STRATIGRAPHIC DIVISION AND LITHOSTRATIGRAPHIC CHARACTERIZATION OF THE ORDOVICIAN SYSTEM IN THE TAZHONG AREA OF THE TARIM BASIN

Ordovician strata are widely distributed in the Tazhong Area. The goal of study is to identify the Cambrian-Ordovician boundary in this area and to determine the age and lithostratigraphic relationship among the Yingshan, Yijianfang, and Lianglitage formations, on the basis of biostratigraphic and lithostratigraphic evidence. Research results are as follows:1) The Yingshan Formation is regarded as Lower-Middle Ordovician, the Yijianfang Formation as Middle Ordovician, and the Lianglitage Formation as Upper Ordovician.2) On the basis of lithostratigraphic data, the Ordovician System in this area is divided into three series and four formations. The Lianglitage Formation is further subdivided into three lithostratigraphic units, and the Yingshan Formation is divided into four lithostratigraphic units. 3) The spatial distribution of the Lianglitage and Yingshan formations is characterized. An unconformity is identified between these two formations.4) The contact between the Lower Ordovician Penglaiba Formation and the Upper Cambrian Qiulitager Formation is a disconformity. 5) The Sangtamu Formation is separated from the overlying strata by an angular unconformity, and the contact between the Sangtamu Formation and the underlying Lianglitage Formation is conformable at some sections but disconformable at others.