The Ruhr coal basin is part of the external fold and thrust belt of the Variscan orogen in Central Europe. Information from extensive coal mining, outcrops in the south of the Ruhr district, reflection seismic surveys and about 800 exploration boreholes in the north, support the interpretation of a mostly molasse-type sequence, more than 6000 m thick, of Namurian and Westphalian age. Both the southwest-northeast trending sedimentary basin structures and the fold structures of the Ruhr Carboniferous were caused by the compressive regime of the Variscan folding in its hinterland, but there is no direct relationship between sedimentary basin structures and the later folding structures. Coal formation started in the Namurian C, reached its maximum during the Westphalian A and B and ended during the Westphalian D. In total, about 250 coal seams were formed, but only 50 of them are of economic importance at present. Strata thicknesses and coal content are generally greater in the southeast of the Ruhr coalfield than in the northwest. An important exception can be observed in the lower part of the Westphalian A, where, in contrast, strata thicknesses are greatest in the northwest (in the Münsterland region), although the coal content remains the greatest in the southeast. Detailed isopach maps covering 100–200 m thick stratigraphic intervals reveal the existence of a southwest-northeast trending zone of reduced subsidence in the Ruhr coalfield that moved from southwest-northeast during the Westphalian. This structure can be interpreted as a peripheral bulge. Coal seems are purer and thicker in the area of this structure, which therefore must have been a paleogeographic element within the Ruhr basin. The general effect of a general decrease in the coal content of the Upper Carboniferous towards the northwest is superimposed on the migration of the coal content maxima of individual formations towards the northwest. During the Namurian C and Westphalian A the coal content maxima were situated in the area of the River Ruhr and during the Westphalian B and C in the area of the River Lippe. The deformation of the Ruhr coal basin is of post-Westphalian age, as demonstrated by the concordant folding of the Devonian and Carboniferous strata. The tectonic structure is mainly characterised by the following elements: stockwerk tectonics, axial elevations and a succession of compressional and extensional tectonics. Due to the general dip of the Ruhr coal basin towards the north, different structural levels (“stockwerks”) can be observed. The southern area displays the lowermost stockwerk, with many minor folds of about constant wavelength and low amplitudes. Thrusts are mainly small and some of them show increasing displacement upwards. The central part of the mining area displays the intermediate stockwerk with large, tight anticlinoria with minor folds separated by open synclines. These are accompanied by folded northwest- and southeast-vergent thrusts. In the northern Ruhr district, high anticlines and broad, trough-shaped synclinoria with only few thrusts represent the uppermost stockwerk. Large fold controlled thrusts die out at this level. Axial culminations and depressions have strongly influenced the structural style of the folding as well. According to this model of stockwerk tectonics, excess volume created by disharmonic folding is redistributed by thrusts. Thrusts dying out downwards at different stratigraphic and structural levels give evidence that there is no regional basal detachment below the Ruhr coal basin. This interpretation fits very well to new results achieved by the deep seismic reflection profile DEKORP 2-N. The section clearly shows thick-skinned tectonics in the Rhenish massif, with a shortening of the whole thickness of crust. The Ruhr coal basin can, therefore, be interpreted in terms of an autochthonous foreland basin in front of a buried thrust front to the south. Investigations on the post-Carboniferous strata of the Ruhr basin indicate different periods of active faulting. Cross and diagonal faults were formed partly at the end of the Variscan folding and partly before and during deposition of the Zechstein strata. A further important period of tectonic movements occurred during the early Kimmerian phase in the Late Triassic. Furthermore, earlier extensional faults in the Ruhr basin have been affected by Late Cretaceous transpression.
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Aug 4, 1995
N F Ukaigwe 
Open Access