Calcified microbial reefs in Cambrian Series 2, North China Platform: Implications for the evolution of Cambrian calcified microbes

Abstract

This study focuses on the microbial reefs of the Zhushadong Formation (Cambrian Series 2) in Shandong Province, China in order to understand the evolution of calcified microbes in the North China Platform during the Cambrian Series 2 and 3. The microbial reefs occur in a thin unit, ca. 3m thick, over an area of 1km2. They consist of three types of thrombolite based on their mesostructures: rimmed, grainstone-patch, and dendritic. The thrombolites mainly occur in various coarse-grained carbonate facies, including crudely stratified oolitic grainstone, stromatolitic grainstone, and disorganized limestone conglomerate. Calcified microbes in the thrombolites include Epiphyton, Kordephyton, a tubiform microbe, Bija, Tarthinia, Renalcis, Amgaina, and Razumovskia. The Zhushadong thrombolites were formed within a grainstone shoal, and experienced repeated burial and exposure. The rimmed thrombolite and grainstone-patch thrombolite experienced abundant input of carbonate grains (forming grainstone patches). In contrast, the dendritic thrombolite formed solely by calcification of microbes that mainly include Epiphyton, Tarthinia, and the tubiform microbe. The outer crusts of the rimmed thrombolite were formed by Amgaina, under high energy conditions.The diverse calcified microbes of the Zhushadong Formation form the earliest assemblage of their type in the North China Platform. Their descendants, mostly Epiphyton, subsequently thrived, forming a ca. 180m thick microbialite–oolite-dominated succession during the Cambrian Series 3 (Zhangxia Formation). Although the reefs in the Zhushadong Formation are much smaller than those of the overlying Zhangxia Formation, their calcified microbes are more diverse. This most likely reflects changes in depositional environments (e.g., abundant siliciclastic input and tidal effects vs. those of a stable carbonate platform), and/or global changes within reef environments (e.g., end-Cambrian Series 2 extinction of archaeocyaths and calcified microbes). A decrease in diversity of calcified microbes in the North China Platform, where archaeocyaths were absent, may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence.