Cambrian to Lower Ordovician reefs on the Yangtze Platform, South China Block, and their controlling factors

Abstract

The Yangtze Platform, South China Block, maintained an epicontinental sea in the Cambrian and Ordovician with oldlands surrounding its northern and western margins and a southeastern slope belt. Many Cambrian to Lower Ordovician reef complexes are known from the Yangtze Platform for which an updated spatiotemporal framework is provided, their biotic composition and architecture is presented, and the reef development is discussed in a global context. The biotic structures of reef units on the Yangtze Platform largely match the evolutionary pattern in terms of composition and architecture. Microbes and calcareous algae are key components in all reefs but metazoan reef-builders are better suited to identifying macroevolutionary patterns. Large-scale dolomitization in Cambrian Series 4 on the one hand prevents recognition of metazoan reefs and on the other hand may indicate hypersaline environments, which prevented metazoan reef growth and only permitted small-scale stromatolites in a few regions. Ordovician metazoans appear to have played substantial roles in reef-building earlier on the Yangtze Platform than elsewhere. Specifically bryozoan and Calathium-lithistid sponge reefs diversified during the Early Ordovician. Climate and eustatic sea-level changes and their related impact on water energy, turbidity, and salinity determined the growth and demise of reefs. The morphology and arrangement of reefs were controlled by the topography of the sea floor.