Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)
Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)
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DOI：
10.1111/sed.12425
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发表时间：
2018-06-01
期刊：
影响因子：
3.5
通讯作者：
Cantelli, Alessandro
中科院分区：
文献类型：
--
作者：
Ghinassi, Massimiliano;D'alpaos, Andrea;Cantelli, Alessandro
潮汐小溪和河道在现代海岸的广泛分布与它们在化石记录中有限的记录形成对比，化石记录中主要记录了由河流和潮流的混合作用产生的点状沙洲元素。潮汐河道弯曲演化的沉积产物相对较少，以前很少有研究集中于现代环境中存在的潮汐点坝的特定相模式。这项研究通过多学科方法增进了对潮汐河道曲折的了解，这些方法结合了历史航空照片的分析、河道内水流速度的测量、沉积岩心的高分辨率相分析和三维建筑模型。所研究的航道弯道(宽12至15米，深2至3米)排干了位于意大利威尼斯泻湖东北部的一片盐沼地区。历史照片显示，在过去的77年里，这条弯道向大海移动了大约15米。结果表明，航道弯道是在无植被的泥滩上形成的，该滩涂逐渐被植被侵占。向海迁移发生在加重条件下，总体迁移率为02~03mYear(-1)，并受到内聚、侵蚀能力较差的外滩沉积的促进。落潮流占主导地位，航道弯道的平移分别促进了沙洲向陆侧和向海侧的冲刷和沉积。潮流在流速分布方面表现出明显的不对称性，它们的偏移模式在沙洲内提供了一种特殊的粒度分布。在洪水期，沙粒淤积发生在沙洲上部，那里是流速最大的地方。在退潮阶段，沙洲经历了二次螺旋流，沙子在沙洲的脚趾处堆积。洪水沉积和落潮沉积的横向叠加分别形成了局部向上粗化和向上细化的沉积组合。
The widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point-bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel-bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi-disciplinary approach that combines analyses of historical aerial photographs, measurements of in-channel flow velocity, high-resolution facies analyses of sedimentary cores and three-dimensional architectural modelling. The studied channel bend (12 to 15m wide and 2 to 3m deep) drains a salt marsh area located in the north-eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77years, the bend has translated seaward ca 15m. Results show that the channel bend formed on a non-vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 02 to 03myear(-1), and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain-size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening-upward and fining-upward sedimentary packages, respectively.