Morphology and growth factors of the Cambrian oncoids in the Shuangquan section, China

Abstract

The strata of the Xuzhuang Formation in the Shuangquan section of the North China Platform show continuous outcrops with various oncoids that lack conspicuous laminates microscopically. This study offers a comprehensive analysis of the distinct oncoids present in the Xuzhuang Formation in the Shuangquan section. Macroscopically, the orientation of the long axis of the oncoids presents different angles with the bedding surface, indicating growth in a turbulent, high-energy environment. Microscopically, the oncoids can be classified into five types based on morphological disparities: suborbicular, elliptical, elongated, rod-shaped, and irregular oncoid, with the nucleus predominantly situated near the center of the oncoid in most instances. Notably, these oncoids reveal filamentous calcified cyanobacteria fossils. This suggests that oncoid formation is the intricate result of calcification processes within biofilms and microbial mats dominated by cyanobacteria. Microscopic examination also reveals residual microbial mats and eroded oncoids. As a result, it is posited that the proper hydrodynamic energy, incident light conditions, and microbial activity constitute the three principal factors driving oncoid development in the Shuangquan section. Consequently, our findings not only present a representative instance of the diversity of Cambrian oncoids on the North China Platform, but also allow for a better understanding of the environment and process of oncoid formation.