Reconstruction Of Palaeovegetation And Palaeoenvironment Community Of Pinghu Formation In Xihu Sag By Geochemical Method

On the scale of global climate change,the 34 Ma Eocene-Oligocene climate transition accompanied the emergence of the Antarctic continent ice sheet,marking the transformation of the earth from an ice-free"greenhouse earth"at the two poles to an ice house earth with ice in the Antarctic.Coal-measure source rocks are widely developed in the sedimentary strata of the Pinghu Formation in the Xihu Sag.The mild paleoclimatic conditions promoted the growth and development of paleo-vegetation.The luxuriant paleovegetation provided a basic source for the enrichment of organic matter in the Xihu Sag.Based on the combination of element geochemistry and paleontology methods,the paleoenvironment and paleovegetation communities of the Pinghu Formation in the Xihu Sag were reconstructed.The source rocks of the Pinghu Formation are characterized by dark mudstone,carbonaceous mudstone and a large number of thin coal interbeds.The geochemical characteristics are mainly of type Ⅱ₂-Ⅲ,reaching mature stages,with the most developed source rocks in the middle section.Saturated hydrocarbon fractions have the characteristics of high Pr/Ph value,high abundance of diterpenoids,low abundance hoopane homologues,and low gammacerane content;in the aromatic hydrocarbon fractions,phenanthrene series compounds dominate,and naphthalene series and The distribution characteristics of dibenzofuran series are the second.At the same time,compounds such as catalene,pyrene and fluoranthene,which have the significance of importing terrestrial higher plants,have been detected.The overall deposition period of the Pinghu Formation was a weakly oxidized-oxidized depositional environment,and the oxidation gradually increased from south to north;the salinity of the water bodies was mostly freshwater-brackish water environment.The large-scale transgression in the middle and late stages of Pinghu Formation sedimentation caused fluctuations in the salinity and redox properties of the water body.The sedimentary organic matter of the Pinghu Formation is dominated by higher plant input.The closer to the low-potential area in the center of the depression,the more input from lower aquatic organisms.Pollen data and paleovegetation parameters AGI,al-AGI,aro-AGI,ar-AGI（Ole）,t-AGI-1,t-AGI-2,etc.reflect that the input of angiosperms has declined along with the sedimentation of the Pinghu Formation The increasing input of gymnosperms supports the view that the paleoclimate has gradually become cold and wet.Plants dominated by coniferous species such as Pinus are grown at relatively high altitudes,with angiosperm species such as Juglans and Quercus among them,and a small amount of ferns such as Polypodiaceae grows in the moist areas of the forest at relatively low altitudes.plant.The northern Kongqueting area is characterized by abundant gymnosperms and angiosperms.The development of angiosperms in the Wuyunting and Baoyunting areas in the central area has decreased,and the input of gymnosperms has a clear advantage.Fern spores in the southern Pinghu area have begun to appear frequently.From north to south,the spread of paleovegetation showed a gradual transition trend from tall gymnosperms to low ferns.As the strata deposits,the temperature and humidity gradually decrease,and the climate gradually changes from tropical-subtropical heat and humidity to temperature and humidity in the temperate zone.Comparing with the global climate change trend,it is found that the climate change in the Xihu Sag in the Mid-Late Eocene has a good correspondence with it.China may also be affected by global tectonic evolution such as the sub-Tethys regression and the uplift of the Qinghai-Tibet Plateau.