Study On The Middle Paleozoic-Devonian Sedimentary Facies And Environment In Rongma, Tibet

The progress in the study of the Paleozoic-Mesozoic sedimentation and paleontology of both north and south Qiangtang basin supported that it’s the ChabuChasang-Shuanghu fault that divides the Qiangtang basin in two. The recent found of the Ordovician Tashishan formation, Slurian Sanchagou formation and Devonian Changsheshan formation is the first found of the Early-Middle Paleozoic sedimental terrane found in the area. The study to their chronostratigraphic framwork and sedimentary environment has just began.Through the study to the Orthocerida found in the Tashishan upper member combined with detrital zircon age from the Tashishan lower member, the Tashishan formation was determined to be Middle Ordovician Series. With the study of the concurrent range of the Graptolithina found in the Sanchagou formation, the author determined the Sanchagou formation to be lower Silurian.The Tashishan formation is divided into two members based on the study on the characteristics of the sediment and fossil of two profiles-the Wenquangou profile(PM008) and the Wenqangoukou profile(PM001). The lower member is terrigenous clastic rock with out any body fossil, and the upper member is crystalline limestone and detrital limestone with Orthocerida and crinoid pluricolumnals. Based on the characteristics of lithology associations, sedimentary structures and the fossils, the Tashishan formation can be plotted into two sedimentary facies area: the shoreface clastic area and the shelf area. The shoreface area is made of 3 subfacies: the foreshore, the shoreface and the transitional zone. The shelf area has 1 subfacies: the offshore. The Sanchagou formation is plotted to the outer shelf subfacies. The Changsheshan formation was deposited at the inner shelf to begin with, and ended up at the outer shelf.Based on the vertical evolution of the sedimentary facies, the sedimentary environment of Tashishan formation evolved from coastal area to the shelf during the middle Ordovician Epoch, and made a complete retrograding sequence. The Changsheshan formation reveals that same kind of sequence happened in Devonian by shifting from inner shelf to the outtershelf.