河北平原中部保定西伯章剖面晚第四纪沉积特征及其环境意义

Late Quaternary depositional characteristics and environment significance of the Xibozhang section in Baoding, central Hebei Plain, China

对河北平原中部保定西伯章厚20.74 m的剖面进行实测,系统采集粒度和光释光(OSL)样品进行分析,以期获得其沉积特征及环境演变信息。光释光测年显示该剖面为102~8.69 ka的沉积,时代属于晚更新世到早全新世。岩性和粒度分析表明该剖面为曲流河冲积沉积,可以进一步划分为河道沉积和河漫滩沉积。河道沉积以含云母碎片的中粗砂为主,发育斜层理,频率曲线以正偏双峰频率曲线为主,分选中等到较差,概率累积曲线以跳跃组分为主的两段式为主。河漫滩沉积以黏土质粉砂为主,含保存良好的腹足类化石和钙质结核,频率曲线以负偏双峰为主,分选差,概率累积曲线以悬浮组分占比大的两段式为主。C-M图显示该剖面以均匀悬浮沉积为主,属典型曲流河沉积。实测剖面与黄土剖面对比分析显示西伯章剖面可以分为三个阶段,下部20.74~16.14 m为末次间冰期晚期沉积,气候温暖湿润,以粉砂质沉积为主;中部16.14~3.18 m为末次冰期沉积,气候干冷,沉积物显示细—粗—细的特征,与末次冰期早冰阶—间冰阶—盛冰阶一一对应;上部3.18~0.2 m为冰后期沉积,与末次间冰期晚期沉积环境类似,以黏土和黏土质粉砂沉积为主。粒度成因分析揭示了河北平原中部保定凹陷区晚第四纪沉积环境主要为河流冲积环境,粒度阶段性分布特征是晚第四纪以来冰期-间冰期气候的物质响应。

This paper aims to ascertain the depositional paleoenvironment since the late Quaternary in the central Hebei plain. We measured the 20.74 m-deep Xibozhang section and analyzed the collected samples for grain size and optically stimulated luminescence(OSL) tests. OSL dating results indicate that the deposits were formed during 102~8.67 ka, belonging to the late Pleistocene to early Holocene. Analysis of lithology and grain size show that, the section is fluvial deposit with typical binary structure, which can be further divided into channel deposit and floodplain deposit. The channel deposits are mainly medium-coarse sand containing mica fragments, with oblique bedding. Frequency curves is characterized by positive bias bimodal with moderate to poor sorting, and the probability cumulative curves are mainly two-stage dominated by jump components. The floodplain deposit is mainly clayey silt with well-preserved shell fossils and calcareous nodules. Most frequency curves are negative bias bimodal with poor sorting, and the probability cumulative curves are mainly two-stage dominated by suspension components. The C-M diagram shows that the Xibozhang section consists largely of uniformly suspended deposits, belonging to typical meandering river deposits. Comparing the measured section with the loess section of Jingyuan, we found that the Xibozhang section could be divided into three stages from the bottom up. The lower part, 16.14~20.74 m, is the deposits of the late stage of the last interglacial period, with warm and humid climate, dominated by silty deposits;The middle part, 3.18~16.14 m, is the deposits of the last glacial period, with dry and cold climate. The grain size shows the fine-coarse-fine characteristic, individually corresponding to the early interglacial stage, interglacial stage and maximum glacial stage of the last glacial period;The upper part, 0.2~3.18 m, is the deposits of the post glacial stage with similar characteristics of 16.14~20.74 m, dominated by clay and clayey silty sand deposits. Grain size analysis reveals that the late Quaternary depositional environment of the Baoding depression in central Hebei Plain is mainly of fluvial environment, and the distribution of grain size stage is the material respond to glacial-interglacial climate.