Base level change and depositional filling response of Jurassic in the Qiangtang Basin of Tibet

The base level during the deposition of Jurassic in the Qiangtang Basin shows a complete cycle from rising to falling. The base level change is closely connected with tectonic evolution of the basin,especially connected with Bangonghu-Nujiang ocean evolution process in the formation and evolution of the basin. It is also affected by climate. The Jurassic strata correspond to a long-term base level cycle sequence. The sequence is in fact a non-complete symmetrical cycle,consisting of rising hemicycle and falling hemicycle. It can be divided into 6 intermediate-term base level cycle sequences,including 2 carbonate sequences,3 mixture sedimentary sequences of carbonate and clastic rocks and one clastic sedimentary sequence. Depositional filling characteristics during base level change show that Bangonghu-Nujiang ocean spreads in Toarcian-Bajocian ages,and is at the height of spreading of Bangonghu-Nujiang ocean in Bathonian-Oxfordian ages. In that process,sea area became smaller because of the dry climate. Eventually,marine depositional filling is ended with the subduction and collision of Bangonghu-Nujiang ocean.