# 浊流及其相关的深水底形研究进展

1.中国科学院深海科学与工程研究所,海南省海底资源与探测技术重点实验室,海南 三亚 572000
2.中国科学院大学,北京 100049
3.青岛海洋科学与技术国家实验室,山东 青岛 266237

# The Research Progress of Turbidity Currents and Related Deep-water Bedforms

Wang Dawei1, Bai Hongxin12, Wu Shiguo123

1.Laboratory of Marine Geophysics and Georesource, Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China
2.University of Chinese Academy of Sciences, Beijing 100049,China
3.Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China

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

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： Deep-water ; Turbidity currents ; Bedform ; Cyclic steps ; Bouma sequence.

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Wang Dawei, Bai Hongxin, Wu Shiguo. The Research Progress of Turbidity Currents and Related Deep-water Bedforms[J]. Advances in Earth Science, 2018, 33(1): 52-65 https://doi.org/10.11867/j.issn.1001-8166.2018.01.0052

## 2 浊流及其相关底形

### 2.1 浊流分类

Fig.1   Conceptual subdivision of supercritical-flow phenomena on the basis of Reynolds number and Vedernikov number[25]

### 2.2 浊流底形

Fig.2   Schematic drawing of a series of asymmetrical cyclic steps (downstream from left to right) and hypothetical densimetric Froude number (Frd) variability[15]

### 2.3 浊流底形分布

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)。

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]

(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

## 3 深水底形研究进展

### 3.1 水槽实验

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。

(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[35]
(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

(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[35]
(a) Antidunes. (b)Unstable antidunes. (c)Chutes-and-pools. (d)Cyclic steps

### 3.2 数值模拟

N=f(Frd0,Rf,S0C0,p,λ, $LbreakL$, $vsU0$,Δτ*,r0,),(1)

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/U0Δτ*。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]

(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[37]
(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)

### 3.3 南海实例

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。

## 4 浊流与鲍马序列的关系

### 4.1 鲍马序列与实验模拟

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。

(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[16]

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[16]

Fig.8   Three dimensional bed-form stability diagram for 1 and 2-phase suspension flows[43]

Fig.9   The flow process of 1-phase suspension flow observed in experiment (a) and the outcrop of turbidite bed (b)[43]
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)

### 4.2 浊流底形与鲍马序列

Fig.10   Summary of flow dynamic and large-scale architrcture for high-concentration and low-concentrationturbidity currents, flow is from left to right[40]

Fig.11   Facies produced by deposition on cyclic step[40]

## 5 结 论

(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。

The authors have declared that no competing interests exist.