班戈错晚第四纪湖泊发育、湖面变化与藏北高原东南部末次大湖期湖泊演化

Late Quaternary Lake Development and Denivellation of Bankog Co as well as Lake Evolution of Southeastern North Tibetan Plateau during the Last Great Lake Period

班戈错是因湖面阶段性下降而于晚更新世末期从母湖色林错东部分离出来的小离湖。2003年5—7月,我们对班戈湖沿岸进行了详细的地貌与第四纪地质调查,湖面高程及6条剖面湖岸阶地的水准测量,并采集了沿岸及邻区的湖相沉积物样品进行U系年龄测定。研究结果表明,班戈错湖岸阶地共6级,其中T1为与色林错分离后所形成,T2至T6的拔湖高度与U系年龄分别为:T2,11.8～29.8m/10.9±1.9～14.8±1.1ka BP;T3,32.4～59.8m/31.1±2.5ka BP;T4,52.4～83.7m/36.3±3.1～38.8±3.7ka BP;T5,82.2～112.9m/35.1±2.9ka B.P.;T6,121.8～139.8m/47.9±3.8ka B.P.,色林错各阶地的高度则应减去与班戈错湖面的高差9m。色林错—班戈错沿岸与纳木错沿岸的6级湖岸阶地在形成时代上可以很好地对比。此外,在藏北高原东南部,还发现了多处晚更新世早中期的高位湖相沉积。据此,可以将藏北高原东南部自MIS5以来的末次大湖期的湖泊演化,划分为125～30ka BP/MIS5e～MIS3a的羌塘大湖期和30～0ka BP/MIS2～1的藏北湖群期。前者又可进一步划分为125～96ka BP/MIS5e～c的羌塘大湖亚期,96～75ka BP/MIS5b～a的羌塘东湖亚期,75～37ka BP/MIS4～MIS3a的大湖退缩亚期及37～30ka BP/MIS3a残留大湖亚期。后者则可分为30～10ka BP/MIS2的湖泊咸化亚期和10～0ka BP/MIS1的盐湖发育亚期。本文还根据自1959年以来历年所测的地形图、遥感图像或数据、水位观测记录、三次水深测量和2003年5月26日的湖水位高程测量等资料,勾绘了班戈错近50年来的湖岸线变迁和湖面高程变化曲线图。研究表明,自1959年至2003年9月的近50年间,班戈错湖面总体上升,但其间经历了先降后升再略有下降的过程,即自1959年至1973年或稍晚,班戈错一直收缩、湖面下降了0.25m,此后湖面总体上升,其幅度可达1.75m。

The Bankog Co is a small lake separated from the eastern part of the Silin Co during the late Late Pleistocene.From May to July,2003,the authors made a detailed investigation of geomorphology and Quaternary geology along the bank of Bankog Co,which included the leveling survey of lake level elevations,the measurement of lake terrace heights along six profiles and the collection of lake deposit samples for U-series dating.The study results show that there are 6 level terraces（T1-T6） along the Bankok Co.Except that T1 was formed after separation from the Silin Co,the heights above the lake level and U-series ages of the other terraces are 11.8～29.8 m and 10.9±1.9～14.8±1.1 ka BP for T2,32.4～59.8 m and 31.1±2.5 ka BP for T3,52.4～83.7 m and 36.3±3.1～38.8±3.7 ka BP for T4,82.2～112.9 m and 35.1±2.9 ka BP for T5,and 121.8～139.8 m and 47.9±3.8 ka BP for T6,respectively.Correspondingly,for the heights of individual terraces of the Silin Co,9 meters should be subtracted from the heights of the Bankok Co terraces,which is the lake level height difference between the two lakes.The 6 level terraces of the Silin Co and the Bankok Co can be well correlated in the formation time with the 6 level terraces of the Nam Co.In addition,some high lake deposits of the early and middle Late Pleistocene may be found in southeastern North Tibetan Plateau.Therefore,the lake evolution of southeastern North Tibetan Plateau since MIS 5 of the last great lake period can be divided into two stages：the Qiangtang great lake stage of 125～30 ka BP,MIS 5e～MIS 3 and the North Tibetan lake group stage during 30～0 ka BP,MIS 2～1.The former can be further divided into four substages：the Qiangtang great lake substage of 125～96 ka BP,MIS 5e～c,the east Qiangtang lake substage of 96～75ka BP,MIS 5b～a,the retreated great lake substage of 75～37 ka BP,MIS 4～MIS 3a and the remnant great lake substage of 37～30 ka BP,MIS 3a.The latter can be further divided into two substages：the lake salting substage of 30～10 ka BP,MIS 2 and the saline lake forming substage of 10～0 ka BP,MIS 1.The study results also show that,from 1959 to 2003,the lake surface of the Bankog Co was overall rising,reaching up to 1.75 m,although from 1959 to 1973 or a little later,there occurred a process of first falling,then rising and again slight falling,with a fall of 0.25 m,and an accompanied contraction of the lake area.