Suitable environments for middle Cambrian microbial-sponge reef development: Reef complexes formed in shoal deposits of the Daegi Formation near the Dongjeom Industrial Complex, Taebaek, Korea

초록

Discoveries of microbial-sponge reef communities from Miaolingian (middle Cambrian) strata of Korea over the past decade have advanced our understanding on the history of early Paleozoic reef evolution. Despite recent progress, it still remains unclear what environmental conditions were suitable for development of these communities. In this study, microbial-sponge reefs and their surrounding deposits were examined from the Daegi Formation of Korea, where a large number of reefs has recently been exposed. The decimeter- to meter-scale Daegi bioherms are composed of thromboid-sponge and localized microbial crust boundstones, forming columnar, bulbous, lenticular to irregular structures. These structures are either coalesced or spaced within a few tens of centimeters, and comprise a larger reef complex. The majority of the thromboid-sponge boundstone is made up of thromboids, that are often merged laterally and vertically, and subordinately of lithistid sponges surrounded by thromboids and micrite. The microbial crust boundstone is dominated by platy to arcuate microbial crusts which are probably formed by Girvanella. Inter-reef sediments are mainly bioclastic packstone to grainstone and sporadic ooid-peloid and microbial crust packstone to grainstone, the former of which contains slightly higher proportion of micrite. Altogether, the dense population of microbial-sponge bioherms franked by grainy carbonate deposits indicates that shoal environments near the fair-weather wave base were one of the optimal habitats for microbial-sponge reef communities. This finding would enhance our understanding of Cambrian microbial-metazoan reef communities in paleoecological dimension.

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