Abstract
With a significant rise of metazoan reef-builders in shallow marine environments, the Late Ordovician saw a substantial increase in precipitation of crystalline crusts in reef systems, which may reflect a dramatic environmental change in the ocean. Isopachous crystalline crusts occur abundantly in boundstone of Katian sphinctozoan-bearing microbial reefs on the Zhe-Gan Platform, South China. These deposits are characterized by in-place carbonate textures that are interpreted as associations of abiotic, bioinduced and biocontrolled fabrics, considered here to be hybrid carbonates. Little previous work has been done to investigate how these hybrid carbonates are related to the transition in biotic composition during the Ordovician. These fabrics occur in well-preserved sphinctozoan-bearing reefs in the lower member of the Sanqushan Formation at the Jitoushan Section in Yushan County, Jiangxi Province. The meter-scale reefs are dominated by Corymbospongia, Amsassia and microbial components, and are cemented by fibrous sparry crusts. The intertwined Corymbospongia and Amsassia build the primary skeletal framework. Microbes and subsequent cementation enhance the stability of the framework by covering most of the macrofossil skeletons. The studied skeletal-microbial-cement reefs share a lot of similarities with contemporaneous reefs of North China and Central Nepal, but differ from Early Ordovician reefs, which lack sparry crusts. Our results provide evidence for extensive synsedimentary cementation on the seafloor, which could have played a crucial role in the consolidation of Late Ordovician reefs during the icehouse conditions before the Hirnantian glacial maximum.
Published Version
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