鄂尔多斯盆地西缘上奥陶统拉什仲组深水等深流沉积

为缩小等深流沉积研究中的现代海洋研究与地层记录研究之间的不平衡和探索中-晚奥陶世鄂尔多斯盆地西缘大洋环流体系在海相深水油气勘探中的意义,在详细的野外观测的基础上,对有关沉积构造进行了古水流和流体动力学分析.结果表明:（1）非水道环境下约有1/3的小型交错层理古水流方向与区域斜坡方向垂直;（2）水道环境下普遍发育双向递变沉积构造,包括平行层（沙纹层）-均匀层-平行层（沙纹层）、条纹条带和双向粒序层3种类型.上述特征可解释为深水斜坡环境下的等深流沉积,并识别出非水道型和水道型2类,后者又分为部分改造型和完全改造型2亚类.      

Contourites: Their recognition in modern and ancient sediments

We suggest that publication of evidence that the continental rise of the western North Atlantic has been shaped by bottom currents flowing parallel to bathymetric contours (Heezen, Hollister and Ruddiman, 1966) marked the beginning of a revolution in sedimentology comparable to the turbidite revolution launched by Kuenen and Migliorini in their classical 1950 paper.Intensification of slow, thermohaline circulation on the western margins of the ocean basins leads to high-velocity, deep, boundary currents, capable of eroding, transporting and depositing fine-grained sediment. Long-period, direct current measurements suggest a complex, periodic flow for these currents, while bottom photographs indicate their influence on the sediment surface. Sediment ridges in the North Atlantic can be closely related to the deep-water circulation pattern. Other morphological features (ripples, furrows, waves), echogram characteristics, and the presence of well-developed nepheloid layers cannot be uniquely attributed to the action of bottom currents.Critical review of marine-based investigations reveals a lack of generally accepted criteria for the recognition of contourites on the basis of sediment character. We discuss the problems in establishing such criteria and recognize that: (a) a continuum may exist between dilute turbidity flows, bottom currents and hemipelagic settling; (b) interbedded turbidites, contourites and hemipelagites are common, especially in a rise environment; and (c) composition and other criteria may be only locally applicable. However, we can identify two main contourite groups, muddy contourites and sandy contourites, and have proposed new criteria for their recognition. Muddy contourites are generally bioturbated, have poorly defined bedding, and contain biogenic sand often concentrated into irregular layers. They may be texturally and compositionally distinct from interbedded turbidites, and have relatively high CaCO₃ and organic carbon contents. Sandy contourites occur as thin, bioturbated, irregular lag-deposits, or as reworked tops of sandy turbidites. In the latter case they may be clean, well sorted, parallel- or cross-laminated, but show no offshore trends or vertical structural sequence. Grain orientation shows the bottom current direction, often superimposed upon the original turbidite fabric.Reveiw of land-based work shows that there is growing recognition of the need for a new concept to complement turbidity-current theory, but that there have been relatively few claims of firm contourite identification. Recognition of ancient contourites has been based either on the application of previous sedimentological criteria, or on an interpretation of the broader environmental framework. It is suggested that it is lack of suitable criteria for the identification of contourites rather than a true scarcity of these rocks that has led to such a restricted literature.Mindful of problems created by diagenesis, tectonic activity and the limited preservation potential of many diagnostic features of marine contourites, we do suggest criteria, and a procedure, for the recognition of contourites in land-based work. Sandy contourites of the reworked-turbidite variety may be the most easily recognised; the presence of bimodal palaeocurrent directions at about 90° is an important indicator of this type.The geological significance of contourites in palaeo-oceanographical, palaeogeographical and tectonic reconstructions is emphasised, especially in relation to work on passive (Atlantic-type) continental margins. We also refer to the possible economic significance of contourites as exploration for hydrocarbons moves into deeper waters.     

深水等深流与重力流交互作用沉积（2000—2022年）研究进展

等深流与重力流在深水环境中较为常见,两者在地质历史时期中可存在相互作用进而形成交互作用沉积。结合近20年研究成果,对深水等深流与重力流交互作用的沉积类型、鉴别标志、形成机理及地质意义进行了总结。1）等深流与重力流交互作用沉积可分为等深流与重力流沉积互层、等深流改造重力流及等深流与重力流同时作用沉积。2）等深流沉积和重力流沉积的有效鉴别是等深流与重力流沉积互层沉积研究的前提。3）等深流改造重力流沉积发育重力流和牵引流沉积构造,双向交错层理最为典型;常具顺斜坡向下及大致平行斜坡的两个水流方向;概率累积曲线呈1~3段式等特征。4）等深流与重力流同时作用沉积主要发育单向迁移水道、不对称的水道—堤岸体系及偏转型朵叶。5）交互作用形成过程主要受等深流与重力流相对能量大小的影响。当重力流活跃时,发育重力流沉积,在重力流末期及间歇期,等深流沉积发育,进而形成重力流与等深流沉积互层。等深流能量较强时,可改造重力流沉积,形成等深流改造重力流沉积。高能等深流在重力流能量较弱时,可对重力流沉积物进行横向搬运,形成迁移水道、不对称水道—堤岸体系及偏转型朵叶。6）主要问题及下一步的主攻方向主要包括四个方面:①重视综合研究,增加实例分析;②完善鉴别标志,推广研究成果;③多方法、多尺度、多条件、多维度综合探讨交互作用沉积过程及主控因素;④加强油气勘探潜力、古环境演化及地质灾害预防等方面的研究。     

MATRIX OF TURBIDITES: EXPERIMENTAL APPROACH

The matrix (< 40 μ) of turbidites forms a possible clue to the density of turbidity currents and the origin of the graywacke matrix. Experiments in a circular flume provide a mechanism to study the relation between composition of suspensions at various speeds and their deposits. There is a close analogy to the lower part of turbidity currents. The lutum content of samples with median diameters greater than 400 or 500 μ is found to correspond to the suspended load of the pore water. The higher value for finer deposits can be recalculated to suspension concentration by use of the “sedimentation factor”. Hence, each turbidite carries, as it were, a sample of its depositing current. The lutum content depends not on the ratio of sand to lutum in the current, as tacitly assumed by many authors, but mainly on the ratio lutum to water, although also influenced by velocity. The average lutum density of coarser recent deep-sea sands is 1-2%. This indicates turbidity currents with 5-10% lutum by weight (density 1.03–1.07). The sand must be added to ascertain the current density. In first approximation turbidity currents tend to have densities at their nose of 1.1–1.2, but higher and much lower values also occur. The maximum original lutum percentage of coarse turbidites is below 10%. Higher values are very scarce and are due to post-depositional mixing, or we are dealing with slides. However, in fine-grained turbidites there is more matrix up to 20% for a median of 100 p. Hence, coarse graded marine graywackes with 20 or more per cent matrix are presumably weakly metamorphic turbidites, that originally held the same modest amount of lutum as recent turbidites of the same grain size. The Trask sorting of the experimental deposits is very good, like the average of natural turbidites. Most cumulative curves of turbidite grain-size analyses on arithmetic probability paper show a characteristic bend in fine sand or silt sizes.     

浊流沉积研究的新进展:鲍马序列、海底扇的重新审视

本文在总结前人对浊流沉积研究的基础上,分析前人对浊流与浊积岩、浊流沉积与浊流相模式的对应关系之间的认识,并对鲍马序列进行重新审视。在海底扇研究过程中,鲍马序列已经不能充分反映浊流沉积的全过程。鲍马序列所反应的沉积模式其实是由碎屑流、浊流、底流等多种形式流体组合和改造后的结果,海底扇沉积模式不能笼统归结为浊流沉积作用的结果。在完善重力流、底流等沉积作用的同时,建立一个与沉积作用相互联系的深海沉积系统,以对深海研究提供更好地指导和预测。     

Quaternary seismic facies of the atlantic continental rise

High-resolution seismic profiles collected by Parasound and SES-2000 deep profilers during Cruise 26 of the R/V Akademik Sergei Vavilov (2009) along the continental slope base of South America, NW Africa, and West Europe are correlated with the deep-sea drilling boreholes. Lithofacies interpretation of the Quaternary deep-water seismic facies of hemipelagites, bottom current deposits (contourites), and gravitites (turbidites, mud flow deposits) is presented. The data obtained reveal the domination of contourites in the accumulation of continental rise apron under conditions of relatively scarce terrigenous material supply. It is shown that acoustically stratified seismic facies under these conditions commonly reflect interbedding of the terrigenous clay and biogenic calcareous ooze related to the Pleistocene glacial/interglacial cycles.     

鲍玛序列的多解性

鲍玛序列作为浊积岩的识别标志被广泛认可,但随着对深水沉积过程认识的深入,鲍玛序列逐渐被重新认识。近年来对深水沉积物重力流的研究发现:鲍玛序列不是浊流的唯一产物,深水环境中其它沉积过程也可形成鲍玛序列。因此,在野外识别浊积岩的过程中,要慎重使用鲍玛序列进行判别。     

碎屑流与浊流的流体性质及沉积特征研究进展

受浊流沉积模式(即鲍马序列和浊积扇模式)的驱动和浊积岩思维定势的影响,自1970s浊流与浊积岩的概念逐渐扩大,特别是通过"高密度浊流"术语的引入,以及将水下浊流与陆上河流的错误类比,使得一部分碎屑流与底流的沉积被认为是浊积岩。随着现代观测设备的应用以及详细的岩芯观察,碎屑流(特别是砂质碎屑流)和浊流被重新认识。浊流是一种具牛顿流变性质和紊乱状态的沉积物重力流,其沉积物支撑机制是湍流。碎屑流是一种具塑性流变性质和层流状态的沉积物重力流,其沉积物支撑机制主要是基质强度和颗粒间的摩擦强度。浊流沉积具特征的正粒序韵律结构,底部为突变接触而顶部为渐变接触;碎屑流沉积一般具上、下两层韵律结构,即下部发育具平行碎屑结构的层流段,上部发育具块状层理的"刚性"筏流段。但当碎屑流被周围流体整体稀释改造且改造不彻底时,强碎屑流可变为中—弱碎屑流,相应自下而上可形成逆—正粒序的沉积韵律结构,其中发育有呈漂浮状的石英颗粒和泥质撕裂屑等碎屑颗粒,明显区别于浊流沉积单一的正粒序韵律结构特征。碎屑流沉积顶、底部均为突变接触。浊流的沉积模式为简单的具平坦盆底的坡底模式,而碎屑流则为复杂的斜坡模式。     

Results of research into Holocene sediments of the South and Central basins of Lake Baikal (BDP-97 and short cores)

Results of investigations of Baikal bottom sediments from a long core (BDP-97) and several short (0–1 m) cores are presented. It has been shown that the Holocene sediments in the Baikal basins consist of biogenic-terrigenous muds, accumulated under calm sedimentation conditions, and of turbidites, formed during catastrophic events. The turbidites can be distinguished from the host sediments by their enrichment in heavy minerals and thus their high magnetic susceptibility. Often, Pliocene and Pleistocene diatom species observed in the Holocene sediments (mainly in the turbidites) point to redeposition of ancient offshore sediments. Our results indicate that deltas, littoral zones, and continental slopes are the source areas of turbidites. The fact that the turbidites occur far from their sources confirms the existence of high-energy turbidity currents responsible for long-distance lateral-sediment transport to the deep basin planes of the lake.     

Sand waves, Echinocardium traces and their bathyal depositional setting (Monte Torre Palaeostrait, Plio-Pleistocene, southern Italy).

ABSTRACT
Stacked cross-sets, up to 2.5 m thick, produced by sand wave migration and meniscate trace fossils produced by Echinocardium cordatum , both considered in the literature as typical of shallow-water marine depositional settings, commonly occur in the bathyal Plio-Pleistocene deposits of Monte Torre (Calabria, southern Italy).
The Plio-Pleistocene sediments form two coarsening-upward depositional sequences, separated by an unconformity and by a palaeobathymetric gap of at least 300 m. The lower sequence passes upwards from hemipelagic marls and thin-bedded turbidites to thick-bedded sandy turbidites, then to sand wave deposits alternated with sandy turbidites, and finally to base-of-slope megabreccias. Facies characteristics and relationships, and the occurrence of deep-sea faunal associations, indicate deposition in the bathyal zone. The facies of the upper sequence reflect a fan-delta environment, no deeper than a few tens of metres.
The depositional setting of the lower sequence, where the sand wave deposits and meniscate trace fossils occur, appears to have been a tectonically controlled seaway, connecting the Tyrrhenian and Ionian Seas. This seaway became progressively narrower with time, evolving into a strait. The overall coarsening-upward trend reflects the upward transition from a low to a high-energy environment, possibly caused by the tectonic narrowing of the seaway. Deposition and erosion from high-concentration turbidity currents and from tidal bottom currents were important processes. Periods of tectonic activity, producing first the uplift of the seaway margins and culminating with the uplift of the strait sequence itself, are marked by-scattered rockfall deposits.
The strait setting, causing the development of powerful, oxygenated bottom currents, produced optimal conditions in the bathyal zone for the colonization of sandy bottoms by a single infaunal r -selected species, Echinocardium sp.     

Numerical simulation of turbidity current flow and sedimentation: II. Results and geological applications

A process-based, forward computer model of turbidity current flow and sedimentation, termed the TCFS model, has been developed to trace the downslope evolution of individual turbidity flows. Details of the model itself have been presented in a preceding paper. We here outline a series of tests of the TGFS model. The sensitivity tests of the TCFS model to general geological controls reveal the quantitative relationship between these controls and the behaviour of turbidity flows and the geometry and textural features of the resulting turbidites. Experimental turbidity currents on relatively steep slopes accelerate more rapidly and reach higher velocities than those on gentle slopes. Flows with larger initial volumes have higher initial velocities, travel further downslope, and form beds of greater thickness and downslope extent than smaller flows. Experimental high-concentration flows with suspended-sediment concentrations of 25% accelerate more rapidly and reach higher downslope velocities than dilute flows with 5% suspended sediment. The higher velocities and enhanced hindered-settling effects of the high-concentration flows lead to much greater transport distances and reduced vertical and lateral sediment size grading in the resulting turbidites. Beds formed by experimental high-concentration flows are massive or show coarse-tail grading whereas beds formed by low-concentration flows show distribution-grading. Experimental flows fed by coarse sediment sources tend to deposit the bulk of their suspended sediment loads on the proximal slope, resulting in more rapid flow deceleration and sedimentation than flows fed by silt-rich, fine-grained sediment sources. Turbidites formed by coarse-sediment flows tend to have a wedge-shaped geometry, with low downslope extent and high surface relief, whereas turbidites formed by fine-sediment flows tend to have a tabular geometry, with greater downslope extent and lower surface relief. A specific geological test of the TCFS model is based on studies of modern turbidity currents in Bute Inlet, British Columbia, Canada. With the input initial and boundary conditions estimated from Bute Inlet, the model predicts the downslope velocity evolution of turbidity currents comparable to those of modern and ancient turbidity flows measured in Bute Inlet. Model-calculated vertical and downslope grain-size properties of turbidites are similar to those exhibited by surface and cored Bute Inlet turbidites. Model flows tend to decelerate more rapidly than some stronger turbidity currents in the Bute Inlet system, and model beds tend to decrease in grain-size downslope more rapidly than observed bottom sediments. This is probably because the TCFS model flows lacked clay, which is abundant in Bute Inlet; they do not fully simulate turbulent mixing of suspended sediments; and they better represent the unsteady, depositional stage of turbidity-currents than the preceding stage of more-or-less steady-flow conditions. These tests demonstrate that the TCFS model provides a semi-quantitative method to study the growth patterns of submarine turbidite systems. It can serve as a predictive tool for analysing the facies architecture of ancient turbidite systems through simulating multi-depositional events by improving its erosion function, and the compatibility between its numerical components.     

The formation of large mudwaves by turbidity currents on the levees of the Toyama deep-sea channel, Japan Sea

The development of mudwaves on the levees of the modern Toyama deep‐sea channel has been studied using gravity core samples combined with 3·5‐kHz echosounder data and airgun seismic reflection profiles. The mudwaves have developed on the overbank flanks of a clockwise bend of the channel in the Yamato Basin, Japan Sea, and the mudwave field covers an area of 4000 km². Mudwave lengths range from 0·2 to 3·6 km and heights vary from 2 to 44 m, and the pattern of mudwave aggradation indicates an upslope migration direction. Sediment cores show that the mudwaves consist of an alternation of fine‐grained turbidites and hemipelagites whereas contourites are absent. Core samples demonstrate that the sedimentation rate ranged from 10 to 14 cm ka^?1 on the lee sides to 17–40 cm ka^?1 on the stoss sides. A layer‐by‐layer correlation of the deposits across the mudwaves shows that the individual turbidite beds are up to 20 times thicker on the stoss side than on the lee side, whereas hemipelagite thicknesses are uniform. This differential accretion of turbidites is thought to have resulted in the pattern of upcurrent climbing mudwave crests, which supports the notion that the mudwaves have been formed by spillover turbidity currents. The mudwaves are interpreted to have been instigated by pre‐existing large sand dunes that are up to 30 m thick and were created by high‐velocity (10°ms^?1), thick (c. 500 m) turbidity currents spilling over the channel banks at the time of the maximum uplift of the Northern Japan Alps during the latest Pliocene to Early Pleistocene. Draping of the dunes by the subsequent, lower‐velocity (10^?1ms^?1), mud‐laden turbidity currents is thought to have resulted in the formation of the accretionary mudwaves and the pattern of upflow climbing. The dune stoss slopes are argued to have acted as obstacles to the flow, causing localized loss of flow strength and leading to differential draping by the muddy turbidites, with greater accretion occurring on the stoss side than on the lee slope. The two overbank flanks of the clockwise channel bend show some interesting differences in mudwave development. The mudwaves have a mean height of 9·8 m on the outer‐bank levee and 6·2 m on the inner bank. The turbidites accreted on the stoss sides of the mudwaves are 4–6 times thicker on the outer‐bank levee than their counterparts on the inner‐bank levee. These differences are attributed to the greater flow volume (thickness) and sediment flux of the outer‐bank spillover flow due to the more intense stripping of the turbidity currents at the outer bank of the channel bend. Differential development of mudwave fields may therefore be a useful indicator in the reconstruction of deep‐sea channels and their flow hydraulics.     

Subglacial and subaqueous processes near a glacier grounding line: sedimentological evidence from a former ice-dammed lake, Achnasheen Scotland

Subglacial and subaqueous sediments deposited near the margin of a Late-glacial ice-dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre-existing glaciolacustrine deposits, and the subaqueous sediments consist of ice-proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment-gravity flows on prograding outwash fans. Beyond the ice-marginal environment, deposition was from turbidity currents, ice-rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.     

Turbiditic and non-turbiditic mudstone of Cretaceous flysch sections of the East Alps and other basins

Recognition of the occurrence and extent of hemipelagic and pelagic deposits in turbidite sequences is of considerable importance for environmental analysis (palaeodepth, circulation, distance from land, hemipelagic or pelagic versus turbidite sedimentation rates) of ancient basins. Differentiation between the finegrained parts (E-division) of turbidites and the (hemi-) pelagic layers (F-division of turbidite-pelagite alternations) is facilitated in basins where carbonate turbidites were deposited below the carbonate compensation depth (CCD) such as the Flysch Zone of the East Alps but may be difficult in other basins where less compositional contrast is developed between the fine-grained turbidites and hemipelagites. This difficulty pertains particularly in Palaeozoic and older basins. For Late Mesozoic-Cenozoic oceans with a relatively deep calcite compensation level three other types of turbidite basins may be distinguished for which differentiation becomes increasingly more difficult in the sequence from (1) to (3): (1) terrigenous turbidite basins above the CCD; (2) carbonate turbidite basins above the CCD; (3) terrigenous turbidite basins below the CCD. Criteria and methods useful for the differentiation between turbiditic and hemipelagic mudstone in the Upper Cretaceous of the Flysch Zone of the East Alps include calcium carbonate content, colour, sequential analysis, distribution of bioturbation, and microfaunal content. In modern turbidite basins clay mineral content, organic matter content, plant fragments, and grain-size (graded bedding, maximum grain diameter) have reportedly also been used as criteria (see Table 3). Deposition of muddy sediment by turbidity currents on weakly sloping sea bottoms such as the distal parts of deep-sea fans or abyssal plains is not only feasible but may lead to the accumulation of thick layers. Contrary to earlier speculation it can be explained by the hydrodynamic theory of turbidity currents, if temperature differences between the turbidity current and the ambient deep water as well as relatively high current velocities for the deposition of turbiditic muds (an order of magnitude higher on mud surfaces than commonly assumed) are taken into consideration. The former add to the capacity of turbidity currents to carry muddy sediment without creating a driving force on a low slope.     

The hyperpycnite problem

Sedimentologic, oceanographic, and hydraulic engineering publications on hyperpycnal flows claim that(1) river flows transform into turbidity currents at plunge points near the shoreline,(2) hyperpycnal flows have the power to erode the seafloor and cause submarine canyons, and,(3) hyperpycnal flows are efficient in transporting sand across the shelf and can deliver sediments into the deep sea for developing submarine fans. Importantly, these claims do have economic implications for the petroleum industry for predicting sandy reservoirs in deep-water petroleum exploration. However, these claims are based strictly on experimental or theoretical basis, without the supporting empirical data from modern depositional systems. Therefore, the primary purpose of this article is to rigorously evaluate the merits of these claims.A global evaluation of density plumes, based on 26 case studies(e.g., Yellow River, Yangtze River, Copper River,Hugli River(Ganges), Guadalquivir River, Rio de la Plata Estuary, Zambezi River, among others); suggests a complex variability in nature. Real-world examples show that density plumes(1) occur in six different environments(i.e., marine,lacustrine, estuarine, lagoon, bay, and reef);(2) are composed of six different compositional materials(e.g., siliciclastic,calciclastic, planktonic, etc.);(3) derive material from 11 different sources(e.g., river flood, tidal estuary, subglacial, etc.);(4) are subjected to 15 different external controls(e.g., tidal shear fronts, ocean currents, cyclones, tsunamis, etc.); and,(5) exhibit 24 configurations(e.g., lobate, coalescing, linear, swirly, U-Turn, anastomosing, etc.).Major problem areas are:(1) There are at least 16 types of hyperpycnal flows(e.g., density flow, underflow, high-density hyperpycnal plume, high-turbid mass flow, tide-modulated hyperpycnal flow, cyclone-induced hyperpycnal turbidity current, multi-layer hyperpycnal flows, etc.), without an underpinning principle of fluid dynamics.(2) The basic tenet that river currents transform into turbidity currents at plunge points near the shoreline is based on an experiment that used fresh tap water as a standing body. In attempting to understand all density plumes, such an experimental result is inapplicable to marine waters(sea or ocean) with a higher density due to salt content.(3) Published velocity measurements from the Yellow River mouth, a classic area, are of tidal currents, not of hyperpycnal flows. Importantly, the presence of tidal shear front at the Yellow River mouth limits seaward transport of sediments.(4) Despite its popularity, the hyperpycnite facies model has not been validated by laboratory experiments or by real-world empirical field data from modern settings.(5) The presence of an erosional surface within a single hyperpycnite depositional unit is antithetical to the basic principles of stratigraphy.(6) The hypothetical model of "extrabasinal turbidites", deposited by river-flood triggered hyperpycnal flows,is untenable. This is because high-density turbidity currents, which serve as the conceptual basis for the model, have never been documented in the world's oceans.(7) Although plant remains are considered a criterion for recognizing hyperpycnites, the "Type 1" shelf-incising canyons having heads with connection to a major river or estuarine system could serve as a conduit for transporting plant remains by other processes, such as tidal currents.(8) Genuine hyperpycnal flows are feeble and muddy by nature, and they are confined to the inner shelf in modern settings.(9) Distinguishing criteria of ancient hyperpycnites from turbidites or contourites are muddled.(10) After 65 years of research since Bates(AAPG Bulletin 37: 2119-2162, 1953), our understanding of hyperpycnal flows and their deposits is still incomplete and without clarity.     

Late Quaternary mass movement on the lower continental rise and abyssal plain off Western Sahara

The late Quaternary development of part of the lower continental rise off Western Sahara has been determined from an investigation of short (< 2 m) gravity cores collected from a deep-sea channel, the interchannel areas and an abyssal hill, between 30 and 33°N. Stratigraphic analysis is based on systematic variations in abundances of particular coccolith species and pelagic sediment types, referenced to the oxygen isotope time-scale. During the last 73 000 years deposition in the channel has included volcaniclastic sand/silt turbidites and minor marl turbidites as well as pelagic sediments. The interchannel area has fewer turbidites, and the sands present were probably deposited from turbidity currents which spilt over the channel sides. The last‘event’ to give rise to sands in the channel and interchannel area occurred about 45 000 years ago. Although the channel has been inactive as an area of turbidity current deposition for the last 20 000 years, sands were deposited elsewhere on the lower rise, indicating that turbidity current transport routes have varied in time. Turbidity current deposition on the abyssal plain and low-lying continental rise appears to be related to distinct sliding events involving transport of material from various sources. Thin marl turbidites are interbedded with pelagic sediments in the area of sediment drape. There is a strong correlation between these and the thick marl turbidites on the abyssal plain, suggesting that the same turbidity current‘events’, occurring about once every 25 000 years, gave rise to both sets of deposits. The thinner units probably represent deposition from the outer parts or tails of the large turbidity flows. The turbidites occur at glacial/interglacial transitions, suggesting that the slides that created them were triggered by mechanisms related to climatic change. Several volcaniclastic sand/silt units within the channel and in interchannel areas occupy mid-stage stratigraphic positions, perhaps indicating a different triggering mechanism for slides around volcanic islands. A debris flow deposit (debrite), between 30°N, 21°W and 31°N, 24°W, is related to the Saharan Sediment Slide, a major mass movement feature on the continental slope over 1000 km to the southeast. Stratigraphic correlations indicate that this slide produced a large turbidity current as well as a debris flow.     

Quaternary sedimentation,margin architecture and ocean circulation variability around the Faroe Islands,North Atlantic

The Quaternary development offshore the Faroe Islands has been studied using high-resolution seismic and core data from the R/V DANA 2000 cruise and previous cruises. Several glacial-related features and deposits are observed, all bearing witness to former extensive glaciations of the Faroe area. On the shelves, overlaying a mid-Pleistocene glacial erosional surface, glacial and glacimarine deposits form a sheet geometry interrupted by ridges of sediment that are likely to represent ice-front deposits. An iceberg turbate north of the Faroe Islands provides evidence of large-scale drift of ultra-deep draft (>600 m) icebergs in the Nordic Seas at pre-Weichselian glacial stage(s). Marginal and transverse troughs found on the eastern and western shelf are suggested to have formed during the same glacial period(s) as the iceberg turbate. Iceberg plough-marks and abundant ice rafted material of non-Faroese origin, together with the relict moraine ridges encircling the Faroe Islands at around the 100 and 200 m water depth contours, indicate that the outer shelf was probably ice free during the Weichselian ice age. On the slopes and basinal parts, the formation of fine-grained contourites was favoured during (Weichselian) glacial stages when bottom currents were reduced. Sediment overloading during these glacial stages resulted in repeated slope instability, causing mass failures of the contourite deposits.     

Sustained turbidity currents and their interaction with debrite-related topography; Labuan Island, offshore NW Borneo, Malaysia

The Temburong Fm (Early Miocene), Labuan Island, offshore NW Borneo, was deposited in a lower-slope to proximal basin-floor setting, and provides an opportunity to study the deposits of sustained turbidity currents and their interaction with debrite-related topography. Two main gravity-flow facies are identified; (i) slump-derived debris-flow deposits (debrites) — characterised by ungraded silty mudstones in beds 1.5 to > 60 m thick which are rich in large (> 5 m) lithic clasts; and (ii) turbidity current deposits (turbidites) — characterised by medium-grained sandstone in beds up to 2 m thick, which contain structureless (T_a) intervals alternating with planar-parallel (T_b) and current-ripple (T_c) laminated intervals. Laterally discontinuous, cobble-mantled scours are also locally developed within turbidite beds. Based on these characteristics, these sandstones are interpreted to have been deposited by sustained turbidity currents. The cobble-mantled scours indicate either periods of intense turbidity current waxing or individual flow events. The sustained turbidity currents are interpreted to have been derived from retrogressive collapse of sand-rich mouth bars (breaching) or directly from river effluent (hyperpycnal flow). Analysis of the stratal architecture of the two facies indicates that routing of the turbidity currents was influenced by topographic relief developed at the top of the underlying debrite. In addition, turbidite beds are locally eroded at the base of an overlying debrite, possibly due to clast-related substrate ‘ploughing’ during the latter flow event. This study highlights the difficulty in constraining the origin of sustained turbidity currents in ancient sedimentary sequences. In addition, this study documents the importance large debrites may have in generating topography on submarine slopes and influencing routing of subsequent turbidity currents and the geometry of their associated deposits.     

马尼拉海沟1.4 ka B.P.以来浊流事件沉积及其成因机制*

浊流是远距离沉积物运输的一种重要方式,海底浊流广泛存在于海底峡谷或海沟。马尼拉海沟位于南海东北部,是一条正在活动的板块汇聚边界。独特的地理位置(亚热带—热带)和气候条件(台风频发),使得马尼拉海沟浊流频发,然而,现今对马尼拉海沟的浊流研究甚少。本研究通过对马尼拉海沟北部水深3747 m处重力柱岩心(GEO6)进行高精度的粒度及沉积学特征分析,探讨马尼拉海沟浊流沉积规律。GEO6岩心底部细颗粒沉积物中浮游有孔虫的¹⁴C的AMS年龄为1405 a B.P.。高精度的粒度分析(0.25 cm)和沉积学特征显示: GEO6岩心记录有至少11次浊流沉积(T1-T11),且这些浊流都有明显的底部粒度最粗(砂质粉砂或砂)、向上粒度逐渐变细的正粒序特征,只有T8沉积体为反粒序特征,可能为异重流沉积。结合区域地质资料,本研究认为1.4 ka B.P.以来,研究区频繁的台风带来了大量陆源松散沉积物堆积在马尼拉海沟上游(高屏峡谷),不稳定的构造环境及地震频发导致这些松散沉积物垮塌并向下游马尼拉海沟输送,在海沟内形成频繁发育的浊流沉积体。     

马尼拉海沟1.4 ka B.P.以来浊流事件沉积及其成因机制*

浊流是远距离沉积物运输的一种重要方式,海底浊流广泛存在于海底峡谷或海沟。马尼拉海沟位于南海东北部,是一条正在活动的板块汇聚边界。独特的地理位置(亚热带—热带)和气候条件(台风频发),使得马尼拉海沟浊流频发,然而,现今对马尼拉海沟的浊流研究甚少。本研究通过对马尼拉海沟北部水深3747 m处重力柱岩心(GEO6)进行高精度的粒度及沉积学特征分析,探讨马尼拉海沟浊流沉积规律。GEO6岩心底部细颗粒沉积物中浮游有孔虫的¹⁴C的AMS年龄为1405 a B.P.。高精度的粒度分析(0.25 cm)和沉积学特征显示: GEO6岩心记录有至少11次浊流沉积(T1-T11),且这些浊流都有明显的底部粒度最粗(砂质粉砂或砂)、向上粒度逐渐变细的正粒序特征,只有T8沉积体为反粒序特征,可能为异重流沉积。结合区域地质资料,本研究认为1.4 ka B.P.以来,研究区频繁的台风带来了大量陆源松散沉积物堆积在马尼拉海沟上游(高屏峡谷),不稳定的构造环境及地震频发导致这些松散沉积物垮塌并向下游马尼拉海沟输送,在海沟内形成频繁发育的浊流沉积体。