Grain Fabric in Hummocky Cross-Stratified Storm Beds: Genetic Implications
Grain Fabric in Hummocky Cross-Stratified Storm Beds: Genetic Implications
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DOI：
10.1306/d426769a-2b26-11d7-8648000102c1865d
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发表时间：
1991
影响因子：
2
通讯作者：
R. Cheel
中科院分区：
文献类型：
--
作者：
R. Cheel
摘要对来自丘状交叉分层（HCS）风暴床的样品中的颗粒结构进行了详细分析，结果表明，这些浅海砂岩是由逐渐减弱的混合（振荡/单向）流沉积而成的。沉积物中的颗粒a轴取向，在平行于可见分层的平面图中，变化很大，但平均取向大致垂直于单向古流指示器（唯一标记），并与区域海岸线平行。这种对齐的a轴是由于沉积下的组合流与多方向的振荡组件产生的风引起的表面重力波与模态波峰方向大致平行于海岸线。由于流动的振荡分量引起的边界剪应力的增减，颗粒的叠瓦在HCS存储器床的水平层状和HCS间隔中变化。在HCS风暴床的基底水平层状间隔中，颗粒叠瓦状角与水平方向的平均值约为13°，并浸入根据鞋底标记推断的古水流中。这种优选的叠瓦方向归因于受组合流的单向分量影响的边界剪应力。然而，准周期性变化的叠瓦角在很宽的范围内和周期性反转的叠瓦方向表明，振荡分量也是显着的沉积过程中的水平层状间隔。在HCS层段，叠瓦角的平均值与可见纹层平行，表明振荡流已占主导地位。通过这些HCS风暴床的结构和变化模式的序列是一致的沉积下减弱的组合流。侵蚀冲刷，形成尖锐的基地和单向唯一的标志，发生在一个有效的单向组件的组合流。单向分量继续影响边界剪应力的方向，在沉积过程中的水平层积砂，但减弱，成为可以忽略不计的一个强大的振荡组件在HCS的形成。
ABSTRACT A detailed analysis of grain fabric in samples from hummocky cross-stratified (HCS) storm beds suggests that these shallow marine sandstones were deposited from waning combined (oscillatory/unidirectional) flows. Particle a-axis orientations in the deposits, in plan view parallel to visible lamination, vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators (sole marks) and subparallel to regional shoreline. This alignment of a-axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind-induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline. Imbrication of particles varies in both the horizontally-laminated and HCS intervals of the HCS stor beds, due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow. In the basal, horizontally-laminated interval of an HCS storm bed, grain imbrication angles vary about a mean of approximately 13° from the horizontal, dipping into the paleoflow inferred from sole marks. This preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow. However, quasi-cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally-laminated interval. In HCS intervals, imbrication angles vary about a mean that is parallel to visible laminae, suggesting th t oscillatory flow had become predominant. The sequence of structures and changing patterns of fabric through these HCS storm beds is consistent with deposition under waning combined flows. Erosional scour, forming the sharp base and unidirectional sole marks, occurred in response to combined flows with an effective unidirectional component. The unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally-laminated sands but waned and became negligible relative to a strong oscillatory component during formation of HCS.