FURONGIAN (LATE CAMBRIAN) SPONGE-MICROBIAL MAZE-LIKE MACERIATE MICROBIALITE IN THE NORTH CHINA PLATFORM: IMPLICATIONS TO THE EVOLUTION OF REEFS DURING THE EARLY PALEOZOIC

Metazoan reef-builders were generally dominant in the Phanerozoic reefs except for the periods after the extinction events, such as the middle–late Cambrian, early Silurian, late Devonian–early Carboniferous, and early Triassic. Instead, organosedimentary deposits formed by cyanobacteria (microbialite) flourished during these periods. In this study, we focus on the Furongian (late Cambrian) microbialites, a period between the early Cambrian archaeocyathan extinction and the Ordovician radiation (Great Ordovician Biodiversification Event; GOBE).

Maceriate microbialites, characterized by centimeter- to decimeter-scale branching maze-like structures (maceria), widely developed in both Laurentia and Gondwana during the Furongian and Early Ordovician. However, organisms responsible for formation of the maceria structures are still poorly known. We report for the first time the occurrence of siliceous sponges together with microbial components within the Furongian maceriate microbialites of the North China Platform. The maceria structures consist of sponge spicule networks as well as microbial components such as microstromatolites, Girvanella, and Renalcis-like forms, whereas the inter-macerial space is occupied by lime muds and some bioclasts. The sponges and microbes played various roles within the reef, as constructors, bafflers, and binders. The accumulative growth of maceria structures with balanced deposition of lime mud and bioclasts formed meter-scale bioherms and biostromes.

Temporally restricted occurrence of the maceria structures (Furongian-Early Ordovician) in various depositional environments suggests that similar biological process could have formed the structures elsewhere. Revealing the constituents of Chaomidian maceriate microbialites therefore shed light on the formation processes of these enigmatic microbialites as well as to the previously unknown metazoan participation within the Furongian reefs.