Interactions between submarine channel systems and deformation in deepwater fold belts: Examples from the Levant Basin, Eastern Mediterranean sea

Submarine channel levee systems form important hydrocarbon reservoirs in many deep marine settings and are often deposited within a structurally active setting. This study focuses on recent submarine channels that developed within a deepwater fold and thrust belt setting from the Levant Basin, eastern Mediterranean Sea. Compressional deformation within the study area is driven by the up-dip collapse of the Nile cone above the ductile Messinian Evaporites. Structures such as folds and strike slip faults exert a strong control on channel location and development over time. From this study four end-member submarine channel–structure interactions can be defined: Confinement, diversion, deflection and blocking. Each of these channel–structure interactions results in a distinct submarine channel morphology and pattern of development compared to unconfined channel levee systems. Each interaction can also be used to assess timing relationships between submarine channel development and deformation.     

南海北部西沙海槽北侧陆坡块体搬运沉积体系的发育特征及演化

Triple mass-transport deposits(MTDs) with areas of 625, 494 and 902 km2, respectively, have been identified on the north slope of the Xisha Trough, northern South China Sea margin. Based on high-resolution seismic reflection data and multi-beam bathymetric data, the Quaternary MTDs are characterized by typical geometric shapes and internal structures. Results of slope analysis showed that they are developed in a steep slope ranging from 5° to 35°. The head wall scarps of the MTDs arrived to 50 km in length(from headwall to termination). Their inner structures include well developed basal shear surface, growth faults, stepping lateral scarps, erosion grooves, and frontal thrust deformation. From seismic images, the central deepwater channel system of the Xisha Trough has been filled by interbedded channel-levee deposits and thick MTDs. Therefore, we inferred that the MTDs in the deepwater channel system could be dominated by far-travelled slope failure deposits even though there are local collapses of the trough walls. And then, we drew the two-dimensional process model and threedimensional structure model diagram of the MTDs. Combined with the regional geological setting and previous studies, we discussed the trigger mechanisms of the triple MTDs.     

海盆沉积“源-汇”系统分析:南海北部珠江海谷-西北次海盆第四纪深水浊积扇

运用近年来采集的高分辨率地震资料和多波束测深数据,在珠江海谷及西北次海盆深海平原区发现大规模发育的第四纪重力流沉积体系,该沉积体系沿珠江海谷以北西-南南东方向贯穿整个北部陆坡,进入西北次海盆后呈扇形展开,形成珠江海谷-西北次海盆大型深水浊积扇系统。据沉积体系空间展布特征差异,将珠江海谷划分为北、中、南三段,北段为过路侵蚀和水道下切,中段以水道充填和天然堤沉积为主,南段以水道-天然堤和朵叶体沉积共存为特征,揭示出北部陆坡珠江海谷是珠江口外陆缘物质输送海盆深海平原的主要通道;海盆区总体以朵叶体发育为特色,呈扇形展布。深水扇系统可分为三期次沉积体,其区域结构记录了重力流沉积物从侵蚀、卸载到南海海盆作为限制性盆地接收陆源沉积物的全过程,为“源-渠-汇”的研究构建了一个完美的范例。本文以珠江海谷-西北次海盆第四纪深水浊积扇沉积体系为例,完整地揭示了水道-扇体的组构和特征,清晰呈现了陆坡-海盆砂体展布的规律,可为建立南海北部新近纪早期深水扇形成模式提供参考,有助于指导南海深水油气勘探工作。     

Sedimentological criteria to differentiate submarine channel levee subenvironments: Exhumed examples from the Rosario Fm. (Upper Cretaceous) of Baja California, Mexico, and the Fort Brown Fm. (Permian), Karoo Basin, S. Africa

Two scales of levee confinement are commonly recognised from submarine channel-levee systems on the seafloor and in the subsurface. Large-scale external levees bound the entire system whilst smaller-scale internal levees bound individual thalweg channels within the channel-belt. Although thin beds are commonly identified in core and well logs, their origin, and consequently their stratigraphic significance is currently poorly understood. This knowledge gap stems, in part, from the lack of unambiguously identified outcrop analogues of channel-levees, and in particular the lack of identifiable internal and external levees. Here we report from two exhumed channel-levee systems where both scales of confinement can be recognised: the Rosario Fm. of Baja California, and the Fort Brown Fm. of South Africa. A suite of characteristic sedimentary features are recognised from internal and external levees respectively: internal levees are characterised by structures indicative of complexity in the waxing-waning style of overspill, interactions with topography and flow magnitude variability; in contrast, external levees are characterised by structures indicative of simple surge-like waning flows, relatively uniform flow directions, laterally extensive beds, and a lack of erosive events. Using these observations, together with published literature, we propose a simple nomenclatural scheme for levee sub-environments, and criteria to differentiate between levee sub-environments in core or outcrop.     

The Ebro Deep-Sea Fan system

The Ebro Fan System consists of en echelon channel-levee complexes, 50×20 km in area and 200-m thick. A few strong reflectors in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee complexes prevent the development of distributary channels and midfan lobe deposits.     

The Ebro Deep-Sea Fan system

The Ebro Fan System consists of en echelon channel-levee complexes, 50×20 km in area and 200-m thick. A few strong reflectors in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee complexes prevent the development of distributary channels and midfan lobe deposits. Margin setting represents fan and/or source area     

Small turbidite systems in a complex tectonic setting (SW Mediterranean Sea): morphology and growth patterns

Three small turbidite systems (Almeria, Sacratif, and Guadiaro), each tens of kilometres long, are developed in the complex morpho-structural setting of the northern Alboran Sea and have similar primary architectural elements (canyons, channel-levee systems, lobes). However, comparison reveals differences in the axial gradients of their canyons, depth/physiographic location, morphological framework, and lateral and longitudinal sedimentary shifts of turbidite deposition. The depositional architecture and sedimentary evolution from late Pliocene to Quaternary seems to be conditioned by number of submarine feeding sources (canyons), sea-level fluctuations and local tectonic (e.g. margin/canyon-channel gradients, faults). We group the Alboran turbidite systems into two models: mud/sand-rich submarine point-source and mud/sand-rich multiple submarine source ramp.     

南沙头通道及横沙通道对长江口深水航道的影响分析

根据长江口南沙头通道、横沙通道和南北槽分汊口的断面水深变化及长江口南北港和南北槽的分流比变化实测资料,分析了长江口北槽深水航道淤积的原因。结果表明,北槽深水航道上段淤积受多种因素影响,其中,南沙头通道和横沙通道的发展对深水航道影响最大。南沙头通道的发展在加大落潮流量的同时,对南港南岸会产生一定的冲刷,后经沙洲的阻挡,把泥沙带向南港北岸,在北槽进口段处落淤,直接影响了进入深水航道的落潮量;横沙通道由于直接贯通了北港北槽的水沙交换,因而削弱了南港和北槽之间的水沙交换,促使北槽深水航道上段产生淤积,这也是南槽河道上段刷深的一个主要原因,而南槽河道的发展必然减少了进入北槽的落潮量,进一步加剧了北槽深水航道上段的淤积。同时,科氏力与北槽南导堤分流口鱼咀工程对深水航道也造成了一定的不可忽视的影响。研究成果对治理北槽深水航道淤积问题保障深水航道正常运行具有十分重要的科学实践意义。     

Sedimentology, stratigraphic occurrence and origin of linked debrites in the West Crocker Formation (Oligo-Miocene), Sabah, NW Borneo

The West Crocker Formation (Oligocene–Early Miocene), NW Borneo, consists of a large (>20 000 km²) submarine fan deposited as part of an accretionary complex. A range of gravity-flow deposits are observed, the most significant of which are mud-poor, massive sandstones interpreted as turbidites and clast-rich, muddy sandstones and sandy mudstones interpreted as debrites. An upward transition from turbidite to debrite is commonly observed, with the contact being either gradational and planar, or sharp and highly erosive. Based on their repeated vertical relationship and the nature of the contact between them, these intervals are interpreted as being deposited from one flow event which consisted of two distinct flow phases: fully turbulent turbidity current and weakly turbulent to laminar debris flow. The associated bed is called a co-genetic turbidite–debrite, with the upper debrite interval termed a linked debrite. Linked debrites are best developed in the non-channellised parts of the fan system, and are absent to poorly-developed in the proximal channel-levee and distal basin floor environments. Due to outcrop limitations, the genesis of linked debrites within the West Crocker Formation is unclear. Based on clast size and type, it seems likely that a weakly turbulent to laminar debris-flow flow phase was present when the flow event entered the basin. A change in flow behaviour may have led to deposition of a sand-rich unit with ‘turbidite’ characteristics, which was subsequently overlain by a mud-rich unit with ‘debrite’ characteristics. Flow transformation may have been enhanced by the disintegration and incorporation into the flow of muddy clasts derived from the upstream channel floor, channel mouth or from channel-levee collapse. Lack of preservation of this debrite in proximal areas may indicate either bypass of this flow phase or that the available outcrops fail to capture the debris flow entry point. Establishing robust sedimentological criteria from a variety of datasets may lead to the increasing recognition of co-genetic turbidite-debrite beds, and an increased appreciation of the importance of bipartite flows in the transport and deposition of sediments in deepwater environments.     

Flow processes and sedimentation in submarine channel bends

Turbidity currents in sinuous submarine channels are an important mechanism for transporting terrestrial sediments to deep water, and their deposits are of increasing importance as hydrocarbon exploration targets. Despite this, the architecture and dynamics of submarine channel systems are not well understood. Analogies are often drawn with fluvial systems due to similarities between their planform shapes even though differences in channel evolution and hydrodynamics have been noted. A key question is the nature of deposition within submarine channel bends; in particular at inner bends where point bars form in alluvial meandering rivers. Recent experimental and numerical work has demonstrated that the fluid dynamics of submarine channel bend flow are markedly different from rivers. Notably, a reversal in the orientation of secondary (helical) flow at bend apices occurs in submarine channels. The potential influence of these differences in fluid dynamics on deposition within submarine channel bends is investigated herein. We report the results of a series of physical experiments in which solute-driven gravity currents were run through pre-formed sinuous channels containing mobile beds. These experiments reveal sedimentation patterns characterised by accumulation zones downstream of bend apices and erosion zones at outer bends. These patterns are broadly analogous to the point bars and outer-bank pools observed in meandering rivers, demonstrating that the longitudinal flow component dominates over the cross-stream component, as also occurs in rivers. However, the data suggest that the reversal in direction of the cross-stream flow component compared with subaerial flows is important in determining the position and morphology of ‘point bars’ relative to bend apices. From analogy with fluvial compound channels, and fluvial theory, this reversal in secondary flow cell orientation is also expected to influence the spatial variations of grain size in submarine channel ‘point-bar’ deposits.     

A submarine channel confluence classification for topographically confined slopes

High-quality 3D seismic data are used in this paper to: a) investigate the geometry of Miocene–Holocene submarine channels in confluence regions, and b) to correlate the geometry of channel confluences with the styles of topographic confinement imposed by salt structures in the Espírito Santo Basin (SE Brazil). A new method is used to analyse geomorphic parameters of three channel intervals (Units 1–3) and a modern channel. 5348 Channel Points (CP) are recorded on a sinusoidal slope interrupted by salt structures. In the upper-slope region, diapir confinement directly controls the location of channel confluences. These are characterised by a sharp increase in channel width and height at their junctions. Scale relationships show an increase in channel width of the order of 1.19 < W < 1.22 in confluence regions. Ratios of channel height between confluence and pre-confluence regions are 1.09 < H < 1.40, with ratios of <1 observed in post-confluence regions. This work proves the existence of a direct relationship between topographic confinement, confluence location and relative channel distribution. The studied submarine confluences are located in regions with high confinement created by salt diapirs. The results of this work are significant, as they show clustered distribution patterns in channels, with higher channel densities being observed in pre-confluence regions. Thus, we propose a new classification for submarine confluences based on a combined analysis of channel geometry and seismic attribute data. Confluences may be symmetric or asymmetric based on the equality of the angles the tributaries bear to the post-confluence channel. Symmetric confluences can be left or right symmetric, based on whether the dominant flow path is in the left- or right-hand tributary. Asymmetric confluences are pure asymmetric or secondary asymmetric depending on whether the dominant flow takes place along the main tributary and the post-confluence channel alignement, or along the secondary tributary which is at an angle with the main alignement.     

Sedimentation styles and depositional processes in a Middle to Late Jurassic slope environment, Bowser Basin, northwestern British Columbia, Canada

The Middle to Upper Jurassic Todagin assemblage in northwestern British Columbia, Canada, was deposited in the Bowser Basin above arc-related rocks of the Stikine terrane. Sedimentary structures indicate that a variety of gravity flow processes were involved in transport and deposition in deep-water slope environments. At Mount Dilworth, laterally continuous and channelized turbidites are interbedded with and overlain by mass-transport deposits in which sedimentary clasts are supported in a mudstone matrix. More than 50% of the succession consists of mass-transport deposits, indicating significant slope instability. A 300 m thick mass-transport complex exposed near the top of the succession is interpreted to result from tectonic activity, which triggered a major change in sediment supply from a local source area. At Todagin Mountain, a channel complex displays three successive channel-fills with associated overbank sedimentation units. Mass-transport deposits are rare, and confined to channel axes. Channels 1 and 2 are characterized by 40-50 m thick, ungraded pebble clast-supported conglomerate while the uppermost Channel 3 contains graded beds and occasional traction structures. The gradual change from erosive and amalgamated channel deposits at the base, to more aggradational channels at the top, is related to elevation of the equilibrium profile. Creation of accommodation favored aggradation on the mud-dominated slope succession and construction of well-developed channel-levee systems. The vertical succession exposed at Todagin Mountain is consistent with normal progradation of the slope under high sedimentation rates. In the Mount Dilworth area, extensional faulting associated with development of the restricted Eskay rift in the early Middle Jurassic produced a dissected basement above which the Todagin assemblage was deposited. These structures were inverted during collision of the Stikine and Cache Creek terranes, and likely played a major role in the stratigraphic evolution of the deep-water architectures.     

南海北部白云深水区东北部小型峡谷内的块体搬运

The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea(SCS), and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of –500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements.Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.     

Late glacial fans in the eastern Skagerrak; depositional environment interpreted from swath bathymetry and seismostratigraphy

The origin of acoustically transparent fan deposits overlying glacial till and ice-proximal sediments on the southern margin of the Norwegian Channel has been studied using high-resolution seismic-reflection profiles and multibeam bathymetry. The first deposits overlying glacigenic sediments are a series of stacked, acoustically transparent submarine fans. The lack of glaciomarine sediments below and between individual fans indicates that deposition was rapid and immediately followed the break up of the Late Weichselian ice cover. The fans are overlain by stratified glaciomarine sediments and Holocene mud. Because of the uniformity of this drape, the upper surface of the fan deposits is mimicked at the present seafloor, and the bathymetric images clearly show the spatial relationship of the fans to bedrock ridges and the presence of braided channel-levee systems on the surface of the youngest fans. The acoustically transparent character of the fan deposits indicates that they comprise silt and clay, and their lobate form and lack of internal stratification indicates that they were deposited by debris flows. The channel-levee morphology indicates deposition from more watery hyperconcentrated fluid flows. The fan sediments were either derived from 1) erosion of Mid Weichselian lake deposits in southern Skagerrak or 2) from Late glacial ice-margin lake deposits, ponded against the Norwegian Channel ice stream, which collapsed catastrophically when the lateral support was removed as the ice disintegrated. Fans composed almost exclusively of fine-grained sediment need not, therefore, rule out an origin in a deglacial setting relatively close to the former margins of glaciers and ice sheets.     

台湾海峡南部海底地形的新发现

我们于1975年对台湾海峡南部进行海底地形和底质调查研究时发现:1.台湾浅滩发育着水下沙丘群。2.陆架地形受现代环境的作用,正经历着冲刷、侵蚀和堆积的再改造过程。3.陆架边缘曾发生了巨大的滑坡。其原因是陆源物质供应严重缺乏,在风浪和各种流系作用下,使沉积物不断进行分异作用,再加上自1938年以来多次发生5—7级海底地震而导致滑坡,滑坡又促使沉积物向深水搬运,加速陆架地形的冲刷。     

Modes of development of slope canyons and their relation to channel and levee features on the Ebro sediment apron, off-shore northeastern Spain

Six submarine slope canyons in an area of the northwestern Mediterranean, offshore from the Ebro River and Delta, were surveyed with bathymetric swathmapping (SeaBeam) and mid-range side-looking sonar (SeaMARC I). All of the canyons have slightly winding paths with concave-upwards gradients that are relatively steep shallower than 1,200 m. Two major types of canyons are identified on the basis of their morphologic character at the base of the slope; Type-I canyons lead to an unchannelled base-of-slope deposit and Type-II canyons are continuous with channel-levee systems that cross the rise.Four Type-I canyons were surveyed in the area. Two of these are broad, U-shaped, steep (average gradients of 1:14), do not indent the shelf, and terminate downslope at debris-flow deposits. These two canyons, the most northern in the area, have rounded heads with extensive gullies separated by knife-edge ridges. Relief of the canyon walls is about equal on both sides of the canyons, although the right-hand walls (looking downslope) are generally steeper. The other two Type-I canyons in the area are similar in that they do not indent the shelf, but they are much smaller and shallower and coalesce before terminating in the base-of-slope region. The two Type-II canyons that feed leveed-channels are U-shaped with flatter floors, longer profiles and gentler gradients than Type-I canyons. They are closer to the Valencia Valley and have relatively small cross-sectional areas.We propose a four-stage evolutionary sequence to explain the development of the canyons observed in this section on the prograding Ebro margin. During the initial stage, slumping and erosion on the slope creates a network of small gullies. During the next stage, headward growth of one (or more) gully leads to a major indentation of the shelf. This is the critical factor for developing a channel that will incise the slope and provide a major conduit for moving sediment to the basin. Stage 3 is characterized by the development of a continuous channel accompanied by levee growth across the lobe. In the final stage, the channel-levee system becomes inactive either through destruction by mass wasting, infilling of the channel, or loss of the major sediment source.     

The Bengal Fan: morphology, geometry, stratigraphy, history and processes

The Bengal Fan is the largest submarine fan in the world, with a length of about 3000 km, a width of about 1000 km and a maximum thickness of 16.5 km. It has been formed as a direct result of the India–Asia collision and uplift of the Himalayas and the Tibetan Plateau. It is currently supplied mainly by the confluent Ganges and Brahmaputra Rivers, with smaller contributions of sediment from several other large rivers in Bangladesh and India.The sedimentary section of the fan is subdivided by seismic stratigraphy by two unconformities which have been tentatively dated as upper Miocene and lower Eocene by long correlations from DSDP Leg 22 and ODP Legs 116 and 121. The upper Miocene unconformity is the time of onset of the diffuse plate edge or intraplate deformation in the southern or lower fan. The lower Eocene unconformity, a hiatus which increases in duration down the fan, is postulated to be the time of first deposition of the fan, starting at the base of the Bangladesh slope shortly after the initial India–Asia collision.The Quaternary of the upper fan comprises a section of enormous channel-levee complexes which were built on top of the preexisting fan surface during lowered sea level by very large turbidity currents. The Quaternary section of the upper fan can be subdivided by seismic stratigraphy into four subfans, which show lateral shifting as a function of the location of the submarine canyon supplying the turbidity currents and sediments. There was probably more than one active canyon at times during the Quaternary, but each one had only one active fan valley system and subfan at any given time. The fan currently has one submarine canyon source and one active fan valley system which extends the length of the active subfan. Since the Holocene rise in sea level, however, the head of the submarine canyon lies in a mid-shelf location, and the supply of sediment to the canyon and fan valley is greatly reduced from the huge supply which had existed during Pleistocene lowered sea level. Holocene turbidity currents are small and infrequent, and the active channel is partially filled in about the middle of the fan by deposition from these small turbidity currents.Channel migration within the fan valley system occurs by avulsion only in the upper fan and in the upper middle fan in the area of highest rates of deposition. Abandoned fan valleys are filled rapidly in the upper fan, but many open abandoned fan valleys are found on the lower fan. A sequence of time of activity of the important open channels is proposed, culminating with formation of the one currently active channel at about 12,000 years BP.     

A model of submarine channel-levee evolution based on channel trajectories: Implications for stratigraphic architecture

Channel-levee systems are frequently interpreted as having a long history of cut-and-fill by channel-shaped features of different scales. Results from a simple geometric model based on a centerline migration algorithm combined with a vertical channel trajectory show that an incising-to-aggrading trajectory of a single channel can produce realistic morphologies similar to systems observed on the seafloor and subsurface, including features such as a basal erosional surface, coeval inner and outer levees, internal erosional boundaries, and terraces draped by inner levee deposits. Channel migration results in composite erosional surfaces that are distinct from topographic surfaces, and their formation does not require larger than usual erosional flows. Many submarine channels interpreted as underfit were probably carved by flows similar to the ones that eroded and deposited the entire channel system. We suggest that the features of most submarine channel-levee systems do not require large temporal variations in flow magnitude but can be explained by a simpler model whereby incision, migration and aggradation of a single channel form over time results in an apparently complex system.     

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 Bengal Submarine Fan, Northeastern Indian ocean

Bengal Submarine Fan, with or without its eastern lobe, the Nicobar Fan, is the largest submarine fan known. Most of its sediment has been supplied by the Ganges and Brahmaputra Rivers, probably since the Early Eocene. The “Swatch-of-No-Ground” submarine canyon connects to only one active fan valley system at a time, without apprent bifurcation over its 2500-km length. The upper fan is comprised of a complex of huge channel-levee wedges of abandoned and buried older systems. A reduction of channel size and morphology occurs at the top of the middle, fan, where meandering and sheet flow become more important. Margin setting represents fan and/or source area