Sedimentary Characteristics And Paleoclimate Of The Zhoutian Formation In The Guangfeng Basin Of Jiangxi Province

The Cretaceous red beds are widespread in southeastern China.The sedimentary environments of the red beds are controversial,and the quantitative study of paleoclimate is relatively insufficient.Abundant paleosols are observed in the Late Cretaceous Zhoutian Formation of the Guangfeng Basin,which is of great significance for reconstructing paleoclimate and studying sedimentary environment of the Cretaceous red beds.In this study,the sedimentary environments and paleoclimatic characteristics of the late Cretaceous Zhoutian Formation were studied by means of outcrop measurements,pebble counting,sandstone stone composition and grain size analysis,pedogenic carbonate carbon and oxygen isotopes and elements analysis.The red beds of Zhoutian Formation have small grain size and develop normal grain sequence bedding,reverse grain sequence bedding,horizontal bedding.According to sediment composition and sedimentary structural characteristics,lithofacies units are divided,including non-sedimentary structural conglomerate facies,normal sequence bedding conglomerate facies,inverse sequence bedding conglomerate facies,sandstone facies and paleosol facies.The clastic composition of sandstone samples show that the content of quartz range from 40%to 65%of the clastic composition,is the highest content,the content of feldspar range from 6%to 25%,is the minimum content,and range from 8%to 29%of the total grains,the values of climatic index?F/Q?range from 0.05 to 0.76,the values of tectonic index?L/Q?range from 0.38 to 1.66.The results of grain size analysis of sandstone show that the sorting is poor,and the roundness is medium,which generally reflects the medium structural maturity.The results of pedogenic carbonate carbon and oxygen isotope measurements of the samples from Zhoutian Formation show that the value of ¹³C ranges from5.987 to 2.693,with an average value of 2.84.The values of¹⁸O ranged from-11.684 to 8.509,with an average of 10.173,and pCO₂ ranged from 482 ppmv to 1207 ppmv,with an average of 792ppmv,about 2 to 3 times the current pCO₂ concentration.The values of CIA ranged from60 to 83,the values of CIW ranged from 63.11 to 92.23,the values of PIA ranged from58.7 7to 90.16,and the values of ICV ranged from 1.76 to 15.87,which are calculated by major elements.The Paleoprecipitation ranges from 275.23 mm to 821.19 mm,with an average value of 590.52 mm and a paleotemperature range from 12.35 degrees to 13.28degrees,with an average value of 12.94 degerees.Therefore,the sedimentary environment of Zhoutian Formation in the early Late Cretaceous of Guangfeng Basin,was dominated by flood plain of meandering river and dry salt lake,and the paleoclimate was dominated by drought.After sedimentation,the sediments may be exposed to the atmosphere for a long time and undergo strong pedogenesis,resulting in the development of red viscous and calcareous deposits in the red beds of the study area.By comparing with the adjacent areas,it is considered that the Cretaceous red beds in southeastern China may be mainly deposited in the dry oxidation environment of continental surface.