Facies and palaeoecology of the upper member of the Aisemont Formation (Late Frasnian, S. Belgium): an unusual episode within the Late Frasnian crisis

The upper member of the Aisemont Formation – also known as the “second biostrome” - is the last significant Upper Frasnian carbonate unit in the northern part of the Namur-Dinant Basin (Southern Belgium). It consists of bioclastic limestone, often dolomitized, with numerous oncoids and corals. Despite its local name, the member is not a biostrome because only one thin bed is constructed by corals. It is mainly composed of limestone with numerous oncoids and a rich fauna of opportunistic organisms (bryozoans, brachiopods, gastropods, sponges, etc.) where Phillipsastrea and Frechastraea are the dominant coral taxa, associated with Alveolites. Ragged colonies of corals, as well as multi-encrusted bodies, show that the sea floor was soft and the rate of sedimentation was seasonal. These factors were unfavourable to common reef builders (stromatoporoids), thus the occurrence of one bed constructed by corals is a remarkable event that corresponds to the colonization of a hard ground defining the base of a falling stage systems tract. This confirms the model of the Aisemont sequence (third-order transgression-regression cycle). The abundant development of microbial structures in the member (oncoids) and in its lateral equivalent in stromatolites and thrombolites of the Petit-Mont Member (Les Valisettes Formation) in the Philippeville Anticlinorium is interpreted as an evidence of the environmental deterioration corresponding to the Late Frasnian Crisis