Reefs in the Early Paleozoic Taebaek Group, Korea: A Review

Abstract

Various Early Paleozoic (Cambrian Series 3？Middle Ordovician) reefs occur in the Taebaek Group, Taebaeksan Basin, Korea, which is located in the eastern margin of Sino-Korean Block. The reefs occur from every carbonate-dominant lithologic units of the Taebaek Group, but their morphology and composition markedly differ. The Daegi Formation (Cambrian Series 3) contains siliceous sponge-Epiphyton (calcified microbe) patch reefs formed in shallow subtidal environments. The Hwajeol Formation (Furongian) contains sporadic dendrolites consist of Angulocellularia (calcified microbe) that developed in relatively deep subtidal environments. The Early Ordovician Dumugol Formation comprises several Archaeoscyphia (lithistid sponge)-microbialite-calathid patch reefs developed in shallow subtidal environments. The Early？Middle Ordovician Makgol Formation contains extensive microbial mats and some stromatolites formed in inter- to supratidal conditions. The Middle Ordovician Duwibong Formation contains Cystostroma (stromatoporoid)-Nicholsonella (bryozoan) patch reefs developed around shallow shoal. These reefs reflect ongoing development and evolution of reef building biotas and sea-level change during the early Paleozoic. Siliceous sponge-Epiphyton reef in the Daegi Formation is one of the earliest metazoan-bearing microbial reefs that formed after the archaeocyath extinction. The dendrolites in the Hwajeol Formation represent a rare example of deeper water form which flourished in shallower platform during the Cambrian Series 3？Furongian. At the onset of the Great Ordovician Biodiversification Event (GOBE), newly emerged reef-builders began to flourish, forming the Archaeoscyphia-microbialite-calathid reefs in the Dumugol Formation. Subsequent relative sea-level fall in the late Early Ordovician resulted in shallowing of carbonate platform and establishment of peritidal environments, forming the extensive microbial mats and stromatolites of the Makgol Formation. Ensuing Ordovician radiation enabled development of metazoan-dominated reefs of the Duwibong Formation. These reefs constructed by stromatoporoids and bryozoans are the prototype of similar Middle Paleozoic reefs. The reefs in the Taebaek Group warrant detailed investigations in order to understand the Early Paleozoic evolution and global-scale changes.