The succeeding part of the treatise discusses in terms of its characterization a complex deposit, namely the hard-coal deposit of the Ruhr area.
4 Hard coal deposits of the Ruhr region
From Ireland through to Russia, significant deposits of hard-coal were formed in Europe during the geological age of the Carboniferous (Fig. 1). In many countries, including Great Britain, France, Belgium and Germany, for example, these deposits formed the basis for industrial development and are even today the industrial backbone of many national economies, as is the case in Poland and in Ukraine.
The Ruhr region also features other deposits, with potassium and rock salt in its north-western sector and strontianite deposits at the north-eastern transition to the Münsterland region. These, and also the iron-ore deposits historically so important for the iron and steel industry, and the also still considerable reserves of lead and zinc, will not be examined within the scope of this treatise, however. Only the rock salt deposits are nowadays still mined. The Borth mine produces rock salt of the Werra Sequence from the Zechstein Formation.
Derivation of the geological features of the Ruhr Basin‘s hard-coal deposits is made possible by an understanding of their genesis. For this reason, the evolution of these deposits will here firstly precede their geological characterization by means of their features. The “Ruhr” (or “Ruhr industrial region”) in the narrower sense makes up only one sector of the Ruhr Basin.
4.1 The hard-coal deposits of the Ruhr Basin
Within this region, the Ruhr Coal Basin extends in the south from the northern boundary of the Rhenish Massif, where the seams frequently diverge toward the surface and outcrop at certain points in the valley of the Ruhr river, from which the region takes its name, up to an area to the north of the Lippe river, where these strata are found at depths of more than 1000 m. Two structures, the Lippstadt and the Krefeld highs, bound the Ruhr Coal Basin in the east-to-west direction (Fig. 2).
18.104.22.168 Pre- and syncarboniferous development in the lithosphere
The Variscan mountain range was formed from the convergent movement of Gondwana and Laurussia. The Variscan Orogeny encompassed in this context the region between the southern boundary of Laurussia and the northern boundary of Gondwana, including the intervening Avalonia and Armorica microplates. This orogeny ultimately resulted, during the Upper Carboniferous, in the formation of a high mountain range on the territory of the present-day Rhenish Massif (a part of the Rheno-Hercynian Zone) and of a subvariscan foredeep adjacent to the north, which was part of a larger basin structure extending up to the southern perimeter of present-day Scandinavia and Scotland. The Ruhr basin was a part of this foredeep. It was formed in the outlying coastal region of the northern boundary of the Variscan Orogeny (Fig. 4).
The erosion of the Variscan mountain range and the filling of the foredeep with clastic sediment resulted in the formation of a molasse basin of many thousand metres in thickness, the load exerted by which caused it to sink further (Fig. 5). Swamps, the biomass of which provided the matter from which coal was created, were able to form temporarily during the basin‘s descent. Subsequent covering by water and sediments prevented the degradation of the biomass and caused coalification as a result of rising pressures and temperatures during the further descent of the now filling trough.
22.214.171.124 Carboniferous development in the atmosphere and biosphere
Plants spread on the land area, and trees developed in the course of the Devonian. The territory of the present-day Ruhr region was located in the southern hemisphere and migrated further to the north, into the vicinity of the Equator, during the period of the Carboniferous. The climate became moist and hot, i.e. tropical. Enormous tropical forests, consisting primarily of plants of the fern family (lycopodiaceae, equiseta, ferns) were able to propagate in the coastal region, in particular [12, 33]. This process was accompanied by rapidly rising photosynthesis and silicate weathering, causing the CO2 content of the atmosphere to fall. Carbon dioxide, and thus also carbon, were fixed to a very large extent in the form of biomass and thus, ultimately, in the form of coal, and also chemically bonded in animal skeletons and shells, and therefore ultimately in the form of carbonate rocks. Climatic conditions during the Carboniferous were thus shaped not only by geographical location, but also by the steep increase of oxygen in the atmosphere, which rose to a historic peak of above 35 %, and by the virtually complete disappearance of carbon dioxide  (Fig. 6).
126.96.36.199 Carboniferous development in the hydrosphere and biosphere
188.8.131.52 Syn- und post-Carboniferous development in the lithosphere
The continuous descent of the basin permitted adequate accumulation and adequate quantities of organic materials in the maritime littoral region, which then provided the basis for the formation of hard-coals (paralic deposit formation). Within a – in geological terms – relatively short period (Namurian, Westphalian) of not even nine million years, several thousand meters of sediments were deposited [1, 9] (Fig. 10). Diagenesis then ultimately resulted in the formation from this, at corresponding pressures and temperatures, of mudstones, siltstones and sandstones, and also coals (Fig. 11). A total of more than two hundred coal seams of various thicknesses (Fig. 12) can thus be differentiated and make up, taken together, around 2 to 3 % of the entire coal-bearing strata sequence of Namurian C and of Westphalian A to C [8, 9, 16]. With the beginning of Westphalian D, the 5 to 7 km thick basin filling has up to today then been exposed to mechanical and thermal loads.
The intensity and direction of these loads have had extremely differing courses, however. Phases exposed to virtually uniform and constant static loading alternated with phases with locally severe and differing stressing, which resulted in corresponding fractures and thrusts within the native rock (Fig. 13). Reversal of the direction of stressing even resulted in inverse thrusts. Plutons rose locally, as a result of magmatic events. For this reason, it is not possible even today to assign all geological processes precisely to the individual phases . Despite these difficulties, the most important tectonic processes in derivation of geological features will be examined here.
During the late phase of the formation of the Variscan mountain range, the coal-bearing sediments of the Upper Carboniferous deposited in the subvariscan foredeep were exposed to stressing by pressure of a north-western orientation as a consequence of the continued movement of Gondwana towards Laurussia. The seams were shortened by up to 50 % in this process. The strata of the Ruhr Basin were thus deformed to folded and overthrust structures, the intensity of folding decreasing toward the north. Synclines and anticlines striking south-west to north-east were formed, as were overthrusts of great extent in some cases, such as the 80 km long Sutan Overthrust, which features overthrust magnitudes of up to 1.3 km. The deformation ultimately came to an end, and terminated at the deformation front.
The Ruhr Basin, as a macrotectonic axis depression zone, was subdivided by a less severely elevated axis combination, the Dortmund Axial Upheaval. The low points of the axes, the Gelsenkirchen and Hamm axial depressions, are located between these culminations [9, 37] (Fig. 15).
Normal and lateral faults, their fault heights and spans of which in some cases reached several hundred metres, also evolved in several subvariscan and post-Variscan phases. The oldest movements can be ascertained during the Variscan Orogeny and then during the Rotliegend and in particular the Keuper geological periods. The Pangaea supercontinent fragmented during these periods and the North Atlantic opened. In this process, the strata of the Ruhr Basin were elongated by up to approx. 5 %, particularly in the east-to-west direction. In the context of the later Alpine Collision between the African and European plates during the Upper Cretaceous, the strata were again subjected primarily to compressive loading of a north-to-south orientation. Finally, reactivation of the fracture tectonics can be ascertained during the Tertiary period.  (Fig. 13).
The result was the occurrence of transverse faults perpendicular to diagonal to the strike of the fold axes, these transverse faults dividing the Ruhr Basis into horsts and grabens (Fig. 16). The direction of strike of the fractures is predominantly north-west to south-east with fault heights of up to above 1000 m (e.g. the “Sekundus-Sprung” in the vicinity of the Gelsenkirchen Main Anticline). Lateral faults also occur in the north-to-south direction, but these most frequently have an east-to-west orientation. The folded structures of more than 1000 m may be dextrally faulted as a consequence of this lateral fault (e.g. the “Wambeler Blatt” in the Witten Syncline).
The coal-bearing strata of the Ruhr Basin were thus shaped during and after the Variscan Orogeny by folded structures, axis corrugation and fractures. Tectonic stockworks formed in the vertical direction as a consequence. In general three tectonic levels can be differentiated with the same orogenic confinement [8, 9] (Fig. 17).
In the first, top, level there occur a few folds of great span and high amplitude, these forming wide trough synclines and high main anticlines with steep sides. Special folds and overthrusts can be found only in the main anticlines. The second, medial, level is characterized by folds of short to moderate spans and of low amplitude, and by special folds and frequent and pronounced overthrusts in the synclines. The third, bottom, level exhibits short-span special folds of low amplitude, with the result that it is not possible to differentiate main anticlines and main synclines in structural terms. Overthrusts are an only subordinate feature here.
The article will be continued in the next issue, AT 01-