Sedimentary Characteristics And Paleoclimate Of The Late Cretaceous Tangbian Formation Of The Xinjiang Basin In Jiangxi Province,SE China

The Xinjiang Basin is a red continental fault basin generated by crustal extension in the middle part of the Gan-hang Tectonic Belt located between the Yangtze and Cathaysia blocks.The strata is the material foundation of Danxia landform and mainly composed of a red clastic rock series of continent.Based on measurement and description of the outcrop sections,statistics for pebbles in the field,identification of rock thin sections and the statistics for clastic particle,carbon and oxygen isotopic analysis of pedogenic carbonates.This paper implemented a detailed research on sedimentary characteristics,provenance and paleoclimate of Late Cretaceous Tangbian Formation,which obtained the following research achievements:Fluvial-lacustrine-aeolian deposits from the Tangbian Formation,in the Xinjiang Basin.It developed braided fluvial facies in early stage,included braided channel and braided bar;in middle stage,it developed lacustrine facies facies;as climate aridification in late stage,it developed aeolian facies.The component of gravel was complex,the sorting of the gravels is poor and the gravels have the characteristics of low maturity.The rose diagram indicated that the paleocurrent was mainly from northeast,north,east and north of the basin.The statistics of sandstone clastic particles indicated that,quartz content was the highest,followed by rock fragment and feldspar was the lowest.C_m=3.72,which indicated that,ingredient maturity was medium,separation rounding was above average and in medium maturity degree.According to Dickinson diagram points,sediments were mainly derived from the recycling orogeny provenance area,subduction complex rock provided abundant provenance with ingredients of arc orogeny provenance.The Precambrian strata of low-level metamorphic rocks series and the Yanshanian movement mid-term acid volcano--intrusive complex and Upper Cretaceous stratum on both sides of the basin period provide massive provenances for Tangbian Formation deposits.The isotope composition of pedogenic carbonates of paleosols is an effectiveapproach to quantitatively reconstruct atmospheric CO₂ partial pressure.Stable isotope analysis of pedogenic carbonates from the Tangbian Formation was conducted to understand the paleoclimatic conditions.The carbon isotope compositions range from-4.30‰ to-2.10‰?PDB?with an average of-2.84‰ and the oxygen isotope compositions range from-6.62‰ to-1.14‰?PDB?with an average of-3.62‰.The CO₂ partial pressure was calculated to be from782 to 1420 ppmv based on Cerling's formula with an average of 1181 ppmv during the Campanian Stage?⁷5 Ma?,which is about 2⁴ times more than that of the present-day level.Therefore,a marked high value of CO₂ partial pressure probably occurred during the late Campanian Stage,possibly reflecting the undulation of pCO₂ during the Late Cretaceous time.