Modern fan deltas in the western Gulf of Corinth,Greece

Rivers in the western Gulf of Corinth have built gravelly fan deltas into waters 100–300 m deep. Seismic profiles (3.5 kHz) and gravity cores show that sand-filled channels extend seaward of modern distributary mouths and lead to sandy depositional lobes in deeper water. Much of the fan delta slope is underlain by mud. Closely spaced gullies cut the interchannel ridges on either side of the main channels. Incised slope valleys occur on east-facing slopes, where the wave fetch is greatest, and may be formed by rip-current-induced turbidity currents.     

南海东北部海底沉积物波的形态、粒度特征及物源、成因分析

在南海东北部广泛发育沉积物波。通过高分辨率多波束数据、地震剖面以及重力柱状样,对沉积物波的形态特征、粒度特征、物源以及形成机制进行了分析。研究表明大致以台湾浅滩南海底峡谷为界,北侧为近北东向展布,南侧为近南北向展布。对其分布规律、地貌和形态特征及重力柱状样粒度分析表明这些沉积物波为浊流成因。沉积物波的发育与新生代晚期研究区的构造活动密切相关,自距今6.5 Ma以来台湾造山运动使台湾岛强烈抬升剥蚀,这些剥蚀物为研究区提供了大量的陆源物质,而在南海东北部陆坡区大量发育的峡谷-冲沟系统为陆缘物质向下陆坡的输送提供了良好的通道。研究区西侧的东沙隆起长期处于抬升剥蚀状态,这种抬升剥蚀也为研究区沉积物波的发育提供了部分物源。随着坡度的减缓,浊流沉积物开始堆积,在台湾浅滩南海底峡谷的北侧形成了展布方向与冲沟垂直的沉积物波,而在南侧由于台湾浅滩南海底峡谷发生转向,浊流从水道中漫溢出来,沉积物堆积下来,形成了与原先水道近于垂直的近南北向的沉积物波。     

The Laurentian Fan: Sohm abyssal plain

The 0.5- to 2-km thick Quaternary Laurentian Fan is built over Tertiary and Mesozoic sediments that rest on oceanic crust. Two 400-km long fan valleys, with asymmetric levees up to 700-m high, lead to an equally long, sandy, lobate basin plain (northern Sohm Abyssal Plain). The muddy distal Sohm Abyssal Plain is a further 400-km long. The sediment supplied to the fan is glacial in origin, and in part results from seismically triggered slumping on the upper continental slope. Sandy turbidity currents, such as the 1929 Grand Banks earthquake event, probably erode the fan-valley floors; but thick muddy turbidity currents build up the high levees.     

The Laurentian Fan: Sohm Abyssal Plain

The 0.5- to 2-km thick Quaternary Laurentian Fan is built over Tertiary and Mesozoic sediments that rest on oceanic crust. Two 400-km long fan valleys, with asymmetric levees up to 700-m high, lead to an equally long, sandy, lobate basin plain (northern Sohm Abyssal Plain). The muddy distal Sohm Abyssal Plain is a further 400-km long. The sediment supplied to the fan is glacial in origin, and in part results from seismically triggered slumping on the upper continental slope. Sandy turbidity currents, such as the 1929 Grand Banks earthquake event, probably erode the fan-valley floors; but thick muddy turbidity currents build up the high levees. Margin setting represents fan and/or source area     

Bottom-current control on sedimentation in the western Bellingshausen Sea, West Antarctica

A set of single- and multi-channel seismic reflection profiles provide insights into the younger Cenozoic sedimentation history of the continental rise in the western Bellingshausen Sea, west and north of Peter I Island. This area has been strongly influenced by glacially controlled sediment supply from the continental shelf, interacting with a westward-flowing bottom current. From south to north, the seismic data show changes in the symmetry and structure of a prominent sediment depocentre. Its southernmost sector provides evidence of sediment drift whereas northwards the data show a large channel-levee complex, with a western levee oriented in the opposite direction to that of the drift in the south. This pattern indicates the northward-decreasing influence of a westward-flowing bottom contour current in the study area. Topographic data suggest the morphologic ridges at Peter I Island to be the main features responsible for variable bottom-current influence, these acting as barrier to the bottom current and entrained sedimentary material. West of Peter I Island, the east-orientated Coriolis force remains effective in deflecting the suspended load of the turbidity currents towards the west, thereby promoting growth of the western channel levee. Calculated sediment accumulation rates based on seismic data reveal Depocentre C to consist of younger Cenozoic material supplied by glacial transport and modified by contour currents in the western Bellingshausen Sea. These findings demonstrate that the shape, structure and distribution of sediment mounds and estimates of sediment accumulation rates can be associated to the influence of bottom currents and their long-term evolution in response to tectonic movements, ice-sheet dynamics and deep-water formation.     

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.     

Recognition of contour-current influence in mixed contourite-turbidite sequences of the western Weddell Sea,Antarctica

Sedimentary processes and structures across the continental rise in the western Weddell Sea have been investigated using sediment acoustic and multichannel seismic data, integrated with multibeam depth sounding and core investigations. The results show that a network of channels with associated along-channel ridges covers the upper continental slope. The seismic profiles reveal that the channels initially developed as erosive turbidite channels with associated levees on their northern side due to Coriolis force. Later they were partly or fully infilled, probably as a result of decreasing turbidite activity. Now the larger ones exist as erosive turbidite channels of reduced size, whereas the smaller ones are non-erosive channels, their shape being maintained by contour current activity. Drift bodies only developed where slumps caused a distinctive break in slope inclination on the upper continental rise, which served to initiate the growth of a drift body fed by contour currents or by the combined action of turbidites and contourites. The history of sedimentation can be reconstructed tentatively by correlation of seismo-stratigraphic units with the stages of evolution of the drifts on the western side of the Antarctic Peninsula. Three stages can be distinguished in the western Weddell Sea after a pre-drift stage, which is delimited by an erosional unconformity at the top: (1) a growth stage, dominated by turbidites, with occasional occurrence of slumps during its initial phase; (2) during a maintenance stage turbiditiy-current intensity (and presumably sedimentation rate also) decreased, probably as a result of the ice masses retreating from the shelf edge, and sedimentation became increasingly dominated by contour current activity; and (3) a phase of sheeted-sequence formation. A southward decrease in sediment thickness shows that the Larsen Ice Shelf plays an important role in sediment delivery to the western Weddell Sea. This study shows that the western Weddell Sea has some characteristics in common with the southern as well as the northwestern Weddell Sea: contour currents off the Larsen Ice Shelf have been present for a long time, probably since the late Miocene, but during times of high sediment input from the shelves as a result of advancing ice masses a channel-levee system developed and dominated over the contour-current transport of sediment. At times of relatively low sediment input the contour-current transport dominated, leading to the formation of drift deposits on the upper continental rise. Seaward of areas without shelf ice masses the continental rise mainly shows a rough topography with small channels and underdeveloped levees. The results demonstrate that sediment supply is an important, maybe the controlling factor of drift development on the Antarctic continental rise.     

Deep-lacustrine sandy debrites and turbidites in the lower Triassic Yanchang Formation,southeast Ordos Basin,central China: Facies distribution and reservoir quality

The deep lacustrine gravity-flow deposits are widely developed in the lower Triassic Yanchang Formation, southeast Ordos Basin, central China. Three lithofacies include massive fine-grained sandstone, banded sandstone, and massive oil shale and mudstone. The massive fine-grained sandstones have sharp upper contacts, mud clasts, boxed-shaped Gamma Ray (GR) log, but no grading and Bouma sequences. In contrast, the banded sandstones display different bedding characteristics, gradational upper contacts, and fine-upward. The massive, fine-grained sandstones recognized in this study are sandy debrites deposited by sandy debris flows, while the banded sandstones are turbidites deposited by turbidity currents not bottom currents. The sediment source for these deep gravity-flow sediments is a sand-rich delta system prograding at the basin margin. Fabric of the debrites in the sandy debris fields indicates initial formation from slope failure caused by the tectonic movement. As the sandy debris flows became diluted by water and clay, they became turbidity currents. The deep lacustrine depositional model is different from the traditional marine fan or turbidite fan models. There are no channels or wide lobate sand bodies. In the lower Triassic Yanchang Formation, layers within the sandy debrites have higher porosity (8–14%) and permeability (0.1–4 mD) than the turbidites with lower porosity (3–8%) and permeability (0.04–1 mD). Consequently, only the sandy debrites constitute potential petroleum reservoir intervals. Results of this study may serve as a model for hydrocarbon exploration and production for deep-lacustrine reservoirs from gravity-flow systems in similar lacustrine depositional environments.     

Morphology and Late Quaternary sedimentation in the Gulf of Oman Basin

The morphology of the Gulf of Oman Basin, a 3,400 m deep oceanic basin between Oman and southern Pakistan and southern Iran, ranges from a convergent margin (Makran margin) along the north side, a passive type (Oman margin) along the south side, translation types along the basin's west (Zendan Fault-Oman Line) and east (Murray Ridge) sides and a narrow continental rise and a wide abyssal plain in the centre of the basin. Sediment input into the basin during the Late Quaternary has been mainly from the north as a result of the uplift of the Coast Makran Mountains in the Late Miocene-Pliocene. Today most of this detritrus is deposited on the shelf and upper continental slope and perched basins behind the fold/fault ridges on the lower slope. The presence of fans and channels on the continental rise on the north side of the basin indicate, however, that continental derived debris was, and possibly is, being transported to the deep-sea by turbidity currents via gaps in the ridges on the lower slope. In addition to land derived terrigenous sediments, the basin deposits also contain biogenic (organic matter and calcium carbonate), eolian detritus and hydrates and authigenic carbonates from the tectonic dewatering of the Makran accretionary wedge. The eolian sediment is carried into the Gulf of Oman Basin from Arabia and the Mesopotamia Valley by the northwesterly Shamal winds. This type of detritus was particularly abundant during the glacial arid periods 21,000–20,000 and 11,000 (Younger Dryas) years ago when exposure of the Persian (Arabian) Gulf increased the area of dust entrainment and shifted the position of the source of the eolian sediments closer to the basin.     

Evolution and depositional structure of earthquake-induced mass movements and gravity flows: Southwest Orphan Basin, Labrador Sea

A series of submarine canyons on the southwest slope of Orphan Basin experienced complex failure at 7–8 cal ka that resulted in the formation of a large variety of mass-transport deposits (MTDs) and sediment gravity flows. Ultra-high-resolution seismic-reflection profiles and multiple sediment cores indicate that evacuation zones and sediment slides characterize the canyon walls, whereas the canyon floors and inner-banks are occupied by cohesive debris-flow deposits, which at the mouths of the canyons on the continental rise form large, coalescing lobes (up to 20 m thick and 50 km long). Erosional channels, extending throughout the length of the study area (<250 km), are observed on the top of the lobes. Piston cores show that the channels are partially filled by poorly sorted muddy sand and gravel, capped by inversely to normally graded gravel and sand. Such deposits are interpreted to originate from multi-phase gravity flows, consisting of a lower part behaving as a cohesionless debris flow and an upper part that was fully turbulent.The Holocene age and the widespread synchronous occurrence of these failures indicate a large magnitude earthquake as their possible triggering mechanism. The large debris-flow deposits on the continental rise originated from large failures on the upper continental slope, involving proglacial sediments. Retrogression of these failures led to the eventual failure of marginal sandy till deposits on the upper slope and outer shelf, which due to their low cohesion disintegrated into multi-phase gravity flows. The evacuation zones and slide deposits on the canyon walls were triggered either by the earthquake, or from erosion of the canyon walls by the debris flows. The slides, debris-flows, and multi-phase gravity flows observed in this study are petrographically different, indicating different sediment sources. This indicates that not all failures lead through flow transformation to the production of a multi-phase gravity flow, but only when the sediment source contains ample coarse-grained material. The spatial segregation of the slide, debris-flow, and multi-phase gravity-flow deposits is attributed to the different mobility of each transport process.     

Seismic features and origin of sediment waves in the Qiongdongnan Basin, northern South China Sea

Sediment waves have been documented around the world for several decades, and their origins are still debated because of their various characteristics in different settings. Based on numerous high-resolution seismic profiles and two boreholes, sediment waves are identified in deepwater areas of the eastern Qiongdongnan Basin, and their distribution and seismic features are illustrated. Combined with the bathymetry, the potential origins of these sediment waves are discussed. Drilling in the central canyon revealed that the channel infill comprises some along-slope fine-grained turbidites, which are good reservoir for gas plays. The sediment waves are distributed on the banks of the central canyon and their seismic features indicate that most of them are caused by turbidity current overflows along the canyon. Although previous researches on these sediment waves suggested that they were of westward-flowing contourite origin, detailed topographic map derived from the seafloor reflector on seismic data shows that there is a N–S trending ridge at the east part of sediment wave zones, which could block and divert the bottom current. According to the geometry of sediment waves, the flow thicknesses across the entire wave field are calculated as 280–560 m, and the current velocity falls in the range of 30–130 cm/s, which would favor a fine-grained composition and could be a good reservoir because of the better sorting of turbidites than contourites or other gravity flow deposits.     

Fine-grained Pleistocene deepwater turbidite channel system on the slope of Qiongdongnan Basin, northern South China Sea

Recently, as oil exploitation has become focused on deepwater slope areas, more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW–NE to W–E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel–levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting.     

深水浊流形成过程的水动力学模拟:以南海北部琼东南盆地浊流形成过程为例

Turbidity currents represent a major agent for sediment transport in lakes, seas and oceans. In particu-lar, they formulate the most significant clastic accumulations in the deep sea, which become many of the world＇s most important hydrocarbon reservoirs. Several boreholes in the Qiongdongnan Basin, the north-western South China Sea, have recently revealed turbidity current deposits as significant hydrocarbon res-ervoirs. However, there are some arguments for the potential provenances. To solve this problem, it is es-sential to delineate their sedimentary processes as well as to evaluate their qualities as reservoir. Numerical simulations have been developed rapidly over the last several years, offering insights into turbidity current behaviors, as geologically significant turbidity currents are difficult to directly investigate due to their large scale and often destructive nature. Combined with the interpretation of the turbidity system based on high-resolution 3D seismic data, the paleotophography is acquired via a back-stripping seismic profile integrated with a borehole, i.e., Well A, in the western Qiongdongnan Basin; then a numerical model is built on the basis of this back-stripped profile. After defining the various turbidity current initial boundary conditions, includ-ing grain size, velocity and sediment concentration, the structures and behaviors of turbidity currents are investigated via numerical simulation software ANSYS FLUENT. Finally, the simulated turbidity deposits are compared with the interpreted sedimentary bodies based on 3D seismic data and the potential provenances of the revealed turbidites by Well A are discussed in details. The simulation results indicate that a sedimen-tary body develops far away from its source with an average grain size of 0.1 mm, i.e., sand-size sediment. Taking into account the location and orientation of the simulated seismic line, the consistence between normal forward simulation results and the revealed cores in Well A indicates that the turbidites should ha     

南海东北部峡谷体系的地貌特征与发育控制因素

海底峡谷在全球陆缘广泛分布,是浅海沉积物向深海运移的主要通道,对于理解深海浊流触发机制、深海沉积物的搬运模式、深海扇的发育历史和深海油气资源勘探等均具有重要意义。本文基于高分辨率高精度的多波束测深数据,首次对南海东北部海底峡谷体系进行了研究,精细刻画了高屏海底峡谷、澎湖海底峡谷、台湾浅滩南海底峡谷和东沙海底峡谷等4条大型海底峡谷的地貌特征并分析其发育控制因素。海底坡度、构造运动、海山与海丘是影响南海东北部峡谷群走向与特征的重要因素,其中,海底坡度对于峡谷上游多分支与"V"字特征有显著的控制作用;构造运动是控制高屏海底峡谷走向的因素,澎湖海底峡谷的走向则与菲律宾海板块与欧亚板块碰撞有关,东沙海底峡谷的走向则与东沙运动相关,台湾浅滩南海底峡谷上段受NW向断裂构造的控制;海山的阻挡作用造成峡谷局部走向和特征改变。海底峡谷群输送大量陆源沉积物到深海盆并形成大面积的沉积物波,海山和沉积物波的发育导致东沙海底峡谷下段"回春"和转向。     

Reconstruction of turbidity currents in Amazon Channel

Quantifying the characteristics of the turbidity currents that are responsible for the erosion, lateral migration and filling of submarine channels maybe useful for predicting the distribution of lithofacies in channel fill and levee reservoirs. This paper uses data from a well-studied submarine channel in Amazon Fan in an attempt to reconstruct the velocity, thickness, concentration, duration, recurrence rates and vertical structure of turbidity currents in this long sinuous channel. Estimates of flow conditions are derived from the morphology of the channels and the characteristics of the deposits within them. In particular, the availability of information on the sediment distribution with respect to the channel topography at the time of deposition allows for insights into the vertical structure of the flow, a key property that has been so far poorly understood. Integration of flow constraints from well and seismic data or from detailed analysis of outcrop with numerical flow models is a critical step toward a complete understanding of the flow and associated deposits. Turbidity currents in sinuous submarine channels, exemplified by Amazon Channel, are found to last for tens of hours and occur on a regular, quasi-annual basis. Model results suggest that these flows had, on average, velocities ranging from 2 to 4 m/s in the canyon/upper fan which decreased to 0.5–1 m/s in the lower fan, travelling in excess of 800 km. The model turbidity currents were subcritical over most of the channel length, indicating a low degree of water entrainment and low rate of deceleration down the channel. The formation of such long, sinuous channels is intrinsically associated with frequent, long-duration, subcritical turbidity currents carrying a silt-dominated sediment load.     

Deep-water geomorphology of the glaciated Irish margin from high-resolution marine geophysical data

The continental margin offshore of western Ireland offers an opportunity to study the effects of glacial forcing on the morphology and sediment architecture of a mid-latitude margin. High resolution multibeam bathymetry and backscatter data, combined with shallow seismic and TOBI deep-towed side-scan sonar profiles, provide the basis for this study and allow a detailed geomorphological interpretation of the northwest Irish continental margin. Several features, including submarine mass failures, canyon systems and escarpments, are identified in the Rockall Trough for the first time. A new physiographic classification of the Irish margin is proposed and linked to the impact of glaciations along the margin. Correlation of the position and dimensions of moraines on the continental shelf with the level of canyon evolution suggests that the sediment and meltwater delivered by the British–Irish Ice Sheet played a fundamental role in shaping the margin including the upslope development of some of the canyon systems. The glacial influence is also suggested by the variable extent and backscatter signal of sedimentary lobes associated with the canyons. These lobes provide an indirect measurement of the amount of glaciogenic sediment delivered by the ice sheet into the Rockall Trough during the last glacial maximum. None of the sedimentary lobes demonstrates notable relief, indicating that the amount of glaciogenic sediment delivered by the British–Irish Ice Sheet into the Rockall Trough was limited. Their southward disappearance suggests a more restricted BIIS, which did not reach the shelf edge south of 54°23′ N. The various slope styles observed on the Irish margin represent snapshots of the progressive stages of slope development for a glacially-influenced passive margin and may provide a predictive model for the evolution of other such margins.     

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.     

Turbidites deposited on Southern Central Chilean seamounts: Evidence for energetic turbidity currents

Gravity cores obtained from isolated seamounts located within, and rising up to 300 m from the sediment-filled Peru–Chile Trench off Southern Central Chile (36°S–39°S) contain numerous turbidite layers which are much coarser than the hemipelagic background sedimentation. The mineralogical composition of some of the beds indicates a mixed origin from various source terrains while the faunal assemblage of benthic foraminifera in one of the turbidite layers shows a mixed origin from upper shelfal to middle-lower bathyal depths which could indicate a multi-source origin and therefore indicate an earthquake triggering of the causing turbidity currents. The bathymetric setting and the grain size distribution of the sampled layers, together with swath echosounder and sediment echosounder data which monitor the distribution of turbidites on the elevated Nazca Plate allow some estimates on the flow direction, flow velocity and height of the causing turbidity currents. We discuss two alternative models of deposition, both of which imply high (175–450 m) turbidity currents and we suggest a channelized transport process as the general mode of turbidite deposition. Whether these turbidites are suspension fallout products of thick turbiditic flows or bedload deposits from sheet-like turbidity currents overwhelming elevated structures cannot be decided upon using our sedimentological data, but the specific morphology of the seamounts rather argues for the first option. Oxygen isotope stratigraphy of one of the cores indicates that the turbiditic sequences were deposited during the last Glacial period and during the following transition period and turbiditic deposition stopped during the Holocene. This climatic coupling seems to be dominant, while the occurrence of megathrust earthquakes provides a trigger mechanism. This seismic triggering takes effect only during times of very high sediment supply to the shelf and slope.     

Knickpoint migration in submarine channels in response to fold growth,western Niger Delta

Several knickpoints have been identified along the present-day thalweg of a sinuous submarine channel–levee system (CLS) on the slope of the western Niger Delta using 3D seismic data. The knickpoints form as a result of gradient changes caused by the uplift of a thrust and fold belt orthogonal to the CLS. The channel gradient is lower locally upstream of folds causing turbidity currents within the channel to decelerate and deposit the coarsest sediment load. The basinward dipping fold limb causes local steepening of the gradient, which leads to increased flow velocity and turbulence within the turbidity currents. This enhances erosion at the base of the channel and leads to the formation of a knickpoint. If preserved, e.g., as a result of channel avulsion or abandonment, the deposits upstream of the knickpoints could constitute an important hydrocarbon reservoir element. They can, however, also be partially eroded by headward-migrating knickpoints, as the channel strives to regain its equilibrium profile, leaving remnant sand pockets preserved on channel margins. Although knickpoints are difficult to recognise from subsurface seismic or outcrop data, it is anticipated that they can form at any stage of the evolution of a channel–levee system and may be particularly important in controlling 3D channel architecture where channels intersect dynamically changing seabed bathymetry.     

Transport of terrestrial organic matter in the Ogooué deep sea turbidite system (Gabon)

In order to define the nature and distribution of the organic matter (OM) preserved in the modern Ogooué deep sea turbidite system (Gabon), bulk geochemical techniques (Rock-Eval pyrolysis, elemental and isotopic analyses) and palynofacies were applied to three piston cores collected in the Cape Lopez Canyon and lobe and on the continental slope, north of the canyon.The hemipelagic sedimentation in the study area is characterized by high accumulations of well-preserved OM (∼2-3 wt. TOC %). Bulk geochemical and palynofacies analysis indicate both a marine and terrestrial origin of the OM. Contribution of the marine source is higher on the slope than in the canyon and lobe.OM accumulation in turbidites is strongly controlled by the combined influence of the Cape Lopez Canyon and littoral drift. In the canyon and lobe, turbidites show generally low TOC content (0.5 wt. %) and OM is oxidized. The origin of the OM is interpreted as both marine and terrestrial, with a higher contribution of continental source versus marine source. The low TOC contents are due to the large siliciclastic fraction transported by the littoral drift and diverted in the Cape Lopez Canyon during high energy processes (e.g. storms) which tend to dilute the OM in the turbidites. Transport by long-shore currents and/or turbiditic flows leads to oxidation of the OM.On the continental slope located north of the Cape Lopez Canyon, large amounts of OM are deposited in turbidites (up to 14 wt. %). The OM is predominantly derived from terrestrial land plants and has not been subjected to intense oxidation. These deposits are characterized by high hydrocarbon potential (up to 27 kg HC/t rock), indicating a good potential as gas-prone source rock. Because Cape Lopez Canyon captures a significant part of the sediment transported by the littoral drift, the siliciclastic sedimentary flux is reduced north of the canyon; OM is thus concentrated in the turbidites. Variation in TOC content within turbidite laminae can be explained by the burst and sweep deposition process affecting the boundary layer of the turbulent flow.This study confirms that gravity flows play a preponderant role in the accumulation and preservation of OM in deep water and that deep sea turbidite systems could be regarded as an environment where organic sedimentation occurs.