Frasnian atolls and reefs from the Dinant Synclinorium: sedimentology and palaeoceanography

The facies architecture, sedimentary dynamics and palaeogeographical evolution were reconstructed for a number of Middle to Late Frasnian buildups developed on a carbonate platform on the south side of the Dinant Synclinorium (Belgium, France). Nine facies were documented in the buildups, each characterized by a specific range of textures and assemblage of organisms. Sedimentological evidence suggests that facies (1) and (2) correspond to iron bacteria-sponge-dominated communities, developing in a quiet aphotic and hypoxic environment. (3) developed between the storm wave base and the fairweather wavebase, in a oligophotic environment. (5), with stromatolitic coatings and thrombolitic bushes developed close to the fairweather wave base. (6) and the fenestral limestone. (7) correspond to an environment with slightly restricted water circulation. (8) developed at subtidal depths in a quiet, lagoonal environment. The main differences between the Middle and Late Frasnian mounds concerns facies architecture and are a consequence of different palaeoceanographic setting. The large flattened Middle Frasnian Arche and Lion buildups show limited vertical differentiation, large-scale progradation features, extensive exportation of material towards off-reef environment and development of inner lagoonal facies. They grew offshore from a well-developed carbonate platform with a healthy carbonate factory. Middle Frasnian sea level fluctuations were relatively mild, and sedimentation was able to counterpart sea-level rise. At the opposite, during Late Frasnian, severe eustatic rises, together with rising oceanic hypoxic conditions were responsible for frequent collapses of the carbonate factory, drowning of the Middle Frasnian carbonate platform and development of buildups with relatively limited lateral extension, high vertical facies differentiation, low potential for material exportation and high content in microaerophilic iron bacteria