Keratose sponge–microbial consortia in stromatolite-like columns and thrombolite-like mounds of the Lower Ordovician (Tremadocian) Mungok Formation, Yeongwol, Korea

Abstract Lower Ordovician stromatolite-like columns and thrombolite-like mounds, composed of fossilised keratose sponges (keratolites) and microbial carbonates, are reported from the Tremadocian Mungok Formation, Yeongwol, Korea. The stromatolite-like columns, which are up to 10 cm wide and high, consist of an inner core with low-angled (10–45°) layers that are covered by high-angled (>45°) layers. The inner core is made up of millimetric layers of alternating keratolite and microbial carbonate, and microbial carbonate dominantly comprises the outer cover. The entire columns are surrounded by bioclastic packstone to grainstone. The thrombolite-like mounds are domes a maximum of 100 cm high and 40–60 cm wide embedded within lime mud and shale. These mounds consist of keratolite–microbial carbonate clots and minor lithistid sponge–microbial carbonate clots. The stromatolite-like columns were formed in a high-energy subtidal setting, in which laminoid keratolite and microbial carbonate formed the tight laminar frame columns. Continued growth of the column narrowed the intercolumnar space, resulting in higher-energy hydrodynamic conditions that limited the growth of sponges but promoted growth of microbial organisms. In contrast, thrombolite-like mounds developed in a low-energy environment below fair-weather wave base, where irregular to bulbose keratose sponges with minor lithistid sponge–microstromatolite associations formed cluster reefs. There appear to have been ecological and/or environmental factors that affected the distribution of these sponges; keratose and lithistid sponges rarely occur together in the Mungok reefs, whereas lithistids are pervasive within coeval intermediate-energy microbial reefs elsewhere. These results demonstrate the importance of hydrodynamic controls on overall reef morphology and configurations during the Early Ordovician, and suggest that keratosan–microbial consortia may have been an integral component of the Great Ordovician Biodiversification Event, together with the lithistid sponge–microbial consortium.