GEOLOGICA BELGICA (2011) 14/3-4: 249-264 THE PRESUMED UPPER ORDOVICIAN GREEN ROCKS AT REBECQ RE- INTERPRETED AS A RESURFACING OF THE CAMBRIAN OISQUERCQ FORMATION (SENNE VALLEY, ANGLO-BRABANT DEFORMATION BELT, BELGIUM) Timothy DEBACKER1, Alain HERBOSCH2 & Jacques VERNIERS1 (8 figures, 1 table, 1 plate) 1Department of Geology and Soil Science, Ghent University, Belgium. E-mail: [email protected], [email protected] 2Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles, Belgium. E-mail: [email protected] ABSTRACT. Lithological, magnetic and geochemical data do not allow distinguishing the homogenous, green cleaved mudstone at Rebecq (Senne Valley), defined as the Upper Ordovician Hospice de Rebecq Formation by Herbosch (2005), from those of the Asquempont Member of the Lower to lower Middle Cambrian Oisquercq Formation. Moreover, structural geological considerations and new mapping show that the geology of the Rebecq area is better explained by re-considering the homogenous, green cleaved mudstone at Rebecq as a local resurfacing of the Asquempont Member of the Oisquercq Formation in the footwall of the Asquempont Detachment System. Using this idea, a new geological map is presented of the Rebecq area. As the rocks at Rebecq, in the type locality of the Hospice de Rebecq Formation, are better re-considered as belonging to the Asquempont Member of the Oisquercq Formation, the name Hospice de Rebecq Formation should be abandoned. This, in turn, implies that also the green rocks of the Grand-Manil area (Orneau Valley), previously considered as belonging to the Hospice de Rebecq Formation, should be renamed. For the latter, we propose the name Cimetière de Grand-Manil Formation. KEYWORDS: Asquempont Detachment System, Asquempont Member, geochemistry, Lower Palaeozoic, magnetic properties, Oisquercq Formation, stratigraphy 1. Introduction Valley) and in the Grand-Manil area (Orneau Valley) (Fig. 1), Herbosch (2005) introduced a new Ordovician On the basis of detailed mapping and lithological and formation. This formation consists of a green, slightly to sedimentological observations in the Rebecq area (Senne very homogenous, cleaved mudstone and Herbosch 228 34 50 66 82 98 114 130 146 162 178 194 3°E 4°E 5°E Middle and Upper X BAL X Devonian Y The Netherlands TICA 218 LAURENTIA Silurian North Sea AVALONIA Antwerpen Oostende ABDB Ordovician 208 V MM aris ont Cambrian c r a F n 198 De f n Interstratified volcanics Noetrematio Gent Di plutonic body jle 188 30051°00’N km Asquempont G er e Ijz te Detachment System 178 e Nieuwpoort-Asquempont n n e Fault zone Z 168 Brussel Position of locality Rebecq area X referred to in text: France Leie Gette A: Asquempont 158 B: Bierghes L X, Y in Lambert coordinates M F: Fauquez Marcq B Q Grand-Manil area L: Lessines 148 fault contacts S A M: Marcq c h Mehaig N e D F ne ld e Q: Quenast other geological e nd re contacts 138 Thyle Senne S O Dyle e r n 0 10 20km n n scale e e Y a t u t 128 France e Figure 1. Geological subcrop map of the Brabant Massif (after De Vos et al., 1993 and Van Grootel et al., 1997) showing the position of the two main outcrop areas of the Hospice de Rebecq Formation described by Herbosch (2005): the Rebecq area in the Senne Valley and the Grand-Manil area in the Orneau Valley. Also the position of localities referred to in the text is indicated. The upper right inset shows the position of the Brabant Massif within the Anglo-Brabant Deformation Belt (ABDB) along the NE-side of the Midlands Microcraton (MM) in the context of Avalonia, Baltica and Laurentia. 250 T. DEBACKER , A. HERBOSC H & J. VERNIERS Ma Formations Motorway A8 Stages Rhuddanian Brutia 444 N QUENAST Hirnantian Madot Ferme 14 Katian Fauquez du Bel Air Faille du Chateau Huet 151 Hospice de Rebecq Fm. 15 2 Bornival b 16 13 Ittre Ferme REBECQ 19 Haute Cabbecq 18 3 Sandbian a 461 c d 12 20 1 4 22 23 5 7 Rigenee e 21 11 Darriwilian 6 Quenast plug 150 9 Tribotte 24 10 Abbaye s t r o de Villers Ferme 8 Dapingian q Ste Catherine ORDOVICIAN Moulin p La Grande n 472 Faille Sud de Fauquez d'Hou Haie Floian m Ferme 149 Le Croiseau Tremadocian 500m Chevlipont 130 131 132 133 488 river/brook fault Outcrop of Hospice de Rebecq Fm. railway fault below cover Outcrop of Bornival Fm. and/or Ittre Fm. road limit of Palaeozoic Outcrop of Madot Fm. exposure Inferred cartographic extent of Hospice de Rebecq Fm. Figure 2. The Hospice de Rebecq Formation in the Senne Valley after Herbosch (2005). Left: Outcrop map of the Hospice de Rebecq Formation, of the Bornival Formation and of the Madot Formation in the Rebecq area between Quenast and Moulin d’Hou. Right: Stratigraphic column of the Ordovician in the Senne Valley. Observation hiatus zones are in white and the stratigraphic position of the Hospice de Rebecq Formation is indicated by an arrow. (2005) called this formation the Hospice de Rebecq position seems a logical choice, considering the Formation, after a well-preserved outcrop in the garden of cartographic occurrence and the existing stratigraphy of the hospice at Rebecq (Senne Valley). Stratigraphically, the Brabant Massif. Indeed, cartographically, the Quenast this formation was placed in the Upper Ordovician, plug at Rebecq (Senne Valley) is flanked towards its overlying the Bornival Formation and underlying the southern and western parts by this homogenous, green Huet Formation (see also Verniers et al., 2001) (Fig. 2). mudstone of the Hospice de Rebecq Formation of Although there is no fossil evidence, this stratigraphic Herbosch (2005), and both are completely surrounded by N 4°07’30’’E 4°10’E A 80 Quenast 1 K3 80 Quenast 2 Figure 3. Simplified Quenast Quenast 4 topographic map of the Quenast-Rebecq area, Ferme 14 du Bel Air Rebecq 6 K4 showing the outcrop Rebecq 5 Ferme 15 Rebecq 7 2 Quenast 3 distribution of the different b 50°40’N Haute 60 13 Cabbecq 16 Rebecq lithostratigraphic units, 18 3 Gralex 2 based on mapping by 50°40’N a 19 c d 12 Gralex 1 Debacker (2001), Herbosch 20 Senne 1 4 23 Rebecq 10 Quarry 22 5 Gralex 3 (2005), Lammertyn (2005), e E 11 7 21 Rebecq 8a 9 Rebecq 2 Herbosch et al. (in press) Rebecq 8b 60 C 10 6 and new observations. Rebecq 4 Quenast plug Outcrop numbers of r Ferme o q 8 100 80 Herbosch (2005) are p Ste Catherine n Rebecq 3 F Ferme Rebecq 1 circled, and outcrop names Rebecq 9 La Grande D Haie B of Debacker & Sintubin m (2008) are underlined. Also Ferme shown are three section Le Croiseau lines, showing the position 4°07’30’’E 1 km 4°10’E Railway of the three subsections of outcrop of outcrop of Tribotte outcrop of outcrop of Ittre outcrop of green rocks outcrop of Huet outcrop of Madot contour of expected Oisquercq or Abbaye de Villers Rigenée or Bornival considered as the or Fauquez Formation outcrop occurrence Formation formations Formation formations Hospice de Rebecq formations of Lower Palaeozoic the composite section of Formation Fig. 7. TH E PRESUME D UPPER OR D O V ICIAN GREEN ROCKS A T REBECQ RE -IN T ERPRE T E D 251 Ordovician formations (Figs 2 & 3). Especially towards southeasternmost, Belgian part of the Anglo-Brabant the southern parts of the area, Upper Ordovician formations Deformation Belt, one of the deformation belts of eastern abound (e.g. Bornival Formation, Madot Formation, Avalonia (Van Grootel et al., 1997; Verniers et al., 2002). Herbosch et al., in press) (Figs 2 & 3). Moreover, in the There is only evidence for one single progressive Grand-Manil area (Orneau Valley), a very similar, green deformation event, now called the Brabantian deformation mudstone is flanked by the Bornival Formation towards event (Debacker et al., 2005a). This deformation event the North and the Huet Formation towards the South (fig. was already happening during the late Llandovery, and 4 of Herbosch, 2005). In addition, according to the continued until the Emsian, possibly even Eifelian stratigraphy of the Brabant Massif shown in Verniers et al. (Verniers et al., 2002; Debacker et al., 2005a). This event (2001) and later confirmed by Vanmeirhaeghe et al. involved the inversion of a pre-existing basin (the Brabant (2005), a biostratigraphical hiatus exists between the Basin), and mainly resulted in the development of a Bornival Formation and the Huet Formation (Fig. 2). regional cleavage and cogenetic folding (Debacker et al., Hence, the positioning of the Hospice de Rebecq 2005a). There is no evidence of wide-spread reverse Formation by Herbosch (2005) within this hiatus, this faulting at the present-day erosion level (Giese et al., being compatible also with the cartographic distribution 1997; Debacker et al., 2004a), and thus far only one of the different formations, was accepted by most people significant reverse shear zone has been documented, this involved with the Belgian Lower Palaeozoic and was also being at Marcq in the Marcq Valley (Debacker, 1999). formally taken into consideration by the Belgian Palaeozoic stratigraphic committee. The Rebecq area is situated in the Senne Valley and is However, within the last two years, new data caused regarded as one of the classical outcrop areas of the the first author of this paper to start questioning the actual Brabant Massif, mainly because of the presence of the stratigraphic position of the green rocks at Rebecq (e.g. Quenast plug (Figs 2 & 3). The Quenast plug is a quartz Debacker et al., 2010).