A NEW MIDDLE ORDOVICIAN SKELETAL-DOMINATED REEF ASSOCIATION FROM KOREA

Decimeter-scale patch reefs primarily composed of stromatoporoids and bryozoans are reported from the Duwibong Formation (Darriwilian), Taebaeksan Basin of Korea, located in eastern margin of Sino-Korean (North China) Block. The reefs are constructed by numerous centimeter-scale columns and masses made up of ragged thin laminae consisting of stromatoporoids (34%), bryozoans (16%), probable calcimicrobes and algae (4%) and siliceous sponge (<2%). Stromatoporoids (cystostroma sp.) are poorly preserved in general but the skeletons composed of characteristic small vesicular cyst plates are recognizable in parts. They frequently alternate with bryozoans which resulted in globular to columnar masses forming the key characteristics of the Duwibong reefs. The bryozoans, identified as a Nicholsonella sp., are characterized by unobvious granular walls and abundant acantho- and mesozooecia. This genus has been widely reported from Russia, North America and North China during Middle to Late Ordovician. The calcimicrobes and algae sporadically occur as tiny patches or thin encrusting laminae. Siliceous sponges encrust the surface of stromatoporoids and bryozoans as well as grow downward from the ceiling of cavities. Both stromatoporoids and bryozoans first emerged as reef building organisms during Early Ordovician from South China, associated with lithistid sponges, pelmatozoans, calcimicrobes and microbialites. The Duwibong reef built mainly by consortium of stromatoporoids and bryozoans is a new Middle Ordovician skeletal-dominated reef association not previously known, indicating the onset of widespread development of skeletal-dominated reefs afterwards.