Paleoenvironmental Evolution Of The Lower Cretaceous Dongshan Formation In The Eastern Depression Of The Boli Basin

The Boli Basin is one of the most important Mesozoic fault basins in Heilongjiang Province.The northern part is the denudation boundary,the southwest is the Yilan-Boli fault,the Xingnong~Xingkai fault is located on the south side of the basin,and the southeast side is the Jude fault..The basin basement is mainly composed of Xingkai block and Jiamusi block.The basin is divided into western depression,central depression and eastern depression.The faults in the basin are relatively developed and the geological structure is very complicated.This paper is based on the eastern depression of the Boli Basin.The Lower Cretaceous Dongshan Formation is the main research target.By analyzing the sedimentary facies,paleoclimate,paleo-saltitude and redox properties of the study area,the paleo-climate and sedimentary facies evolution and distribution characteristics are obtained.Correspondence relationship.This paper provides a reference for deepening understanding of the paleoclimate,paleoenvironment and sedimentary evolution of the Dongshan Formation in the eastern Heilongjiang Basin.This paper takes the core observation as the main method and the log data analysis as a supplement.The sedimentary facies of the Lower Cretaceous Dongshan Formation in the eastern Deli of the Boli Basin are analyzed.The longitudinal extension of the study area is studied by using the single well facies and the connected well sedimentary facies.The law of cloth is studied,and the characteristics of the distribution of sedimentary facies in the study area are discussed.Through the analysis of the geochemical characteristics of trace elements and rare earth elements,combined with the analysis of paleosomal sporopollen assemblages,U-Pb dating,the sedimentary time of the Dongshan Formation in the eastern Deli basin,the paleoclimate and paleo-salinity of the sedimentary period Systematic analysis,the relationship between sedimentary evolution law and sedimentary climate is discussed.The age of the bottom boundary of Dongshan Formation is 107.15±0.65 Ma,and the upper age is 105.0±0.81 Ma.The Lower Cretaceous Dongshan Formation in the eastern Depression of the Boli Basin is dominated by delta facies and lacustrine facies,of which the delta can be further divided into delta fronts;the lacustrine facies is further divided into shallow facies and semi-deep subfacies,further Seven sedimentary microfacies are divided.Through the analysis of the single well sedimentary facies of the D4 and Zk2 drilling wells in the study area,the lateral and longitudinal distribution characteristics of the sedimentary facies are analyzed according to the sedimentary facies profile of the well and the equal thickness map of the sandstone and the sand-ground ratio contour map.Determine the evolution characteristics of the sedimentary facies in the study area.The paleoclimate and paleo-salt characteristics of the Dongshan Formation in the study area were restored by the combination of elemental geochemistry and paleontology sporopollen fossils.The sequence temperature of the Dongshan Formation is lower,the climate in the sequence II is relatively hot,and the sequence III is gradually changed from a cold and humid climate to a hot and humid climate.According to the analysis,the temperature of the sequence of the Dongshan Formation is lower,the lake facies and delta facies are deposited in the study area,the sequence II of the Dongshan Formation becomes dry,the sedimentary range of the delta front is increased,and the area of the lake is reduced.The sedimentary range of the lacustrine facies in the sequence III period is small,and the sedimentary range of the delta front facies is increased.During the sedimentary period of the Dongshan Formation,the overall temperature of the climate was low until the temperature increased and then the temperature decreased.