THE TAPHONOMIC CHARACTER AND OCCURRENCE OF UPPER PERMIAN–LOWER TRIASSIC PLANT ASSEMBLAGES IN THE MID-PALEOLATITUDES, BOGDA MOUNTAINS, WESTERN CHINA

The continental paleobotanical record is paramount to understanding the patterns and processes that affected ecological systems in deep time. The Bogda Mountains, Xianjiang Uygur Autonomous Region, western China, contains a geochronometrically constrained latest Permian–Triassic succession of continental deposits in the NE Pagean mid-paleolatitudes from which such patterns may be discerned. Three low-order cycles (Wutonggou, Jiucaiyuan, Shaofanggou) are identified in a stratigraphic succession comprised of >4700 m, subdivided into 1838 high-order cycles (HC) using repetitive environmental changes. The plant taphonomic character of these is assessed across 15 measured section, exposed in 8 localities, distributed across a ~100 km west–east transect. Assemblages include: autochthonous and allochthonous permineralized wood; parautochthonous and allochthonous macrofloral adpressions, identifiable to systematic affinity; allochthonous unidentifiable phytoclasts on bedding surfaces; and autochthonous rooting structures.

Similar to paleobotanical results in other fully continental basins, macrofloral elements are rarely preserved. Wood and adpressions occur in <2% of meandering river and littoral and sublittoral lake HCs, deposited under semi-arid to humid conditions. Although paleosols occur in all HCs, developed under arid to humid conditions, physical rooting structures are encountered in only 23% of strata. Allochthonous phytoclasts are the most common taphonomic assemblage, preserved in association with micaceous minerals on bedding in fine-grained lithofacies. The presence of phytoclast assemblages throughout the Permian–Triassic succession evidences the presence of riparian vegetation during a time when models propose the catastrophic demise of land plants. These mesofossil and microfossil (palynological) assemblages offer the best opportunity to understand the crisis affecting the base of terrestrial ecosystems during the lastest Permian and earliest Triassic.