Permian attributes and tectonic evolution of the west Junggar Basin
ZHANG Yuanyuan, ZENG Yuke, TANG Wenbin
Key Laboratory of Orogenic Belts and Crustal Evolution of the Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China
The Mahu sag in the northwestern margin of the Junggar Basin is one of the most important hydrocarbon-rich sags in the Junggar Basin. The understanding of basin attributes and tectonic evolution of the Junggar Basin during the Permian-early
Triassic is significant for oil and gas exploration and development. The Permian to early Triassic terrestrial successions in the Mahu sag record the tectonic evolution of the northwestern margin of the Junggar Basin. Comprehensive studies of structures,
sequence stratigraphy, sedimentary infill, sediment dispersal and paleogeographic construction were performed on the Permian-early Triassic successions. 2D seismic profiles show the occurrences of basin-bounding normal faults and half-graben
structures during the Early Permian and the tectonic inversion during the late Permian to early Triassic. Widespread soft-sediment deformation structures in the lower Permian Fengcheng Formation with lateral continuity correspond with the main activity on basin-bounding normal faults. The Permian to Triassic strata are organized into two second-order transgressive-regressive (T-R) sequences. During the early Permian, sediments of transgressive systems tracts 1 recorded depositional system transition from fan delta to deep lacustrine setting, and the depocenter was distributed along the boundary normal fault. At this time, tectonic mechanical subsidence exceeded sediment supply. During the middle Permian, the depositional system of regressive system tracts 1 transitioned from lacustrine to fan delta setting, and the location of the depocenter shifted to the center of the Mahu sag, where thermal subsidence played a great role. During the late Permian-early Triassic, fan-deltas were distributed along the HongChe, Ke-Bai and Wu-Xia fault zones. Therefore, three stages of the tectonic evolution of the Permian-early Triassic Mahu in the northwestern margin of Junggar Basin are recognized: early Permian syn-rift mechanical subsidence, middle Permian post-rift thermal subsidence, and late Permian to early Triassic tectonic inversion. The dynamic mechanism of the tectonic evolution of the northwestern margin of the Junggar Basin is consistent with the evolutionary pattern of the late Paleozoic post-accretionary orogenic stage in the southwestern margin of the Central Asian Orogenic Belt.
