Depositional environments and cyclo- and chronostratigraphy of uppermost Carboniferous-Lower Triassic fluvial-lacustrine deposits, southern Bogda Mountains, NW China - A terrestrial paleoclimatic record of mid-latitude NE Pangea

摘要

Two uppermost Carboniferous-Lower Triassic fluvial-lacustrine sections in the Tarlong-Taodonggou half-graben, southern Bogda Mountains, NW China, comprise a 1834m-thick, relatively complete sedimentary and paleoclimatic record of the east coast of mid-latitude NE Pangea. Depositional environmental interpretations identified three orders (high, intermediate, and low) of sedimentary cycles. High-order cycles (HCs) have five basic types, including fluvial cycles recording repetitive changes of erosion and deposition and lacustrine cycles recording repetitive environmental changes associated with lake expansion and contraction. HCs are grouped into intermediate-order cycles (ICs) on the basis of systematic changes of thickness, type, and component lithofacies of HCs. Nine low-order cycles (LCs) are demarcated by graben-wide surfaces across which significant long-term environmental changes occurred. A preliminary cyclo-stratigraphic framework provides a foundation for future studies of terrestrial climate, tectonics, and paleontology in mid-latitude NE Pangea. Climate variabilities at the intra-HC, HC, IC, and LC scales were interpreted from sedimentary and paleosol evidence. Four prominent climatic shifts are present: 1) from the humid-subhumid to highly-variable subhumid-semiarid conditions at the beginning of Sakamarian; 2) from highly-variable subhumid-semiarid to humid-subhumid conditions across the Artinskian-Capitanian unconformity; 3) from humid-subhumid to highly-variable subhumid-semiarid conditions at early Induan; and 4) from the highly-variable subhumid-semiarid to humid-subhumid conditions across the Olenekian-Anisian unconformity. The stable humid-subhumid condition from Lopingian to early Induan implies that paleoclimate change may not have been the cause of the end-Permian terrestrial mass extinction. A close documentation of the pace and timing of the extinction and exploration of other causes are needed. In addition, the semiarid-subhumid conditions from Sakamarian to Artinskian-Kungurian (?) and from middle Induan to end of Olenekian are in conflict with modern mid-latitude east coast meso- and macrothermal humid climate. Extreme continentality, regional orographic effect, and/or abnormal circulation of Paleo-Tethys maybe are possible causes. Our work serves as a rare data point at mid-latitude NE Pangea for climate modeling to seek explanations on the origin(s) of climate variability in NE Pangea from latest Carboniferous to Early Triassic.