Sedimentary Characteristics And Paleoclimate Of The Late Cretaceous Zhoutian Formation In The Huichang Basin Of Jiangxi Province

The Huichang Basin is located in the southern part of Jiangxi Province,which is one of the Cretaceous continental faulted basins in Southeast China.Research on the sedimentology of the late Cretaceous red beds in these red basins is significant for the understanding of the Cretaceous greenhouse paleoclimate.In this study,the paleoenvironment,paleoclimate and provenance of the Zhoutian Formation were studied by field observation,and microscope observation,grain size analysis,detrital grain counting,geochemical element analysis and Chroma characteristics analysis of samples.The red beds profile of the Zhoutian Formation can be divided into two parts based on lithological characteristics.The lithology of the lower section is mainly composed of purple-red-brick red medium-thick fine sandstone intercalated with thin mudstone.The lithology of the upper section is dominated by purple,khaki,gray-green and a few other variegated mudstones.The mudstone layer is thick,up to tens of meters.There is less sandstone in this section,and it is sandwiched in the mudstone in the form of thin layers.Horizontal bedding is developed in the rock stratum,and wavy bedding,oblique bedding and trace fossils are also seen,and wave marks can be seen in some layers.The sandstone samples have the highest quartz content,followed by feldspar,and the least detritus content.Quartz is mainly single crystal quartz,occasionally vein quartz.The feldspar is dominated by potassium feldspar,with a small amount of plagioclase distributed,and the surface of the feldspar is seriously clayed and sericitized.The detritus is dominated by sedimentary rock detritus.Mainly iron and calcium cement.Microscopic rock and mineral identification and the triangular projection of major elements（（Fe₂O₃+Mg O）-Na₂O-K₂O）indicate that the Zhoutian Formation sandstone is mainly feldspar sandstone.The probability accumulation curve consists of two stages:jumping and levitating.Rolling Component Missing and two jumping components,indicating that the hydrodynamic force during the depositional process is generally weak and affected by the transformation of lake water waves.In general,the sandstone in the Zhoutian Formation in the study area is fine particle size and well-sorting.The results of geochemical element analysis and colorimetric analysis of the samples show that the source systems of clastic rocks are diverse,and the parent rocks are dominated by continental mature multi-cycle quartz sources and felsic rocks,accompanied by the addition of some ancient sediment sources.The field observation,sample microscope observation and counting,sample element geochemistry rations including&U（0.64～1.01）,V（V+Ni）（0.51～0.86）,andδCe（0.61-1.11）collectively indicate that the Zhoutian Formation was generally deposited in a weak oxidization depositional environment.The fluctuation of the&U curve increased and the fluctuation of the Th/U curve decreased,indicating that the oxidative environment of the upper Zhoutian Formation was weaker than that of the lower section.Multiple chemical weathering indices（CIA,CIW,PIA and WIP）calculated from geochemical ratios in mudstones and derived paleotemperature estimates.In the lower section,the paleotemperature is 5.3℃～17.11℃,and the average paleotemperature is 11.83℃,the chemical weathering indices of the CIA,CIW and PIA are all in the low range,the WIP,Sr/Ba ratio,Sr/Cu ratio is in the high range,and the increased fluctuation of the chromaticity“L^*”value and the decreased fluctuation of the“a^*”value indicates a relatively dry and cold condition.In the upper section,the paleotemperature is 9.83℃～19.80℃,and the average paleotemperature is 16.62℃,the chemical weathering indices of the CIA,CIW and PIA are all in the high range,the WIP,Sr/Ba ratio,Sr/Cu ratio is in the low range,and the decreased fluctuation of the chromaticity“L^*”value and the increased fluctuation of the“a^*”value indicates a relatively hot and cold humid.As a whole,the Zhoutian Formation was generally deposited in a weak oxidization depositional environment,and the oxidation degree of the upper section was weaker than the lower section.the paleoclimate might have changed from dry and cold to hot and humid from the early to late depositional stage of the Zhoutian Formation.