Early recovery of sponge framework reefs after Cambrian archaeocyath extinction: Zhangxia Formation (early Cambrian Series 3), Shandong, North China

Abstract

Reefs dominated by the anthaspidellid sponge <em>Rankenella zhangxianensis</em>, the calcimicrobe <em>Epiphyton</em> and the stem-group cnidarian <em>Cambroctoconus orientalis</em>, together with encrusting microstromatolites occur early in the middle Cambrian (Series 3, late Stage 5) of Shandong, eastern China. In the Zhangxia Formation, these in situ components created a tight framework, with centimeter-scale growth cavities mainly filled by fine-grained matrix. Among them, <em>R. zhangxianensis</em> and <em>C. orientalis</em> mutually attached and locally formed metazoan-dominated frameworks. These metazoan-microbial reefs form thin lenses b2m wide within microbial mounds, and probably developed at least a fewcentimeters of synoptic relief above these surrounding structures. With an age of N505 Ma, these Rankenella reefs indicate recovery of framework-buildingmetazoans within ~5 million years of the archaeocyath reef decline. In structure, they resemble archaeocyath reefs as well as Early Ordovician lithistid sponge-microbial reefs, having conjoined conical macroskeletons thickly veneered by calcimicrobes and microbial carbonate with largely matrix-filled intervening cavities. In combination with other sponge-microbial reefs reported from Australia, Iran, Korea and the USA, they demonstrate that an anthaspidellid sponge-microbial reef consortium was widespread throughout the mid-late Cambrian.