Early Cambrian microbial reefs, archaeocyathan inter-reef communities, and associated facies of the Yangtze Platform

A sedimentological and paleoecological study of part of the northern Yangtze Platform of China was conducted. Three sections of the Early Cambrian (Atdabanian/Botomian Stage) Xiannudong Formation where studied, and reveal that stromatolite and thrombolite reefs flourished in close association with an inter-reef community of archaeocyaths and other organisms. Adjacent to the reef environment were migrating ooid shoals, oncoids, peloids, and silty/sandy lime mud.No archaeocyathan reefs were observed in any of the measured sections, but irregular archaeocyaths were found in some microbial reefs, and archaeocyaths form the bulk of the debris in float-rudstone that was deposited between microbial reefs. We interpret the archaeocyath-rich float-rudstone to represent a storm-churned inter-reef deposit. Typically, ‘regular’ type archaeocyaths dominated the float-rudstone, while ‘irregular’ type archaeocyaths were found in abundance dwelling within the microbial reefs. This trend changed upsection as ‘irregular’ type archaeocyaths dominated both the float-rudstone and in the microbial reefs.The oncolite and peloidal mudstone are interpreted as representing a quiet water, lagoonal environment created by the presence of the microbial reefs and ooid shoals. During large storms material from the ooid shoals or microbial reefs would wash into the lagoon. The silty/sandy micrite represents a quiet water setting, which could either be a deep-subtidal environment or a restricted lagoonal environment.The facies associations of these three sections, along with other published data indicate that the Yangtze Platform, at least during the Early and Middle Cambrian, was a carbonate platform and equatorially located.