1. INTRODUCTION

The Carboniferous system derives its name from the coal-bearing sedimentary deposits (“coal measures”) in Western Europe. This is evidently the case in Belgium. The coal industry has long been the major industrial sector of Belgium and has greatly contributed to the shaping of the country. Coal has been mined since the twelfth century but attained highest levels between 1880 and 1960. Coal production peaked at 30 million tons annually; altogether 2,500 million tons of hard coal has been mined from Belgium. Decline of coal mining from the 1960’s till complete termination in 1992 was hastened by high exploitation costs and not by depletion of coal reserves.

Coal deposits of Upper Carboniferous ‘Westphalian’ age extend across the national boundaries of Belgium into the Netherlands, Germany and France. They originally formed part of the extensive Northwest European paralic coal basin, now structurally divided by uplift zones where coal has been removed by erosion, and by former foredeep basins and grabens, where coal is covered by a thick pile of more recent deposits.

Two major coal basins occur in Belgium, separated by the Anglo-Brabant Massif. The southern or Wallonian basin extends along the Sambre-Meuse axis from the French Nord – Pas-de-Calais coalfield in the west to the southern parts of the German Aachen and Ruhr coalfields in the east. It is subdivided into several districts: Liège, and the Borinage (or Mons), Centre, Charleroi and Basse-Sambre districts, which are all located in the most prolific Hainaut province. Smaller depleted coal deposits occur near Namur and Huy along the Meuse river, in the Dinant and Theux basins. Coal seams crop out in these southern basins, hence the early start of mining.

The concealed northern basin or Campine basin, which forms the continuation of the equally abandoned South Limburg coalfield of the Netherlands and the northern part of the Aachen and Ruhr coalfields in Germany, was discovered in 1901. Two major parts can be distinguished: the larger, western part or Antwerp Campine contains only Namurian and Lower Westphalian deposits, with limited coal reserves; the eastern part or Limbourg Campine contains a fully preserved coal-rich Westphalian section. The boundary between both parts is constituted by the NNW-SSE Beringen - Rauw fault systems. The Limbourg coal basin is further subdivided in an eastern and western coalfield by the N-S Donderslag fault system, which has markedly affected burial history and coal rank. The east Limburg coalfield adjoins the Dutch South Limburg coalfield.

The structuration of the paralic Coal Measures into different basins mainly occurred in two steps, associated with the variscan deformation at the end of the Carboniferous and with the cim­meric deformation during the Jurassic. Out of a common sedimentary origin developed two structurally different basins which greatly influenced the mining history.

With the exception of some tiny basins in the deepest synclines of the Dinant Synclinorium and in the overturned Theux tectonic window, the Wallonian coal basin is located in the Namur-Verviers Syclinoria (used in the geometrical sense, encompassing all autochtonous and allochtonous beds that may be present within this frame) at the front of the variscan mobile belt. It was strongly affected by variscan folding and thrusted against the rigid Lower Paleozoic Brabant Massif to its north. Folds are steep and asymmetrical; mechanised mining was only possible in the flat fold limbs (plateures). Deformed allochtonous overthrust sheets (‘nappes’) wherein a more complete section of the Coal Measures has been preserved, complete the Hainaut mining districts. Tectonic style and ‘northern’ thickness and coalification patterns suggest gravity sliding of the superficial massifs from the north as a result of uplift of the Brabant massif and evaporitic dissolution and subsidence in the Lower Carboniferous to Devonian strata underlying the Hainaut coal basin (Delmer, 1997). The Midi Overthrust forms the southern limit to this basin: Lower Devonian rocks are thrusted with a low angle over the parautochtonous coal basin whose real southern extension is yet unknown. In this way compressive tectonics dominated the southern basin; later deformation was possibly associated with dissolution of underlying carbonatic-evaporitic beds.

Variscan deformation of the Campine basin, which overlies the Brabant Massif, was relatively minor. It was associated with block-faulting movements along the major strike-slip faults affecting the Brabant Massif. Cimmeric uplift of the Brabant Massif caused a downwarp of the Campine basin towards the northeast. In this direction the Campine basin passes into the tectonically active Roer Valley Graben (or Roermond Graben) which forms the junction between the Lower Rhine Graben and the Central Netherlands Basin. In this way extensional tectonics and a more complete post-Carboniferous sedimentary record characterise the Campine basin.

The stratigraphic subdivision of the Carboniferous in Belgium, as in the remainder of Western Europe, continues to follow the classic distinction between carbonate-dominated marine ‘Dinantian’ and siliciclastic continental ‘Silesian’ containing the coal deposits (the latter subdivided in Namurian and Westphalian). Thickness of eroded sediments of uppermost Westphalian to Stephanian age may attain 1800 to 2500 m (Helsen & Langenaeker, 1999; Littke et al., 2000). The chronostratigraphic standard scale for the Carboniferous, with Upper Carboniferous stages defined in the marine deposits on the Russian Platform and the Urals, as proposed at the latest international geological congresses, is not finding any application in Belgium. Chronostratigraphic correlations mentioned here are indicative only. Radiometric ages of both chronozonations are not always concordant. For practical convenience, the following correlations for base of corresponding series/stages are accepted:

The Upper Carboniferous stratigraphy of Belgium was last revised by Paproth et al. (1983), after previous subdivisions by Renier (1912, 1928, 1930) and Delmer & Ancion (1954a, b). Nevertheless, some modifications have been made, based on the following principles:

- a lithostratigraphic framework applicable to all Upper Carboniferous deposits in Belgium;

- a subdivision better adapted to the mapping exercise for the new geological map of Wallonia, and revised for the Westphalian C as stratigraphic insight has been improved;

- clear distinction between lithostratigraphic units (formations and members) on the one hand, and marker horizons (used as the base of the units) on the other hand

- relying on the original definition of the different terms (‘assises’ and ‘horizons’) to avoid confusion.

Mapping practices, however, do not widely use the formal stratigraphic subdivision. They traditionally represent coal seam bundles (‘faisceaux’) and follow marker horizons, such as marine bands. Detailed cross-sections or subcrop maps in coal basins show the succession of coal seams.