CAMBRIAN SERIES 3 MICROBIAL REEF-DOMINATED CARBONATE PLATFORM ON THE EASTERN SINO-KOREAN BLOCK

The Cambrian Series 3 witnessed widespread development of microbial reefs after the first Phanerozoic reef crisis, which has largely been documented from Laurentia. This study reports various Cambrian Series 3 microbial-dominated reefs from the Daegi Formation of Korea toward comprehension of the regional variability of the Cambrian Series 3 reefs. The cumulative thickness of these reefs comprises over 20% of 180 m-thick carbonate-dominated Daegi Formation. The Daegi reefs are composed of prevailing thrombolite-siliceous sponge boundstone, subordinate Girvanella crust-Epiphyton boundstone, and minor stromatolites. Thrombolite-siliceous sponge boundstone is characterized by pervasive mesoclots and Epiphyton thalli, where interstitial space of the framework often contains siliceous sponges. The siliceous sponges are occasionally encrusted by mesoclots and Epiphyton, providing substrates for microbial attachments, hence interpreted as dweller and local frame-builder. Girvanella crust-Epiphyton boundstone is composed of vertical to laterally aligned, sheet-like to arcuate Girvanella crusts. The framework of boundstone is primarily constructed by Girvanella crust, where Epiphyton thalli and minor siliceous sponges attached on top of the crusts. Coalescing of columnar and upward-widening stromatolites constitute rare biostrome. The Daegi carbonate platform dominated by the thrombolitic and subordinate microbial reefs appears to have extensively developed on the eastern margin of Sino-Korean Block located at the peri-Gondwanan region, which are comparable to Laurentian counterparts with an exception of common metazoan incorporation into reefs. The current study suggests that Cambrian Series 3 carbonate platform was largely dominated by microbial reefs, and probably represents the peak of microbialites throughout the Phanerozoic.