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Lacustrine Carbonate-Siliciclastic Clinoforms in the Lower-Permian Lucaogou Low-Order Cycle, Southern Bogda Mountains, NW China


Complex lithofacies and geometry of a carbonate-dominated clinoforms package in lower Permian Lucaogou (LCG) low-order cycle provide clues on sedimentary processes, paleoclimate, and lake level change in an intermountain lake basin, Bogda Mountain, NW China. The package is in the lower LCG LC, ~5.2 m in stratigraphic thickness, extends ~200 m down dip and ~4 km laterally, and dips to NE-NW with a maximum angle of 15o. Field and petrographic results of 30 samples show that the package is founded on microbialites with trapped clasts, and terminated by overlying transgressive profundal and deltaic deposits. Each clinoform consists of a couplet of a lower siliciclastic-rich and an upper carbonate-rich bed, ~20-50 cm thick. Both beds contain ~10% of siliciclastic or carbonate constituents, respectively, and are mixed siliciclastic-carbonate rocks. Carbonate-rich beds contain lithic packstone and wackestone with volcanic clasts, intraclasts, ooids, aggregate grains, peloids, and bioclasts. Siliciclastic grains are more than carbonate grains, but rich micrite and microcrystalline cements make the carbonate constituents as dominant. Siliciclastic-rich beds are dominantly basaltic lithics with rare carbonate grains. Grains in both lithofacies are medium to coarse-sand-sized and moderately sorted. Diagenetic alteration is common and variable in carbonate-rich beds. Abrasion and microbial textures of carbonate grains indicate current erosion and transport of intra and extra-clasts and a microbial origin. The grains may have been derived from a closed shallow carbonate flat where carbonate productivity was high, away from areas of major siliciclastic influx. The clinoform package is interpreted as a barrier bar or spit formed by current-transported sediments, which were deposited at an embayment. Alternation of carbonate-rich and siliciclastic-rich beds may have been caused by oscillation between subhumid to semi-arid conditions, which affected the carbonate productivity and detrital grain supply. The overall trend suggests a lake level change from a relatively deep and quiet condition when microbialite was formed, to a relatively agitated condition when littoral sediments were accumulated. Extensive diagenetic alteration is indicative of a lake margin environment affected by oscillating groundwater table and lake level. The documentation of rare lacustrine carbonate clinoforms provides insights on lake sedimentation in rift basins.