The failure propagation of weakly stable sediment: A reason for the formation of high-velocity turbidity currents in submarine canyons

The long-distance movement of turbidity currents in submarine canyons can transport large amounts of sediment to deep-sea plains.Previous studies show obvious differences in the turbidity current velocities derived from the multiple cables damage events ranging from 5.9 to 28.0 m/s and those of field observations between 0.15 and 7.2 m/s.Therefore,questions remain regarding whether a turbid fluid in an undersea environment can flow through a submarine canyon for a long distance at a high speed.A...     

尼日利亚西部大陆边缘海底峡谷下部结构研究

Multi-beam,sub-bottom and multichannel seismic data acquired from the western Nigerian continental margin are analysed and interpreted to examine the architectural characteristics of the lower parts of the submarine canyons on the margin.The presence of four canyons: Avon,Mahin,Benin,and Escravos,are confirmed from the multi-beam data map and identified as cutting across the shelf and slope areas,with morphological features ranging from axial channels,moderate to high sinuosity indices,scarps,terraces and nickpoints which are interpreted as resulting from erosional and depositional activities within and around the canyons.The Avon Canyon,in particular,is characterised by various branches and sub-branches with complex morphologies.The canyons are mostly U-shaped in these lower parts with occasional V-shapes down their courses.Their typical orientation is NE–SW.Sedimentary processes are proposed as being a major controlling factor in these canyons.Sediments appear to have been discharged directly into the canyons by rivers during the late Quaternary low sea level which allows river mouths to extend as far as the shelf edge.The current sediment supply is still primarily sourced from these rivers in the case of the Benin and Escravos Canyons,but indirectly in the case of the Avon and Mahin Canyons where the rivers discharge sediments into the lagoons and the lagoons bring the sediments on to the continental shelf before they are dispersed into the canyon heads.Ancient canyons that have long been buried underneath the Avon Canyon are identified in the multichannel seismic profile across the head of the Avon Canyon,while a number of normal faults around the walls of the Avon and Mahin Canyons are observed in the selected sub-bottom profiles.The occurrence of these faults,especially in the irregular portions of the canyon walls,suggests that they also have some effect on the canyon architecture.The formation of the canyons is attributed to the exposure of the upper marginal area to incisions from erosion during the sea level lowstand of the glacial period.The incisions are widened and lengthened by contouric currents,turbidity currents and slope failures resulting in the canyons.     

Effect of Development of Caofeidian Harbor Area in Bohai Bay on Hydrodynamic Sediment Environment

Based on the characteristics of waves, tidal currents, sediment and seabed evolution in the Caofeidian sea area in the Bohai Bay, a 2D sediment mathematical model of waves and tidal currents is employed to study the development schemes of the harbor. Verification of spring and neap tidal currents and sediment in the winter and summer of 2006 shows that the calculated values of tidal stages as well as flow velocities, flow directions and sediment concentration of 15 synchronous vertical lines are in good agreement with the measured data. Also, deposition and erosion of the sea area in front of Caofeidian ore terminal induced by suspended load under tidal currents and waves are verified; it shows that the calculated values of depth of deposition and erosion as well as their distribution are close to the measured data. Furthermore, effects of reclamation scheme of island in front of the land behind Caofeidian harbor on the hydrodynamic environment are studied, including changes of flow velocities in the deep channels at the south side of Caofeidian foreland and Laolonggou and in various harbor basins, as well as changes of deposition and erosion of seabed induced by the project.     

A buried submarine canyon in the northwest South China Sea:architecture,development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous, undocumented submarine canyon(Huaguang Canyon) in the Qiongdongnan Basin, northwest South China Sea. The Huaguang Canyon with a NW orientation is 140 km in length, and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach. The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the central canyon. This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene(～10.5 Ma). The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections, indicating that this ancient submarine canyon was filled with coarse-grained sediments. The sediment was principally scourced from the Xisha carbonate platform. In contrast to other buried large-scale submarine canyons(central canyon and Zhongjian Canyon) in the Qiongdongnan Basin, the Huaguang Canyon displays later formation time, smaller width and length, and single sediment supply. The coarse-grained deposits within Huaguang Canyon provide a good environment for reserving oil and gas, and the muddy fillings in Huaguang Canyon have been identified as regional caps. Therefore, Huaguang Canyon is potential area for future hydrocarbon exploration in the northwest South China Sea. Our results may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.     

Submarine canyons on the north of Chiwei Island: influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified:the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea (ECS) slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initiated during the middle Pleistocene and guided by F4 that is a N-S trending fault on the slope and F1, a large NW-SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE-trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by F5, a N-S trending fault, diverted and blocked by the submarine terrace.     

Morphology and architecture of the typical erosion-genesis submarine canyons on the East China Sea slope

The Yithi submarine canyons,composed of four canyons less than 60 km in length,are located on the narrowest part of the East China Sea(ECS) slope.They extend from the shelf break at 160 m down to water depth of 1 500 m with an average gradient(along the canyon axis) of 3°(<1 000 m) and 0.7°(>1000 m).The sinuosity of the canyons ranges form 1.02 to 1.14 and their pathways extend radially from the shelf break to the axis of the Okinawa Trough.Structural and evolution pattern of the Yithi canyons are mainly controlled by sediment mass-movements and turbidity current and similar with that of the canyons in Ebro continental slope.The whole canyon system consists of three parts:the canyon,the channel and the fan.Slumps and slides often develop in the upper part of canyon where the water depth is less than 1000 m,and the turbidities usually developed on the fan.The scale of turbidites becomes smaller and their inner structures become more regular towards the ends of the canyons.Canyon-fans are often associated with small angle progradational reflection.Most canyon-fans and levees were transversely cut by active normal faults with NEE-SWW trending that are coupled to the modern extension of the Okinawa Trough.According to the age of formation of canyon-fans and sediments incised by canyons,we can infer that the Yithi canyons were formed since the middle the Medio-Pleistocene.     

Submarine canyons on the north of Chiwei Island:influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified： the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea （ECS） slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initia- ted during the middle Pleistocene and guided by F4 that is a N--S trending fault on the slope and F1, a large NW--SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE -trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by FS, a N-S trending fault, diverted and blocked by the submarine terrace.     

Sediment Deposition and Resuspension in Mouth Bar Area of the Yangtze Estuary

—A comprehensive analysis is conducted based on observations on topography.tidal current.salinity.suspended sediment and bed load during the years of 1982.1983.1988.1989.1996 and 1997 in theYangtze Estuary.Results show that the deformation of tidal waves is distinct and the sand carrying capaci-ty is large within the mouth bar due to strong tidal currents and large volume of incoming water and sedi-ments.Owing to both temporal and spatial variation of tidal current.deposition and erosion are extremelyactive.In general a change of up to 0.1 m of bottom sediments takes place during a tidal period.The maxi-mum siltation and erosion are around 0.2 m in a spring to neap tides cycle.The riverbed is silted duringflood when there is heavy sediment load.eroded during dry season when sediment load is low.The annualaverage depth of crosion and siltation on the riverbed is around 0.6 m.In particular cases.it may increaseto 1.4 m to 2.4 m at some locations.     

The rule of sediment transport on the Nanhui tidal flat in the Changjiang Estuary

The Nanhui tidal flat is located in the area of slow current where the ebb currents from the Changjiang Estuaryand the Hangzhou Bay converge and the flood current from the sea diverges into the estuary and the bay. The flat extends seaward in tongue shape and has a wide and gentle surface with a marked difference of tidal levels on its two sides, which results in the sediment longitudinal transport on the flat. The water-sediment conditions are diverse at different locations. The velocity and sediment concentration in intertidal zone are higher during the flood tide than those during the ebb tide. The net sediment transport is landward, resulting in a large amount of deposition of sediments on the shoal. However, the ebb current is the dominant one in deep-water area where the net sediment transport is seaward. There exist two circulation systems in plane view on the shoal and in its adjacent deep-water area, which results in the sediment exchanges between the flat and channel and between the estuary an     

Difference in full-filled time and its controlling factors in the Central Canyon of the Qiongdongnan Basin

Based on the interpretation of high resolution 2D/3D seismic data,sedimentary filling characteristics and fullfilled time of the Central Canyon in different segments in the Qiongdongnan Basin of northwestern South China Sea have been studied.The research results indicate that the initial formation age of the Central Canyon is traced back to 11.6 Ma(T40),at which the canyon began to develop due to the scouring of turbidity currents from west to east.During the period of 11.6–8.2 Ma(T40–T31),strong downcutting by gravity flow occurred,which led to the formation of the canyon.The canyon fillings began to form since 8.2 Ma(T31) and were dominated by turbidite deposits,which constituted of lateral migration and vertical superposition of turbidity channels during the time of8.2–5.5 Ma.The interbeds of turbidity currents deposits and mass transport deposits(MTDs) were developed in the period of 5.5–3.8 Ma(T30–T28).After then,the canyon fillings were primarily made up of large scale MTDs,interrupted by small scale turbidity channels and thin pelagic mudstones.The Central Canyon can be divided into three types according to the main controlling factors,geomorphology-controlled,fault-controlled and intrusionmodified canyons.Among them,the geomorphology-controlled canyon is developed at the Ledong,Lingshui,Songnan and western Baodao Depressions,situated in a confined basin center between the northern slope and the South Uplift Belt along the Central Depression Belt.The fault-controlled canyon is developed mainly along the deep-seated faults in the Changchang Depression and eastern Baodao Depression.Intrusion-modified canyon is only occurred in the Songnan Low Uplift,which is still mainly controlled by geomorphology,the intrusion just modified seabed morphology.The full-filled time of the Central Canyon differs from west to east,displaying a tendency of being successively late eastward.The geomorphology-controlled canyon was completely filled before3.8 Ma(T28),but that in intrusion-modified canyon was delayed to 2.4 Ma(T27) because of the uplifted southern canyon wall.To the Changchang Depression,the complete filling time was successively late eastward,and the canyon in eastern Changchang Depression is still not fully filled up to today.Difference in full-filled time in the Central Canyon is mainly governed by multiple sediment supplies and regional tectonic activities.Due to sufficient supply of turbidity currents and MTDs from west and north respectively,western segment of the Central Canyon is entirely filled up earlier.Owing to slower sediment supply rate,together with differential subsidence by deep-seated faults,the full-filled time of the canyon is put off eastwards gradually.     

Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme.The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. There is little evidence for erosion, mass-wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes.Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, highly aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass-wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.     

Characteristics and occurrence of submarine canyon-associated landslides in the middle of the northern continental slope,South China Sea

High-resolution and high-density 2-D multichannel seismic data, combined with high-precision multibeam bathymetric map, are utilized to investigate the characteristics and distribution of submarine landslides in the middle of the northern continental slope, South China Sea. In the region, a series of 19 downslope-extending submarine canyons are developed. The canyons are kilometers apart, and separated by inter-canyon sedimentary ridges. Numerous submarine landslides, bounded by headscarps and basal glide surfaces, are identified on the seismic profiles by their distorted to chaotic reflections. Listric faults and rotational blocks in head areas and compressional folds and inverse faults at the toes of the landslides are possibly developed. Three types of submarine landslides, i.e., creeps, slumps, and landslide complexes, are recognized. These landslides are mostly distributed in the head areas and on the flanks of the canyons. As the most widespread landslides in the region, creeps are usually composed of multiple laterally-coalesced creep bodies, in which the boundaries of singular component creep bodies are difficult to delineate. In addition, a total of 77 landslides are defined, including 61 singular slumps and 16 landslide complexes that consist of two or more component landslides. Statistics show that most landslides are of a small dimension (0.53–18.09 km² in area) and a short runout distance (less than 3.5 km). Regional and local slope gradients and rheological behavior of the displaced materials might play important roles in the generation and distribution of the submarine landslides. A conceptual model for the co-evolution of the canyons and the associated landslides in the study area is presented. In the model it is assumed that the canyons are initiated from gullies created by landslides on steeper sites of the continental slope. The nascent canyons would then experience successive retrogressive landsliding events to extend upslope; at the same time canyon downcutting or incision would steepen the canyon walls to induce more landslides.     

南海北部陆坡一统峡谷群地貌特征及控制因素分析

基于最新的高分辨率多波束全覆盖测深数据、单道地震和多道地震剖面数据,对南海北部陆坡一统峡谷群9条峡谷的地形地貌及沉积特征进行了分析：峡谷群自陆坡向深海盆方向呈聚敛型,横断面主要呈“V”型,谷壁对称发育,坡度较陡; 研究区海底地层受多条断裂控制,呈典型阶梯状发育,海底断陷、重力滑塌面和小型滑坡体等海底不稳定地质灾害高度发育,说明峡谷群海底环境处于极不稳定状态。在研究区海底峡谷群地貌演化过程中,西沙海槽区域沉降等新生代构造运动控制着峡谷群地貌格局的形成; 来自北部陆架的充足沉积碎屑物质的输入往往伴随着高密度浊流、海底滑坡、坍塌等海底灾害的发生,控制着峡谷群的进一步发育; 相对海平面变化直接改变了研究区的沉积环境,为陆源碎屑物质的搬运提供了更加直接的通道,这也是诱发陆坡海底失稳、塑造峡谷群地貌特征的重要因素之一。     

Sediment dynamics and geohazards off Uruguay and the de la Plata River region (northern Argentina and Uruguay)

The continental margin off Uruguay and northern Argentina is characterized by high fluvial input by the de la Plata River and a complex oceanographic regime. Here we present first results from RV Meteor Cruise M78/3 of May?CJuly 2009, which overall aimed at investigating sediment transport processes from the coast to the deep sea by means of hydroacoustic and seismic mapping, as well as coring using conventional tools and the new MARUM seafloor drill rig (MeBo). Various mechanisms of sediment instabilities were identified based on geophysical and core data, documenting particularly the continental slope offshore Uruguay to be locus of submarine landsliding. Individual landslides are relatively small with volumes <2km³. Gravitational downslope sediment transport also occurs through the prominent Mar del Plata Canyon and several smaller canyons. The canyons originate at a midslope position, and the absence of buried upslope continuations strongly suggests upslope erosion as main process for canyon evolution. Many other morphological features (e.g., slope-parallel scarps with scour geometries) and abundant contourites in a 35-m-long MeBo core reveal that sediment transport and erosion are controlled predominantly by strong contour currents. Despite numerous landslide events, their geohazard potential is considered to be relatively small, because of their small volumes and their occurrence at relatively deep water depths of more than 1,500?m.     

Characteristics of migrating submarine canyons from the middle Miocene to present: Implications for paleoceanographic circulation,northern South China Sea

Previously undocumented, migrating submarine canyons have developed in the Pearl River Mouth Basin along the northern continental margin of the South China Sea from the middle Miocene to present. A grid of high-resolution, 2-D multi-channel seismic profiles calibrated by borehole information permits documentation of these northeastward migrating submarine canyons, as the result of the interplay of gravity flows and bottom currents. The modern canyons have lengths of 30–60 km, widths of 1–5.7 km, and relief of 50–300 m in water depths of 450–1500 m. Buried ancient submarine canyon successions were originally eroded by basal erosional discontinuities and partially filled by canyon thalweg deposits. These are overlain by lateral inclined packages and hemipelagic drape deposits. Basal erosional discontinuities and thalweg deposits are probably created principally by turbidity currents and filled with turbidites. Lateral inclined packages likely were formed by along-slope bottom currents. The evolution of these migrating submarine canyons reveals that northeastward bottom currents have consistently occurred at least from the middle Miocene to present in the study area. It might further imply that thermohaline intermediate water circulation of the South China Sea has been anti-cyclonic from the middle Miocene to present. The initiation of migrating submarine canyons possibly signals commencement of strong bottom currents after the middle Miocene in the South China Sea. The intensification of bottom currents also possibly may reflect shoaling of the major ocean seaways and increased vigor in oceanic circulation forced by global cooling after the middle Miocene.     

南海深水沉积过程之大洋钻探目标

多个航次的大洋钻探已经在南海成功实施,航次科学目标主要聚焦于古海洋、海盆形成过程以及岩石圈破裂过程等,钻遇了多种类型的深水沉积物,而且近些年来的物理海洋调查也已证实了其复杂的深水沉积动力环境,尤其是南海北部陆缘区海底地形地貌复杂,发育了深水水道、块体流等重力流沉积以及多种类型的等深流沉积。南海独特的构造环境和复杂底流活动所导致的不同深水沉积体系类型和丰富的沉积记录,使南海北部陆缘区成为研究深海沉积过程的最佳场所。通过对2—3个剖面多站位不同地形地貌条件下多类型深水沉积物的钻探对比分析,将有助于深刻理解重力流-底流相互作用特征,揭示块体流、浊流和底流形成和发育演化过程,查明岩石圈破裂和海底扩张的沉积响应等科学问题,丰富深水沉积动力学理论,深化对南海大陆边缘沉积格局演变和深层海流演变的整体性和全面性认识。     

Recent transfer of coastal sediments to the Laurentian Channel,Lower St. Lawrence Estuary (Eastern Canada), through submarine canyon and fan systems

Multibeam sonar data, acoustic sub-bottom profiles and box cores were used to study the activity of submarine canyons and fans near the city of Les Escoumins, on the North Shore of the Lower St. Lawrence Estuary (Eastern Canada). The multibeam data were used to generate a high-resolution digital terrain model that reveals the presence of a large number of canyons and fans along the northern slopes of the Laurentian Channel. This paper focuses on two of the larger canyons, and their associated submarine fans. The sub-bottom profiles on the fans reveal high-amplitude reflections at the sediment/water interface and near the seafloor surface, indicating the occurrence of layers of coarse material. A turbidite was observed in a box core sampled in one of the fans, confirming the nature of the coarse layer. Geophysical and sedimentological data indicate that the canyons and fans play an important role in transferring coastal sandy sediments to the deeper marine environments by longshore drift-initiated turbidity flows, and thereby contribute to the negative sediment budget along the coast. The morphology of the canyons indicates that they were produced by a combination of erosive turbidity flows and retrogressive failures. The two box cores sampled on the fans reveal a recent (~last 60 years) quasi-exponential increase in sand content near the surface of the cores, possibly reflecting recent deforestation and/or increased coastal erosion.     

南海东北部陆缘浊流活动的地貌记录及其形成机制分析

超临界浊流影响下的周期阶坎广泛分布在南海东北部台西南盆地的西澎湖峡谷中。由于频繁的构造活动, 常年的台风影响, 以及来自中国台湾河流的大量沉积物供给, 导致这个区域的浊流活动经常发生。本文利用高分辨率的地貌资料, 对西澎湖峡谷发育的23个净侵蚀周期阶坎和10个净沉积周期阶坎进行了形态的定量分析, 并且统计了流经这些周期阶坎的浊流流速。结果显示, 浊流在流经净侵蚀周期阶坎过程中的速度有明显突变, 而在净侵蚀到净沉积周期阶坎的过程中速度也发生了明显的降低, 前者由峡谷中的坡折带导致, 后者则是由限制性环境到非限制性环境的转变造成的。此外, 净沉积周期阶坎主要分布于靠近峡谷口外的西南侧, 这是由于科氏力对浊流影响的结果。研究此区域周期阶坎的形态特征和形成机制能够帮助我们更好地理解海底地形地貌与浊流的相互关系, 对于理解峡谷中的地貌演化具有重要意义。     

白云凹陷陆坡峡谷沉积与迁移特征及其对天然气水合物成藏的影响

利用高分辨率地震资料,研究了南海北部白云凹陷中新世以来的陆坡峡谷沉积和迁移特征及其对动态似海底反射（BSR）的影响。白云凹陷陆坡区浊流和底流共同作用形成了大型单向迁移峡谷沉积体系。峡谷的沉积过程包括侵蚀为主阶段、侵蚀-沉积共同作用阶段及沉积为主阶段。峡谷沉积相主要包括峡谷侵蚀基底、谷底沉积、谷内滑塌块体搬运沉积及侧向倾斜沉积层等4个单元。峡谷的迁移造成含天然气水合物脊部两侧不同的侵蚀-沉积环境,因此,脊部两侧BSR反射特征也不同。随着峡谷迁移的进行,在峡谷侵蚀侧翼处,沉积物被侵蚀,天然气水合物稳定带底界将发生下移,BSR反射特征为多轴较连续反射;而峡谷沉积侧翼处,沉积物增厚,天然气水合物稳定带将发生上移,BSR反射特征为单轴连续反射。     

Deposition processes from echo-character mapping along the western Algerian margin (Oran–Tenes), Western Mediterranean

The westernmost Algerian margin (south Algero-Provençal basin) depicts a few offshore active faults, moderate to rare seismicity, and generally very steep slopes (>16°). We classified and mapped 12 echo types according to their sub-bottom acoustic facies observed on this margin on 2–5.2 kHz Chirp echo-sounder data (MARADJA 2003 cruise). The echo-character maps are interpreted in terms of sedimentary processes: the B1 echo type (parallel to subparallel high- to low-amplitude sub-bottom reflections), mainly in the deep basin, corresponds to hemipelagic sedimentation; R1 (prolonged single echo with no sub-bottoms) and R2 (small irregular overlapping hyperbolae) echo types, generally near or in canyon systems, are associated with turbidity currents or more rarely to contour currents or mass-transport deposits such as slumps, slides and debris flows; the transparent echo types (T1–T5) and R3 (chaotic lens of low-amplitude reflections on top of higher amplitude), often located at the foot of the slope or canyons walls, typically indicate mass-transport deposits (like slides) or turbidites. Large zones that display a large variety of echo types are evidenced in the study area and are generally associated with turbidity currents, but could also be associated with bottom currents. It appears that active tectonics plays a significant role in this part of the margin which presents a few active faults offshore but also a strong and relatively frequent seismicity onland. The general pattern of the distribution of mass-transport deposits is particular – i.e. many but small slides all along the margin – and suggests a probable triggering by recurrent earthquake shakings. However, active tectonics is not the only factor influencing the deposition pattern, as some zones seem characterized by predominant strong turbidity currents transporting sediments far away from the foot of the margin, whereas others depict retrogressive erosion features on the slope, i.e. small slides scarps in gullies rather than transport by turbidity currents. In particular, the rivers sediment discharge fluxes and the geomorphologic characteristics of the margin seem to be very important factors too.