Résumé
De tous les récifs dévoniens, les édifices frasniens du Synclinorium de Dinant ont été parmi les plus anciennement et les plus souvent étudiés. Cet intérêt séculaire est lié en partie à la qualité des affleurements : actuellement, près de 75 récifs sont connus et la plupart ont été exploités en carrière par la technique du fil hélicoïdal, livrant au géologue des parois sciées de plusieurs dizaines de mètres carrés. Ces extraordinaires conditions d'affleurement, combinées à de bonnes conditions d'accès et à un canevas stratigraphique récent (Boulvain et al., 1999) font de ces édifices des références internationales. L'objet de ce travail est de synthétiser la sédimentologie et les conditions de développement des récifs du Frasnien du Synclinorium de Dinant. Les relations entre les types de récifs et l'évolution paléocéanographique du bassin de sédimentation sont également mises en évidence.
Mots-clés : Sédimentation récifale, Frasnien, Microfaciès, Atoll, Paléo-océanographie, Belgique, Synclinorium Dinant
Abstract
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 wavebase and the fairweather wavebase, in a oligophotic environment. (5), with stromatolitic coatings and thrombolitic bushes developed close to the fairweather wavebase. (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.
Key words: Reef sedimentation, Frasnian, Microfacies, Atolls, Paleo-oceanography, Belgium, Dinant Synclinorium
Dernière mise à jour le 30.06.2015