| As a product of the geological action during the geological history,the Diexi palaeo-dammed lake contains abundant information.The study of its sediments is helpful to explore the origin and evolution of Diexi palaeo-dammed lake,which also has good guiding significance for the study of the regional paleoclimate environment and the development and governance of current dammed lakes.The upper reaches of the Minjiang River are located in the active area of neotectonic movement on the eastern edge of the Qinghai-Tibet Plateau.There are numerous high-altitude valleys,strong erosion and palaeo-dammed lakes.In this paper,we collected 17 chronological samples and 152 granular samples from the 10 lacustrine profiles of Diexi palaeo-dammed lake and 3 outburst deposits in the upper reaches of Minjiang River.The OSL ages of 17 samples were measured by the method of Single Aliquot Regeneration(SAR)to study OSL chronology of Diexi paleo-dammed lake.Particle size experiments of 152 samples of lake sediments were performed using a laser particle size analyzer to study the grain size characteristics of the sediments in Diexi palaeo-dammed lake.The main conclusions are as follows:(1)The grain composition of the sediments in the Diexi palaeo-dammed lake is dominated by silt,and the average grain size()is between 3.76 and 7.42,withan average grain size of 6.51.The trend of particle size changes from theintersection of the river and the lake to the location of the dam is consistent with the trend of changes in ordinary lakes from lakeside to lake core.The standard deviation(?)ranged from 1.28 to 2.86 with an average of 1.62.The sorting was poor or poorer.The skewness()ranges from-0.25 to 0.43 with an average value of 0.1.Most ofthem are between-0.1 and 0.2,and the sediment particle size is positive or nearly symmetrical.From the intersection of the river and the lake to the location of the dam,the particle size frequency curve gradually transitions from a double peak to a symmetrical single peak.(2)Through the preheating plateau experiment,dose recovery experiment and light-emission characteristic curve analysis,it was determined that the method of SAR is suitable for the measurement of the equivalent dose(De)value of sediment samples from the Diexi palaeo-dammed lake.The final experimental preheat temperature was240°C.At the same time,the degree of sun retiring of 17 OSL samples was tested by the De-distribution feature of the samples.Only 3 samples had bad sun retreat.The rest of the samples had good sun retreat.The test pieces of 3 samples which had bad sun retreat were screened by relative standard deviation(RSD).The difference sample was measured and the mean value method was used to obtain the sample De value.(3)Doserate and OSL age of samples were Calculated,and the age-stratum relationship map established.The average sedimentary rate of the upper section of the Diexi palaeo-dammed lake was about 7.0mm/a.Comparing with the data of predecessors,this paper drew a conclusion that the age of the dammed lake was about30,000 aBP,and it died at 10,000 aBP.(4)During the last glacial ice age(40-30kaBP),the warm and moist climate in the upper reaches of the Minjiang River,the abundant rainfall,the strong undercutting effect of the river and tectonic movements are the possible reasons for the formation of the Diexi paleo-dammed lake.During the 25-10 kaBP entering the ice age,the river's handling and erosion capacity weakened,and the accumulation capacity increased,which is conducive to the preservation of the dammed lake.During the Holocene(about 10000aBP),the climate became warmer,and a large number of glacier meltwaters flowed into the river,increasing the erosion and unloading capacity of the river and prompting the rapid disappearance of the lake. |
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