Study On The Relationship Between Dammed Lake Inthe Jishi Gorge,Yellow River And Disasters In The Lajia Ruins

The occurrence of natural hazards such as earthquake,landslide and dammed lake are a serious threat to the safety of life and property of people,especially in prehistoric times when the ability of prevention and prediction is in a low level.Lajia Ruins,located in Minhe county,Qinghai province,was honored as one of the top 10 archaeological discoveries of China in 2001 due to the excavation of its large prehistoric disaster scene of Qijia culture.The study of Lajia Ruins and its disaster event will contribute to understand the prehistoric relationship between human activities and climate environment in Gansu-Qinghai region,upper Yellow River,make a good preparation for danger and disasters,so as to provide reference for the development of the current relationship between human activities and climate environment.And many scholars have been attracted by the disasters of Lajia Ruins.But about the disasters of Lajia Ruins named “Oriental Pompeii”,there is still a large controversy.So in this paper,based on the ancient dammed lake and special clastic sediments in the upstream of the Yellow River among Xunhua basin,Jishi gorge,Guanting basin,13 profiles of HBC and HSJ were selected,17 samples of OSL were collected and tested.On the basis of chronology framework using Single Aliquot Regeneration and Standard Growth Curve of OSL dating method,it could be discussed that the period time and life span of dammed lake in the Jishi gorge,and its relationship with disasters in the Lajia Ruins,the cause of abandon about prehistoric Lajia settlement,from the insight of the geomorphology and sedimentology,archaeology and chronology of the combination.The main conclusions of this paper are as follows:(1)According to the analysis results of the sedimentary characteristics of the TGK profile and MEP profile,it is indicated that the fan delta deposit from the nearby gully in the two profiles,is not a part of ancient dammed lake,which originated from landslide or flashflood deposits.And the charcoal age of deposit can't represent the age of outburst flood,it can be regarded as the age of landslide or flashflood event after disappearance of dammed lake.(2)Through OSL dating of the dammed lake from Xunhua basin to JiShi gorge,combined with the previous researches,three dammed lake sedimentary were identified: the first one is about roughly 50 ka,the second one is around roughly 30 ka,and the third one began roughly at 8.8 ka,disappeared roughly at 5 ka.The results show that lifetspan of dammed lake is earlier than the catastrophe time of Lajia site.What's more,the dammed lake did not outburst quickly,but gradually fill and become shallow and disappear.(3)It is very likely that there have been many landslides,many dammed lakes in the Jishi gorge area.As a result,it is difficult to make a “dam” linked with dammed lake sediments from the eastern region of Xunhua basin to the western region of Jishi gorge,and outburst flood at no direct dating evidence.In addition,the determination of remnant dam in great bend of Jishi gorge,and the relationship between remnant dam and destruction of prehistoric Lajia settlement need further inquiry.(4)Based on the analysis of the sedimentary characteristics and OSL dating results of the special clastic sediments,it shows that the special clastic sediments from Jishi gorge to Guanting basin maybe avalanche debris cone,slope sediments,flashflood sediments,etc,which are different causes,different sources and different ages.No sedimentary features and chronological features were found to be reliable in the outburst flood deposits.In summary,from Xunhua basin to Guanting basin,upper Yellow River,no sedimentology,geomorphology,and chronology were found to meet the dammed lake sediments,dam body and outburst flood sediments.That is,there was no evidence of the trinity that could support the occurrence of outburst flood event roughly at 4.0 ka BP.Furthermore,the obtained evidence temporarily does not support the conclusion that the catastrophes of prehistoric Lajia settlement were caused by dammed lake in Jishi gorge,it maybe ruined by multiple natural disasters,including earthquakes,flashfloods,mudflows coming along the gullies in the northern Guanting basin.