Sedimentary Characteristics And Paleoclimatic Significance Of The Late Cretaceous Red Beds In Nanxiong Basin Of Guangdong Province

The terrestrial Cretaceous red beds are widely distributed in the Nanxiong Basin,northeastern Guangdong Province.This basin is a typical continental faulted red basin in Southeast China.In the study area,much progress has been made in stratigraphic division and paleontology,but the study of paleosol sequence and paleoclimate is relatively insufficient.This paper studied the sedimentary characteristics,paleoenvironment and Paleoclimate of the Cretaceous main field profile in Nanxiong basin in detail by means of field outcrop profile observation,gravel statistics,grain size analysis of sandstone samples and paleoclimate alternative index analysis.The applicability of the climate alternative index in the study area was tested.The Zhutian section includes Dafeng formation,main field formation and Zhenshui formation.The Dafeng formation is characterized by the development of brownish red conglomerate characterized by erosion erosion surface,grain sequence bedding,parallel bedding and imbricate structure.The Zhutian formation is characterized by the development of thick purplish red mudstone,argillaceous siltstone and paleosol.The Zhenshui formation is characterized by the development of purplish red conglomerate with erosion erosion surface and grain sequence bedding structures,including intercalated medium-thin mudstone,argillaceous siltstone and paleosoil.Viscous layers,calcium slabs,and calcareous nodules are developed in ancient soil,and a large number of trace fossils and biologically disturbed structures were visible to the naked eye.The results of gravel statistics show that the gravel composition of Zhenshui formation is similar to that of Dafeng formation.T The average contents of sandstone and quartz are about 86%and 11%,respectively,while the contents of granite and limestone are relatively small.The sandstones of Zhutian section are mainly lithic quartz sandstones and feldspathic lithic sandstone,with medium sorting property,sub angular and sub circular roundness.The cement is mainly siliceous or calcareous,and the content of ferruginous is relatively small.In the particle size analysis,the probability cumulative curve is in three-stage form and muti-segment.H*,?(CaCO₃)%,?_lfand TOC%of the climatic alternative indexes all showed the trend of first increasing and then decreasing with the depth of the profile.However L*,a*,b*,c*tend to decrease first and then increase.The correlation analysis of climate substitution index shows that c*,h*in paleosol are highly correlated with?(CaCO₃)%and have a good linear relationship.In the sandstone stratum,L*is highly negatively correlated with TOC%,a*?b*are highly positively correlated and linearly correlated with?(CaCO₃)%,?_lfis highly positively correlated with?(CaCO₃)%and has an excellent linear relationship.The sedimentary environment of Nanxiong basin in the late Cretaceous can be divided into three stages.The sedimentary stage of Dafeng formation is the alluvial fan sedimentary environment dominated by river development.The sedimentary stage of Zhutian formation is the flood plain or dry salt lake environment.The sedimentary stage of Zhenshui formation is alternately affected by the alluvial fan environment and flood plain sedimentary environment.The analysis of climate substitution index shows that the paleoclimate of the Zhutian section in the study area presents the trend of semi humid-arid-semi arid fluctuation as a whole,which is characterized by wet heat in the early period,dry heat in the middle period,and humid heat in the later period.In general,it is revealed that there were climate fluctuations in the late Cretaceous,one of which peaked in the sedimentary period of the Zhutian formation.