A Methodological Study On The Lake-level Reconstruction Of Lacustrine Basin-a Case Study Of Laizhou Bay Depression In The Southern Bohai Bay Basin

The change of the lake level of the lacustrine basin is one of the hotspots and frontier problems at home and abroad.The change of the lake level controls the formation of sequence stratigraphic interface and the distribution of different types of sediments.Restoring the lake level changes in different periods can not only guide the hierarchical division,and it has important significance for the study of paleoclimate and paleoenvironment.Two sets of strata,minghuazhen formation and guantao formation,were deposited in laizhou bay depression in the south of bohai bay basin during the neogene period.Previous studies believed that there were widely distributed shallow lake and delta sediments in the basin during this period,but the research on paleolake level restoration was relatively insufficient.Therefore,by studying the cores and cuttings of well LZW1 and well LZW2 drilled in laizhou bay depression,sedimentology,geochemistry of main trace elements and palynology are used in this paper to reconstructed the paleo-lake level change in this area in the neogene period with a comprehensive multi-index,and further discussed the controlled factors and the significance of paleoclimate and paleoenvironment.Sedimentology indicators show that the study area in neogene main filling the braided stream,meandering river braided river delta,meandering river delta and littoral and shallow lake sedimentary system,combined with the feature of the facies and sedimentary structure analysis,the study area in the neogene period development4 lake level grades,During the Guantao Formation,the depth rank of the lake is mainly 1-2,and the lake level is shallow;the lake level of the lower part of the Minghuazhen group is increase rapidly and dominated by 3-4 grades.The upper part of the minghuazhen formation shows a slightly decrease.The results of the qualitative indicators of the main trace elements also show that the lake level is generally shallow-deep-shallow in the neogene period,through the analysis of quantitative paleobathymetry empirical formula,the lake level in neogene of the Laizhou Bay Sag is fluctuates frequently between 0 and 6 m.In addition,there is a good couplingrelationship between the lake level change and the water environment change revealed by the main trace element index.The palynological study showed that the guantao formation was dominated by herbaceous plants,the emergent plants developed,and the lake was in the initial stage of expansion.The mountain trees in the lower part of the minghuazhen formation began to appear in large quantity,and the submerged plants gradually increased.Combined with the previous studies and the above analysis,this paper believes that the lake level in the study area developed multiple secondary cycles in the member and the formation during the neogene period,showing the characteristics of lake advection to lake retreat,which are mainly controlled by climate.