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5.27. Molignée Formation – MOL

 

Authors: Groessens, 1975; Mamet, 1964; Conil & Groessens, in Groessens et al., 1976; Paproth et al., 1983; Hance, 1988; Hance et al., 1994).

Description: The formation is characterized by alternations of thin-bedded (centimetric to decimetric), often laminated, black limestones (“black marble” composed of lime mudstones with radiolarians, and packstones) and thick-bedded, dark-grey limestones (lime mudstones to wackestones). These alternations are the poly- and monosequences of Mamet (1964). The ratio black /dark-grey limestones increases upwards. Slumping is common. Nodular chert occurs locally. The formation is famous for the exceptional preservation of the (rare) macrofossils including echinoids, molluscs, graptolites, brachiopods, medusae and fish.

The basal contact with the underlying Leffe Fm can be progressive. The Molignée Fm is overlain by the Salet Fm and does not extend up to the Neffe Fm as described by Delcambre and Pingot (1993).

Stratotype: Road section along the side road leading to the village of Salet from the main road along the Molignée valley, about 4 km WSW of Yvoir. The base of the formation is considered here as corresponding to bed 169 (as suggested by Hance et al., 1994), the level at which the “black marble” facies becomes a significant component, and not to bed 52 as previously stated (Conil et al., 1977; Paproth et al., 1983).

Area: DSA. The Molignée Fm extends in a narrow band, oriented WNW-ESE, bordered by the Waulsortian discontinuous barrier to the south and the flank of the shelf to the north. It can locally overlie the Waulsortian buildups.

Thickness: The thickness varies from a few metres to more than 100 m. It is 57.6 m thick in the stratotype.

Age: Moliniacian (early Visean). In the stratotype, the base of the formation postdates the entry of the foraminifer Eoparastaffella (bed 162) and that of the conodont Gnathodus homopunctatus. It ends at the top of the Cf4bSubzone at Salet, but can extend into the Cf4g Subzone near Dinant. At Salet, the Molignée Fm includes the TST and HST of the third-order sequence 5 of Hance et al. (2001).