Sedimentary and palaeoenvironmental evolution of the Junggar Basin, Xinjiang, Northwest China

Abstract

This review paper summarizes the sedimentary and palaeoenvironmental evolution of the Junggar Basin in Northwest China largely based on hardly accessible Chinese language papers, and complemented by own field observations and a critical survey of key sediment cores from petroleum wells. We have combined this information and updated existing lithofacies and isopach maps for characteristic time slices of basin evolution and palaeoenvironmental change. The Junggar Basin was initiated during the late stage of collisional tectonics in the southern Central Asian Orogenic Belt (Altaids) since the Early Permian. According to studies in surrounding mountain chains and geophysical surveys, the basement consists of a collage of oceanic basins, intraoceanic island arcs, and microcontinents of Precambrian to Palaeozoic age. The basin fill is subdivided into three tectonically controlled stratigraphic sequences which are separated by two regional angular unconformities. The first cycle in the Permian and Triassic is characterized by an Early Permian extensional strike-slip and a Late Permian to Triassic compressional foreland setting. After an Early Permian marine regression, persistent nonmarine fluvio-lacustrine conditions were established containing probably the thickest organic-rich mudstone interval in the world, which act as major source rocks of the basin. Starting with four depocenters, the basin was unified during the Triassic. The preserved total maximum thickness of this cycle is about 8,500 m in the southern depocenter. During the second intracontinental depression cycle, subsidence slowed down and the depocenter migrated towards the basin center reaching a maximum thickness of 6,000 m. The palaeoenvironment was dominated by a large oscillating freshwater lake receiving changing quantities of clastic sediments from the surrounding mountain ranges and forming alluvial fans, braid plains, and deltas partly containing coal seams of economic interest. Sedimentary facies, pollen, and palaeobotanical plant fossils show an overall aridization trend and a shrinking lake cover. During the Neogene cycle, the depocenter migrated back to the south and the former asymmetric foreland basin was reactivated due to thrusting and rapid uplift of the Tian Shan. The maximum thickness of these molasse-type deposits exceeds 5,000 m. Despite its strong potential, there is still a lack of high resolution bio- and cyclostratigraphy, sequence stratigraphy, and palaeoclimate studies in the Junggar Basin to elucidate local versus regional palaeo-environmental patterns and to better constrain far-distance tectonic forcing.