Sediment Wave Fields Research Articles

Mediterranean bottom-current deposits: an example from the Southwestern Adriatic Margin

Abstract The identification of bottom-current deposits is a key to understanding the long-term deep-sea circulation and its changes through geological times. The Southwestern Adriatic Margin (SAM) is a small Mediterranean sub-basin that represents a key site to study bottom-current deposits in a Mediterranean context and hence to improve our knowledge of changes in Mediterranean deep-water circulation during the recent geological past. The SAM is characterized by complex stratification and circulation related to an interaction between two south-flowing bottom water masses: the cold North Adriatic Dense Water (NAdDW), formed in the shallow northern Adriatic through cold wind forcing and winter heat loss, and the highly saline Levantine Intermediate Water (LIW), generated in the Eastern Mediterranean through intense evaporation and flowing along the slope in a depth range of 200–600 m. Chirp-sonar profiles, TOBI mosaics and sediment cores acquired along the SAM reveal distinctive sediment drift types (elongated, plastered and isolated drifts) and extensive fields of sediment waves. Non-depositional and erosional features related to bottom-current activity include moats between drifts and the steep slope, widespread upper-slope erosional areas and extensive furrowed areas, which are particularly developed where change in slope orientation blocks the current circulation. The distribution, morphology and size of bottom-current features along the SAM result from an interaction between current regime and slope morphology, characterized by structural highs perpendicular to the slope contour (e.g. Dauno Seamount), multiple slope incisions (e.g. Bari Canyon and slump scars) and extensive block-slide deposits. Morphobathymetric and seismic stratigraphic data on the SAM show that bottom-current deposits are best developed where the regional slope flattens seaward of a very steep, often erosional, upper slope. The roughness of the lower slope, in particular, seems to correlate with the complexity and decreasing size of the bottom-current deposits. Like other land-locked basins, the Adriatic underwent dramatic palaeogeographical and palaeoceanographic rearrangements during the Late Quaternary sea-level oscillations. Indeed, during the Last Glacial Maximum (LGM), most of the areas where NAdDW is formed today were subaerially exposed. Concurrently, during glacial times the LIW production was probably reduced compared with the present-day conditions. The SAM slope is a key site to study the impact of changing current regime on late Quaternary slope deposits. Other Mediterranean late Quaternary contourite deposits are either in water depths compatible with the LIW, particularly in the case of shallow sill basins (e.g. Sicily, Corsica Channel), or at the slope base reflecting the flow of Mediterranean deep waters. The SAM bottom-current deposits, instead, seems to record the changing interaction between two distinctive bottom-hugging currents along the same pathway.

Sedimentary processes in the Wilkes Land margin: a record of the Cenozoic East Antarctic Ice Sheet evolution

Multichannel seismic data collected off Wilkes Land (East Antarctica) reveal four main units that represent distinct phases in the evolution of the Cenozoic depositional environment. A Cretaceous synrift succession is overlain by hemipelagic and distal terrigenous sequences deposited during Phase 1. Sediment ridges and debris-flow deposits mark the transition to Phase 2. Unit 3 records the maximum sediment input from the continent and is characterized by the predominance of turbidite deposits. During Phase 4 the sediment supply from the continental margin was reduced, and draping and filling were the dominant processes on the continental rise. Unit 4 also contains the deposits of sediment wave fields and asymmetric channel-levee systems. These four units are a response to the Cenozoic evolution of the East Antarctic Ice Sheet. During Phase 1, small ice caps were formed in the innermost continental areas. The ice volume increased under temperate glacial regimes during Phases 2 and 3, when large volumes of melt-water production led to high sediment discharge to the continental rise. Change to a polar regime occurred through Phase 4, when a thick prograding wedge developed on the continental shelf and slope and the sediment transport to the rise diminished, producing general starvation conditions.

Short-distance variability in slope bed-forms along the Southwestern Adriatic Margin (Central Mediterranean)

The slope of SW Adriatic Margin (SAM) is impinged today by two bottom-water masses, of thermohaline origin, that interact constructively along a preferred path: the Levantine Intermediate Water (LIW), generated in the eastern Mediterranean through intense evaporation and flowing along the contour, and the North Adriatic Dense Water (NAdDW), formed on the shallow north Adriatic and cascading episodically off the shelf. These two southward-flowing bottom-water masses impact the upper slope, which is characterized by an irregular physiographic setting, and together create the bottom-current deposits (e.g. fine-grained contourite drifts and sediment waves) and related erosional features (e.g. furrows, scours, comet marks). This study is focussed on a small open-slope area of the SAM (ca. 35 × 25 km), where high-resolution seismic data allow observe elongated contour-parallel scours, widespread erosion surfaces (accompanied by wide crescent-shape scours and furrow fields) and giant comet-marks spatially associated and genetically linked along a common current path. Further downslope, and away from this highly energetic current path, a field of sediment waves migrates upcurrent and shows bifurcated crests. In some cases the downcurrent limbs of the sediment waves display a well-defined erosional character on seismic profiles and furrows on side scan sonar mosaics. All bottom-current features observed are in equilibrium with the present day oceanographic regime, as documented by the available current-metre data. Moreover, sediment cores suggest that the sediment waves are actively migrating during the present interglacial, possibly implying that bottom waters formation was enhanced after the shallow Adriatic shelf was drowned during the late-Quaternary eustatic rise. The SAM slope represents, therefore, an ideal site to improve our understanding of the depositional impact of bottom currents along the Mediterranean margins and may provide useful hints on the activity of bottom currents elsewhere on the Mediterranean margins.

3D seismic analysis of the West Shetland Drift system: Implications for Late Neogene palaeoceanography of the NE Atlantic

Detailed seismic stratigraphic analysis of the West Shetland Drift, located on the Shetland Margin of the Faeroe–Shetland Channel, has been undertaken using commercial 3D seismic data. Emphasis was placed on the interpretation of a significant plastered contourite drift body, the West Shetland Drift–Slope section, with the aim of reconstructing the palaeoceanographic regime of the West Shetland slope during the past 5 Ma. Although limited in areal extent, the West Shetland Drift Slope section provides a key record of the depositional environment and bottom current activity on the West Shetland slope during the Late Neogene. The drift body was divided into three seismic–stratigraphic units and depositional processes responsible for each unit were interpreted, resulting in proposal of an evolutionary model in which a mixed downslope–alongslope depositional system was succeeded by alongslope current derived contourite drift deposition during the Early Pliocene to Mid-Pleistocene. During this time SW flowing alongslope currents impinged upon the West Shetland slope between the basin floor and a mid- to upper slope position. The final stage of deposition consisted of slope wedge progradation associated with Mid- to Late Pleistocene glaciation. Contourite deposition on the West Shetland Slope was punctuated by periods of glaciation activity, during which alongslope current activity appears to have been disrupted, suggesting a link between alongslope current activity and glaciation. Detailed mapping of key, smaller scale features, including a spectacular bottom current sediment wave field and iceberg ploughmarks, both of which were identified for the first time during this study, was instrumental in constraining the palaeocurrent regime, and would not have been possible without the high spatial resolution of the 3D seismic data. This study therefore highlights the efficacy of 3D seismic data as a tool for large scale spatial and temporal analysis of contourite drifts palaeoceanographic research.

Seabed morphology and the bottom-current pathways around Rosemary Bank seamount, northern Rockall Trough, North Atlantic

Rosemary Bank is a broadly domed and elongate seamount with a diameter of 70 km, occurring in water depths of between 300 and 2300 m, 120 km west of the UK mainland in the northern Rockall Trough. Recent multibeam bathymetry and sub-bottom profiles, together with pre-existing current meter and CTD data, seismic reflection profiles and seabed core samples were examined in order to evaluate past and present bottom-current pathways and processes. The multibeam data image volcanic parasitic cones, concave slide scars and the terraced slopes of the bank. Bottom-current sedimentation is interpreted as producing a drift–moat complex surrounding the entire seamount and including two sediment wave-fields, developed to the west and east of the bank in water depths of 1500–2000 m. The western drift covers an area of over 1000 km 2. Sediment waves to the west of the bank are up to 150 m high with wave lengths of 1.5–2 km. Four 100 m deep, 3 km wide, linear depressions, bisect the waves and are interpreted as 25–30 km long extensions of the moat. Seismic reflection profiles show the main phase of drift construction was during the mid-Miocene to Pliocene with the Pliocene to Holocene being an interval of drift maintenance. Cores from sediments draping over and adjacent to the seamount contain sandy and gravelly contourites interbedded with hemipelagites of late Pleistocene to Holocene age. Current meter and CTD data from the western moat indicate Labrador Sea Water flowing northwest, in contrast to the previously assumed anticlockwise circulation pattern around the seamount.

Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and the Weddell Gyre flows

New swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift, sediment wave morphologies, and seismic facies in the central Scotia Sea. The deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean: the eastward flowing Antarctic Circumpolar Current (ACC) and the northward outflow of the Weddell Sea Deep Water (WSDW). The contourite drifts are wedge-like deposits up to 1 km thick, that exhibit aggradational reflectors along axis thinning towards the margins. The contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles. The elongate-mounded drifts are best developed along the left-hand margins of channelized bottom current flows, due to the Coriolis force. A contourite fan has a main channel and two distributary channels that expand over a gentle seafloor. The proximal fan exhibits sediment waves with the distal fan incised by furrows. Sediment wave fields are well developed in areas of intensified bottom flows without channels, particularly at the confluence of the ACC and the WSDW. Small sediment waves occur where unidirectional bottom current flows predominate. Sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea-bottom relief. The stratigraphic sequence above the oceanic crust of Early to Middle Miocene age contains six seismic units separated by major reflectors. All the units were shaped under the influence of strong bottom current flows, although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time. The age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep-sea deposits in the region. The oldest, Units VI–IV, are of Early to Middle Miocene age and developed under the influence of the ACC. They are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5–8 cm/ky. Unit III, with an estimated Middle Miocene age, evidences the first incursion of WSDW into the central Scotia Sea, when plate movement caused openings in the South Scotia Ridge and allowed the connection with the northern Weddell Sea through Jane Basin and gaps in the ridge. Unit II, estimated to be of Late Miocene to Early Pliocene age, extends over the area and is characterized by internal unconformities. A major unconformity at the base of Unit II records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the Antarctic Peninsula shelf. Unit I, of Late Pliocene to Quaternary age, shows intensified bottom currents. The unconformity at the base of Unit I probably predates the onset of major Northern Hemisphere glaciations and the greater expansion of Antarctic ice sheets during the Late Pliocene. The extensive distribution of contourite deposits above the oceanic crust testifies to the long-term production of Antarctic Bottom Water. Cold, deep water was swept northward from the Weddell Gyre, interacting with the ACC, and possibly exerting profound influences on the global circulation system and the onset of major glaciations.

Upper Quaternary deposits on the Sao Tomé deep-sea channel levee system (South Brazilian Basin): major turbidite versus contourite processes

This paper is an attempt to show how one may discriminate turbiditic and contouritic processes in the deposition of deep-sea accumulations. The case study is the Sao Tomé deep-sea ‘channel levee’ system in the South Brazilian Basin. This system is elongated parallel to the margin contour, and was first interpreted as being controlled by contour current activity. Detailed analyses of 3.5-kHz profiles and piston cores allowed to demonstrate that the Upper Quaternary sediments are predominantly deposited by turbidite and hemipelagic–pelagic sedimentation processes. On the levee and the transitional area towards the deeper rise, frequent fine-grained turbidites, accounting for 25–45% of the entire depositional series, are interbedded with the hemipelagic–pelagic muds. In the channel, thicker and coarser turbidites (15%) are associated with debris-flows (20%). Deposit deformation in the form of slides, slumps and diapir-like structures largely affect the distal transitional area. Slight evidence of contour current activity only consists of some sediment wave fields and manganiferous-rich layers or some top-truncated sequences and foraminiferal sandy layers.

Deep‐water sediment wave fields, bottom current sand channels and gravity flow channel‐lobe systems: Gulf of Cadiz, NE Atlantic

Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s−1 to <0·5 m s−1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope‐oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave‐covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well‐sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings.

Upper slope sediment waves in the Cilician Basin, northeastern Mediterranean

Large sediment waves have been observed within the upper slope deposits of the Cilician Basin (northeastern Mediterranean), at the interfluve between two submarine canyons present offshore of the Mersin Shelf. There are several generations of sediment waves stacked within the sedimentary sequence, with the most recent bedforms found on the seabed in an area consisting of fine-grained sediments. The surficial sediment wave field, estimated to cover an area of ∼55 km2, is found at water depths between 250 and 310 m. The buried sediment wave fields have similar dimensions, but they are located further downslope. Wave dimensions increase with water depth and depth in the sedimentary sequence. The largest bedforms reach 40 m in height and 1.8 km in length. Most waves appear to have been migrating upslope, i.e. towards the north/northeast, and this migration direction is mostly consistent throughout the sedimentary sequence. This consistency indicates similar mechanisms of formation and maintenance over a considerable time interval. The morphology and migration pattern of the observed bedforms suggests that sedimentation in the Cilician Basin during wave formation has been controlled by near-bed flows resembling those generated by the present Asia Minor Current, although these flows may have been stronger in the past than they are at present.

Initiation and evolution of turbidity current sediment waves in the Magdalena turbidite system

This study describes an extensive sediment–wave field in the Magdalena Turbidite System, Caribbean Sea, Colombia, which occupies an area of at least 15 000 km2 on the continental slope (3330–>3800 m). The waves display wavelengths up to 1.9 km, wave heights up to 18 m, and crestlines that are aligned roughly parallel to the regional bathymetric contours. Preferential deposition on the upslope wave flank has led to upslope migration, in the manner of antidunes. The Magdalena sediment waves are interpreted as forming beneath unconfined turbidity currents, which may result from the downslope evolution of slumps and mass flows. The unconfined turbidity currents are derived from several point sources along the continental slope and spread laterally as they flow downslope. This has led to the formation of a laterally extensive sediment–wave field. Simple numerical modelling estimates that the turbidity currents responsible for wave generation were near- or super-critical, with flow thickness and velocity estimated at 40–160 m, and 36–82 cm s−1 respectively. However, later phases of wave growth were not dependent on specific flow conditions. The most important aspect of this study is that the entire sediment–wave unit, from the basal boundary to the present-day seafloor, has been investigated using ultra high-resolution seismic profiles. The sediment–wave unit rests upon an irregular discontinuity that marks a recent change in the sedimentary regime of the Magdalena Turbidite System, from channelised to unchannelised gravity flows. Above this boundary, the sediment waves display a growth pattern characterised by an increase in wave dimensions. In addition, the wave dimensions appear to become more regular through time. However, breaks of slope in the lower bounding surface of the wave field have produced variations in wave morphology that are still visible at the present-day seafloor. This indicates that there is a close relationship between variations in slope angle and turbidity current flow characteristics, which in turn leads to variations in wave morphology.

Turbidity current sediment waves on irregular slopes: observations from the Orinoco sediment–wave field

The Orinoco sediment–wave field covers an area of at least 29 000 km2 on the southern margin of the Orinoco Valley, at a water depth of 4400–4825 m. Wave dimensions are highly variable across the wave field, with wavelengths of 110–2600 m, and wave heights of 1–15 m. Slope gradients are also very variable, with values of 0.14–0.48°. Overall, the sedimentary sequence on the upslope wave flanks is about 40% thicker than that on the downslope flanks, leading to upslope wave migration in the manner of antidunes. In addition, reflectors on the upslope flanks generally display higher reflectivity than those on the downslope flanks, suggesting that a higher proportion of coarser sediment occurs on the upslope flank. An unconfined turbidity current origin is proposed for the Orinoco sediment waves, based upon detailed analysis of regional stratigraphic/seismic facies, and sediment wave distribution, morphology and dimensions. Sediment waves are not related to flows passing along (or spilling out of) the Orinoco Valley, or to bottom currents flowing parallel to the slope. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela, Guyana and Suriname continental margins. Simple numerical modelling has enabled turbidity current flow characteristics across the Orinoco sediment waves to be estimated: internal Froude number=0.7–1.1, flow thickness=24–645 m, and flow velocity=31–82 cm s−1. A key finding of this study is that there appears to be a close relationship between the changes in slope gradient and those in wave dimensions across the wave field. The irregular gradient of the present-day wave field is partly a reflection of the irregular bounding surface of the sediment waves, which is represented by mass-flow deposits and associated mud diapirs. The changes in slope gradient along this lower boundary lead to variations in the flow thickness and flow velocity of passing turbidity currents, which in turn control the wave dimensions. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.

Classification and characterisation of deep-water sediment waves

Deep-water sediment waves can be classified using a combination of grain size and wave-forming process, although in some cases one or other of these criteria may be indeterminable. Sediment waves are generated beneath currents flowing across the seabed, in the form of either downslope-flowing turbidity currents or alongslope-flowing bottom currents. Waves formed by either process show varying characteristics, depending on whether they are constructed of coarse- or fine-grained sediments. Sediment wave studies over the last five decades are reviewed, and clear trends can be discerned. Early descriptive studies in the 1950s and 1960s relied almost exclusively on seismic reflection profiles, and the wave-forming process was often a subject of much debate. In the 1970s and 1980s the quality of sediment wave datasets increased, with sidescan sonar, deep-sea drilling and numerical modelling all applied to sediment wave studies. Consequently, the wave-forming process became more easily identifiable, and models for the growth of bottom current and turbidity current sediment waves were introduced. Most studies from the 1990s onwards have focussed on turbidity current sediment waves, in response to the increasing demand for data from turbidite systems from the hydrocarbon exploration and production industry. Studies of bottom current sediment waves during this period have focussed on the applications to palaeoceanography, in response to the recent boom in climate change studies. The main focus of this paper is the characterisation of both fine- and coarse-grained, turbidity and bottom current sediment waves, including the depositional environment, wave morphology, wave sediments and migration, and the wave-forming process. In addition, criteria for distinguishing between fine-grained bottom current and turbidity current waves are discussed, and also for identifying other wave-like features formed by different processes, such as creep folds. Although in many sediment wave studies the dominant wave-forming process is easy to determine, in others it is likely that a more complex combination of processes has occurred. Further studies should concentrate on methods for identifying these processes and how they interact, and also investigate the exact mechanisms for the initiation and evolution of sediment wave fields.

Generation and migration of coarse-grained sediment waves in turbidity current channels and channel–lobe transition zones

Large-scale sediment waves, composed of gravels and sands, have been studied using deep-water sidescan systems. New data are presented from submarine channels off the Canary Islands and from canyon mouths off Portugal. Data from other areas are briefly reviewed, including a re-interpretation of data from Laurentian Fan, in order to summarise the varied morphology and setting of these bedforms. Coarse-grained sediment waves are found in the proximal, dominantly bypassing areas of deep-water turbidite systems, within canyons, channels and channel–lobe transition zones. Wave heights are in the region of 1–10 m, and wavelengths are up to several hundred metres. The distribution of waves, and sparse sedimentological evidence from modern and ancient sediment wave fields, suggests that initial transport and deposition of coarse sediment occurs within a high-density turbidity current, and not as a non-Newtonian debris flow. In some cases the development of pronounced wave asymmetry, and evidence of wave disruption and reworking, suggests that the wave morphology is at least partially controlled by a later phase of low-density turbidity flow. Grain size also appears to exert some control on wave morphology, for example, gravel-rich waves have a greater height for the same wavelength than sand-rich waves. Coarse-grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents, and are equally difficult to recognise in outcrop because of their large size.

Quaternary sedimentation and origin of the Orinoco sediment–wave field on the Demerara continental rise (NE margin of South America)

New core data from a sediment–wave field on the Demerara Outer Ridge show that the waves are mainly built from turbidity currents that flow northwards down the Demerara margin. The contour currents circulating parallel and obliquely to the margin are apparently not as important to wave formation as previously proposed. Detailed analysis of the sedimentary facies of cores taken on the crest and flank of a sediment wave indicates that fine-grained turbiditic deposits are dominant. Hemipelagic/pelagic deposits are interbedded with the turbidites. Sediment redistribution by contour currents was only evidenced by the presence of peculiar clay mineral assemblages and microfaunal associations, as well as some sedimentary structures such as truncations and cross-bedding. The turbiditic processes were active during the Last Glacial. In contrast, the Holocene is characterised by pelagic sedimentation. A combination of seismic and core data suggests that the sediment waves result from predominantly turbiditic depositional processes interacting with minor contour currents and synsedimentary deformation processes.

Laboratory experiments and numerical simulation of sediment-wave formation by turbidity currents

This paper presents the results of laboratory experiments and numerical simulations carried out in order to understand the mechanism of sediment-wave formation by turbidity currents. Experimental turbidity currents were generated in a 10-m-long laboratory flume, so as to investigate the topographic effects of slope and ridges on turbidite deposition. The results indicate that preferential deposition occurs on the upstream side of the ridges. This preferential deposition is considered to result in possible upstream migration of the topography, a common feature of deep-sea sediment waves, provided that such deposition were repeated. The preferential deposition occurred under a subcritical turbidity current, implying that antidune flow conditions (0.844<Fr<1.77) are not necessarily required for upstream migration of the bedforms. A numerical simulation using layer-averaged Navier–Stokes equations is successfully applied to the laboratory experiments, and then to deep-sea turbidity currents. The numerical model predicts the formation of a wavy structure in a sequence of thousands of turbidites, with wave dimensions and internal architecture similar to deep-sea sediment waves. The wavy structure is interpreted to form as a succession of mounds, each of which grows individually rather than simultaneously as a sinusoidal wave. Each mound is developed by preferential deposition downstream of a slope break, which in turn generates a new slope break on its downstream side flank. Upstream migration of the waveform results from differential deposition between the upstream and downstream sides of the mounds. These results indicate that neither antidunes nor lee-waves are necessary for the formation and upstream migration of the wavy structure. Wave formation as a series of individual mounds can be considered as an origin for relatively small sediment-wave fields, because no more than 4–5 crests are formed in the model predictions. It is suggested that more extensive sediment-wave fields can be developed by turbidite deposition on an initially undulating bottom, which results in upstream migration by differential deposition, and downstream extension of the wave field by mound formation progressively further downstream.

Fine-grained sediment waves on the western flank of the Zapiola Drift, Argentine Basin: evidence for variations in Late Quaternary bottom flow activity

Extensive fields of deep-water sediment waves, which drape the flanks of the Zapiola Drift in the central Argentine Basin, provide a record of the long-term stability of bottom water flow patterns. Combining observations from a dense grid of high-resolution narrow-beam echosounder data with results of wave growth modelling confirms that a southward current is responsible for the wave geometry on the western flank of the drift. This flow seems to be the western part of an anticlockwise gyre, which is centred around the Zapiola Drift. The regional variation of accumulation rates on the drift flank suggests that the flow velocity gradually decreases upslope. To the north–east of the drift crest, flow velocities increase again, causing a southwestward migration of the crest itself. Changes in the internal geometry of the sediment waves show evidence for variations in flow intensity during the last ∼100 kyr. Four different periods of alternating higher and lower flow velocities can be identified with fluctuations between 10 and 20 cm/s. This caused erosion, non-deposition and deposition on the downslope wave flanks.

Distinguishing sediment waves from slope failure deposits: field examples, including the ‘Humboldt slide’, and modelling results

Migrating sediment waves have been reported in a variety of marine settings, including submarine levee–fan systems, floors of fjords, and other basin or continental slope environments. Examination of such wave fields reveals nine diagnostic characteristics. When these characteristics are applied to several features previously attributed to submarine landslide deformation, they suggest that the features should most likely be reinterpreted as migrating sediment-wave fields. Sites that have been reinterpreted include the ‘Humboldt slide’ on the Eel River margin in northern California, the continental slope in the Gulf of Cadiz, the continental shelf off the Malaspina Glacier in the Gulf of Alaska, and the Adriatic shelf. A reassessment of all four features strongly suggests that numerous turbidity currents, separated by intervals of ambient hemipelagic sedimentation, deposited the wave fields over thousands of years. A numerical model of hyperpycnal discharge from the Eel River, for example, shows that under certain alongshore-current conditions, such events can produce turbidity currents that flow across the ‘Humboldt slide’, serving as the mechanism for the development of migrating sediment waves. Numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor (i.e. numerical steps), sediment waves can form and migrate upslope. Hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves. Physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms. The morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field, including those that were previously described as submarine landslides.

The Magdalena Turbidite System (Caribbean Sea): present-day morphology and architecture model

A detailed analysis of the modern morphology, acoustic facies and architecture of the eastern Magdalena Turbidite System has been based on multibeam bathymetry and backscatter data. In spite of the fact that the Magdalena Turbidite System developed in a tectonically active setting, its present architecture model is comprised of two tecto-sedimentary provinces: deformed-constructional and undeformed-erosive. The deformed province occurs where the deformed prism developed. This prism is characterised by structural highs and associated small-scale ridges and scarps. In this province, the Magdalena Turbidite System is constructional because sediment coming from the Colombian hinterland (via the Magdalena River and from coastal erosion) and sediment remobilised from the walls of the structural highs are transported trough canyons and gullies, and emplaced as gravity deposits into the deformed prism. In the undeformed province, by contrast, the Magdalena Turbidite System displays an architecture model similar to a passive continental margin, and is characterised by large-scale mass-flow deposits, leveed channel complexes, scars of slumps, and sediment waves. This province is presently erosive because the existing leveed channels are partially or totally destroyed and cannibalised by mass-flow deposits, cut by small-scale scars of slumps, and their morphology is subdued or modified by the formation of a sediment wave field.

Multi-process generated sediment waves on the Landes Plateau (Bay of Biscay, North Atlantic)

Detailed analyses of recently collected bathymetric and sparker seismic data, support a new interpretation of the Landes Plateau field of sediment waves located on the Aquitaine upper continental slope (Bay of Biscay). The wave geometry, previously described as the result of a major sediment failure, is interpreted as a structure with a complex origin including the interaction of depositional and gravity deformation processes. Depositional processes are mainly recorded by the upslope migrating pattern of the waves resulting from oblique or sigmoid downlap reflections on the upslope flank of the waves and by toplapping and truncated reflections on the downslope flank. Hemipelagic and turbiditic sedimentation may be involved in the wave building as well as contouritic processes that could be related to the existing northward polar current and internal waves. Gravity deformations are syndepositional, discontinuous and of low amplitude, affecting thick layers which alternate with undeformed layers. They seem to correspond to gentle sediment creeping or stretching associated with minor listric or compaction-like faults, and possible limited back rotation of sediment blocks. These multi-process generated sediment waves could be rather common on the continental margins as they could have been mistaken with either depositional or deformational structures.

On the architecture of high-latitude continental margins: the influence of ice-sheet and sea-ice processes in the Polar North Atlantic

Abstract The presence of ice during the Late Cenozoic distinguishes the nature and rates of processes on high-latitude margins from those elsewhere. Ice sheets terminating in marine waters deliver icebergs, meltwater and debris to high-latitude seas. Sea ice influences ocean salinity structure and downslope water and sediment transfer, and also transports fine-grained sediments over long distances. These cryospheric processes have led to the development of a distinctive sedimentary architecture on modern high-latitude continental margins. Large submarine fans made up almost entirely of stacked debris flows are present around the Norwegian-Greenland Sea. Large slides are located in a variety of settings relative to rates of sediment delivery from Quaternary ice-sheet margins, but no large slides have been mapped on the East Greenland margin. However, extensive channel systems and sediment-wave fields are present in the Greenland Basin, probably related to intermittent downslope flow of dense water and turbidity currents. The extensive NE Greenland shelf was not innundated by ice-sheet advance during recent full-glacial conditions, allowing sea-ice and deep-water production during both interglacials and full-glacials. Changes in the nature and rate of sedimentation within the Greenland Basin should provide clues on the rate of dense-water production, with implications for thermohaline circulation in the North Atlantic. Other erosional and depositional features on the Norwegian-Greenland Sea margins include canyons and contourite drifts. High-relief tectonic features influence sediment reworking by turbidity currents at abyssal depths. A simple conceptual model for glacier-influenced marine sedimentation summarizes the role of cryospheric processes in high-latitude margin sedimentary environments.