Study On The Sedimentary Environment Of Sema Section In Nagarze Area Of Tibet

The determination of Jurassic/Cretaceous(J/K)boundary is an unsolved global geoscience problem.The Marine strata about J/K are well outcropped in Tibet,China,and a set of volcanic strata is outcropped in Nagarze,southern Tibet,which provides a possibility for the study of J/K boundary.Through the analysis of stratigraphy,petrology,isotope chronology and elemental geochemistry of Sema section in Nagarze,and the paleooceanographic environment of this period is restored.The determination of stratigraphic age of Sangxiu Formation can provide reliable age evidence for the determination of J/K boundary in Tibet,and also provide basis for the determination of sedimentary time and sedimentary environment evolution of Cretaceous strata in Tibet.The Sangxiu Formation in Sema section is a group which includes clastic sedimentary rocks,pyroclastic rocks,the sedimentary rocks include sandstone,siltstone,mudstone and shale,grain from coarse to fine along the stratum,which reflect the transgression,horizontal bedding is developed;the pyroclastic rocks are tuff.And observing rock chips under the microscope,the main components are quartz,feldspar,etc,clastic particles are sub-angular,moderately rounded,sorted well,elongated directional distribution,which show that the study area is shallow sea sediments with weak hydropower.U-pb isotopic dating of zircons from tuff intercalation in the 2nd,16 th and 20 th layers of the Sangxiu Formation in Sema section shows that the weighted average ages are 140.8±1.9Ma,140.7±1.9Ma and 140.0±1.3Ma.Considering the strata with certain thickness in the lower sediments,it is inferred that the bottom age of the Sangxiu Formation is close to the internationally recognized J/K boundary 145 Ma age.Based on the index elements of Al,Ti,Zr,Th,etc and discriminant diagrams of CIA,ICV,HF-La/Th,La/SC-Co/Th,etc,it is concluded that the parent rocks in the study area are mainly felsic rocks,which subject to moderate weathering,combining with the evolution of the sedimentary environment in the Early Cretaceous,the Sangxiu Formation originates from oceanic island arc structure.By analyzing the ratios of elements Si/Al,K/Na,Zr/Rb,V/Cr,δCe/δEu,etc,it can be concluded that the provenance supply stability in the study area,the shallow sea what gradually deep water body,weak hydrodynamics,reduced paleosalinity,accelerated deposition rate and warm-humid climate.The change of sea level leads to the alternation of weak oxidation-weak reduction conditions of Sangxiu Formation.Figure 33;Table 20;Reference 67...