Grain-size cycles in Salawusu River valley since 150 ka BP

Abstract

The Milanggouwan stratigraphical section in the Salawusu River Valley takes down 27 alternately evolutionary sediment sequences of the aeolian dune layers with the fluvio-lacustrine facies and palaeosols since 150 ka BP. Based on grain-size analyses of the 86 samples from the section, some views have been put forward: (1) Grain-size accumulative centigrade contents of the aeolian sands in the past 150 ka in Milanggouwan section are mainly characterized by the aeolian palaeo-mobile dune sands, and those of the fluvial facies dominated by very fine sands also contain some quantities of silt and clay. (2) The grain-size general distributional characteristics - Mz (particle diameter) and σ (standard deviation) clearly show that the fluvio-lacustrine facies and palaeosols usually become finer in particles and worse in sorting than the aeolian sands. As regards Sk (bias angle) and Kg (kurtosis) values, the former two increase obviously compared with the latter, displaying positive bias and the latter often proximately symmetrical distributed, only few in negative bias. (3) At vertical direction of the section, the grain-size distributional coarse and fine rhythms vary very pronouncedly with the alternation of sedimentary facies. Mz, σ, Sk, Kg and SC/D (ratio of the sum of silt and clay to sand) of fluvio-lacustrine, palaeosols and aeolian sands separately constitute a commulative between peak and valley values. (4) The values of grain-size parameters lie on their eigenvalues. Taking the eigenvalue Φ50 as a boundary, the Φ75, Φ84 and Φ95 above the eigenvalue have wide flounce range and high sensitivity responding to the sedimentary facies, which may be called 'active eigenvalues'. In contrast with them, those below the above-cited eigenvalue, i.e., Φ5, Φ16 and Φ25, which have relatively narrow flounce range and low sensitivity, are called 'inert eigenvalues'.

萨拉乌苏河流域米浪沟湾地层剖面150 ka BP 以来古流动砂丘砂与河湖相和古土壤在粒度上具有多次显著的粗细韵律交替变化, 粒度参数- M z、R、Sk、Kg 和SC/ D 亦随之响应为多波动峰谷交替的态势。同时, 粒度特征值- 5 5、5 16、5 25、5 50、5 75、5 84、5 95 相应地表现为非常有节奏地跳动。据此, 将米浪沟湾剖面划分为27 个粒度粗细沉积旋回, 并认为其是150 ka BP 以来在东亚冬夏古代季风交替影响下沙漠演变的一个颇为完整的气候- 地质过程记录。