RELATIONSHIP BETWEEN LITHO- AND ORGANO-FACIES OF LOWER PERMIAN LUCAOGOU LOW-ORDER CYCLE, SOUTHERN BOGDA MOUNTAINS, GREATER TURPAN-JUNGGAR RIFT BASIN, NW CHINA

Litho-and organo-facies of four high-order cycles (HCs) at a meter scale have been identified through sedimentologic and geochemical approaches in lower-Permian Lucaogou low-order cycle. Types of organic matter (OM), lake water chemistry, and biological communities interpreted from geochemical proxies, such as biomarkers and organic carbon isotope (δ¹³C), signify three distinct organofacies (OF) within the four HCs. OF-Ⅰ is dominated by siltstone and very fine sandstone, characterized by heavy δ¹³C (~-21.9 ‰) and poor-quality OM (average TOC~0.3%, Type ⅡA kerogen). The OM was mainly stemmed from higher terrestrial plants and partially from aquatic microorganisms, in shallow and oxic/hypoxic-fresh water in open lake. OF-Ⅱ consists of dolomitic shale and dolomite with light δ¹³C (-29.6~-25.1‰), moderate TOC of 0.7-6.4%, and Type ⅠA and ⅡA kerogen. The OM was derived from mixed contribution of aquatic algae, microorganism, and higher terrestrial plant debris highly modified by microbial activities. It was deposited in hypoxic fresh-brackish/alkaline lake water, shallow to deep littoral environments with restricted hydrology. OF-Ⅲ is composed of black shale, distinguished by moderate δ¹³C (-26.7~-24.1‰) and excellent oil-prone OM (average TOC~2.8-4.8%, Type ⅠA kerogen). The OM was derived from a major contribution of aquatic phytoplankton and algae, mixed with minor input from macrophytes and higher plants. It was formed in acid-dysoxic profundal environments with a stratified saline water column. Bulk-geochemical proxies and molecular biomarkers vary systematically as lake expansion/deepening and contraction/shallowing, suggesting a genetically linked relationship between litho- and organo-facies in highly fluctuating lake systems. The OFs are governed by the same sedimentary processes controlling lithofacies, fluctuate as the shift of lake level and shoreline trajectories in terms of hydrology and sedimentation. Besides, the OFs oscillate with the variation of relative abundance of OM input between higher terrestrial plant and aquatic algae, as well as redox and salinity of lakewater. The highly variable lacustrine cycles suggest frequent oscillations of climate between subarid and humid.