., 2002; Masson et al., 2004). On the opposite side of the channel, at equivalent water depths, small-scale features, including comet marks, show that the flow was actually to the northeast with velocities mostly in the range of 0.4–1.0 m s 1 (Figure 15.9); this flow is interpreted to result from a recirculation cell within the NSDW (Masson et al., 2004).

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Table 15.1 Bottom-current velocities associated with various types of bedforms observed in the deep ocean Bedform

Sediment type

Peak current velocity (m s 1)

Reference

Sediment waves, contourite drifts

Fine-grained, often pelagic/ hemipelagic Silt

0.05–0.2

Manley and Flood (1993a, b)

0.05–0.15

Hollister and McCave (1984) Flood (1983) Belderson et al. (1982) Southard and Boguchwal (1990) Baas (1999) Masson (2001) Lonsdale and Malfait (1974) Kenyon and Belderson (1973) Kenyon (1970, 1986) Kuijpers et al. (2002) Kenyon (1986) Belderson et al. (1982) Kuijpers et al. (2002) Kenyon (1970) Kenyon and Belderson (1973) Belderson et al. (1988) Flood (1983) Belderson et al. (1988) Belderson et al. (1982) Kenyon and Belderson (1973) Belderson et al. (1982)

Lineations

Fine-grained, cohesive mud Sand, often rippled

<0.3 0.7 0.3–0.4

Foraminiferal sand Clastic sand

>0.3 0.4–1.0

Comet marks

Sand/gravel lag

0.6 to >1.0

Sand ribbons

Sand

0.75–1.5

Furrows

Gravel

1.0 to >1.5

Erosional scours

Gravel, rock

1.0–2.5

Furrows Contourite sheet

Barchan dunes

ACKNOWLEDGEMENTS This study contains data obtained from a large number of scientific research cruises, and we thank the scientists, officers and crews for their assistance in data collection and subsequent processing and interpretation. We are grateful to Peter Talling and Michael Frenz for their review comments.