Microbialite Textures and Their Geochemical Characteristics of Middle Triassic Dolomites, Sichuan Basin, China

Abstract

Microbialite textures, such as microbial mats and biofilms, were observed in the Middle Triassic dolomite in the Sichuan Basin, western China, using core examination, thin section petrography, scanning electron microscopy (SEM), and geochemical analyses. The dolomite texture, consisting of fibrous and spherulitic structures, is similar in morphology and size distribution to those observed in microbial culture experiments. Extracellular polymeric substances (EPS) were identified based on the occurrence of fibers forming a reticular pattern and nanometer-sized spheroids. The rare earth element (REE) and stable isotope (C, O, and Sr) compositions of the Middle Triassic dolomite were measured to determine their geochemical characteristics. Using seawater as a standard, the dolomitic microbialites (MD) exhibited significantly positive La and Eu anomalies and higher REE concentrations and (Nd/Yb)sn values than associated limestones, and these patterns are inferred to be related to initial complexation on organic ligands in the biofilm, as proposed by previous researchers. The ambient temperature during dolomite precipitation was estimated to be within the 23 °C to 50 °C range, as indicated by the δ18O values of the dolomite. This study suggests that various microbial effects can significantly affect diagenetic processes in the Middle Triassic dolomite.