LAUSR

Abstract Skeletal–microbial–cement reefs are a triple hybrid carbonate that mainly formed during the Pennsylvanian to Mid-Triassic, when a marked increase in microbial carbonate formation coincided with extensive precipitation of crystalline crusts on the seafloor. We report a new type of reef-building association from Middle–Upper Ordovician strata of western North China, in which erect thin tubes of tetradiids (coralomorph) are encrusted by the calcimicrobes Renalcis and Angusticellularia and then by a large amount of early marine cement that is presumably high-Mg calcite or aragonite in composition. The resulting meter-scale mound is embedded within intraclastic–bioclastic grainstone, implying high-energy shallow-marine conditions. The thin tetradiid tubes, which would have been unable to physically withstand strong waves and currents, are interpreted to have been consolidated by encrusting calcimicrobes and then by extensive early marine cementation. Tetradiid-bearing reefs have generally been reported from muddy successions; the results of the present study suggest that consolidators were important in reef-building in high-energy environments during the later Ordovician. Considering also the coeval bivalve–sponge–microbial–cement reef reported from the same area and a sponge–microbial–cement reef from Arctic Canada, early marine cementation appears to have been at least locally important in the late Ordovician, similar to the Pennsylvanian through the Mid-Triassic. These triple hybrid carbonates may have formed by a combination of: (1) emergence of newly evolved skeletal reef-builders during the Great Ordovician Biodiversification Event; (2) development of CO2-concentrating mechanisms in calcimicrobes induced by a decrease in atmospheric CO2; and (3) an increase in the calcium saturation state in seawater resulting in extensive abiotic cementation as well as calcification of microbes. All of these factors might have been induced by global cooling throughout the Mid–Late Ordovician.