The Study On The Datong Volcano Active Stages And Paleolake Changes Based On The Sedimentary Records

Volcano eruptions and paleolake changes occurred in Datong basin had closely related to the living environment of human.Datong volcano eruptions have always been one of the threatening factors for the living environment of mankind,and it is a regional natural catastrophes.Many researchers have been interested in Datong volcano and did a large number of research since the early last century,especially about the study on the time of volcano eruptions.However,the active stages of Datong volcano eruption have not been revealed clearly until now.On the other hand,paleolake covered the Datong basin for a long time during the Quatemary.Volcano ash and many weathered material provided much nutrient content to the paleolake,which resulted in vegetation lush and animal diversity.Those were very important for ancient humans.Therefore,paleolake’s transgression and regression were the controlling factors of ancient humans living.Field investigation,magnetic susceptibility,chemical components,grain size and paleomagnetism of the regional sediment show some facts.:1)Volcanic clastic layers deposited in different stages near former lakeside display incline with four different dip angles,which respectively are 125∠30o、137∠21o、45∠9o、95∠4o.And the degree of dip angle of lower layer is larger than the higher layer’s.2)There are four fine volcanic clastic layers in the grayish-green lacustrine sediment in the former inshore.3)There exist four sediment layers in the center of former lake which display high magnetic susceptibility value,implying some volcanic ash may deposited in them.Besides,the grain size of the four lacustrine layers with high magnetic susceptibility is much coarser than that of others.4)The black sand lied in the lacustrune section has the similar element contant of Ti and Fe,and magnetic value with the regional volcanic ash.5)There exist three sediment layers in the loess section in the land away from the former lake,which display high magnetic susceptibility value and low Rb/Sr one,implying some volcanic ash may deposit in them.6)The magnetic boundaries of B/M and Jaramillo Positive Subzone were determined in the regional lacustrine section.Based on these facts,the paper deduced that 1)Datong volcanoes experienced four active stages in the past and there took place many volcanic eruptions during each stage;the beginning times of four active stages are earlier than 0.90 Ma B.P.,~0.51 Ma B.P.,~0.32 Ma B.P.and ~0.09 Ma B.P.,respectively;and other stage of ~0.19 Ma B.P.was also recorded by loess sediment.2)The grain size distribution curve reflects four sedimentary rhythms,which implied there had occurred four lake regressions.3)Each of the active stages is synchronous with that of the lake regressions in the basin,respectively,implying there are some relation between the volcanic movement and lake regression.The above results reveal the active stages of Datong volcano eruptions.Besides,they also offer some proof for the study on the environmental impact from Datong volcano eruptions and paleolake changes,and find a new way to study the Datong volcanic movements,the regional lake regressions-lake transgression in the basin.