New insight into the evolution of large‐volume turbidity currents: comparison of turbidite shape and previous modelling results
New insight into the evolution of large‐volume turbidity currents: comparison of turbidite shape and previous modelling results
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对大体积浊流演化的新见解：浊积岩形状与先前模拟结果的比较
DOI：
10.1111/j.1365-3091.2007.00858.x
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发表时间：
2007
期刊：
影响因子：
3.5
通讯作者：
R. Wynn
中科院分区：
文献类型：
--
作者：
P. Talling;L. Amy;R. Wynn
Marnoso Arenacea 地层提供了古代浊积岩系统中迄今为止记录的最广泛的单个流动沉积物（床）的相关性。这些相关性对床形状提供了异常详细的约束，用于推断流动演化并评估数值和实验室模型的有效性。床体积具有近似对数正态的频率分布；少数水流主导了这个非渠道化盆地平原的沉积物供应。小体积（<0·7 km3）内的浊积砂岩以近似指数的方式使下流变薄。这种形状是空间耗尽流的特性，并且已被先前的数学模型和实验室实验再现。较大体积（0·7–7 km3）层中的砂岩层段在其近端部分具有较宽的最大厚度。在这个最大厚度范围内的粒度趋势表明，尽管流动在时间上不稳定，但在~30公里的距离内，流动几乎是均匀的。以前的数学模型和实验室实验尚未重现这种类型的沉积形状。这可能是因为模型无法模拟强大水流中近床沉积物浓度趋于恒定值（饱和）的方式。或者，这种差异可能是由于海底流中水流厚度和沉积物沉降速度的比率相对较高，以及海底梯度的非常缓慢的变化造成的。床内侵蚀、随时间变化的排放以及将悬浮沉降物重新加工为床质也有助于解释沉积物形状的差异。大多数大体积岩床都包含一个内部侵蚀面，其下方是反级配砂岩，记录了流量的增减。先前已推断这些特征是异重洪水排放产生的浊积岩的诊断。这些浊积岩体积太大，不可能是由异重流形成的，除非这种流能够侵蚀立方千米的海底沉积物。这些流动更有可能源自海底斜坡破坏。两个层由仅由浊积泥岩分隔的多个砂岩层段组成。这些特征表明，海底斜坡失稳要么以盈亏事件的形式发生，要么分多个阶段发生。
The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log‐normal frequency distribution; a small number of flows dominated sediment supply to this non‐channelized basin plain. Turbidite sandstone within small‐volume (<0·7 km3) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger‐volume (0·7–7 km3) beds have a broad thickness maximum in their proximal part. Grain‐size trends within this broad thickness maximum indicate spatially near‐uniform flow for distances of ∼30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea‐floor gradient. Intra‐bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large‐volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea‐floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages.