INTRICATE MICROBIAL-SILICEOUS SOPNGE FRAMEWORKS: NEW INSIGHTS ON SPONGE BIOCONSTRUCTION TOWARD THE EARLY ORDOVICIAN REEFS

The Early Ordovician witnessed increasing influence of skeletal organisms in microbial-dominant reef construction, which preceded Middle to Late Ordovician expansion of skeletal-dominant reefs. We provide detailed petrographic analysis of microbial-siliceous sponge reefs from the Early Ordovician Dumugol Formation, Korea with emphasis on the contributions of siliceous sponges, i.e., anthaspidellid sponge Archaeoscyphia and unidentified, non-anthaspidellid spiculate sponges, in the reef development. Dumugol reefs are mainly composed of microbialites, Archaeoscyphia with the subordinate spiculate sponges and calathids. Two types of boundstone textures were recognized: microbial-dominated and sponge-microbial boundstone. The former was mainly formed by upward and laterally stacking microbialites, whereas the latter by siliceous sponges and surrounding microbialites. The incorporation of microbialites to both types of boundstone demonstrates that microbialites were primarily responsible for the reef construction, both framework building and encrusting other components. Siliceous sponges contributed in diverse ways for construction of sponge-microbial boundstone. Archaeoscyphia attached each other and formed local sub-decimeter sponge framestones creating framework, shelter, and intraskeletal cryptic spaces. Spiculate sponges played diverse roles including encrusting other constituents, dwelling in cryptic spaces, stabilizing reef-flank sediments, and also rarely building framework. Overall development pattern and constituents of the Dumugol reefs are similar to coeval microbial-sponge reefs elsewhere. However, the diverse roles of siliceous sponges in construction of Early Ordovician reefs have not previously been described. Framework-building siliceous sponges reported herein represent one of the early phase of metazoan bioconstruction in addition to recently reported Early Ordovician skeletal-dominant reefs, which announce part of the prelude of changeover from microbial- to skeletal-dominated reefs afterward in the Phanerozoic.