Establishment Of Quaternary Stratigraphic Sequence And Its Environmental Significance In Yanji City

Yanji city is located at the north of Changbai Mountain in Jilin Province.Yanji city is not only the capital of Yanbian Korean Autonomous Prefecture,but also the central of political,economic,cultural,transportation and international communication in Yanbian.Yanji basin is a Mesozoic graben fault basin in Jilin Province with area of 1670km2.In target zone,Quaternary deposit is generally relatively thin,and the Quaternary strata was mainly composed of the first terrace,the second terrace and the third terrace,as well as river bed and floodplain.The second terrace and the third terrace are relatively developed.At present,there are few system research about the deposits sequence,genesis and formation age of Quaternary deposition.Therefore,based on the field geological survey,combined with remote sense,heavy magnetic and aeromagnetic data and previous research results,this paper carried out controlling drilling detection around Buerhatong river.Via Quaternary age dating,section analysis and comparison and pollen analysis,this paper established the sedimentary sequence of the Quaternary strata in the target zone,and also studied the paleoenvironment.The results can lay the foundation for the composite drilling section and provide scientific basis for seismic hazard assessment for active faults,can provide some reference for forecasting future climate and environment change as well.The main conclusions of this paper are as follows:1.The late period of Holocene deposit was mainly distributed in floodplain and riverbed,and the lithology was mainly gravel,sand and clay.2.Holocene deposit was mainly distributed in the first terrace,floodplain andriverbed of Buerhatong river and its branches.Deposit at the first terrace was typical river sediment with obvious dual structure.The bottom layer was brownish yellow gravel and the top layer was grayish dark clay.By Optically Stimulated Luminescence Dating,this sediment belong to Holocene sediment.3.Deposit in late Pleistocene was mainly distributed in the second terrace of Buerhatong river and its branches.It was typical river sediment.deposit was sand or sandy gravel at the bottom,yellowish brown clay with a little gravel at the middle part,brownish gray clay at the top.By Optically Stimulated Luminescence Dating,this sediment was equivalent to the sediment in the last glaciation.The top of the second terrace was covered by the Holocene dark soil,grayish dark clay,with the thickness of1.2~1.5m.4.Middle Pleistocene deposit was mainly distributed at the third terrace of Buerhatong river and its branches.The bottom was gravel layer and the top was sand and clay layer.5.Pollen composition of the sediment since the late period of the late Pleistocene in Yanji basin could be divided into 6 pollen belts from the bottom up.The pollen fossils of the ?-? pollen belts represented desert vegetation landscape of the last glaciation and the pollen fossils of the ? pollen belts represented savanna meadow vegetation landscape under warm humid climate of Holocene.The regional climate has experienced a cycle of relatively cold,dry cold,relatively cold,dry cold,relatively cold and wet humid,since the late period of the late Pleistocene...