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

The 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 (Belgium). 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 specific range of textures and assemblage of organisms: 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 limestone (facies A8-L8). The 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. This facies contains lenses of facies A5-L5, with stromatolitic coatings and Renalcis-rich thrombolitic bushes. These lenses were developed in greatest abundance closest 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, which were deposited in a moderately protected subtidal to intertidal area. Facies A8-L8 developed at subtidal depths in a quiet, lagoonal environment. The buildups started with the development of facies A3-L3, with microbial lenses and algal facies becoming progressively more abundant upwards. Above about 20m in each buildup, more protected facies are found in the buildup’s central part. This atoll-like geometry suggests the development of restricted sedimentation in this central area, sheltered by bindstone or floatstone facies. The initial development of the lower part of a buildup during a transgression and subsequent highstand would have been followed by reefal growth along the edge of the buildup during the succeeding lowstand; an atoll crown would then have started to develop during the following transgressive stage. The presence of restricted facies can be seen as the consequence of the balance between sea level rise and reef growth.