LACUSTRINE BIOGENIC AND ABIOGENIC COATED GRAINS IN LOWER PERMIAN LUCAOGOU LOW-ORDER CYCLE IN ZHAOBISHAN SECTION, BOGDA MOUNTAINS, NW CHINA

Coated grains (CGs) in a microbialite-rich rift lake show unique texture and composition of nuclei and cortices and are grouped into 4 types to infer bio- and abiogenic processes and factors. Type 1 CGs have a ragged clotted micritic cortex and form packstones. CGs are 0.3-2 mm in size and of variable shape. Nuclei are carbonate. A cortex has 1-3 laminae and are 0.1-1 mm thick and mostly micritic. Micritic laminae are uneven and clotty; rare sparry ones are calcitic and even. The micritic laminae formed by microbial encrusting and mediated precipitation in weakly agitated water. Type 2 CGs have a cortex of inner micritic laminae and an outer chert lamina and comprise packstones. CGs are rounded, 0.2-0.5 mm in size. Siliciclastic nuclei dominate. A cortex has 2-3 laminae, 0.05-0.15 mm thick. Micritic laminae are even and contain some clots and trapped clasts. The morphology indicates frequent rolling in agitated water with large clastic influx. The micritic laminae have a microbial origin. Type 3 CGs have thick cortices and comprise packstone-grainstone. CGs are 1-2.5 mm in size, well-rounded and broken. Nuclei are mainly peloids. A cortex has 3-8 alternating organic-rich micritic and siliceous laminae. Micritic ones are 0.005-0.2 mm thick, clotted, ragged and discontinuous. Siliceous laminae are even. CGs are covered by clotted micritic films. The large number of laminae, roundedness, and broken grains indicate strong agitation. However, the thick cortices indicate fast microbially-mediated precipitation in Ca-rich lake water. Type 4 CGs have a thin non-clotted cortex and form grainstone. CGs are 0.2-0.5 mm in size. Siliciclastic nuclei dominate. A cortex is ~0.1 mm thick and contains a micritic lamina and rare underlying sparry lamina. The former is ~0.01 mm thick and consists of isopachous micrite; the latter is syntaxial. The thin non-clotted micritic laminae suggest weak inorganic precipitation and microbial activities due to large influx of freshwater and clastics. Overall, biogenic CGs (Type 1) formed in weakly agitated water with little clastic influx and strong microbial activities. Abiogenic CGs (Type 4) formed in highly agitated freshwater with large clastic influx. Types 2 and 3 CGs formed by alternating bio- and abiogenic processes. A variety of bio- and abiogenic processes formed a spectrum of CGs.