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Pétrologie sédimentaire, B20, Université de Liège, Sart Tilman, B-4000 Liège, Belgium (e-mail: fboulvain{at}ulg.ac.be)
The facies architecture, sedimentary dynamics and palaeogeographic evolution were reconstructed for a number of middle-late Frasnian carbonate mounds from the south side of the Dinant Synclinorium (Belgium). Nine facies were recognized in the buildups, each characterized by a specific range of textures and assemblage of organisms: spiculitic wackestone with stromatactis (facies Pm1), which becomes progressively enriched in crinoids and corals (Pm2); grey or pinkish limestone with stromatactis, corals and stromatoporoids (A3–L3, Pm3); grey limestone with corals, peloids and dasycladales (A4–L4, Pm4); grey, microbial limestone (A5–L5, Pm5); grey limestone with dendroid stromatoporoids (A6–L6); grey, laminar fenestral limestone, (A7–L7); and grey, bioturbated limestone (A8–L8). Sedimentological evidence suggests that facies Pm1 and Pm2 correspond to iron bacteria-sponge-dominated communities, developing in a quiet aphotic and hypoxic environment. A3–L3 developed between storm and fair-weather wave base, in an oligophotic environment. Facies A5–L5 developed close to fair-weather wave base. Facies A6–L6 and the fenestral limestone A7–L7 correspond to an environment with slightly restricted water circulation. Facies A8–L8 developed at subtidal depths in a quiet, lagoonal environment. The main differences between the middle and late Frasnian mounds concern facies architecture, and are a consequence of different palaeoceanographic settings. 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 keep up with sea-level rise. At the opposite extreme, during the 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.
