Search for: All records

ABSTRACT The Bogda Mountains, Xianjiang Uygur Autonomous Region, western China, expose an uppermost Permian–Lower Triassic succession of fully continental strata deposited across three graben (half graben) structures in the mid-paleolatitudes of Pangea. A cyclostratigraphy scheme developed for the succession is subdivided into three low-order cycles (Wutonggou, Jiucaiyuan, Shaofanggou). Low-order cycles are partitioned into 1838 high-order cycles based on repetitive environmental changes, and their plant taphonomic character is assessed in > 4700 m of high-resolution, measured sections distributed across ∼ 100 km. Four taphonomic assemblages are represented by: permineralized wood (both autochthonous and allochthonous), megafloral adpressions (?parautochthonous and allochthonous) identifiable to systematic affinity, unidentifiable (allochthonous) phytoclasts concentrated or disseminated on bedding, and (autochthonous) rooting structures of various configurations (carbon films to rhizoconcretions). Their temporal and spatial occurrences vary across the study area and are dependent on the array of depositional environments exposed in any particular locality. Similar to paleobotanical results in other fully continental basins, megafloral elements are rarely encountered. Both wood (erect permineralized stumps and prostrate logs) and adpressions are found in < 2% of meandering river and limnic cycles, where sediment accumulated under semi-arid to humid conditions. The absence of such assemblages in river-and-lake deposits is more likely related to physical or geographical factors than it is to an absence of organic-matter contribution. With such a low frequency, no predictable pattern or trend to their occurrence can be determined. This is also true for any horizon in which rooting structures are preserved, although paleosols occur in all or parts of high-order cycles developed under arid to humid conditions. Physical rooting structures are encountered in only 23% of these and are not preserved equally across space and time. Allochthonous phytoclasts are the most common taphonomic assemblage, preserved in association with micaceous minerals on bedding in fine-grained lithofacies. The consistency of phytoclast assemblages throughout the succession is empirical evidence for the presence of riparian vegetation during a time when models propose the catastrophic demise of land plants, and does not support an interpretation of vegetational demise followed by long-term recovery across the crisis interval in this basin. These mesofossil and microfossil (palynological) assemblages offer the best opportunity to understand the effects of the crisis on the base of terrestrial ecosystems. 

Stratigraphic sections in the Bogda Mountains, NW China, provide detailed records of late Permian–Early Triassic terrestrial paleoenvironmental and paleoclimatic evolution at the paleo-mid-latitude of NE Pangea. The sections are located in the Tarlong-Taodonggou, Dalongkou, and Zhaobishan areas, ~100 km apart, and ~5000 m in total thickness. An age model was constructed using seven high-resolution U-Pb zircon CA-TIMS dates in the Tarlong-Taodonggou sections and projected to sections in two other areas to convert the litho- and cyclo-stratigraphy into a chronostratigraphy. Sediments were deposited in braided and meandering streams, and lacustrine deltaic and lakeplain-littoral environments. A cyclostratigraphy was established on the basis of repetitive environmental changes for high-order cycles, stacking patterns of 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 generally humid-subhumid and gradually became variable toward a seasonally dry condition in the early Induan. Lush vegetation had persisted across the Permo–Triassic boundary into the early Induan. A subhumid-semiarid condition prevailed during the deposition of mid Induan–lower Olenekian Jiucaiyuan and lower Olenekian Shaofanggou LCs. These three LCs are largely continuous and separated by conformities and diastems. 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 stratigraphic variability is mainly 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 the mid Induan to early Olenekian, and served as primary food source for terrestrial ecosystems, including vertebrates. Correlation of the vascular plant evolutionary history from the latest Changhsingian to early Induan in the Bogda Mountains with those reported from Australia and south China indicates a diachronous floral changeover on Pangea. The late Permian–Early Triassic litho-, cyclo- and chrono-stratigraphies, constrained by the age model, providesfoundation for future studies on the evolution of continental sedimentary, climatic, biologic, and ecological systems in the Bogda region. It also provides a means to correlate terrestrial events in the mid-paleolatitudes with marine and nonmarine records in the other parts of the world. 

A silicified trunk,Zhuotingoxylon liaoiWan, Yang, Wang, Liu et Wang gen. et sp. nov., is described from the uppermost part of Guodikeng Formation in South Taodonggou section, Turpan–Hami Basin, Xinjiang Uygur Autonomous Region, northwestern China. It is characterized by a solid pith, endarch primary xylem and pycnoxylic wood. The pith is composed of parenchyma and sclereids. Radial walls of primary xylem tracheids have spiral and scalariform thickenings. Secondary xylem consists of thick‐walled tracheids and parenchymatous rays. Uniseriate rounded pits with oval apertures are distributed on radial tracheidal walls separately. Cell walls of rays are homogeneous and smooth. Rays are 1–10 cells high in tangential section. Cross‐field pits are cupressoid. There are 1–4 bordered pits with slit‐like to oval apertures in each cross‐field. Based on the anatomical features of the pith and xylems, it is proposed that the new stem has a coniferous affinity. The new fossil stem adds to the knowledge of vascular plant diversity close to the Permian–Triassic boundary.