Study On Sedimentary Environments Of The Late Cretaceous Danxia Formation In Mt. Danxiashan Of Northern Guangdong

Mt.Danxiashan is located in the north of the Danxia Basin,Shaoguan City,Guangdong Province,which is famous for well development of spectacular Danxia landscapes.The Danxia Basin is one of the Cretaceous continental faulted basins in Southeast China.Much progress has been achieved in stratigraphic division and correlation,paleontology and geomorphology.However,sedimentary environments of Danxia Formation remain in debate.In this study,the paleoenvironment,paleoclimate and provenance of Danxia Formation are studied by field observation,and microscope observation,detrital grain counting,grain size analysis,environmental magnetism,geochemical analysis of sandstone and mudstone samples.The Bazhai Member of the Danxia Formation is mainly characterized by the development of erosional bases,imbrication structure,graded bedding and parallel bedding.The gravel size changes greatly and the sorting is poor.The Jinshi Rock Member is mainly composed of fine-grained lithic arkose,intercalated with conglomerate and paleosol units,which is featured by development of large-scale tabular and wedge-shaped cross beddings,good sorting,and cemented mainly by calcium and iron oxide minerals.The Baizhai Top Member is mainly composed of conglomerate and sandstone beds sharing similar sedimentary structures with the Bazhai Member.The pebble counting results of conglomerates of the Bazhai Member show pebbles consist of quartzite,limestone,sandstone.The gravels are poorly sorted and medium rounded.The average grain size(M_Z)of the sandstone is concentrated from 45.43?m to 162.93?m,and the frequency curve is normal distribution.Under the scanning electron microscope,the sand grains are loosely cemented with abundant intergranular pores are developed,and the disc-shaped pit,crescent shaped pit and ground glass surface can be seen on the surface of the quartz grains.The chemical alteration index(CIA*)of sandstone and mudstone samples ranges from58.70 to 75.71 with an average of 67.36,indicating an overall low degree of chemical weathering.It reflects dry climate conditions so that feldspars could be preserved in a great quantity.The?Ce _N values are from 0.83 to 1.46 with an average value of 1.00,reflecting oxidizing environments.The total?REE of samples changes from 23.22×10^-6 to 258.96×10^-6,and the average value of LREE/HREE is 9.05.It can be seen that the differentiation of light and heavy rare earth elements is obvious,and there are also Ce element enrichment and relatively obvious negative Eu anomalies.The geochemical plotting diagrams show that the source area might be dominated by passive continental margin as well as active continental margin,indicating a complex sediment source.The trace element plotting illustrates that the source rocks were dominated by felsic rocks.The original sediments of the parent rock were mainly derived from the Upper Crust,and there is a certain degree of sediment recycle.Therefore,the red beds of the Danxia Formation were produced in superaqueous dry and oxidation sedimentary environments in the background of the Cretaceous greenhouse climate.Due to paleoclimatic fluctuations,lithological changes occurred in the Danxia Formation.The conglomerates of the early(Bazhai Member)and late periods(Baizhai Top Member)of Danxia Formation were mainly deposited in river-dominated alluvial fan deposition systems,while the middle period(Jinshi Rock Member)was probably generated in a desert depositional system,which was mainly composed of aeolian sand dunes.