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。     

突然释放型浊流在不同坡折渠道中的流动与沉积

应用雷诺平均纳维尔—斯托克斯模型模拟探讨了等量突然释放型浊流在流经不同坡折渠道的流动及沉积特性,取得如下主要结论:随着底坡增加,浊流流速增长率因卷吸作用的增强而减缓;浊流深度平均速度和浓度形态相似,头部大且向尾部呈线性下降;在小坡折处产生沉积,沉积最厚处离坡折处不远且上下游平均粒径相差不大,往下游厚度呈线性减小;而在大坡折处产生侵蚀,随着坡度的增大侵蚀增加,沉积最厚处离坡折处较远且上下游平均粒径相差大,形态上呈上拱状。这些认识对于根据浊流沉积特征推测其形成环境具有一定的参考作用。     

海底扇沉积相模式、沉积过程及其沉积记录的指示意义

海底扇是由沉积物重力流形成的海底沉积体。其分类学和相模式研究表明,海底扇主要由海底水道、溢岸及朵叶体等沉积单元构成。然而古代和现代海底扇沉积均无法由单一的通用相模式进行解释。以粒度差异所建立的相模式类型涵盖了多方面信息,相对简单实用。海底扇的触发机制主要包括海底沉积物失稳、洪水型异重流、海洋动力过程及复合成因机制等类型。海底扇的主导流体类型（碎屑流与浊流）、海底地貌形态（限制性与非限制性）及海洋动力条件（底流作用）深刻影响了海底扇的沉积作用、平面形态及空间组合特征,整体上分为三类。其中,浊流沉积主导的海底扇在非限制性海底环境中主要表现为扇状或指状形态,在限制性海底环境中则直接受控于盆地的地貌形态;碎屑流沉积主导的海底扇以块体搬运为特征,平面上表现为舌状和叶状展布形态;底流与重力流共同作用形成的混合型海底扇朵叶体沿底流流向侧向偏转,部分受底流改造沉积形成孤立漂积丘状形态。海底扇沉积物记录了环境信号从“源”到“汇”传输效率和保存程度,对构造变形和古气候变化具有重要的指示作用。人类世以来的现代海底扇沉积物同时也是深海微塑料、陆源有机碳的重要储库,定量评估其丰度特征对于环境评价、污染治理与管控及全球碳循环均具有深远的现实意义。     

琼东南盆地陵水区中央峡谷水道沉积数值模拟

深水峡谷是当今海洋油气勘探领域的研究热点。南海西北部琼东南盆地的中央峡谷已部署了多口深水钻井来勘探沉积充填的岩性油气藏,然而,由于这些探井数量相对有限、井间距离大以及地震资料分辨率较低,峡谷内砂体的叠复关系、边界接触关系以及沉积演化等问题尚不清楚,严重制约着储层空间分布与储层物性的精细刻画。对琼东南盆地陵水区中央峡谷开展了沉积数值模拟研究,根据研究区具体地质背景建立了地质模型和数学模型,通过流体动力学软件ANSYS FLUENT正演模拟了多个沉积期次和多套砂组在不同初始条件(如物源和入流速度)下的浊流沉积几何形态,包括砂体平面分布特征和隔夹层分布特征。模拟结果表明:峡谷平直段内,浊流受底床摩擦力的持续影响,流速降低且湍流强度减弱,使得较粗颗粒可沉降于底床,细颗粒可随浊流头部涡流悬浮;峡谷狭窄段内,浊流头部的湍流较强,侵蚀峡谷壁并使峡谷走向发生偏移,悬浮颗粒受离心力作用形成溢岸沉积;峡谷内砂体展布具有垂向分异性,砂体内部泥岩以夹层为主,厚度一般较小且横向连续性差。与现有地震、钻井资料的对比分析显示本次数值模拟结果具有有效性。本研究成果揭示了中央峡谷不同沉积期次和不同砂组的沉积水动力学过程,进而预测了砂体的空间展布特征,为储层物性预测提供了坚实支撑。     

西秦岭直合隆地区三叠系深水水道沉积模式分析

西秦岭三叠系地层主要由深水沉积物组成。选取西秦岭直合隆地区出露较好的三组剖面进行研究,按重力流类型及静水条件下所发生的沉积现象,总结出六种岩石相类型,即滑塌岩相（F1）、碎屑流相（F2）、超高密度流相（F3）、高密度浊流相（F4）、低密度浊流相（F5）、深海泥岩相（F6）。通过对三处剖面的横向追踪对比,结合不同沉积环境下表现出特定的岩石相占比及砂体堆叠样式,共识别出限制性水道、弱限制性水道及水道天然堤三类沉积单元。限制性水道中砂体叠置关系复杂,以超高密度流相占主导地位,次为碎屑流相;弱限制性水道中砂体叠置较规整,以超高密度流相占主导地位,次为低密度浊流相;而水道天然堤表现为砂泥互层（总厚度约15 m）,主要为低密度浊流相和深海泥岩相。最后建立了研究区内深水水道沉积演化阶段模式图——早期限制性环境下水道较顺直,水道较窄,砂体叠置关系复杂;中期限制性环境相对早期有所减弱,水道弯曲度增大,水道变宽,出现溢岸沉积,砂体叠置关系复杂;晚期为弱限制性环境,弯曲水道两侧发育天然堤,砂体叠置规整。本次研究利用野外露头建立的深水水道沉积演化模式,在一定程度上还原其相关的重力流演化过程及发育期次,对全球其他类似研究具有一定的参考意义。     

北祁连志留系复理石的再探讨

本文通过对北祁连冰沟和肃南两地下志留统复理石的研究．认为它们都是深海系的浊流沉积，包括了盆地平原、外扇、中扇和内扇等不同环境。其中夹有多层砾岩，但它们不代表海水变浅或滨海环境，而是深海扇中的海底峡谷沉积。最后，本文进而讨论了有关的古地理面貌和古构造格局。     

北祁连志留系复理石的再探讨

本文通过对北祁连冰沟和肃南两地下志留统复理石的研究．认为它们都是深海系的浊流沉积，包括了盆地平原、外扇、中扇和内扇等不同环境。其中夹有多层砾岩，但它们不代表海水变浅或滨海环境，而是深海扇中的海底峡谷沉积。最后，本文进而讨论了有关的古地理面貌和古构造格局。     

海底冲沟——深水沉积输运系统的"毛细血管"

海底冲沟作为比海底峡谷、水道地貌尺度小1个数量级的微地貌,与海底峡谷、水道共同构成了完整的深水沉积输运系统。由于调查数据分辨率的限制,在前人研究中,海底冲沟的重要作用一直被忽视。海底冲沟是深水沉积输运系统的“毛细血管”,数量巨大,主要分布在大陆坡、岛礁边缘、河口冲积扇前缘、海底峡谷和水道内部,与深水工程安全、岛礁安全、深水沉积体系和深水油气储层预测息息相关。从识别特征、沉积环境、影响因素和形成机理等方面,系统介绍了海底冲沟的研究进展;从分布区域、坡度、形态和沉积特征等方面,探讨了海底冲沟与海底峡谷和水道之间的区别与联系,并给出了区分三者的方法。通过归纳不同沉积环境的海底冲沟特征,将其定义为：在坡度大于2°的地形中,重力流成因、直线型的微地貌尺度沟槽,具有宽度小、深度浅、长度短和形态笔直（弯曲度接近于1）的特点,其中,片状浊流成因的海底冲沟还具有相互平行和等间距分布的集群特征。     

科学与技术并进——近20年来海底峡谷浊流观测的成就和挑战

自1993年至今,美国地质调查局的科学家及其合作伙伴在美国西海岸的Monterey海底峡谷进行了针对现代浊流过程的一系列基础性研究,并成功地在世界上首次实地测量到高精度浊流流速及粒度参数.近20年来的数据和知识积累为解释海底峡谷内沉积物和其他颗粒物质输运的机理,以及浊流在维持深海峡谷中生机勃勃的生态系统所起的重要作用提供了直接依据.通过展示把海底观测应用于海洋沉积动力学研究过程中的成果、经验、教训,以及介绍目前还在讨论中的研究计划,以期达到以下宏观论点:在海洋科学里,只有科学与技术不脱节的科研团队才有希望获得成果和突破.     

深海弯曲水道内部一种特殊的沉积单元:凹岸坝

在深海水道研究过程中,识别出深海弯曲水道内部存在一种特殊的沉积单元--凹岸坝,基于尼日尔三角洲陆坡区浅层高频三维地震资料,利用地震相分析技术,探究了凹岸坝的沉积结构特征和形成机制,讨论了其与水道弯曲丘(nested mounds)、曲流河凹岸滩坝之间的差异。研究结果表明:凹岸坝是分布于曲率较大、以垂向加积为主的末期水道弯曲凹岸处的坝体沉积单元,该沉积单元在地震剖面上表现为强振幅、连续性较好的反射特征,其内部沉积界面倾向于水道弯曲凸岸处,倾角约1°~20°,且在凹岸弧顶处达到最大值。凹岸坝形成的关键在于惯性作用,其造成水道内部重力流流体在弯曲处发生溢岸,导致流量减少,流体动能相应降低,流体携砂能力小于沉积物负载,造成沉积物快速沉降,从而形成凹岸坝。由于凹岸坝是一种连续性较好的砂体沉积,所以其可成为潜在的、储集性能较好的油气储集体。     

The orientation of helical flow in curved channels

The planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open‐channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density‐driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near‐bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open‐channels.     

An ancient submarine canyon in the Oligocene-Miocene of the western Niger Delta

ABSTRACT
An anomalous sequence of thick fossiliferous marine shale of late Oligocene-early Miocene age, here termed the Opuama Shale Member, occurs within the paralic Agbada Formation in the subsurface of the western Niger Delta. The Opuama Shale fills a deep palaeochannel which was cut into Eocene-early Oligocene paralic beds. Planktonic and benthic foraminifera suggest that deposition in the channel began in the late Oligocene-early Miocene at outer shelf-slope depths, and that by the late early Miocene the channel had filled to shallow neritic depths. The channel, which is believed to be of submarine origin, is termed the Opuama canyon. The Opuama canyon originated during the pronounced early-Middle Oligocene drop in sea-level, and was cut by turbidity currents. It was filled, during the major late Oligocene-early Miocene rise of sea-level.
The location of the Opuama canyon in the western re-entrant of the Niger Delta supports Burke's (1972) conclusion that the western and eastern (where ancient submarine canyon fills are known) re-entrants of the delta are potential sites for ancient submarine canyons. Burke based his conclusion on the observation that the eastern and western re-entrants of the modern delta are areas where opposing longshore drifts converge and generate turbidity currents which cut submarine canyons.     

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.     

Lateral accretion in a deep-marine channel complex: implications for channellized flow processes in turbidity currents

Some deep-marine channels show striking similarities to fluvial channels, despite major differences in the properties of the flows that they conduct. Some field observations from deep-marine channel deposits within a Late Cretaceous palaeo-canyon in the Rosario Formation of Baja California, Mexico, that bear on these comparisons have been reported. These channel deposits contain erosively based lateral accretion sets. These sets are generally a few metres thick and resemble fluvial point bar deposits. Sediment movement and deposition within these accretion deposits was induced by turbidity currents several to many times thicker than the depth of the channel, moving at several metres per second. The inclined sets define laterally migrating and sinuous channels locally at a high angle to the confining canyon. The instantaneous channel widths varied from 6 to 39 m, the depths from 2·5 to 4 m and the sinuosities from 1·3 to 3·1. Palaeocurrent data, taken mostly from clast imbrication in conglomerates, indicates current modes along the channel thalweg, but with other directions representing either secondary flow (oriented primarily up the point bar) or over-passing canyon-confined flow. It is suggested that, at times, the lower part of the turbidity currents flowing down the channels behaved similarly to within-bank fluvial currents, with a cross-channel component of flow towards the cut bank, and return flow at the bed sweeping up the point bar. At other times, this secondary circulation may be absent or reversed, which may be related to changes in flow thickness, coupling with the overriding flow and possible flow separation.     

Sealers Bay submarine fan complex, Oligocene, southern New Zealand

The Oligocene Balleny Group of Chalky Island, southwestern Fiordland, comprises a typical continental margin sequence 900 m in thickness. Thin nearshore traction deposited sediments at the base are overlain by submarine canyon and fan lithofacies that were deposited by the full range of subaqueous mass-transport processes. A steep-walled channel within Balleny Group is interpreted as a fossil proximal fan-channel. The sedimentary fill of the channel is texturally similar to sediments moving by slump-creep in Recent submarine canyons and fan-valleys. The field data presented indicate (1) that a small canyon complex at Sealers Bay was initially cut by subaqueous debris-flows derived from an adjacent cliffed continental coast; (2) that transport within the upper parts of the canyon and fanchannel complex was primarily by inertia-flow and slump-creep; and (3) that these more proximal types of mass-transport gave way gradationally and successively to fluxoturbidity and turbidity currents at locations further down-slope, with consequent deposition of sediment in more distal fan-channel and fan-surface environments as fluxoturbidites and turbidites, with lesser contributions from inertia-flows.     

Turbidite depositional patterns and flow characteristics, Navy Submarine Fan, California Borderland

The late Pleistocene and Holocene stratigraphy of Navy Fan is mapped in detail from more than 100 cores. Thirteen ¹⁴C dates of plant detritus and of organic-rich mud beds show that a marked change in sediment supply from sandy to muddy turbidites occurred between 9000 and 12,000 years ago. They also confirm the correlation of several individual depositional units. The sediment dispersal pattern is primarily controlled by basin configuration and fan morphology, particularly the geometry of distributary channels, which show abrupt 60° bends related to the Pleistocene history of lobe progradation. The Holocene turbidity currents are depositing on, and modifying only slightly, a relict Pleistocene morphology. The uppermost turbidite is a thin sand to mud bed on the upper-fan valley levées and on parts of the mid-fan. Most of its sediment volume is in a mud bed on the lower fan and basin plain downslope from a sharp bend in the mid-fan distributary system. Little sediment occurs farther downstream within this distributary system. It appears that most of the turbidity current overtopped the levée at the channel bend, a process referred to as flow stripping. The muddy upper part of the flow continued straight down to the basin plain. The residual more sandy base of the flow in the distributary channel was not thick enough to maintain itself as gradient decreased and the channel opened out on to the mid-fan lobe. Flow stripping may occur in any turbidity current that is thick relative to channel depth and that flows in a channel with sharp bends. Where thick sandy currents are stripped, levée and mid-fan erosion may occur, but the residual current in the channel will lose much of its power and deposit rapidly. In thick muddy currents, progressive overflow of mud will cause less declaration of the residual channelised current. Thus both size and sand-to-mud ratio of turbidity currents feeding a fan are important factors controlling morphologic features and depositional areas on fans. The size-frequency variation for different types of turbidity currents is estimated from the literature and related to the evolution of fan morphology.     

不同近岸河床组成情况下岸坡崩塌试验

在弯道水槽中开展6组试验,分别用非黏性土及黏性土填筑河床,研究相同水力作用下近岸河床组成对黏性岸坡崩塌的影响规律。研究发现,在试验给定的岸坡及河床组成情况下,非黏性河床凹冲凸淤且总体表现为淤积,近岸河床及凹岸岸坡冲刷强度大,凸岸附近床面上泥沙掺混较明显;黏性河床及凹岸岸坡均被冲刷,河床主流区冲刷强度比近岸河床及凹岸岸坡大。相较于黏性河床,非黏性河床近凹岸处较易冲刷,水流结构重新调整,凹岸坡脚处水流流速及紊动能可增至2倍左右,环流强度可增至11倍,加速了岸坡崩塌及崩塌体的分解输移;非黏性河床近凹岸坡脚处变形以及凹岸岸坡崩塌量均相对较大,岸坡崩塌强度为河床淤积强度的2~4倍,崩塌物质可充分补给河床的泥沙来源;经水力冲刷后非黏性河床组成情况下形成的河道滩槽高差相对较小,河道横断面相对宽浅。     

水沙调节后荆江典型河道横向调整过程的响应——Ⅱ.上、下荆江调整差异初探

三峡水库2003年6月蓄水运用后,下游河道水沙过程已发生大幅变化,对于岸滩抗冲性较弱的荆江河道来说,河道的横向调整过程势必受到影响。采用考虑岸滩崩塌的河势研究数值模型,针对三峡水利枢纽工程运行所引发的水沙变异过程,初步探讨了荆江典型河道横向调整及河势变化对水沙条件变化的响应。对于下荆江石首河段来说,来沙减少后,冲刷加剧,局部河岸坍塌及平面变形加剧,主要发生在受弯道水流顶冲的位置,但河势演变趋势及平面变形总体上基本一致,并未发生较大变化。对于上荆江沙市—新厂河段来说,来沙量减小后,河道平面变形幅度总体上减小,局部最大减幅可达50%左右。