CYCLOSTRATIGRAPHY OF UPPERMOST PERMIAN-LOWER TRIASSIC TERRESTRIAL DEPOSITS IN BOGDA MOUNTAINS, GREATER TURPAN-JUNGGAR BASIN, NW CHINA – UPDATES FROM SEDIMENTARY, GEOCHEMICAL, AND PALEOBOTANICAL DATA

The upper Permian-Lower Triassic fluvio-lacustrine deposits in Bogda Mountains provide clues on PTB terrestrial mass extinction and recovery in mid-latitude NE Pangea. Six sections, 1-100 km apart, present a solid stratigraphic framework to interpret evolution of sedimentary, climatic, and eco-systems. However, rapid lateral facies and thickness changes in a rift setting challenge stratigraphic correlation. A cyclostratigraphic approach can decipher regional climatic and tectonic trends for improved correlation. Systematic depositional environment changes define three orders of cycles. The upper Permian-early Induan Wutonggou low-order cycle (WTG LC) records a period of humid-subhumid climate, lush vegetation, and gradual increase in seasonality. It changes to Induan JCY LC with a major increase in Calcisols and onset of fluvial deposition. The JCY contains mainly lake-margin and flood plain mudrocks hosting mature Calcisols in Tarlong-Taodonggou and Dalongkou sections, except the thick coarse fluvial sediments in Zhaobishan section. It is 60-73 m thick, but 182 m in Zhaobishan, which is proximal to northern Tianshan suture zone. The LC records an initial flux of coarse clastics followed by landscape stability, arid climate, greatly reduced fossil plants, but common Lystrosaurus. The overlying Olenekian Shaofanggou (SFG) LC also starts with a coarse clastic influx, followed by mud-rich lake-margin and fluvial deposits with mutli-story Calcisols in the upper part, indicating a source uplift followed by great landscape stability. Abundant rootlets and few fossil woods suggest lush vegetation that were not well preserved. It is 85-180 m thick, depending on depositional environments, and shows similar general climatic and tectonic trends to those of JCY. The juxtaposition with thick conglomerates of Middle Triassic Karamay LC indicates another major source uplift and drainage basin reorganization. Tectonics is the primary and climate secondary controls on low-order cyclicity. Correlation of δ¹³C_org excursions suggests possible PTB in the uppermost WTG LC with gradual environmental and climatic shift across PTB. Finally, albeit a reduction in floral diversity across PTB, the Bogda region was compositionally stable and not barren during Early Triassic, when some vertebrates had thrived.