Assessment of Backwater Controls On the Architecture of Distributary-Channel Fills In A Tide-Influenced Coastal-Plain Succession: Campanian Neslen Formation, U.S.A.
Assessment of Backwater Controls On the Architecture of Distributary-Channel Fills In A Tide-Influenced Coastal-Plain Succession: Campanian Neslen Formation, U.S.A.
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DOI：
10.2110/jsr.2016.33
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发表时间：
2016-05
影响因子：
2
通讯作者：
L. Colombera;M. N. Shiers;N. Mountney
中科院分区：
文献类型：
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作者：
L. Colombera;M. N. Shiers;N. Mountney
河流的回水区是指河床高程达到海平面点下游的远水河段。回水水力学被认为对河流-三角洲地貌动力学具有重要的控制作用，但在保存的地层记录中记录的表达方式还不清楚。现代河流的向海河段在高流量时，由于河口附近的河道内水面相对于海岸线的固定水面下降，可能会发生水流加速和侵蚀。随着沿海平原分流河道接近海岸线，它们往往受到横向流动性减少的影响，这可能与低流量时沉积物通量减少有关。根据数值模型和现代沉积系统的观测，目前对与回水效应相关的通道形态动力学的理解，在这里用于预测岩石记录中沿海分流河道的结构。根据现有知识，分流河道充填物的典型特征为：宽厚比低、冲刷面向其底部聚集、加积充填类型、具有受水位下降影响的冲刷充填证据的相组织，可能导致潮汐信号向其底部叠加，以及产状、厚度和含砂量有限的同生易砂漫滩单元。为了检验这些预测，进行了实地考察，以检查从坎帕尼亚Neslen组（美国犹他州东部），其中包括一系列的砂岩，碳质泥岩，煤，沉积在沿海平原设置的间隔通道机构的沉积学特征，并在其中保存了重要的证据，潮汐的影响。三种类型的通道机构被认为是在所研究的间隔，在岩性和形成通道形态动力学：砂倾向于横向增生通道元素，heterolithic横向增生通道元素，砂倾向加积带状通道元素。这项研究集中在带状通道机构，因为他们拥有一个几何形状兼容的横向稳定的分流区的下降。这些机构的沉积学和建筑特征进行了分析和比较，提出的模型的建筑分流河道填料。虽然决定性的证据，回水过程的影响，在控制相结构的分流河道填充没有达到，所研究的机构显示一个整体的内部架构，岩性组织，边界表面的性质，并与其他单位，符合所提出的模型在一定程度上的关系。所分析的带状砂体均具有侵蚀切岸、泥岩沉积物比例非常有限、缺乏成因相关的条形单元、底部冲刷充填物群集以及缺乏与同生河流补给的漫滩砂岩的关系等特征。这项工作为今后的研究提供了指导，需要更好地了解回水过程在控制分流河道体的结构中的作用，它们的下倾变化，以及这些变化如何在沿海平原的地层演化中表达。
Abstract The backwater zone of a river is its distal reach downstream of the point at which the streambed elevation reaches sea level. Backwater hydraulics is believed to exert an important control on fluvio-deltaic morphodynamics, but the expressions with which this may be recorded in the preserved stratigraphic record are not well understood. The seaward reaches of modern rivers can undergo flow acceleration and become erosional at high discharges due to drawdown of the in-channel water surface near the river mouth, in relation to the fixed water surface at the shoreline. As coastal-plain distributary channels approach the shoreline they tend to be subject to a reduction in lateral mobility, which could be related to diminished sediment flux at low flow. Current understanding of channel morphodynamics associated with backwater effects, as based on observations from numerical models and modern sedimentary systems, is here used to make predictions concerning the architecture of coastal distributary channel fills in the rock record. On the basis of existing knowledge, distributary-channel fills are predicted to be typically characterized by low width-to-thickness aspect ratios, by a clustering of scour surfaces toward their base, by an aggradational infill style, by a facies organization that bears evidence of drawdown-influenced scour filling, possibly resulting in the overprint of tidal signals toward their base, and by co-genetic sand-prone overbank units of limited occurrence, thickness, and sand content. To test these predictions, fieldwork was carried out to examine sedimentological characters of channel bodies from an interval of the Campanian Neslen Formation (eastern Utah, USA), which comprises a succession of sandstone, carbonaceous mudstone, and coal, deposited in a coastal-plain setting, and in which significant evidence of tidal influence is preserved. Three types of channel bodies are recognized in the studied interval, in terms of lithology and formative-channel morphodynamics: sand-prone laterally accreting channel elements, heterolithic laterally accreting channel elements, and sand-prone aggradational ribbon channel elements. This study concentrates on the ribbon channel bodies since they possess a geometry compatible with laterally stable distributaries developed in the zone of drawdown. Sedimentological and architectural characteristics of these bodies are analyzed and compared with the proposed model of the architecture of distributary-channel fills. Although conclusive evidence of the influence of backwater processes in controlling the facies architecture of distributary-channel fills is not reached, the studied bodies display an ensemble of internal architecture, lithological organization, nature of bounding surfaces, and relationships with other units that conforms to the proposed model to a certain extent. The analyzed ribbon sandbodies are all characterized by erosional cut-banks, very limited proportions of mudstone deposits, a lack of genetically related barform units, clustering of scour fills at their base, and a lack of relationships with co-genetic river-fed overbank sandstones. This work provides a guide to future research, which is required to better understand the role of backwater processes in controlling the architecture of distributary channel bodies, their down-dip variations, and how these are expressed in the stratigraphic evolution of prograding coastal plains.