Cambrian microbial reefs in Shandong Province, China

Abstract

During the Cambrian, a typical epeiric platform (North China Platform) formed on the Sino-Korean Block. Various reefs formed on the platform since the Cambrian Series 2. The Cambrian Series 2 reefs (Zhushadong Formation) comprise various thrombolites that consist of Epiphyton, Kordephyton, tube-shaped microbe, Bija, Tarthinia, Renalcis, Amgaina, and Razumovskia. Compared with the Cambrian Series 3 reefs on the same platform (Zhangxia Formation), the Zhushdong thrombolites yield more diverse calcified microbes than that of the Zhangxia Formation, although the reefs were much smaller. The difference was most likely due to changes in depositional environments (abundant siliciclastic input and tidal effects vs. stable carbonate platform) and global changes within reefal environments (end-Cambrian Series 2 extinction of archaeocyaths and calcified microbes). The Furongian (late Cambrian) reefs are characterized by maze-like maceriate reefs that form several bioherms and biostromes. Among them, there is an extensive biostromal microbialite (10–20 m in thickness and over 6,000 km2 in area) which is overlain by grainstones. The lower part of the biostromal microbialite is characterized by flat-bedded megastructure that deposited during sea-level highstand. With rapid rise in sea level, the microbialites caught up with the sea-level rise in the topographic highs, forming domal megastructures, whereas the domal microbialites deposited together with abundant non-microbial sediment in topographic lows. Subsequent deposition and migration of coarse-grained non-microbial sediment during sea-level rise progressively terminated the entire microbialites. Microfacies analysis was done on the maze-like maceriate reefs of the Furongian succession. The maze-like maceria structure was formed by siliceous sponge spicule networks and microbial components including microstromatolites, Girvanella, and Tarthinia. The Cambrian Series 3 and Furongian reefs were compared. The Cambrian Series 3 reefs are characterized by thrombolites and dendrolites, which were mainly constructed by calcified microbe Epiphyton and Renalcis. On the other hand, many of the Furongian reefs are maze-like maceriate reefs formed by siliceous sponges and some calcified microbes including Girvanella and Tarthinia, and columnar stromatolites mainly constructed by Girvanella. The Cambrian Series 3 and Furongian are separated by a major geologic event that occurred across the series boundary, characterized by positive carbon and sulfur isotope excursion (Steptoean Positive Carbon Isotope Excursion), eustatic sea-level drop, extinction and diversification of organisms, and increase in characteristic sedimentary facies. Reefs were also greatly affected by the event

calcified microbe Epiphyton and Renalcis greatly decreased, which were later gradually substituted by sponge-microbial association and calcified microbes (mostly Girvanella).