Coral reefs and growth dynamics of a low-angle Carboniferous platform: Records from Tianlin, southern China

Metazoan reef building receded globally during the Carboniferous after the Late Devonian extinction events. Shallow-water bioconstructionswere generally small and scarce, and corals played aminor role in their construction. However, in southern China, one large (50 m high) and three small (6–16 m high) coral reefs, dating from the Late Viséan–Serpukhovian, have recently been discovered in two localities in Guangxi. To understand the occurrence of these coral reefs and to constrain the factors that controlled their growth, three sections around Langping (Guangxi) were measured, and selected samples were analysed using petrography, biostratigraphy,and chemostratigraphy. The depositional environment of these exceptional Mississippian reefs was interpreted as a low angle platform (1–2.5°), recording oligotrophic conditions and normal salinity. The three small reefs have grown around the fair-weatherwave-base,whereas the large coral reef has grownin a deeper environment, between the fair-weather and storm wave-base. During Carboniferous, the global temporal reef distribution can be partly ascribed to paleoclimate changes. However, in southern China, the growth of extended Mississippian coral reefs is restricted to a short timewindow, the Late Viséan-Serpukhovian, which coincides with high amplitude and high frequency sea-level fluctuations most likely related to glacial pulses. Recent studies revealed that several rugose coral species survived the mid Carboniferous glaciation event thanks to the persistence of warm water ocean currents maintaining tropical conditions on the platform. Due to the scarcity of extended coral reefs elsewhere, it cannot be excluded that Tianlin provided an ecological refuge for corals, related to oceanic currents. Another potential favorable factor could be the mid ramp depositional setting. The deep-water platform areas could potentially provide more stable conditions, less affected by global stress events. However, the ‘deep reef refugia' hypothesis remains debatable as only very few studies have explored its validity at the community level. Even if the causes of the occurrence of an extended coral reef in southern China cannot be fully constrained, the existence of this large coral framework in the Late Viséan-Serpukhovian provides additional evidence that coral communities did build reefs after the Late Devonian extinction events