Paleoenvironment And Paleoclimate Of The Middle Jurassic In The Northeastern Of Ordos Basin

Over the past half century,the really big rise in global temperature,ozone layer depletion,and natural disasters with greenhouse gas emission,climate variation and living environment have drawn great attention.The issues of climate change process and driving mechanism under global warming are necessary to explore in depth.It is crucial for finding solutions to predict and respond to various climate emergencies.Jurassic period is a special period of geological history with a typical greenhouse effect,the climate characteristics and changes in the definition of the study reveals the evolutionary rules.The study of continental is closer to human life,and the types are more complex.However,continental facies is constrained by regional scale factors,leading to the dispersion of sedimentary records,which has hindered the study of paleoclimate.Ordos Basin is one of the large continental sedimentary basins in Mesozoic and Cenozoic of the eastern region in China.The northeastern part of the Ordos Basin is an important coal enrichment zone,and the Middle Jurassic series Yan'an Formation is one of important coal-bearing formation.The development of a large number of climate-indicating minerals and fossils of ancient plants has provided facilities for the study of the ancient climate of the Middle Jurassic.This paper is focused on the interpretation of geological-section,rock-mineral determination and geochemical analysis,the sedimentary environment types,redox conditions and palaeoclimatic divisions of the Yan'an and Zhiluo Formations of the Middle Jurassic in the northeastern Ordos Basin are determined.The color,petrology,and sedimentary structure indicate that the sandstone has the characteristics of abundant lithoclast and feldspar with medium to coarse structure,bad-sorted,and bad-rounded.Zhiluo Formation is characterized by the sedimentary environment of the distributary channel-floodplain of the meandering river,and it mainly develops two types of subfacies,they are channel and flood plain,and two microfacies,they are point bar and arrested deposits on valley floor;Yan'an Formation is characterized by the sedimentary environment of braided river-lake delta,and it mainly develops three subfacies,they are delta plain,delta front,prodelta,and two microfacies,they are channel bar and interchannel area.According to geochemical characteristics,the ratio of the oxides in Zhiluo Formation indicates arid-hot climate in the main elements Ca/MgO?5.33?.SiO₂/Al₂O₃?5.05?.FeO/Fe₂O₃?0.39?.and trace elements Sr/Ba?0.42?.Sr/Cu?36.32?.Cu/Zn?0.18?,while some ratios are positive anomaly,and the occurrence of extreme thermal events is presumed.The value of U/Th.V/Cr.Fe²⁺/Fe³⁺reflects the oxidation environment,and the ratio of the oxides in Yan'an Formation indicates warm-humid climate correspondingly in the main elements?0.27.4.15.0.37?and trace elements?0.16.4.57.0.26?,it reflects reducing environment.The relationship between sandstone and coal seams provides sedimentary support for further study of the coal accumulation regulation,and the climatic condition of wet and arid verifies the rationality of the global Jurassic climate,it corresponds the current climate.