The Cretaceous Paleoenvironment Evolution Of The Taohe Basin,Xichuan,Henan Province

Cretaceous is a typical representative of extreme greenhouse climate in geological history,and it is also an era of frequent volcanoes,large-scale continental drift and important tectonic changes.Due to the accretion of mid ocean ridges and the expansion of new oceans,most of the areas were submerged by seawater during the Cretaceous period,and marine strata were mainly developed in the world.However,East Asia still maintains the continental environment,especially in China,where the continental strata are well exposed,well developed and highly studied,mainly distributed in the North China platform.These continental sediments can provide important evidence for reconstructing the global Cretaceous paleoenvironmental changes and have great research significance.The late Cretaceous basins in Southwest Henan Province are located in the joint part of Qinling,North China and Yangtze plate tectonic units.They are the main basin controlling faults in the late Cretaceous basin,with long-term complex dynamic changes and structural deformation characteristics.The Cretaceous strata of Taohe basin in Xichuan County are relatively well developed,containing a large number of dinosaur bones and dinosaur egg fossils.Predecessors have made in-depth research on lithofacies paleogeography through sedimentary strata,structural characteristics and dinosaur egg fossils,but the exploration on Cretaceous paleoclimate of Taohe basin has not been in-depth.Therefore,this paper takes the Taohe Township PM03 section as the research object,using geochemical analysis as the main method and petrography as the auxiliary research method,through the carbonate carbon and oxygen isotope composition,total organic matter content,ferrous ion content and major and trace element characteristics of the section sediment samples,to reconstruct the late Cretaceous paleoclimate evolution process in the study area,It provides new data for the study of continental Cretaceous in China.Based on the geochemical analysis of the sediment samples of PM03 section in Taohe basin,the following conclusions are obtained:1.The content of SiO₂ in sediments is the highest,ranging from 11.06%to 61.44%;The content of Cao was high,ranging from 6.54%to 49.85%;The content of alumina is 1.15%to 15.17%;The content of Fe₂O₃ is 0.53%to 5.97%;The content of K₂O ranged from 0.21%to 3.17%;The content of Mg O ranged from 0.85%to 18.77%;The content of Na₂O is 0.01%to 1.88%.The NASC normalized values of Cl,Co,Cu and Zn in trace elements are less than 1,showing a significant deficit;The contents of P,Ba,Cr,La,Mn and Nd are high,and they are enriched in the profile.The carbon isotopic composition of carbonate is between-5.82‰to 2.92‰(PDB),and the oxygen isotopic composition is between-9.57‰to-2.37‰(PDB);The content of calcium carbonate ranged from 22.97%to 72.99%;The TOC content ranged from 0.02%to 1.00%;The content of ferrous ion ranged from 0.57 to 2.14?g/kg.2.The salinity index Sr content,Sr/Ba,Rb/K and carbon and oxygen isotopic compositions indicate that the sediments were formed in terrestrial freshwater environment;The redox indexes V/Cr,Ni/Co and Cu/Zn all indicate that the sedimentary environment of the profile sediments is weak oxidation environment;The values of ICV,CIA,Na/K,Ba/Sr and Na₂O/K₂O-CIA diagram indicate that the sedimentary area belongs to medium to strong weathering environment,and the overall weathering degree is medium,indicating that the water body activity in the sedimentary environment is strong;The low value of Drought Index C,the high value of Sr/Cu and the bivariate diagram of climate indicate that the whole study area belongs to arid environmental conditions.3.The Gaogou formation of PM03 profile is alluvial fan meandering fluvial facies and channel floodplain microfacies;Majiacun formation is meandering river facies and channel floodplain microfacies;The Sigou formation is braided fluvial facies,and channel floodplain microfacies occur alternately.The paleoclimate experienced a cycle of extremely hot and dry warm wet rainy semi-arid arid.