民勤盆地12kaBP以来的沉积旋回

Sediment cycles in Minqin Basin since 12 kaBP

ZY剖面沉积物的粒度特征反映了民勤盆地降水、水动力搬运强度及湖面水位高低的变化.利用剖面沉积物的粒度资料;;发现该剖面自12kaBP以来风成沉积与湖相和浅湖相沉积在粒度上具有多次显著的粗细韵律交替变化;;粒度参数Mz;;σ;;Sk;;Kg和SC/S亦随之相应为多波动峰谷交替的态势.同时;;粒度特征值φ5;;φ16;;φ25;;φ50;;φ75;;φ84;;φ95相应地表现为非常有节奏地跳动.据此;;将ZY剖面分为2个风成沉积、2个湖相沉积和2个浅湖相沉积;;并认为其是12kaBP以来在东亚冬夏古季风交替影响下沙漠演变的一个颇为完整的气候-地层记录.本剖面不仅可以作为古环境与古气候变化的自然记录;;而且剖面所反映的环境变化为研究本区土地退化、荒漠化等问题提供了自然背景.

The Minqin Basin lies in the environment-sensitive zone in North China, and the grainsize char- acteristics of its sediments indicate variations of precipitation, hydrodynamics and the lake level changes. The ZY section in the Minqin Basin has taken 14 alternately evolutionary sediment sequences of the wind deposition with low-lake deposition and lake deposition since 12 kaBP. Based on grain-size analyses of the 292 samples from the section, some views have been put forward: Some possible major depositions are discussed. So are wind deposition, low-lake deposition and lake deposition through the analysis of the characteristics of the grain-size frequency curves of the sediments. At vertical direction of the section, the grain-size dis- tribution with coarse and ?ne rhythms vary very pronouncedly with the alternation of sedimentary faces. Mz(particle diameter), σ(standard deviation), Sk(bias angle),Kg(kurtosis) and SC/S(ratio of the sum of silt and clay to sand) of wind deposition with low-lake deposition and lake deposition separately constitute a commulation between peak and valley values. 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 ?ounce range and high sensitivity responding to the sedimentary facies.In contrast with them , those below the above-cited eigenvalue, i.e., φ5, φ16 and φ25, have relatively narrow ?ounce range and low sensitivity. In brief, the evolu- tion of the ZY section in the Minqin Basin is characterized by sedimentary continuity and faster sedimentary rate, which can not only indicate the paleoenvironmental and climate change in Holocence, but also provide natural background for the soil regeneration and deserti?cation.