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3.2.3. Grands Breux Formation - GBR

Authors: Tsien, 1974: Bieumont, Lion and Boussu-en-Fagne Members; Coen-Aubert & Boulvain, 1999.
Description: In its type locality the Grands Breux FM is divided in ascending stratigraphic order into the Bieumont Mbr and the Boussu-en-Fagne Mbr. Large bioherms are developed in the uppermost part of the former member and the main part of the latter member. The Bieumont Mbr consists of greyish micritic, sometimes bioclastic, well bedded to nodular limestones with shaly intercalation in its upper part. The Boussu-en-Fagne Mbr essentially consists of greenish shales with calcareous nodules, alternating at some levels with nodular or bedded limestones. The Lion Mbr bioherms are made up mainly of greyish massif, micritic limestones with local development of stromatactis, stromatoporoids and tabulate corals. Talus-like structures of bedded argillaceous and bioclastic limestones indent the flanks of the bioherm in its upper part.
Stratotype:Railway cut Couvin-Charleroi south of Frasnes (Bieumont and Boussu-en-Fagne Mbrs); the Lion Mbr is exposed in the nearby Lion Quarry.
Area: The Bieumont and Boussu-en-Fagne Mbrs occur along the southern and south-eastern borders of the Dinant Synclinorium up to Barvaux. Bioherms representing the Lion Mbr are well developed in the western part of southern border of the Dinant Synclinorium, between Dourbes and Trélon (NE France).
Isolated occurrences are known from Rochefort and the south-western part of the Philippeville Anticlinorium.
Thickness: About 120 m for the Bieumont and Boussu-en-Fagne Mbrs together in the type area; thickness of the Lion Mbr bioherms is estimated at 120 m - 250 m. The formation is gradually thinning to the east and farther to the north-east.
Age: Middle Frasnian, containing conodonts of the hassi sl, jamieae and lower part of the Lower rhenana Zones (Mouravieff, 1982; Coen-Aubert & Boulvain, 1999).