SEDIMENTARY AND ORGANIC GEOCHEMICAL EVIDENCE ON CONTINENTAL CLIMATE VARIABILITY IN MID-LATITUDE NORTHERN PANGEA, LOWER PERMIAN LUCAOGOU AND HONGYANCHI LOW-ORDER CYCLES, SOUTHERN BOGDA MOUNTAINS, GREATER TURPAN-JUNGGAR BASIN, NW CHINA

A preliminary early Permian climatic record in NW China was interpreted from depositional systems, biological communities, and lake water chemistry. Two types of depositional cycles, fluctuating profundal mixed siliciclastic and carbonate cycle in Lucaogou low-order cycle (LCG LC) and lacustrine deltaic cycle in Hongyanchi (HYC) LC were delineated using outcrop and petrographic data. Five organofacies (OF) were identified on the basis of type of organic matter (OM), biomarkers, and organic carbon isotopes (δ¹³C) of 139 samples from 6 sections: OF1 has Type ⅡA kerogen and low total organic carbon (TOC), formed in fresh oxic fluvial environment; OF2 has types ⅠA and ⅡA kerogen, low TOC, and on fresh oxic lakeplain; OF3 has Type ⅡA kerogen, low TOC, in fresh-brackish hypoxic deltaic environment; OF4 has mixed types ⅠA and ⅡA kerogen, moderate TOC, in dysoxic brackish sublittoral setting; OF5 has Type ⅠA kerogen, high TOC, in anoxic profundal environment with a stratified saline water column. The contrasts in shale thickness and types of cycle and organofacies between Tarlong and Taodonggou areas in a single half graben suggest that N Tarlong is both the depositional and subsidence centers for LCG LC, where profundal deposits with average TOC~3.4% and Type ⅠA kerogen dominate, with minor deltaic deposits. Shales in N Taodonggou and SW Tarlong sections have Type ⅡA kerogen and average TOC of 0.04% and 0.09%, respectively; they were deposited at the ramp margin of the half-graben. However, the depositional and subsidence centers for HYC LC are in S Tarlong, where profundal shales are thicker and more organic-rich than those in N Tarlong. Repetitive lake expansion–contraction suggests climate oscillated dominantly between subhumid to semiarid. The absence of thick coarse-grained facies and plant-rich deltaic deposits suggests limited river inflow. The presence of evaporite minerals and gammacerane and dominance of Type ⅠA kerogen in LCG LC indicate a semiarid-arid climate. Deltaic deposits with abundant plant debris, dominance of ⅡA kerogen, and absence of gammacerane in HYC LC suggest a large influx of river water and sediments, indicating a humid climate.