Establishment Of Middle Jurassic Chawula Group And Its Geological Effect In Tanggula Moutain South Qiangtang Basin

The south Qiangtang basin situates in the central part of the Qinghai-Tibetplateau. Middle Juassic marine sedimentary strata are widespread in the basin.Previously, south and north Qiangtang basins are regarded as one basin, neglecting theboundary between Yunnan-Tibet region and south China region as well as the latePaleozoic Gondwana and Eurasia continent. Correspondingly the strata successionsand frameworks of the Juassic were confusion.Based on middle Jurassic distribution, rock and main biotic assemblage,depositional sequence and sedimentary construction, this study divides the Qiangtangarea into two twice-order stratigraphic regions: north Qiangtang-Changdu-Lanpingregion and south Qiangtang-Baoshan region.The north Qiangtang-Changdu-Lanpingregion is composed of three third-order stratigraphic sub-regions: the north Qiangtang,south Changdu and Lanping –Simao. The Qiangtang-Baoshan region are recognizedas other three third-order stratigraphic regions: Duoma-Anduo,Suoxian-Zuogong andBaoshan.In the Suoxian-Zuogong region, the Chawula Group has been established by thiswork, and subdivided into three formations. In ascending order they are the ChawusiFormation, Longren Formation and Jiabulong Formation. They are referred to as theBajocina-Callovian of Middle Jurassic in age. The Chawula Group is a group of finedetrital rock and carbonate, coarse grained arkosic sandstone, gypsum brecciamudstone and white fibrous gypsum. The Longren Formation contains abundantfossils of bivalve, brachiopoda and starfish, in which the bivalve is a typical B-Hassemblage. It gives a age of Bathonian, Middle Jurassic. The Chawula Group isunderlain angular unconformablly by the pre-Carboniferous base metamorgraphicrock system , and overlain by the upper Niubao Formation Paleocene-Eocene.Thestratigraphy is of important significances for regional structures and paleogeography.The Chawula Group consists of a group of detrital and carbonate rocks.According to rock assembly and carbonate micro-facies, as well as fossils, three kindsof sedimentary environments can be recognized. They are littoral, shallow –tidal flatand inner ramp. Two sedimentary cyles have been distinguished, which constitues aingression sequence of gradual ascending in relative sea-level. This study haspreliminarily established the curve of relative sea-level changes of Chawula Group,which is correlated with the curve of sea-level changes in Tibet and the world. It issuggested that the tectonic movements are the primary factor that resulted in thelong-term sea-level change of Tibet Tethys region, and the climate is the secondaryfactor.