Frasnian carbonate buildups of southern Belgium: the Arche and Lion members interpreted as atolls

(13 fi gures and 1 plate) ABSTRACT. O e facies architecture, sedimentary dynamics and paleogeographic evolution were reconstructed for a number of Frasnian buildups developed on a carbonate platform on the south side of the Dinant Synclinorium (Bel-gium). Bed-by-bed sampling and detailed petrography were complemented by magnetic susceptibility analysis, allowing for high-precision lateral correlation. Six facies were recognised in the buildups, each characterized by a specifi c range of textures and assemblage of orga-nisms: grey, pinkish or greenish limestone, with stromatactis, corals and stromatoporoids (facies A3-L3); grey limestone with corals, peloids and dasycladales (facies A4-L4); grey, microbial limestone (facies A5-L5); grey limestone with dendroid stromatoporoids (facies A6-L6); grey, laminar fenestral limestone, (facies A7-L7); grey, bioturbated lime-stone (facies A8-L8). O e time-equivalent off-buildup sediments include a large amount of transported material that originally came from the buildups. Sedimentological evidence suggests that facies A3-L3 developed between the storm wavebase and the fairweather wavebase, in a oligophotic environment. O is facies contains lenses of facies A5-L5, with stromatolitic coatings and Renalcis-rich thrombolitic bushes. O ese lenses were developed in greatest abundance clos-est to the fairweather wavebase, and they became anastomosing. Facies A6-L6 was developed in an environment with slightly restricted water circulation; there is a steady transition between this facies and the fenestral limestone A7-L7