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首页> 外文期刊>Journal of Asian earth sciences >Late Pleistocene alluvial plain sedimentation in Lower Narmada Valley, Western India: Palaeoenvironmental implications
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Late Pleistocene alluvial plain sedimentation in Lower Narmada Valley, Western India: Palaeoenvironmental implications

机译：印度西部下纳尔默达河谷晚更新世冲积平原沉积：古环境意义

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摘要

Late Pleistocene fluvial sediments that were deposited in a slowly sinking basin are now exposed as 30-50 m high incised vertical cliffs all along the Lower Narmada Valley in western India. The exposed fluvial deposits have been classified into two sediment packages, alluvial fan sediments overlain by alluvial plain sediments. The alluvial plain sequence has not been studied previously. It consists mainly of sands and silts and is dominated by overbank deposits. Occurrence of large scale bedforms in the alluvial plain sequence points to the existence of a large sand bed river in an alluvial plain setting. The major sedimentary facies in stratigraphic order include large channel fills, giant epsilon cross bedded strata, overbank fines occurring in horizontal, massive and undulatory stratified forms associated with crevasse splay and backswamp deposits, and a reddish brown palaeosol overlain by thinly stratified sands and silts at the top of the exposed sediment succession. Large sized channel fills occur at two stratigraphic levels, which are morphologically similar and are indicative of high rates of deposition and avulsion. The large channel fill structures and the giant epsilon cross bedded strata indicate a large single channel river that was consistently 10-15 m deep and about 70-80 m wide even during the dry seasons. These dimensions are larger than those of the present day Narmada River at low discharge levels. The overbank sediments indicate rapid deposition through frequent overbank floods and floodplain aggradation by a laterally shifting river. Available chronologic data suggests that the reddish brown palaeosol correlates with a regional phase of pedogenesis in the alluvial plain of Gujarat prior to the Last Glacial Maximum (LGM), The thinly stratified sands and silts overlying the palaeosol were deposited by a considerably depleted but perennial river during the arid phase of the Last Glacial Maximum. Overall, the alluvial plain sediments of the Lower Narmada valley, particularly those below the palaeosol, have been attributed to a hyper-avulsive large river with low sinuosity whose high discharge levels were determined primarily by a large catchment area further to the east and not by the semiarid climate prevailing in the Gujarat alluvial plain during the upper part of the Late Pleistocene. The study concludes that the Narmada River has maintained a large catchment at least since the last 100 ka, however, the river was characterised by a much bigger channel during much of the Late Pleistocene with discharge levels higher than the present day.
机译：沉积在一个缓慢下沉的盆地中的晚更新世河流相沉积物现在暴露在印度西部整个下纳尔默达河谷的30-50 m高的垂直峭壁上。裸露的河床沉积物已分为两类沉积物，即冲积扇沉积物和冲积平原沉积物覆盖。以前没有研究过冲积平原层序。它主要由沙子和淤泥组成，并且以过高的沉积物为主。在冲积平原序列中出现了大规模的床形，这表明在冲积平原环境中存在着一条大的沙床河。地层顺序的主要沉积相包括大的河道填充物，巨大的ε交叉层状地层，与裂缝缝和背水沉积物相关的水平，块状和起伏的分层形式出现的高溢位细屑，以及棕褐色的古土壤覆盖着薄层状的沙土和淤泥裸露的沉积物序列的顶部。大尺寸的河道充填发生在两个地层上，这两个地层在形态上相似，表明沉积和撕脱率很高。大河道填充结构和巨大的ε交叉层状地层表明，即使在干旱季节，单河道也始终深10-15 m，宽约70-80 m。这些尺寸比今天的纳尔默达河低排水量大。堤岸沉积物表明，由于频繁的堤岸洪水和横向移动的河床使洪泛区聚集而迅速沉积。现有的年代学数据表明，红褐色的古土壤与古吉拉特邦冲积平原在最后冰川期（LGM）之前的成岩作用的区域阶段有关。在最后一次冰川最大值的干旱阶段。总体而言，下纳尔默达河谷的冲积平原沉积物，特别是古土壤以下的冲积沉积物，被归因于一条高冲积性，低曲率的大河，其高水位主要由更远的东部集水区决定，而不是由晚更新世上部古吉拉特河冲积平原普遍存在半干旱气候。研究得出的结论是，至少自最后100 ka以来，纳尔默达河就一直保持着较大的流域，然而，该河的特征是在晚更新世的大部分时间内，河道要大得多，排水量要高于今天。

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