Microbial Imprints On Ooids From Cambrian Oolitic Limestones Of China

Microfacies analysis of Cambrian oolitic limestones from Yangtze and North China regions reveals that microbes not only involve in the formation of ooids, but also play important roles in ooid desctruction.The Cambrian strata are nearly complete and well exposed in the Huangshandong-Wang jiaping section of Three Gorge area. The oolitic limestones sampled from the bottom of Tianheban Formation show little diagenetic alterations. The internal structures of ooids are well preserved. The Cambrian strata are well exposed in the Tushan section of Hancheng and the Zhoujiaqu section of Longxian, and totally collected 13 samples.During the study, the methods of microfacies analysis are widely used. A precise field record of geological data as well as scientific sampling is carried out during the field work. In the laboratory, the study focus on the microscopic analysis of thin sections, combined with staining, scanning electron microscopy and fluorescence microscopy techniques for comprehensive analysis.The study of microbial imprints preserved in the oolitic limestones is the core of this thesis. In-depth analysis indicates that microbial activities not only can construct ooids, but also destruct them.Microbial construction relates to:â‘ Ooids involved with Girvanella fossils, which subdivided into four types (Girvanella core ooids, Girvanella cortex ooids, Girvanella ooids, and Girvanella cryptomorphic ooids);â‘¡Calcified biofilms;â‘¢Constructive micrite envelope.Microbial destruction includes:â‘ Microboring produced by microbial activities in the ooids surface;â‘¡Destructive micrite envelope referred to microbial micritization.Different types of microbial imprints on ooids from oolitic limestones are distingushed and studied in this paper. And the study has obtained a preliminary understanding on the role of microbes during the formation of ooids. Microbial metabolism can change the physical-chemical conditions of microenvironments, which promote carbonate precipitation on ooids surface and form various types of constructive deposition; also microbial activities can destroy the original structures of ooids by microboring and microbial micritization.