Sedimentary Sequence And Lithofacies Paleogeography Of Lower Jurassic In Eastern Junggar Basin, Xinjiang

Junggar basin is a large-scale intracontinental superimposed basin in the Late Carboniferous-Neogene in Northwest China. The study area is locates in the eastern part of Junggar basin. in which the Lower-Middle Jurassic contains abundant mineral resources, such as coal, petroleum and natural gas.Based on previous studies and updated geological data, this study aim to reveal sedimentary characteristics and lithofacies paleogeography of the East Junggar basin through studying sequence stratigraphy, sedimentology, paleontology and logging data. Which provide geological basis for further scientific research and evaluation of mineral resources exploration.In this study, the lower Jurassic Badaowan formation and the Sangonghe formation in the Eastern Junggar basin are systematically divided. The Badaowan formation is divided into two lithologic sections (i.e., J1b1 and J1b2), and the Sangonghe formation is taken as one lithologic section (i.e., J1s). Furthermore, we think that the lower Jurassic sequence stratigraphic frame indicates a long cycle (tectonic sequences) and three long-term cycles (the third order sequence). We also proposed a comprehensive classification scheme for Jurassic sedimentary system of Junggar basin. There are four main types of sedimentary facies, i.e., braided river facies, meandering river facies, meandering river delta facies and lacustrine facies. Through the analysis of palaeoclimate and palaeo flow, we think the Eastern Junggar basin was mainly controlled by subtropical moist climate during lower Jurassic, while only the climate at the early stage of the Sangonghe formation was arid and semi-arid. The direction of the ancient water flow was from north to south, and the sediments were mainly from the northern part of the study area. In addition, by taking the long-term cycle as a mapping unit, we present sequence lithofacies and paleogeography map, which reflects the palaeogeographic pattern and evolution of the Eastern Junggar basin during Lower Jurassic.The early Badaowan formation mainly developed the braided river, meandering river facies and meandering river delta, and was interfered by a short period of lake invasion. During the middle and late period of the Badaowan formation, the basement of the basin rose rapidly, and the Northeastern Junggar basin supplied abundant materials for the study area, resulting in a stable thick coal layer. Afterwards, the basin basement began to decline. As a result, the southwest part of our study area present the shore-lake and shallow-lake sub-facies, while the north part change to the meandering river delta sedimentary environment. During the Sangonghe formation, the basement of the Eastern Junggar basin subsided rapidly and was interfered by a large-scale lake invasion. The meandering river delta sedimentary facies belts retreated and the phase zone become to be narrow towards northeast as a result of this transgression. The southwest part of our study area developed a semi deep-deep lake sedimentary environment, and the lake facies was the main sedimentary environment.