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2.5.15. Ronquières Formation -RON

Authors: Malaise, 1883, emend. Michot, 1954, enlarged by Legrand, 1967, described in detail by Louwye et al., 1992 and Verniers et al., 1992.

Description: Dominated by grey mud-grade turbidites with compact or finely parallel laminated, fine silty slate and mudstone with quartzic pelites in the e-divisions, finely parallel laminated or obliquely stratified very fine sandstone; occasionally parallel laminated anoxic laminated hemipelagites, sometimes calcareous; very rarely oxic laminated hemipelagite, metabentonite or calcareous nodules. The Ronquières Fm varies from dark to light grey, caused by the quartzic and illitic composition. Changes in the relative frequency of the different Bouma mud-grade turbidites and laminated hemipelagites and of the thickness of each of the divisions allow in the type area the distinction of 11 units divided into subunits (Louwye et al., 1992).

The formation is different from the Llandovery formations and the Corroy Fm which have a light greenish or olive colour caused by the chloritic composition. The middle and upper Wenlock Les Vallées, Vissoul and Fumal Fms have the same colours and lithology as the Ronquières Fm, but lack the typical high frequency of Tbcde sequences, the thick Tb divisions and thin Tc divisions. The Vichenet Fm is characterised by much thicker often lightly calcareous Tde sequences. The upper boundary is not observed due to the cover of the Devonian unconformity. The lower boundary is not observed either.

Stratotype: outcrops around the village of Ronquières, Sennette valley; sections east of the bridge over the canal at Ronquières (along the canal, the road and the hill side called Mont Godart) (Michot, 1954) and sections on both sides of the large excavation for the inclined shiplift “Plan Incliné de Ronquières” (Legrand, 1967).

Area: Sennette valley, Landenne area and Burdinale valley.

Thickness: Minimum recorded thickness in the Sennette valley: 538 m (Louwye et al., 1992); observed min. 9 m, estimated >125 m in the Mehaigne area (Verniers, 1983a).

Age: Graptolites in Ronquières in the Sennette valley indicate the Neodiversograptus nilssoni Biozone (Leriche, 1912) or the Neodiversograptus nilssoni, Lobograptus scanicus and possibly the Pristiograptus tumescens biozones forming together the Gorstian stage (B. Rickards, 1984; pers. comm. in Louwye et al., 1992). The chitinozoans in Ronquières belong to the Cingulochitina convexa-C. serrata biozone and have an early Ludlow (Gorstian) and possibly a latest Wenlock age (Van Grootel, 1990 ms; Van Grootel in André et al., 1991; Louwye et al., 1992). The same biozone was found in the Mehaigne area (Verniers, 1982) and in the Landenne area (De Winter, 1998 ms).

Remarks: Junior synonym of the Velaine fm (De Winter, 1998 ms) in the Landenne area and of the MB9 fm (Verniers, 1983a) or the Boin fm in the Burdinale valley, Mehaigne area (Verniers, 1976 ms; Verniers & Van Grootel, 1991: fig. 7). The >61 m thick Mont Godart Fm (Louwye et al. 1992; Verniers et al. 1992), distinguished by its high frequency in Tc divisions and relatively thick laminated hemipelagite beds is reduced here in rank and considered to be the lowest of the 11 unnamed units of the Ronquières Fm.

(J. VERNIERS)