Downstream rounding rate of pebbles in the Himalaya

IF 2.8 2区 地球科学 Q2 GEOGRAPHY, PHYSICAL Earth Surface Dynamics Pub Date : 2024-04-17 DOI:10.5194/esurf-12-515-2024
Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, Mark Naylor
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Abstract

Abstract. Sediment grains are progressively rounded during their transport down a river. For more than a century, Earth scientists have used the roundness of pebbles within modern sediment, and of clasts within conglomerates, as a key metric to constrain the sediment's transport history and source area(s). However, the current practices of assessment of pebble roundness are mainly qualitative and based on time-consuming manual measurement methods. This qualitative judgement provides the transport history only in a broad sense, such as classifying distance as “near” or “far”. In this study, we propose a new model that quantifies the relationship between roundness and the transport distance. We demonstrate that this model can be applied to the clasts of multiple lithologies including modern sediment, as well as conglomerates, deposited by ancient river systems. We present field data from two Himalayan catchments in Nepal. We use the normalized isoperimetric ratio (IRn), which relates a pebble's area (A) to its perimeter (P), to quantify roundness. The maximum analytical value for IRn is 1, and IRn is expected to increase with transport distance. We propose a non-linear roundness model based on our field data, whereby the difference between a grain's IRn and the maximum value of 1 decays exponentially with transport distance, mirroring Sternberg's model of mass loss or size reduction by abrasion. This roundness model predicts an asymptotic behaviour for IRn, and the distance over which IRn approaches the asymptote is controlled by a rounding coefficient. Our field data suggest that the roundness coefficient for granite pebbles is 9 times that of quartzite pebbles. Using this model, we reconstruct the transport history of a Pliocene paleo-river deposit preserved at the base of the Kathmandu intermontane basin. These results, along with other sedimentary evidence, imply that the paleo-river was much longer than the length of the Kathmandu Basin and that it must have lost its headwaters through drainage capture. We further explore the extreme rounding of clasts from Miocene conglomerate of the Siwalik zone and find evidence of sediment recycling.
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喜马拉雅山卵石的下游成圆率
摘要沉积物颗粒在沿河搬运的过程中会逐渐变圆。一个多世纪以来,地球科学家一直将现代沉积物中卵石的圆度和砾岩中碎屑的圆度作为一个关键指标,以确定沉积物的运移历史和来源地区。然而,目前对卵石圆度的评估主要是定性的,并以耗时的人工测量方法为基础。这种定性判断只能提供广义上的迁移历史,如将距离分为 "近 "或 "远"。在这项研究中,我们提出了一个新模型,可以量化圆度与运输距离之间的关系。我们证明,该模型可应用于多种岩性的碎屑,包括现代沉积物以及古代河流系统沉积的砾岩。我们展示了尼泊尔两个喜马拉雅流域的实地数据。我们使用将卵石面积(A)与周长(P)相关联的归一化等周率(IRn)来量化圆度。IRn 的最大分析值为 1,预计 IRn 会随着运输距离的增加而增加。我们根据实地数据提出了一个非线性圆度模型,根据该模型,谷物的 IRn 与最大值 1 之间的差值会随着传输距离的增加而呈指数衰减,这与 Sternberg 的磨损导致质量损失或尺寸减小的模型如出一辙。这种圆度模型预测了 IRn 的渐近行为,IRn 接近渐近线的距离由圆度系数控制。我们的实地数据表明,花岗岩卵石的圆度系数是石英岩卵石的 9 倍。利用这一模型,我们重建了保存在加德满都山间盆地底部的上新世古河流沉积的迁移历史。这些结果以及其他沉积证据表明,这条古河流的长度远远超过加德满都盆地的长度,而且它的源头一定是通过排水捕获而消失的。我们进一步探讨了西瓦利克地区中新世砾岩中的极度圆滑的碎屑,并发现了沉积物循环的证据。
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来源期刊
Earth Surface Dynamics
Earth Surface Dynamics GEOGRAPHY, PHYSICALGEOSCIENCES, MULTIDISCI-GEOSCIENCES, MULTIDISCIPLINARY
CiteScore
5.40
自引率
5.90%
发文量
56
审稿时长
20 weeks
期刊介绍: Earth Surface Dynamics (ESurf) is an international scientific journal dedicated to the publication and discussion of high-quality research on the physical, chemical, and biological processes shaping Earth''s surface and their interactions on all scales.
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