THREE-ORDER FLUVIAL-LACUSTRINE CYCLOSTRATIGRAPHY AND TECTONIC AND CLIMATIC SIGNALS IN PERMIAN-LOWER TRIASSIC TERRESTRIAL SEDIMENTARY SUCCESSIONS, BOGDA MOUNTAINS, NW CHINA

Fluvial-lacustrine deposits in Bogda Mountains, NW China, record Permian-Early Triassic terrestrial environmental, tectonic, climatic, and biological evolutions in the mid-paleolatitude of NE Pangea. Six sections in south and north Bogda are 1000-2100 m thick, deposited in half grabens of the greater Turpan-Junggar rift basin. Three orders of sedimentary cycles are defined. High-order cycles are 0.1-1s m thick, composed of siliciclastics, carbonates, and paleosols, indicating repetitive lake expansion and contraction or fluvial erosion and deposition. Intermediate-order cycles are 1-10s m thick of stacked HCs forming depositional sequences. 10 low-order cycles (LCs) are 10-100s m thick, have persistent tectonic and/or climatic conditions, and are constrained by zircon ages and biostratigraphy. The upper Gzhelian to mid-Sakmarian lower, middle, and upper Daheyan LCs are 150-300 m thick; Calcisols in fluvial deposits indicate an arid climate. The upper Sakmarian Lucaogou LC contains 80-180 m of fluvial-lake deposits formed in a highly-variable arid-humid climate. The Artinskian Hongyanchi LC is 51-117 m thick and similar to the Lucaogou and capped by a major unconformity. The Wordian to basal Capitanian lower Quanzijie LC contains 2-95 m of fluvial deposits and Calcisols, indicating an arid climate. The Capitanian upper Quanzijie LC is 60-220 m thick and contains fluvial and loess deposits and Gleysols at the top, indicating a dramatic arid-to-humid transition. The Wuchiapingian-basal Induan Wutonggou LC contains 330-829 m of fluvial-lake deposits and humid paleosols, indicating a humid-subhumid climate. The PTB is confined to a 90-m interval. The Induan Jiucaiyuan LC contains 98-183 m thick of fluvial-lake deposits and Calcisols, indicating an arid-semiarid climate. The Olenekian Shaofanggou LC is 112-2.5 m thick and similar to the Jiucaiyuan. The entire succession comprises one overall and several LC-scale fluvial-lacustrine tripartite sequences. Several intervals of arid-semiarid conditions are in conflict with modern mid-latitude east-coast humid climate. Extreme continentality, orography, or abnormal Paleo-Tethys circulation may be the causes. The humid-subhumid condition across PTB implies that climate may not be a major cause of end-Permian terrestrial mass extinction.