Cenozoic basin sedimentary record and basin-controlling mechanism of northern Nyima, Tibet

On the basis of field investigation and stratigraphic correlation, the Cenozoic strata of the northern Nyima basin are classified as Niubao Formation. According to an analysis of rock assemblage and sedimentary features, fan delta facies, lacustrine facies and alluvial fan facies are identified in Niubao Formation in northern Nyima basin. The fan delta facies can be subdivided into fan delta front subfacies and pre-fan delta subfacies, whereas the lacustrine facies can be subdivided into semi-deep lake subfacies, shore-shallow lake subfacies and shore lake subfacies. It is believed that the sedimentary setting went through three evolution stages, i.e., initial period of basin formation, expansion period and shrinking period of the basin, The evolution stages of basin correspond to various sections of Niubao Formation. Stages of development of the basin are related to paleoclimate and tectonics in northern Nyima. It was not a single factor that controlled the basin development but a combined factor that affected all stages. Based on previous researches, the authors hold that the formation of basin began in the period from the end of Early Cretaceous to earliest Late Cretaceous. Based on the sedimentary characteristics that the margin facies is accompanied by semi-deep lake subfacies in the basin, and volcanic rock occurs on the bottom of Niubao Formation, it is concluded that Nyima basin has the characteristics of strike-slip fault systems.