1 Introduction
The Lower Palaeozoic formations in Belgium crop out in six areas: the Brabant Massif in the north and north-west, the Condroz inlier, also called the “Bande condruzienne” or “Bande de Sambre-et-Meuse” (Sambre-et-Meuse Strip) in the centre of the country, and four inliers in the Ardennes, which traditionally are called “Massifs” in the regional literature: the two larger Stavelot and Rocroi inliers and the two smaller Givonne and Serpont inliers (figs. 1 and 2). Four of these massifs extend into the surrounding countries: the Brabant Massif into the Netherlands and France, the Stavelot Massif into Germany where it is called (Stavelot-) Venn Massif, the Rocroi Massif and the Givonne inlier into France. All the massifs or inliers are unconformably covered by Devonian rocks. The unconformity has been called Caledonian in the literature and results from orogenic deformations caused by the Avalonia-Baltica-Laurentia collisions during the Late Ordovician to Middle Devonian times.
The Brabant Massif contains a thick siliciclastic often turbiditic and rather complete Lower Palaeozoic sequence, from the lowest Cambrian to the lowermost Devonian (Lochkovian). Its prolongation below the Devonian cover can be traced by boreholes and geophysical data to the north under the Campine Basin until the Roermond Graben, in the south at least under the northern half of the Brabant Parautochthon and to the west under the North Sea into the concealed Caledonides of East-Anglia. The entire fold belt is called the Anglo-Brabant fold belt (Pharaoh et al., 1993).
The Condroz Inlier is composed of at least four tectonic wedges carried along the Midi Overthrust in the Variscan deformation front: a northern part at Ombret, a major central part and two smaller southern parts, the Oxhe Inlier in the east and the Puagne area in the south-west. It contains only Ordovician and Silurian siliciclastic sediments of a deeper shelf facies, generally not turbiditic, except at Ombret.
The four Ardennes inliers are similar in composition: they contain a thick siliciclastic, often turbiditic sequence from the lowest Cambrian to the top of the Middle Ordovician. They are all situated in the Variscan Ardennes Allochton, previously also called Dinant Nappe, thrusted from the south and south-east about ten to about one hundred kilometres into their present position.
The Stratigraphic Subcommission on the Lower Palaeozoic of Belgium has evaluated the descriptions of formations in the older and younger literature, the descriptions of units in publications in press or under study. It has put several units into synonymy with earlier described units, it abandoned units which are too large or too thin and it agreed on the following formations described below and located in the chronostratigraphy in fig. 3. Syntheses on the Brabant Massif can be found in Legrand (1968) and De Vos et al. (1993), on its Ordovician in Servais et al. (1993), on its Silurian in Verniers and Van Grootel (1991), on the Stavelot Massif in Geukens (1986, 1999), on the Rocroi Massif in Beugnies (1963) and on the dating with acritarchs of the Ardennes Massifs in Vanguestaine (1992).
The boundaries of the Silurian system and its stages and substages have been accepted by the International Stratigraphical Commission (see Holland & Bassett, 1989). For the definition of the chronostratigraphical boundaries of the Cambrian and Ordovician systems work is still in progress. For the Ordovician System the global chronostratigraphical version by Webby (1998) is used, next to the traditionally better known British chronostratigraphy as redefined by Fortey et al. (1995). The geological time scale of Gradstein and Ogg (1996) is used for the Cambrian series and stages and for the absolute time scale. No new abbreviations were attributed to unnamed members. In the updated figures 7 and 8 the global chronostratigraphy of Ogg et al. (2008) is used.
It should be mentioned that Precambrian rocks have never been proven in Belgium, except for some xenoliths in Ordovician intrusive rocks in the Brabant Massif dated with Proterozoic ages obtained by isotopic studies (André, 1991). The basement underlying the Lower Palaeozoic rocks in Belgium remains unknown.
It should also be mentioned that although some of the lowermost Devonian units have their bases in the uppermost Silurian, they will be discussed in the Lower Devonian section. The Les Tailles Fm in the Stavelot Massif with an age between Mid Ordovician and earliest Devonian is however described here.
The lithostratigraphy of the Condroz inlier has not been studied in detail during the last twenty years. A restudy will probably improve much of its descriptions and divisions. Most figures of the thickness of the units are estimates from geometry because only rarely the entire column of the unit is observable. The thickness is not corrected for compaction nor for tectonic thickening and hence should be regarded with a substantial range in error.
Figure 1. Top of Palaeozoic basement in Belgium. 1: outcrop area of Palaeozoic rocks; 2: isobath of the top of Palaeozoic basement under Mesozoic or Cenozoic cover; 3: faults (after Legrand, 1968 and Marechal, 1992).
Figure 2. Large structural units in the Palaeozoic basement in Belgium. A: outcrop area of Palaeozoic rocks; B: Variscan Midi thrust fault and northern limit of the Variscan deformation front; C: Lower Palaeozoic outcrop area; D: Lower Palaeozoic Brabant Massif under Mesozoic/Cenozoic cover. 1: Campine Basin; 2: Brabant Massif with 2a: Western Flanders borehole area; 2b: Dendre valley; 2c: Senne basin, 2d: Dyle basin; 2e: Gette valley; 2f: Orneau valley; 2g: Landenne area; 2h: Mehaigne area; 3: Synclinorium of Namur; 4: Condroz inlier, with 4a: Ombret area; 4b: Fond d’Oxhe window; 4c: Puagne area, 4d: western end: Acoz area; 5: Synclinorium of Dinant; 6: Anticlinorium of the Ardennes with 6a: Rocroi inlier; 6b: Serpont inlier; 6c: Stavelot (-Venn) inlier; 7: Synclinorium of Neufchâteau or of the Eifel; 8: Givonne inlier. Units 2, 4, 6 and 8 contain Lower Palaeozoic rocks, the units 1, 3, 5 and 7 Devonian and Carboniferous rocks. The units 5, 6 7 and 8 are part of the Allochton of the Ardennes. (after Marechal, 1992)
Figure 3. Chronostratigraphical position of the Lower Palaeozoic lithostratigraphic units of Belgium. The genesis of the sediments is not studied in the Givonne and the Serpont area and only the lithology is shown. The succession of the two units in the Serpont area is debated (Beugnies, 1960 and Geukens & Richter, 1962). The latter authors consider the succession to be inverted from what is shown in the column. The fat rectangle in the Series column of the chronostratigraphy shows under 1 the British Ordovician chronostratigraphy revised by Fortey et al. 1995 and under 2 the position of the Llanvirn and Llandeilo stages, before the latter was abolished in 1995. Abbreviations: Prec.: Precambrian; Moridu.: Moridunian; Whitla.: Whitlandian; Fenn.: Fennian; Abereid.: Abereiddian; Llan.: Llandeilian; Aurel.: Aurelucian; Bur: Burrellian; Chen: Cheneyan; Stref: Streffordian; Dev: Devonian. See figures 7 and 8 for the updated Cambrian and Ordovician stratigraphy of the Brabant Massif. The global chronostratigraphy of Ogg et al. (2008) is followed.
Figure 4. Chronostratigraphical position of the Lower Palaeozoic lithostratigraphic units in the Brabant Massif (detail of fig. 3).
Figure 5. Chronostratigraphical position of the Lower Palaeozoic lithostratigraphic units in the Condroz inlier (detail of fig. 3).
Figure 6. Chronostratigraphical position of the Lower Palaeozoic lithostratigraphic units in the Ardennes inliers (detail of fig. 3).