Late Quaternary Stratigraphic Division And Paleolake Evolution In The Hetao Basin

Quaternary strata are the carrier of information on climate change,sedimentation and geomorphic processes are also the key to the study of Quaternary palaeogeography and paleoenvironmental changes.The Hetao area in the Inner Mongolia is a Cenozoic fault basin,which has a huge quaternary stratum.Because core materials are difficult to obtain,there has been a lack of systematic research till now.Previous studies have found that the Hetao Basin developed a long-term lake environment in the Quaternary,and once there was a great unified lake.The evolution of the Hetao paleolake is an important part of the Quaternary Geology of the Hetao Basin.Geologists have done a great deal of work before.However,due to the limitation of dating methods,the lacustrine sedimentary sequence in the basin has not been clearly defined,and the evolution of the Hetao paleolake since the late Middle Pleistocene has not been studied deeply.That makes it difficult to compare the strata in the Hetao Basin with adjacent areas during the late Pleistocene.Therefore,it is of great significance to analyze the sedimentary environment during the late Pleistocene in the basin and reconstruct lake evolution history on the basis of Quaternary stratigraphic division and regional stratigraphic correlation.Based on field investigation in the study area,we got rich data of core,age and climate.The stratigraphic division in Hetao Basin during middle late Pleistocene was detailed,and a multiple stratigraphic classification system was established.In view of the key problems in the evolution of lakes in the basin,the spatial distribution of lake sediments,the time series and the lake water level information reflected by sediment properties are extracted,and the evolution stages of lakes are divided.Combined with tectonic events and paleoclimate characteristics,the driving factors of paleolake evolution are discussed.The main achievements are:1)Through the analysis of the characteristics of the lithology of the Hetao Basin Sediment drilling acquisition,combining with a variety of dating methods and theclimatology method,we established multiple stratigraphic classification system including lithostratigraphy,chronostratigraphy,climate stratigraphy of the late Quaternary in the Hetao Basin.The late Quaternary strata of the Hetao basin can be divided into middle Pleistocene Gouziban formation,upper Pleistocene Sara Wusu formation and Chengchuan formation,and Holocene.In addition,the Sara Wusu formation is further divided into the upper and lower sections,and the precise chronological constraints are made for the SaraWusu and the Chengchuan formation.2)According to the distribution and sedimentology characteristics of lacustrine facies in boreholes,two lake-river cycles in the basin are divided,and the coverage of lakes in each period is clarified.We divided the sediment size of the paleolake into four types to illustrate the water level changes.This paper expounds the evolution history of Hetao paleolake.The lake evolution process since 300 ka is divided into five stages:1.The Great-Lake Period in the Middle Pleistocene(320ka~160ka),there is a great unified paleolake,and the basin is in a deep water with reducing environment.2.Lake Forming Period in the Late Pleistocene(160ka~120ka),the rivers and lakes co-exist,with the Yellow River growing in the north and lake water confined to the middle of the basin.3.The Great-Lake Period in the Late Pleistocene(120ka~100ka),there is a great unified paleolake,which is a closed lake,and the water level is not as high as that of the Middle Pleistocene.4.Lake shrinking Period in the Late Pleistocene(100ka~11ka),the Yellow River began to flow out at100 kaB.P.,the Great Lake began to disintegrate,and the basin margin continued to sink and develop lakes,all the lakes disappeared till 11 kaB.P..5.The Modern Yellow River Period(11ka~now),there are a series of fluvial lakes and wind erosion lakes on the river flood plain,which have no relation with the late Pleistocene Great Lake.3)By comparing the phase characteristics of the paleo lake evolution with the landmark events of paleoclimate change,we analyzed the climatic factors that influenced the evolution of the lake.Based on the related literature of neotectonic movement in the basin margin fault zone,we analyzed the tectonic setting of the formation and disappearance of the paleolake.The study shows that the development and disappearance of lakes in the basin are controlled by the neotectonic movement inthe form of differential uplift and decrease in the basin margin fault zone.The fluctuations of paleoclimate can affect the water level of lakes to a certain extent.