The Chitinozoan Biostratigraphy of the Silurian of the Ronquie'res^Monstreux Area (Brabant Massif, Belgium)
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Review of Palaeobotany and Palynology 118 (2002) 287^322 www.elsevier.com/locate/revpalbo The chitinozoan biostratigraphy of the Silurian of the Ronquie'res^Monstreux area (Brabant Massif, Belgium) Jacques Verniers *, Geert Van Grootel, Stephen Louwye, Bregt Diependaele Research Unit Palaeontology, Department of Geology and Pedology, Ghent University, Krijgslaan 281 S8, B-9000 Gent, Belgium Abstract A major outcrop area in the Silurian of the Brabant Massif (part of the Anglo^Brabant Deformation Belt) lies along the Sennette valley, from the hamlet Fauquez to the area around Ronquie'res, and along the Samme and the Thisnes valleys, around the city of Nivelles and its hamlets Monstreux and Bornival. Recent lithostratigraphical mapping in the study area has shown the presence of nine Silurian formations (from bottom to top the Brutia, Bois Grand-Pe're, Fallais, Corroy, Petit Roeulx, Steenkerque, Froide Fontaine, Vichenet and Ronquie'res formations, the latter in its type locality). One sample from presumably the Huet Formation, one of the three Upper Ordovician formations present in the Monstreux area is also studied. The presumed presence of Upper Ordovician in an area poor in outcrops in the north of the Monstreux area is confirmed by the chitinozoans in that one sample. Fifty one samples from seven Silurian formations were dissolved to extract chitinozoans and 37 proved to be fossiliferous. Two formations remain unstudied. The chitinozoan assemblages are poorly to moderately preserved due to the high anchizonal metamorphism in the outcrop area. The assemblages are diverse and can be abundant, with a content of between 0.1 and 23 chitinozoans per gram of rock. They can be correlated with the global Silurian chitinozoan biozonation (the Angochitina longicollis, Margachitina margaritana, Cingulochitina cingulata biozones are recognised), with the Welsh chitinozoan biozones for the Wenlock in the Builth district where the graptolite biozonation was described at first, for the Ludlow with the chitinozoans biostratigraphy in the chronostratigraphical type sections of the Ludlow series at Ludlow, Shropshire, and with the local Silurian chitinozoans biozonation in the Brabant Massif. The presumed Late Ordovician age of these three poorly outcropping and previously poorly dated formations in the Monstreux area is confirmed with the chitinozoans. Four of the nine Silurian formations in the study area were previously dated with graptolites (Brutia and Ronquie'res Formations), with acritarchs (Fallais Formation) or with chitinozoans (Steenkerque Formation). The ages of the latter two formations was confirmed or refined with the chitinozoans. Three other formations (Corroy, Petit Roeulx and the combined Froide Fontaine and Vichenet formations) are dated for the first time in the study area. The Ronquie'res Formation was dated previously with graptolites as lower Ludlow, which is confirmed by the chitinozoans and refined to the lower middle part of the Gorstian. A very rapid sedimentation of this thick turbiditic formation is deduced. Only one Silurian formation (Bois Grand-Pe're Formation) remains undated in the study areas. A new species is described: Conochitina pumilio sp. nov. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: chitinozoans; Llandovery; Wenlock; Ludlow; biozonation; Brabant Massif 1. Introduction and area description * Corresponding author. Tel.: +32-9-264-4614; Fax: +32-9-264-4608. The results of a biostratigraphical study of chi- E-mail address: [email protected] (J. Verniers). tinozoans from two Silurian outcrop areas in the 0034-6667 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S0034-6667(01)00120-8 PALBO 2421 22-5-02 288 J. Verniers et al. / Review of Palaeobotany and Palynology 118 (2002) 287^322 Brabant Massif, Belgium, a part of the Lower of Ronquie'res, and the 1.6-km long trench dug Palaeozoic Anglo^Brabant Deformation Belt, for the Plan Incline¤ (inclined ship lift) of Ron- are presented. The ¢rst area is called herein the quie'res. In the latter the unconformity is clearly Ronquie'res area and contains the Silurian and visible between the folded and cleaved ¢ne silici- outcrops in the lower parts of the valley of the clastic rocks of the Silurian and the covering Mid- Sennette River and the Samme River, around the dle Devonian conglomerates, sandstones and village of Ronquie'res. In the upper parts of the shales (Givetian). The second area, called herein valleys, Palaeogene sediments are present. The the Monstreux area, outcrops in the lower part of area is bounded to the north by the Fauquez the Rhisnes Valley, an tributary of the Samme Fault, which brings the Silurian into contact River, near the village of Monstreux, near the with the Upper Ordovician, and unconformably city of Nivelles (Fig. 2). This area is bounded to to the south by Devonian strata (Fig. 1). There the south by the unconformable covering of Dev- are two well known large outcrops: the Mont onian and elsewhere by the Palaeogene. The out- Godart section, which outcrops east of the bridge crops in the area are mostly Silurian, but the Fig. 1. Location of the sections and samples in the Ronquie'res study area, on a topographical and geological map after own mapping. The inset shows the position within Belgium. The sampling place of Martin and Rickards (1979) is also shown (num- bers beginning with FM77.-). PALBO 2421 22-5-02 J. Verniers et al. / Review of Palaeobotany and Palynology 118 (2002) 287^322 289 Fig. 2. Location of sections and samples in the Monstreux study area, on a topographical and geological map after our own mapping. The numbers 1^3 indicate the graptolite localities in the literature or archives of the Belgian Geological Survey (see text). This ¢gure is situated east of Fig. 1 with a strip 950 m wide in between. Upper Ordovician is present also in the north, These ¢ndings are probably from outcrops 1.3 although poorly exposed. to 1.4 km north of the bridge section, north of Until recently palaeontological data from the the Ferme des Che'vres (Louwye et al., 1992). two study areas were sparse. Dumont (1848) Three authors (Gosselet, 1888; Cuvelier and Pa- was the ¢rst author who described the Ronquie'res quet, 1908) wrongly attributed these graptolites to outcrops, but Malaise (1873) was the ¢rst to men- the Mont Godart section. Leriche (1912) was the tion the presence of fossils: badly preserved grap- ¢rst to ¢nd graptolites in the Mont Godart sec- tolites, which resemble ‘Graptolithus priodon’. tion: Monograptus bohemicus Barrande, M. cf. PALBO 2421 22-5-02 290 Table 1 Amount of chitinozoan species or genera, the total number of chitinozoans counted, the total weight of dissolved rock examined and the number of chitinozoans per gram of rock J. Verniers et al. / Review of Palaeobotany and Palynology 118 (2002) 287^322 PALBO 2421 22-5-02 Table 1 (Continued) J. Verniers et al. / Review of Palaeobotany and Palynology 118 (2002) 287^322 PALBO 2421 22-5-02 The position of the samples in the Steenkerque Formation are given per section, below the Ferme des Che'vres section and above the section in Rue de Hiernoulet. The relative position of the samples in the two sections could not be established accurately. 291 292 J. Verniers et al. / Review of Palaeobotany and Palynology 118 (2002) 287^322 PALBO 2421 22-5-02 J. Verniers et al. / Review of Palaeobotany and Palynology 118 (2002) 287^322 293 nilssoni Barrande and M. colonus Barrande, which lychian age for her sample FM 77.6.1 from the are characteristic species for the lower Ludlow. outcrops north of the Ferme des Che'vres (Fig. 1), With this ¢nd, he also established for the ¢rst which we mapped as the top of the Fallais For- time the presence of lower Ludlow rocks in the mation. The chitinozoans indicate the middle to Brabant Massif. A fact to note is that Leriche Upper Wenlock for samples FM 77.5.1 to 3 from (1912) found specimens, which he attributed to the outcrops east of the Ferme des Che'vres (Fig. M. cf. nilssoni, but that afterwards no other au- 1), which we mapped as the Steenkerque Forma- thor (including Leriche, 1922) retains this infor- tion. mal species assignment. Mailleux (1933) mentions The area has been investigated extensively by the presence of Saetograptus colonus (Barrande), our team, with ¢eldwork, mapping and detailed M. bohemicus (Barrande), Lobograptus scanicus logging of the lithostratigraphy. Detailed descrip- (Tullberg) and Pristiograptus dubius (Suess) in tions of all the outcrops of the Ludlow part of the the Ronquie'res area, but it is not clear, however, Ronquie'res area, the detailed stratigraphical logs if the two latter species were collected at the Mont and a study on the metabentonites, are presented Godart outcrop (Louwye et al., 1992). Michot in Verniers et al. (1992). The re-evaluation and (1954, 1957) assigns the lower part of the Mont rede¢nition of the Ronquie'res Formation and Godart outcrop to the M. Nilssoni (sic)(Barrande) the interpretation of the sedimentary environment graptolite zone, sensu Elles and Wood (1901^ is shown in Louwye et al. (1992). In the Mont 1918). Godart and the Plan Incline¤ sections, 21 grapto- During the enlargement of the canal between lites levels were discovered. We refer to the latter 1963 and 1966 Legrand (1967), found Neodiver- two publications for the location of these levels in sograptus nilssoni (Barrande), M. bohemicus Bar- the detailed logs and maps. The state of preserva- rande and P. dubius (Suess) in the Mont Godart tion of the graptolites is mostly very poor and section. He did not recover many fossils from the species characteristic for the Gorstian biozones large Plan Incline¤ section: only graptolites attrib- Neodiversograptus nilssoni, Lobograptus scanicus uted to P. dubius (Suess) near pro¢le 75 and one and possibly Pristiograptus tumescens are reported specimen of a Siphocrinites, without precise loca- (Rickards, personal communication in Louwye et tion.