Resumen de Katian oncoids of the lower Ordovician Lianglitage Formation in the Tarim Basin, Northwest China
Shi Pingzhou, Tang Hongming, Wang Zhengyu, Sha Xuguang
This study examines oncoids and their diagenetic structures in the lower Ordovician (Katian Stage) Lianglitage Formation in the Tarim Basin, Northwest China. Oncoid morphologies range from spherical and sub-spherical to irregular. Individual oncoids generally range from 4 to 10 mm in diameter, with an average of 8 mm and a maximum of 18 mm. Microbial fossils, including gastropods, sponges, trilobite skeletons, and bryozoans, are abundant in oncoid nuclei. Densely packed, calcified microbial clumps are also observed in some nuclei. Microbial remnants in oncoid cortices appear mainly as filaments. Alternating light and dark laminae are obvious in the oncoid cortices, and both sets of laminae contain extracellular polymeric substances, nanoparticles, and microcrystalline calcite. This suggests that oncoid formation was the result of interaction between microbial chemical processes and the external environment. Microbes and heterotrophic organisms provide the building material for oncoid nuclei formation, and frequent fluctuations in sea level provide favorable hydrodynamic conditions.
