MICROBIAL PARTICIPATION IN FORMATION OF LACUSTRINE DOLOMITE OF THE MIDDLE PERMIAN LUCAOGOU FORMATION, URUMCHI, XINJIANG, CHINA

The Middle Permian Lucaogou Formation, exposed in the Hongyanchi sections at Urumchi, Northern Xinjiang, NW China, is characterized by lacustrine dark gray dolostones interbedded with gray limestones, sandstones, thick black mudstones and oil shales. The dolostones are mainly composed of micro-crystalline dolomite, quartz and calcite, rich in organic matter. We recognized four types of carbonate micromorphologies in petrographic thin sections and under SEM analysis: microspheres (Both calcispheres and dolospheres, 6-20 μm in diameter) and nano-sized structures: granules (<120 nm), nano-rods (250 -1000 nm) and micron-sized subhedral and anhedral dolomite (0.5-20 μm). The nano-sized structures formed the external coating of the microspheroids in contact with the sedimentary matrix. The microspheres are of unknown origins, variable in size with simple organic rich outlines and crystalline lumens. The nano-sized carbonate structures compare closely to anhedral structures in modern lacustrine and experimental settings associated with anaerobic and aerobic microorganisms and their metabolic activities and products. The isotopic composition, δ¹³C_PDB of dolostones ranged from 6.2 ‰ to 14.6‰; the δ¹⁸O_PDB ranged from -9.5‰ to -2.8‰; the positive δ¹³C values suggest isotope fractionation by methanogens. A poor degree of ordering, absence of CaCO₃ replacement texture, enrichment with Sr and organic matter, positive δ¹³C and presence of clay-sized quartz, collectively suggest that the micron-sized subhedral and anhedral dolomite precipitated by rapid primary deposition from the solution mediated by archeal methanogenesis.