LOWER PERMIAN TO LOWER TRIASSIC FLUVIAL-LACUSTRINE DEPOSITIONAL SYSTEMS, CYCLO-STRATIGRAPHY, AND CONTROLLING PROCESSES, SOUTHERN BOGDA MOUNTAINS, NW CHINA

1,200-m superbly-exposed Lower Permian-Lower Triassic rocks in Tu-Ha intermontane basin were measured at a cm-dm scale at two locations 6 km apart and mapped in a 50-km² area in Bogda Mountains, China. 752 high-order cycles (HCs) composed of braided and meandering stream and lacustrine deltaic and mixed siliciclastic and carbonate deposits are divided into five types, representing cycles of fluvial erosion-deposition and lake expansion-contraction. Their stacking patterns delineate 113 intermediate-order and 15 low-order cycles (ICs and LCs). Cycle correlation using lithologic markers, physical tracing, and cycle stacking pattern suggest that LCs and many ICs are correlative in 6 km and ICs and many HCs are correlative in 1-2 km.

Three Kungurian Daheyan LCs of mainly braided stream HCs filled topographic lows on Artinskian volcanoclastics. Fluctuating profundal HCs of organic-rich shale, dolomitic shale, limestone, and sandstone of Roadian Lucaogou LC thin westward into fluvial and littoral siliciclastic and carbonate HCs. Fluvial and littoral HCs of Wordian Hongyanchi LC thin westward into dominantly littoral HCs. Eastward-thinning Capitanian Quanzijie LC of fine-grained fluvial HCs overlies a major Calcisol, indicating a major semi-arid-humid climate shift into Late Permian. Eastward-thickening Lopingian Wutonggou LC contains alternating meandering-stream and lacustrine deltaic ICs with Spodosols and Histosols indicating humid climate. Induan Jiucaiyuan LC contains thick super meandering stream systems with immature Calcisols, indicating enlarged catchment and a gradual shift to semi-arid climate. Olenekian Shaofanggou LC contains persistent littoral-lakeplain HCs of sandstone, shale, and Calcisols formed under stable tectonic and semi-arid conditions. In general, catchment evolution and long-term climate change controlled types of HCs in LCs and some ICs. Alternating westward and eastward thickening of LCs suggests tectonically partitioned catchment basins and changing provenances. 1:5 bundling of HCs in some intervals suggests a Milankovitch climatic origin for HCs and ICs, while others may be autogenic. The 3-order cyclostratigraphic framework is critical to accurately deciphering paleogeographic changes and controlling processes in a nonmarine intermontane basin.