柴达木盆地夏日哈河中游古洪水沉积物的粒度特征

Grain size Characteristics of the Palaeoflood Sediments in the middle stream of the Xiariha River in the Qaidam Basin

洪水是干旱区重要的地表过程,柴达木盆地东部位于亚洲夏季风与西风作用的边界带,广泛分布的古洪水沉积对了解区域环境变化和地貌演化有重要意义,但是目前相关研究却十分有限。本文选择柴达木盆地东部夏日哈河中游的一处古洪水—风成砂剖面进行研究,以认识柴达木盆地东部洪水沉积的粒度特征及其与其它类型沉积物的差异,并探讨其古环境意义。结果表明古洪水沉积粒度频率曲线呈单峰、双峰两种类型,分选程度较差,极负偏分布,峰态尖窄,并且与河流相沉积、风成砂、风成黄土有明显差异,可以作为识别沉积相的重要参考。洪水沉积与其它类型沉积在搬运方式上也存在显著差异。古洪水沉积中常见的粗细旋回经证明是由粒径成分及其比例差异造成的,并且粗细层在粒度曲线的形态(粗层为单峰,细层为双峰)和搬运方式上(粗层为递变悬移,细层为均匀悬移)都有差异,代表了洪水过程中的水动力变化,因此可以作为识别和划分该区域古洪水事件的依据之一。同时,在不同阶段洪水有频次和规模上的差异,并且有由洪水环境向风沙环境过渡的变化,可能受地貌演化和气候变化的共同影响。

In this study, a paleoflood-aeolian sand section in the middle reaches of the Xiariha river in the eastern of Qaidam Basin was studied to understand its grain size characteristics and paleoenvironmental implications. The results displayed two types of grain size distribution curves, namely single-peak distribution and bimodal distribution. The flood sediments were very poorly sorted, and its grain size characteristics are obviously different with fluvial sediments, aeolian sand and loess, which could be used as a criterion to identify different sediments. They are different in transpotational processes as well. The coarse-fine cycles which are common in flood sediments were proved to be caused by different grainsize fractions, which was caused by the changes of the hydrodynamic changes during the flood. The frequency and magnitude of the floods and the sedimentary environment changes from flood to aeolian sand might be associated with both the geomorphological and climatic changes.