Study On Genetic Mechanism Of Limestone-marl Alternations Of Devonian Tuqiaozi Fomation In Longmenshan Area,Sichuan

The rhythm layer has always been an important object for stratigraphers and paleoclimatologists.It is also an important carrier for the study of high resolution cycleostratigraphy.The study on limestone-marl alternations is mainly concentrated in South China and the main area of research is differential diagenesis.However,comprehensive research of limestone-marl alternations is relatively rare.On both sides of maximum flooding surface of Devonian Tuqiaozi Formation in Longmenshan Area,there are three sets of limestone-marl alternations with obvious characteristics.By profile survey,slice observation and geochemical features,this paper focuses on the formative factors of limestone-marl alternations of Tuqiaozi Foamation.Based on the measured data of field profile and sedimentary characteristics,there are developed three mainly types of sedimentary facies in the Tuqiaozi Formation: carbonate platform,carbonate gentle slope and basin facies.The three types appear alternately in the sedimentary facies evolution of Tuqiaozi Formation.Based on the analysis of major elements,such as SiO₂,Fe₂O₃,FeO,Al₂O₃,TiO₂,CaO,MgO,K₂O,Na₂O,MnO and P₂O₅,the chemical composition of the major elements shows a basically consistent trend both in limestone and marl.The proportions of CaO,SiO₂ and Al₂O₃ are large,and the other major elements are very low.Based on the analysis of trace elements of V,Cr,Co,Ni,Cu,Zn,As,Rb,Sr,Zr,Cd,Ba,Th and U,it shows that the compositions and proportions of trace elements have a great similarities both in limestone and marl.Through analysis of correlation between acid insoluble residues and burned residues and elements in limestone and marl,it is found that contents of SiO₂,Al₂O₃,K₂O,MnO,TiO₂ and Cr are highly correlated with acid insoluble content.In particular,the correlation between SiO₂ and acid insoluble residues is up to 0.93.On the basis of the above analyses,the source of the elements is analyzed,which provides a reliable elements basis for the study on limestone-marl alternations.Based on the analyses of paleoxygenation（U/Th）,palaeosalinity（Sr/Ba）,paleo-water depth（MnO）and paleoclimate（Sr/Cu）,this paper studies the paleoenvironment and paleoclimate of limestone-marl alternations forming.The limestone-marl alternations were formed in such depositional environment of high system tract marine with poor oxygen under hot climate.Paleoclimate and differential diagenesis are the main causes of the formation of limestone-marl alternations.From macro point of view,the rhythm and wave of paleoclimate controls the paleoenvironment and geochemical characteristics of limestone-marl alternations.From micro point of view,the differential diagenesis controls and further affects the differentiation and rhythmicity of geochemical characteristics of limestone-marl alternations.