LOWER-PERMIAN LACUSTRINE MICROBIAL BUILDUPS IN A CYCLOSTRATIGRAPHIC CONTEXT, BOGDA MOUNTAINS, NW CHINA

Microbial buildups in lake deposits of lower Permian Lucaogou low-order cycle in Zhaobishan, Bogda Mtn., NW China, occur in 12 high-order cycles (HC) in a 20-m succession. They overlie transgressive oolite or conglomerate and underlie regressive deltaic sandstone. They are mound-shaped and coalesce, 10s of cm wide and 10-50 cm high. Data from 40 thin sections show that mounds contain thrombolite, stromatolite, wackestone, and packstone. Thrombolites consist of lumps separated by cements and sediments. Lumps comprise clustered clots and calcite cements, are 1-4 cm in size, and show upward and lateral growth patterns. Clots are 10-100 microns in size and equant. Sediments between lumps include micrite, extra- and intraclasts, and patches of clots without a growth pattern. Stromatolites are laminated or digitated, domal or wavy. Wackestone is mud rich and contains extra- and intraclasts and clots. Packstone contains oncoids and extraclasts, lumpy micritic matrix, and calcite cement. Oncoids are mm in size, have uneven cortices of clotted micrite and rare sparry calcite with a wrinkly surface. In a mound, thrombolite is dominant; stromatolite and wackestone are mainly in the lower part; and packstone in the top part. Upward in the lower part of the succession, mounds become larger and higher in relief; extraclasts decrease. In the upper part, the trend reverses and oncoids increase upward. The thrombolites formed in quiet shallow lake; and episodic storm agitation and clastic influx and water depth change produced stromatolite and wackestone. The presence of clastic nuclei, agitated water, and fast micritic precipitation induced by microbial activities formed oncoids. Lake water was likely rich in Ca to provide nutrients for microbes and to precipitate calcite. The mounds occur in the maximum-transgressive interval of HCs when the lake was deepest and clastic influx smallest. The thickness of buildups is inversely related to that of overlying deltaic deposits in HCs. The increased flux of freshwater and clastics during delta formation reduced the Ca concentration and hampered calcite precipitation and microbial activities. This implies that climate was relative arid during transgression and humid during regression. The trend is applicable to the lower and upper parts of the succession, respectively.