Records Of Datong Volcanic Activity In Regional Loess Sedimentation

Volcanic activity should be phased in time,that is,there are active periods and calm periods.The Datong Basin is located in the northernmost part of the NNE-to-shear-tensioning fault basin-mountain(west)-Shaanxi(west)mantle system in China.During the Quaternary period,regional tectonic movements and accompanying volcanic activities formed and evolved,it has important control role.There are many volcanoes in the Datong Basin.Since the 1930 s,different scholars have studied the history of their activities.However,due to the different methods of dating,the difference of field sampling levels and the number of volcanic eruptions,the academic community has not yet formed a unified understanding of its stage activity history,and its activity is still controversial.The staged history of Datong volcanic activity is a geoscience problem that has not yet been revealed.During the Quaternary period,thick layers of loess-paleosol sediments were deposited in the Datong Basin;these sediments recorded regional climate changes and geological historical events.Field trips have revealed that during a volcanic activity,large amounts of volcanic debris are produced and traces are left on numerous geological records in vast areas.The volcanic debris produced by the Quaternary volcanic activity is also interspersed in the loess-paleosol layer.Searching for sedimentary horizons containing volcanic debris in the loess-paleosol profile is one of the effective ways to reveal the history of volcanic activity in the region.In order to explore the stage history of the Datong volcanic activity,the author conducted a field survey of the Datong volcanic group in the Datong Basin and looked for a loess-paleosol sedimentary profile that was significantly affected by volcanic activity.Through the detailed field physical description,sampling and indoor physical and chemical analysis of these sections,it is expected to find the record of regional volcanic stage activities in the deposition of low-lying loess.In this paper,the Nangou Valley,Jinshan Northwest Ditch and Heisan Nangou loess-paleosol sedimentary profile,which are close to the volcanic cone,are selected as the target profile.The magnetic susceptibility and chemical element physicochemical characteristics are measured and analyzed.Multiple records have stratification of active volcanic periods and some research conclusions have been obtained.(1)In the loess sedimentary layer,due to the influence of volcanic activity,the physicochemical properties such as magnetic susceptibility,Ti and Fe content will change,the magnetic susceptibility value will increase abnormally,and the Ti and Fe contents also show high values.(2)Through the study of the loess-paleosol sedimentary profiles of the Nangou Valley,the Jinshan Northwest Ditch and the Nanshan of the Heshan Mountain,the records of the Datong volcano in the S1 development period of the paleosol,the L3 sedimentary period of the loess layer and the S5 development period of the paleosol were recorded.The active period of the first stage of the volcano;the loess sedimentary layer has a good record for the active period of the Datong volcano.(3)Take samples for photoluminescence dating in the northwest of Jinshan,the west side of Jinshan Northwest Expressway and the lava layer of Heishan or the bake layer below the lava flow.The dating result is the Datong volcano in 90,000 years ago.There are two active periods of volcanic activity around 270,000 years ago.Corresponding to the active period of the second stage volcanic period of the paleosol S1 development period and the loess layer L3 deposition period.Through the volcanic activity corresponding to the loess-paleosol sequence and the traditional photoluminescence dating,it is concluded that the Datong volcanic activity recorded at least three active periods of volcanic activity in the regional loess deposit.