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
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.