GEOLOGICA BELGICA (2013) 16/1-2: 49-65 Stratigraphy of the Lower Palaeozoic of the Brabant Massif, Belgium. Part I: The Cambro-Ordovician from the Halle and Ottignies groups. Alain HERBOSCH1 & Jacques VERNIERS2 1 Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles, Belgium, [email protected] 2 Department of Geology and Soil Science, Ghent University, Belgium, [email protected] ABSTRACT. Multidisciplinary research in the last 25 years and recent geological mapping in the Brabant Massif, have completely changed our knowledge about one of the poorly known part of Belgian geology. The sedimentary succession is surprisingly complete by comparison with the literature before the 1970s, from the lower Cambrian to the top of the Silurian, and very thick (>13 km), thus highlighting the need to produce an up-to-date stratigraphic nomenclature. In this first paper about the Cambrian and the lowest Ordovician, we describe in detail the formations, which are classified into two new groups, how the description of the units evolved through time, their lithology, sedimentology, boundaries and contacts, thickness, fossil content and type sections or most typical outcrop areas. The lower Halle Group comprises the Blanmont, Tubize and Oisquercq formations that consist of sandstone, siltstone and pale coloured slate. The overlying Ottignies Group comprises the Jodoigne, Mousty and Chevlipont formations formed of a more argillaceous and notably darker lithology. The two groups constitute a very thick (> 9 km) lower Cambrian to lowermost Ordovician siliciclastic succession, mostly pelagic and turbiditic. The cumulative thickness curve is concave-upwards which shows an extensional rift basin. A comparison with the Condroz Inlier shows that only the uppermost part of the sedimentary pile was observed in boreholes (Chevlipont Formation). KEYWORDS: Brabant Massif, stratigraphy, Cambrian, Avalonia, Condroz Inlier , rift. 1. Introduction A detailed 1/10.000-scale bedrock mapping project ran between 1993 and 2007 (13 maps published at a scale of 1/25.000: Multidisciplinary studies in the last 20 years show that the Doremus & Hennebert, 1995; Herbosch & Lemonne, 2000; Brabant Massif contains a very thick siliciclastic, often turbiditic Delcambre et al., 2002; Hennebert & Eggermont, 2002; Pingot and quite complete Lower Palaeozoic sequence, from the upper & Delcambre; 2006; Delcambre & Pingot, 2008; Herbosch & lower Cambrian to the uppermost Silurian and very probably Blockmans, 2012; Herbosch et al., accepted a, b; Blockmans to the lowermost Devonian (lowest Lochkovian; Verniers & et al., accepted; Hennebert & Delaby, submitted a, b). This Van Grootel, 1991; Steemans, 1994). Its prolongation below mapping combined with stratigraphic, palaeontological, tectonic, the Devonian cover can be traced by boreholes and geophysical geophysical, geochemical and geochronological work (e.g. Ph.D.: data to the NE under the Campine Basin as far as the Roermond Verniers, 1976; André, 1983; Servais, 1993; Everaerts, 2000; Graben, to the S at least under the northern half of the Brabant Debacker, 2001; Larangé, 2002; Dewaele, 2004; Vanmeirhaeghe, Parautochthon and to the NW under the North Sea into the 2006) has highlighted the need to produce a detailed and up-to- concealed Caledonides of East Anglia. The entire fold belt is date new stratigraphic scheme for the Brabant Massif. All these now called the Anglo-Brabant fold belt (Pharaoh et al., 1993) or studies allow the identification of mappable lithostratigraphic Anglo-Brabant Deformation Belt (Verniers et al., 2002). units for the Cambrian, Ordovician and Silurian of the whole SE Fig. 1 gives an overview of the stratigraphic evolution of outcropping zone of the Brabant Massif (Fig. 3). the Brabant Massif from the pioneering work of Malaise (last The purpose of this paper is to present the new stratigraphic version 1911) to the modern synthesis of Verniers et al. (2001). terminology and to describe of the new groups, formations and It should also be noted that before the 1970s the stratigraphic members, to explain how the description of the units evolved nomenclature was different in each river basin and the thickness through time, their lithology, sedimentology, boundaries and of the sedimentary succession has been much underestimated. contacts, thickness, fossil contents and, where possible, type A complete history of the geological discovery of the Brabant sections or most typical outcrop areas. This new stratigraphic Massif is given in Debacker (2001). scheme has been recently accepted by the National Commission MALAISE C. (1911) ANTOINE A. et R. (1943) LEGRAND R. (1968) MICHOT P. (1978) VERNIERS et al. (2001) Geolgical map n°120 (1910) Sm et Rv inverted succession ORDOVICIAN (Lower Silurian) ORDOVICIAN Madot Formation Huet Formation Sl 1b - Assise de Gembloux Ashgillien: schistes fossilifères Bornival Formation O et volcanites Ittre Formation R Sl 1a - Assise de Rigenée Assise de Gembloux Carodoc: schistes noirs Assise de Rigenée Rigenée Formation D O SALMIAN Grès et psammites V Assise de Strichon I Fm. Assise du Tribotte Formation C Sm1 - Assise de Villers-la-Ville de Undifferentiated Lower and I Psammites du Middle Ordovician de Tribotte A Villers- Tribotte N Villers- Abbaye de Villers la Ville Quartzophyllades Formation siliceux de Villers la REVINIAN Quartzophyllades de CAMBRIAN Couches de Chevlipont Ville Assise de lʼAbbaye Assise Sm1a - quartzozophyllades Laroche Couches de Chevlipont Formation Grès et schistes Mn zonaires micacé Chevlipont de Sm1a - quartzites et phyllades Rv - Assise de Mousty Schistes noirs de REVINIAN Mousty Faux Rvc - Assise de Mousty Assise de Mousty Mousty Formation Schistes noirs zonés C de Glory Rvb - Assise de Jodoigne A M Assise d'Oisquercq Rva - Assise d'Oisquercq Oisquercq Formation B R DEVILLIAN DEVILLIAN I A Dv2 - Assise de Tubize Assise de Tubize Dvb - Assise de Tubize Tubize Formation N Figure 1: Historic evolution of the Dv1 - Assise de Blanmont Dva - Assise de Dongelberg Blanmont Formation Cambro-Ordovician stratigraphy of (et Dongelberg) (= Blanmont) Jodoigne Formation the Brabant Massif from Malaise Dvo - Assise de Jodoigne (1911) to Verniers et al. (2001). 50 A. HERBOSCH & J. VERNIERS Figure 2: Chronostratigraphic position Age (Ma) Series Global Brabant Massif lithostratigraphy of the Cambrian and Lower Ordovician Stages Groups Formations lithostratigraphic units in the Brabant Massif (chronostratigraphy after Gradstein . I REBECQ ABBAYE DE VILLERS Fm. et al., 2012). Base “M. C.” at 509 Ma and MIDDLE DARRIWILIAN Group C 467.3 base “U. C.” at about 499 Ma are base of ORDOVICIAN I DAPINGIAN the traditional “Middle Cambrian” and 470 V 470 “Upper Cambrian” respectively (Peng et al., 2012 p. 477). O FLOIAN hiatus D LOWER 477.7 ORDOVICIAN R 480 TREMADOCIAN O CHEVLIPONT Fm. 485.4 GSSP C/O Stage 10 Tangissart Mbr. 490 490 FURONGIAN MOUSTY Fm. JIANGSHANIAN unnamed Mbr. 494 OTTIGNIES Franquenies Mbr. 497 PAIBIAN Group ? base "U. C" GUZHANGIAN Jodoigne Unit N 500 501 Jod.-Souveraine Unit SERIE 3 JODOIGNE Fm. A DRUMIAN Orbais Unit 505 Maka Unit I Stage 5 ? base "M. C" 509 Asquempont Mbr. R OISQUERCQ Fm. 510 Ripain Mbr. Stage 4 ? B Les Forges Mbr. SERIE 2 514 HALLE Group TUBIZE Fm. Rogissart Mbr. M Mont-St-Guibert Mbr. Stage 3 ? A 520 521 BLANMONT Fm. C Stage 2 ? 529 530 TERRENEU- VIAN FORTUNIAN 540 541 GSSP C/E EDIACARAN SYSTEM of Stratigraphy (sub-commission Lower Palaeozoic; NCS, 2009, Belgium (Fourmarier, 1920; Legrand, 1968; De Vos et al., 2012). In this first part, we begin with the description of the 1993; Piessens et al., 2005). At first sight, it appears as a gently six formations that cover a large part of the Cambrian and the ESE plunging broad anticlinal structure, with a Cambrian core lowest part of the Ordovician. This time interval corresponds flanked on both sides by Ordovician to Silurian strata. To the to Megasequence 1 (Vanguestaine, 1992; Verniers et al., 2002; S, SW and SE it is unconformably overlain by the Devonian to Linnemann et al., 2012) and is interrupted by a large stratigraphic Carboniferous deposits of the Brabant Parautochthon (Mancy et hiatus (Fig. 2). The chronostratigraphy and time scale of Gradstein al., 1999). To the S, the Brabant Parautochthon is tectonically et al. (2012) are followed throughout the paper. overlain by the Ardenne Allochthon along the Midi Fault 2. Regional geology: a short introduction System (Variscan overthrust). To the NW, the massif continues beneath the North Sea and links up with the East-Anglia Basin The Brabant Massif (Fig. 3) consists of a largely concealed (Lee et al., 1993). Both areas form part of the Anglo-Brabant WNW-ESE directed fold belt developed during Early Palaeozoic deformation belt (Pharaoh et al., 1993, 1995), the eastern branch times, documented in the sub-surface of central and north of a predominantly concealed slate belt moulded around the 3°E 4°E The Netherlands North Sea Oostende Antwerpen Nieuwpoort Gent Di jle 50°00'N LCS er Ijz e n n e Z Brussel e e France Lei ett Fig. 7 G northern limit of outcrops Fig. 10 S c fault contacts h e D Mehaign ld e e other geological e nd re S Ma e D contacts O T S n y r h n e r cq l n e y n e e l t n 0 10 20 km e t a 50°30’N e e Fig. 6 France u Figure 3: Geological subcrop map of the Middle and interstratified Brabant Massif (after De Vos et al., 1993; Cambrian Ordovician Silurian plutonic body volcanics Upper Devonian Debacker et al., 2004a) with the location low-angle unconformity contact location of Figure 7 and 10 Nieuwpoort-Asquempont = Asquempont Detachment System Fault Zone (NAFZ) of the Figs. 6, 7 and 10. STRATIGRAPHY OF THE LOWER PALAEOZOIC OF THE BRABANT MASSIF, BELGIUM 51 The Brabant Massif shows a very thick siliciclastic 20 pile, with an abundant turbiditic and pelagic sequences, ranging from the upper part of the lower Cambrian (Terreneuvian) 19 in the antiformal core to the upper Silurian and even to the lowermost Devonian along the rims (Figs.