TECTONIC AND CLIMATIC CONTROLS ON CYCLIC SEDIMENTATION OF LOWER PERMIAN TO LOWER TRIASSIC FLUVIAL AND LACUSTRINE DEPOSITS, TARLONG VALLEY, SOUTHERN BOGDA MOUNTAINS, NORTHWESTERN CHINA

A 1178-m superbly-exposed Permo-Triassic cyclic fluvial-lacustrine section in southern Bogda Mountains, NW China was studied at a cm-dm scale to evaluate factors controlling intermontane basin filling at mid to high paleolatitude. Primay cycles have 5 types: 1) braided stream cycles (BS) of conglomerate, sandstone, thin mudrock, 1-11 m thick; 2) coarse meandering stream cycles (CMS) of conglomerate, sandstone, thin mudrock, 1-6 m thick; 3) classic meandering stream cycles (MS) of sandstone, conglomerate, thick mudrock, 2-12 m thick; the fluvial cycles contain calcitic or hydromorphic paleosols; 4) lacustrine deltaic cycles (LD) of upward-coarsening shale, sandstone, conglomerate capped by calcitic or hydromorphic paleosols, 1-13 m thick; and 5) lacustrine mixed carbonate and siliciclastic cycles (LM) of upward littoral sandstone, profundal shale, limestone, dolomitic and gypsum-bearing (?) shale, and calcitic paleosols, 0.2-3 m thick. The cycles reflect environment changes during lake expansion and contraction, controlled by intra-cycle tectonic and/or climatic changes. BS and CMS cycles indicate active provenance uplift, whereas MS and LD cycles indicate filling of over-filled lakes during tectonic quiescence. Calcitic and hydromorphic paleosols suggest intra-cycle humid to semi-arid climatic changes. LM cycles indicate filling of large balance or under-filled lakes in semi-arid climate during tectonic quiescence.

373 primary high-order cycles are grouped into 32 intermediate and 5 low-order cycles, 10s - 100s m thick. They reflect variable long-term dominance of provenance uplift, basin subsidence, and climate. Rifting-drifting transition in late Early Permian separated older BS and CMS-dominated rocks from younger LD, LM, and MS-dominated rocks; semi-arid to humid climate change in early Late Permian separated older calcitic paleosol-rich fluvial-lacustrine rocks from younger hydromorphic paleosol-rich fluvial-lacustrine rocks. Stacking of different types of high-order cycles suggests that lake deepening may not correlate with expansion, and lake expansion may not correlate with a high P/E ratio and/or sediment influx. Sediment-starved and sediment-copious over-filled lakes can to be differentiated.