Unconfined Turbidity Currents Research Articles

Unconfined gravity current interactions with orthogonal topography: Implications for combined‐flow processes and the depositional record

ABSTRACTTurbidity current behaviour is affected by interactions with seafloor topography. Changes in flow dynamics will depend on the orientation and gradient of the topography, and the magnitude and rheology of the incoming flow. A better understanding of how unconfined turbidity currents interact with topography will improve interpretations of the stratigraphic record, and is addressed herein using three‐dimensional flume tank experiments with unconfined saline density currents that enter a horizontal basin before interacting with a ramp orientated perpendicular to flow direction. The incoming flow parameters remained constant, whilst the slope angle was independently varied. On a 20° slope, superelevation of the flow and flow stripping of the upper, dilute region of the flow occurred high on the slope surface. This resulted in a strongly divergent flow and the generation of complex multidirectional flows (i.e. combined flows). The superelevation and extent of flow stripping decreased as the slope angle increased. At 30° and 40°, flow reflection and deflection, respectively, are the dominant flow process at the base of slope, with the reflected or deflected flow interacting with the parental flow, and generating combined flows. Thus, complicated patterns of flow direction and behaviour are documented even on encountering simple, planar topographies orientated perpendicular to flow direction. Combined flows in deep‐water settings have been linked to the interaction of turbidity currents with topography and the formation of internal waves with a dominant oscillatory flow component. Here, combined flow occurs in the absence of an oscillatory component. A new process model for the formation and distribution of hummock‐like bedforms in deep‐marine systems is introduced. This bedform model is coupled to a new understanding of the mechanics of onlap styles (draping versus abrupt pinchout) to produce a spatial model of gravity‐current interaction, and deposition, on slopes to support palaeogeographical reconstructions.

Three-Dimensional Numerical Modeling of Confined and Unconfined Turbidity Currents – Model Development and Validation

Three-Dimensional Numerical Modeling of Confined and Unconfined Turbidity Currents – Model Development and Validation

Sequential bedform development in mixed turbidite–contourite systems: An example from the Cosmonaut Sea, East Antarctica

Mixed turbidite–contourite depositional systems are commonly found on continental margins, but their bedforms and associated sedimentary processes have not been studied in depth. In this work, we used multibeam echo-sounder, sub-bottom profiling, and multichannel seismic data from the continental rise of the Cosmonaut Sea, East Antarctica, to (1) identify primary bedforms in a combined-current (i.e., turbidity current + contour current) channel–levee system and (2) infer bedform-associated sedimentary processes. Within turbidite channels and on adjacent levees and distal overbank deposits, scours, furrows, and sediment waves of varying dimensions and trends were identified. These bedforms are interpreted to have formed in two steps, which have been likely repeated over and over again through time. First, scours and sediment waves within the channels were formed by turbidity currents, while sediment waves on adjacent levees were likely formed by synchronous interactions between overspilled unconfined turbidity currents and the westward Antarctic Bottom Water (AABW) contour current. Second, after waning of the episodic turbidity currents, AABW flow created a field of erosional furrows on a distal overbank, with these furrows truncating the large field of sediment waves earlier generated by the combined flow of interacting currents. Bedform locations, orientations, and truncating relationships are key for identifying the likely origins of mixed-system bedforms.

Turbidity currents generating lobes: flow rate influence on 3D experiments without slope break

Turbidity current hydrodynamic properties are evaluated experimentally to understand the formation of turbidity current depositional lobes and the relationship with flow properties, in particular, flow rates. This study focused on the depositional behavior of unconfined turbidity currents through the analysis of three-dimensional experiments performed in a large-scale channel-basin tank without slope break. Three flow rates were simulated when flow velocities, both in longitudinal and transversal directions, were measured and resulting depositional features were evaluated. The three-dimensional physical experiments carried out in this work allowed the identification of two flow rate models with different hydrodynamic characteristics and two distinct lobes. Lower flow rates produced elongated lobate deposits, with characteristic lower flow regime plane bed on the surfaces, characteristic downstream sediment fining that resulted from lower flow velocities, and visibly less turbulent flows from less competent and waning turbidity currents. Higher flow rates showed a more characteristic radial and downstream fining sediment with lobe surfaces displaying ripples and dunes, generated by the higher flow velocities, presumably more turbulent, and more competent turbidity currents.

Distribution, characteristics, and processes of formation of sediment waves along the Indian north-eastern continental margin

Muddy, upslope-migrating sediment waves, with their crestal lines parallel to the bathymetric contours, are the dominant geomorphic elements in the entire lower slope along the Indian north-eastern continental margin. Wave dimensions are maximum with high but differing degrees of sediment accretions on the upslope and downslope sides of wave crests and with most upslope wave migrations in the northeastern part of the study area, having relatively low seafloor gradients. The wave dimensions decrease with lower as well as differing degrees of sediment accretions on both sides of wave crests and with lower upslope wave migrations in the southwestern parts of the study area, having steeper seafloor gradients. Within each area of the lower slope, starting from updip, wave dimensions increase to their maximum at some distance downdip with optimal low seafloor gradients. Farther downdip, the wave dimensions decrease even with lower seafloor gradients continuing. Also, within each area, at the base of the wave intervals, the waves are small and grow into larger features with more sediment accretions in the upper intervals.The distribution and characteristics of sediment waves observed along the Indian margin are indicative of deposition by down-slope moving, unconfined turbidity currents that have evolved from other sediment gravity flows derived from several point sources updip. Along-slope bottom currents are least likely agents of wave deposition here. We interpret that the spatial and temporal variations in wave dimensions are due to the variations in seafloor gradients, turbidity current flow characteristics, frequency, duration and total number of flow events in space and time.

Re-channelization of turbidity currents in South China Sea abyssal plain due to seamounts and ridges

Turbidity currents can be characterized as net-erosive, net-depositional or net-bypassing. Whether a flow is erosive, depositional or bypasses depends on the flow velocity, concentration and size but these can also be impacted by external controls such as the degree of confinement, slope gradient and substrate type and erodibility. Our understanding of the relative importance of these controls comes from laboratory experiments and numerical modelling, as well as from field data due to the proliferation of high-resolution 3D seismic and bathymetric data, as well as the outcrop and rock record. In this study, based on extensive multibeam and seismic reflection surveys in combination with International Ocean Discovery Program cores from the South China Sea, we document a new mechanism of turbidity current transformation from depositional to erosive resulting in channel incision. We show how confinement by seamounts and bedrock highs of previously unconfined turbidity currents has resulted in the development of seafloor channels. These channels are inferred to be the result of confinement of flows, which have traversed the abyssal plain, leading to flow acceleration allowing them to erode the seafloor substrate. This interpretation is further supported by the coarsening of flow deposits within the area of the seamounts, indicating that confinement has increased flow competency, allowing turbidity currents to carry larger volumes of coarse sediment which has been deposited in this region. This basin-scale depositional pattern suggests that pre-established basin topography can have an important control on sedimentation which can impact characteristics such as potential hydrocarbon storage.

Ambient Entrainment Mechanisms of Partially Unconfined Turbidity Currents Interacting with a Continuous Rigid Obstacle

Abstract In this study, the entrainment mechanisms of unconfined turbidity currents that interact with a linear rectangular obstacle will be investigated and compared with confined studies. Laborat...

Quantifying propagation characteristics of unconfined turbidity currents interacting with an obstacle within the slumping regime

ABSTRACTExperimental studies of confined turbidity currents interacting with obstacles have become popular over the last three decades; however, little work has sought to quantify the characteristics of unconfined turbidity currents interacting with obstacles. In the present study, unconfined turbidity currents colliding with a rectangular basal obstruction are investigated. With a focus on the initial theoretical slumping regime, flow characteristics are quantified and characterized. The currents varied with initial density and substrate roughness. Unobstructed currents propagated at a constant velocity, confirming previous theory that the current is within the slumping regime; however, a typical reverse ambient bore was not present. Froude and Reynolds numbers were shown to decrease over the obstacle by 43–54% and 17%, respectively. Deflected lateral propagation along the obstacle face was found to entrap ambient fluid and encourage mixing, providing a starting point for understanding the implications of lateral propagation along submerged pipelines and reservoir barriers.

Flow structure of unconfined turbidity currents interacting with an obstacle

Driven by a growing importance to engineered structures, investigating the flow characteristics of turbidity currents interacting with a basal obstruction has become popular over the last three decades. However, research has focused on confined studies or numerical simulations, whereas in situ turbidity currents are typically unconfined. The present study investigates experimentally the velocity and turbulence structure of an unconfined turbidity current, in the immediate regions surrounding a rectangular obstacle. Initial density of the current, and substrate condition is varied. Through a novel technique of installing ultrasonic probes within the obstacle, the presence of a velocity recirculation region immediately upstream and downstream of the obstacle is revealed and confirmed with high-resolution imagery. This was found to be comparable to previous confined studies, suggesting that stream-wise velocity profile structure is somewhat independent of confinement. The obstacle was found to reduce velocity and turbulence intensity maxima downstream of the obstacle when compared with unobstructed tests.

Coupling of Ultrasonic and Photometric Techniques for Synchronous Measurements of Unconfined Turbidity Currents

By synchronizing data collection, such as photometric and ultrasonic Doppler profiling (UVP) measurement techniques, new insights can be obtained into environmental flows, such as highly dynamic turbidity currents. We introduce a combined experimental setup, which ultimately allows a time reduction in testing programmes, and discuss the measurement advances with the help of four surface conditions we tested for unconfined turbidity currents: (a) a smooth surface; (b) a smooth surface with an obstacle present; (c) a rough surface; and (d) a rough surface with an obstacle present. We show that data from both measurement techniques indicate that a rough surface reduces global current velocities and the magnitude of turbidity current phenomena, including Kelvin-Helmholtz instabilities and lobe-and-cleft formation. However, by coupling the techniques, photometric data give valuable insight into the spatial development of instabilities, such as the grouping of lobe and cleft formations. The presence of an obstacle causes local regions of an increased and decreased velocity, but does not affect the global current velocity. Additionally, the obstacle created three local intensity maxima upstream, dissipating to two maxima downstream, supporting the presence of local eddies. The study shows that the combination of UVP and photometry is an effective way forward for obtaining detailed qualitative and quantitative insights into turbulent flow characteristics and we highlight the potential for future research.

Gravitational, erosional and depositional processes on volcanic ocean islands: Insights from the submarine morphology of Madeira Archipelago

The submarine flanks of volcanic ocean islands are shaped by a variety of physical processes. Whilst volcanic constructional processes are relatively well understood, the gravitational, erosional and depositional processes that lead to the establishment of large submarine tributary systems are still poorly comprehended. Until recently, few studies have offered a comprehensive source-to-sink approach, linking subaerial morphology with near-shore shelf, slope and far-field abyssal features. In particular, few studies have addressed how different aspects of the subaerial part of the system (island height, climate, volcanic activity, wave regime, etc.) may influence submarine flank morphologies. We use multibeam bathymetric and backscatter mosaics of an entire archipelago – Madeira – to investigate the development of their submarine flanks. Crucially, this dataset extends from the nearshore to the deep sea, allowing a solid correlation between submarine morphologies with the physical and geological setting of the islands. In this study we also established a comparison with other island settings, which allowed us to further explore the wider implications of the observations. The submarine flanks of the Madeira Archipelago are deeply dissected by large landslides, most of which also affected the subaerial edifices. Below the shelf break, landslide chutes extend downslope forming poorly defined depositional lobes. Around the islands, a large tributary system composed of gullies and channels has formed where no significant rocky/ridge outcrops are present. In Madeira Island these were likely generated by turbidity currents that originated as hyperpycnal flows, whilst on Porto Santo and Desertas their origin is attributed to storm-induced offshore sediment transport. At the lower part of the flanks (−3000 to −4300 m), where seafloor gradients decrease to 0.5°–3°, several scour and sediment wave fields are present, with the former normally occurring upslope of the latter. Sediment waves are often associated with the depositional lobes of the landslides but also occur offshore poorly-developed tributary systems. Sediment wave fields and scours are mostly absent in areas where the tributary systems are well developed and/or are dominated by rocky outcrops. This suggests that scours and sediment wave fields are probably generated by turbidity currents, which experience hydraulic jumps where seafloor gradients are significantly reduced and where the currents become unconfined. The largest scours were found in areas without upslope channel systems and where wave fields are absent, and are also interpreted to have formed from unconfined turbidity currents. Our observations show that tributary systems are better developed in taller and rainy islands such as Madeira. On low-lying and dry islands such as Porto Santo and Desertas, tributary systems are poorly developed with unconfined turbidite currents favouring the development of scours and sediment wave fields. These observations provide a more comprehensive understanding of which factors control the gravitational, erosional, and depositional features shaping the submarine flanks of volcanic ocean islands.

Response of unconfined turbidity current to relay‐ramp topography: insights from process‐based numerical modelling

AbstractThis natural‐scale experimental study combines structural modelling of soft‐linked normal‐fault relays with a CFD (computational fluid dynamics) numerical simulation of a range of unconfined turbidity currents overrunning the relay‐system topography. The flow, released from an upslope inlet gate 2000‐m wide and 50‐m to 100‐m high, rapidly expands and adjusts its thickness, velocity and sediment load to the substrate slope of 1.5°. A lower initial sediment concentration or smaller thickness renders the quasi‐steady flow slower and its sediment‐transport capacity lower. A 3D pattern of large interfering Kelvin‐Helmholtz waves causes fluctuations of the local flow velocity magnitude and sediment concentration. Four zones of preferential sediment deposition are recognized: a near‐gate zone of abrupt flow expansion and self‐regulation; a flow‐transverse zone on the counter‐slope of fault footwall edges; a flow‐transverse zone at the fault‐scarp toes and a similar transverse zone near the crest of the hanging wall counter‐slopes. The sand deposited on the counter‐slope tends to be re‐entrained and fed back to the current by a secondary reverse underflow. The spatial extent and sediment accumulation capacity of depozones depend upon the released current volume. The impact of relay system on an overrunning current depends upon the fault separation distance and stage of tectonic evolution. An early‐stage relay system, with small vertical displacement and little overlap of faults, is bypassed by the current with minimum flow disturbance and no pronounced deposition. An advanced‐stage system, with greater fault displacement and overlap, gives a similar hydraulic effect as a single fault segment if the fault separation is small. If the separation is relatively large, the flow tends to be internally redirected sideways from the ramp into the hanging wall synclinal depressions. Since normal‐fault relays are common features in extensional basins, the study bears important implications for turbiditic slope‐fan models and for the spatial sand prediction in subsurface exploration of faulted submarine slopes.

Response of unconfined turbidity current to normal‐fault topography

AbstractTurbidity currents descending the slopes of deep‐water extensional basins or passive continental margins commonly encounter normal‐fault escarpments, but such large‐magnitude phenomena are hydraulically difficult to replicate at small scale in the laboratory. This study uses advanced computational fluid dynamics numerical simulations to monitor the response of large, natural‐scale unconfined turbidity currents (100 m thick and 2000 m wide at the inlet gate) to normal‐fault topography with a maximum relief of nearly 300 m. For comparative purposes, the turbidity current is first released on a non‐faulted pristine slope of 1·5° (simulation model 1). The expanding and waxing flow bypasses the slope without recognizable deposition within the visibility limit of 8 vol.% sand grain packing. Similar flow is then released towards the tip (model 2) and towards the centre (model 3) of a normal‐fault escarpment. In both of these latter models, the sand carried by flow tends to be entrapped in four distinct depozones: an upslope near‐gate zone of flow abrupt expansion and self‐regulation; a flow‐transverse zone at the fault footwall edge; a flow‐transverse zone at the immediate hangingwall; and a similar transverse zone near the crest of the hangingwall counter‐slope, where some of the deposited sand also tends to be reshuffled to the previous zone by a secondary reverse underflow. The near‐bottom reverse flow appears to be generated on a counter‐slope of 1·1°, increased to 2·0° by deposition. The Kelvin–Helmholtz interface instability plays an important role by causing three‐dimensional fluctuations in the flow velocity magnitude and sediment concentration. The thick deposits of large single‐surge flows may thus show hydraulic fluctuations resembling those widely ascribed to hyperpycnal flows. The study indicates further that the turbiditic slope fans formed on such fault topographies are likely to be patchy and hence may differ considerably from the existing slope‐fan conceptual models when it comes to the spatial prediction of main sand depozones.

Sediment waves and depositional implications for fine-grained rocks in the Cerro Toro Formation (upper Cretaceous), Silla Syncline, Chile

The Cerro Toro Formation contains a variety of lithofacies including pebble and cobble conglomerate, coarse-grained sandstone, thin-bedded mudstone and sandstone, and slumped to chaotic mudstone. Coarse-grained rocks in the Cerro Toro have been described and well documented, whereas characterization and description of mudstone-prone lithofacies are not well documented. These mudstone-prone rocks exhibit two architectural patterns: 1) broad (>200 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap) and downlap patterns, and 2) laterally persistent (>400 m), horizontal, thin-bedded mudstone and sandstone. Adjacent to Channel Complex 3 (Paine C Member), these facies exhibit a stratigraphic transition from horizontal to wavy and curved beds concurrent with pronounced aggradation of laterally equivalent, and possibly coeval, channel facies. Sandstone and mudstone beds within the wavy-bedded facies exhibit turbidite lithofacies that include current-ripple lamination (Tc), planar lamination (Tb) massive, graded intervals (Ta), and laminated to structureless silt- and clay-rich beds (Tde). Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1 m thick. These thick sandstone bedsets display inclined bedding or lamination associated with mudstone rip-up clasts and are confined to troughs or swales within large-scale wavy-bedded units. Erosion surfaces within this thin-bedded fine-grained lithofacies are spaced vertically at 10–15 m, commonly associated with the crest of curved bedding and display at least 5 m of relief. Based on map patterns, channel-form architectural elements, and stratal-stacking relationships, the coarse-grained rocks in the Cerro Toro have been interpreted as the fill within deep-water channels. In contrast, the thin-bedded mudstone and sandstone lithofacies located adjacent to and eroded into by the channel facies are interpreted as coeval levee facies. Planar, tabular bedsets located at the base of this fine-grained lithofacies are interpreted as initial overbank deposition associated with the upper part of Channel Complex 3, whereas the curved, wavy beds in the fine-grained lithofacies are interpreted as sediment waves developed on the backside of a levee. The curved bedding, lenticular, medium-grained sandstone bedsets, scattered erosional surfaces, and onlap-downlap stratal geometries in the Cerro Toro bear resemblance to sediment waves associated with Quaternary coarse-grained, channel-levee systems. In contrast, sediment waves generated in other deep-water settings, such as upper slope, continental rise or basinal settings, usually are dominated by silt- and very fine-grained sand, very continuous layering (over 1 km), relatively long wavelength (>1 km) and relatively high amplitudes (>10 m). A possible analog for the coarse-grained and fine-grained rocks in the Cerro Toro is the Quaternary Var Ridge and associated fan valley located in the Mediterranean Sea. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size clasts, and thin-bedded mud and sand in levees with documented sand bed thickness up to 40 cm and up to medium-grained sand. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap downlap, and toplap, similar to the Cerro Toro. The fine-grained material can be linked to deposition from unconfined turbidity currents that were generated via overbank flows from the Var fan channel. The development of these architectural elements requires out of channel turbidity currents and relatively thick flows. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation.

Laboratory experiments on the formation of subaqueous depositional gullies by turbidity currents

Gullies and furrows are shallow, sub-parallel features widespread on the seafloor and some lacustrine environments. Vast fields of linear channels of erosional origin called furrows, with spacings of meters and up to hundred of meters, are common in settings with relatively low seafloor slopes and have been previously ascribed to helical motion in benthic boundary layers. Submarine gullies are linear, donwslope-trending features of erosional, depositional or mixed nature, commonly observed in the upper continental slope, submarine canyon walls, and other relatively steep submarine environments. While gully fields have been associated to the action of unconfined turbidity currents and other cascading dense flows, their relatively large spacings (ranging from several hundreds of meters to 1–2 km) suggest that these features might be the result of other mechanisms different than boundary layer helical motion. In this paper we propose an alternative mechanism for the initiation of gullies observed in relatively steep seabed gradients, downslope of a seabed slope change such as those observed in continental margins (self-slope break) or submarine canyon walls. Our working hypothesis is that gullies could be the result of a centrifugal instability that sets in interfaces of gravity currents as they flow through a bedslope transition. Since for these cases the centrifugal force excess is of the same order of magnitude of the driving force, and therefore a potential source of flow instability, we propose that gully spacing might be controlled by the magnitude of the centrifugal force, scaling roughly with the radius of curvature of the transition (slope break). We also present experimental evidence on the formation of longitudinal features resembling slope gullies that were created in a laboratory tank after running successive supercritical, depositional turbidity currents through a slope change similar to that observed in continental margins. Gullies in the tank developed through differential deposition only, and showed spacing an order of magnitude larger that that expected from a boundary-layer helical motion origin. These experimental results along with the observation that gullies were not formed in the tank without a bed slope change (low to high) support the hypothesis of a centrifugal instability origin for longitudinal features in relatively steeper submarine settings. Our experimental results also support our first-order approximate gully spacing rule, which is in general agreement with field observations of gully spacing in upper continental slopes and submarine canyon walls.

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.

Late Quaternary evolution of the San Antonio Submarine Canyon in the central Chile forearc (∼33°S)

Hydrosweep swath-bathymetry and seismic-reflection data reveal the morphology, sedimentary processes, and structural controls on the submarine San Antonio Canyon. The canyon crosses the forearc slope of the central Chile margin for more than 150 km before it empties into the Chile Trench near 33°S latitude. In its upper reaches, the nearly orthogonal segments of the San Antonio Canyon incise ∼1 km into thick sediment following underlying margin-perpendicular basement faults and along the landward side of a prominent margin-parallel thrust ridge on the outer mid-slope. At a breach in the outer ridge, the canyon makes a sharp turn into the San Antonio Reentrant. Resistance to erosion of outcropping basement at the head of the reentrant has prevented the development of a uniformly sloping thalweg, leaving gentle gradients (<2°) up-canyon and steep gradients (>6°) across the lower slope. Emergence of an obstruction across the head of the San Antonio Reentrant has trapped sediment in the mid-slope segments of the canyon. Presently, little sediment appears to reach the Chile Trench through the San Antonio Canyon. The development of the San Antonio Canyon was controlled by the impact of a subducted seamount, which formed the San Antonio Reentrant and warped the middle slope along its landward advancing path. Incision of the canyon landward of the outer mid-slope ridge may be ascribed to a combination of headward erosion and entrenchment by captured unconfined turbidity currents. Flushing of the canyon was likely enhanced during the lowered sea level of the last glaciation. Where the canyon occupies the triangular embayment of the reentrant at the base of the slope, sediment has ponded behind a small accretionary ridge. On the trench floor opposite the San Antonio Canyon mouth, a 200-m-thick levee–overbank complex formed on the left side of a distributary channel emanating from a breach in the accretionary ridge. Axial transfer of sediment was inhibited to the north of the San Antonio Canyon mouth, which left the trench to the north sediment starved. Between ∼32°40′S and 33°40′S, the Chile Trench axial turbidite channel deeply incises the San Antonio distributary complex. This entrenchment may have been initiated when the barrier to northward transport was eliminated.

Sedimentary processes in the Selvage sediment‐wave field, NE Atlantic: new insights into the formation of sediment waves by turbidity currents

An integrated geophysical and sedimentological investigation of the Selvage sediment‐wave field has revealed that the sediment waves are formed beneath unconfined turbidity currents. The sediment waves occur on the lower continental rise and display wavelengths of up to 1 km and wave heights of up to 6 m. Wave sediments consist of interbedded turbidites and pelagic/hemipelagic marls and oozes. Nannofossil‐based dating of the sediments indicates a bulk sedimentation rate of 2·4 cm 1000 years–1, and the waves are migrating upslope at a rate of 0·28 m 1000 years–1. Sediment provenance studies reveal that the turbidity currents maintaining the waves are largely sourced from volcanic islands to the south. Investigation of existing models for sediment‐wave formation leads to the conclusion that the Selvage sediment waves form as giant antidunes. Simple numerical modelling reveals that turbidity currents crossing the wave field have internal Froude numbers of 0·5–1·9, which is very close to the antidune existence limits. Depositional flow velocities range from &lt;6 to 125 cm–1. There is a rapid increase in wavelength and flow thickness in the upper 10 km of the wave field, which is unexpected, as the slope angle remains relatively constant. This anomaly is possibly linked to a topographic obstacle just upslope of the sediment waves. Flows passing over the obstacle may undergo a hydraulic jump at its boundary, leading to an increase in flow thickness. In the lower 15 km of the wave field, flow thickness decreases downslope by 60%, which is comparable with results obtained for other unconfined turbidity currents undergoing flow expansion.

Turbidity current sediment waves on the submarine slopes of the western Canary Islands

Two sediment wave fields have been identified on the flanks of the western Canary Islands of La Palma and El Hierro, using a high-quality 2-D and 3-D dataset that includes GEOSEA and TOBI imagery, 3.5-kHz profiles, and short sediment cores. The La Palma sediment wave field covers some 20,000 km2 of the continental slope and rise, and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2.4 km. The wave crestlines have a complex morphology, with common bifurcation and a clear sinuosity. Waves have migrated upslope through time. Cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic/hemipelagic layers. The wave field is interpreted as having formed beneath unconfined turbidity currents. A simple, previously published, two-layer model is applied to the waves, revealing that they formed beneath turbidity currents flowing at 10–100 cm/s−1, with a flow thickness of 60–400 m and a sediment concentration of 26–427 mg/l. The El Julan sediment wave field lies within a turbidity current channel on the southwest flank of El Hierro. The sediment waves display wave heights of about 6 m and wavelengths of up to 1.2 km. The waves are migrating upslope, and migration is most rapid in the centre of the channel where the flow velocity is highest. This wave field has been formed by channelised turbidity currents originating on the flanks of El Hierro.

Late Quaternary sedimentation patterns of the Ebro turbidite systems (northwestern Mediterranean): Two styles of deep-sea deposition

Seven sediment types are identified from the analysis of 60 piston cores in the Ebro turbidite systems (1200–1800 m deep) between the Ebro shelf and the Valencia Trough in the northwestern Mediterranean Sea. These sediments are grouped into five facies associations: channel floor of erosive-depositional channel-levee complex, levee of erosive-depositional channel-levee complex, depositional channel-levee complex, interchannel, and apron facies. Unconfined slides, debris flows, channeled turbidity currents and differential hemipelagic settling are the main depositional processes which develop these facies. Deposition in the Ebro turbidite systems occurred primarily during the Pleistocene when turbidity currents funneled coarsegrained sediments through canyon/channel systems. During the Holocene, most sediment was trapped in deltaic and nearshore environments and turbidite deposition was replaced by hemipelagic settling. In the hemipelagic sequence thus created, grain size, terrigenous components, and chlorite/kaolinite and illite/kaolinite ratios decrease upwards while planktonic and calcite/quartz ratios increase. Two styles of growth patterns are present during periods of low sea level. Most common are base-of-slope channel-levee complexes, each of which is developed alongside an older complex. The complexes were formed by sediment supplied from different canyons as the systems migrated from north to south with time. In addition, in the central sector, base-of-slope aprons have been formed in gullied slope terrains by unconfined slides, and less commonly by unconfined turbidity currents. The major differences between the Ebro and other base-of-slope systems are: (a) the incomplete development of channel-levee complexes as compared to typical deep-sea fans, (b) the coexisting growth of aprons, and (c) multiple canyon sources. Low slope stability, resulting in many sediment supply point sources, is one of the primary factors which encourages development of base-of-slope aprons.