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The Ordovician of the Condroz Inlier, Belgium: Trilobites from the Southeastern Margin of Avalonia ⁎ Robert M

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The Ordovician of the Condroz Inlier, Belgium: Trilobites from the Southeastern Margin of Avalonia ⁎ Robert M Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 272–294 www.elsevier.com/locate/palaeo The Ordovician of the Condroz Inlier, Belgium: Trilobites from the southeastern margin of Avalonia ⁎ Robert M. Owens a, , Thomas Servais b,c a Department of Geology, National Museum of Wales, Cardiff, CF10 3NP, Wales, UK b Institut für Paläontologie, Universität Erlangen, Loewenichstraβe 28, D-91054 Erlangen, Germany c Laboratoire de Paléontologie et Paléogéographie du Paléozoïque, UMR 8014 du CNRS, USTL, SN5, Cité Scientifique, F-59655 Villeneuve d'Ascq Cedex, France Received 11 May 2005; accepted 2 February 2006 Abstract During the Early Palaeozoic, Belgium was located in the eastern part of the microcontinent of Avalonia, which moved from high to low latitudes during the course of the Ordovician. In the Belgian Condroz Inlier (“Bande de Sambre-et-Meuse”), Ordovician trilobites have been reported from four formations: the early Llanvirn Huy Formation, the Caradoc Sart-Bernard and Oxhe formations, and the Ashgill Fosses Formation. These assemblages are some of the easternmost occurrences of trilobites in Avalonia, and they include aspects that are particularly interesting from a palaeobiogeographical standpoint. The trilobite faunas of the Huy and Sart-Bernard formations are revised in detail, while the others are discussed more briefly, having been redescribed in recent years. The Huy Formation belongs to a biofacies characteristic of the outer shelf/upper slope, and its faunas are typically Avalonian, almost identical to those recorded from the Skiddaw Group of northern England, and from the Ebbe and Remscheid anticlines in the Rhenish Massif, Germany. Pricyclopyge binodosa, which is inferred to be mesopelagic, is common to all these areas, and also occurs in Bohemia and Armorica. Placoparia cambriensis has a remarkably wide distribution in the early Llanvirn, and has been recorded from a range of biofacies across Avalonia, Armorica, Morocco and Bohemia. It is suggested that this species may have been epipelagic. Re-examination of the trilobites from the Sart-Bernard Formation has confirmed that all are conspecific with, or compare closely with Bohemian taxa. The trilobite assemblages of the Oxhe Formation are typical eastern Avalonian faunas, while the Ashgill Fosses Formation assemblage has much in common with those of other parts of Armorica, as well as with Baltica and Bohemia, attesting to the increasing proximity of Avalonia, Baltica and the Prague Basin during the latest Ordovician. © 2006 Elsevier B.V. All rights reserved. Keywords: Trilobites; Ordovician; Avalonia; Condroz Inlier; Belgium 1. Introduction one of the areas of outcrop of Lower Palaeozoic rocks in Belgium. While trilobites have not been reported from The Condroz Inlier (“Bande de Sambre-et-Meuse”), the Ordovician of the Ardennes, and only from the along with the Brabant Massif and the Ardenne inliers, is uppermost Ordovician (Ashgill) of the Brabant Massif, several assemblages have been recorded from the ⁎ Corresponding author. Tel.: +44 29 2057 3255; fax: +44 29 2066 Condroz Inlier, from four formations: the early Llanvirn 7332. (early Abereiddian) Huy Formation, the Sart-Bernard E-mail address: [email protected] (R.M. Owens) Formation (attributed herein to the middle Caradoc, 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.02.025 R.M. Owens, T. Servais / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 272–294 273 broadly equivalent to the British Burrellian Stage), the rocks in Belgian literature, was summarized by Verniers middle Caradoc (late Burrellian, Longvillian Substage) et al. (2001), who listed all the lithostratigraphical units. Oxhe Formation and the early to middle Ashgill While the investigations on the Brabant Massif and the (Pusgillian to Rawtheyan) Fosses Formation. Ardenne inliers provided a detailed knowledge of the It is today widely accepted that the Belgian Ordovician litho- and biostratigraphy of these areas (Verniers et al., sediments were deposited on the southeastern border of 2001; Herbosch and Verniers, 2002), the Condroz Inlier the microcontinent of Avalonia (Cocks et al., 1997; has remained poorly known, and the lithological suc- Servais and Fatka, 1997; Verniers et al., 2002). The cession is still partly unresolved. This area of outcrop is trilobites of the Condroz Inlier should therefore palaeo- currently in the progress of being restudied and re- geographically reflect the rifting from Gondwana during mapped. Detailed definitions and descriptions of its the early Ordovician and the migration of Avalonia constituent formations, with precise biostratigraphical northwards towards collision with Baltica during the late dating should therefore be available in the coming years. Ordovician. The Ordovician palynomorphs (acritarchs The Condroz Inlier is a long and narrow band of and Chitinozoa) of Belgium clearly indicate a shift from outcrops, directed WSW–ENE, parallel to and south of peri-Gondwanan (cold-water) assemblages in the early the Sambre and Meuse Rivers (Fig. 1). It is about 65 km and early middle Ordovician towards (temperate to warm long, but only 0.5 km to 4 km wide. The westernmost water) Baltic assemblages in the late Ordovician (Servais outcrops are to the east of Charleroi; eastwards, it and Maletz, 1992; Samuelsson and Verniers, 2000; extends almost as far as Liège. The outcrops consist of Servais et al., 2003; Vanmeirhaeghe and Verniers, 2004). sedimentary rocks ranging from early Ordovician to late The objective of this paper is to discuss the affinities, in Silurian age, predominantly siliciclastics, generally not terms of taxonomy, palaeobiogeography and palaeoecol- turbiditic and partially calcareous in the uppermost ogy, of the trilobite faunas of the Ordovician of the Ordovician (Ashgill). Mostly deposited in a deeper shelf Condroz Inlier. The assemblages from the Huy, Sart- setting, the sediments were attributed to the Condroz Bernard, Oxhe and Fosses Formations are compared with Shelf (Verniers et al., 2001, 2002). those from England and Wales, and also with those de- The various components of the Condroz Inlier are scribed from western Germany (Ebbe Anticline, Rhenish brought together as at least four inliers along the Midi Massif), the Prague Basin and from Armorica. The assem- Overthurst that separates the Namur and Dinant blages are integrated into a model of trilobite biofacies and synclinoria, which are composed of Devonian–Carbon- are plotted on a palaeogeographic reconstruction. iferous sediments. They all result from Variscan deformation, and together comprise the Condroz Inlier. 2. Geological setting They are (Fig. 1): 2.1. The Condroz Inlier (“Bande de Sambre-et-Meuse”) (1) a small northern inlier at Ombret (the Ombret Inlier); Cambrian to Silurian sediments in Belgium crop out in three different areas: in the northern part in the Brabant Massif; in the southeastern part in the Ardenne inliers, traditionally called “massifs” (from west to east: the Rocroi, Givonne, Serpont and Stavelot Inliers), and in the central part, south of the Sambre and Meuse rivers, in the Condroz Inlier, that is also named “Bande condrusienne” or “Bande de Sambre-et-Meuse”. All these massifs or inliers are overlain unconform- ably by Upper Palaeozoic sediments. Belgian geologists have focused most of their studies on these world-famous Devonian and Carboniferous sequences, that have pro- vided many names for internationally used geological series and stages. In contrast, the Lower Palaeozoic has been investigated to a lesser extent, and detailed rein- Fig. 1. Location of the Condroz Inlier, Belgium. The shaded area represents the ‘Bande de Sambre-et-Meuse’, and includes the main vestigations started only during the 1980s. Ordovician inlier, extending approximately from Vitrival-Bruyère to The stratigraphical succession of the Cambrian to the east of Huy, together with the smaller subsidiary inliers at Ombret, Silurian sediments, traditionally named the Caledonian Oxhe and Puagne. 274 R.M. Owens, T. Servais / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 272–294 (2) a major central inlier (the Condroz Inlier sensu some of them is not yet fully understood; acritarchs and stricto); Chitinozoa give the best prospects for a refined stra- (3) a small southeastern inlier between Huy and tigraphy and correlation. Liège: the Oxhe Inlier (also named the Fond d'Oxhe Window); 2.2. The Ordovician sequence of the Condroz Inlier (Fig. 2) (4) a small inlier in the southwestern part near Charleroi: the Puagne Inlier. The British regional series and stages have tradition- ally been used in the Belgian Lower Palaeozoic. Fig. 2 The original research on the numerous discontinuous shows the correlation of the different formations of the outcrops during the 19th century was followed by more Condroz Inlier with them, and also with the Global detailed investigations towards its end and during the Series and Stages and with the time-slices of Webby et al. earlier part of the 20th century. A stratigraphical suc- (2004). Taking into account that the lithological units cession and a first general overview was published by occur in four different inliers and that the formation Malaise (1910). Subsequently, after studying additional names can change from one inlier to another, the outcrops, Maillieux (e.g. 1926, 1938, 1939) modified the stratigraphy of the different units of the Condroz Inlier stratigraphical scheme by adding some new formation
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