A NEW MIDDLE ORDOVICIAN REEF-BUILDING CONSORTIUM OF BIVALVE-SILICEOUS SPONGE-MICROBE FROM NORTH CHINA

A new reef-building consortium of bivalve-siliceous sponge-microbe from the Middle Ordovician succession of the western North China Platform is reported. The reefs are mainly composed of bivalves, siliceous sponges, and microbial carbonates (calcified microbes Epiphyton, Renalcis, and Girvanella, and microcrystalline microstromatolite), in addition to minor brachiopods, Amsassia, and tetradiids. Bivalves are very thin walled and articulated, mainly horizontal to upward in growth direction. Siliceous sponges are characterized by regularly spaced spicule networks embedded within micrite, which partly transform into peloidal texture. Bivalves are encrusted by sponges and/or filled with sponges, together with smaller bivalves occasionally embedded within sponges. Microbial carbonates either cover upper surfaces of siliceous sponges and bivalves, or occur independently as centimeter-scale patches. The reefs were constructed mainly by bivalves with subordinate siliceous sponges and microbes, which were subsequently encrusted and stabilized by sponges and microbes. The co-occurrence of bivalve-siliceous sponge is possibly analogous to their modern counterparts, in which sponges encrust bivalves or bivalves live within sponges. The bivalve-siliceous sponge-microbial reef in this study, together with other Ordovician reefs, represents ongoing changeover of microbial- to skeletal-dominated reefs during the Middle Ordovician.