246 / 2019-06-30 00:33:42

Sedimentary Characteristics of Microbial Carbonates in Intermittent Period of Volcanic Eruption -A Case Study from Lower Cretaceous Shipu Group in Zhejiang Province, East China

Microbial carbonates, Lower Cretaceous, Shipu Group, Stromatolites

This study describes a sequence of microbial carbonates and volcanics of Lower Cretaceous Shipu Group, an example of microbialites influenced by volcanic activity. It is located at Shipu town in eastern Zhejiang Province on the coast of southeast China along the coastal lowlands of the western Pacific Ocean. Based on macroscopic observation of outcrop, microscopic examination of thin sections, Field Emission Scanning Electron Microscopy(FESEM) imaging analysis，Electron Probe microanalysis(EPMA) and Energy Dispersive X-ray Spectrometry (EDS) analysis, nine units of microbial carbonates have been recognized in the section and their thickness increased gradually upwards as volcanism decreased. There are ooids, bioclastic grains, intraclasts and serpulites in the grainstone with local dolomitization. Microbial reefs developed either individually or conjoining with the adjacent one which consisted of stromatolites and serpulid tubes with common recrystallization. Due to intensive silicification, microorganisms were preserved in crystal lattice so well that the original microstructure even the chamber of microorganism could be observed clearly through FESEM imaging analysis. A two-dimensional depositional model was established to illustrate the palaeoenvironmental evolution for the development of the various lithofacies of the Shipu Group: grain bank including spherulite developed in intertidal zone; microbial reef occurred in subtidal zone and laminated microbialites emerged in supratidal zone. Volcanism could affect development of microbialites both positively and negatively, because volcanic activities could provide abundant nutrition for microorganisms or kill them.
Keywords: Microbial carbonates, Lower Cretaceous, Shipu Group, Stromatolites