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An integrated process‐based model of flutes and tool marks in deep‐water environments: Implications for palaeohydraulics, the Bouma sequence and hybrid event beds
Sedimentology ( IF 3.5 ) Pub Date : 2020-05-11 , DOI: 10.1111/sed.12727
Jeff Peakall 1 , Jim Best 2 , Jaco H. Baas 3 , David M. Hodgson 1 , Michael A. Clare 4 , Peter J. Talling 5 , Robert M. Dorrell 6 , David R. Lee 1
Affiliation  

Flutes and tool marks are commonly observed sedimentary structures on the bases of sandstones in deep‐water successions. These sole structures are universally used as palaeocurrent indicators but, in sharp contrast to most sedimentary structures, they are not used in palaeohydraulic reconstructions or to aid prediction of the spatial distribution of sediments. Since Kuenen’s famous 1953 paper, flutes and tool marks in deep‐water systems have been linked to turbidity currents, as reflected in the standard Bouma sequence taught to generations of geologists. Yet, these structures present a series of unaddressed enigmas. Detailed field studies in the 1960s and early 1970s observed that flutes are typically associated with thicker, more proximal beds, whilst tools are generally prevalent in thinner, more distal, beds. Additionally, flutes and tool marks are rarely observed on the same surfaces, and flutes are seen to change downstream from larger wider parabolic to smaller narrower spindle shaped forms. No model has been proposed that explains these field‐based observations. This contribution undertakes a radical re‐examination of the formative flow conditions of flutes and tool marks, and demonstrates that they are the products of a wide range of sediment gravity flows, from turbulent flows, through transitional clay‐rich flows, to debris flows. Flutes are not solely the product of turbulent flows, but can continue to form in transitional flows. Grooves are shown to be formed by debris flows, slumps and slides, not turbidity currents, and in many cases the debris flows are linked to the debritic component of hybrid flows. Discontinuous tool marks, including skim (bounce) marks, prod marks and skip marks, are shown to be formed by transitional mud‐rich flows. Consequently, the observed spatial distribution of flutes and tool marks can be explained by a progressive increase in flow cohesivity downstream. This model of flutes and tool marks dovetails with models of hybrid flows that predict such a longitudinal increase in flow cohesivity. However, some deposits show grooves preferentially associated with Bouma TA beds, and these are likely formed by flows transforming from higher to lower cohesion, and are present in basins where hybrid beds are absent or rare. The recognition that sole structures may have no genetic link to the later overlying turbidity current deposits, and can be formed by a wide range of flow types, indicates that the existing pictorial description of the Bouma sequence is incorrect. A modified Bouma sequence is proposed here that addresses these points. In utilizing the advances in fluid dynamics since Kuenen’s pioneering research, this study demonstrates that it is possible to use flutes and tool marks to interpret flow type at the point of formation, the nature of flow transformations, and the mechanics of the basal layer. These advances suggest that it is then possible to predict the nature of deposit type down‐dip. This new understanding, in combination with further testing in outcrop of the proposed relationships between sole marks and palaeohydraulics, opens up a wealth of possibilities for improving the understanding of deep‐water clastic environments, with implications for developing more complete facies models, assessing subaqueous geohazards and the resilience of seafloor infrastructure, and advancing our understanding of deep‐water sediments as archives of palaeoenvironmental change.

中文翻译:

深水环境中基于过程的综合槽纹和工具痕迹模型:对古水力学、Bouma 层序和混合事件床的影响

槽纹和工具痕迹是深水层序砂岩基底上常见的沉积构造。这些唯一的结构普遍用作古水流指标,但与大多数沉积结构形成鲜明对比的是,它们不用于古水力重建或帮助预测沉积物的空间分布。自从 Kuenen 在 1953 年发表著名的论文以来,深水系统中的凹槽和工具痕迹就与浊流有关,这反映在传授给几代地质学家的标准 Bouma 序列中。然而,这些结构呈现出一系列未解之谜。1960 年代和 1970 年代初期的详细实地研究观察到,长笛通常与更厚、更近端的床层相关,而工具通常在更薄、更远端的床层中普遍存在。此外,在同一表面上很少观察到凹槽和刀具痕迹,并且可以看到凹槽在下游从较大的较宽的抛物线形变为较小的较窄的纺锤形形状。没有提出模型来解释这些基于实地的观察结果。这一贡献对槽纹和工具痕迹的形成流动条件进行了彻底的重新检查,并证明它们是大范围沉积物重力流的产物,从湍流,到过渡富粘土流,再到泥石流。长笛不仅是湍流的产物,而且可以在过渡流中继续形成。凹槽被证明是由泥石流、坍塌和滑坡形成的,而不是由浊流形成的,并且在许多情况下,泥石流与混合流的碎屑成分有关。不连续的工具痕迹,包括脱脂(弹跳)痕、刺痕和跳跃痕,显示为过渡富泥流形成。因此,观察到的凹槽和工具痕迹的空间分布可以通过下游流动凝聚力的逐渐增加来解释。这种凹槽和工具模型与混合流动模型相吻合,预测流动凝聚力的这种纵向增加。然而,一些矿床显示出优先与 Bouma TA 层相关联的凹槽,这些很可能是由流动从较高的凝聚力转变为较低的凝聚力形成的,并且存在于不存在或稀有混合层的盆地中。认识到唯一的结构可能与后来上覆的浊流沉积物没有遗传联系,并且可以由多种流动类型形成,表示 Bouma 序列的现有图形描述不正确。这里提出了一个修正的 Bouma 序列来解决这些问题。本研究利用自 Kuenen 开创性研究以来流体动力学的进展,证明可以使用凹槽和工具标记来解释地层的流动类型、流动转变的性质和基底层的力学。这些进展表明,然后可以预测存款类型向下倾斜的性质。这种新的理解,结合对所提出的唯一标记和古水力学之间关系的进一步测试,为提高对深水碎屑岩环境的理解开辟了丰富的可能性,对开发更完整的相模型具有影响,
更新日期:2020-05-11
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