王大伟
, 白宏新
Wang Dawei, Bai Hongxin
中图分类号: P736.21
文献标识码: A
文章编号: 1001-8166(2018)01-0052-14
收稿日期: 2017-09-30
修回日期: 2017-12-25
网络出版日期: 2018-01-10
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
First author:Wang Dawei(1976-), male, Suihua City, Heilongjiang Province, Associate professor. Research areas include marine geophysics, submarine geohazard.E-mail:wangdawei@idsse.ac.cn
作者简介:王大伟(1976-),男,黑龙江绥化人,副研究员,主要从事地震沉积学、深水油气和海洋地质灾害研究.E-mail:wangdawei@idsse.ac.cn
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摘要
从19世纪发现海底浊流现象开始,这一重要的深水沉积过程就引起了地学界的广泛关注。研究发现深水浊流成因的周期阶坎等超临界流底形广泛分布于海底,且与鲍马定义的浊积岩序列有着密切联系。由于浊流事件的不可预测、破坏力强,直接观测设备和技术也比较匮乏,造成对浊流事件进行直接海底观测的工作较少。总结了国内外浊流及其相关底形的研究成果,对浊流分类、底形的形成与演化进行了讨论,列举了国内外几个典型深水区浊流及其相关超临界流底形的研究案例,包括实验、数值模拟和深水底形研究,详细介绍了周期阶坎这一主要的超临界流底形,讨论了周期阶坎的形成与演化过程及其与鲍马序列的对应关系。最后,对浊流及其相关的超临界流底形研究进行了展望。
关键词：
Abstract
Since turbidity current was reported in the 19th century, its flow dynamics, depositional processes and products have drawn much attention of geoscience community. In the last decades, with the help of rapid development of geophysical technology in deep-water areas, superficial bedforms formed by turbidity currents like cyclic steps have been widely documented on the seafloor, and they have been interpreted to be closely related to turbidite facies defined by the Bouma sequence. However, there is still a lack of direct observation on turbidity currents due to difficulties in the design and deployment of flow-measuring instruments under the sea. Such difficulties also result in much uncertainties in the explanations for the formation of bedforms and related flow processes. This paper summarized and discussed current research status of turbidity-currents classification, the formation and evolution of bedforms. Examples of supercritical-bedform studies using various methods such as experiments, numerical simulation, bathymetric data and seismic data, were shown in this paper. As one of main supercritical flow bedforms, cyclic steps were described in detail in this paper, including its formation, evolution and relationship with Bouma sequence. The variations in initial bed morphology and hydrodynamic parameters are responsible for the changes in the shapes of bedforms. Turbidites formed under different hydrodynamic conditions correspond to different units of Bouma sequence. Not all turbidity events can form a complete Bouma sequence. Therefore, traditional Bouma sequence cannot be applied to all turbidite studies. A more complete turbidite facies model must be established through studies from modern deep-sea sediments, outcrops, physical and numerical simulations. Additionally, turbidity currents and related supercritical bedforms are receiving more and more attention. They are important components of understanding the dynamic evolution of deep-water continental slope. The study of cyclic steps and other bedforms related to turbidity currents not only helps to characterize flow dynamics, but also provides a theoretical basis for the research of turbidite reservoirs. Finally, we proposed future research directions of turbidity currents and their related supercritical bedforms.
Keywords：
在过去很长一段时间里,海底被认为是有机质及其他沉积物最终的沉积场所,而且认为被厚厚一层海水与波涛汹涌的海表面隔离开的海底世界是非常平静的。直到19世纪末,Milne[1]通过一系列的海底观测证据得出海底是在不断变化的,有些变化比较缓慢,有些变化非常剧烈且具有破坏性,这一理论的提出颠覆了传统观念中关于海底的认识。1929年,在纽芬兰岛南部陆坡上,由地震造成的浊流破坏了多家通讯公司的海底电缆[2],这一事件引起了人们对海底地质活动的关注,也极大地提高了科学家对海底浊流探索的兴趣[3~6]。但是,由于技术的限制,使得早期的研究仅局限在实验室[7]、数值模拟[8]和露头研究[9],一直没能捕捉到浊流的流速及内部结构。随着地球物理探测技术及海上勘探工作的不断发展,发现广袤的深水海底存在着丰富多样的形态。随后,科学家们陆续在世界各地发现了海底峡谷[10],在远离海岸的海底峡谷一端发现了源自海岸的砂体沉积,Smith[11]判定其可能是深水浊流将海岸沙沿着海底峡谷搬运到了深海,“浊流”这一概念也被越来越多的人知晓。从1993年开始,美国地质调查局(U.S. Geological Survey, USGS)在Monterey海底峡谷深水区布放潜标对浊流进行实地观测,截至2011年,一共对该区域进行了3次观测,每次观测时长1年左右,共捕捉到7次浊流事件,成功地收集了浊流过程中悬浮/沉积物样品以及流动参数,为浊流相关的研究提供了宝贵资料[12]。随着深水油气、大洋矿产等深海资源的开发和深水工程作业的广泛开展,深水探测技术越来越成熟,勘探范围更加广泛,勘探精度也越来越高。海底各种各样的底形(bedforms)逐渐被发现,深水底形成为了目前深海科学领域的一个研究热点。Normark等[13]在Monterey峡谷发现了沉积物波,并对其做了初级的物理学解释。Wynn等[14]在加那利岛的海底斜坡上也发现了浊流成因的波状沉积物底形。Cartigny等[15]通过水槽实验观察到浊流形成的底形,研究了底形的外部形态和内部结构,进行了数值模拟。2015年,Zhong等[16]利用高分辨率的多波束海底测深数据和地震数据,发现了南海东北陆坡的周期阶坎底形,研究了周期阶坎的发育特征和内部结构。
本文介绍了目前国际上研究较成熟的几个典型深水底形实验和具体实例,对超临界流底形的形成与演化过程进行了分析,并讨论了周期阶坎等深水底形与鲍马序列之间的关系,对浊流及其相关底形研究的发展方向做了展望。
浊流一般起源于大陆架边缘或大陆坡上游,沿着海底峡谷或水道等重力流流体通道,将海岸沙等陆源碎屑物质向下游搬运,在搬运过程中随着流体动力学参数的改变,在海底形成诸如逆行沙丘(antidunes)、不稳定逆行沙丘(unstable antidunes)、流槽和冲坑(Chutes and pools)、周期阶坎(cyclic steps)①等深水底形[15]。底形一般指底床(河床或海床)沉积物在流体作用下形成的波状形态。本文主要探讨海底浊流成因的底形。浊流研究具有极大的挑战性,因为它们多发生在深水区,发生时间不可预测,而且极具破坏性,对其进行观测研究十分困难,目前也只观测到少数低速浊流的流动过程[17~19]。
浊流是沿着水下斜坡或海底峡谷流动、携带有大量泥砂的高密度流体,是重力流的一种特殊形式[20]。Lowe[21,22]根据流体内沉积物颗粒的支撑机制将重力流细分为:浊流、流化流(fluidized flow)、颗粒流(grain flow)、泥流(mud flow)或黏性碎屑流(cohesive debris flow)。自然界出现的重力流通常包含一种以上的支撑机制,在不同的搬运阶段表现不同,而且在流动过程中也可以相互转化。通常在流动过程中,重力流都可转化为浊流,关于浊流的分类也一直存在着争论[23,24]。
浊流的流体动力学影响了深水沉积体系结构单元的演化,可以分为超临界流和亚临界流2种。超临界流在海底峡谷和水道的陡坡处较常见,亚临界流在盆底和陆隆位置发育的水道和朵体中较常见。Cartigny等[25]通过实验对具有不同动力学参数的流体及其对应的底形进行了研究和分类(图1)。
水下超临界流至亚临界流的转换过程是不连续的,需要通过内水跃(internal hydraulic jump)这个不连续过程来实现。水跃(hydraulic jump)是流体力学中的一个现象,当明渠中的流体从高速超临界流状态进入低速亚临界流状态时,流体的速度突然减小,流体的部分动能被紊流消散,部分动能转换为位能,造成液面明显升高的现象称为水跃,内水跃是指流体处在一个密度更低的环境流体中时发生的水跃[26]。
图1 以雷诺数和维德尼科夫数为依据的超临界流底形概念上的细分图[
Fig.1 Conceptual subdivision of supercritical-flow phenomena on the basis of Reynolds number and Vedernikov number[
随着人工地震资料和多波束测深数据精度的不断提高,在海底发现了大量长波状(波长/波高≫1)且向上游迁移的底形,这类底形往往成串分布在峡谷、水道和水道—朵体过渡带等深水沉积结构单元中[27],这类底形之前也被称为沉积物波或冲刷底床(scour)[28]。沉积物波被广泛用于描述大型的、对称或非对称的、向上游迁移的波状底形。冲刷底床是切入海底、呈新月形—近圆形的凹槽,通常在天然堤和水道—朵体过渡带形成一系列不对称波状底形,该底形属于侵蚀型底形,通常在地层记录中很少保存下来,但在现今海底之上特征比较明显[29]。
沉积物波或冲刷底床大多属于超临界浊流形成的周期阶坎[30]。周期阶坎是浊流沉积中长波状(波长/波高≫1)且向上迁移的高流态底形,以内水跃为界,在海底陡坡和坡折带常见。周期阶坎的出现,说明通过内水跃,超临界流转变为亚临界流,然后再次加速成超临界流,重复上述转变,这一过程周而复始直到流体无法加速成超临界流(图2)。
图2 不对称周期阶坎和密度弗劳德数(Frd)变化示意图(从左至右)[
Fig.2 Schematic drawing of a series of asymmetrical cyclic steps (downstream from left to right) and hypothetical densimetric Froude number (Frd) variability[
超临界流及相关的深水底形(如逆行沙丘、周期阶坎),已经成为深水沉积体系中重要的组成部分[27,30,31]。这类底形在峡谷—水道、天然堤漫溢和水道—朵体过渡带沉积与演化中发挥着重要作用[15,27,30~33]。虽然已有个别机构可以直接观测浊流的流动过程并采集相关参数,但是由于技术上的限制和浊流事件的不可预测性,目前还未能在世界范围内对大规模的浊流事件进行直接有效的监测和数据采集。对于周期阶坎这种浊流成因的底形,国内外学者已经开展了大量的室内实验、数值模拟和海底观测研究[15,25,34~37],对其内部结构特征、形成与演化规律,已经有了较完善的理论体系和研究经验。
Mutti等[38]根据浊流主要颗粒的大小,提出了2个水道—朵体过渡带的沉积模式(图3a,b)。
模式一:以粗粒沉积为主的浊流,在陆坡坡折的位置发生内水跃,产生大型冲刷底形。由于水跃期间的能量损失和水流能力的降低,在刚超过陆坡坡折的水道口位置便沉积形成砂体楔(图3a)。
模式二:砂、泥混合的浊流,不会剧烈冲刷海底,并且由于泥的存在降低了沉降速度,使大部分沉积物搬运至更远的位置(图3b)[38,42]。Wynn等[39]根据海底和浅剖成像资料,支持了这些早期的水道—朵体过渡带沉积模型,侵蚀和沉积特征都得到了很好的印证。
Hamilton等[32]和Postma等[33]通过物理模拟实验,展示了水道—朵体过渡带浊流的地貌动态特征。超临界流在水道口发生内水跃产生沉积,水道口的沉积进一步促使内水跃的发生,导致沉积物在水道内向上游沉积充填,最终导致水流改道。考虑到大陆边缘浊流中发生内水跃的可能性,最新的研究已经在深水沉积露头的解释过程中,加入了浊积岩的周期阶坎成因[40,43,44]。每个阶坎的相组合包括块状、相对粗粒的砂岩层(鲍马序列Ta段)[45],其底部可见塑性沉积变形,处于冲刷底床的下游、迎流面的上游;以及相对细粒、平行层理砂岩(图3d)[40,46,47]。在野外露头中,阶坎表现为平坦至透镜状的地层,单个阶坎可以发育几百米,且阶坎向上游迁移[40,48]。
图3 水道—朵体过渡带超临界浊流内水跃过程及其沉积形成的底形(据参考文献[14,39~41]修改)
(a)小体积、富砂流体,上面是流动过程,下面是沉积物;(b)大体积、砂泥混合流体,上面是流动过程,下面是沉积物,C-C’和D-D’是(c)和(d)示意图的位置(据参考文献[39]修改);(c)水道—朵体过渡带平面图(据参考文献[40]修改);(d)周期阶坎沉积的浊积岩相
Fig.3 Processes and deposits of supercritical turbidity currents undergoing an internal hydraulic jump at the channel-lobe transition zone(modified after references [14,39~41])
(a) Relatively small-volume, sandrich current. Above: Process, Below: Resulting deposit. (b) Large-volume, mixed sand and mudcurrent. Above:Process;Below: Resulting deposit. The regions of C-C’ and D-D’ are schematically illustrated inparts (c) and (d) (modified after reference [39]).(c)Map view of channel-lobe transition zone(modified after reference [40]). (d) Turbidite facies produced by deposition on a single cyclic step
基于岩相特征来识别野外露头中长波状的周期阶坎仍然存在挑战,因为许多露头的规模比较有限,能够完全展现周期阶坎几何形状的露头实例非常少[49,50]。另外,浊积岩中层理的发育和缺失主要受沉积物供给和沉降速率的影响[34,51~53],只要沉积物的供给量或沉降速率足以抑制其层理的发育,厚层、粗粒、块状或正粒序的浊积岩,可以出现在沉积的任何位置,且这些位置并不一定与内水跃相关。大量的深水砂岩沉积在坡度较低的广阔的盆底区[54,55],但是,在这些环境中超临界流不可能占主导地位。即使是位于大型海底水道中的浊流,在角度较低的陆隆和盆底位置也只能是亚临界流(如亚马逊水道[56])。所以,仅凭借某个单一岩相很难判断浊积岩的成因和形成与演化过程。
近年来,随着实验室模拟手段和深水地球物理技术的不断发展,深水底形引起了广泛关注,该领域学者相继做了许多超临界流及其相关底形的实例研究和实验模拟,下面将列举其中几个典型模拟实验和研究实例,旨在说明深水超临界流及其相关底形的研究进展,为后续该方面的研究提供理论基础。
Cartigny等[35]2014年在Utrecht University的水槽实验室进行了超临界流及其相关底形的实验(图4),得到了关于超临界流底形的外部几何形态、内部结构、形成与演化的研究成果。
通过实验观察,对超临界流流动过程及其相关底形的形成与演化有了很直观的认识,并建立了单一流向超临界流底形形成与演化的理想模式(图5)。
在水槽实验中,发现从逆行沙丘到周期阶坎底形的演化是由流体初始能量的增加导致。而且实验表明稳定逆行沙丘(本文称逆行沙丘)和周期阶坎是与超临界流相关底形的最主要表现形式,根据它们相似的周期和渐变的地貌动力学关系,可认为不稳定逆行沙丘、流槽和冲坑底形是逆行沙丘与周期阶坎底形之间的过渡阶段。在该实验过程中,虽然逆行沙丘这种动力学特征的底形占重要地位,但是,随着流体动力的减弱,在其上部会叠覆低振幅、长波长的周期阶坎。所以,周期阶坎往往是在实际地质调查和地球物理勘探中观察到的与浊流相关最多的深水底形。
图4 超临界流底形水槽实验[
(a)Utrecht University的Eurotank水槽实验室可循环水槽示意图,长12 m,宽0.48 m,高0.6 m;(b)水槽中正在流动的流体形成了周期阶坎底形,水槽下方灰色的是再循环管道;(c)从水槽头部向下游看到的周期阶坎底形形成环境
Fig.4 Flume experiment of supercritical bedforms[
(a) Re-circulating flume, 12 m long, 0.48 m wide and 0.6 m long in the Eurotank Flume Laboratory at Utrecht University. (b) Flume in action with flow directed to the left and development of cyclic steps, notice gray recirculating pipe underneath the main tank.
(c) Downstream view from the flume head during cyclic-step conditions
图5 单向超临界流流动过程及其底形结构和演化的4个阶段示意图[
(a) 稳定的逆行沙丘; (b)不稳定逆行沙丘; (c)流槽和冲坑; (d)周期阶坎底形
Fig.5 Representative overview of four stages in unidirectional supercritical flows[
(a) Antidunes. (b)Unstable antidunes. (c)Chutes-and-pools. (d)Cyclic steps
周期阶坎形成的控制因素主要通过数值模拟方法来分析。2011年,Kostic等[30,57]建立了无量纲参数N,来控制周期阶坎的地貌动态变化:
N=f(Frd0,Rf,S0C0,p,λ,
式中:Frd0,C0和U0分别是流体的密度弗劳德数、浓度和速度;S0是初始底形的斜率;p是表征底形抗侵蚀能力的系数;λ为底形孔隙度;Lbreak/L为坡折位置长度相对于整个研究区长度的比值;vs/U0是颗粒沉降速度与流速的比值;Δτ*为底床的希尔兹数;r0是一个常数,是近河床沉积物浓度和平均浓度的比值;为河床摩擦系数。
Kostic[30]发现沉积型周期阶坎迁移和地层特征的控制因素(图6),包括了初始斜率的影响S0、底形抗侵蚀能力p、底形孔隙度λ、陆坡坡折长度Lbreak、流体浓度C0、密度弗劳德数Frd0。在峡谷和水道的近端,较陡的坡度和较高的沉积物浓度更有利于形成超临界流,这种环境有利于产生较短波长的周期阶坎;相比之下,限制小、角度低的天然堤和远端水道—朵体过渡带产生的周期阶坎波长较长[15,27,30,31]。另外,D(沉积物的平均颗粒尺寸)和p的值,对于确定沉积型周期阶坎的侵蚀沉积程度也非常重要。控制周期阶坎开始和沉积的关键无量纲参数分别是vs/U0和Δτ*。沉积物沉降速度与流体速度的比率,确定了浊流内水跃沉积物颗粒的大小, 而河床希尔兹数Δτ*,确定了水跃是否足以留下沉积记录[30]。
图6 周期阶坎演化的控制因素[37]
(a)基础方案:颗粒大小80 μm,抗侵蚀能力p=0.04; p的范围为0~1,分别对应固结岩床到未固结松散沉积物;(c)~(h)的条件除了选中的控制因素外,其他和(a)基本一致;底形演化(绿线)沿初始陆坡(红线)变化;(b)基础方案弗劳德数向下游的变化情况;(c)初始陆坡的影响(S0从1.3%增加到2.5%);(d)河床孔隙度的影响(λ从0.5增加到0.7);(e)悬浮沉积物浓度的影响(C0从0.01增加至0.03);(f)沉积物提供的影响(抗侵蚀能力p从0.04增加至0.06);(g)坡折带位置的影响(在模拟域内从6 km增加至15 km);(h)流体弗劳德数的影响(流体深度从20 m增加至100 m)
Fig.6 Controlson the evolution of cyclic steps[
(a) Base case in which grain size was set equal to 80 μm and the entrainment limiter p was set equal to 0.04. The entrainment limiter p ranges from 0 for consolidated bedrock to 1 for unconsolidated, loose sediment. Parts (c)~(h) pertain to the same conditions as part (a), except for the value of one selected control parameter. Bed elevation (green lines) change along a hypothetical initial slope (red line) in response to a sequence of overriding turbidity currents.(b) Downstream variation in the Froude number for the base case in part A.(c) Effect of the initial slope (S0 increased from 1.3% to 2.5%).(d) Effect of bed porosity (λ increased from 0.5 to 0.7).(e) Effect of the inflow concentration of suspended sediment (C0 increased from 0.01 to 0.03).(f) Effect of the sediment availability (entrainment limiter p increased from 0.04 to 0.06).(g) Effect of the slope-break location (increased from 6 to 15 km within the model domain). (h) Effect of the inflow Froude number (inflow depth increased from 20 to 100 m)
南海是西太平洋地区最大的边缘海,南海海盆起源于始新世—中新世[58]。南海沉积物波的研究可以追溯至19世纪60年代末,当时Fox等[59]提出盆地沉积物波的存在,Damuth[60]指出马尼拉海沟附近存在大约25 000 km2的沉积物波区域,并将其解释为浊流成因。在过去10年,南海识别出了越来越多的沉积物波[61~66]。Zhong等[16]利用多波束测深数据和人工地震剖面,在典型峡谷中识别出了大量阶坎状底形。这些底形波长1.2~10 km,波高5.4~80.9 m,依据其发育特点:呈链状排列、较大的波长/波高比,以及典型的向上游迁移特征,将其解释为超临界流形成的周期阶坎。
Zhong等[16]对研究区发育的周期阶坎底形的触发、形成演化和内部结构进行了系统研究,通过流体数值模拟研究,发现上游较陡峭区域的周期阶坎是浊流净侵蚀成因,而下游较平缓区域的周期阶坎是浊流净沉积成因。此外,坡度也是控制周期阶坎分布的一个重要因素,随着峡谷中泓线坡度的增加,周期阶坎的波长和波长/波高比都会增大,并且发现周期阶坎在南海东北部陆坡的出现有一定的坡度范围限制,一般只能出现在坡度变化在0.26°~1.24°的斜坡环境,如果坡度太小或太大都不会出现周期阶坎。
目前,由于对海底重力流的直接监测仍然存在很大困难,只能通过露头和岩芯资料对其在时间和空间上的演化进行研究。用岩芯和露头的浊积相观测结果,估算重力流发生时的弗劳德数和悬浮颗粒物的辐射率(suspension fall-out rate)[41],但是,迄今为止,浊积相组合与大规模流体动力学之间的关系,尚未通过普通的浊积相模式解决。Postma等[40]提出了一个简单的相标准,既可以用来区分超临界与亚临界浊流,也可以定义不同浊流动力学特征的相组合。利用这些相组合得出的原始流体动力学参数,可以用来验证和改进已有模型,也可以帮助石油地质学家更好地预测深水油气储层特征,指导深水油气勘探的生产与实践。
深水浊流的作用过程十分复杂,很多现象并不能用经典的鲍马序列来解释,而且由于沉积物性质各不相同,浊流的沉积作用也存在很大差别。在陆坡环境中,浊流一般都会转化成超临界流,并且随着流体从超临界流转化成亚临界流,一定会经历水跃现象。
Postma等[43]通过实验观察、野外露头等资料的分析对比,研究了周期阶坎及其与鲍马序列Ta之间的对应关系。在单相悬浮流中,所有颗粒都是悬浮的,当这些悬浮物质有足够的剪切力侵蚀底床、增加负载时,从流体表面向基底会呈现连续的密度变化梯度。在双相悬浮流中,流体是分层的,并且底部高浓度牵引流和上部较低浓度的悬浮流之间有一个明显的密度界面,该界面标志上下两部分支撑机制的不同。根据水槽实验和野外观测到的证据,建立了一个浊流成因底床稳定性图(图8)。它以水跃为关键点,对于建立沉积构造与弗劳德数、粒径和粒子辐射率之间的对应关系非常重要。
周期阶坎是向上游迁移的底形,具有明显的背流面和迎流面(沙丘状),流经背流面一侧的流体往往是超临界流,在低谷位置会发生一次水跃,然后流体流过迎流面一侧。Mutti 等[38]描述的关于大型沙波迎流面一侧的鲍马序列Ta段与周期阶坎非常吻合。通过将试验观察到的浊流水动力情况与野外观察到的沉积地层进行对比,发现二者有很好的匹配性,单相悬浮流从斜坡倾泻而下,由超临界流转变为亚临界流,在水跃位置冲刷底床并形成典型的鲍马序列Ta段沉积(图9)。
图7 周期阶坎波列分布图
(a)典型峡谷1水深及周期阶坎波列分布图;(b)典型峡谷2水深及波列分布图[
Fig.7 Distribution diagrams of cyclic steps
(a)Bathymetric image and cyclic steps along typical canyon 1. (b)Bathymetric image and cyclic steps along typical canyon 2[
图8 单相和双相悬浮流三维底形稳定性图[
Fig.8 Three dimensional bed-form stability diagram for 1 and 2-phase suspension flows[
图9 试验中观察到的单相悬浮流流动过程(a)和野外观察到的浊积岩(b)[
浊积岩包括:递变层理段Ta(2)、平行层理段Tb(1和3)、爬升波状交错层理段(4)和被牵引毯沉积覆盖的波状层理段(5)
Fig.9 The flow process of 1-phase suspension flow observed in experiment (a) and the outcrop of turbidite bed (b)[
The flow process of 1-phase suspension flow observed in experiment (a) and the outcrop of turbidite bed (b). The turbidite bed including: Graded Ta unit (2), plane bed laminated Tb units (1 and 3), climbing ripples cross sets (4), the ripples are covered by traction carpet deposits (5)
浊流沉积底形的形态特征主要是由其流体动力学特征控制,流体动力学特征可以用沉积物浓度、雷诺数和弗劳德数3个参量来表示,雷诺数决定流体是层流或湍流,弗劳德数决定流体是超临界流或亚临界流(图1)。因此,流体沿着斜坡向下,随着坡度的减小,流体动力学参数的变化控制浊流经历几次流体转换,其中重力转换(颗粒物沉积过程中形成了分层流)和体转换(从纯流体到塑性粘滞流体的流变学转换)是2个最重要的部分。这些动力学参量的不同导致了4种不同类型的流动过程(图10),每一种都对应典型的浊积相[41]。
图10 高浓度浊流(类型一和类型二)和低浓度浊流(类型三和类型四)流体动力学和大尺度构造特征,流动方向从左向右[
Fig.10 Summary of flow dynamic and large-scale architrcture for high-concentration and low-concentrationturbidity currents, flow is from left to right[
类型一:具有超临界基底层的分层流。流动过程中的不同阶段对应不同的沉积相,水跃位置的沉积相可以被描述为软沉积变形(火焰状构造)和无结构的正粒序沉积,对应鲍马序列Ta段(图11)。向下坡方向逐渐远离水跃位置,流体沿底床的剪切力逐渐增大,沉积相的分层现象更加明显,从几乎无构造的Tb4沉积相,经过粗糙的Tb3a相和间隔分层的Tb3b相到层间隔小于0.5 cm的水平层状Tb2沉积相(图11)[40]。
类型二:具有亚临界基底层的分层流。其底部亚临界流可以产生一个几乎无侵蚀的扁平状结构基底,底床中包含了Tb4~Tb2范围的分层类型。由于流动过程中不发生水跃现象,其沉积底床中不存在类型一中的Ta段,只出现了无构造无粒序的Tb4沉积相、粗糙的Tb3沉积相和间隔分层的Tb3b和Tb2层理,这说明该类型流体在沉积过程中主要受颗粒物辐射率控制。
类型三:不分层的超临界流。类似于爬升波痕的长波长构造,具有扁平的迎流面,产生不连续侵蚀的背流面,其波长符合深度范围在100~200 m的超临界悬浮流通过流体分离溢出堤岸而形成的超临界流底形。通过对沉积物波取岩芯,发现沉积相主要由薄层的Ta相(具有火焰状构造)、Tb2~Tb1和微波状的Tc(鲍马序列Tc段)单元构成,背流面两侧由交替的泥质层和细砂层组成,沿着迎流面向上游方向粒度是最粗糙的,波峰位置粒度稍细一些,在背流面粒度最细[67]。
类型四:不分层的亚临界流。在盆地平原比较常见,流体在该处逐渐减弱并完全沉积,从上游端至末端通常形成Tb1,Tc,Td和Te相(鲍马序列Te段)[22],一般Te相会覆盖在整个浊积地层之上。
浊流及相关超临界流底形正受到越来越多的关注,它们是陆坡深水沉积体系地貌动态演化的重要组成部分。诸如逆行沙丘、周期阶坎等海底广泛分布的底形,它们的形成、演化过程与发育在陆架边缘斜坡和峡谷水道中的浊流体系密不可分,通过物理实验和数值模拟,得出浊流流体动力学参数随环境发生改变,从而导致在浊流流动过程中产生不同类型的底形。这些物理实验和数值模拟的底形结果与野外观察到的浊积岩相一一对应,分别对应于鲍马序列中的某个或某几个相组合。这说明鲍马序列目前仍然是一种广泛使用的浊积岩序列,但是,显然已不能用来解释所有的浊积岩问题。
不同流体动力学参数的组合导致浊流的流动过程更为复杂,例如超临界浊流、亚临界浊流、分层流、不分层流等,它们与底床的作用也各不相同,因此,形成各种各样的沉积或侵蚀底形。但是,就目前已经发现的深水底形来看,周期阶坎是在世界范围内分布最广泛的超临界流底形。由于周期阶坎一般具有较长的波长和较小的波高,在小范围的现代海底不容易识别,在小尺度的地质露头中也很难观察到,所以,在之前很长一段时间的研究中都没有将其纳入浊流沉积体系。对周期阶坎及其他浊流相关底形的研究,不仅有助于认识浊流搬运过程中流体动力学特征的变化情况和搬运体系的沉积特征,而且可以为浊流沉积的油气储层研究提供理论依据和实践指导。
由于海底的浊流事件难以预测,目前的观测技术和设备还不够成熟,对于浊流成因的深水底形与浊流流体动力学之间的关系,只能通过物理实验和数值模拟展开更深入的研究。在未来的发展方向上,作者认为浊流及其相关的深水底形研究,可能有以下几个方面:
(1) 加大对浊流沉积体系的勘探和识别。将目前已经发展成熟的计算技术应用到地球物理数据的处理与解释、流体数值模拟中,获取尽可能多的沉积体参数,利用更合理、更精确的方法对浊流及其相关的超临界流底形的动态演化进行数值模拟和实验研究,获取更详细可信的实验结果。
(2) 大力发展深海观测技术。研发高性能的深海观测设备,培养相关的技术人员,实现世界范围内对大规模深水地质过程(浊流、海底滑坡等)的直接监测,用仪器设备获取浊流流动过程中的动力学参数、流体样品,观测底形的形成与演化,并与实验模拟结果进行对比,改进现有的实验及模拟方法,不断加深对浊流、海底滑坡等地质现象的认识。
(3) 加强野外露头研究(包括陆上地质露头和现代海底底形观测)。由于直接观测大规模深海浊流存在很大难度,短时间内在技术上无法实现,所以,通过野外露头对其进行研究是行之有效的方法。浊流体系与鲍马序列有着密不可分的联系,尤其是现代海底观测到的如周期阶坎等大型超临界流成因的深水底形,在未来的研究中应尽可能从更大规模的露头上去研究,在鲍马序列的基础上建立更加完善的浊积岩相模式。
致 谢:感谢中国科学院深海科学与工程研究所秦永鹏博士给出的写作建议,荷兰Utrecht University的George Postma教授、英国Durharm University的Matthieu Cartigny博士提供的文献支持,以及审稿专家给出的中肯意见。
The authors have declared that no competing interests exist.
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Subaqueous sediment density flows: Depositional processes and deposit types [J].
Abstract Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (T E-3 ) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (T E-2 ) and finely laminated mud (T E-1 ) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (T D ) and ripple cross-laminated (T C ) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (T C ) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (T B-1 ), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (T B-2 ). More widely spaced planar lamination (T B-3 ) occurs beneath massive clean sand (T A ), and is also formed by high-density turbidity currents. High-density turbidite deposits (T A , T B-2 and T B-3 ) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (T D and T C ,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (D CS ), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (D M-2 ), or result from more local flow transformation due to turbulence damping by cohesive mud (D M-1 ). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.
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| [10] |
1. Submarine valleys on the Pacific Coast of the United States. 2. Standard geodetic data. 3. Early Spanish voyages of discovery on the coast of California [J].
During pneumococcal infection of mice, nucleic-acid products, including deoxynucleotides, may be released into the serum from cellular disintegration in at least three organs, the lungs, spleen and liver. The serum, after sterilisation to remove contaminating pneumococci, stimulated multiplication of virulent but not avirulent pneumococci in vitro. It also stimulated growth of virulent pneumococci in serum from uninfected animals and could be replaced, at least in part, by certain nucleic-acid degradation products at concentrations found in infected serum. The effects of the serum were lost after dialysis or dilution.
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| [11] |
The submarine valleys of the California coast [J].
Smith WS.
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| [12] |
Accomplishments and challenges in measuring turbidity currents in submarine canyons [J].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 [J].
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。
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| [13] |
Stow D A V, et al. Sediment waves on the monterey fan levee: A preliminary physical interpretation [J].
Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km 2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.
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| [14] |
Stow D A V, et al. Turbidity current sediment waves on the submarine slopes of the western Canary Islands [J].
Two sediment wave fields have been identified on the flanks of the western Canary Islands of La Palma and El Hierro, using a high-quality 2-D and 3-D dataset that includes GEOSEA and TOBI imagery, 3.5-kHz profiles, and short sediment cores. The La Palma sediment wave field covers some 20,000 km 2 of the continental slope and rise, and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2.4 km. The wave crestlines have a complex morphology, with common bifurcation and a clear sinuosity. Waves have migrated upslope through time. Cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic/hemipelagic layers. The wave field is interpreted as having formed beneath unconfined turbidity currents. A simple, previously published, two-layer model is applied to the waves, revealing that they formed beneath turbidity currents flowing at 10–100 cm/s 611 , with a flow thickness of 60–400 m and a sediment concentration of 26–427 mg/l. The El Julan sediment wave field lies within a turbidity current channel on the southwest flank of El Hierro. The sediment waves display wave heights of about 6 m and wavelengths of up to 1.2 km. The waves are migrating upslope, and migration is most rapid in the centre of the channel where the flow velocity is highest. This wave field has been formed by channelised turbidity currents originating on the flanks of El Hierro.
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| [15] |
A comparative study of sediment waves and cyclic steps based on geometries, internal structures and numerical modeling [J].
Although sediment waves cover many levees and canyon floors of submarine fan systems, their relation to the turbidity currents that formed them is still poorly understood. Over the recent years some large erosional sediment waves have been interpreted as cyclic steps. Cyclic steps are a series of slowly upslope migrating bedforms (steps), where each downward step (the lee side of the bedform) is manifested by a steeply dropping flow passing through a hydraulic jump before re-accelerating on the flat stoss side. Here, a general comparison is made between sediment waves and cyclic steps. First, the analogies between their geometries and internal structures are explored. Secondly, a basic numerical model is used to construct stability fields for the formation of cyclic steps. These stability fields are compared with large, existing datasets of both fine- and coarse-grained sediment waves. The numerical results enable an explanation of geometrical trends found over series of sediment waves in the upper part of the Monterey Canyon, on the middle Amazon Fan and on a leveed channel in the Makassar Strait in terms of changes in flow properties of the overriding turbidity current. Based on sedimentological arguments and numerical analysis it is concluded that cyclic steps form a potential alternative for the existing interpretations on the origin of upslope migrating sediment waves.
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| [16] |
Cartigny M J B, Kuang Z, et al. Cyclic steps along the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea [J].
Large-scale step-like features within the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea are investigated by integrating high-resolution multibeam bathymetric data and multichannel seismic profiles. These step-like features, ranging from 1.2 to 10.0 km in wavelength and 5.4–80.9 m in wave height, are mostly interpreted as cyclic steps formed by turbidity currents flowing through the canyons, based on their characteristic step-like morphology, in-train alignment, large wavelengths and aspect ratios (ratio of wavelength to wave height), and typical upstream-sloping backset bedding, among others. A train of 19 continuous steps delineated along the thalweg of the South Taiwan Shoal canyon measures up to 100 km and may be the longest ever reported. Nine short trains of scours identified on a terrace of the South Taiwan Shoal canyon are oriented parallel to the distributaries draining over the terrace and roughly perpendicular to the main canyon thalweg, indicating a complicated flow pattern within the canyon valley. Two trains of scours separated by an intracanyon high in the steeper middle reach of the West Penghu canyon are interpreted as transitional bed forms between antidunes and cyclic steps, which develop downstream into a train of five net-depositional cyclic steps with typical backset bedding in the gentler-sloping lower reach of the canyon. Average slope gradients for the canyon reaches with cyclic steps range from 0.26° to 1.24°. Along each thalweg step train, a slope break is identified to separate the net-erosional cyclic steps in the steeper upstream segment from the net-depositional ones in the gentler downstream segment. Rough estimations indicate that the paleoflows are 100 to 300 m thick with maximum velocities of up to 10 m s–1. The estimated flow depths match well with those inferred from geomorphologic analysis. Estimated paleodischarges of ?7–23 × 105 m3 s–1 are equivalent to ten times the discharge of the modern Amazon River.
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| [17] |
Currents in Monterey submarine canyon [J].
[1] Flow fields of mean, subtidal, and tidal frequencies between 250 and 3300 m water depths in Monterey Submarine Canyon are examined using current measurements obtained in three yearlong field experiments. Spatial variations in flow fields are mainly controlled by the topography (shape and width) of the canyon. The mean currents flow upcanyon in the offshore reaches (>1000 m) and downcanyon in the shallow reaches (<800 m) of the canyon. Tidal currents, especially the semidiurnal components, are dominant and account for more than 90% of total energy. Pulses of strong currents near the canyon floor, which last several days at a time and have a magnitude as high as 60+ cm/s, are attributed to intense baroclinic processes occurring within the canyon. The V-shaped canyon walls and the near-critical slope of the canyon floor focus the baroclinic tides of semidiurnal and higher frequencies to the canyon bottom to produce the >100-m amplitude isotherm oscillations and associated high-speed rectilinear currents. The 15-day spring-neap cycle and a 0900083-day090009 band are the two prominent frequencies in subtidal flow field. Neither of them seems directly correlated with the spring-neap cycle of the sea level.
|
| [18] |
Normalized velocity profiles of field-measured turbidity currents [J].
Multiple turbidity currents were recorded in two submarine canyons with maximum speed as high as 280 cm/s. For each individual turbidity current measured at a fixed station, its depth-averaged velocity typically decreased over time while its thickness increased. Some turbidity currents gained in speed as they traveled downcanyon, suggesting a possible self-accelerating process. The measured velocity profiles, first in this high resolution, allowed normalizations with various schemes. Empirical functions, obtained from laboratory experiments whose spatial and time scales are two to three orders of magnitude smaller, were found to represent the field data fairly well. The best similarity collapse of the velocity profiles was achieved when the streamwise velocity and the elevation were normalized respectively by the depth-averaged velocity and the turbidity current thickness. This normalization scheme can be generalized to an empirical function Y = exp(–αXβ) for the jet region above the velocity maximum. Confirming theoretical arguments and laboratory results of other studies, the field turbidity currents are Froude-supercritical.
|
| [19] |
Turbidity current research in the past century: An overview [J].海底浊流研究百年回顾 [J]. |
| [20] |
Sediment gravity flows: Mechanics of flow and deposition [M] |
| [21] |
Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents [J].
Paediatric craniofacial surgery has evolved since the innovative work of Tessier in the 1960s. The development of the team approach has greatly helped this progress. Technical advances of osteosynthesis and better radiological examinations have further improved the predictability and stability. Microvascular free tissue transfer is playing an increasing role in certain malformations.
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| [22] |
Sediment gravity flows: Their classification and some problems of application to natural flows [J].
Highly sophisticated and practical algorithms have been developed through this research. Using the developed methods, the amount of human intervention required will be significantly reduced: chromosomes are reliably and accurately segmented from the background, pixels are accurately classified, and clusters of overlapping and touching chromosomes are automatically decomposed.
|
| [23] |
Syvitski J P M, Migeon S, et al. Marine hyperpycnal flows: Initiation, behavior and related deposits. A review [J]. |
| [24] |
Ten turbidite myths [J]. |
| [25] |
Morphodynamics and sedimentary structures of bedforms under supercritical-flow conditions: New insights from flume experiments [J].
Supercritical-flow phenomena are fairly common in modern sedimentary environments, yet their recognition and analysis remain difficult in the stratigraphic record. This fact is commonly ascribed to the poor preservation potential of deposits from high-energy supercritical flows. However, the number of flume data sets on supercritical-flow dynamics and sedimentary structures is very limited in comparison with available data for subcritical flows, which hampers the recognition and interpretation of such deposits. The results of systematic flume experiments spanning a broad range of supercritical-flow bedforms (antidunes, chutes-and-pools and cyclic steps) developed in mobile sand beds of variable grain sizes are presented. Flow character and related bedform patterns are constrained through time-series measurements of bed configurations, flow depths, flow velocities and Froude numbers. The results allow the refinement and extension of some widely used bedform stability diagrams in the supercritical-flow domain, clarifying in particular the morphodynamic relations between antidunes and cyclic steps. The onset of antidunes is controlled by flows exceeding a threshold Froude number. The transition from antidunes to cyclic steps in fine to medium-grained sand occurs at a threshold mobility parameter. Sedimentary structures associated with supercritical bedforms developed under variable aggradation rates are revealed by means of combining flume results and synthetic stratigraphy. The sedimentary structures are compared with examples from field and other flume studies. Aggradation rate is seen to exert an important control on the geometry of supercritical-flow structures and should be considered when identifying supercritical bedforms in the sedimentary record
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| [26] |
Hydraulic jump in triangular channel [J].
The hydraulic jump in the triangular channel is analysed using an elementary approach. The results regarding the sequent depth ratio are confirmed by model observations. Furthermore, the length characteristics, the surface profiles and typical velocity distributions are discussed. Finally, these results are compared with the hydraulic jump in the rectangular channel, and interesting conclusions end the paper.
|
| [27] |
Channel formation by flow stripping: Large-scale scour features along the Monterey East Channel and their relation to sediment waves [J].
The Monterey East system is formed by large-scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour-shaped depressions ranging from 3路5 to 4路5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude-supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude-supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream-migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net-erosional or net-depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream-migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.
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| [28] |
Large-scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows [J].
61Novel statistical analysis of global seafloor bedform database61Simplification of existing classification bedform schemes61Evidence that supercritical flows are more prevalent than previously appreciated.61Bimodal size of bedforms relates to environmental confinement.
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| [29] |
Huvenne V A I, et al. New insights into the morphology, fill, and remarkable longevity (>0.2 m.y.) of modern deep-water erosional scours along the northeast Atlantic margin [J]. |
| [30] |
Modeling of submarine cyclic steps: Controls on their formation, migration, and architecture [J].
Abstract Submarine cyclic steps are a newly recognized manifestation of fundamental morphodynamic instability of Froude-supercritical flow over an erodible bed. There is a growing recognition of the global presence and importance of cyclic steps. An attempt was made here to: (1) outline submarine cyclic steps in the context of the sediment waves of various origins; (2) elucidate the physics and key parameters governing their formation, migration, and architecture; and (3) summarize selected numerical experiments on netdepositional and net-erosional cyclic steps in a useful form. The paper also addresses frequent terminology confusion between net-depositional cyclic steps and sediment waves in general.
|
| [31] |
Upper flow regime bedforms on levees and continental slopes: Turbidity current flow dynamics in response to fine-grained sediment waves [J].
ABSTRACT Our knowledge of submarine upper flow regime bedforms is very limited. Numerical experiments presented herein were designed to broaden our understanding of the origin and dynamics of fine-grained upper flow regime bedforms in turbidite systems, particularly on levees and continental slopes. The experiments combine field information in order to: 1) examine the hydrodynamic response of a bed consisting of fine-grained sediment waves to a wide range of turbidity current conditions, and 2) demonstrate that fine-grained sediment waves most likely form and evolve as upper flow regime bedforms, i.e., cyclic steps, transitional bedforms, or antidunes. The results of the study provide valuable information for the reconstruction of turbidity current flow dynamics on levees, continental slopes, and in other submarine settings that display a slope break.
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| [32] |
Hoyal D C J D. Hydraulic and sediment transport properties of autogenic avulsion cycles on submarine fans with supercritical distributaries [J].
Abstract Submarine fans, like other distributive systems, are built by repeated avulsion cycles. However, relative to deltas and alluvial fans, much less is known about avulsions in subaqueous settings. In this study, we ran a set of subaqueous fan experiments to investigate the mechanics associated with autogenic avulsion cycles of self-formed channels and lobe deposits on steep slopes. The experiments used saline density currents with crushed plastic to emulate sustained turbidity currents and bed load transport. We collected detailed hydraulic and bathymetric measurements and made use of a 1-D laterally expanding density current model to better understand different aspects of the avulsion cycle. Our results reveal three major components of the avulsion cycles: (1) distributary channel incision, extension, and stagnation; (2) mouth bar aggradation and hydraulic jump initiation; and (3) hydraulic jump sedimentation and upstream retreat. Interestingly, in all but one experiment, the avulsion cycles led to fans that remained perched above the basin slope break. Experimental data and hydraulic theory were used to unravel actual mechanics associated with cycles. We found that channels stopped extending into the basin due to a decay in sediment transport capacity relative to sediment supply and that the reduction in capacity was primarily an outcome of expansion-driven velocity reduction; dilution played a secondary role. Once channel extension ceased, mouth bar deposits aggraded to a thickness approximately equal to the critical step height needed to create a choked flow condition. The choke then initiated a hydraulic jump on the upstream side of the bar. Once formed, the jump detained a majority of the incoming sediment and forced the channel-to-lobe transition upstream, filling the channel with steep backset bedding and capping the entire channel with a mounded lobate deposit. These intrinsic processes repeated through multiple avulsion cycles to build the fan.
|
| [33] |
|
| [34] |
Emplacement of massive turbidites linked to extinction of turbulence in turbidity currents [J].
Submarine turbidity currents are controlled by gravity acting on suspended sediments that pull water downslope along with them. In addition to suspended sediments, turbidity currents also transport sediments at the base of the flow, which causes the reorganization of basal sediments prior to the settling of suspended grains. However, as turbidity currents reach areas with minimal slope, they cross a fall-velocity threshold beyond which the suspended sediments begin to stratify the flow. This process extinguishes the turbulence near the bed. Here we use direct numerical simulation of turbidity currents to show that this extinction of turbulence eliminates the ability of the flow to re-entrain sediment and rework the sediment at the base of the flow. Our simulations indicate that deposits from flows without basal reworking should lack internal structures such as laminations. Under appropriate conditions, then, sustained delivery of fine sediments will therefore result in the emplacement of massive turbidites. We suggest that this mechanism can explain field observations of massive deposits that were emplaced gradually by dilute but powerful turbidity currents. We also conclude that turbulence in submarine turbidity currents is more fragile than river systems, and more sensitive to damping by the stratification of suspended sediment in the flow.
|
| [35] |
Morphodynamics and sedimentary structures of bedforms under supercritical-flow conditions: New insights from flume experiments [J].
Supercritical-flow phenomena are fairly common in modern sedimentary environments, yet their recognition and analysis remain difficult in the stratigraphic record. This fact is commonly ascribed to the poor preservation potential of deposits from high-energy supercritical flows. However, the number of flume data sets on supercritical-flow dynamics and sedimentary structures is very limited in comparison with available data for subcritical flows, which hampers the recognition and interpretation of such deposits. The results of systematic flume experiments spanning a broad range of supercritical-flow bedforms (antidunes, chutes-and-pools and cyclic steps) developed in mobile sand beds of variable grain sizes are presented. Flow character and related bedform patterns are constrained through time-series measurements of bed configurations, flow depths, flow velocities and Froude numbers. The results allow the refinement and extension of some widely used bedform stability diagrams in the supercritical-flow domain, clarifying in particular the morphodynamic relations between antidunes and cyclic steps. The onset of antidunes is controlled by flows exceeding a threshold Froude number. The transition from antidunes to cyclic steps in fine to medium-grained sand occurs at a threshold mobility parameter. Sedimentary structures associated with supercritical bedforms developed under variable aggradation rates are revealed by means of combining flume results and synthetic stratigraphy. The sedimentary structures are compared with examples from field and other flume studies. Aggradation rate is seen to exert an important control on the geometry of supercritical-flow structures and should be considered when identifying supercritical bedforms in the sedimentary record
|
| [36] |
Submarine channel initiation, filling and maintenance from sea-floor geomorphology and morphodynamic modelling of cyclic steps [J].
Advances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea-floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high-resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon-channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent-shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea-floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long-wave, upstream-migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude-supercritical over the lee side of the bedform and Froude-subcritical over the stoss side. Numerical modelling and seismic-reflection imagery support an interpretation of weakly asymmetrical to near-symmetrical aggradation of predominantly fine-grained net-depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, proto-San Mateo channel comprises a series of relatively coarse-grained net-erosional cyclic steps, which nucleated out of sea-floor perturbations across the tectonically active lower slope. Thus, the interaction between turbidity-current processes and sea-floor perturbations appears to be fundamentally important to channel initiation, particularly in high-gradient systems. Offshore of southern California, and in analogous deep-water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.
|
| [37] |
Cyclic steps and related supercritical bedforms: Building blocks of deep-water depositional systems, western North America [J].
Cyclic steps are long-wave (the ratio of wavelength to height is 62021), upstream-migrating, upper-flow-regime bedforms bounded by internal hydraulic jumps (i.e., transition from densimetric Froude supercritical to subcritical flow) in turbidity currents. They commonly occur in regions with high gradients and slope breaks. Here we review the morphodynamic evolution and depositional products of cyclic steps and related supercritical bedforms (e.g., antidunes). We present examples from high-resolution geophysical surveys and monitoring of continental margins in western North America integrated with physically based numerical modeling of turbidity currents and associated bedforms. We compare numerical modeling results to direct monitoring of turbidity currents in the Squamish prodelta, British Columbia, Canada. Cyclic steps and antidunes influence phases of canyon-channel evolution, levee-overbank deposition, and channel-lobe-transition-zone sedimentation, thereby advancing channels and lobes into the basin. Bedforms range from relatively small cyclic steps and antidunes within active submarine canyons and channels (~0210 1 02m wavelength; ~0210 0 02m height) to large cyclic steps in less confined levee-overbank environments and the channel-lobe transition zone (~0210 3 02m wavelength; ~0210 2 02m height). Cyclic steps and related supercritical bedforms are important to the morphodynamic evolution of architectural elements of some deep-water depositional systems, especially those located along tectonically active margins with high gradients and slope breaks that can promote internal hydraulic jumps in turbidity currents.
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| [38] |
An Integrated Approach to the Study of Turbidite Systems [M]. |
| [39] |
Characterization and recognition of deep-water channel—Lobe transition zones [J].
The channel-lobe transition zone (CLTZ) is an important, but commonly overlooked, element of many deep-water turbidite systems. Recognizing this zone is difficult in both modern and ancient environments and depends largely on the quality and resolution of the data obtained. In this article, three case studies of modern CLTZs are presented, largely based on high-resolution side-scan sonar imagery. These data are then compared to other well-defined CLTZs, both modern and ancient, and the common characteristics identified.CLTZs occur at canyon/channel mouths and are commonly associated with a break of slope. Most sediment bypasses this zone, and consequently only coarse sands and gravels are deposited, although these are commonly patchily distributed and extensively reworked. The CLTZ is characterized by abundant erosional features, including isolated spoon- and chevron-shaped scours up to 20 m deep, 2 km wide, and 2.5 km long. In areas of more widespread erosion, these merge to form amalgamated scours several kilometers across. Depositional bed forms include sediment waves with wavelengths of 1-2 km and wave heights up to 4 m. The presence or absence of a CLTZ has important implications for hydrocarbon exploration and development, especially in terms of the connectivity between sandy channel-fill and lobe facies.
|
| [40] |
Cartigny M J B. Supercritical and subcritical turbidity currents and their deposits—A synthesis [J]. |
| [41] |
Stow D A V. Classification and characterisation of deep-water sediment waves [J].
Deep-water sediment waves can be classified using a combination of grain size and wave-forming process, although in some cases one or other of these criteria may be indeterminable. Sediment waves are generated beneath currents flowing across the seabed, in the form of either downslope-flowing turbidity currents or alongslope-flowing bottom currents. Waves formed by either process show varying characteristics, depending on whether they are constructed of coarse- or fine-grained sediments. Sediment wave studies over the last five decades are reviewed, and clear trends can be discerned. Early descriptive studies in the 1950s and 1960s relied almost exclusively on seismic reflection profiles, and the wave-forming process was often a subject of much debate. In the 1970s and 1980s the quality of sediment wave datasets increased, with sidescan sonar, deep-sea drilling and numerical modelling all applied to sediment wave studies. Consequently, the wave-forming process became more easily identifiable, and models for the growth of bottom current and turbidity current sediment waves were introduced. Most studies from the 1990s onwards have focussed on turbidity current sediment waves, in response to the increasing demand for data from turbidite systems from the hydrocarbon exploration and production industry. Studies of bottom current sediment waves during this period have focussed on the applications to palaeoceanography, in response to the recent boom in climate change studies. The main focus of this paper is the characterisation of both fine- and coarse-grained, turbidity and bottom current sediment waves, including the depositional environment, wave morphology, wave sediments and migration, and the wave-forming process. In addition, criteria for distinguishing between fine-grained bottom current and turbidity current waves are discussed, and also for identifying other wave-like features formed by different processes, such as creep folds. Although in many sediment wave studies the dominant wave-forming process is easy to determine, in others it is likely that a more complex combination of processes has occurred. Further studies should concentrate on methods for identifying these processes and how they interact, and also investigate the exact mechanisms for the initiation and evolution of sediment wave fields.
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| [42] |
Comparing Examples of Modern and Ancient Turbidite Systems: Problems and Concepts [M]. |
| [43] |
Structureless, coarse-tail graded Bouma Ta formed by internal hydraulic jump of the turbidity current? [J]. |
| [44] |
Cartigny M J B. Recognition of cyclic steps in sandy and gravelly turbidite sequences, and consequences for the Bouma facies model [J].
Abstract Preservation of cyclic steps contrasts markedly with that of subcritical-flow bedforms, because cyclic steps migrate upslope eroding their lee face and preserving their stoss side. Such bedforms have not been described from turbidite outcrops and cores as yet. A conceptual block diagram for recognition of cyclic steps in outcrop has been constructed and is tested by outcrop studies of deep water submarine fan deposits of the Tabernas Basin in south-eastern Spain. Experimental data indicate that depositional processes on the stoss side of a cyclic step are controlled by a hydraulic jump, which decelerates the flow and by subsequent waxing of the flow up to supercritical conditions once more. The hydraulic jump produces a large scour with soft-sediment deformation (flames) preserved in coarse-tail normal-graded structureless deposits (Bouma Ta), while near-horizontal, massive to stratified top-cut-out turbidite beds are found further down the stoss side of the bedform. The architecture of cyclic steps can best be described as large, up to hundreds of metres, lens-shaped bodies that are truncated by erosive surfaces representing the set boundaries and that consist of nearly horizontal lying stacks of top-cut-out turbidite beds. The facies that characterize these bedforms have traditionally been described as turbidite units in idealized vertical sequences of high-density turbidity currents, but have not yet been interpreted to represent bedforms produced by supercritical flow. Their large size, which is in the order of 20聽m for gravelly and up to hundreds of metres for sandy steps, is likely to have hindered their recognition in outcrop so far.
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| [45] |
Sedimentology of Some Flysch Deposits: A Graphic Approach to Facies Interpretation [M]. |
| [46] |
Quaternary development of migrating sediment waves in the Var deep-sea fan: Distribution, growth pattern, and implication for levee evolution [J].
A field of sediment waves built by turbidity currents on the Var deep-sea fan has been studied using a large amount of seismic-reflection data and cores. To understand the spatial organisation and evolution of the sediment waves, maps of both surficial and an older subsurface wave field were prepared. Three different sediment wave geometries (symmetrical to asymmetrical cross-section) were recognised and can be linked to particular styles of evolution through time (from simple vertical aggradation to upslope and upcurrent progradation). Each geometry appears to be linked with: (i) a particular location on the Var Sedimentary Ridge; (ii) the type of turbidite deposits; (iii) the local gradient slope; and (iv) the height of the Ridge. Several factors control the nature and distribution of sediment waves and these factors do not change significantly through time, as suggested by the common geometries, evolution and distribution of the sediment waves between the present and fossil fields. Supercritical flow conditions and high sediment supply allow the development of well-developed and prograding sediment waves, while subcritical flow conditions and low sediment supply allow the formation of smaller and aggrading sediment waves. The impact of these two factors is also influenced by the morphology of the fan valley, the height of the Ridge, and the type of transported sediment. The evolution of sediment wave amplitude is also strongly influenced by the distance of sediment waves from the Ridge crest, while wavelength evolution is more influenced by slope gradient. The building of the Var Sedimentary Ridge and particularly the asymmetry and the overall morphology of the levee are directly controlled by the growth pattern of the sediment waves.
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| [47] |
Piper D J W, Posamentier H, et al. Variability in form and growth of sediment waves on turbidite channel levees [J].
Fine-grained sediment waves have been observed in many modern turbidite systems, generally restricted to the overbank depositional element. Sediment waves developed on six submarine fan systems are compared using high-resolution seismic-reflection profiles, sediment core samples (including ODP drilling), multibeam bathymetry, 3D seismic-reflection imaging (including examples of burried features), and direct measurements of turbidity currents that overflow their channels. These submarine fan examples extend over more than three orders of magnitude in physical scale. The presence or absence of sediment waves is not simply a matter of either the size of the turbidite channel-levee systems or the dominant initiation process for the turbidity currents that overflow the channels to form the wave fields. Both sediment-core data and seismic-reflection profiles document the upslope migration of the wave forms, with thicker and coarser beds deposited on the up-current flank of the waves. Some wave fields are orthogonal to channel trend and were initiated by large flows whose direction was controlled by upflow morphology, whereas fields subparallel to channel levees resulted from local spillover. In highly meandering systems, sediment waves may mimic meander planform. Larger sediment waves form on channel-levee systems with thicker overflow of turbidity currents, but available data indicate that sediment waves can be maintaned during conditions of relatively thin overflow. Coarser-grained units in sediment waves are typically laminated and thin-bedded sand as much as several centimetres thick, but sand beds as thick as several tens of centimetres have been documented from both modern and buried systems. Current production of hydrocarbons from sediment-wave deposits suggests that it is important to develop criteria for recognising this overbank element in outcrop exposures and borehole data, where the wavelength of typical waves (several kilometres) generally exceeds outcrop scales and wave heights, which are reduced as a result of consolidation during burial, may be too subtle to recognise.
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| [48] |
The stratigraphic expression of decreasing confinement along a deep-water sediment routing system: Outcrop example from southern Chile [J].
The products of sediment-laden turbidity currents that traverse areas of decreasing confinement on submarine slopes include erosional and depositional features that record the inception and propagation of deep-sea channels. The cumulative stratigraphic expression and deposits of such transitions, however, are poorly constrained relative to depositional settings dominated by end-member confined (i.e., submarine channel fill) and unconfined (i.e., lobe) deposits. Upper Cretaceous strata of the Magallanes foreland basin in southern Chile are characterized by a variety of stratigraphic architectural elements in close juxtaposition both laterally and vertically, including: (1) low-aspect-ratio channelform bodies attributed to slope channel fills; (2) high-aspect-ratio channelform bodies interpreted as the deposits of weakly confined submarine channels; (3) lenticular sedimentary bodies considered to represent the infill of laterally coalesced scours; (4) discontinuous channelform bodies representing isolated scour fills; and (5) a cross-stratified, positive-relief sedimentary body, which is interpreted to record an upslope-migrating depositional bed form. These elements are interpreted to have formed at a submarine sediment routing system segment characterized by a break in slope, and an accompanying decrease in confinement. The various architectural elements examined are interpreted to record a unique stratigraphic perspective of turbidite channels at various stages of development, from early-stage discontinuous and isolated scour fills to low-aspect-ratio channel units.
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| [49] |
Facies architecture of channel-levee deposits, Lago Nordenskjold and Laguna Mellizas Sur, Cerro Toro Formation, Chile [J].
react-text: 164 Understand and model the lateral and vertical distribution of microbial and oolite carbonate factories in the Smackover Formation in the eastern Gulf Coast, USA. /react-text react-text: 165 /react-text
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| [50] |
Sediment waves and depositional implications for fine-grained rocks in the Cerro Toro Formation (upper Cretaceous), Silla Syncline, Chile [J].
A possible analog for the coarse-grained and fine-grained rocks in the Cerro Toro is the Quaternary Var Ridge and associated fan valley located in the Mediterranean Sea. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size clasts, and thin-bedded mud and sand in levees with documented sand bed thickness up to 40cm and up to medium-grained sand. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap downlap, and toplap, similar to the Cerro Toro. The fine-grained material can be linked to deposition from unconfined turbidity currents that were generated via overbank flows from the Var fan channel. The development of these architectural elements requires out of channel turbidity currents and relatively thick flows. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation.
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| [51] |
Suspended-load fallout rate as an independent variable in the analysis of current structures [J].
The dynamic interpretation of most current-structure sequences derives directly from experiments on the succession of bedforms produced by flows in flumes. The results of these and related studies have been used to construct stability field diagrams in which the fields of individual bedforms are usually expressed as a function of flow intensity (power, velocity, bed shear stress, etc.) and grain size. The data underlying existing stability-field diagrams were collected largely from the study of flows carrying coarse-grained sediment entrained through particle-by-particle bed erosion. Many flows, however, do not entrain sediment through simple bed erosion. Most turbidity currents originate by the development of turbulence in slumps, slides, and other slope failures. Such flows generally form with highly concentrated suspended loads and their bed-load layers derive sediment from the collapsing suspended-sediment clouds. Because the collapse properties of such clouds may be related as much to suspended particle concentration, size distribution, particle interactions, and other factors as to flow intensity, the stability fields of bedforms developed beneath such flows may differ in flow intensity-grain-size relationships from those beneath flows deriving sediment from bed erosion alone. Useful stability-field diagrams for turbidity currents must include suspended-load fallout rate as a third variable, independent of flow intensity and mean grain size. A preliminary stability-field diagram of this type indicates that Bouma T abc sequences may theoretically form with essentially no velocity variation of the attendant flow. This type of analysis may have considerable relevance to the interpretation not only of turbidites but also of other deposits formed where bed-load layers are fed from above rather than below. These include shallow-shelf storm units deposited from highly concentrated flows and volcaniclastic layers formed where pyroclastic debris falls directly into moving water.
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| [52] |
Deposit structure and processes of sand deposition from decelerating sediment suspensions [J].
Turbidity currents are notoriously difficult to monitor directly, therefore interpretation of their deposits forms the basis for much of our understanding of these flows. The deceleration rate of a flow is a potentially important yet poorly understood control on depositional processes. A series of experiments were conducted in an annular flume, in which fast (up to 3.5 m/s) and highly turbulent flows of sand (up to 250 碌m) and water were decelerated at different rates and processes of deposition and deposit character analyzed. Previously poorly documented depositional processes were observed in the experiments. This is because the flows were initially unusually fast and of prolonged duration, with sustained periods of sediment fallout as the flow slowed down. The conditions in these flows are thus likely to be closer to those at the base of a waning turbidity current than is achieved in other relatively slow experimental flows. The collapse of high-concentration, moving, thin (< 5 mm) near-bed layers (laminar sheared layers) were an important mechanism by which the bed aggraded beneath these unsteady flows. At bed aggradation rates in excess of 0.44 mm/s the sequential collapse of laminar sheared layers produced a structureless, poorly graded and poorly sorted deposit (Bouma Ta). When bed aggradation rates fell below 0.44 mm/s the collapsing laminar sheared layers were reworked by turbulence to form planar laminae (Bouma Tb). These laminae are formed in a very different manner than the planar laminae attributed to bedwaves in previous open-channel flow experiments. Collapse of laminar sheared layers is therefore an alternative process for generating the Bouma Tb division. Inverse grading developed at the base of the deposits of slowly decelerated flows. This inverse grading was probably a result of grain sorting in a high-concentration layer that persisted at the base of the flow for many minutes prior to the onset of deposition.
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| [53] |
Textural trends in turbidites and slurry beds from the Oligocene flysch of the East Carpathians, Romania [J].
Deep-water sandstone beds of the Oligocene Fusaru Sandstone and Lower Dysodilic Shale, exposed in the Buzu Valley area of the East Carpathian flysch belt, Romania, can be described in terms of the standard turbidite divisions. In addition, mud-rich sand layers are common, both as parts of otherwise normal sequences of turbidite divisions and as individual event beds. Eleven units, interpreted as the deposits of individual flows, were densely sampled, and 87 thin sections were point counted for grain size and mud content. S/Tdivisions, which form the bulk of most sedimentation units, have low internal textural variability but show subtle vertical trends in grain size. Most commonly, coarse-tail normal grading is associated with fine-tail inverse grading. The mean grain size can show inverse grading, normal grading or a lack of grading, but sorting tends to improve upward in most beds. Fine-tail inverse grading is interpreted as resulting from a decreasing effectiveness of trapping of fines during rapid deposition from a turbidity current as the initially high suspended-load fallout rate declines. If this effect is strong enough, the mean grain size can show subtle inverse grading as well. Thus, thick inversely graded intervals in deep-water sands lacking traction structures do not necessarily imply waxing flow velocities. If the suspended-load fallout rate drops to zero after the deposition of the coarse grain-size populations, the remaining finer grained flow bypasses and may rework the top of the Sdivision, forming well-sorted, coarser grained, current-structured Tunits. Alternatively, the suspended-load fallout rate may remain high enough to prevent segregation of fines, leading to the deposition of significant amounts of mud along with the sand. Mud content of the sandstones is bimodal: either 3 13% or more than 20%. Two types of mud-rich sandstones were observed. Coarser grained mud-rich sandstones occur towards the upper parts of S/Tdivisions. These units were deposited as a result of enhanced trapping of mud particles in the rapidly deposited sediment. Finer grained mud-rich units are interbedded with ripple-laminated very fine-grained sandy Tdivisions. During deposition of these units, mud floccules were hydraulically equivalent to the very fine sand- and silt-sized sediment. The mud-rich sandstones were probably deposited by flows that became transitional between turbidity currents and debris flows during their late-stage evolution.
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| [54] |
Anatomy of turbidites and linked debrites based on long distance (120 km×30 km) bed correlation, Marnoso Arenacea Formation, Northern Apennines, Italy [J].
Much of our understanding of submarine sediment-laden density flows that transport very large volumes ( ca 1 to 100 km 3 ) of sediment into the deep ocean comes from careful analysis of their deposits. Direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic. In order to understand how submarine sediment-laden density flows evolve in space and time, lateral changes within individual flow deposits need to be documented. The geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events. This study of the Miocene Marnoso Arenacea Formation (Italy) provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence. One hundred and nine sections were logged through a ca 30 m thick interval of time-equivalent strata, between the Contessa Mega Bed and an overlying 'columbine' marker bed. Correlations extend for 120 km along the axis of the foreland basin, in a direction parallel to flow, and for 30 km across the foredeep outcrop. As a result of post-depositional thrust faulting and shortening, this represents an across-flow distance of over 60 km at the time of deposition. The correlation of beds containing thick (> 40 cm) sandstone intervals are documented. Almost all thick beds extend across the entire outcrop area, most becoming thinly bedded (< 40 cm) in distal sections. Palaeocurrent directions for flow deposits are sub-parallel and indicate confinement by the lateral margins of the elongate foredeep. Flows were able to traverse the basin in opposing directions, suggesting a basin plain with a very low gradient. Small fractional changes in stratal thickness define several depocentres on either side of the Verghereto (high) area. The extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle, substantially less than the thickness of flows. Thick beds contain two distinct types of sandstone. Ungraded mud-rich sandstone intervals record evidence of en masse (debrite) deposition. Graded mud-poor sandstone intervals are inferred to result from progressive grain-by-grain (turbidite) deposition. Clast-rich muddy sandstone intervals pinch-out abruptly in downflow and crossflow directions, in a fashion consistent with en masse (debrite) deposition. The tapered shape of mud-poor sandstone intervals is consistent with an origin through progressive grain-by-grain (turbidite) deposition. Most correlated beds comprise both turbidite and debrite sandstone intervals. Intrabed transitions from exclusive turbidite sandstone, to turbidite sandstone overlain by debrite sandstone, are common in the downflow and crossflow directions. This spatial arrangement suggests either: (i) bypass of an initial debris flow past proximal sections, (ii) localized input of debris flows away from available sections, or (iii) generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and/or abrupt flow deceleration. A single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud-rich and mud-poor sandstone.
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| [55] |
Basin-wide turbidites in a Miocene, over-supplied deep-sea plain: A geometrical analysis [J].
<P>Eighteen stratigraphic sections, 200 m thick on average, were logged in basin plain deposits of the Marnoso-arenacea Formation (Miocene, northern Apennines) over an area of 123 脳 27 km. Turbidites form 80-90% of the facies association, hemipelagites the remainder. Thin and thick-bedded turbidites are separated by an approximate statistical boundary at 40 cm; most prominent beds (> 1 m thick) are qualified as megaturbidites. With reference to the main supply-dispersal system (NW to SE), the basin plain can be axially subdivided into proximal, intermediate and distal segments by means of the following parameters: bulk sand content, sand/shale ratio in turbidites, mean thickness of individual layers and component beds, and frequency of thick layers. Almost 40% of thick-bedded turbidites can be traced over the whole study area. These basin-wide deposits form the bulk of the basin fill. Geometrical reconstruction shows that some sandstone beds taper downcurrent from the proximal plain or the adjacent fan area while others thin upcurrent suggesting sand by pass of the fan. Mudstone beds in general thicken towards the end and the margins of the plain indicating that turbidite mud, besides bypassing the fan as a rule, was affected by ponding in the plain. Thin-bedded turbidites have a low sand/shale ratio or are completely muddy representing either tails of sandier turbidites of the outer fan (lobe and fringe deposits) or sheets extending to a great part of or to the whole plain. Sandstone lobes advanced from fans into the plain for 40-50 km gradually thinning and shaling out over a transitional zone of 10-20 km. Their internal geometry shows simple and complex growth patterns: end members are defined as progradational and aggradational. Estimates of original length, width and volume of individual turbidites strongly suggest that flows were usually confined and deflected by basin slopes regardless of source location. Basinal deposits are thus characterized by great thickness and volume, abundance of mud and fine sand, extremely low lateral gradients of thickness and grain size (but rapid wedging near the sides). The basin plain developed as a part of an elongated, oversupplied basin with a 'highly efficient', probably delta-fed, dispersal system.</P>
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| [56] |
Reconstruction of turbidity currents in Amazon Channel [J].
Quantifying the characteristics of the turbidity currents that are responsible for the erosion, lateral migration and filling of submarine channels maybe useful for predicting the distribution of lithofacies in channel fill and levee reservoirs. This paper uses data from a well-studied submarine channel in Amazon Fan in an attempt to reconstruct the velocity, thickness, concentration, duration, recurrence rates and vertical structure of turbidity currents in this long sinuous channel. Estimates of flow conditions are derived from the morphology of the channels and the characteristics of the deposits within them. In particular, the availability of information on the sediment distribution with respect to the channel topography at the time of deposition allows for insights into the vertical structure of the flow, a key property that has been so far poorly understood. Integration of flow constraints from well and seismic data or from detailed analysis of outcrop with numerical flow models is a critical step toward a complete understanding of the flow and associated deposits. Turbidity currents in sinuous submarine channels, exemplified by Amazon Channel, are found to last for tens of hours and occur on a regular, quasi-annual basis. Model results suggest that these flows had, on average, velocities ranging from 2 to 4 m/s in the canyon/upper fan which decreased to 0.5-1 m/s in the lower fan, travelling in excess of 800 km. The model turbidity currents were subcritical over most of the channel length, indicating a low degree of water entrainment and low rate of deceleration down the channel. The formation of such long, sinuous channels is intrinsically associated with frequent, long-duration, subcritical turbidity currents carrying a silt-dominated sediment load.
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| [57] |
The response of turbidity currents to a canyon-fan transition: Internal hydraulic jumps and depositional signatures [J].
Turbidity currents often carve canyons into the continental slope, and then deposit submarine fans on lower slopes farther downstream. It has been hypothesized here that this slope decline can cause a turbidity current to (a) undergo an internal hydraulic jump near the canyon芒聙聯fan transition, and (b) leave a depositional signal of this transition. These hypotheses are studied with a numerical model. Rapidly depositing turbidity currents need not undergo a hydraulic jump at a slope break. When a jump does occur, it can leave a depositional signal in terms of an upstream-facing step. A previous attempt to capture this signal failed because the current was treated as purely depositional. In the present model both sediment deposition and entrainment are included. An upstream-facing step appears when deposition dominates erosion. The step requires entrainment since the deposition rate is continuous through the jump, whereas the sediment entrainment rate is not. Therefore, the step is caused by enhanced net deposition due to reduced entrainment rate across the jump. Under certain circumstances, a single step can be replaced by a train of upstream-migrating cyclic steps, each separated by a hydraulic jump. The numerical model is verified against experiments, and then applied at field scale
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| [58] |
The South China Sea [J]. |
| [59] |
Abyssal anti-dunes [J]. |
| [60] |
Migrating sediment waves created by turbidity currents in the northern South China Basin [J]. |
| [61] |
Geomorphology,grain-size charicteristics,matter source and forming mechanism of sediment waves on the ocean bottom of the northeast South China Sea [J].南海东北部海底沉积物波的形态、粒度特征及物源、成因分析 [J].
在南海东北部广泛发育沉积物波.通过高分辨率多波束数据、地震剖 面以及重力柱状样,对沉积物波的形态特征、粒度特征、物源以及形成机制进行了分析.研究表明大致以台湾浅滩南海底峡谷为界,北侧为近北东向展布,南侧为近 南北向展布.对其分布规律、地貌和形态特征及重力柱状样粒度分析表明这些沉积物波为浊流成因.沉积物波的发育与新生代晚期研究区的构造活动密切相关,自距 今6.5Ma以来台湾造山运动使台湾岛强烈抬升剥蚀,这些剥蚀物为研究区提供了大量的陆源物质,而在南海东北部陆坡区大量发育的峡谷-冲沟系统为陆缘物质 向下陆坡的输送提供了良好的通道.研究区西侧的东沙隆起长期处于抬升剥蚀状态,这种抬升剥蚀也为研究区沉积物波的发育提供了部分物源.随着坡度的减缓,浊 流沉积物开始堆积,在台湾浅滩南海底峡谷的北侧形成了展布方向与冲沟垂直的沉积物波,而在南侧由于台湾浅滩南海底峡谷发生转向,浊流从水道中漫溢出来,沉 积物堆积下来,形成了与原先水道近于垂直的近南北向的沉积物波.
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| [62] |
Sediment waves on the South China Sea slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific deep water into the South China Sea [J].
The results from this study also provide the further evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in South China Sea at least since Quaternary.
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| [63] |
Seismic features and origin of sediment waves in the Qiongdongnan Basin, northern South China Sea [J].
Sediment waves have been documented around the world for several decades, and their origins are still debated because of their various characteristics in different settings. Based on numerous high-resolution seismic profiles and two boreholes, sediment waves are identified in deepwater areas of the eastern Qiongdongnan Basin, and their distribution and seismic features are illustrated. Combined with the bathymetry, the potential origins of these sediment waves are discussed. Drilling in the central canyon revealed that the channel infill comprises some along-slope fine-grained turbidites, which are good reservoir for gas plays. The sediment waves are distributed on the banks of the central canyon and their seismic features indicate that most of them are caused by turbidity current overflows along the canyon. Although previous researches on these sediment waves suggested that they were of westward-flowing contourite origin, detailed topographic map derived from the seafloor reflector on seismic data shows that there is a N–S trending ridge at the east part of sediment wave zones, which could block and divert the bottom current. According to the geometry of sediment waves, the flow thicknesses across the entire wave field are calculated as 280–56002m, and the current velocity falls in the range of 30–13002cm/s, which would favor a fine-grained composition and could be a good reservoir because of the better sorting of turbidites than contourites or other gravity flow deposits.
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| [64] |
Channel-related sediment waves on the eastern slope offshore Dongsha Islands,northern South China Sea [J].
The characteristics and origin of the sediment waves on the eastern slope offshore Dongsha Islands, northern South China Sea (SCS) were analyzed by integrating high-resolution multibeam bathymetric, seismic and piston core data. Four sediment wave fields were identified, which are the West Dongsha Channel (WDSW), West Taiwan Channel (WTSW), South Taiwan Channel (STSW) and West Penghu Channel (WPSW) sediment wave fields, respectively. They are distributed on the right levees (looking downstream) and adjacent overbank areas of related channels (WDSW, WTSW, and STSW), or developed outside the channel mouths (WPSW). Each sediment wave field consists of several to tens of rows of sediment waves. These sediment waves are up to 2.8–7.202km in wavelength and 30–6002m in wave height. With wave crests orthogonal to or oblique to the orientation of the channels, the sediment waves gradually decrease in dimension both in downslope direction and with the distance increasing away from the channel. Most sediment waves are asymmetric in cross sections with a thicker upslope flank and a thinner downslope flank, and therefore migrate upslope. Piston cores suggest that sandy turbidites developed with normal size grading dominate the channels, while massive muds with thin sandy or silty turbidite interbeds prevail in the inter-channel regions on which the sediment waves situated. The sediment waves on the channel levees and related overbank areas (WDSW, WTSW, and STWD) were presumably built by the turbidity currents overspilled from the channels; while those in WPSW, located outside the mouth of the West Penghu Channel, are suggested generated by unconfined sheet-like turbidity currents outside the channel mouth.
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| [65] |
Development and its tectonic activity’s origin of turbidity current sediment wave in Manila Trench, the South China Sea [J].南海东北部台湾浅滩陆坡的浊流沉积物波的发育及其成因的构造控制 [J].
<FONT face=Verdana>南海东北部马尼拉海沟发育有面积达35 000 km2的、壮观的沉积物波波域，水深2 600～4 100 m。基于波域的特征、区域分布、物质组成等，判断其为重力流成因。该波域于1.2 Ma开始起动，从而形成了和下伏平行反射截然不同的波状反射，二者之间为沉积物波的起动面。该波域的发育史受控于台湾造山运动的发育史，吕宋岛弧在北西西向运动的菲律宾海板块的携带下，于6.5 Ma左右和欧亚大陆边缘发生斜向弧—陆碰撞，碰撞焦点持续向南转移，于1.2 Ma左右转移至台湾南部，造成台湾南部的物理剥蚀、乃至向毗邻深水的碎屑供应急剧增加，改变了深水沉积动力学状态，浊流活动的强度和频率极大加强，从而开始了该波域的起动和发育。该沉积物波的发育和台湾造山运动构成了完美的构造活动和沉积响应关系。</FONT>
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| [66] |
Current status of deep-water sediment wave studies and the South China Sea perspectives [J].深水沉积物波及其在南海研究之现状 [J].
深水沉积物波的研究始于20世纪50年代。根据成因和结构特征，可以将深水沉积物波划分为细粒底流、细粒浊流、粗粒底流和粗粒浊流等类型。不同类型的沉积物波具有不同的形态、物质组成及分布特征。已提出的深水沉积物波的形成模式主要有背流波模式、逆行沙波模式、内波模式及底形和斜坡失稳混合模式等。1994年太阳号95航次和1999年ODP184航次揭示并证实，南海北部东沙岸外1144站所处的深水陆坡区发育有一高速沉积物牵引体。根据最新的地震资料分析发现，该牵引体实际上由一系列逆陆坡向上倾方向迁移的沉积物波组成，这一发现对于南海北部大陆边缘古海洋、古环境和古气候研究，以及南海深水油气勘探具有重要意义。
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| [67] |
Detailed seismic-reflection and sedimentary study of turbidite sediment waves on the Var Sedimentary Ridge (SE France): Significance for sediment transport and deposition and for the mechanisms of sediment-wave construction [J]. |
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