GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 159-2
Presentation Time: 9:00 AM-6:30 PM


LU, Yiran, Geology and Geophysics Program, Missouri University of Science and Technology, 1870 Miner Circle, Rolla, MO 65409 and YANG, Wan, Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, MO 65409,

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 15o, 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.