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Dune-scale cross-strata across the fluvial-deltaic backwater regime: Preservation potential of an autogenic stratigraphic signature
Geology ( IF 5.8 ) Pub Date : 2020-12-01 , DOI: 10.1130/g47601.1
Chenliang Wu 1 , Jeffrey A. Nittrouer 1 , Travis Swanson 2 , Hongbo Ma 1, 3 , Eric Barefoot 1 , Jim Best 4 , Mead Allison 5
Affiliation  

Dune-scale cross-beds are a fundamental building block of fluvial-deltaic stratigraphy and have been recognized on Earth and other terrestrial planets. The architecture of these stratal elements reflects bed-form dynamics that are dependent on river hydrodynamic conditions, and previous work has documented a multitude of scaling relationships to describe the morphodynamic interactions between dunes and fluid flow. However, these relationships are predicated on normal flow conditions for river systems and thus may be unsuitable for application in fluvial-deltaic settings that are impacted by nonuniform flow. The ways in which dune dimensions vary systematically due to the influence of reach-averaged, nonuniform flow, and how such changes may be encoded in dune cross-strata, have not been investigated. Herein, we explored the influence of backwater flow on dune geometry in a large modern fluvial channel and its implications for interpretation of systematic variability in dune cross-strata in outcrop-scale stratigraphy. This was accomplished by analyzing high-resolution channel-bed topography data for the lowermost 410 km of the Mississippi River, which revealed that dune size increases to a maximum before decreasing toward the river outlet. This spatial variability coincides with enhanced channel-bed aggradation and decreasing dune celerity, which arise due to backwater hydrodynamics. An analytical model of bed-form stratification, identifying spatial variability of cross-set thickness, indicates a prominent downstream decrease over the backwater region. These findings can be used to inform studies of ancient fluvial-deltaic settings, by bolstering assessments of proximity to the marine terminus and associated spatially varying paleohydraulics.

中文翻译:

河流三角洲回水区的沙丘尺度跨地层:自成岩地层特征的保存潜力

沙丘规模的横床是河流三角洲地层学的基本组成部分,已在地球和其他地球行星上得到公认。这些地层元素的结构反映了取决于河流水动力条件的床层动力学,并且先前的工作已经记录了许多比例关系,以描述沙丘和流体流动之间的形态动力学相互作用。但是,这些关系是根据河流系统的正常流量条件预测的,因此可能不适用于受流量不均匀影响的河流三角洲地区。沙丘尺寸由于到达距离平均,不均匀流量的影响而系统变化的方式,以及如何在沙丘跨地层中编码这种变化的方法尚未得到研究。在这里 我们探讨了在大型现代河道中回水流量对沙丘几何形状的影响及其对露头规模地层中沙丘跨层系统变率的解释意义。这是通过分析密西西比河最低的410 km的高分辨率河床床面地形数据来完成的,该数据揭示了沙丘尺寸在向河出口减小之前先增大到最大值。这种空间变异性与由于回水流体动力学而引起的河床水床凝结作用增强和沙丘速度降低相吻合。床型分层的分析模型,确定了跨定厚度的空间变异性,表明在回水地区下游显着下降。这些发现可用于为古代河流三角洲地区的研究提供参考,
更新日期:2020-12-18
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