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2.5.11. Les Vallées Formation - LVA

 

Authors: Verniers, 1976 ms; Verniers & Van Grootel, 1991.

Description: Grey mudslate, mudstone, siltstone and fine sandstone with non-calcareous quartzic pelite in the e-divisions; alternating distal turbidites with thin bedded laminated hemipelagites. In the lower 125 m the Tde sequences are medium thick to thick (15 to 30 cm); between 125 and 185 m the Tde sequences are thick and very thick (up to 60 cm), and in the upper 35 m the Tde sequences are again medium thick to very thick (30 to 60 cm) with relatively thick d-divisions. T(b)cde sequences are sometimes present. The lower boundary with the underlying Corroy Fm is observed, the upper boundary is not observed in the field, but it is probably the Vissoul Fm.

Stratotype: The formation has only few outcrops in the Mehaigne area; the whole formation was visible in a temporary pipeline gully(1975), north of Bois Tier à Mehaigne, between the hamlet Dreye and the village Fumal; the base of the formation and the lowest 5 m are defined in the road section north of the old “Moulin de Bounia”; the upper part of the formation is well visible in the high energy turbidite unit in the northern part of the railway section of the Orneau valley, 350-500 m east and north-east of the “Ferme de Chenémont” (parastratotype section); Les Vallées is a local place name in the village of Burdinne, in the vicinity of which the unit is present.

Area: Mehaigne and Burdinale valleys.

Thickness: Recorded sections: 225 m; estimated thickness: >225 m.

Age: Middle Wenlock, based on the relative position of the formation: above the underlying formation which contains at its top graptolites of a post-riccartonensis biozone (middle Wenlock) and below the covering formation also with middle Wenlock graptolites. The chitinozoans from the lower part of the formation belong to an interzone, subzone C4 (Verniers, 1982, 1999).

Remarks: Synonyms: MB5 Fm in Verniers (1981, 1983a); “partie moyenne de l’Assise de Corroy” (Verniers, 1983b).

(J. VERNIERS)