First report of Cystostroma (Stromatoporoidea; Ordovician) from Sino-Korean Craton

A primitive stromatoporoid Cystostroma is reported from the Middle Ordovician Duwibong Formation of Taebaeksan Basin for the first time in Korea, which occupies the eastern margin of Sino-Korean Craton. It occurs as oncoid-like structures with hemispheroidal thin laminae and stromatolite-like columns composed mainly of stacked domal to bulbous encrusting laminae alternating with bryozoans. The former mainly occur within grainstone facies whereas the latter are surrounded by muddy facies. These contrasting growth forms may have resulted from the differences in energy-level and/or effect of intergrowth with other encrusting organisms. Though internal texture of Cystostroma is poorly preserved, application of the “white card technique” reveals imbricated small convex-upward cysts which are diagnostic of the genus. Previously reported occurrences of early Cystostroma are confined to the upper Middle Ordovician Chazy Group of North America which is more-or-less contemporaneous with the Duwibong Formation in age. The Sino-Korean Craton contains considerable records of early stromatoporoids and the present study adds another occurrence of a primitive stromatoporoid Cystostroma which may have possibly been related to the origination of stromatoporoids in the region. This finding is expected to provide a better understanding for the origin and dispersal of early stromatoporoids during the Early Paleozoic.