Subaqueous sediment density flows: Depositional processes and deposit types
Subaqueous sediment density flows: Depositional processes and deposit types
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DOI：
10.1111/j.1365-3091.2012.01353.x
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发表时间：
2012-12
期刊：
影响因子：
3.5
通讯作者：
P. Talling;D. Masson;E. Sumner;G. Malgesini
中科院分区：
文献类型：
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作者：
P. Talling;D. Masson;E. Sumner;G. Malgesini
海底沉积物异重流是沉积物在地球上移动的最重要过程之一，但很难直接监测。全世界只有五个地点直接测量了长期运行的海底异重流的速度，而且从未直接测量过它们的泥沙浓度。大多数异重流的唯一记录是它们的沉积物存款。本文总结了密度流存款沉积物的过程，并提出了一个新的单一分类的存款类型。细粘性泥浆的胶体特性确保了泥浆沉积的复杂性，大量泥浆有时会长距离淤积或回流到盆地低洼处。未分级泥浆（TE-3）的沉积最有可能最终是由于相对较薄和较密的流动中的固结作用，尽管泥浆的初始粒度分选表明稀释和膨胀流动的早期阶段。分级泥浆（TE-2）和精细分层泥浆（TE-1）最有可能是在较低泥浆浓度下絮凝物沉降的结果。泥浆层段下的粒度突变是常见的，并记录了由于胶态泥浆行为而导致的中间粒度的旁路。平面层状（TD）和涟漪交叉层状（TC）非粘性粉砂或细砂通过稀释流沉积，外部存款形状与先前的空间减速（耗散）稀释流模型一致。在涟漪交错叠层（TC）间隔下的粒度破碎是常见的，并记录了一段时间的沉积物改造（有时成为沙丘）或旁路。细平面层积砂可以在稀流（TB-1）中由低振幅底波沉积，但它最有可能主要由高密度流（TB-2）下的高浓度近床层沉积。更宽间隔的平面纹层（TB-3）出现在大块清洁砂（TA）下方，也是由高密度浊流形成的。高密度浊积岩矿床（TA、TB-2和TB-3）具有与受阻沉降一致的扁平形状，通常被更广泛的低密度浊积岩（TD和TC）覆盖。这种核心和褶皱的形状表明，事件有时包括两个不同的流动分量。大块干净砂很少被液化泥石流（DCS）堆积，在这种情况下，干净砂是未分级的或具有片状粒度纹理。干净的沙子碎屑可以延伸几十公里，然后突然变尖。上升流转换表明，净沙泥石流有时通过高密度浊流的转换形成。粘性泥石流可存款三种类型的可能含有碎屑的不级配泥砂。厚的粘性碎屑往往发生在更近端的设置，并从最初的边坡破坏延伸。较薄且高度移动的低强度粘性泥石流产生局限于远端区域的广泛沉积物。这些低强度泥石流可能包含碎屑并传播很长距离（DM-2），或者由于粘性泥浆的湍流阻尼而导致更多的局部流动转变（DM-1）。单个流动沉积物（床）的绘图强调了单个事件如何包含几种流动类型，以及流动类型之间的转换。流动转变可以是从稀流到密流，也可以是从密流到稀流。流动状态、存款类型和流动转变强烈地依赖于流动中粘性细泥的体积分数。最近的实地观察表明，与以前广泛引用的模型有很大的偏差，许多将流动类型与存款类型联系起来的假设都没有得到很好的检验。关于这些引人注目的流动，还有很多东西需要了解。
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 (TE‐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 (TE‐2) and finely laminated mud (TE‐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 (TD) and ripple cross‐laminated (TC) 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 (TC) 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 (TB‐1), but it is most likely to be deposited mainly by high‐concentration near‐bed layers beneath high‐density flows (TB‐2). More widely spaced planar lamination (TB‐3) occurs beneath massive clean sand (TA), and is also formed by high‐density turbidity currents. High‐density turbidite deposits (TA, TB‐2 and TB‐3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low‐density turbidite (TD and TC,). 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 (DCS), 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 (DM‐2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM‐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.