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2.5.14. Vichenet Formation - VCH

Author: Malaise, 1910; Mortelmans, 1953; Legrand, 1961.

Description: Grey mudslate, mudstone, siltstone and rare fine sandstone with often calcareous quartzic-chloritic pelite in the thick e-divisions; alternating distal thick-bedded turbidites with thin-bedded laminated hemipelagites. In the Mehaigne area the formation was described in detail: thick to very thick Tde sequences (on average between 21 and 51 cm), with T(b)cde sequences (0 to 35% of all sequences), an average 50% thicker than the average Tde sequences; c-divisions range between 0.8 and 13 cm. Characteristic are the very thick Tde turbidite sequences, grey to greenish colour sometimes calcareous. Neither the lower nor the upper boundary have been observed.

Stratotype: 350 m long road section north of the abandoned station of Vichenet-Bossière and east of the castle of Vichenet, Orneau valley; the section exposes the lower part of the formation.

Area: Orneau valley, type area; Burdinale valley, Mehaigne area; Landenne area.

Thickness: Estimated thickness in the Orneau valley: >200 m; Mehaigne area: recorded sections: 50 m; thickness of outcropping sections: 86 m; estimated thickness of the formation: >210 m (Verniers, 1983a); Landenne area: recorded sections: 12 m; estimated thickness of the formation: >350 m (De Winter, 1998 ms).

Age: No fossils found or studied in the Orneau valley; estimated middle to upper Wenlock, based on its relative position in relation to a fossil locality below the unit (Rickards, pers comm. in Verniers, 1983b). Chitinozoans are the only fossils found in the Mehaigne area in the middle of the formation. The assemblage belongs to the Sphaerochitina lycoperdoides global biozone of Verniers et al. (1995), lower to middle part of the Homerian (upper Wenlock) (Verniers, 1982; 1999). In the Landenne area the chitinozoans belong to the same biozone for most of the formation and for the lower part of the formation to the Cingulochitina cingulata biozone, middle of the Sheinwoodian (middle Wenlock) to the middle Homerian, upper Wenlock (De Winter, 1998 ms).

Remarks: Unit created by Malaise (1910) with type locality at the Vichenet railway station (Orneau valley) to represent the Ludlow in the Brabant Massif. A graptolite locality described by him, could not be located, nor could the fossils in his collection. Michot (1954) proposed the name Ronquières Fm for the Ludlow and the "Assise de Vichenet" was abandoned. The type locality in Vichenet was not described in detail until now. The Oteppe Fm described in detail by Verniers (1976 ms) and the MB8 fm (Verniers, 1981, 1982, 1983a; Verniers & Van Grootel, 1991), both in the Mehaigne area, are considered to be junior synonyms. The Landenne Fm described in the Landenne area by De Winter (1998 ms) is also considered a junior synonym.

(J. VERNIERS)