LACUSTRINE CARBONATE CLINOFORMS IN THE LOWER-PERMIAN LUCAOGOU LOW-ORDER CYCLE, SOUTHERN BOGDA MOUNTAINS, NW CHINA

Complex lithofacies of a carbonate dominated clinoform package in lower Permian Lucaogou (LCG) low order cycle, Bogda Mountains, NW China, provide clues on forming processes in a half-graben lake. The package was deposited on a flat foundation of sandstone-limestone and truncated by a surface overlain by prodeltaic shale. It is ~5.2 m thick, progrades from S to N for ~200 m, with maximum 15^o, and spans ~4 km laterally. A clinoform consists of a lower poorly-cemented siliciclastic-rich and an upper well-cemented carbonate-rich bed, forming a clinoform cycle. 19 thin sections are studied from a dip section of the package, 8 of which from a clinoform cycle from top to toe. The foundation is composed of micritic peloidal lithic wacke, lithic wackestone, interbedded with lithic thrombolitic wackestone. The lower part of a clinoform cycle consists of peloidal lithic subarenite with abundant and imbricated blackened clasts, which is coarse-very coarse-grained, very calcareous, and current laminated. The upper part consists of meteorically-modified lithic wackestone (30-50%), which is slightly grainier upward and highly autobrecciated by displacive calcite. At the toe, algal laminated oncoidal wackestone with chertification dominates. Framework grains in wackestones include abundant basic-intermediate volcanic lithics, carbonate aggregate grain, intraclast, ooid, peloid, and bioclast, and some unidentifiable highly altered grains. The ratio of detrital vs. carbonate grains is 5:1 on average. Clinoform wackestones are coarser and more poorly sorted than those in the foundation, contain less very coarse sand sized grains upward along a clinoform. The volcanic lithics were derived from basement or N Tianshan suture zone to the south, whereas the carbonate clasts from a nearby carbonate factory at lake margin. Abundant meteoric overprints suggest frequent lakelevel fluctuation. Coarse grains and current structures in the package indicate high-energy conditions during shoreface accretion forming clinoforms, whereas in-situ microbial deposits indicate periods of low-energy conditions, especially common at the toe. The Lucaogou lake was probably saline in an arid climate and had persistent longshore currents caused by strong wind and wave conditions.