BIOMARKER ANALYSIS IN PALEOENVIRONMENTAL AND PALEOGEOGRAPHIC RECONSTRUCTION OF LACUSTRINE SHALES, LOWER PERMIAN LUCAOGOU LOW-ORDER CYCLE, SOUTHERN BOGDA MOUNTAINS, NW CHINA

Petrographic and geochemical characteristics of lacustrine shales provide critical clues about their origins. The characteristics of fluvial and lacustrine shales in Lower-Permian Lucaogou low-order cycle in southern Bogda Mountains, greater Turpan-Junggar intracontinental rift basin, NW China, are used to interpret the provenance and depositional environments and processes of those shales. Lithology, composition, sedimentary texture and structures, stratal geometry, and boundary relationship were used to identify profundal, prodeltaic, and fluvial shale lithofacies. The profundal shales are organic-rich, dolomitic, and/or calcareous, sub-mm to mm laminated, and represent maximum transgressive deposits. Saturated hydrocarbons and aromatic hydrocarbons extracted from 74 shale samples were geochemically analyzed by Gas Chromatography-Mass Spectrometer. Relative high abundance of carotene, gammacerane, and pregnane show hypersaline lakewater. Relative low ratios of pristane/phytane (Pr/Ph < 0.5) suggest anoxic depositional environment. This hypersaline and anoxic lake probably was underfilled. Rock-Eval analyses of 74 samples indicate that the total organic carbon content ranges from 0.015 to 25.00 wt%, average 2.89%, and kerogens are mostly IIA and rarely IA and IIIA. 15 shale samples collected from a 5.4-m thick interval show variations from relative low (< 3.00) to high (3.00-7.00) total organic carbon content, indicating systematic changes from prodeltaic to profundal shale facies at a high-order cycle scale. Lakewater salinity, oxygen content, temperature and acidity are also interpreted from the organic geochemical data to aid in interpretation of depositional environments, provenance, and their changes and, ultimately paleogeographic evolution of the rift lake.