Evolution Of Late Quaternary Sedimentary Environment And Its Engineering Geological Significance In Baiyangdian Area

Xiongan New Area is a major national strategy.In order to establish transparent Xiongan,realize the integration of ground and underground,and make more rational and effective use of underground space,it is urgent to provide basic sedimentology and climate change background for scientific urban planning and construction.Baiyangdian is the largest lake in North China Plain.The Quaternary sediments are deep and the sedimentary stratigraphic structure is complex in Baiyangdian area.The origin and evolution of Baiyangdian are still controversial.Quaternary geology is an important basis for engineering geology research.It is of great practical and scientific significance to study the evolution of Quaternary sedimentary environment and the significance of engineering geology for the construction of Xiongan New Area.In order to find out the geological structure characteristics and resource and environment endowment of the New Area,The China Geological Survey carried out geological survey in Xiongan area and obtained more than 500 geological borehole data.In this paper,three boreholes,GB005,GB039 and G7315,are selected as research objects.G7315 is located in the middle of Baiyangdian modern lake,and GB005 and GB039 are located in the juma river basin in the north of the lake and the Tanghe river basin in the south of the lake,which can basically represent the main sedimentary environment of Baiyangdian area.By collecting core deposition samples at 0-50 m depth of the borehole,analyzing the environmental indicating significance of sediment granularity and loss of ignition index,and combining with the results of chronology,the evolution process of the sedimentary environment since the late quaternary in the research area was reconstructed,and its relationship with the engineering characteristics of soil mass in the engineering geological layer was discussed.The main conclusions were as follows:1.The particle size composition characteristics of four typical sedimentary environments were identified by neighborhood spread clustering analysis: the dynamic environment of lake-center facies or limnary facies was similar,and the frequency distribution curve of sediments with fine silt as the main particle size presented as a single peak normal distribution with a peak value of 5~20 m;The lakeshore facies is dominated by coarse silt,and the frequency distribution curve of grain size is bimodal.Fluvial particles are coarse,mainly fine and medium sand,with a single peak size distribution,the peak value of more than 100 m;Diluvial sediments are poorly sorted,hydrodynamic conditions are rapidly changing,and grain size characteristics of sediments are usually bimodal.2.Use the meta model of G7315,GB005 GB039 drilling sediment granularity and data analysis,is the number of each borehole granularity and yuan representatives on either side of the environmental implications,borehole reveals that the research on G7315 lake area since the late quaternary sedimentary environment evolution process: 177.7 ~ 126 ka,Baiyangdian lake level as a whole is low,mainly lake facies,after 137.9 ka,hydrodynamic suddenly stronger,sedimentary environment is given priority to with fluvial;In the MIS5 period,the climate was warm and humid,mainly in the transitional facies environment from lakeshore to lake center.At the stage of MIS4,the early fluvial sedimentation was enhanced,and then the lake sedimentary environment was dominant.In the warm and wet stage of MIS3,the change of end-element composition revealed the sedimentary environment dominated by the transition phase between lakeshore and lakeshore,and the lake expanded compared with the previous stage.Since 31 ka,the sedimentary environment has gradually changed from lakeshore to fluvial facies,and entered the holocene,and then to lakeshore or lakeshore facies sedimentary environment.3.Based on the established evolution process of sedimentary environment and combined with the results of engineering geological survey,the influence of sedimentary environment change on engineering geological characteristics is analyzed.The change of climatic conditions affects the hydrodynamic strength of the carrying medium in rivers and lakes,and then affects the physical and mechanical properties of soil.The stratum mainly composed of alluvial deposition has fast deposition rate,coarser grain size and better physical and mechanical indexes,so it is suitable for bearing stratum.Lacustrine sedimentation-dominated strata have continuous sedimentation,high organic matter content and relatively weak soil bearing capacity.