水下沉积重力流沉积的细粒沉积物平面层理的起源

IF 1.9 3区 地球科学 Q1 GEOLOGY
Omar N. Al-Mufti, R. William C. Arnott
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引用次数: 0

摘要

在水下沉积重力流沉积的现代和古代细粒沉积物中,平面层理是一个普遍存在的组成部分。这些沉积物通常交替出现厚度在亚毫米以下、边界清晰的富淤泥层和富粘土层,在至少几十米的横向距离内,厚度或沉积物质地变化不大。富含淤泥的地层具有中等至良好的分选,厚度从几十微米到几百微米不等,甚至只有一个淤泥颗粒。相比之下,富含粘土的地层分选较差,厚度在几十到几百微米之间。这些质地不同的夹层的厚度和有规律的交替,以及没有表明迁移绕过或崎岖床层沉积的特征,都表明在紧靠床层上方的近床层区域有交替的物理过程和相关的沉积模式。以前的解释侧重于粘土絮凝,这与近床区域的高流体剪切条件难以协调。本文认为,在水力平稳的湍流底部毫米至亚毫米厚的粘性次层中,高流体剪切力和沉积物浓度的共同作用最初会导致剪切变薄,并增强流体下部的流动性,使淤泥继续沉降到床面,形成分选良好的淤泥层。随着淤泥沉降和粘粒沉积物浓度的增加,流体动力润滑力增强,降低了水流近床部分的流动性。随后,床面向外的流动扰动会扰乱这种状态,从而大幅增加摩擦应力和有效流体粘度,最终产生剪切卡阻,导致胶凝和富含分选不良的粘土层沉积。这一过程会重复多次,最终形成由富淤泥层和富粘土层交替组成的沉积层,其上覆盖着从悬浮液中直接沉积下来的无结构泥浆。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

The origin of planar lamination in fine-grained sediment deposited by subaqueous sediment gravity flows

The origin of planar lamination in fine-grained sediment deposited by subaqueous sediment gravity flows

Planar lamination is a ubiquitous component of modern and ancient fine-grained sediments deposited by subaqueous sediment gravity flows. These sediments commonly exhibit alternating sub-millimetre-thick, sharply bounded silt-rich and clay-rich layers that change little in thickness or sediment texture over lateral distances that range up to at least several tens of metres. Silt-rich layers are moderately to well-sorted and a few tens to hundreds of microns to a single silt-grain thick. In contrast, clay-rich layers are more poorly sorted, and a few tens to hundreds of microns thick. The thickness and regular alternation of these texturally differentiated interlayers, in addition to the absence of features suggesting transport bypass or deposition by migrating rugged bed forms, suggest alternating physical processes and related modes of deposition in the near-bed region immediately above the bed. Previous interpretations have focussed on clay flocculation, which is difficult to reconcile with the high fluid shear conditions in the near-bed region. Here it is suggested that in the millimetre to sub-millimetre-thick viscous sublayer at the base of a hydraulically smooth turbulent flow, a combination of high fluid shear and sediment concentration initially leads to shear thinning and enhanced mobility in the lower part of the flow, and for silt to continue settling to the bed forming a well-sorted silt lamina. As silt settles and clay-size sediment increases in concentration, hydrodynamic lubrication forces strengthen and reduce mobility of the near-bed part of the flow. This condition is then perturbed by a bedward-directed outer flow disturbance that dramatically increases frictional stresses and effective fluid viscosity and ultimately shear jamming that causes gelling and deposition of a poorly sorted clay-rich layer. This process is then repeated multiple times to build up a deposit composed of alternating silt-rich and clay-rich laminae overlain by structureless mud deposited directly from suspension.

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来源期刊
CiteScore
4.10
自引率
16.70%
发文量
42
审稿时长
16 weeks
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