Study On The Luminescence Time Of Sediments In The Golongbao Landslide And Its Barrier Lake In The Upper Reaches Of The Yellow River

The Gelongbu large-scale landslide (50.4 * 106 m3) in the upper reaches of the Yellow River was located in the reservoir zone of Jishi Valley Hydropower Station and is of important geological engineering significance. Meanwhile, the outburst of the dammed lake of Gelongbu landslide has had an important impact on the cultural environment of Lajia of Guanting Basin and thereout has become a hot research subject. However, the forming ages of the landslide has been controversial. Initial researches suggest that it is formed in 8,000-9,000 a BP and most of the following studies deduce that the dammed lake is formed in about 7,000-9,000 a BP according to the result of 14C and OSL dating. Some researchers even think that its formation age reached 1,0000 a BP. Furthermore, there are some scholars who conclude that the landslide lake and its outburst flood occurred in around 3,700 a BP.Being the material of Quaternary environment dating, Lacustrine sediments have been widely used, for the commonly used 14C dating means has shown some restrictions due to its narrow scope of dating, unwarrantable material gain and the tendency of being easily affected by the antique carbon. Development of OSL techniques has provided another alternative means of dating lacustrine sediment, applicability of which has been tested through a large number of researches at home and abroad. But there are still some problems to restrict the applications of OSL techniques in lacustrine sediment dating, including the uncertainty that if the OSL samples are completely faded and the influence of water content on dose rate.In order to further determine the exact forming age of the dammed lake, we have collected the relevant landslide pond sediments, small dammed lake sediments and dammed lacustrine sediments of the Yellow River for OSL dating. Sun faded degree of the samples would be detected through comparisons of various De value distribution diagrams and the test of samples with different particle size so that those poorly faded Des would be deleted to get more real dating results. Besides, saturation moisture content and measured moisture content of samples would be calculated and the water content of samples in different stages of the dammed lake would also be considered. The following results are obtained according to the experimental results:(1) De value of the medium-grained (38-63μm) quartzs in most samples is smaller than that of the fine-grained (4-11 μm) quartzs through comparisons of the De value and results of OSL dating of the fine-grained quartzs and medium-grained quartzs of the same sample collected from the Gelongbu landslide dammed lake sediments and landslide pond sendiments. The previous result has proved that sun faded degree of medium-grained quartzs is better than that of the fine-grained quartzs and hence is suitable for the OSL dating.(2) The water content of lacustrine sediments has obvious impact on dating during the existence of Gelongbu landslide dammed lake and there is a large difference on the result of OSL dating by employing two algorithms. Studies in this paper have considered the change of water content over the life cycle of the dammed lake and has got a older OSL dating result by about 5%-17%.(3) The span of OSL dating results of lacustrine sediments is large,it is related with the source and handling ways of samples as well as the handling distance. Materials of the landslide pond for OSL dating should go to the coarse-grained quartzs and those lacustrine sediments of the Yellow Rvier under long distance transport are suitable for the OSL dating because of their better sun faded degree.(4) The OSL dating result of Gelongbu landslide pond was no earlier than 7.55 ±0.53 ka while the OSL dating result XTC-1 and YS-2 of the bottom of the dammed lake of the Yellow River was 7.64±0.68 ka and 7.52±0.17 ka. The exact OSL dating result of the occurrence of GeLongbu landslide should be around 7,000-8,000 a BP considering the previous two results.