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2.2.5 Flénu Formation

Authors: Renier, 1912, 1928.

Description: Originally defined as “Assise du Flénu” is largely similar to the Charleroi Formation, also characterised by a rhythmic succession of coal-mudstone-sandstone sequences. Coal seams are frequent and may be rather thick (up to 5 m), though mining was mostly limited to the Borinage and the eastern Campine. Weakly marine incursions but especially tonsteins (volcanic ash layers in coals) allow a further subdivision.

The following members have been distinguished (names and definition according to Renier, 1928; Delmer, 1963; Paproth et al., 1983), from bottom to top:

- Meeuwen Member, according to Delmer (1963); base: Maurage = Petit Buisson = Aegir marine band, boundary Westphalian B - Westphalian C; named Maurage Member by Renier (1928) and Paproth et al. (1983). Stratotype: borehole KB121 Meeuwen-Bullen;

- Wasmes Member, according to Renier (1928); base Tonstein Hermance (Nord) = Hanas (Borinage) = Hagen 1 (Campine, Ruhr);

- Neerglabbeek Member, according to Dusar (1989); Dusar et al. (in press); named Hornu zone by Renier (1928); base weakly marine ‘Geisina’ band, overlying the coal seam with Tonstein Nibelung. Stratotype: borehole KB146 Neerglabbeek

The Flénu Formation is conformably overlying the Charleroi Formation along the Maurage = Aegir marine band. This marine band of glacio-eustatic origin is recognised everywhere and forms the best marker horizon within the Westphalian, above the marine bands of the Châtelet Formation. The boundary with the overlying Neeroeteren Formation is marked by the onset of coarse grained stacked sandstones.

Stratotype: Mons (Borinage) coal basin.

Area: The eastern Campine basin and nappes in the Borinage coalfield (western part of the Wallonian basin) in Belgium and extending beyond the national boundaries into Carboniferous basins of neighbouring countries.

Thickness: 1100 m preserved thickness in the Borinage; max 950 m in the Campine.

Age: Lower Moscovian; Westphalian C and basal Westphalian D according to traditional subdivision.