Macro- and microfeatures of Early Cambrian dolomitic microbialites from Tarim Basin, China

Chong-Hao Sun,Guo Yang,Ya-Sheng Wu,Ying Li,Hong-Xia Jiang,Wen-Qing Pan,Bao-Shou Zhang

doi:10.1186/s42501-020-00082-w

Abstract

The fabrics of microbialites preserved in limestones are generally better than in dolostones. What are the fabrics of the microbialites preserved in heavily dolomitized dolostones? This paper presents an example of a strongly dolomitized Cambrian microbialite profile. The Xiaoerblak Formation (Cambrian Series 2 Stage 3 and lower Stage 4) of the Sugaitblak section in Aksu, Xinjiang Uygur Autonomous Region, China is mainly composed of microbial dolostones. Due to strong alteration by diagenesis, their features, formation and environments have not been fully understood. Here, based on detailed observation on outcrops and thin sections, we show that this formation comprises four kinds of microbialites: laminite, thrombolite, thrombolitic laminite, and Renalcis framestone, in five intervals (Interval I to Interval V). We identified three main types of microbialite fabrics, i.e., clotted fabric, laminated fabric and skeletal fabric, and established a high-resolution vertical evolution sequence of the microbialites. The clotted fabric and the laminated fabric were further divided into subtypes. We found that the original fabrics were mainly affected by dolomitization, recrystallization and dissolution, and the alteration degree of the microbialite fabric is stronger in the lower part of this formation. The laminated fabric has the strongest resistance to diagenesis, followed by the clotted fabric. Based on studies of different rock types and sedimentary structures, we concluded that the sedimentary environment of Xiaoerblak Formation consists of three settings: a) Intervals I to III formed in restricted tidal flat environments, b) Interval IV and the lower part of Interval V in restricted deep subtidal environments, and c) upper part of Interval V in shallowing-up open subtidal environments.

Highlights

The term ‘microbialite’ was first proposed by Burne and Moore (1987) and later modified by Riding (1991) to ‘microbolite’, referring to the organosedimentary deposits formed by benthic microbial communities through trapping, binding detrital particles, and microbially-induced calcification
A total of 120 petrographic thin sections were made from the samples collected at 1.5 m interval on average, and were analyzed under the plane polarized light (PPL) and cross polarized light (XPL)
It seems that the laminated fabric has the strongest resistance to diagenesis, secondly the clotted fabric

Summary

Introduction

The term ‘microbialite’ was first proposed by Burne and Moore (1987) and later modified by Riding (1991) to ‘microbolite’, referring to the organosedimentary deposits formed by benthic microbial communities through trapping, binding detrital particles, and microbially-induced calcification. Cyanobacteria, bacteria, archaea and microalgae are considered as the main microorganisms involved in microbialite formation (Golubic 1973; Riding 1991). Microbialites are widely distributed throughout most of the history of the Earth. Wu et al (2018) proposed the term caststone for the rocks mainly composed of cast fossils of microbes, which is replaced here by castolite. The fabrics of microbialites are the basis for recognizing, understanding, and classifying microbialites, as well as determining their formation environments, and predicting their spatial and temporal distribution

Methods

Findings

Discussion

Conclusion