Formative mechanisms, depositional processes, and geological implications of Furongian (late Cambrian) reefs in the North China Platform

The Furongian reefs are markedly different, in terms of macro- and micro-fabrics, from the Cambrian Series 3 reefs that are dominated by thrombolites and dendrolites and were constructed mainly by Epiphyton in the Shandong region. This difference is also recognized in the Beijing region, ca. 500 km away. The abrupt transition from the Cambrian Series 3-type to Furongian-type reefs, coincidently with a decrease in calcified microbe diversity, was most likely due to global euxinic oceanic conditions and a possible eustatic sea-level drop, rather than the highly diachronous, platform-wide drowning event (i.e., drowning of the Cambrian Series 3 carbonate platform). The abundant occurrence of sponge spicule networks in the Furongian reefs suggests that metazoan reef builders (i.e., sponges) resurged and became actively involved in the reefal systems prior to the Great Ordovician Biodiversification Event. This study may provide an important basis for further investigation into the evolution of reefal systems during the middle to late Cambrian when metazoan reef-builders were known to be scarce.