THE FEATURES AND AGES OF LAKE BEACH ROCK AROUND SAYRAM LAKE IN WESTERN TIAN SHAN AND ITS SIGNIFICANCE ON LAKE LEVEL FLUCTUATION DURING THE LAST INTERGLACIAL EPOCH MIS3
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摘要: 在对西天山赛里木湖盆地进行第四纪地质调查与5万填图基础上,发现沿该湖泊的不同湖岸阶地上都不同程度地发育了可指示湖面变化的湖滩岩。水准测量结果表明,典型的湖滩岩最常见于高出现今湖面7.1~9.4 m和33.4~39.4 m的低、高两级湖积台地上。对湖滩岩样品进行岩石学和矿物学研究进一步揭示,湖滩岩主要由内碎屑、藻团块、陆源碎屑、胶结物和填隙物等构成,胶结物主要为亮晶方解石,夹少量文石,表明赛里木湖周边的湖滩岩为典型的方解石胶结砂屑砾屑岩。湖滩岩样品的U系年代测试结果表明,低、高两级台地上的湖滩岩主要形成于距今24.8±1.5 ka至27.6±1.5 ka和55.4±3.8 ka的晚更新世晚期,大致对应末次冰期间冰阶MIS3阶段早期和末期的相对暖湿气候阶段。湖滩岩及其测年结果指示,赛里木湖最近一期最高湖面出现在距今55.4 ka左右末次间冰阶早期,其后由于气候的干旱化,湖面整体处于逐步下降过程,在相对暖湿期间经历了多次湖面相对稳定期并形成湖滩岩。Abstract: Lake beach rock is one of the products which can reflect the land and sea interactions, of great significance for studying the ancient lake level fluctuations, lakeshore zone evolution and paleoclimatic change. On the basis of Quaternary geological survey and 1 :50000 scale geological mapping for Sayram Lake Basin in western Tianshan, we found lake beach rocks on the different shore terraces, indicating lake level fluctuations. The levelling result showed that the concentration distribution of typical lake beach rock was mainly on the fluvial terraces above the lake level 7.1~9.4 m and 33.4~39.7 m. The Petrologic and mineralogical study further showed that the lake beach rocks were mainly composed of intraclast, algal briquette, terrigenous clastic, cement and filler content, etc., and the cement mainly for the sparry calcite with a small amount of aragonite, which proved that the Sayram Lake beach rocks were typical calcite cementation sand gravel chip rocks. According to the U series dating results of lake beach rock samples, the lake beach rocks on the fluvial terraces were formed from 24.8±1.5 ka to 27.6±1.5 ka and 55.4±3.8 ka, late of the late Pleistocene, roughly corresponding to the relatively warm climate stage of early and late in last glacial interglacial epoch MIS3 stage.The lake beach rocks and dating results indicated that the recent highest level of Sayram Lake appeared in the distance of about 55.4 ka, early in the last interglacial epoch. Due to the subsequent drying climate, the lake level was overall in the process of decline gradually. During the relatively warm moist period, the lake level underwent multiple relatively stable periods, and resulted in lake beach rock formed.
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Key words:
- the West Tianshan /
- the Sayram Lake /
- lake level fluctuation /
- lake beach rock /
- the last glacial period
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图 4 赛里木湖周缘湖滩岩显微特征
a—湖滩岩手标本,可观察到其结构为典型的砂质含砾胶结岩,块状构造,接触式胶结;b—镜下观察湖滩岩薄片,岩石为含砂砾质砂屑砾屑结构(-),基底式接触式胶结。内碎屑由泥晶灰岩、粉晶灰岩等构成,次棱角—浑圆状,具方向性分布。陆源碎屑由岩屑、石英构成。岩屑主见硅质岩、石英岩、(含铁炭质)粘土质板岩、花岗斑岩、蚀变岩等,大小不等。胶结物由亮晶方解石及少量文石构成,粒径<0.1mm。亮晶方解石、文石主呈纤状、刃状等;c—镜下观察湖滩岩薄片,岩石由内碎屑、藻团块、藻球粒、陆源碎屑、填隙物构成。陆源碎屑由岩屑、石英构成,棱角-次棱角状为主,零散分布。岩屑主见硅质岩、石英岩、粘土质板岩等,大小0.02~8.0 mm不等;d—镜下观察湖滩岩样品中的藻团块特征。藻团块呈团状,有的边缘不规则,0.2~1.0 mm,由泥晶方解石及藻类构成,色暗;e—镜下观察湖滩岩胶结物特征,胶结物由亮晶方解石及少量文石构成,亮晶方解石、文石主要呈纤状、刃状等,垂直碎屑生长,构成栉壳结构,局部由多层纤状方解石与有机质混杂构成玛瑙纹状构造,基底式及接触式胶结;f—镜下观察湖滩岩的泥晶填隙物。湖滩岩中的内碎屑及陆源碎屑之间的填隙物由亮晶方解石胶结物、文石及泥晶方解石填隙物构成
Figure 4. Micro-characteristics of the lacustrine-beach rock around south-east Sayram Lake
表 1 赛里木湖湖滩岩出露高程及铀系测试结果
Table 1. Elevations and U-series dates of the lacustrine-beach rock around the Sayram Lake
样品编号 剖面地点与采样部位 拔湖高度/m 高程/m 岩性 年龄/ka S0811-7 S1剖面最高湖岸堤顶部 34.72 2107 钙质胶结砂砾石 55.4±3.8 S0814-5 S7剖面第9测点小湖岸堤顶 9.38 2082 钙质胶结砾石 27.6±1.5 S0814-9 湖岸阶地后缘上覆洪积扇前缘 44.85 2118 钙质胶结含砂砾石 37.0±2.2 S0814-17 S8剖面2080 m湖岸堤顶部 7.14 2080 钙质胶结砾石 24.8±1.5 -
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