Development of Middle Cambrian leiolitic bioherms dominated by calcified microbes: A case study of the Xinji Section (North China Platform)

Abstract

Among the varieties of microbial carbonate rocks, leiolites are rare in ancient rock records and have only few examples in the modern environment, which makes them difficult to study. This study documents the sedimentary environment, characteristics and composition of microbial carbonates, dominated by calcified microbes, which developed in the Cambrian Miaolingian Xuzhuang Formation on the North China Platform. The upper part of the formation in the Xinji section of Lushan City, Henan Province is bounded by a drowning-unconformity surface of a third-order depositional sequence. Beside the development of an ooid shoal in response to sea-level fall, a bioherm dominated by leiolite also developed in the upper part of the Xuzhuang Formation. These leiolites represent special sedimentary phenomena due to their occurrence with high-energy ooid shoals. Microscopically, they are characterized by abundant calcified cyanobacteria fossils, such as dichotomous-branching Epiphyton and filamentous cyanobacteria, thus showing the diversity and complexity of the cyanobacterial microbial mats that predominate the leolitic bioherm. Four groups are confirmed for Epiphyton based on the morphological characteristics of calcified filaments. The filamentous cyanobacteria can be divided into Girvanella and Subtifloria , and their possible forming mechanisms are discussed. The macroscopic and microscopic characteristics of the current leiolites provide important research clues and typical examples for understanding the origin of the Miaolingian leiolitic bioherm and its paleoenvironmental significance. • The Cambrian leiolitic bioherm developed in a high-energy environment during a drop in sea-level on the North China Platform. • The present study delineates the chambered, shrub-like Epiphyton and filamentous cyanobacteria (Girvanella and Subtifloria) • The microbial calcification occurred prior to early diagenesis, thus opposing the possibility of Epiphyton as a “diagenetic fossil”.