A new reef-building consortium from a Middle Ordovician succession of the western North China Platform is described, consisting mainly of bivalves, siliceous sponges, and microbial carbonates (Epiphyton-like, Renalcis-like and Girvanella calcified microbes, and microcrystalline microstromatolites), in addition to minor brachiopods, Amsassia and Rhabdotetradium. The bivalves are thin-walled and mostly articulated, indicating in situ preservation. The siliceous sponges are characterized by regularly spaced spicule networks embedded within micrite, which partly grade into peloidal textures. Three main types of bivalve-sponge associations are found: (1) larger bivalves (2-13 mm) encrusted by sponges, (2) sponges occupying internal spaces of larger bivalves, and (3) smaller (0.2-0.4 mm) bivalves embedded within sponge spicule networks. Microbial carbonates either cover the upper surfaces of siliceous sponges and bivalves, or occur independently as centimeter-scale patches. The reefal boundstones were constructed mainly by bivalves, siliceous sponges and microbes, which were subsequently encrusted and stabilized by additional sponges and microbes. Extensive early marine cementation forming fibrous cement helped stabilize the reef framework. The co-occurrence of bivalves and siliceous sponges is possibly analogous to modern-day counterparts in which sponges encrust bivalves or bivalves are living within sponges, suggesting a symbiotic relationship. The bivalve-siliceous sponge-microbial reefs of this study, together with other Ordovician reefs, represent the changeover from microbial- to skeletal-dominated reefs during the Middle Ordovician. The current example may represent an ancestral association of bivalve-siliceous sponge-microbe reefs, similar to those in the Mesozoic and Cenozoic, thus shedding light on the roots of such associations.
Bibliographical noteFunding Information:
JHL thanks R. Riding for discussion on the nature of microbes within the Chenluzhen reefs and K. Marenco for careful comments which helped improve this manuscript. The assistance provided by X. Yuan (China Petroleum Changqing Oilfield Branch) during fieldwork is greatly appreciated. This study was supported by the National Research Foundation of Korea ( 2015R1A6A3A03019727 ) to JHL, ( 2013R1A2A2A01067612 ) to DJL, and ( 2015R1A2A2A01007063 ) to SJC, and by a Korea University Grant to JHL and JH. This study is a contribution to the IGCP 653 Project ‘The onset of the Great Ordovician Biodiversity Event’.
© 2016 Elsevier B.V.
- Middle Ordovician
- North China Platform
- Siliceous sponge
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes