南海北部陆坡区海底峡谷地貌、沉积特征及控制因素

南海北部陆坡发育众多海底峡谷,其形成、发育、演化过程都存在较大差异。本文选取南海北部陆坡典型的珠江口外海底峡谷群、东沙海底峡谷、台湾浅滩南海底峡谷和澎湖海底峡谷进行研究,通过高分辨率多道地震数据和多波束测深数据,结合前人研究成果,对4条典型海底峡谷的形态特征、沉积充填特征及结构、形成发育过程及控制因素进行研究。结果表明,南海北部陆缘各个海底峡谷的形成受多个控制因素的影响,其影响程度及方式都有差别。构造活动、海平面变化及沉积物重力流与海底峡谷的演化密切相关,而陆地河流和局部构造因素也以不同方式影响着海底峡谷的发育。对于发育在主动大陆边缘的台湾岛东南侧的澎湖海底峡谷,其板块运动和岩浆活动活跃,其上发育的海底峡谷的控制因素以内营力地质作用为主。而具有被动大陆边缘属性的其他3条峡谷,由于构造运动较少或停止,其上发育的海底峡谷的控制因素以外营力地质作用为主。     

Transtensional tectonism and its effects on the distribution of sandbodies in the Paleogene Baiyun Sag, Pearl River Mouth Basin, China

The Baiyun Sag, situated at the north continental slope of the South China Sea, is a main sub-unit in the Southern Depression Belt of the Pearl River Mouth Basin. In this Sag, the middle Eocene Wenchang and upper Eocene–lower Oligocene Enping Formations had developed in the evolution stage of continental faulted basin. Seismic stratigraphic sequences and fault structures revealed that the Baiyun Sag was short of long-reaching boundary faults, and that it was a rifted basin greatly influenced by basement faults rather than a typical half-graben. Different from the sags in Northern Depression Belt of the Pearl River Mouth Basin which controlled by large-scale NEE-strike faults, the Baiyun Sag had been controlled by two groups of NWW-strike en echelon fault belts with approximate opposite dips, which developed in the southwest and northeast of this Sag respectively and had played the roles of boundary faults. These en echelon faults, together with narrow synclines, partial flower structures and fluid diapirs, indicated the left-lateral transtensional activities, which had resulted in subsidence center departing to main faults and stretching S-shaped. Moreover, the en echelon faults had constructed many composite transfer zones of relay ramps, and controlled the distribution of sandbodies. The en echelon fault belts are located in accordance with Nw-striking Mesozoic basement faults. Hence the left-lateral transtensional activities were responsible for the Western Pacific Plate subducting and strike slip reactivation of the basement faults. Significantly, NW-striking basement faults had forcefully determined the development of not only the Baiyun Sag but also the Xingning Sag.     

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。     

Submarine canyon development in the Izu-Bonin forearc: A SeaMARC II and seismic survey of Aoga Shima Canyon

SeaMARC II sidescan (imagery and bathymetry) and seismic data reveal the morphology, sedimentary processes, and structural controls on submarine canyon development in the central Izu-Bonin forearc, south of Japan. Canyons extend up to 150 km across the forearc from the trench-slope break to the active volcanic arc. The canyons are most deeply incised (1200–1700 m) into the gentle gradients (1–2°) upslope on the outer arc high (OAH) and lose bathymetric expression on the steep (6–18°) inner trench-slope. The drainage patterns indicate that canyons are formed by both headward erosion and downcutting. Headward erosion proceeds on two scales. Initially, pervasive small-scale mass wasting creates curvilinear channels and pinnate drainage patterns. Large-scale slumping, evidenced by abundant crescent-shaped scarps along the walls and tributaries of Aoga Shima Canyon, occurs only after a channel is present, and provides a mechanism for canyon branching. The largest slump has removed >16 km³ of sediment from an 85 km² area of seafloor bounded by scarps more than 200 m high and may be in the initial stages of forming a new canyon branch. The northern branch of Aoga Shima Canyon has eroded upslope to the flanks of the arc volcanoes allowing direct tapping of this volcaniclastic sediment source. Headward erosion of the southern branch is not as advanced but the canyon may capture sediments supplied by unconfined (non-channelized) mass flows.Oligocene forearc sedimentary processes were dominated by unconfined mass flows that created sub-parallel and continuous sedimentary sequences. Pervasive channel cut-and-fill is limited to the Neogene forearc sedimentary sequences which are characterized by migrating and unconformable seismic sequences. Extensive canyon formation permitting sediment bypassing of the forearc by canyon-confined mass flows began in the early Miocene after the basin was filled to the spill points of the OAH. Structural lows in the OAH determined the initial locus of canyon formation, and outcropping basement rocks have prevented canyon incision on the lower slope. A major jog in the canyon axis, linear tributaries, and a prominent sidescan lineament all trend NW-NNW, reflecting OAH basement influence on canyon morphology. This erosional fabric may reflect joint/fracture patterns in the sedimentary strata that follow the basement trends. Once the canyons have eroded down to more erosion-resistant levels, channel downcutting slows relative to lateral erosion of the canyon walls. This accounts for the change from a narrow canyon axis in the thickly sedimented forearc basin to a wider, more rugged canyon morphology near the OAH. About 9500 km³ of sediment has been eroded from the central, 200 km long, segment of the Izu-Bonin forearc by the formation of Aoga Shima, Myojin Sho and Sumisu Jima canyons. The volume of sediment presently residing in the adjacent trench, accretionary wedge, and lower slope terrace basin accounts for <25% of that eroded from the canyons alone. This implies that a large volume (>3500 km³ per 100 km of trench, ignoring sediments input via forearc bypassing) has been subducted beneath the toe of the trench slope and the small accretionary prism. Unless this sediment has been underplated beneath the forearc, it has recycled arc material into the mantle, possibly influencing the composition of arc volcanism.     

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.     

南海西沙海域永乐海底峡谷形成演化过程分析*

远海孤立碳酸盐台地周缘发育了碳酸盐岩峡谷, 对其开展研究有助于深刻理解碳酸盐碎屑沉积物的“源-汇”体系及深水油气成藏等方面。文章利用多波束测深、高分辨率二维多道地震等数据, 精细刻画南海西沙海域永乐海底峡谷的地貌形态及内部充填特征, 揭示该峡谷沉积演化过程, 分析峡谷成因控制因素及稳定性。永乐海底峡谷形成演化可分为萌芽、汇聚和拓展3个阶段, 随着演化过程的发展, 峡谷规模及对沉积物输运作用增加。永乐海底峡谷形成及演化主要受古地貌隆起形成的负地形和沉积物重力流侵蚀作用影响。峡谷在第四纪以后仍有较明显的活动迹象。分析显示永乐海底峡谷是西北次海盆的重要物质输送通道, 其沉积演化过程及稳定性对研究碳酸盐台地沉积物输运等深水沉积过程及岛礁工程建设具有一定参考意义。     

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

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

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.     

Geology and biology of Oceanographer submarine Canyon

We investigated Oceanographer Canyon, which is on the southeastern margin of Georges Bank, during a series of fourteen dives in the “Alvin” and “Nekton Gamma” submersibles. We have integrated our observations with the results of previous geological and biological studies of Georges Bank and its submarine canyons. Fossiliferous sedimentary rocks collected from outcrops in Oceanographer Canyon indicate that the Cretaceous—Tertiary boundary is at 950 m below sea level at about 40°16′N where at least 300 m of Upper Cretaceous strata are exposed; Santonian beds are more than 100 m thick and are the oldest rocks collected from the canyon. Quaternary silty clay, deposited most probably during the late Wisconsin Glaciation, veneers the canyon walls in many places, and lithologically similar strata are present beneath the adjacent outer shelf and slope. Where exposed, the Quaternary clay is commonly burrowed by benthic organisms that cause extensive erosion of the canyon walls, especially in the depth zone (100–1300 m) inhabited by red crabs (Geryon) and/or jonah crabs (Cancer). Bioerosion is minimal on high, near-vertical cliffs of sedimentary rock, in areas of continual sediment movement, and where the sea floor is paved by gravel. A thin layer of rippled, unconsolidated silt and sand is commonly present on the canyon walls and in the axis; ripple orientation is most commonly transverse to the canyon axis and slip-faces point downcanyon. Shelf sediments are transported from Georges Bank over the eastern rim and into Oceanographer Canyon by the southwest drift and storm currents; tidal currents and internal waves move the sediment downcanyon along the walls and axis. Large erratic boulders and gravel pavements on the eastern rim are ice-rafted glacial debris of probable late Wisconsinan age; modern submarine currents prevent burial of the gravel deposits. The dominant canyon megafauna segregates naturally into three faunal depth zones (133–299 m; 300–1099 m; 1100–1860 m) that correlate with similar zones previously established for the continental slope epibenthos. Faunal diversity is highest on gravelly sea floors at shallow and middle depths. The benthic fauna and the fishes derive both food and shelter by burrowing into the sea floor. In contrast to the nearby outer shelf and upper slope, Oceanographer Canyon has not been extensively exploited by the fishing industry, and the canyon ecosystem probably is relatively unaltered.     

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.     

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

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

Slope-confined submarine canyons in the Baiyun deep-water area,northern South China Sea: variation in their modern morphology

On the basis of newly collected multibeam bathymetric data, chirp profiles and existing seismic data, we presented a detailed morphological interpretation of a series of slope-confined canyons in water depths of 300–2000 m in the Baiyun deep-water area, northern margin of the South China Sea. Although these canyons are commonly characterized by regular spacing and a straight-line shape, they vary in their lengths, starting and ending water depths, canyon relief, slope gradients, wall slope gradients and depth profiles along the axis. The eastern canyons (C1–C8) have complex surface features, low values in their slope gradient, canyon relief and wall slope gradient and high values in their length and starting and ending depth contrasting to the western ones (C9–C17). From the bathymetric data and chirp profiles, we interpret two main processes that have controlled the morphology and evolution of the canyons: axial incision and landsliding. The western part of the shelf margin where there were at least four stages of submerged reefs differs from the eastern part of the shelf margin where sedimentary undulations occurred at a water depth of ~650 m. We consider that the variation in morphology of submarine canyons in the study area is the result of multiple causes, with the leading cause being the difference in stability of the upper slope which is related to the submerged reefs and sedimentary undulations.     

Geomorphological evidence for upslope canyon-forming processes on the northern KwaZulu-Natal shelf,SW Indian Ocean,South Africa

A geomorphological and statistical analysis of slope canyons from the northern KwaZulu-Natal continental margin is documented and compared with submarine canyons from the Atlantic margin of the USA. The northern KwaZulu-Natal margin is characterized by increasing upslope relief, concave slope-gradient profiles and features related to upslope growth of the canyon forms. Discounting slope-gradient profile, this morphology is strikingly similar to canyon systems of the New Jersey slope. Several phases of canyon incision indicate that downslope erosion is also an important factor in the evolution of the northern KwaZulu-Natal canyon systems. Despite the strong similarities between the northern KwaZulu-Natal and New Jersey slope-canyon systems, key differences are evident: (1) the concavity of the northern KwaZulu-Natal slope, contrasting with the ∼linear New Jersey slope; (2) the relative isolation of the northern KwaZulu-Natal canyons, rather than the dense clustering of the New Jersey canyons; and (3) the absence of strongly shelf-breaching canyons along the northern KwaZulu-Natal margin. In comparison with the New Jersey margin, we surmise a more youthful stage of canyon evolution, a result of either the canyons themselves being younger or the formative processes being less active. Less complicated patterns of erosion resulting from reduced sediment availability have developed in northern KwaZulu-Natal. The reduction in slope concavity on the New Jersey margin may be the result of grading of the upper slope by intensive headward erosion, a process more subdued—or less evident—on the KwaZulu-Natal margin.     

The Central Submarine Canyon in the Qiongdongnan Basin, northwestern South China Sea: Architecture, sequence stratigraphy, and depositional processes

Submarine canyons have been the subject of intense studies in recent years because of their close link to deepwater systems. The Central Canyon is a large unusual submarine canyon in the northwestern margin of the South China Sea, has a total length of about 425 km and is oriented sub-parallel to the continental slope. Using integrated 2D/3D seismic, well log, core, and biostratigraphy data, the current study documents the stratigraphic framework, internal architecture, depositional processes, and controlling factors of the segment of the Central Canyon located in the Qiongdongnan Basin.The integrated analysis shows that the canyon fill consists of four 3^rd-order sequences, SQ4, SQ3, SQ2, and SQ1. Each of them is bounded by regionally important erosional surfaces (3^rd-order sequence boundaries). Within each 3^rd-order sequence there is maximum regressive surface separating a regressive systems tract in the lower part and a transgressive systems tract in the upper part. Nine facies are identified and are further grouped into five depositional units, DU1 through DU5.The canyon evolved through four cut-and-fill stages, with a change from predominantly axial cut-and-fill to primarily side cut-and-fill. Axial cut-and-fill dominated during the first stage, and the slope-subparallel paleo Xisha Trough was intensely eroded by large-scale axial gravity flows. During the second cut-and-fill stage, the Central Canyon experienced both axial and side cut-and-fill. The third stage was dominated by side cut-and-fill. The canyon was eroded and fed by slope channels that transported sandy sediments from the shelf to the north during regression, and was covered by side-derived muddy MTCs during transgression. The last stage was also dominated by side cut-and-fill. The canyon, however, was filled predominantly by side-derived muddy MTCs.Evolution and depositional processes in the Central Canyon were likely controlled by slope-subparallel negative-relief induced by paleo-seafloor morphology, structural inversion of the Red River Fault and the slope-subparallel basement faults. Additionally, Coriolis force, sea-level fluctuations, high sedimentation rate, and rapid progradation of the slope also controlled and influenced the depositional processes, and internal architectures of the canyon.     

Sediment transport in Washington and Norfolk submarine canyons

A sediment study suggests that Washington and Norfolk canyons off the Mid-Atlantic States are not inactive, but have served periodically since the Late Pleistocene as conduits of sediment originating on the adjacent shelf and upper slope. Large quantities of sand occur in the canyon heads as thin beds and laminae, and on the continental slope as mixtures of sand (to $\text{>40\%}$), silt and clay that are extensively reworked by burrowing organisms. Sandy turbidites occur in the canyons on the rise. Basinward dispersal, from the outer shelf and uppermost slope, is recorded by heavy mineral suites and bioclastic components, primarily foraminifera of shallow marine origin, in the lower slope and upper continental rise canyon cores. The down-axis movement of material, presumably episodic, in the Holocene to recent results from offshelf spillover into canyon heads, failure on the steep walls bordering canyons on the slope, and resuspension by bottom currents.     

Numerical simulations of the internal tide in a submarine canyon

Three-dimensional numerical simulations of the generation and propagation of the semidiurnal internal tide in a submarine canyon with dimensions similar to those of the Monterey Canyon are carried out using a primitive equation model. Forcing with just sea level at the offshore boundary in an initially horizontally homogeneous ocean with realistic vertical stratification, internal tides are generated at the canyon foot and rim, and along portions of the canyon floor. The results compare favorably with observations, both indicating enhancement of energy along the canyon floor propagating at an angle consistent with linear internal wave theory. Due to the earth's rotation, internal tide energy is distributed asymmetrically in the cross-canyon direction, favoring the southern side. The effect of canyon floor slope is explored, with the finding that small changes in the slope result in large changes in the amount and distribution of the internal tide energy. Canyons whose floors are subcritical with respect to the semidiurnal frequency along their entire length have very little baroclinic energy, whereas canyons that are near-critical along much of their length, such as the Monterey Canyon, develop strong internal tides that propagate shoreward. Canyons that are near-critical at their mouths but supercritical further inshore generate the most internal tidal energy overall, although little of it makes it onto the continental shelf shoreward of the canyon head. The effects of internal tides within the canyons can be seen outside the canyons as well. Water is transported from depth onto the adjacent continental shelf along the canyon rims. This tidal pumping can be responsible for alongshore internal tide propagation and tidal-period surface currents with relatively small horizontal scales of variability.     

珠江口盆地白云凹陷海底峡谷沉积模式

利用高分辨率三维地震资料,在珠江口白云凹陷三维工区中发现形态以及发育特征相似的7个海底峡谷.通过对其现今地貌特征研究,将其分为上、中、下段.综合地震反射振幅、连续性及外部结构形态等信息,识别出了峡谷侵蚀基底、谷底沉积、谷壁滑塌等峡谷地震相单元.结合峡谷不同位置处的各类地震相的发育情况,可知峡谷的这3个地貌单元具有不同的...     

南海北部陆坡峡谷区海底沉积物工程地质特征及区域稳定性评价

The upper part of the continental slope in the northern South China Sea is prone to submarine landslide disasters,especially in submarine canyons. This work studies borehole sediments, discusses geotechnical properties of sediments, and evaluates sediment stability in the study area. The results show that sediment shear strength increases with increasing depth, with good linear correlation. Variations in shear strength of sediments with burial depth have a significantly greater rate of change in the canyon head and middle part than those in the canyon bottom. For sediments at the same burial depth, shear strength gradually increased and then decreased from the head to the bottom of the canyon, and has no obvious correlation with the slope angle of the sampling site. Under static conditions, the critical equilibrium slope angle of the sediments in the middle part of the canyon is 10° to 12°, and the critical slope angle in the head and the bottom of the canyon is 7°. The results indicate that potential landslide hazard areas are mainly distributed in distinct spots or narrow strips on the canyon walls where there are high slope angles.     

Physiographic and bathymetric characteristics of the continental slope,northwest Gulf of Mexico

Bathymetric charts of the continental slope of the northwestern Gulf of Mexico reveal the presence of over 90 intraslope basins with relief in excess of 150 m. The evolution and the general configuration of the basins are a function of halokinesis of allochthonous salt. Intraslope-interlobal and intraslope-superlobal basins occupy the upper and lower continental slope, respectively. Other structures on the slope associated with salt tectonics are the Sigsbee Escarpment, the seaward edge of the Sigsbee salt nappe, and the Alaminos and Keathley canyons. Major erosional features are the Mississippi Canyon and portions of a submarine canyon on the southern extreme of the Sigsbee Escarpment.