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.     

印度河扇水道堤岸外侧更新世沉积物波发育特征与形成过程分析*

印度河扇更新世发育的沉积物波结构复杂、形态多样,其形成过程的认识程度低。本次研究通过高分辨率地震数据和地震解释技术,研究了印度河扇沉积物波的波长、形态、波峰变化等形态特征;阐述了沉积物波与沉积物变形特征的差异、识别了两者的区分标志;总结了水道堤岸斜坡和区域斜坡上沉积物波的分布规律;在此基础上,讨论了沉积物波的形成机理和控制因素,分析了沉积物波的形成过程,并建立了印度河扇沉积物波的形成模式。研究表明: (1)研究区沉积物波波长平均为486.84 m,最大1473 m;波高在10~60 m之间,平均30 m。(2)沉积物波的形态有对称型和非对称型,其迁移方式有上坡迁移型、加积型和下坡迁移型;沉积物波主要发育在水道堤岸的斜坡上,在区域斜坡上也发育少量的沉积物波,这2种沉积物波波脊的走向差异很大,水道堤岸斜坡上的沉积物波主要分布于水道凹岸堤岸的外侧,距离水道越远其规模(波长、波高)越小,波脊走向近于NE-SW方向,与水道的走向平行或斜交;区域斜坡上的沉积物波波脊的走向多为NW-SE向,平行于区域斜坡的走向,离源区越远规模越大。(3)水道堤岸斜坡上的沉积物波是由水道型浊流在离心力的作用下,溢出水道的凹岸,在堤岸外侧的斜坡上沉积形成的,堤岸斜坡的角度对沉积物波的发育规模影响不大,浊流的强度和输沙量对其规模影响大;区域斜坡上发育的沉积物波是由顺坡而下的非水道化的浊流沉积形成;滑塌变形造成的起伏地貌以及早期沉积物波的存在,也都影响了后期沉积物波的发育。     

印度河扇水道堤岸外侧更新世沉积物波发育特征与形成过程分析*

印度河扇更新世发育的沉积物波结构复杂、形态多样,其形成过程的认识程度低。本次研究通过高分辨率地震数据和地震解释技术,研究了印度河扇沉积物波的波长、形态、波峰变化等形态特征;阐述了沉积物波与沉积物变形特征的差异、识别了两者的区分标志;总结了水道堤岸斜坡和区域斜坡上沉积物波的分布规律;在此基础上,讨论了沉积物波的形成机理和控制因素,分析了沉积物波的形成过程,并建立了印度河扇沉积物波的形成模式。研究表明: (1)研究区沉积物波波长平均为486.84 m,最大1473 m;波高在10~60 m之间,平均30 m。(2)沉积物波的形态有对称型和非对称型,其迁移方式有上坡迁移型、加积型和下坡迁移型;沉积物波主要发育在水道堤岸的斜坡上,在区域斜坡上也发育少量的沉积物波,这2种沉积物波波脊的走向差异很大,水道堤岸斜坡上的沉积物波主要分布于水道凹岸堤岸的外侧,距离水道越远其规模(波长、波高)越小,波脊走向近于NE-SW方向,与水道的走向平行或斜交;区域斜坡上的沉积物波波脊的走向多为NW-SE向,平行于区域斜坡的走向,离源区越远规模越大。(3)水道堤岸斜坡上的沉积物波是由水道型浊流在离心力的作用下,溢出水道的凹岸,在堤岸外侧的斜坡上沉积形成的,堤岸斜坡的角度对沉积物波的发育规模影响不大,浊流的强度和输沙量对其规模影响大;区域斜坡上发育的沉积物波是由顺坡而下的非水道化的浊流沉积形成;滑塌变形造成的起伏地貌以及早期沉积物波的存在,也都影响了后期沉积物波的发育。     

Linking submarine channel–levee facies and architecture to flow structure of turbidity currents: insights from flume tank experiments

Submarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.     

Temporal and spatial changes of the submarine Cretaceous paleoslope in Northern Tunisia,inferred from slump folds analysis

This paper presents the first comprehensive, non-exhaustive, study of the genetic relationship between slump folds and the synsedimentary paleoslope during Cretaceous time in northern Tunisia. Slump folds occur mainly in the Cretaceous marl-dominated lithofacies, which exposes numerous slump folds structures. In addition, fault kinematic analysis is conducted to define the paleostress fields and the stress states characterizing the Cretaceous extension that triggers soft-sediment deformation and slumping. The MAM and the APM methods are used to deduce the paleoslope in several localities. The calculated values of paleoslope trend derived from MAM and APM methods precise the variation of the paleoslope trend during Cretaceous times in northern Tunisia. This paleoslope is ～NW-dipping during Berriasian, ～SSW-dipping during Valanginian, ～NW-dipping during the Barremian and ～N- to ～NNE or ～S- to ～SSW-dipping during Aptian–Albian period. The results of the back-tilted fault diagram show a ～North to ～Northeast-trending tectonics extension. The back-tilting of Cenomanian slump axis and poles of axial planes (MAM and APM methods) give close results with ～Southward or ～Northward-dipping paleoslope. The restored fault diagrams show ～North to ～Northeast-trending extension during Cenomanian times. Coniacian-Santonian marls deposits seal all the gravity-driven deformation structures. North Tunisian area exposes evidences for abundant soft-sediment deformation and slumping atop a northward facing submarine slope, which was probably dominant from the Early Cretaceous to Santonian with ～North-South tectonic extension related to the Southern Tethyan rifted continental margin evolution.     

Stratigraphic hierarchy and three-dimensional evolution of an exhumed submarine slope channel system

Submarine slope channel systems have complicated three-dimensional geometries and facies distributions, which are challenging to resolve using subsurface data. Outcrop analogues can provide sub-seismic-scale detail, although most exhumed systems only afford two-dimensional constraints on the depositional architecture. A rare example of an accessible fine-grained slope channel complex set situated in a tectonically quiescent basin that offers seismic-scale, down-dip and across-strike exposures is the Klein Hangklip area, Tanqua-Karoo Basin, South Africa. This study investigates the three-dimensional architecture of this channel complex set to characterise the stratigraphic evolution of a submarine channel-fill and the implications this has for both sediment transport to the deep-oceans and reservoir quality distribution. Correlated sedimentary logs and mapping of key surfaces across a 3 km² area reveal that: (i) the oldest channel elements in channel complexes infill relatively deep channel cuts and have low aspect-ratios. Later channel elements are bound by comparatively flat erosion surfaces and have high aspect-ratios; (ii) facies changes across depositional strike are consistent and predictable; conversely, facies change in successive down depositional dip positions indicating longitudinal variability in depositional processes; (iii) stratigraphic architecture is consistent and predictable at seismic-scale both down-dip and across-strike in three-dimensions; (iv) channel-base-deposits exhibit spatial heterogeneity on one to hundreds of metres length-scales, which can inhibit accurate recognition and interpretations drawn from one-dimensional or limited two-dimensional datasets; and (v) channel-base-deposit character is linked to sediment bypass magnitude and longevity, which suggests that time-partitioning is biased towards conduit excavation and maintenance rather than the fill-phase. The data provide insights into the stratigraphic evolution and architecture of slope channel-fills on fine-grained continental margins and can be utilised to improve predictions derived from lower resolution and one-dimensional well data.     

Comparative study of sampling methods and in situ and laboratory analysis for shallow-water submarine hydrothermal systems

A primary aim for sampling of submarine hydrothermal vents is to minimize inclusion of ambient seawater. Here, we compare the results of three different sampling methods (air displacement, two-valve bottle, and syringes) for shallow submarine systems. Mixing of hydrothermal fluid with seawater is unavoidable; however, calculations based on linear mixing models allow estimation of the hydrothermal fluid end-member composition. The results show that sampling with a two-valve bottle and syringes are the best options because they allow collection of samples with a large proportion of hydrothermal fluid. Additionally, we compare the results of in situ and laboratory analyses of the fluid samples, and demonstrate that determination of chemical composition in situ is the best option for some components, as re-equilibration affects some component concentrations (i.e. bicarbonate). Conversely, silica determination in situ usually underestimates the concentration in the fluid, as it does not account for polymeric silica. Other components can be measured either in the field or in the laboratory.     

安徽绩溪新元古代海底扇沉积环境分析

研究区位于安徽绩溪县杨溪镇,区内主要发育一套新元古代浅变质碎屑岩系,厚度为639.7 m。碎屑岩以泥岩、泥质粉砂岩、砂岩、含砾砂岩和砾岩为主,发育水平层理,粒序层理,韵律层理等沉积构造,可见鲍马层序Ta、Tc、Td、Te段。碎屑组分以跃移组分为主,含量为35%～58%。地层上段厚为254.8 m,包含粉砂岩、泥质粉砂岩和粉砂质泥岩,水平层理和粒序层理发育,为中扇-外扇扇叶沉积,中段厚为192.5 m,主要为细砂岩、粉砂岩和泥质粉砂岩,砂岩条带广泛发育,属于中扇水道沉积,下段厚为192.4 m,为一套砾岩层,砾石分选较差,砾岩层间夹有粉砂质泥岩,为上扇水道滞留沉积。总体上,形成一套退积的海底扇沉积序列。     

海底透水通道发育带隧道施工注浆技术研究

通过对海底透水通道的特征分析,采用裂隙超前预注浆和裂隙补充注浆方法成功地解决了裂隙注浆堵水难题,为海底隧道安全施工提供了保障。提出了适于海水条件下的裂隙注浆原则、参数、设备、方法、工艺、材料及注浆效果检查方法,并成功地应用于胶州湾海底隧道穿越断裂破碎带时裂隙岩体注浆实践,其研究结果可为类似工程提供参考     

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。     

Multi-factor sensitivity analysis on the stability of submarine hydrate-bearing slope

There are many factors affecting the instability of the submarine hydrate-bearing slope (SHBS), and the interaction with hydrate is very complicated. In this paper, the mechanical mechanism of the static liquefaction and instability of submarine slope caused by the dissociation of natural gas hydrate (NGH) resulting in the rapid increase of pore pressure of gas hydrate-bearing sediments (GHBS) and the decrease of effective stress are analyzed based on the time series and type of SHBS. Then, taking the typical submarine slope in the northern South China Sea as an example, four important factors affecting the stability of SHBS are selected, such as the degree of hydrate dissociation, the depth of hydrate burial, the thickness of hydrate, and the depth of seawater. According to the principle of orthogonal method, 25 orthogonal test schemes with 4 factors and 5 levels are designed and the safety factors of submarine slope stability of each scheme are calculated by using the strength reduction finite element method. By means of the orthogonal design range analysis and the variance analysis, sensitivity of influential factors on stability of SHBS are obtained. The results show that the degree of hydrate dissociation is the most sensitive, followed by hydrate burial depth, the thickness of hydrate and the depth of seawater. Finally, the concept of gas hydrate critical burial depth is put forward according to the influence law of gas hydrate burial depth, and the numerical simulation for specific submarine slope is carried out, which indicates the existence of critical burial depth.     

The role of submarine groundwater discharge in coastal biogeochemistry

The advection of fluids from sediments into the coastal ocean provides a source of nutrients, metals, carbon, and chemical tracers. Of these tracers radium isotopes have proved to be useful in quantifying fluxes of the other components and evaluating sources. Models that use radium must evaluate assumptions that are inherent in the models. The results of these studies demonstrate that submarine groundwater discharge must be considered as a significant source of nutrients, carbon, and metals to the coastal ocean.     

鄂尔多斯盆地西缘奥陶系拉什仲组复合水道沉积特征及演化

鄂尔多斯盆地西缘奥陶系拉什仲组发育一套砂泥互层的深水重力流沉积,其下部重力流复合水道发育。综合野外实测、薄片和粒度分析等,探讨复合水道沉积特征、形成过程及主控因素,最终建立其沉积模式。结果表明：(1)研究区发育5种岩相和3种岩相组合,即块状层理砾屑灰岩相(Cm)、粒序层理细—粉砂岩相(Sg)、平行层理砂岩相(Sp)、交错层理粉砂岩相(Sc)、水平层理(泥)页岩相(Sh),分别代表碎屑流沉积(Cm)、浊流沉积(Sg, Sp, Sc)及深水原地沉积(Sh);(2)根据复合水道内部单一水道形态、岩相组合及粒度等将复合水道分为4个期次,反映重力流能量由强到弱的变化过程;(3)复合水道多期次发育与相对海平面升降、沉积物供给、构造运动和重力流规模及能量变化密切相关;(4)重力流沉积为斜坡—盆地环境中复合水道沉积模式。该研究可补充对研究区重力流水道认识,为油气勘探提供借鉴。     

Sedimentary processes in the Selvage sediment-wave field, NE Atlantic: new insights into the formation of sediment waves by turbidity currents

An integrated geophysical and sedimentological investigation of the Selvage sediment-wave field has revealed that the sediment waves are formed beneath unconfined turbidity currents. The sediment waves occur on the lower continental rise and display wavelengths of up to 1 km and wave heights of up to 6 m. Wave sediments consist of interbedded turbidites and pelagic/hemipelagic marls and oozes. Nannofossil-based dating of the sediments indicates a bulk sedimentation rate of 2·4 cm 1000 years^–1, and the waves are migrating upslope at a rate of 0·28 m 1000 years^–1. Sediment provenance studies reveal that the turbidity currents maintaining the waves are largely sourced from volcanic islands to the south. Investigation of existing models for sediment-wave formation leads to the conclusion that the Selvage sediment waves form as giant antidunes. Simple numerical modelling reveals that turbidity currents crossing the wave field have internal Froude numbers of 0·5–1·9, which is very close to the antidune existence limits. Depositional flow velocities range from <6 to 125 cm^–1. There is a rapid increase in wavelength and flow thickness in the upper 10 km of the wave field, which is unexpected, as the slope angle remains relatively constant. This anomaly is possibly linked to a topographic obstacle just upslope of the sediment waves. Flows passing over the obstacle may undergo a hydraulic jump at its boundary, leading to an increase in flow thickness. In the lower 15 km of the wave field, flow thickness decreases downslope by 60%, which is comparable with results obtained for other unconfined turbidity currents undergoing flow expansion.     

陆上、水下喷发成因火山岩储层发育特征和成藏控制因素对比分析

火山岩油气藏的研究一般以陆上喷发沉积的火山岩体为主,事实上全世界四分之三的火山活动是在水下发生的,只是水下喷发沉积的火山岩在各个方面的研究还比较少,在勘探中尚未引起注意。本文以三塘湖盆地石炭系火山岩油藏为例,对比分析了陆上与水下喷发沉积形成的火山岩在岩性、颜色、结构构造以及储集空间特征和地震反射特征等方面的差异,指出了三塘湖盆地石炭系哈尔加乌组火山岩为水下喷发环境形成。通过对研究区两种类型的火山岩油藏系统的研究,总结出了水下喷发沉积的火山岩储层的形成机理以及其形成规模油气藏的条件,为今后的火山岩油气藏的勘探指出了新的方向。     

陆上、水下喷发成因火山岩储层发育特征和成藏控制因素对比分析

火山岩油气藏的研究一般以陆上喷发沉积的火山岩体为主,事实上全世界四分之三的火山活动是在水下发生的,只是水下喷发沉积的火山岩在各个方面的研究还比较少,在勘探中尚未引起注意。本文以三塘湖盆地石炭系火山岩油藏为例,对比分析了陆上与水下喷发沉积形成的火山岩在岩性、颜色、结构构造以及储集空间特征和地震反射特征等方面的差异,指出了三塘湖盆地石炭系哈尔加乌组火山岩为水下喷发环境形成。通过对研究区两种类型的火山岩油藏系统的研究,总结出了水下喷发沉积的火山岩储层的形成机理以及其形成规模油气藏的条件,为今后的火山岩油气藏的勘探指出了新的方向。     

Transformation of debris flows into turbidity currents: mechanisms inferred from laboratory experiments

Three sets of lock exchange experiments were run to look at the generation of turbidity currents from debris flows. The flows ranged from reasonably dilute (4% volumetric concentration) to dense (40% volumetric concentration) with cohesive, non-cohesive and mixed cohesive/non-cohesive sediment. Concentration was measured at one height using an Ultrasonic High Concentration Meter. Velocity was measured using Ultrasonic Doppler Velocimetry Profiling at 10 different heights in each run. The resulting flows range from plug flows to well mixed flows. Comparison of the concentration profiles, velocity time–height plots and vertical profiles of downstream velocity and root mean square velocity showed several different transformation mechanisms. Depending on the concentration and composition of the flow, transformation took place through one or more of the following processes: erosion of material from the dense mass, breaking apart of the dense underflow, breaking of internal waves and turbulent mixing. The extent of transformation depends on the viscosity and density of the flow. Initially very dense and viscous flows experience minor transformation only at the surface, resulting in a dilute turbidity current. Flows that are initially not so dense and viscous are churned up entirely, undergoing the different transformation processes. For these flows, transformation processes work throughout the entire flow, not just at the surface. Transformation of the less-dense flows is efficient with all or most material ending up in the resulting turbidity current.     

Seasonal variability of physico-chemical characteristics of the Haldia channel of Hooghly estuary, India

Physical and chemical characteristics of the Hooghly estuary during winter (December 1997–January 1998), summer (May 1998) and post-monsoon (November 1998) seasons have been studied. Salinity varied spatially and temporally and seasonally during ebb and flood tide conditions. Water temperature showed a difference of 10‡C in winter to summer. Temperature did not vary much vertically as it is a well-mixed estuary. Strong currents exceeding 100 cm S^-1 were observed during peak ebb and flood tide conditions irrespective of the season. Longitudinal eddy diffusion coefficient (K _x ) was estimated as 757m S^-1 and 811m² S^-1 during summer and post-monsoon seasons, respectively. The vertical eddy diffusion coefficient (ε_v) was estimated as 0.0337 m² S^-1 during post-monsoon season. The salinity and current observations are compared with those obtained from models reported earlier. Values of pH, Dissolved Oxygen and Biological Oxygen Demand are within the threshold limits of the estuarine environment. Nutrients show seasonal variation in the estuarine environment. High values (160-2686 mg l^-1) of total suspended matter were noticed both at surface and bottom in the study region showing the impact of fresh water and sediment transportation.     

Analysis of drivers governing temporal salinity and temperature variations in groundwater discharge from Altug Submarine Karst Cave (Kas-Turkey)

Salinity and temperature variations in groundwater discharge from the Altug submarine karst cave have been observed at 28 m below sea level for every 10 min between November 2004 and August 2005 to determine the drivers that govern the salinization. Comparisons between temporal trends of salinity and temperature with those of precipitation, air pressure, sea level and wind velocity revealed an apparent dominance of precipitation regime on the salinity and temperature variations. Spectral analyses applied to observations showed that the air pressure and sea level oscillations are affected by sun and moon tides which do not have an appreciable impact on the salinity and temperature variations. Annual rate of salinization in Altug cave seems inversely related to the inland groundwater head so that the maximum and minimum fresh water contributions occur at mid-spring and late-summer, respectively.