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The detailed structure and composition of turbiditic channel-mouth lobes is still largely unknown because they commonly lie at abyssal water depths, are very thin and are therefore beyond the resolution of hull-mound acoustic tools. The morphology, structure and composition of the Congo turbiditic channel-mouth lobe complex (90×40 km; 2525 km 2 ) were investigated with hull-mounted swath bathymetry, air gun seismics, 3.5 kHz sub-bottom profiler, sediment piston cores and also with high-resolution multibeam bathymetry and video acquired with a Remote Operating Vehicle (ROV). The lobe complex lies 760 km off the Congo River mouth in the Angola abyssal plain between 4740 and 5030 m deep. It is active and is fed by turbidity currents that deposit several centimetres of sediment per century. The lobe complex is subdivided into five lobes that have prograded. The lobes are dominantly muddy. Sand represents ca. 13% of the deposits and is restricted to the feeding channel and distributaries. The overall lobe body is composed of thin muddy to silty turbidites. The whole lobe complex is characterized by in situ mass wasting (slumps, debrites). The 1-m-resolution bathymetry shows pervasive slidings and block avalanches on the edges of the feeding channel and the channel mouth indicating that sliding occurs early and continuously in the lobe build-up. Mass wasting is interpreted as a consequence of very-high accumulation rates, over-steepening and erosion along the channels and is therefore an intrinsic process of lobe building. The bifurcation of feeding channels is probably triggered when the gradient in the distributaries at the top of a lobe becomes flat and when turbidity currents find their way on the higher gradient on the lobe side. It may also be triggered by mass wasting on the lobe side. When a new lobe develops, the abandoned lobes continue to collect significant turbiditic deposits from the feeding channel spillover, so that the whole lobe complex remains active. A conceptual lithostratigraphic model is proposed for five morpho-sedimentary environments: lobe rims, lobe body, distributaries, levees, feeding channel. This study shows that high-resolution bathymetry ROV observations are necessary to fully understand the build-up processes of modern channel-mouth lobes.

The Algerian margin is a seismically active region, where during the last century, several large magnitude earthquakes took place. This study combines geotechnical and sedimentological data with numerical modelling to quantitatively assess the present-day slope stability of the Algerian margin. Geotechnical laboratory tests, such as cyclic triaxial tests, oedometric tests and vane shear tests were carried out on sediment cores collected on the study area. The liquefaction potential of a sediment column located about 30 km from the Boumerdes earthquake epicentre of 21st May 2003 was evaluated theoretically for an earthquake of M w = 6.8. We show that thin sand and silt beds such as those described on recovered sediment cores are the main cause of sediment deformation and liquefaction during earthquakes. Numerical calculations showed that the slope failure may occur during an earthquake characterised by a PGA in excess of 0.1g, and also that, under a PGA of 0.2g liquefaction could be triggered in shallow silty–sandy deposits. Moreover, comparison of the predicted slope failure with failure geometries inferred from seafloor morphology showed that earthquakes and subsequent mass movements could explain the present-day morphology of the study area.

Sandy lobe deposits on submarine fans are sensitive recorders of the types of sediment gravity flows supplied to a basin and are economically important as hydrocarbon reservoirs. This study investigates the causes of variability in 20 lobes in small late Pleistocene submarine fans off East Corsica. These lobes were imaged using ultra-high resolution boomer seismic profiles (

The present study is focused on the latest Pleistocene distal lobe of a confined turbidite system. Several scales of depositional structures are seen using very high-resolution seismic data, which are quite comparable with those of onland outcrops. The lobe is a sedimentary complex and was built by numerous successive events. This study allows for the first time to describe a comprehensive model for the inter-relationship of different architectural elements within a lobe complex. We demonstrate that the three-dimensional geometry of the lobe is directly controlled by the degree of confinement of gravity currents and thus to the pre-existing morphology that evolved during lobe growth. The confinement and lithology of the lobe has a great impact on the reservoir physical properties.

New high-resolution seismic reflection data collected along the eastern margin of Corsica have been analysed to describe the morphology of the turbidite systems located seaward of the Golo River mouth. The boomer data reveal that there is not only one turbidite system directly associated with the river, but four additional, non-coalescing systems which grew simultaneously. In the south, the system has the typical morphology of a turbidite deposit rich in mud and sand with a well-developed meandering canyon and channel morphology. In the north, they have the morphology of sand-rich turbidite systems with shorter straight channels. The southernmost deposits are interpreted to represent a more advanced stage of turbidite system development. Terraces, recognised by their particular seismic facies on boomer profiles which clearly differs from the surrounding levee facies, are observed in the channel meanders. They are interpreted as confined levees built by vertical accretion due to deposition from low-energy flows. Despite limited penetration, boomer seismics are demonstrated to be a useful complement to lower-resolution sparker data. The boomer data are superior (1) for the characterisation of fine-grained turbidite deposits by extending seawards the limits of the turbidite systems commonly defined by the acoustic response of sands, (2) in demonstrating the persistence of turbiditic processes farther towards the basin, and (3) for proposing conceptual models for the formation of terraces in fan valleys.

Multichannel seismic data newly acquired during two ZaAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

Hyperpycnal flows form in the marine environment when river discharge enters the ocean with suspended concentrations in excess of 36 kg m 23 due to buoyancy considerations, or as little as 1- 5 kg m 23 when convective instability is considered. They form at a river mouth during floods of small to medium size rivers including extreme events such as jokulhaups, dam breaking and draining, and lahars. Associated with high-suspended concentration, they can transport considerable volume of sediment to ocean basins. The typical deposit or hyperpycnite sequence is a compound of a basal coarsening-up unit, deposited during the waxing period of discharge, and a top fining-up unit deposited during the waning period of discharge. Hyperpycnites differ from other turbidites because of their well-developed inversely graded facies and intrasequence erosional contacts. These observations lead to a complete redefinition and interpretation of fine-grained turbidites. Hyperpycnite stacking can locally generate high-sedimentation rates, in the range of 1 - 2 m per 100 year. Because hyperpycnites are related to climate through flood frequency and magnitude, their record should vary with sea level and climate change. They can also be associated with proximal ice-melting settings. Hyperpycnal flows could also be involved in the formation of meandering canyons and channels. q 2003 Elsevier Ltd. All rights reserved.

The Zaire deep-sea fan is one of the largest fans in the world still affected by turbidite sedimentation along a unique active meandering turbidite channel. This active channel is fed by turbidity currents, which are generated at the Zaire River mouth, and flow via a deeply entrenched canyon across the shelf and the continental slope. Based on a detailed study of the morphology and architecture of the present Zaire Canyon/Channel, several main zones can be defined (the canyon, the upper-fan valley, the upper and the lower channel-levee system leading into distal lobes). They are characterised by different behaviours in terms of erosion, transport and sedimentation within the canyon/channel. An important characteristic of the Zaire Canyon/Channel is the deep incision of the thalweg, well below the regional sea floor along its whole path. The origin of this entrenchment may be linked to the incision of the Zaire Canyon back across the shelf during the last sea-level rise. This incision of the canyon has allowed the continuity of turbidite activity during the Holocene, in maintaining the connection between the canyon head and the river mouth (in contrast to most of other large deep-sea fans, which are generally inactive during highstands). The entrenchment of the Zaire Canyon/Channel limits the overflow of turbidity currents and the turbidite sedimentation over levees, and prevents avulsion along the upper part of the Zaire Channel. Most of the sediment transported in turbidity currents are probably led down to the lower channel-levee system and the distal lobes.

Turbidity currents generated during floods of small and medium rivers have been demonstrated to be an important process of sediment transport from continent to abyss. They produce fine-grained turbidite deposits. No deposit related to these flood-related turbidity currents has yet been described in the deep sea. In this paper, we present some unusual sandy to muddy turbidite beds cored in the Var turbidite system (NW Mediterranean). They show a coarsening-upward basal unit capped with a classical fining-upward unit which are related to the periods of increasing and decreasing discharge at the river mouth, respectively. The two units are separated by a contact which can be gradational to erosional. This intrabed contact is interpreted as resulting from erosion during peak flood conditions. This intrabed contact can be confused with classical basal contacts of turbidite beds. The frequency of hyperpycnal turbidite beds can be used to relate climatic changes inland to the deep-sea sedimentary record, as an increase corresponds to periods of enhanced flooding at the river mouth.

We propose to apply in this note a simple mathematical model for the sedimentation of gravity deposits in deep water. An inverse method has been developed in order to characterize the parameters associated with the submarine flow. This method also provides the initial conditions, which constitute a major uncertainty in geoscience modelling. Application of the inverse method to the 1979 Nice airport collapse (southern France), constrained by cable breaks and/or deposit thicknesses, leads, for example, to the estimation of the initial volume and/or the physical parameters of the flow (friction coefficient, modified turbulent Schmidt number, etc.). The application of the model to this real case shows that only the simultaneous inversion of dynamic (cable break times) and sedimentological (deposit) data allows a satisfactory reconstruction of the flow.

Deposits observed in cores collected on the south levee of the active middle Zaire deep-sea fan are fine-grained turbidites containing abundant thin silt laminae, alternated with clay laminae. Each sequence represents successive spillover of the dilute upper part of a continuous channelised turbidity current. Pulsatory overflows create a low velocity (0.1 m·s−1) overspill current on the levee with duration from a few days to a few weeks. Variations of the hydraulic regime within the overflow generate laminae.

Sediment waves have been observed on the backside of levees in deep-sea turbidite systems where they are built by turbidity currents that spill out of channels and spread sediment over the levees. In an attempt to understand the way in which, sediment waves are initiated, prograde and/or aggrade, two sediment waves were selected within the present Var turbidite system (Ligurian Sea, north-western Mediterranean) for a detailed sedimentary study. The data include high-resolution seismic-reflection profiles, 3.5-kHz echo-sounder profile, high-resolution side-scan imagery and cores collected from the upstream flank, crest, and downstream flank of both sediment waves. Core-to-core correlations allow interpretation of the dynamic of gravity-flows across the sediment waves. The asymmetrical internal structure of the waves results from higher rates of sediment deposition on the upstream flank and lower rates of deposition and erosion on the downstream flank and further results from active progradation stages during periods of greater gravity-flow activity. The strong differences of depositional processes across the sediment waves result through time in the individualisation of large and interconnected sand bodies in the distal part of the levee.

This paper concerns the study of the over-consolidation state of a marine sediment extracted from the continental slope in the Gulf of Guinea. Special attention was devoted to the physicochemical phenomena. The study was carried out 1) at a microscopic level using the theory of the diffuse double layer [6, 8, 12] and 2) at a macroscopic level using an elastoplastic model considering the suction as a stress state variable [1]. These models have provided a physical explanation as well as an adequate description of the phenomenon of over-consolidation.

Two scientific submarine exploration cruises were realised in 1998 offshore Congo and Angola within the ZaiAngo project. The main goal of the project is the study of the present and recent Zaire submarine fan. The survey followed the present channel down to its distal end, 760 km from the river mouth at a water depth of 4 800 m. Preliminary results indicate that it feeds a distal lobe complex. Only one turbiditic channel is active at one time. The channels are highly meandering in their upper course. More than 80 palaeochannels have been identified both south and north of the present channel. The estimated area of the Zaire deep-sea fan is close to those of the Amazon fan.

A field of sediment waves built by turbidity currents on the Var deep-sea fan has been studied using a large amount of seismic-reflection data and cores. To understand the spatial organisation and evolution of the sediment waves, maps of both surficial and an older subsurface wave field were prepared. Three different sediment wave geometries (symmetrical to asymmetrical cross-section) were recognised and can be linked to particular styles of evolution through time (from simple vertical aggradation to upslope and upcurrent progradation). Each geometry appears to be linked with: (i) a particular location on the Var Sedimentary Ridge; (ii) the type of turbidite deposits; (iii) the local gradient slope; and (iv) the height of the Ridge. Several factors control the nature and distribution of sediment waves and these factors do not change significantly through time, as suggested by the common geometries, evolution and distribution of the sediment waves between the present and fossil fields. Supercritical flow conditions and high sediment supply allow the development of well-developed and prograding sediment waves, while subcritical flow conditions and low sediment supply allow the formation of smaller and aggrading sediment waves. The impact of these two factors is also influenced by the morphology of the fan valley, the height of the Ridge, and the type of transported sediment. The evolution of sediment wave amplitude is also strongly influenced by the distance of sediment waves from the Ridge crest, while wavelength evolution is more influenced by slope gradient. The building of the Var Sedimentary Ridge and particularly the asymmetry and the overall morphology of the levee are directly controlled by the growth pattern of the sediment waves.

The distribution of surficial sediments and sediment dispersion patterns on the steep continental slope off Nice (SE France) have been studied using side-scan sonar, 3.5 kHz profiles, short piston cores and bottom photographs. The input of terrigenous material to the Baie des Anges, a submarine embayment bounded by two prominent ridges, is dominated by fluvial input from the Var River, the Paillon River being only a minor source. The Var River provides very coarse bedload material (gravel and cobble) directly to the head of the Var Canyon. Gravel and cobble deposits are found all along the Var Canyon and the Upper Fan Valley of the Var submarine fan and have been shaped into gravel waves. The fine particles (suspension load) are separated from the coarse bedload upon entering the sea and form up to 60-m thick deposits on the uppermost continental slope of the Baie des Anges. These deposits are formed by settling out of sediment plumes. The presence of silt and fine sand laminae that decrease in thickness and frequency away from the Var River mouth indicate the influence of meso- and hyperpycnal flows on these plume deposits. Areas outside the Baie des Anges are not connected to major fluvial input and receive only hemipelagic sediments. These primary deposits are highly unstable and sediment failure due to seismic loading, sedimentary loading or undercutting is frequent. Sediment failure produces secondary sediment gravity flows that export most of the material to the basin, but also produce turbidity-current over-spill deposits on ridges bounding the slope canyons and on terraces within the Var Canyon. Slump and debris-flow deposits are also observed. At least some of these secondary flows erode the continental slope as cross-cutting chutes on the upper continental slope and erosional scours in the Upper Fan Valley demonstrate. Modern sediment dispersal patterns on the continental slope off Nice are proposed as a modern analogue to lowstand conditions on continental margins. In fact, the absence of a continental shelf together with a steep slope strongly reduces the influence of sealevel on the physiography of the margin.

During the SEDIFAN 1 cruise we surveyed the bathymetry and the acoustic properties of the surface sediment of the Celtic Deep Sea Fan. We also collected Kullenberg cores in order to study recent sedimentary processes. From the bathymetry survey it is relatively easy to recognize the main areas of modern fan. The upper fan included a large sedimentary ridge which constitutes the right levee of the prominent meandering Whittard valley. After its confluence with the Shamrock valley the course of the Whittard valley is abruptly deflected to the south. At a short distance to the south the valley divides into two upper-fan channels, the Celtic channel to the west being the deeper one. This point constitutes the centre of a radiating pattern which is developed on a 150 degrees quadrant and a radius of about 100 km. The acoustic imagery displays contrasted features, related to change in lithology within the first metre beneath the sea bottom and to the sea floor roughness. The Austell ridge exhibits a contrasted pattern of elongated areas with high and low acoustic backscattering levels. This pattern is related to the development of abyssal dunes, the amplitude of which is of metric order. Particularly remarkable is a lobe-shaped low back-scattering area in the western part of the middle fan, also noteworthy are a lineated facies to the west and a braided facies to the east of the fan. The laminated silty-clayey sequences deposited on the Whittard ridge and on the Trevelyan levee were deposited during the deglaciation. We interpret these as turbidity currents overflow deposits from the Whittard valley. At the end of isotopic stage 3 and during stage 2, the English Channel was a large plain flooded by the Channel River. During this period a broad delta developed at 100 m below the present-day depth and a wide spectrum of material was bound to be supplied to the deep sea and contributed particularly to the deposition of the Whittard ridge silty-clayey sequences. The stage 2 deposits are characterized by rhythmic levels enriched in monosulfides. These types of deposits are common in areas affected by fluvial discharges. Excluding the sedimentary ridge and the channel levees the surface deposits sampled with the Kullenberg corer are sandy. These sands are deposited in various contexts on the interfluve between the western and eastern channels and at channel mouths. They were emplaced during high sea level stands as a result of high energy gravity processes. The precise sources of these sands have not yet been identified, however benthic foraminifers from included ooze pebbles have living depths of between 500 and 1 000 m. The gravity processes which eroded this marry ooze may have been triggered on the upper slope. The Celtic shelf is presently a high energy platform where the conjunction of storms and spring tides call lead to enhanced sediment transport from near-shore to the deep sea. The relict or palimpsest deposits of the glacial delta also constitute a large reservoir of sandy material which can also be subject to reworking., Le programme Enam 2 (European North Atlantic Margin) concerne l'étude des processus sédimentaires quaternaires du Spitzberg au golfe de Gascogne. Dans le cadre de ce programme, la reconnaissance de l'Éventail profond celtique était l'objectif de la campagne Sedifan 1 au cours de laquelle nous avons établi la morphologie de l'éventail et obtenu une image acoustique des fonds sédimentaires. La morphologie permet de mettre en évidence une organisation en éventail. On note aussi le développement remarquable d'une ride sédimentaire au niveau de l'éventail supérieur. Les sédiments prélevés révèlent la présence de dépôts sableux, témoins d'une activité récente qui pourrait être liée à l'importance de l'hydrodynamisme sur les Grands Bancs de la plate-forme celtique.

Seismic data collected on the Celtic Fan during Sedifan cruises (European Program Enam II), stratigraphically correlated to DSDP site 400 (leg 48) show that fan deposition was initiated during the Early Miocene. It progressively extended towards the middle of the Bay of Biscay during the Plio-Quaternary. The fan includes 3 main seismic units with several channel/levee bodies disturbed by mass-movement deposits. Depocenters shifted several times, and the present-day morphology ultimately results from a last shift of detritic inputs to the Whittard System.

Core description is used extensively by sedimentologists to yield critical information from sediments. It consists of a naturalistic description of the visual and textural characteristics of the sediment. It is generally a manual procedure. However, digital core descriptions are now required re manage data processing. Hand-written description can be keyed into a computer, but it is tedious and time consuming. Moreover, recording and typing errors can occur. A system is presented for digital description of sediment cores using a bar code principle. It can be used in the laboratory as well as on board vessels. It comprises a box inside which the half-core can be placed. A ruler and several sets of bar codes are printed on the inside box top. Bar codes are read by a waterproof sensor pencil that is interfaced with a PSION 3A Docket computer. The software mimicks the traditional method of cope description, the geologist answering questions using the optical pencil. It offers versatility and is designed with the geoscientist's need for flexibility and ease of use in mind. It allows computerised core data management and is highly upgradable. It can help to harmonise core description and to compare it with core data provided by the new multiparameter measurement systems. Detailed log plots no longer require tedious coding. Logmeter data can be used to automatically create customized, full-featured log graphs displaying lithology, descriptions, curves, histograms, graphic symbols and attractive legends, using commercial or dedicated softwares., Le logmètre est un outil de saisie de données basé sur l'utilisation de codes-barres. Il peut être utilisé indifféremment à bord des navires ou en laboratoire pour la description des carottes de sédiments. Le système se présente comme un boîtier à l'intérieur duquel on place la demi-carotte à décrire. Le couvercle intérieur du boîtier comporte une règle graduée et différents ensembles de codes-barres qui sont lus au moyen d'un stylo laser interfacé avec un ordinateur de poche de type Psion 3A. L'ordinateur est programmé pour reproduire une séquence de description type. L'opérateur répond aux questions en utilisant soit le crayon optique, soit le clavier. Ce système simple et évolutif permet d'harmoniser les descriptions de carottes en vue de leur traitement informatique et apporte un gain de temps et de fiabilité par rapport à la procédure manuelle classique.

The 1979 Nice turbidity current is modelled using a visco-plastic analysis of flow velocity because the initial flow concentrations are expected to have been very high. The complete history of the failed sediment from debris flow to turbidity current plume is therefore addressed. The turbidity current portion is considered as a steady state flow divided into a dense bottom flow and an upper plume. Model results show that a dense flow can be generated from the debris flow by the disaggregation of the initial slide. The dense flow would be strongly erosive and able to create and maintain a low-density plume at its surface. The depth of erosion of the channel floor by the dense flow is predicted to reach 6–11 m in overconsolidated sediments, with the main erosion taking place in Var Canyon and the Upper Fan Valley. The eroded volume (108 m3) provides additional material to the sediment mass of the initial failure. The dense flow appears able to inject fine sand and silt into the overlying plume during 90 km, and would disintegrate before being able to deposit sediment. The extensive sand layer along the travel path of the turbidity current may have been deposited from the tail of the trailing plume: a result of the velocity difference between the plume and the dense flow. Observations on sedimentary structures, erosion features and distribution of the sand deposit are quite in agreement with our modelling approach. For example, gravel waves can be generated when loose deposits are reworked by the supercritical dense flow. The methodology and equations presented here provide a good estimate of the geological consequences of a high-velocity gravity flow undergoing rheological transition.

The Pasisar seismic acquisition system combines a source at the sea surface and a deep-towed single channel streamer. This unconventional device geometry reduces the width of the first Fresnel zone which increases the lateral resolution. However, the device acquisition geometry generates artifacts on seismic profiles and induces large incidence angles of the seismic signal. A specific processing sequence must be applied to the data to obtain a readable seismic section. Penetration of the seismic signal depends on the energy of the signal reaching the seafloor and on its incidence angle. Because of smaller source energy, 800 Joules Sparker data cannot be acquired in water depth larger than 1500 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seismic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution of any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as signal-to-noise ratio obtained with both the Pasisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep-towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal reflector.

Conventional near‐surface seismic systems provide limited resolution of ocean bottom sedimentary horizons. This problem is related to the size of the Fresnel zone and is particularly important in deep water or topographically rugged regions. The French Institute for Marine Research and Exploitation (IFREMER) has recently investigated methods to overcome this limitation.

International audience; The Var Turbidite System (NW Mediterranean Sea) is fed during the present-day highstand sea level by large earthquake-induced ignitive turbidity currents, low-density turbidity currents resulting from retrogressive failures triggered on the upper continental slope, and hyperpycnal flows related to the Var River floods. Using a large dataset including bathymetric data, side-scan sonar images, seismic-reflection profiles, cores and photographs of the seafloor, this paper attempts to better constrain the hydrodynamic behaviour of debris flows and turbidity currents along the Upper and Middle Valley of the Var Turbidite System. The drastic change of the seafloor morphology between the Upper and the Middle Valley suggests that gravity flows undergo rapid transformation from cohesive to fully turbulent behaviour. This transformation is related to a hydraulic jump caused by an abrupt decrease in slope angle at the transition between the Upper and the Middle Valley and is associated with en masse deposition and elevation of the seafloor. Strong seafloor erosion prevails in the Middle Valley, suggesting that, for a low and constant slope angle, turbulent flows must regain a balance between concentration and flow thickness rapidly after they experience hydraulic jump. The internal stratification and vertical grain-size distribution within turbulent flows are inferred from the distribution of fine- to coarse-grained turbidites found in cores located along the crest of the Var Sedimentary Ridge with a decreasing elevation above the floor of the Middle Valley. The theoretical vertical velocity profile deduced from the vertical grain-size distribution exhibits a general trend and an inflection of the gradient curve different from those of the velocity profiles classically obtained using numerical modelling.

Recent oceanographic surveys revealed the existence of five volcaniclastic deep-sea fans off La Reunion Island. The Cilaos fan is a large volcaniclastic submarine fan, connected to rivers that episodically experience torrential floods through a narrow and steep shelf-slope system. New piston cores presented in this study together with echosounder profiles give new insight into the evolution, of this extensive and sand-rich turbidite system. The Cilaos fan extends over 15,000 km(2) on an abyssal plain and is compartmentalized by topographic highs. Located southwest of the island, the sedimentary system consists of a canyon area and a deep sea fan divided into a proximal and a distal fan. The proximal fan is characterized by its wide extent and coarse-grained turbidites. The distal fan is characterized by elongated structures and fine-grained turbidites. A detailed morphological study of the fan which includes the analysis of swath bathymetry, backscatter, echosounder, and piston core data shows that the Cilaos fan is a complex volcaniclastic deep-sea fan, highly influenced by preexisting seafloor irregularities. The canyons and the slope area show a complex and evolving sediment feeding system with a direct sediment input by the river and irregular sediment supply by submarine landslide. Three main construction stages are identified for this system: (1) an old incision phase of the channels forming wide turbidites extending over the entire distal fan; (2) a period of no or low activity characterized by a thick layer of hemipelagic mud; and (3) a local reactivation of the channel in the proximal fan. Each stage seems to be linked to a different sediment source with a progressively increasing contribution of hemipelagic sediment and mud in younger stages. (C) 2011 Elsevier B.V. All rights reserved.

The present study provides an overview of recent sedimentation patterns on the central Algerian continental margin. Recent sedimentation patterns were assessed from morphological analysis, which is based on swath bathymetry and echo-facies mapping. It appears that sedimentation along the Algerian margin is controlled by two processes:(1) gravity-induced processes, including both mass-transport deposits and turbidity currents, and (2) hemipelagic sedimentation. Mass-transport deposits occur on the Algerian margin at the canyon heads and flanks, in the interfluve areas between canyons, along the seafloor escarpments, and on the flanks of salt diapirs. Mass-transport deposits (MTDs) sampled by coring consist of a variety of soft and hard mud-clast conglomerate and turbidite deposits. MTDs are mostly localized at the toes of steep slopes, where thrust faults were previously identified and mapped. Analysis of the spatial distribution of MTDs and their recurrence in time help reconstruct the main predisposing factors and triggering mechanisms, and evaluate their impact on evolution of the Algerian margin

International audience; A new geophysical data set off La Réunion Island (western Indian Ocean) reveals a large volcaniclastic submarine fan developing in an open-ocean setting. The fan is connected to a torrential river that floods during tropical cyclones. Sediment storage at the coast is limited, suggesting that the sediments are carried directly to the basin. The fan morphology and turbidites in cores lead us to classify it as a sand-rich system mainly fed by hyperpycnal flows. In the ancient geological record, there are many examples of thick volcaniclastic successions, but studies of modern analogues have emphasized mechanisms such as debris avalanches or direct pyroclastic flow into the sea. Because the Cilaos deep-sea fan is isolated from any continental source, it provides information on architecture and noncatastrophic processes in a volcaniclastic deep-sea fan.

Article history:Received 31 May 2010Received in revised form 25 October 2010Accepted 25 October 2010Available online 31 October 2010Communicated by D.J.W. PiperKeywords:last deglaciationoverbank depositsturbidity currentsVar Sedimentary RidgeLigurian Sea The Var turbiditic system located in the Ligurian Sea (SE France) is an intermediate mud/sand-rich system.TheparticularityoftheVardeep-seafanisitssinglechannelwithabruptbendsanditsasymmetricandhyper-developed levee on the right hand side: the Var Sedimentary Ridge. Long-term sediment accumulation on theVar Sedimentary Ridge makes this an ideal target for studying the link between onshore climate change anddeep-sea turbidite stratigraphy. This paper focuses on the establishment of the first detailed stratigraphy ofthe levee, which is used to analyze the timing of overbank deposition throughout the last deglaciation. Mainresults indicate that high variability in turbidite frequencies and deposition rates along the Var SedimentaryRidgearedeterminedbytwomainparameters:1)theprogressivedecreaseoftheleveeheightcontrollingtheabilityofturbiditycurrentstospilloutfromthechannelontothelevee,and2)climaticvariationsaffectingthedrainage basin, in particular changes in glacial condition since late Last Glacial Maximum to early Holocene.Compared to other deep-water areas, this study confirms the ability of turbiditic systems to record pastclimatic events on millennial timescales, and underlines the influence of European deglaciation on theobserved decrease in turbidite activity in the Var canyon. The presence of a very narrow continental shelf andasingle,largechannel-leveesystemmakestheVarSedimentaryRidgeauniqueexampleofclimate-controlledturbiditic accumulations.© 2010 Elsevier B.V. All rights reserved.

The Var deep-sea fan was deposited during the Pliocene and Quaternary seaward progradation of the Var delta within a steep-sided, flat-floored basin formed during the Messinian salinity crisis. The prominent palaeo-Var canyon was cut on the slope at the same time. Following the Early Pliocene transgression, Var sediments were initially trapped in the coastal ria formed by sea-level incursion of the head of the Messinian valley. By Middle Pliocene time Gilbert-type deltas had prograded to the break of slope (modern coastline) allowing coarse sediment to reach the basin. Quaternary coastal uplift and sediment supply from the glaciated Alps resulted in a supply of coarse sediment to the fan by the braided Var River throughout the Quaternary, even during sea-level highstands such as the present. In the earliest Pliocene, hemipelagic clays accumulated while coarse sediment was trapped in the Var ria. Through the rest of the Pliocene, a leveed fan valley extended southward from the Var canyon, which periodically switched locations through avulsion to the east. At about the beginning of the Quaternary, this eastward spillover channel became the main distributary and re-occupation of the older south-trending channels was blocked by rapid levee progradation and aggradation of mud waves by overbanking of turbidity flows on the Var sedimentary ridge. During the Quaternary, channel shifting to the north occurred. The Quaternary Var fan has an unusually steep concave longitudinal profile compared with most passive margin fans. The Laurentian fan has a similar profile to the Var; both have a high levee and a gravel and pebble-floored fan valley that is essentially a bypass zone, compared with the aggradational channel-levee systems (with lower levees) in most passive margin fans. This is a consequence of the constant supply of coarse sand, gravel and pebbles to the fan, even during highstands of sea level. The erosive capability of this coarse sediment in ignitive turbidity currents resulted in flushing out of the upper and middle fan valley system. Thick muddy turbidity currents, probably resulting from slumping caused by canyon incision, produced the prominent mud waves on the levee. The concave longitudinal profile led to rapid sediment deposition at critical points in the valley system as a result of turbidity flow expansion; this in turn promoted the development of a left hook in the valley system.

High-resolution seismic boomer profiles, with a vertical resolution of less than 1 m, together with piston cores and previous side-scan sonar data, are used to describe late Quaternary sedimentation on the Var deep-sea fan. Chronological control is provided by foram biostratigraphy and radiocarbon dating in cores, and is extended over the fan by seismic correlation. Regional erosional events correspond to the oxygen isotopic stage 2 and 6 glacial maxima. Cores and seismic data define a widespread surface sand layer that is correlated with prodelta failure in 1979 and subsequent submarine cable breaks. Numerical modelling constrains the character of this 1979 turbidity current. It originated from a relatively small slide on the upper prodelta that put sufficient material in suspension to form an accelerating turbidity current which eroded sand from the Var Canyon. The turbidity current was only 30 m thick on the Upper Valley, but experienced significant flow expansion in the Middle Valley to thicknesses of more than 120 m, where it spilled over the eastern Var Sedimentary Ridge at a velocity of about 2·5 m s−1. Other Holocene turbidity currents (with a recurrence interval of 1000 years) were somewhat muddier and thicker, but also deposited sand on the levees of the Middle Valley, and are inferred to have had a similar slide-related origin. Late Pleistocene turbidity currents deposited thick mud beds on the Var Sedimentary Ridge. The presence of sediment waves and the mean cross-flow slope inferred from levee asymmetry indicates that some of these flows were many hundreds of metres thick and flowed at velocities of about 0·35 m s−1. This contrast with Holocene turbidites suggests that a slide origin is unlikely. Estimated times for deposition of thick mud beds on the levees are many days to weeks. The Late Pleistocene flows may therefore result from hyperpycnal flow of glacial outwash in the Var River. The variation in the Late Pleistocene to Holocene turbidite sedimentation is controlled more by variations in sediment supply than by sea-level change.

The slope off Nice (French Riviera) is characterized by the progradation of a large deltaic sedimentary body fed by the Var River. It principally consists of fine‐grained plio‐quaternary deposits interbedded with conglomerates. These deposits are cut by deep valleys and canyons and have been submitted to degradation through active mass‐wasting processes at water depth of 20–500 m. Several sediment facies are differentiated and classified by laboratory‐determined mass physical and geotechnical properties of nineteen gravity core samples. This classification is used in order to produce a geotechnical map of the surficial sediment of the area. The study of consolidation states of these sediments reveals a sedimentary hiatus appearing under Holocene deposits on the eastern wall of the major canyon (Var Canyon) between 900 and 1500 m water depth. This sedimentary hiatus, which extends for several kilometers, corresponds to an erosion surface rather than an omission surface according to the overconsoli...

Thin Quaternary sediments overlie Neogene bedrock on the shelf off Antibes, southeastern France. Sparker seismic profiles show shelf break progradation at 110 m water depth, immediately below the most recent postglacial transgression. An older progradational event at the same depth correlates with the top of an acoustically, well-stratified sequence on the slope dated at 29.6 ka. Cores show the post-glacial transgressive surface overlain by shelly sands (dated at 12.7 ka at 85m) and several meters of mud. At the present, the seabed consists of an ephemeral cover of fine sand. The data set on sea-level rise complements data recently obtained on land on the Holocene transgression.

International audience; Sinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a "reservoir" scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels.

The Var prodelta progrades across a straight, narrow shelf (less than 2 km wide) with a very steep continental slope reaching locally more than 30°. Historically, the Var delta front is sadly famous for the 1979 catastrophic submarine landslide that resulted in several casualties and infrastructural damage. Geotechnical and geophysical investigations carried out in late 2007 to the east of the 1979 landslide scar provide evidence for the possible occurrence of a new important sedimentary collapse and submarine landslide. Geophysical data acquired in the area show the presence of several seafloor morphological steps rooted to shallow subsurface seismic reflections. Moreover, in situ piezocone measurements demonstrate the presence of several shear zones at the border of the shelf break at different depths below the seafloor. The aim of this technical note is to present and discuss acquired geotechnical and geophysical data in terms of failure mechanisms and submarine landslides. Both geophysical and geotechnical data suggest the start-up of a progressive failure mechanism and reveal the possible occurrence of a submarine landslide and the urgent need for mitigation procedures.

International audience; The Congo deep-sea fan is one of the largest fans in the world still affected by presently active turbidity currents. The present activity of deep-sea sedimentary processes is linked to the existence of a direct connection between the Congo River estuary and the Congo canyon head that allows relatively continuous sediment feeding of the deep-sea environment, in spite of a wide continental shelf (150 km). Because of this important activity in terms of sedimentary processes, the deep-sea environment of the Congo–Angola margin presents major interests concerning physical, chemical and biological studies near the seafloor. The main aim of this paper is to present the initial geological context of the BioZaire Program, showing a synthesis of the major results of the ZaïAngo Project including (1) the brief geological setting of the Congo–Angola margin, (2) the structure of the modern Congo deep-sea fan, (3) the sedimentary architecture of the recent Congo turbidite system (from the canyon to the distal lobes) and (4) the recent and present turbidite sedimentation. In order to provide useful information and advice relevant to biological and geochemical studies across the Congo sedimentary system, this article is particularly focused on the present sedimentary processes and the present activity of turbidity current along the Congo canyon and channel.

International audience; The West African continental slope is an important theatre for geological survey prospecting and drilling for hydrocarbons but little is known about local deep-sea biological communities at these depths. While shallow-water reefs are common and well-known features in the tropics, only few records of deep-water corals exist at low latitudes, and most of them have been reported by historical oceanographic cruises undertaking circum-navigations of the world. This study, based on a multi-disciplinary approach, presents a description of newly discovered deep-water coral reef communities along the Angola margin. Data from ROV, multibeam bathymetry, side-scan sonar and seismics from a deep-towed acoustic system (SAR) were used to describe the morphology of the coral mounds and their relationship with the local geological setting. The reef-building scleractinian coral Lophelia pertusa has colonised carbonate mounds that reach heights of ca. 30 in and follow an orientation that is correlated with salt tectonic processes. Recent erosion is suggested as a process that influences the shape of the mounds. Sixteen fish taxa were identified during the ROV video surveys, with some of them likely to have a strong affinity with dense-living corals. The species observed belong to families commonly associated with deep-water corals (i.e. Sebastidae, Berycidae, Lophiidae and Chaunacidae), except an abundant species belonging to the family Zoarcidae, rarely observed in this type of environment. Lucinidae shells were found around mounds. As this bivalve family is indicative of reduced sediment and generally associated with cold-seep environments, this finding could revive the debate over the relationship between the distribution of cold-water coral habitat and gas seeps. However, there is no present-day nutritional relationship between living coral and chemosynthetic-derived biomass. The possible role of fluid expulsion in carbonate precipitation acting as the first step for coral colonisation is nevertheless discussed

Seismic reflection profiles have been used to map the Messinian erosion surface and associated basin-margin deposits on the continental margin off Nice. A palaeovalley extends seaward from the incised Var palaeovalley mapped on land. The lower part of this valley is filled by an acoustically incoherent unit with a flat top and steep slope into the basin: this unit is interpreted as a coarse-grained deltaic deposit that accumulated synchronously with the upper Messinian evaporites in a ponded lake basin. Two submersible dives on strategic outcrops supported the interpretation of the seismic profiles. We examined and sampled outcrops of probably early Tertiary sandstones underlying the Messinian erosion surface and of a fining-up, predominantly conglomeratic sequence that fills Messinian channels on the continental slope and probably overlies the deltaic deposits. These conglomerates pass up into Early Pliocene marls and have turbidite sedimentary structures. These are prodeltaic deposits formed during the transgression at the end of the Messinian. Based on seismic facies mapping, sampling and submersible observations, a palaeogeographic reconstruction of the area is proposed. A basal erosion surface, including the Var palaeovalley, represents extreme dessication of the western Mediterranean and was followed by accumulation of the main Messinian salt. Following this, a lake was ponded in the Ligurian Basin, with accumulation of evaporites and shales. A Gilbert-type delta prograded into the basin from the Var palaeovalley. Relative sea (lake)-level lowering is estimated to be about 1200 m at this time. There has been little faulting on the margin since the Early Pliocene transgression, but there is estimated to have been about 800 m of flexural subsidence due to both thermal subsidence and water loading.

International audience; The Amazon Fan is one of the largest river-fed submarine fan systems in the world. It is located in the Western Equatorial Atlantic and extends 1100 km northward from the Amazon canyon head located at the shelf break off northern Brazil. Numerous previous investigations on the Amazon Fan mostly focused on the upper and middle parts of the fan. Two recent cruises have provided a new dataset based on bathymetric, acoustic imagery and 3.5 kHz sub-bottom profiler data. It is the first detailed exploration of the youngest Amazon channel-mouth lobe complex that remained the “missing piece” of one of the best studied modern deep-sea fans. The stratigraphic framework of the surveyed channel-mouth lobe complex of the Amazon Fan consists of eight channel-levee systems and their associated channel-mouth lobe systems (from Brown to Amazon). These depositional systems record a complex depositional history in a short time period between the Last Glacial Maximum (not, vert, similar 20,000 cal yr BP) and the rise in sea-level until 10,100 cal yr BP, when sediment supply to the fan ceased. The main body of the lobes is characterised by a general low to average backscatter facies (from white to light grey) scattered with higher backscatter fingers (dark grey). Two types of lobes are distinguished based on the planform shape of their backscatter facies: (1) elongated and narrow lobes with branching patterns like the veins of a leaf and (2) radial lobes with crescent-shaped features opened in the downstream direction, that do not have a clear organisation within the lobe. The eight channel-levee systems and associated lobes were built in a time span of not, vert, similar 10 ka and with an average time between successive avulsions of 1250 years. The life span of lobes in the Amazon system is around 600 years which is very short compared to the life span of the fan as a whole. The observed wide range in channel lengths and lobe sizes suggests that each system had a variable time duration and/or sediment supply rate. The variability of the channel lengths could be controlled by internal and/or external forcings linked to processes occurring at different stages in the system in the Amazon drainage basin, on the Amazon shelf, at the canyon head or at the channels bifurcation point. Numerous channel-levee systems and lobes are concentrated in a relatively narrow area. Locally, the pre-existing morphology of the distal depositional area influences the shape and the location of lobes, providing an argument for strong topographical control where local gradient and degree of topographic confinement are key control parameters.

The continental rise southeast of the Grand Banks of Newfoundland was surveyed many times during the last 8 years, especially during the search area for the wreck of the Titanic . High-resolution sidescan sonar images (systeme acoustique remorque, SAR) have shown an area characterised by complex surficial morphology, referred to as in “instability corridor”. New seismic reflection profiling has shown that the instability corridor is more extensive than was previously believed and that it is the expression of the distal part of a large sediment slide. It consists of upslopedipping imbricated sediment blocks which are tens of metres thick and many kilometres in length. Downslope from the slide is an acoustically incoherent unit interpreted as comprising debris flow deposits which could represent the final phase of evolution of the slide. The slide is blanketed by 5–10 m of younger sediment. The upper part of this blanket has been recovered by coring and consists of foram ooze, thin sandy turbidites and muddy beds with mudstone clasts. Radiocarbon dating and foram assemblages show that the upper metre of the core is late Holocene in age, with about eight resedimented beds forming in the last 4000 years. These recent turbidity currents cut erosional furrows and refresh the pre-existing steps of the slide. Three phases characterise the evolution of the instability corridor: (1) the major event involving a giant slide of bedded sediments up to 100 m thick, with a source probably on the steep continental slope of the Grand Banks 100 km to the north, (2) a draping of normal continental margin terrigenous sediments, and (3) erosion by one or a series of high-density turbidity currents. The giant slide created an instability corridor within which a number of channeled and non-channeled events took place during the late Holocene.

International audience; In 1979, a catastrophic event occurred on the Nice continental slope (French Riviera) generating the lost of human lives and important material damages. Part of the new harbour constructed at the edge of the International Airport of Nice collapsed into the sea. The main aim of this work was 1) to present a review of facts and details related to the 1979 accident and a review of the geological setting, and 2) to evaluate the slope stability before and after the new harbour construction, by taking into account new available data such as sediment cores and piezocone CPTU data. The CPTU data were of great value to understand the origin of the 1979 event. They show the existence of a sensitive clay bed between 30 mbsf and 45 mbsf. Under high deviatoric load a sensitive clay layer underwent an important creep, which dramatically decreased its resistance and caused the slope failure. This working hypothesis was supported by the good agreement between the maximum thickness of the failure surface and the depth of the sensitive clay layer. Slope stability assessment using the finite element model Femuslope show the metastable state of the Nice slope before the harbour extension. Numerical calculations demonstrated that creeping of the sensitive clay layer could be at the origin of the 1979 slide. In addition, the exceptionally heavy rainfall which occurred before the accident and consequently the seepage of fresh water probably induced the decrease of the effective stress and accelerated sediment creeping and triggered the Nice slope failure. A progressive and relatively long-term creeping failure scenario is in good agreement with the official report mentioning cracks, settlements, failures and embankment collapses occurred during land filling operations.

The Golo Margin in eastern Corsica is dissected by four canyons and two gullies which fed turbidite systems. Study of the dispersal of surficial sediments and flow dynamic in the Golo system is based on Kullenberg and interface cores interpreted in relation to a previously published seismic dataset. Cores were described in detail and interpreted within a sedimentary and stratigraphic framework. During the last 42,000 years, gravity processes which occurred in the large systems with a canyon source were mainly slide-induced, differentiated turbulent surges and hyperpycnal flows. Processes occurring in the small system with a gully source are mainly hyperconcentrated and concentrated flows. Deposits from the Corsican Margin can intercalate with products of processes triggered on the Pianosa Ridge located in the eastern part of the basin. During relative sea-level lowstands or during periods of rapid or high-amplitude sea-level fall, only large canyons (South and North Golo) are supplied by carbonate-rich hyperconcentrated and concentrated flows which are channelled in incised valleys on the shelf. During periods of slow or low-amplitude sea-level fall and during sea-level rise, sediments are trapped on a shelf delta and intensely winnowed by shelf hydrodynamic processes. Sand-rich hyperconcentrated and concentrated flows occur. All the systems fed by a canyon are active simultaneously. Gullies form and are active only during periods of sea-level rise. During relative highstands of sea level (Holocene), all the system is draped by hemipelagic sediments. Relative sea-level changes and canyon location relative to river mouths have a strong influence on the nature of sediment input, and the initiation and type of gravity flows which, in turn, control morphology and geometry.

We present new results from the MARADJA'03 cruise depicting the geological structures offshore central and western Algeria. Using swath bathymetry and seismic reflection data, we map and discuss the offshore limits of the Internal Zones corresponding to relics of the AIKaPeCa domain that drifted and collided the African plate during the Miocene. We identify large reverse faults and folds that reactivate part of these limits and are still active today. The morphology of the westernmost NE-SW margin suggests a former strike-slip activity accommodating a westward block translation responsible for the shift of the Internal Zones towards the Moroccan Rif., Nous présentons les résultats récents de la campagne MARADJA'03, qui visent à mettre en évidence les structures géologiques dans le domaine marin au nord-ouest de l'Algérie. Grâce aux données de bathymétrie multifaisceau et de sismique réflexion, nous cartographions et discutons les limites en mer des Zones internes correspondant aux reliques du domaine AlKaPeCa qui a dérivé, puis est entré en collision avec la plaque africaine au Miocène. De grandes failles inverses et plis, actifs dans le champ de contrainte actuel, réactivent certaines de ces limites. La marge ouest-algérienne, orientée NE¿SW, indique la présence d'une ancienne activité en décrochement ayant accommodé la translation des Zones internes vers l'ouest.

[1] We investigate the active seismogenic fault system in the area of the 2003 Mw 6.9 Boumerdes earthquake, Algeria, from a high-resolution swath bathymetry and seismic survey. A series of 5 main fault-propagation folds ∼20–35 km long leave prominent cumulative escarpments on the steep slope and in the deep basin. Fault activity creates Plio-Quaternary growth strata within uplifted areas such as a rollover basin on the slope and piggyback basins in the deep ocean. Most thrusts turn to fault-propagation folds at the sub-surface and depict ramp-flat trajectories. We find that the two main slip patches of the 2003 Mw 6.9 Boumerdes earthquake are spatially correlated to two segmented cumulative scarps recognized on the slope and at the foot of the margin. The overall geometry indicates the predominance of back thrusts implying underthrusting of the Neogene oceanic crust.

Reflection seismics has been intensively used for the last four decades by marine geologists and geophysicists for imaging Earth structures below the seafloor. Because their subject of interest is below the sea bottom, solid Earth scientists do not usually consider the seismic signal propagating in the water column and most often do not even record it, in order to save data storage space. Two physical oceanographers, Gonella and Michon [1988],first reported internal waves revealed by reflection seismics in the northeastern Atlantic. Only recently, though, has the scientific community realized the importance of this issue, after Holbrook et al. [2003] published reflection seismic sections of the water column off Newfoundland, Canada, showing reflectors related to the major oceanographic front between the Labrador Current and the North Atlantic Current.

Detailed study of fine-grained migrating sediment waves and processes of sediment deposition along the modern, active, meandering channel of the Zaire turbidite system was made on recent cruises. The data used in this study include EM12 bathymetry and backscatter imagery, 3.5 kHz profiles, high-resolution seismic-reflection profiles and Kullenberg piston cores. Migrating sediment waves are built under several different flow energies and hydrodynamic conditions (subcritical or supercritical condition) according to their location along the modern Zaire channel. The process of sediment-wave initiation is inferred from cores and high-resolution seismic-reflection profiles. Overflow density and sediment supply delivered to levees appear as critical parameters for wave initiation. In other cases, a preexisting wavy topography is observed that acts as a template favoring sediment-wave initiation. Core-to-core correlation between the two flanks of a single sediment wave reveals no major differences in the lithofacies, suggesting no change of the dynamics of overflows during sediment deposition on both flanks of the waves. Comparison of number, thickness, and type of individual beds in cores collected on both flanks of the wave suggests that most lower-velocity and density overflows depositing sediments on the upstream flank bypass on the downstream flank. This process is probably related to a strong decrease then increase of the bed shear stress on the upstream and downstream flank of the wave, respectively. Interpretation of lithofacies suggests that construction of migrating sediment waves is closely linked to repeated successive spillovers from the head and body of a single channelized turbidity current. Construction of a single wave is rapid, less than 5000 years.

A large turbidity current was detected in the Zaire submarine valley at 4000 m water depth. Current meters, turbidimeter and sediment trap deployed on a mooring located in the channel axis, although they were damaged, recorded the signature of a very high energy event. An average velocity of more than 121 cm s−1 was measured 150 m above the channel floor. Coarse sand and plant debris were collected at 40 m height. The turbidity current clearly overflowed the edges of the valley as demonstrated by the large quantity of turbiditic material (464 mg organic carbon m−2 d−1) found in the sediment trap moored 13 km south from the channel axis.

The Celtic Fan (Evans 1996; Droz et al. 1999; Auffret et al. 2000) is a radial turbidite accumulation at the mouth of the two main systems of canyons of the Celtic margin: the Whittard System to the west and the Shamrock System to the east (Fig. 1). Open image in new window Fig. 1. a The Celtic Fan was fed by the two main systems of canyons of the Celtic margin: Shamrock system on the southeastern side, and Whittard system on the northwestern side of the margin. On the outer shelf, the Celtic sand banks are remnants of the large delta of the “Fleuve Manche”, which supplied the fan with coarse sediments during part of its development. The main channel of the fan, the Celtic Channel, is presently connected to the Whittard Canyon. b Fan deposition was periodically disturbed by mass movements (debris-flows) that only affected fan fringes and did not strongly influence the architecture of the fan. SEDIFAN cruises, in the framework of ENAM II program collected, among other data, 6-channel high resolution (blue lines) and PASISAR (red lines) very high resolution seismic profiles.