Stratigraphic Records In The Jurassic-Cretaceous Transition Beds Of Dongla Section, Southern Tibet

The determination of the Jurassic-Cretaceous boundary is a worldwide problem. The Jurassic-Cretaceous strata are exposed in the Dongla section, Langkazi, southern Tibet.The Dongla section can be divided into Weimei, Sangxiu and Jabula formations from the bottom to top. The samples which has been collected in this research are used for the nannofossil and geochemistry study. The samples of volcanic rocks in the Sangxiu Formation have also been collected, which are used for the study of the isotope chronology.For the study of biostratigraphy, a large number of calcareous nannofossils have been found in the top of the Sangxiu formation for the first time. The species diversity of the calcareous nannofossils in Dongla section is lower by the identification. The nannofossils which have been identified include Cyclagelosphaera margerelii,Watznaueria barnesae, Watznaueria manivitae, Watznaueria biporta, Watznaueria fossacincta, Watznaueria ovata, Watznaueria Britannica, Diazomatolithus lehmanii, and Biscutum constans. They belong to nine species of four genera, in which Watznaueria is the dominant genus.Based on the SHRIMP Zircon U-Pb Dating, the age of the andesite is 138 Ma, it belongs to lower Valanginian in Lower Cretaceous and provides an evidence for the determination of the Jurassic-Cretaceous boundary.To deal with the samples from the Dongla section, a geochemical analyses has been used. Based on the REE characters and the figure of log(K2O/Na2O) and Si O2, it can be found that the samples from Dongla section show typical REE characters of sedimentary rock and passive continental margin. Based on Ca O/(Fe+Ca O),(Mg O/Al2O3)×100 andδ Ce, it can be found that the sedimentary environment is marine facies, the ancient ocean is reductive environment, and the salinity of the water is low.