2.4.1. Brutia Formation – BRT (December 2011 revision by A. Herbosch and J. Verniers)
Authors: After a locality south of Gembloux, Orneau valley (Delcambre & Pingot, 2002).
Description: The lower member (unnamed) contains (greenish) medium to dark grey mudstone and slate, compactly bedded. At about one third up in the unit, a grey mudstone that shows dark grey tubular lenses (about 1-2 mm wide and 2-5 mm long) is observed. These structures are interpreted as bioturbations (Chondrites sp. or “fucoïdes”). The upper part of the formation, named the Nivelles Member (Delcambre and Pingot, 2002), consist of a very thick volcano-sedimentary interstratified layer. It is formed by fine-grained quartzitic tuff, very hard, white, light pink or yellow and by weathering it is transformed into kaolinite. It has locally been exploited by the keramic industry (Ladeuze, 1990).
Stratotype: Not yet defined. For the “Eurite de Grand-Manil” type area in the Orneau valley, Gembloux, hamlet of Grand-Manil, between the localities Try-al-Vigne and Brutia (Delcambre and Pingot, 2002 fig. 17). For the “Eurite de Nivelles” type area in the Thines valley, between Monstreux and Nivelles (Herbosch and Lemonne, 2000; Hennebert and Eggermont, 2002.
Area: Outcrop area: Orneau valley and Thines valley (Monstreux-Nivelles); Coeurq valley (Herbosch et al., in press). Absent in the Sennette valley, due to the presence of faults.
Thickness: In the Orneau valley between 80 and 100 m for the entire formation, with about 40 to 60 m for the first member and 40 m for the “Eurite de Grand-Manil” (Delcambre & Pingot, 2002) and about 50 m for the “Eurite de Nivelles” in the Thisnes valley (Herbosch and Lemonne, 2000; Hennebert and Eggermont, 2002).
Age: Trilobites in the lower/middle part of the formation were described by Malaise (1903) without giving an age. The chitinozoans from the mottled mudstone member are dominated by Belonechitina cf gamachiana, indicating a mid or late Ashgill, possibly Hirnantian age (Samuelsson & Verniers, 1999, 2000). Graptolites described from the top of the formation in slates below the Eurite of Grand-Manil in the Orneau valley, belong to the C. cyphus Biozone (Elles in Maillieux, 1930a). Graptolite collections in the same levels mentioned in Gerlache (1956) and determined by Bulman (1950) as C. scalaris indicated the acuminatus Biozone (basal graptolite biozone of the Rhuddanian) or slightly above or below. The graptolites described from within the Nivelles Member indicated undoubtedly a C. vesiculosus Biozone (Rickards in Verniers & Van Grootel, 1991). The determinations from both valleys indicate a Rhuddanian (Early Llandovery) age for the top of the unit. A mid Ashgill ranging to the Rhuddanian age is proposed for the formation.
Remarks: Synonyms: lower part of the “Assise de Grand-Manil”; the upper volcanic layer has been called “Eurite of Grand-Manil” or “Eurite of Nivelles”.