Contourites, Earth Systems and Environmental Sciences

Contourites☆ M Rebesco, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy ã 2014 Elsevier Inc. All rights reserved.

Introduction History Terminology Bottom Currents Sediment Drifts Seismic Characteristics Large Scale (i.e., Sediment Body) Medium Scale (i.e., Unit) Small Scale (i.e., Facies) Facies Model Facies Continuum and Distinguishing Criteria

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Introduction Contourites are sediments deposited or substantially reworked by the powerful action of bottom currents. The study of contourites is important in at least three main respects: palaeoclimatology, hydrocarbon exploration, and slope stability. The continuous and relatively high-resolution record included in these sediments is crucial for palaeoclimatology since it holds the key for priceless information on the variability in circulation pattern, current velocity, oceanographic history, and basin interconnectivity. Bottom currents are a crucial factor in oil and gas reservoir development since weak flows may favour the accumulation of source rocks and robust flows may represent a viable mechanism capable of forming ‘clean’ sands in the deep sea. Low-permeability, fine-grained contourites have been found recently to play a critical role in slope stability by providing potential overpressured glide planes, either when their high-water-content is rapidly loaded or when their rigid biosiliceous microfabric is collapsed by diagenesis. Despite its significance, little is still known about this group of sediments. There are three reasons for the long-lasting disregard: the inherent elusive nature of these complex deposits, the 50-year dominance of the turbidite paradigm, and the controversy that surrounds these sediments since they were first recognized. The elusive and very subtle characteristics of these slowly and continuously accumulated sediments and their occurrence within a spectrum of deep-water deposits (Figure 1) does not allow them to be easily recognized and decoded. The monumental efforts to promote the turbidite systems, the simplicity and predictability of turbidity current concepts, and the sense of confidence given by the routine use of widely applicable models has induced the geologic community to ignore alternative, more complex deep-water models. The never-ending disputes regarding the occurrence of sandy contourites versus reworked turbidites and the early errors in recognition of fossil examples have prevented the establishment of a widely shared consensus on valid diagnostic criteria for the identification of these deposits. The growing level of interest and research in contourites is shown by the recent publication of several special volumes dealing with such systems, covering large parts of present ocean floors and continental margins. An increasing number of fossil occurrences in ancient sediments exposed on land have also been documented. Nevertheless, wide multidisciplinary approaches and the integrated work of different international specialists (e.g., deep-water sedimentologists, seismic interpreters, physical oceanographers, and palaeoclimatic modellers) are still needed to improve our knowledge of these systems and to help tackle the problems that remain.

History The systematic study of deep-sea sediments began at the end of the nineteenth century. At that time the ocean floor was perceived as a tranquil realm receiving only pelagic clays. The possibility that thermohaline bottom currents may influence sedimentation in the deep oceans was suggested by the German oceanographer Wust in 1936. However, the contourite concept was not accepted in marine science until the second half of the 1960s after the American team of Heezen and Hollister had provided geological and oceanographic evidence of this process along the eastern North American continental margin. During the next two decades standard facies models for contourite sediments were developed from coring contourite accumulations. Other important steps forward were achieved by addressing the link between current strength and grain size and by confronting the problem of ☆ Change History: January 2014. M Rebesco updated the “History”, “Terminology” and “Bottom Currents” sections and added Figures 4 and 5 (all following figures were re-numbered accordingly).

Reference Module in Earth Systems and Environmental Sciences

http://dx.doi.org/10.1016/B978-0-12-409548-9.09094-1

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Figure 1 Schematic model of downslope gravity-driven currents and alongslope bottom currents interacting within the continuous spectrum of deepwater processes. Reprinted from Shanmugam, G. (2000). 50 years of the turbidite paradigm (1950s–1990s): deep-water processes and facies models—a critical perspective. Marine and Petroleum Geology 17, 285–342, with permission.

distinguishing contourites from other deep-sea facies like hemipelagites and fine-grained turbidites. Among the projects that have provided significant contributions to contourite research have been the HEBBLE (High-Energy Benthic Boundary Layer Experiment) in the Nova Scotia Rise and the international drilling programs (ODP and predecessors) in most contourite deposits of the world’s oceans (Figure 2). From the end of 2004 to the beginning of 2012 there were 6 Integrated Ocean Drilling Program (IODP) expeditions particularly relevant for contourite research: 303 and 306 on some contourite deposits between Labrador Sea and North Atlantic; 307 on cold-water coral mounds enclosed in contourite deposits; 317 on the current-influenced margin of the Canterbury Basin; 319 on the mixed turbidite and contourite systems of the Wilkes Land margin (Antarctica); 339 on the Gulf of Ca´diz, acknowledged as the world’s premier contourite laboratory (Expedition 339 Scientists, 2012). Expedition 339 in particular was the ultimate testing ground for the contourite paradigm, which in general was found in very good order. However, very interesting modifications are required, for example, to detail very interesting interactions between contourite and turbidite processes that are completely new and different from the current models. The identification of contourite sands that would provide good quality potential reservoirs when buried deeply, and associated contourite muds that could provide potential source rocks and suitable seals, could herald a paradigm shift in oil exploration targets in deepwater settings. Nevertheless, there are many years of research ahead to extract all these details and to decode the full climatic signal from the expedition 339 cores. At the very end of the 1990s the International Geological Correlation Programme (IGCP) Project 432, named ‘Contourite Watch’, was launched to facilitate research on contourites and bottom currents. It included the establishment of a global network of contourite workers, the organization of international workshops, and the publication of selected special volumes, including an atlas of contourite systems. Recently (second half of 2012) two international networking projects established: IGCP 619 (Contourites: processes & products) aimed at improving the visibility of contourites for the large scientific and public community and involving more isolated scientists and teams; and International Union for Quaternary Research (INQUA) 1204 (Quaternary Contourite Log-book) aimed at facilitating the research of Quaternary Contourites scientists around the world especially involving early career and developing country researchers.

Terminology Most workers currently agree on very broad definitions and on the use of most terms related to contourite research. The need to maintain a certain degree of flexibility to allow for development in understanding has been acknowledged; nevertheless the present state of contourite terminology usage is possibly still too loose to allow for a clear description of the products (sediments) and consequently for a clear understanding of the depositional processes. The term ‘contourite’ was originally introduced to define the sediments deposited in the deep sea by contour-parallel bottom currents. The term was subsequently widened to embrace a larger spectrum of sediments that at diverse depths are affected to various extents by different types of currents. Since both the depth and direction of the bottom currents that influenced nonrecent deposits are seldom precisely identifiable, a rigorous restriction of contourite to its original sense would severely limit its application. Such a long-established term should hence be maintained for nonspecific use, and the employment of modifying terms to qualify the deposit in terms of depth, current type and action, and interacting process is recommended (see Table 1). The term ‘contour current’ refers to those thermohaline-driven, deep-water currents that flow approximately parallel to the bathymetric contours. It is widely used synonymously for ‘bottom current’, which in contrast includes all deep currents not driven

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Figure 2 ODP, IPOD, and DSDP sites drilled in contourite deposits of the world’s oceans. Data sources: 1998 to present from ODP web site http://www-odp.tamu.edu/sitemap/sitemap.html; and before 1998 from Stow, D. A. V., Fauge`res, J. C., Viana, A. and Gonthier, E. (1998). Fossil contourites: a critical review. In: Stow, D. A. V. and Fauge`res, J.C. (eds.) Contourites, turbidites and process interaction. Sedimentary Geology 115, 3–31.

Table 1

Examples of qualifying terms for contourite-related deposits

Depth

Deep water

Mid-water

Shallow water

Shelf

Slope

Lacustrine

Type of current

Thermohalinedriven Pirated

Wind-driven

Up- and downwelling Condensed

Up- and downcanyon Polished

Internal waves and tides

Type of action

Contourparallel Resuspended

Interacting process

Turbidite sourcing

Pelagic settling

Faulting

Creeping

Far-off transported Overbanking

Winnowed Downslope resedimentation

N.B.: Not all kinds of terms are to be used concurrently (e.g., deep-water turbidites winnowed by wind-driven bottom currents, or mid-water turbidite-sourced contourites).

by sediment suspension that are capable of eroding, transporting, and depositing sediment on the sea floor. Although bottom current is suitably applicable to different types of currents flowing alongslope as well as upslope and downslope (see below), the term contour current sensu stricto should be applied only to currents whose flow can be recognized as being parallel to contours. Contourites were first described as being at great depths beneath deep-water bottom currents. Subsequently, contourites and bottom current-controlled deposits were recognized in several settings including shallow gateways, outer shelf and slopes, and even lakes. The depth ranges for a three-category subdivision was hence proposed: deep water
2000 m; mid-water ¼ 300–2000 m; shallow water ¼ 50–300 m. However, a rigid definition of the depth confine is of limited value, especially for nonrecent and fossil contourites for which the deposition depth is largely unknown. Nevertheless, the use of a modifying term to qualify the deposit in terms of depth is recommended when a distinction can be made.

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‘Sediment drift’ is the generic term for a sediment accumulation that was appreciably controlled by the action of a bottom current. It is often used synonymously for ‘contourite drift’, which in principle should be specifically used for sediment accumulations deposited principally (though not exclusively) by contour currents. Therefore, the most appropriate term for contourite accumulations is in most cases ‘contourite drift’ or the wide-ranging designation ‘sediment drift’. Whereas drift is a well-established term that refers to a depositional feature, only quite recently (second half of the years 2000) a certain attention is being given to the various types of contourite erosional features. Such erosional features include: moats, contourite channels, marginal valleys, large furrows (isolated or in fields), and erosional terraces. These features are interpreted as the result of interaction between bottom currents and seafloor topography. When alongslope sedimentary processes dominate over a continental margin, a contourite depositional system (CDS) may develop (Herna´ndez-Molina et al., 2006, 2009). It includes genetically linked erosional and depositional features (drifts) located in the same depth range as the contour currents that affect sedimentation. The occurrence of mixed system, however, is the norm rather than the exception with sediment drifts. For this reason, a qualifying modifier like bottom current-controlled (modified, reworked, etc.) is recommended for turbidite systems and hemipelagic deposits that were significantly affected by interaction with bottom currents. Similarly, when mounds, levees, fans, lobes, channels, and other terms closely associated with downslope systems are applied to contourite systems, they must be preceded by the term contourite.

Bottom Currents There is a wide spectrum of bottom currents that operate in deep water. The bottom currents sensu stricto (contour currents) are those that are part of either the thermohaline- or wind-driven major circulation patterns (Figure 3). They are usually persistent for long time intervals as testified by the thick drifts that took million of years to develop. Such currents generally have an overall alongslope flow, but in detail their velocity and direction are extremely variable in both time and space. In fact they are affected by seafloor morphology (obstacles, gateways, and changes in slope direction and steepness), Coriolis force, circular motions (gyres and eddies) unrelated to contours, and eddy kinetic energy changes (seasonal and at different scales). Substantial changes in kinetic energy at the seafloor may be produced by sea-surface topographic variations, resulting in episodic high bottom current velocities, referred to as ‘benthic storms’. Such storms may resuspend large amount of sediment that nourish the bottom ‘nepheloid layer’ (several hundred metres thick, long-living, low-concentration (50–100 mgl1) clouds of sediment particles close to the seafloor). Additional types of deep (bottom) currents not driven by sediment suspension have been observed to flow mostly perpendicular to the slope. These include currents related to internal waves and tides (formed between subsurface water layers of varying density), canyon currents (frequently reversing flow of clear or very low-density turbid waters), and down- and upwelling flows (currents on the continental slopes that may be generated by density, wind, storms, or obstructions).

Figure 3 Global ocean circulation pattern. Reprinted from Steele, J. H., Turekian, K. K. and Thorpe, S. A. (eds.) Encyclopedia of ocean sciences. London: Academic Press, with permission.

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Figure 4 Bottom-current characteristics: a schematic summary of principal features. Reproduced from Stow, D. A. V., Hunter, S., Wilkinson, D., Herna´ndez-Molina, F. J. (2008). The nature of contourite deposition. In: Rebesco, M., Camerlenghi, A. (eds.) Contourites, developments in sedimentology 60, pp 143–156. Amsterdam: Elsevier, with permission from Elsevier.

The present global thermohaline circulation of deep water is originated by the cooling and sinking of surface water masses in the polar areas. The Antarctic bottom water (AABW) forms beneath Antarctic floating ice shelves (especially in the Weddell Sea), descends the continental slope, and hence circulates clockwise in the Southern Ocean from which it escapes northwards into the world’s oceans. The Arctic bottom water (ABW) forms in the Norwegian and Greenland Seas, flows intermittently south through basin gateways, and mixes with the North Atlantic bottom water and Labrador seawater. Subsequently, AABW and ABW gradually mix with other stratified water masses of their own density and circulation pattern (e.g., intermediate depth water produced by evaporation in semi-enclosed seas, like the Mediterranean overflow water – MOW). The present global surface circulation is governed by dominant wind systems. In many cases the action of such currents affects the entire water column down to the seafloor. This is the case for the powerful Antarctic circumpolar current (ACC) and western boundary currents like the Gulf Stream, Brazil, and Agulhas and Kuroshio Currents. All the above currents impinge upon the seafloor (Figure 4) where they may erode, transport, and deposit sediment in the form of contourites. The velocities of these bottom currents are generally very slow (few centimetres per second) and hence just able to slightly affect the action of other depositional processes (hemipelagic settling and low-density fine-grained turbidity currents). However, bottom currents accelerate when constrained by Coriolis force within western boundary undercurrents restricted against the continental slope (commonly with velocities ranging between 10 and 100 cms1) or when constricted in narrow gateways (velocities may even exceed 200 cms1). In many cases the bottom currents are strong enough to profoundly affect deep-water sedimentation by a number of processes including (in order of increasing current strength) the prevention of fine sediment deposition, the entrainment (pirating) and transport of suspended sediments within the nepheloid layer, the reworking of the coarse fraction introduced by episodic down-slope processes, the erosion and transport by traction of silt and fine sand, and sedimentary lag production by polishing and winnowing the seafloor. The wide variety of bedforms sculpted by bottom currents in the deep seafloor can provide important insights into both flow characteristics and depositional and erosional mechanisms of contourites. In a recent paper, Stow et al. (2009) have synthesised a large amount of data into a bedform/velocity matrix (Figure 5), from which one can derive information on flow direction, velocity, variability and continuity. Such processes take place chiefly beneath the core of the main bottom current flow. The velocity decreases away from the axis of the current, and deposition of various types of sediment drifts takes place in the relatively slack waters to the side of sediment-laden bottom currents. In most paleocurrent reconstructions, the axis of the current is hence generally considered subparallel to the elongated direction of the drift and to its crest (if traceable). However, such a relationship between current and accumulation is in most case highly speculative since the associated time-scales are extremely dissimilar. In fact, the time required for a drift development is commonly one order of magnitude larger than the glacial–interglacial cyclicity, which is widely accepted to profoundly affect bottom-water circulation, and exceeds by over four orders the time length of our direct oceanographic observations.

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Figure 5 Bedform-velocity matrix: a schematic distribution of bedform types with respect to flow velocity and grain size. Reproduced from Stow, D. A. V., Herna´ndez-Molina, F. J., Llave, E., Sayago-Gil, M., Dı´az-del Rı´o, V., Branson, A. (2009). Bedform-velocity matrix: the estimation of bottom current velocity from bedform observations. Geology 37(4), 327–330, with permission from the Geological Society of America.

Sediment Drifts There is a large variety of contourites, ranging from those that occur closely interbedded with other deep-water facies to those that build up individually distinct bodies (mounded drifts). Due to their large dimensions, commonly ranging between a few tens and several hundred thousand square kilometres, drifts are essentially identifiable by means of seismic profiles. Although in some instances sediment drifts may be clearly recognized on the basis of seismic profiles alone according to a widely agreed-upon criteria, a rigorous identification generally requires independent supporting data. The large-scale features of the drifts (their morphology and overall geometry) are controlled by a number of interrelated factors including bathymetric setting (physiography, depth, morphology), current conditions (velocity, time length of activity, variability, frequency of changes), sediment availability (amount, type, source type, frequency, and variability of input), and the interaction with other depositional processes (in time and space, both collaboratively or antagonistically). Accumulation rates in sediment drifts are rather variable, from a few tens to hundreds metres per million years, depending on regional conditions, specific age, and evolutionary stage. However, for most drifts their average growth rate is commonly several tens of metres per million years.

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Repeated attempts to categorize the essentially continuous spectrum of contourite accumulations into several types of drift have been made (Figure 6); however, the categorizations are not totally distinct such that some drifts may be classified as more than one type: (1). A ‘sheeted drift’ is characterized by a very broad low-mounded geometry. It has a very low relief (up to a few hundred metres) over a very large area (up to 1  106 km2) and shows a slight decrease in thickness towards the margins. Two kinds of sheet drift may be identified:

Figure 6 Sediment drift types and inferred bottom current paths. Adapted from Rebesco, M. and Stow, D. A. V. (2001). Seismic expression of contourites and related deposits: a preface. In: Rebesco, M. and Stow, D. A. V. (eds.) Seismic expression of contourites and related deposits. Marine Geophysical Researches 22, 303–308, with permission of Kluwer Academic Publishers; Stow, D. A. V., Fauge`res, J. C., Howe, J., Pudsey, C. J. and Viana, A. (2002). Bottom currents, contourites and deep sea sediment drifts: current state-of-the-art. In: Stow, D. A. V., Fauge`res, J. C., Howe, J., Pudsey, C. J. and Viana, A. (eds.) Deep-water contourite systems: modern drifts and ancient series, seismic and sedimentary characteristics. Geological Society, London, Memoir 22, 7–20, with permission.

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(a) ‘abyssal sheet’, which fills basin plains whose margins trap the bottom currents within a complex gyratory circulation, and (b)‘slope plastered sheet’, which is coated (plastered) against continental margins where a gentle and smooth topography favours a broad nonfocussed current. (2). An ‘elongated, mounded drift’ is characterized by distinctly elongated and mounded geometry (Figure 7). This kind of drift is the most spectacular and was among the first to be identified. It may have a relief of several hundred metres above the surrounding seafloor (total thickness up to 2 km) and a very variable extent (103–105 km2) with an elongation ratio of at least 2:1. Elongation is generally parallel to the margin, which means that the crest of the drift is parallel to the current axis. Nevertheless, elongation and progradation trends are variable, being controlled by at least three interacting factors: current system, bathymetry, and Coriolis force. Two types of mounded drift may be identified: (a) A ‘separated drift’ is kept apart (separated) from the adjacent continental slope by a distinct moat in which the main core (axis) of the bottom current is flowing as testified by local nondeposition and/or erosion (Figure 8). Such drifts show an evident upslope progradation with a minor alongslope migration in the downstream direction. (b)A ‘detached drift’ is removed (detached) from that part of the continental slope where it originally enucleated and has oppositely directed currents flowing along its two flanks. It is generated by the interaction of the bottom current with a distinct current system or with a prominent change in the slope orientation, and as a consequence in plain view it generally has an arched shape produced by the dominant downslope progradation.

Figure 7 Seismic profile (a) and interpretation (b) showing an elongated, mounded drift in the north-eastern Canterbury Bight. Adapted from Lu, H., Fulthorpe, C. S. and Mann, P. (2003). Three-dimensional architecture of shelf-building sediment drifts in the offshore Canterbury Basin, New Zealand. Marine Geology 193, 19–47, with permission.

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Figure 8 Seismic profiles showing a separated drift close to the Marion Platform margin (a) and a mounded drift complex, with location of some ODP leg 194 sites. Adapted from Shipboard Scientific Party (2002). Leg 194 summary. In: Isern, A. R., Anselmetti, F. S., Blum, P. et al. (eds.) Proceedings ODP, Initial Reports, Vol. 194. Available from: http://www-odp.tamu.edu/publications/194_IR/chap_01/chap_01.htm.

(3). A ‘channel-related drifts’ is characterized by its relationships with narrow conduits (deep channels, gateways, trenches, moats, etc.) where bottom currents are constricted and drastically accelerated. In addition to significant erosion/nondeposition, two quite distinct types of channel-related drift may develop: (a) A drift deposited within the conduits, either as axial mounds on the floor or as lateral sheets on the flanks of the channel, is known as a ‘patch drift’ (though this name may cause confusion with patch drifts unrelated to channels) and ‘subsidiary drift’, where it occurs within the moat, separating a giant elongated drift from the continental slope. This irregular, discontinuous channel-related drift generally elongates in the flow direction. (b)A ‘contourite fan’ is a much larger, cone-shaped deposit developed at the downcurrent exit of the conduits. It usually overlays major basal erosive disconformities, but for the most part shares many characteristics with medium-sized turbidite fans. (4). A ‘patch drift’ is characterized by a random (patchy) distribution controlled by the intricate interaction of the bottom current system with a complex seafloor morphology. Such a small-scale (a few tens of square kilometres), elongated, irregular drift may be either relatively mounded or thinly sheeted. It occurs plastered against reliefs or within a trough where the irregular topography may modify both direction and velocity of the local current flow. (5). A ‘confined drift’ is characterized by a mounded shape elongated parallel to the basin axis and distinct moats along both flanks. It occurs within relatively small enclosed basins, eventually actively subsiding, and generally shows a complex staking of upward-convex lenticular depositional units. (6). An ‘infill drift’ is characterized by a mounded geometry that progressively infills the topographic depression or irregularity in which it occurs. It typically forms at the head of the scar or at the margin of the toe of slumps developed beneath the path of a bottom current, and generally shows moderate relief and extent, variable shape, and downcurrent progradation. (7). A ‘fault-controlled drift’ is characterized by a certain influence in its development from faulting. This recent addition lacks many well-documented examples and is too little known to allow any generalization regarding deposit geometry and bottom current nature. However, it appears to develop either at the base or at the top of a fault-generated basement relief in response to

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perturbations in the bottom current flow pattern. A supplementary characteristic may be (subsequently reactivated) syn-depositional faulting that affects the relatively steeper side of such drifts. (8). ‘Mixed drift systems’ are characterized by the interactions of alongslope currents with other depositional processes. The most effective interplay is between contourites and turbidites, but drift development may be variously affected by the association with debrite, hemipelagite, and glacigenic systems. On the basis of a large variety of different types of interactions (interfingering, intercalation, imbrication, incorporation, winnowing, entrainment, etc.), a wide assortment of terms has been adopted, including: companion drift-fan, fan-drift, levee-drift, composite slope-front fan, etc.

Seismic Characteristics Although much progress has been made in determining the combination of criteria that best represent contourite deposits, seismic data without additional supporting evidence should not be used to make a firm identification of contourites. This is especially true when contourites are closely interbedded with other deep-water sediment facies (e.g., turbidites, hemipelagites, debrites, or glacigenic deposits). In addition, the vital information derived from detailed bathymetric images (either by multibeam or 3D seismic surveys) of contourite deposits is still relatively rare (Figure 9). Nevertheless, there is a wide consensus on a three-scale set of seismic diagnostic criteria for confident contourite identification.

Large Scale (i.e., Sediment Body) (1). Drift geometry (one of those described in the previous section) beneath a bottom current system; (2). Large dimensions (up to 1  106 km2), larger on average than those of turbidite channel–levee systems; (3). Asymmetric mound-and-moat geometry in contrast to the symmetric gull-wing geometry typical of turbidite channel–levee systems (Figure 10); (4). Overall downcurrent elongation; and (5). Widespread regional discontinuities, especially at the base of the drift.

Medium Scale (i.e., Unit) (1). Overall downcurrent migration of the stacked lenticular, upward-convex, seismic units; (2). Downlapping (onlapping on steep slopes) and sigmoidal progradational reflector patterns where downstream and upslope migration occurred; (3). Uniform reflection pattern, commonly found with extensive subparallel moderate to low amplitude reflectors; (4). Well-layered units with high lateral continuity along both strike and dip; and (5). Distribution of the depositional/erosional areas and lateral migration influenced (significantly at high latitudes) by the Coriolis force.

Small Scale (i.e., Facies) (1). Transparent layers and low-amplitude parallel reflectors, though seismic characteristics are very dependent on acquisition and processing methods; (2). Continuous, discontinuous, chaotic, and wavy reflectors in contourites and in turbidites, though seismic facies exclusively associated with contourites have not been defined yet; (3). Sediment waves, in both contourite and turbidite systems, though initially considered diagnostic of contourites; and (4). Bedforms, including longitudinal furrows, depositional tails in the lee of obstacles, and dunes.

Facies Model Most typically, contourites are composed of fine-grained, structureless, highly bioturbated mud. However, they show a wide range of grain-sizes, composition, and preserved sedimentary structures (Figures 11 and 12). They normally occur as thick (tens to hundreds of metres), uniform, fine-grained sequences (including thin to medium coarsergrained beds), or interbedded with hemipelagites, turbidites, and other resedimented facies, or as coarse lags within gateways. They are dominantly homogeneous (Figure 11(a)), poorly bedded (Figure 12(a)), and mostly bioturbated (Figure 12(b)) and mottled throughout, with little primary structures preserved. Bioturbation is generally considered an essential characteristic of contourites based on the belief that an active bottom current would increase oxygen concentration in the water and in turn increase organic activity, thus allowing contourites to be distinguished from episodic turbidites. However, bioturbation potential is also inferred to be a function of current intensity and other life-favourable environmental conditions. As a matter of fact, nonbioturbated contourites have been described as well (Figure 12(c)). Well-developed but somewhat irregular, fine lamination with indistinct

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Figure 9 Grey-scale seafloor map derived from 3D seismic amplitude (a), detail of the contour map (b), and seismic line (c) showing a mounded drift and bottom current-related deposits adjacent to a NE–SW trending topographic high. (1) Sheet-like coarse-grained deposit; (2) nonconfined, stringlike deposit; (3) erosional furrows; (4) moat between upslope-migrating separated drift and topographic high; (5) basal erosional discontinuity. Reprinted from Viana, A. (2001). Seismic expression of shallow- to deep-water contourites along the south-eastern Brazilian margin. In: Rebesco, M. and Stow, D. A. V. (eds.) Seismic expression of contourites and related deposits. Marine Geophysical Researches 22, 509–521, with permission of Kluwer Academic Publishers.

bioturbation may be evident both in shallow-water and in high-latitude facies (Figure 12(c)), possibly as a result of a hybrid turbidite–contourite deposition. Although cyclicity is common, primary silty parallel and cross lamination (Figures 11 and 12), where present, shows no regular sequence of facies as in turbidites. Contourites may show reverse grading, with coarse lag concentrations and sharp or erosive contacts (Figures 11 and 12). Grain size may vary from (silty) mud to sand. Gravel-rich contourites are common in glacigenic environments due to the presence of ice-rafted debris. Bottom current winnowing and erosion in narrow gateways may also produce gravel lags or shale-chip layers. Sorting is generally medium to poor, but some sandy contourites may be well sorted and relatively free of mud, showing low or negative skewness values. No offshore textural trend may be observed. Composition is generally mixed with a combination of biogenic and terrigenous material. High sand-sized content in muddy hemipelagic contourites is often formed by bioclasts (Figures 11 and 12). The terrigenous component largely reflects the

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Figure 10 Schematic model showing some typical seismic characteristics of the sediment drifts and the ideal difference with a channel–levee system.

nearby local sources, but alongslope mixing and far-off contributions also occur. In contrast the composition is essentially pelagic in biogenic contourites developed in open oceans or in upwelling areas. Manganese dioxides or iron oxides may occur as micronodules, stains, coating, and metal-enriched crusts within the manganiferous contourites. Fabric (when not exceedingly altered by bioturbation) is indicative of alongslope grain orientation, parallel to the bottom current flow. Contourites (Figure 13) are normally arranged in few decimetre-thick sequences showing gradual grain size and/or compositional changes. Five divisions may be eventually identified: (C1) lower mud, (C2) lower mottled silt, (C3) middle sand, (C4) upper mottled silt, and (C5) upper mud. A complete C1–5 sequence is interpreted to represent a gradual long-term current velocity variation and/or sediment supply change. Partial base-only or top-only sequences are also common.

Facies Continuum and Distinguishing Criteria Deep-sea sediments may be interpreted as the product of three main processes: gravity-driven downslope resedimentation, alongslope bottom current activity, and slow pelagic settling through the water column. However, such processes are in fact end

Figure 11 Core photos from ODP Leg 181 in the SW Pacific Gateway. Reproduced from Carter, R. M., McCave, I. N., Richter, C. et al. (eds.) (1999), Proceedings ODP, Initial Reports, Vol. 181. Available from: http://www-odp.tamu.edu/publications/181_IR/181ir.htm. (a) Core 181-1121B-1H-CC, 50–82 cm from the Campbell Drift showing brownish yellow sand with extremely sharp bottom contact (75 cm) with the underlying yellow clay. This sand bed exhibits no grading and is interpreted as a contourite deposite left behind after that an intense winnowing removed the fine material. (b) Core181-1122C-44X-2, 50–85 cm in the left (north) bank levee of the abyssal Bounty Fan showing greenish grey fine sand and silt beds that commonly exhibit sharp scoured top and bottom contacts with the interbedded mottled, dark greenish grey pelagic/hemipelagic bioturbated silty clay. The conspicuous planar and cross laminations, representing concentrations of foraminifera and carbonate debris, suggest a stronger, episodic benthic flow regime, in contrast to that of the decelerating turbidity currents. (c) Core 181-1124C-37X-2, 20–40 cm from the Rekohu Drift showing light greenish grey clay-bearing nannofossil chalk with flaser-like interbeds and laminae suggesting the presence of sediment-moving bottom currents.

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Contourites

Figure 12 Core photos from two different ODP Legs. (a, b) ODP Leg 194 in the sediment drifts adjacent to Marion Plateau. Reproduced from Isern, A. R., Anselmetti, F. S., Blum, P. et al., (2002). Proceedings ODP, Initial Reports, Vol. 194. Available from: http://www-odp.tamu.edu/publications/ 194_IR/194ir.htm. (c) ODP Leg 188, Wild Drift in Cooperation Sea. From Shipboard Scientific Party (2001). Site 1165. In: O’Brien, P. E., Cooper, A. K., Richter, C. et al. (eds.) Proceedings ODP, Initial Reports, Vol. 188. Available from: http://www-odp.tamu.edu/publications/188_IR/VOLUME/ CHAPTERS/IR188_03.PDF. (a) Core 194-1195B-21H-5, 34–54 cm showing a surface scoured into a light greenish grey wackestone by strong bottom currents in a high-energy hemipelagic setting. The sediments above the scoured surface consist of alternations between light grey silt- and very fine sand-sized wackestone and very fine to fine sand-sized skeletal packstone dominated by broken planktonic foraminifera. (b) Core 194-1195B-44X-1, 94–122 cm showing alternation of silt-sized light grey skeletal packstone and greenish gray mudstone with well-preserved Chondrites burrows and top and basal sharp scoured surfaces. (c) Core 188-1165C-3R-3, 55–75 cm showing a dark grey claystone with