Turbidite deposition and the development of canyons through time on an intermittently glaciated continental margin: The Bonanza Canyon system,offshore eastern Canada

Bonanza Canyon is a complex canyon system on the slope from the intermittently glaciated Grand Bank on the south side of Orphan Basin. A 3D seismic reflection volume, 2D high-resolution seismic reflection profiles and ten piston cores were acquired to study the evolution of this canyon system in relation to glacial processes on the continental shelf and the effects of different types of turbidity currents on the development of deep water channels. Mapped reflector surfaces from the 3D seismic volume show that the Bonanza Canyons developed in a depression created by a large submarine slide of middle Pleistocene age, coincident with the onset of glacigenic debris flows entering western Orphan Basin. Two 3–5 km wide, flat-floored channels were cut into the resulting mass-transport deposit and resemble catastrophic glacial meltwater channels elsewhere on the margin. Both channels subsequently aggraded. The eastern channel A became narrower but maintained a sandy channel floor. The western channel, B, heads at a spur on the continental slope and appears to have been rather passively draped by muds and minor sands that have built 1500-m wave length sediment waves.Muddy turbidites recorded by piston cores in the channel and on the inter-channel ridges are restricted to marine isotope stage (MIS) 2 and were deposited from thick, sheet-like, and sluggish turbidity current derived from western Orphan Basin that resulted in aggradation of the channels and inter-channel ridges. Sandy turbidites in channels and on inner levees were deposited throughout MIS 2–3 and were restricted to the channels, locally causing erosion. Some coincide with Heinrich events. Channels with well-developed distributaries on the upper slope more readily trap the sediments on Grand Bank to form sandy turbidity currents. Channel B dominated by muddy turbidity currents has wide and relatively smooth floor whereas channel A dominated by sandy turbidity currents has a sharp geometry.     

Seismic imaging of the Formosa Ridge cold seep site offshore of southwestern Taiwan

Multi-scale reflection seismic data, from deep-penetration to high-resolution, have been analyzed and integrated with near-surface geophysical and geochemical data to investigate the structures and gas hydrate system of the Formosa Ridge offshore of southwestern Taiwan. In 2007, dense and large chemosynthetic communities were discovered on top of the Formosa Ridge at water depth of 1125 m by the ROV Hyper-Dolphin. A continuous and strong BSR has been observed on seismic profiles from 300 to 500 ms two-way-travel-time below the seafloor of this ridge. Sedimentary strata of the Formosa Ridge are generally flat lying which suggests that this ridge was formed by submarine erosion processes of down-slope canyon development. In addition, some sediment waves and mass wasting features are present on the ridge. Beneath the cold seep site, a vertical blanking zone, or seismic chimney, is clearly observed on seismic profiles, and it is interpreted to be a fluid conduit. A thick low velocity zone beneath BSR suggests the presence of a gas reservoir there. This “gas reservoir” is shallower than the surrounding canyon floors along the ridge; therefore as warm methane-rich fluids inside the ridge migrate upward, sulfate carried by cold sea water can flow into the fluid system from both flanks of the ridge. This process may drive a fluid circulation system and the active cold seep site which emits both hydrogen sulfide and methane to feed the chemosynthetic communities.     

厦门湾口外近岸陆架区海底沙波发育特征

利用2014–2017年在台湾海峡西部采集的多波束、单道地震剖面、沉积物粒度样品及海流监测资料,在厦门湾近岸陆架区识别出一系列海底沙波,并对沙波的形态特征、分布规律和沉积物组成特征进行分析,探讨水动力条件及其对沙波发育的影响。结果表明沙波发育区水深一般为10~60 m,地形较平缓开阔,坡度一般为0°~1°;平面上沙波区呈一系列NW-SE向条带状坡地,波脊呈线性或新月形,波脊轴线为SW-NE方向,沙波波长为120~800 m,波高2~12 m,沙波指数较大(>30)。地震剖面显示,波形形态主要分为三类:近对称性沙波、非对称性沙波及叠合沙波。近对称性沙纹的波高较大,沙波指数小;非对称性沙波的波长较长,沙波指数大;稳定沙波经后期水流“改造、激活”形成叠合沙波。砂含量较高,沉积物类型以砂、粉砂质砂及砂质粉砂为主,多为细砂—中砂。厦门湾口外的近岸陆架区水动力较强,流系复杂,总体受浙闽沿岸流、南海表层流和黑潮分支的影响。本区为不正规半日潮,流速为0.3~0.7 m/s,落潮流以S向为主,涨潮流向以NNE向为主,潮流作用对沙波的发育和改造起重要影响。     

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。     

Processes controlling very low sedimentation rates on the continental slope of the Gonone-Orosei canyon system,NE Sardinia—terrestrial and oceanic significance

The narrow shelf and upper slope immediately above the Gonone canyon head off NE Sardinia represent areas of very low sedimentation rates. Along the sides of the canyon head (1,600 m water depth), the sediment deposits are homogeneous but show alternating light-grey intervals rich in carbonate and dark-grey ones rich in organic matter, possibly related to distal turbidite processes. Deposits older than 50,000 years are already encountered at core depths of 2.50 m, the sedimentation rates varying from 6–21 cm/10³ years in the lower parts of two cores and from 1.5–3 cm/10³ years in the upper parts. At about 35,000 years BP, both cores show a simultaneous drop in sedimentation rate by a factor of 3, probably in response to local mechanisms of channel avulsion. Lithological, mineralogical and geochemical properties reveal the environmental factors which are responsible for the extremely slow sediment accumulation. The southernmost sector of the coast, and partly also of the shelf, consists of Jurassic limestones which supply only small amounts of fine-grained material transported in suspension. During the last sea-level highstand, the accumulation of the Cedrino River pro-delta remained restricted to the coast, the low siliciclastic sediment yields resulting in poor shelf sediment trapping. The present morphology of the canyon head prevented the occurrence of gravity processes in the deeper part of the canyon system, including the coring sites. Accordingly, deposition was mainly fed by hemipelagic material of planktonic origin, together with only moderate terrigenous inputs. On a wider late Pleistocene timescale, seismic data indicate the occurrence of a coarse-grained, layered turbidite facies, implying a very different architecture of the canyon drainage system probably prior to 60,000 years BP.     

南海北部水道活动引起的不同类型滑塌体对应的水合物赋存模式

2015~2016年在神狐新钻探区钻遇大量水合物岩心，证实南海北部神狐新钻探区具有较好的水合物成藏环境和勘探前景。结合2008~2009年该区采集的地震资料，我们对晚中新世以来细粒峡谷的沉积特征及其相应的水合物成藏模式进行了分析。通过对大量地震剖面进行解释，发现该区峡谷两侧的隆起上发育大量的滑塌体。本文通过岩心粒度分析，地震相识别分析和水合物测井响应分析等手段综合识别出对水合物成藏有控制作用的三种类型的滑塌体：原生滑塌体、峡谷切割滑塌体、和同生断裂滑塌体。结合沉积速率、流体流速分析和峡谷迁移等沉积学要素对滑塌体成因进行分析，认为峡谷切割滑塌体由于后期峡谷迁移对前期滑塌体切割形成的、同生断裂滑塌体是由于隆起区基底不平引起差异性沉降而形成的。不同类型的滑塌体发育位置不同：原生滑塌体常发育在隆起中坡度较缓的区域、峡谷切割成因滑塌体常发育在不定向迁移的峡谷两侧、同生断裂滑塌体常发育在隆起中坡度起伏较大的区域。三种类型滑塌及其相应的水合物成藏模式不同，其中原生滑塌体有利于水合物成藏，而另外两种类型的滑塌体由于其不能对自由气进行有效封堵而不利于水合物成藏。根据三种滑塌体对水合物成藏的响应指出在粗粒的含有孔虫粉砂岩储层上，覆盖细粒的泥岩对自由气进行封堵有利于水合物成藏，并且多层的泥岩覆盖是造成水合物稳定带中水合物多个分层成矿现象出现的原因。     

Mounded seismic units in the modern canyon system in the Shenhu area,northern South China Sea: Sediment deformation,depositional structures or the mixed system?

The canyon system, including 17 small slope-confined canyons in the Shenhu area, northern South China Sea, is significantly characterized by mounded or undulating features on the canyon flanks and canyon heads. However, the mechanism underlying the formation of these features has yet to be elucidated. In previous studies, most of them were interpreted as sediment deformation on the exploration seismic profiles. In this paper, we collected high-resolution bathymetric data, chirp profiles and geotechnical test data to investigate their detailed morphology, internal structures, and origin. The bathymetric data indicated that most mounded seismic units have smooth seafloors and are separated by grooves or depressions. The distance between two adjacent mounded units is only hundreds of meters. On chirp profiles, mounded seismic units usually exhibit chaotic reflections and wavy reflections, of which the crests migrate upslope. The slope stability analysis results revealed that the critical angle of the soil layers in the study area tends to be 9°, indicating that most mounded seismic units on the canyon flanks and heads are stable at present. The terrain characteristics and seismic configurations combined with the slope stability analysis results indicated that most mounded seismic units are not sediment deformation but depositional structures or mixed systems composed of deformation and depositional structures.     

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.     

Sediment waves on the South China Sea Slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific Deep Water into the South China Sea

Using an integrated multi-beam bathymetry, high-resolution seismic profile, piston core, and AMS ¹⁴C dating data set, the current study identified two sediment wave fields, fields 1 and 2, on the South China Sea Slope off southwestern Taiwan. Field 1 is located in the lower slope, and sediment waves within it are overall oriented perpendicular to the direction of down-slope gravity flows and canyon axis. Geometries, morphology, and internal seismic reflection configurations suggest that the sediment waves in field 1 underwent significant up-slope migration. Field 2, in contrast, is located more basinward, on the continental rise. Instead of having asymmetrical morphology and discontinuous reflections as observed in field 1, the sediment waves in field 2 show more symmetrical morphology and continuous reflections that can be traced from one wave to another, suggesting that vertical aggradation is more active and predominant than up-slope migration.Three sediment wave evolution stages, stage 1 through stage 3, are identified in both field 1 and field 2. During stage 1, the sediment waves are built upon a regional unconformity that separates the underlying mass-transport complexes from the overlying sediment waves. In both of these two fields, there is progressive development of the sediment waves and increase in wave dimensions from the oldest stage 1 to the youngest stage 3, even though up-slope migration is dominant in field 1 whereas vertical aggradation is predominant in field 2 throughout these three stages.The integrated data and the depositional model show that the upper slope of the study area is strongly dissected and eroded by down-slope gravity flows. The net result of strong erosion is that significant amounts of sediment are transported further basinward into the lower slope by gravity flows and/or turbidity currents. The interactions of these currents with bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea and preexisting wavy topography in the lower slope result in the up-slope migrating sediment waves in field 1 and the contourites as observed from cores TS01 and TS02. Further basinward on the continental rise, turbidity currents are waned and diluted, whereas along-slope bottom (contour) currents are vigorous and most likely dominate over the diluted turbidity currents, resulting in the vertically aggraded sediment waves in field 2.The results from this study also provide the further evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in South China Sea at least since Quaternary.     

Glacial and post-glacial sedimentary processes in the Irish Rockall Trough from an integrated acoustic analysis of near-seabed sediments

Near-seabed (<50 m) sediments were studied throughout the Irish sector of the Rockall Trough (ca. 123,000 km²) based on a combined analysis of shallow seismic (3.5 kHz) and multibeam swath data acquired by the Irish National Seabed Survey and reprocessed here at higher resolution. The detailed identification of seven acoustic facies served to classify the Rockall Trough into six main sedimentary provinces, incorporating the well-known Feni Drift, Donegal-Barra Fan and Rockall Bank mass flow. In the northern part of the study area, extensive mass transport deposits from both flanks of the trough are the dominant depositional features. Debris flow deposits formed by ice streaming of the British-Irish ice sheet characterise most of the Donegal-Barra Fan, whereas turbidite deposition occurs towards the toe of the fan. On the western margin of the trough, the post-glacial Rockall Bank mass flow deposit displays a rough topography with no acoustic penetration. Several failure scarps are visible on the flank of the bank where the mass flow originated, and pass downslope into large sediment lobes and smaller debris flow deposits. Smaller-scale mass transport deposits were also discovered close to some canyons indenting the eastern slope. High seismic penetration characterises the Feni contourite drift deposit, and precise mapping of its geographical extent shows that it is considerably wider than previously reported. The sediment waves that drape this drift are interpreted as predominantly relict features, and their varied geometry suggests a complex oceanographic regime. In the deeper part of the trough, the data reveal novel evidence of the widespread occurrence (about 12,000 km²) of distinct seismic and backscatter signatures indicating the possible presence of fluid escape structures within fine-grained sediments of mixed contouritic, hemipelagic and turbiditic origin. Sediment overloading and increased pore pressure resulting from extensive mass wasting to the north of the area is a likely cause of dewatering-rooted fluid migration towards the seabed, but further investigations are required to confirm the nature and origin of such fluids in the Rockall Trough.     

Past and present sedimentary activity in the Capbreton Canyon, southern Bay of Biscay

Located in the south-eastern part of the Bay of Biscay, the Capbreton Canyon incises the continental shelf up to the 30 m isobath contour, and acts as a natural conduit for continental and shelf-derived sediments. EM1000 multibeam bathymetry shows two main features characterising the canyon — a deeply entrenched meandering channel, bordered by fluvial-like terraces constituting large sediment traps. A dataset of cores and seismic profiles together with a multibeam bathymetry map has enabled the characterisation of recent sedimentary activity in the axial channel and on the terraces. Data analysis evidenced the major role of the canyon head in recent sediment dynamics. This part of the canyon is a temporary reservoir for sediments, accumulated by coastal hydrodynamic processes. Exceptional climatic, tectonic or hydrodynamic events can mobilise the sediments and generate gravity-driven flows. Under the present-day sea-level highstand conditions, these flows are not powerful enough to bring their bedload to the deep sea, and are confined mainly to the upper part of the canyon. Turbidity currents model the axial channel pathway and are at the origin of terrace formation. Terraces in the Capbreton Canyon are not typical but rather are reduced to confined levees. Three factors control the vertical growth of a terrace: (1) the amount of overspilled sediments brought by turbidity currents, (2) hemipelagic sedimentation and (3) terrace height. The amount of sediment spilling over a terrace decreases with increased terrace elevation. Concurrently, the proportion of hemipelagic fallout depositing on a terrace increases. Terraces are considered to be fossil when the height of the terrace prevents further deposition by overspilling. The terraces studied in this paper are interpreted as having formed during the Holocene, implying that the sediment dynamics of the Capbreton Canyon is continuous through time. Highstand periods differ from lowstand periods because they show a decrease in the energy of erosive processes. Temporal variations in erosive and depositional processes in the canyon are controlled by the Adour River, which delivers large amounts of sediment to the system.     

General patterns of circulation, sediment fluxes and ecology of the Palamós (La Fonera) submarine canyon, northwestern Mediterranean

Currents, particle fluxes and ecology were studied in the Palamós submarine canyon (also known as the Fonera canyon), located in the northwestern Mediterranean. Seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis, on the canyon walls and on the adjacent slope. Additionally, local and regional hydrographic cruises were carried out. Current data showed that mean near surface and mid-depth currents were oriented along the mean flow direction (NE–SW), although at 400 and 1200 m depth within the canyon current reversals were significant, indicating a more closed circulation inside the canyon. Mean near-bottom currents were constrained by the local bathymetry, especially at the canyon head. The most significant frequency at all levels was the inertial frequency. A second frequency of about three days, attributed to a topographic wave, was observed at all depths, suggesting that this wave was probably not trapped near the bottom. The current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon. The associated vertical velocity ranged between 10 and 20 m/day and was constrained to the upper 300 m. This latter feature, together with other computations, suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow.In the canyon, suspended sediment concentration, downward particle fluxes, chlorophyll and particulate C and N were significantly higher up-canyon from about 1200 m depth than offshore, defining, along with the different hydrodynamics, two canyon domains: one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf-slope front. The higher near-bottom downward total mass fluxes were recorded in the canyon axis at 1200 m depth along with sharp turbidity increases and are related to sediment gravity flows. During the deployment period, the increase in downward particle fluxes occurred by mid-November, when a severe storm took place. On the canyon walls at 1200 m depth, suspended sediment concentrations, downward particle fluxes, chlorophyll and particulate C and N were higher on the southern wall than on the northern wall inversely to the current’s energy. This could be caused by an upward water supply on the southern canyon wall and/or the mean flow interacting with the canyon bathymetry. In the swimmers collected by the sediment traps, the dominant species was an elasipod holothurian, which has not been recorded in other canyons or elsewhere in the Mediterranean, indicating particular speciation.     

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     

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.     

Sediment failures and flows in the Gulf of Cadiz (eastern Atlantic)

Several types of sediment failures in the Gulf of Cadiz were observed using multibeam bathymetry, acoustic imagery and high-resolution seismic. These instabilities are mainly sediment failures and flows. Their width and length vary from 1 to more than 10 km. The failures are mainly related to high sedimentation rates, particularly in places where the Mediterranean Outflow Water (MOW) spills over, such as channel bends and the outer side of the giant contourite levee. Steep slopes are also a trigger for failure at the continental shelf-slope transition, on valley sides, on canyon flanks, and on the sides of bathymetric highs. Other mass movements are related to fluid escape (mud volcanoes) and earthquakes. In areas where the MOW flows along the seafloor, the constant shearing and related erosion can add to the overall stresses. The frequency of failures can be estimated using the deposits resulting of their distal transformations into turbidites.     

花东盆地晚中新世以来沉积演化特征

利用近年来在台湾东部海域采集的多道地震和多波速测深资料，对该海域花东海盆区晚中新世以来的沉积充填演化特征进行描述和分析。通过对花东海盆区域地形特征描述、层序地层格架的建立和地震剖面的解译，在本区晚中新世以来的沉积充填中刻画出6种典型地震相类型，并分析其对应的沉积相类型，包括浊积扇、浊积水道充填、块体流、沉积物波、海底峡谷－伴生沉积物滑塌变形－充填、深水扇沉积。结合地震相平面分布及垂向沉积相叠置关系，将晚中新世－第四纪沉积充填演化划分为3个阶段:晚中新世晚期开始受到块体流冲蚀阶段，到海底峡谷冲刷－沉积物失稳－峡谷充填－再侵蚀阶段，到峡谷输送的大量沉积物在上新世以来主要堆积发育了沉积物波、浊积扇、深水扇等沉积体系阶段。     

Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies

The Neogene and Quaternary sediments of the Faeroe-Shetland Channel and West Shetland shelf and slope rest upon a major regional unconformity, the Latest Oligocene Unconformity (LOU), and have been deposited through the interaction of downslope and parallel-to-slope depositional processes. The upper to middle continental slope is dominated by mass-transport deposits (debris flows), which progressively diminish downslope, and were largely generated and deposited during glacial cycles when ice sheets supplied large quantities of terrigeneous sediment to the upper slope and icebergs scoured sea-floor sediments on the outer shelf and uppermost slope. Large-scale sediment failures have also occurred on the upper slope and resulted in deposition of thick, regionally extensive mass-transport deposits on portions of the lower slope and channel floor. In contrast, large fields of migrating sediment waves and drift deposits dominate most of the middle to lower slope below 700 m water depth and represent deposition by strong contour currents of the various water masses moving northeastward and southwestward through the channel. These migrating sediment waves indicate strong northeastward current flow at water depths shallower than 700 m and strong southwestward current flow at water depths from 700 to >1,400 m. These flow directions are consistent with present-day water-mass flow through the Faeroe-Shetland Channel. The Faeroe-Shetland Channel floor is underlain by thin conformable sediments that appear to be predominantly glacial marine and hemipelagic with less common turbidites and debris flows. No evidence is observed in seismic or core data that indicates strong contour-current erosion or redistribution of sediments along the channel floor.     

Large Eddy Simulation for Plunge Breaker and Sediment Suspension

Breaking waves are a powerful agent for generating turbulence that plays an important role in many fluid dynamical processes, particularly in the mixing of materials. Breaking waves can dislodge sediment and throw it into suspension, which will then be carried by wave-induced steady current and tidal flow. In order to investigate sediment suspension by breaking waves, a numerical model based on large-eddy-simulation (LES) is developed. This numerical model can be used to simulate wave breaking and sediment suspension. The model consists of a free-surface model using the surface marker method combined with a two-dimensional model that solves the flow equations. The turbulence and the turbulent diffusion are described by a large-eddy-simulation (LES) method where the large turbulence features are simulated by solving the flow equations, and a subgrid model represents the small-scale turbulence that is not resolved by the flow model. A dynamic eddy viscosity subgrid scale stress model has been used for the     

Evidence of hyperpycnal flows at the head of the meandering Kaoping Canyon off SW Taiwan

Using seismic and Chirp sonar profiles, this paper tests the hypothesis that hyperpycnal flows are the main factor controlling the formation and maintenance of the meandering Kaoping submarine canyon off SW Taiwan. Cross-section geometries, and erosional as well as depositional features vary along the canyon course. In the proximal, sinuous part of the canyon, down-cutting into the shelf strata has created a relief of 340 m. The cause of this intense erosion of the seafloor is suggested to be the frequent development of hyperpycnal flows. A seismic section across a meander in the distal part of the canyon shows levees formed by overspilled sediments at the outer bend, and a terrace characterized by relatively flat stratified facies at the inner bend. The geological setting and climatic conditions in SW Taiwan (e.g. earthquakes, typhoons, floods), as well as major river–canyon connections (for example, direct input of highly concentrated suspended sediment) would all promote hyperpycnal flow generation. This causes axial incision, canyon wall slumping, and the formation of levees by spill-over deposition in the upper reach of the Kaoping Canyon.     

Stratal architecture and origin of lateral accretion deposits (LADs) and conterminuous inner-bank levee deposits in a base-of-slope sinuous channel,lower Isaac Formation (Neoproterozoic), East-Central British Columbia,Canada

In the Castle Creek study area, a vertically dipping, 2.5 km-thick succession of basin-floor to base-of-slope Neoproterozoic rocks are superbly exposed. In part of that outcrop, inner-bend (point-bar) deposits of sharp-based, laterally accreting sinuous channels are exposed, of which one is described in detail (Isaac Channel unit 2.2—IC2.2). IC2.2 is up to 13 m thick and extends laterally for at least 400 m. Lateral-accretion deposits, or simply lateral accretion deposits (LADs), are inclined at 7–12° toward the channel base and are about 120–140 m long. Grain size changes little obliquely upward along an individual LAD, or vertically upward through the channel-fill. LADs consist of two repeating and interstratified kinds: coarse-grained LADs consisting of strata up to granule conglomerate, and fine-grained LADs composed of thin- to medium-bedded finer-grained turbidites. In the lower part of the channel-fill, strata consist only of amalgamated coarse-grained LADs composed of decimetre-thick beds composed of very coarse sandstone/granule conglomerate that grade upward to medium sandstone. Tractional sedimentary structures are absent and fine-grained strata, specifically mudstone, occur only as isolated patches of intraclast breccia. In the upper part of the channel-fill, however, LADs consist of a rhythmic interfingering of coarse- and fine-grained LADs. Coarse-grained LADs consist of 2–3 bed-thick packages that are separated and then pinch-out rapidly into fine-grained LADs. Close to their up-dip pinch-out these coarse strata consist commonly of poorly sorted, ungraded very coarse sandstone/granule conglomerate overlain abruptly by planar-laminated or medium-scale (dune) cross-stratified, medium-grained sandstone. Fine-grained LADs are composed of mudstone interbedded with thin- and medium-bedded T_bcd and T_cd turbidites that obliquely downward and become truncated as the super- and subjacent coarse-grained LADs amalgamate.The rhythmic intercalation of coarse- and fine-grained LADs is interpreted to be related to temporal changes in the nature of sediment deposition along the point-bar of a deep-marine sinuous channel. Following failure along the cut-bank margin (outer bend), deposition of coarse-grained sediment on the point-bar (inner bend) occurred in order to re-establish an equilibrium channel geometry, and thereby equilibrium sediment transport conditions (i.e. sediment bypass). Once equilibrium was re-established deposition of finer, thinner-bedded strata of the succeeding fine LAD resumed. These strata represent deposition from the dilute tail region of flows that for the most part had already transited that particular channel bend and transported the bulk of its coarse sediment further down-dip. This history of alternating coarse and fine-grained sedimentation was repeated several times in the channel bend as it migrated laterally. Moreover, in coarse LADs, the restricted occurrence of tractional sedimentary structures close to their up-dip pinch-out suggests that although suspension deposition may have dominated over much of the lateral accretion surface, it was succeeded, at least on the upper part of the lateral accretion surface, by sediment reworking and bed-load transport, possibly related to elevated turbulent stresses caused by mixing along the sharp density interface in a strongly stratified turbulent flow.Although seemingly similar to LADs reported from fluvial point-bars, deep-marine LADs of the Windermere exhibit many important differences. Some of these differences are likely related to the differences in the mode of sand (and coarser) sediment transport in deep-marine versus non-marine environments, specifically, suspension versus bed load, respectively. In addition, fundamental differences in the flow structure between subaqueous suspension currents and open-channel flows most probably exert an additional first-order control contributing to these differences.