Relatively deep subtidal microbial–lithistid sponge reef communities in Lower Ordovician rocks reveal early escalation of the Great Ordovician Biodiversification Event

Abstract

The timing and magnitude of early stage of the Ordovician radiation with respect to reef consortia are still not well understood. In this study, we analysed the constructional style and environmental conditions of middle Tremadocian patch reefs in the middle Dumugol Formation of Korea, and assessed the temporal-spatial patterns of Furongian–Lower Ordovician reefs, to obtain a better insight into the transition of reef development during the Early Ordovician. The metre-scale microbial–sponge patch reefs, which are some of the oldest Ordovician reefs in this region, formed when carbonate-prevailing depositional systems were being re-established after deposition of shale-dominated outer shelf systems. These reefs and the enclosing lenses of normally graded packstone to grainstone change laterally to limestone–shale couplets, indicating that the reef-builders had settled in low-energy mid-ramp habitats. The main reef-building organisms are laminar, columnar–domal microbialites (peloidal crusts and stromatolites) and annulated obconical to cylindrical specimens of the anthaspidellid sponge Archaeoscyphia, most of which are mutually attached and form vertically and laterally connected reef frameworks. These reef attributes are comparable to those of their middle–upper Tremadocian counterparts in peri-Gondwana and Laurentia, but differ from Cambrian–lower Tremadocian microbial-lithistid sponge reefs that are largely concentrated in shallow subtidal deposits. Bio- and/or chemostratigraphic data from these Lower Ordovician successions reveal that the occupation of deeper subtidal ecospace by microbial–anthaspidellid consortia occurred rapidly at a global scale during the middle Tremadocian. This appearance, and the nearly contemporaneous emergence of novel skeletal reef-builders, may represents a pulse of the early Great Ordovician Biodiversification Event, shortly after the occurrence of the Top Skullrockian Isotopic Carbon Excursion (TSICE).