Sedimentary Characteristics And Geochemistry Of Late Cretaceous Red Beds In Danxia Basin

The Danxia Basin is a near-diamond-shaped continental fault basin developed under an extensional tectonic setting during Late Mesozoic in South China.The red beds of Late Cretaceous Changba and Danxia formations are well preserved and outcropped in the basin,providing a good location to understand the Cretaceous greenhouse climate and terrestrial sedimentary processes in South China.In this study,depositional environments and geochemical characteristics of the Late Cretaceous Changba and Danxia formations were studied by observation of outcrop profiles,rock and mineral petrographic identification of samples,grain size analysis and major and trace element geochemical analysis.The Changba,Bazhai,Zhanglaofeng,and Baizhaiding profiles were selected for observation and sampling of red beds of the Changba and Danxia formations.The Changba Formation is generally purplish-red,and shows a grain-size change of coarse-fine-coarse-fine from bottom to top.Erosional bases,graded beddings,and cross-beddings are developed in the first and third members of the Changba Formation,while common latent holes,root traces,calcareous nodules,gray-green halos are developed in the second member and fourth member of the Changba Formation.The Danxia Formation is generally brown-red and brownish-red,and lithology is coarse-fine-coarse characterized from bottom to top.Parallel bedding,cross bedding,graded bedding and imbrication structure are developed in the Bazhai Member,which is similar to that in the Baizaiding Member,and large-scale cross-bedding is developed in the Jinshiyan Member.The overall grain size of the Danxia Formation is coarse,and the rock cementation is good,so the Danxia landform is mainly developed in this formation.Field sedimentary characteristics,combined with sample section observation and grain size analysis indicate that the Changba Formation has medium sorting,while the Jinshiyan Member of the Danxia Formation is well sorted,indicating that the red beds of the Changba and Danxia formations were mainly formed in a fluvial and lacustrine sedimentary environment,but the latter also includes alluvial fans and aeolian environments.The samples section identification results were consistent with the triangular projection results of the major element（Fe₂O₃+Mg O）-Na₂O-K₂O of the Changba and Danxia formations,indicating that the rock samples were mainly feldspar sandstone.The geochemistry rations and projection of major elements,trace elements and rare earth elements of rock samples have good indicative significance of paleoenvironment,paleoclimate and source.The Th/U and&U curves indicate that the sedimentation period of the Changba and Danxia formations is an oxidation environment,and the oxidation environment of the first member to second member of the Changba Formation continued to weaken,increased in the second member to third member,and the fluctuation continued to weaken until the Baizhaiding Member of the Danxia Formation.Combined with the Sr/Ba and Sr/Cu curves,the paleoclimatic drought degree was the highest from the late first member to the early second member of the Changba Formation,and the climate changed to humid general during the fourth member of the Changba Formation to the Danxia Formation.Combined with the observation of sedimentary facies,the Jinshiyan member of the Danxia Formation also had a relatively dry period.CIA,CIW,PIA,WIP,ICV curve and paleotemperature curve indicate that the weathering and temperature in the first member of the Changba Formation were relatively stable,and the weathering weakened and the temperature fluctuation decreased in the second member to third member,the weathering continued to strengthen,the temperature fluctuation increased until the Jinshiyan Member of the Danxia Formation,and the weathering weakened and the temperature decreased in the Baizhaiding Member of the Danxia Formation.The diagram of the source identification of Al₂O₃/Si O₂-（Fe₂O₃+Mg O）and the triangular projection of Th-Co-Zr/10,La-Th-Sc and Th-Sc-Zr/10 indicated that the source properties of the sediments in the Changba and Danxia formations were continental environments,the Th/U-Th and Th/Sc-Zr/Sc projections indicated sediment recycle,and the V-Ni-4La and La/Th-Hf projections indicated that the rock samples of the Changba and Danxia formations were closer to the felsic rocks and the samples of the Changba Formation had ancient sediments added.