Sedimentary characteristics and evolution of the Toutunhe–Qingshuihe Formation in the central section of the southern margin of the Junggar Basin

Abstract Objective: In order to clarify the sedimentary characteristics and sedimentary evolution of Tou Tunhe Formation and Qingshuihe Formation in the middle part of the southern margin of Junggar Basin, the depositional characteristics and sedimentary evolution are discussed based on the measured outcrop profile and the paleoenvironment changes. Methods: According to the lithofacies division scheme of Maill, the lithology and sedimentary structure of Toutunhe Formation and Qingshuihe Formation in the southern margin of Junggar Basin were analyzed by using the dual attribute method combining lithology and sedimentary structure. Based on the guidance of sedimentology and the changes of paleoenvironment, the law of vertical sedimentary evolution in the study area is discussed. Results: There are 4 types of conglomerate facies, sandstone facies, siltstone facies and mudstone facies, including 7 types of conglomerate facies, 6 types of sandstone facies, 4 types of siltstone facies and 1 type of mudstone facies. Four sedimentary facies can be divided into lacustrine facies, meander river facies, braided river delta facies and alluvial fan facies, including 16 types of sedimentary microfacies. During the depositional period of the Tou Tunhe Formation in the Middle Jurassic, microfacies and shallow lake subfacies such as distributary channel, natural levee, interdistributary channel and far bank dune developed in the study area, indicating the delta-lake depositional model. Due to the continuous strong tectonic activity in the North Tianshan Mountains, drainage structures were widely developed in the Late Jurassic. During the deposition period of the Qigu Formation, brown red sediments were mostly developed, which was a semi-arid environment with sedimentary microfacies such as channels and point DAMS, and a meanded-river sedimentary model. During the sedimentary period of the Kalaza Formation, the conglomerate changed from reddish-brown to gray, the hot and arid climate ended, and microfacies such as debris flow deposition and channel deposition developed, indicating the sedimentary model of alluvial fan. During the Lower Cretaceous Qingshuihe Formation, the Mongol-Okhotsk Ocean was completely closed, the basin rapidly settled, and the lake transgression was rapid in a large range. The lithology was mainly sandstone and siltstone, and a few fine conglomerates were occasionally found at the bottom, and the sedimentary structures such as small sand-grained bedding, parallel bedding and tabular cross-bedding were widely developed. Conclusion: The sedimentary characteristics of Toutunhe Formation and Qingshuihe Formation in the middle part of the southern margin of Junggar Basin are analyzed, and the sedimentary evolution law is discussed, which provides a new idea for the distribution of underground sedimentary system.