Paleosoil Characteristics And Paleoclimate Evolution Of Maastrichtian-Danian Period In Songliao Basin

Paleosol is generally formed in a long period of geomorphic stability,in which the strata are discontinuous or unconformity,which contains rich information about paleoclimate,paleovegetation,paleogeomorphology and sedimentary environment.The second member of Mingshui Formation is the product of the shrinking stage of Songliao depression.The climate characteristics of the late stage of Songliao depression are recorded in the paleosol developed in this stage.The core of Mingshui Formation is 596.55 m and the core recovery rate is 93.69%.The core section of the second member of Mingshui Formation is 210.57-632.33 m and the thickness is 421.76 m.Based on the core data of the second member of Mingshui Formation of SK-1 in Songliao Basin,this paper studies the paleosol petrology and geochemical characteristics of the second member of Mingshui Formation by core observation,geochemical analysis and X-ray diffraction,and then discusses the paleoclimate evolution in this period.By summarizing the petrological characteristics of the study area,it can be concluded that siltstone and mudstone are the lithology with the highest frequency in the second member of the second member of Mingshui Formation,followed by argillaceous siltstone and silty mudstone,which mainly identify meandering river deposit,shallow lake deposit,nearshore bar deposit,river beach deposit and river lake deposit microfacies,which is a set of rock combination mainly composed of meandering river and shallow lake deposit.According to the lithologic characteristics and V/Cr ratio and ?Ce value,the sedimentary facies can be divided from the bottom to the top: meandering river deposit at the bottom,shallow lake deposit in the middle,meandering river deposit at the top and shallow lake deposit at the top.That is to say,the second member of Mingshui Formation experienced two meandering river to shallow lake sedimentary cycles from the bottom to the top.The paleosol type developed in the second member of the Mingshui Formation is buried paleosol,and the identification marks are mainly calcareous nodule,sliding friction surface and plant root trace.The main content,trace and rare earth elements of Paleosol in the second member of Mingshui Formation are determined by geochemical test.The values of CIA,CIA-K and MAP are obtained by calculating the major elements.Through the vertical change trend,it can be roughly determined that the bottom and middle lower part of the second member of the open water formation of SK-1 are primary chemical weathering,cold and dry climate;the middle upper part,upper part to top are moderate chemical weathering,warm and humid climate,and the top up is likely to be strong chemical weathering,hot and humid climate.The Rb/Sr ratio of trace elements also confirms the above changes;the V/Cr ratio is between 0.54 and 1.43,with an average value of 0.95,indicating that the sedimentary environment in this period is oxidation environment.The vertical variation trend of rare earth element ?LREE/?HREE ratio shows that the second member of Mingshui Formation experienced two weathering cycles from weak to strong vertically from bottom to top;?Ce values were all less than 1,which was Ce negative anomaly and Ce deficit,which was the same as that of V/Cr ratio,indicating that the sedimentary environment of this period was oxidation environment.Through the X-ray diffraction test of clay minerals,it is found that illite disappears at 330.4m near the K/Pg boundary of the second member of Mingshui Formation in SK-1,chlorite disappears at 311 m,and then develops again at 330.4m;quartz clay ratio changes abruptly at the K/Pg boundary,increases from bottom to top,and then decreases abruptly after crossing the boundary;illite clay ratio is above the K/Pg boundary,and generally decreases from bottom to top.The above changes indicate that there is climate change near the K/Pg boundary,which is manifested in the process of chemical weathering from the bottom to the top,and the climate is gradually becoming humid.According to the comprehensive analysis,the second member of Mingshui Formation in SK-1 was in the oxidation environment,which was in the transition from the cold and dry environment of primary weathering and semi-arid to the warm and humid environment of moderate weathering,and finally experienced the hot and humid environment of intense weathering,during which the local climate slightly fluctuated.