Stratigraphic Characteristics Of The Early Paleozoic And Implications Of Sedimentary Evolution In The Southwest Tianshan

Tianshan is located in Eurasia hinterland with a complex structure, composed by a series of mountains, basins and valleys. It is one of the most complex orogenic belts in the world. The southwest area has the most complex geologic structure in Tianshan, which saves the important information of the evolution of the Asian continent. More importantly, this area completely saves the information of the evolution of the oceanic crust of the Paleozoic.Thus, it has become one of the hot spots for geological research in recent years. Besides,the Early Paleozoic strata has gone through a tectonic evolution: stretch to amass, which is the key period of the tectonic evolution in Tarim. As a result, the research has a guiding significance on the oil and gas production.This paper is based on the stratigraphy, the sedimentology and the Basin analysis theory.By studying the stratigraphic characteristics including the rock, the structure, the chemical composition, the sequence etc., we deepen the understanding of the stratigraphic system in the early Paleozoic and the sedimentary facies and sedimentary environment. In Lower Cambrian Xiaoerbulake Formation, a suite of carbonates was deposited at the bottom with slope facies in the carbonate platforms and the upper is dolomitic limestone with lagoonal facies. In the Middle-Upper Cambrian Ava’s Formation, the rock is composed of dolomite and also has a large number of red mudstone. The Ava’s Formation is formed in the evaporative carbonate platform environment. In Lower Ordovician Qiulitage Formation member II、I, the rock is mostly composed by dolomite and the depositional environment is restricted platform. In member Ⅲ, the rock is mostly composed by limestone and the dolomitic content is low, thus relecting the depositional environment is restricted platform.In Middle Ordovician Qilang Formation, the rock is mostly composed by sandstone, and the depositional environment is terrigenous clastic barrier epicontinental sea.In Lower Siluria Kepingtage Formation, the rock is mostly composed of sandstone, reflecting the depositional environment is terrigenous clastic barrier epicontinental sea. In the Middle-Upper Siluria Tataaiertage Formation, the rock is mostly composed slimsy clasolite reflecting the depositional environment is also terrigenous clastic barrier epicontinental sea.According to the stratigraphic characteristics of the Early Paleozoic and combing with the previous researches, the paper restores sedimentary environment and builds the basin-mountain coupling relationship. The sedimentary evolution process can be divided into three stages: epicontinental basin basin- stretching stage from the Late Sinian to the Early Ordovician, the south Tianshan ocean contraction stage in the Middle Ordovician, Thesouth Tianshan ocean continued contracting stage. The strata in the early Paleozoic had gone through the depositional cycle of transfusion and regression from the Cambrian to the Ordovician. It is the same in the Silurian.