PALEOENVIRONMENTAL AND PALEOCLIMATIC EVOLUTION AND CYCLO- AND CHRONO-STRATIGRAPHY OF UPPER PERMIAN-LOWER TRIASSIC FLUVIAL-LACUSTRINE DEPOSITS IN BOGDA MOUNTAINS, NW CHINA – IMPLICATIONS FOR PERMO-TRIASSIC DIACHRONOUS PLANT EVOLUTION ACROSS THE PERMIAN-TRIASSIC BOUNDARY

Outcrop sections in Bogda Mountains, NW China, record late Permian-Early Triassic terrestrial paleoenvironmental and paleoclimatic evolution at paleo-mid-latitude of NE Pangea. Ten sections, ~5,000 m in total thickness, were measured in Tarlong-Taodonggou (TT), Dalongkou, and Zhaobishan areas, ~100 km apart. An age model was constructed using 7 U-Pb zircon CA-TIMS dates in the TT area and projected to other sections to convert litho- and cyclo-stratigraphy into chronostratigraphy. The sections contain fluvial and lacustrine sediments. A cyclostratigraphy was established based on repetitive environmental changes for high-order cycles and long-term climatic and tectonic trends for low-order cycles (LC). Sedimentary evidence from the upper Wuchiapingian-mid Induan Wutonggou LC indicates that the climate was humid-subhumid and gradually became variable toward a seasonally dry condition in early Induan. Lush vegetation had persisted across Permo-Triassic boundary into early Induan. A subhumid-semiarid condition prevailed during deposition of mid Induan-lower Olenekian Jiucaiyuan and lower Olenekian Shaofanggou LCs. The LCs are largely continuous. Intra- and inter-graben stratigraphic variability is reflected by variations in thickness, depositional system, and average sedimentation rate, and results in variable spatial and temporal stratigraphic resolution. Such variability is controlled by paleogeographic location, depocenter shift, and episodic uplift and subsidence in the source areas and catchment basin. A changeover of plant communities occurred during the early Induan, postdating the end-Permian marine mass extinction. However, riparian vegetation and upland forests were still present from mid Induan to early Olenekian, and served as primary food source for terrestrial ecosystems. Correlation of plant evolutionary history from the latest Changhsingian to early Induan with those in Australia and south China indicates a diachronous floral changeover on Pangea. The litho-, cyclo- and chrono-stratigraphy provides a foundation for future studies on the evolution of continental sedimentary, climatic, biologic, and ecological systems in the Bogda area and for global correlation of terrestrial events in the mid-paleolatitudes with other marine and nonmarine records.