Sedimentary Characteristics And Paleoclimate Of The Changba Formation In The Danxia Basin,shaoguan City,Guangdong Province,South China

The Danxia Basin is located in Renhua County at the border of Hunan,Jiangxi and Guangdong provinces.It is a faulted basin developed under the crustal extension tectonic setting in south China.The red beds in the basin are well exposed and the paleosol is widely developed,which is an ideal object for studying sedimentary environment and paleoclimate during Late Cretaceous.In this paper,the clastic composition,sedimentary characteristics,paleosol characteristics and paleoclimate of the Changba Formation in the Danxia Basin are studied through outcrop measurement and description,sandstone composition and grain size analysis,and paleosol carbon-oxygen isotope analysis.The measured profiles show that the Changba Formation is featured by the coarse-fine-coarse-fine grain changes.The Changba Formation is the sedimentary product of alluvial fan facies and meandering river facies.Alluvial fan facies developed in the first and third member,with relatively coarse grains,whereas the meandering river facies with fine sediments developed in the second and fourth member with welded paleosols.The pedogenic carbonates in the paleosol mainly includes brick-red,white carbonate nodules and depsition layers,with a small amount of light halo spots.The results indicate that the sedimentary environments of the Changba Formation changed in response to paleoclimate transformation.Statistics of sandstone clastic particles show that the content of the quartz is the highest,followed by feldspars,feldspar is the least,most of them are feldspathic lithic sandstone.The probability curve shows that the content of jumping component is 35%⁷0%,the sorting is medium,the suspension component is 15%⁴5%,the separation is poor.Some of the samples contained about 15%of the rolling component with medium deviation in sorting.The C-M diagram indicates medium sorting maturity,medium sorting and roundness preference,indicating medium structure maturity.Carbon and oxygen isotopes of pedogenic carbonate in the second and fourth members of the Changba Formation are measured.The results show that the change range of?¹³C values is small,ranging from-3.37‰to-1.36‰,with an average value of-2.19‰.The?¹⁸O value is between-10.11‰and-7.04‰,and the average value is-8.34‰.According to the empirical formula of Cerling?1999?,it is estimated that the p CO₂ in the deposition period of the Changba Formation in Late Cretaceous is between1001 to 1741 ppmv,with an average value of 1397 ppmv,which is about 4 times of the atmospheric CO₂ concentration.The result show that that concentration of atmospheric CO₂ in Cretaceous is decreasing in general,and there is a high concentration of atmospheric CO₂ in Cenomanian-Turonian Stage indicating climate fluctuation in late Cretaceous.The estimation results of paleoprecipitation and paleotemperature show that the paleosol of the Changba Formation has less precipitation and arid climate.However,under the overall arid paleoclimate background,the changes of sediment compositions and sedimentary structures in the four lithologic members of the Changba Formation indicate that there were obvious fluctuations in paleoclimate.