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2.5.3. Bois Grand-Père Formation - BGP

Authors: Delcambre & Pingot, in press a.

Description: Mostly medium to dark grey shale or slate unit, dark greenish grey at the base; at certain levels interbedding of many medium grey laminated siltstone beds and light coloured quartzitic very fine sandstone, a few to 20 cm thick, showing lamination or fine oblique stratification with undulating bedding planes; some beds can be calcareous; at least partly deposited as turbidites. The lower boundary is taken above the Eurite of the Grand-Manil member. The upper boundary is not observed.

Stratotype: Orneau valley, east side; small abandoned quarries and outcrops a few hundred meters south of the hamlet Grand-Manil and the localities Tri à la Vigne and Brûtia.

Area: Brabant Massif: not observed in the Senne and Sennette valleys, probably due to faulting; present in the Thisnes valley (Nivelles) and the Orneau valley; possibly in the 8 km long subcrop infiltration galleries of Voroux-Goreux, where spilitic pillow lava and basalts were described (De La Vallée-Poussin & Renard, 1876). Some similarities of the Bois Grand-Père Fm exist with the Deerlijk and Lust Fms from the subsurface in the SW of the Brabant Massif, e.g., the dominantly grey colour, the thin bedded obliquely stratified fine sandstone beds alternating sharply with the thin shale/slate beds. However the high proportion of dark graptolitic shale in the subsurface of Western Flanders is not observed in the few outcrops present in the Orneau valley. Hence a separate unit is maintained. The emendation by Legrand 1961b, to include also higher laying green shale units into the formation, is not maintained; the latter units are placed in the Fallais Fm.

Thickness: About 200 m (Delcambre & Pingot, in press a); estimated at 375 to 500 m (Verniers, unpublished).

Age: Graptolites have been described earlier from the cyphus, the convolutus and the crispus biozones (Maillieux, 1930a, 1930b, 1933; Michot, 1930). A mid Rhuddanian to mid Telychian age is tentatively proposed.

Remarks: Equivalent to part of the “Assise de Grand-Manil” (Malaise, 1900), which contains too many lithologies to be kept as a unit. The lower part is now incorporated in the Madot and Brûtia Fms, and the highest light green shale or slate part in the Fallais Fm.

(J. VERNIERS)