Slate deposits of the Rhenic Mountains [Germany]
The Rhine Mountains comprise primarily Devonian and Carboniferous rock units of the Rhenohercynian. To the north it is covered by Cretaceous units of the 'Münsterländer Kreidemulde' and to the northwest by the Tertiary of the 'Niederrheinische Bucht'.
The southern edge is built by the Hunsrück and Taunus and is separated from the 'Saar-Nahe basin' and 'Mainzer basin' by the 'Taunus-Hunsrücksüdrandstörung'. To the east in runs under the Permian-Mesozoic cover of the 'Hessian basin'.
The Rhine Mountains or Rhenish Massif is divided by the Rhine River in the Left Rhenish Massif and Right Rhenish Massif. The structural frame is characterized by synclines and anticlines. The Lower-Devonian roofing slate deposits in the Rhenish Massif lie within the 'Mosel syncline' and the 'Hunsrück Anticlinorium'. In the Right Rhenish Massif the Middle-Devonian roofing slate deposits are in Westphalia within the 'Ostsauerländer Hauptsattel'.
During the Devonian two environments of different facies developed which are important for the different slate properties. They are described as 'Rhenish facies' and 'Hercynian facies'. Sedimentary environments showing the 'Rhenish facies' were situated near the southern coast of the 'Old Red continent'. This proximal fresh-water and shallow-marine shelf areas are characterized by coarse grained, psammitic sediments and sometimes by coral reefs.
Sedimentary environments in 'Hercynian facies' show fine grained, pelitic sediments
which were deposited in distal deep-water and still-water areas.
The first complete sedimentary development of the Rhenish Massif began during the Devonian. In this time the Old-Red continent exist in the north with a southernly adjoining shallow marine sedimentation environment in the area of the todays Rhine Mountains. The Lower-Devonian sediments were deposited in 'Rhenish facies'. During the Siegenian the northern part was governed by fluvial and deltaic sedimentation conditions whereas in the southern part, within the Mosel syncline, sediments in Hunsrück facies were deposited(Walther, 1997).
During the Middle-Devonian deeper basins developed, for example, in the 'Sauerländer basin' and led to the sedimentation of sediments in Hercynian facies.
The Right Rhenish Massif is characterized by thin skinned tectonic with a thrusting and concentric fold in the north to large scale listric faults in the southern part (Nierhoff, 1994).
The Hunsrück-nappe is a part of the 'Hunsrück-Soonwald Anticlinorium' and shows an intense imbricate faulting and tectonic piles, building a complicated structural setting.
During this process the movements deformed the cleavage plane at the faults that means a second fanning fracture cleavage and partly a third cleavage developed. The third one mostly occurs as a crenulation foliation (Nierhoff, 1994).
According to Walter (1997), the timely different Variscan folding began in the Phyllitzone (phyllitic zone) at the Taunus-Südrand for about 327-318 Ma (Namurian) and reached the northern edge of the Rhenish Massif for about 305-290 mill. years (Upper Westphalian). During this folding the roofing slates had undergone a low-grade metamorphism.
Simplified map of the Rheinic Mountains with its slate areas.
The Slates of Mosel and Hunsrück
Within the Left Rhenish slate deposits slate was or is mined from Müllenbach, Laubach to Ochtendung, east of Mayen. Slate Mining was also carried out in the areas of Bundenbach and Gmünden, which are very famous for their unique fossils throughout the world. The third slate district lies in the Rhine valley, at Kaub and Bacharach as well as the still active slate mine in the vicinity of Altley (Bartels et al., 1998).
The 'Mayener Dachschieferfolge' ('Mayen roofing slate series') comprises a slate-sandstone interbedding. The mining zone itself cotains four slate bearing zones separated by sandy series. The most important zone here is the 'Katzenberg Zone' which is the bottom of the 'Mayener Dachschieferfolge'. Furthermore, 3 to 4 slate layers exist in the 'Glückauf Zone' which have a thickness between 16-49 ft. as well as the 'Mosella Zone' and the 'Margareta Zone'. The slates show a fanning fracture cleavage and the angle SS-S1 is about 20°.
In the traditional slate mining area of the Hunsrück only in Altley slate is still mined. In Bundenbach slate is mined for flooring, walls and grave stones. In former times slate was mined above ground as well as subsurface.
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The Slates of Westphalia
The roofing slate deposits of Westphalia are situated in the north-eastern part of the Rhine Mountains in the area of Meschede/Brilon and Bad Berleburg. According to Dienemann & Burre (1911), three main slate districts can be distinguished (comp. Fig. 3). The slates lie in Lower- and Middle-Devonian rock units of the (comp. Table below).
The slate deposits are within the northern flank of the 'Ostsauerländer anticline' with the north-westernly adjoining 'Nuttlar main syncline', comprising the strongly folded Flözleeren ('divided slate') of the Namurian B.
The Fredeburg district runs from the Hessian Willingen in the northeast to Schmallenberg in the southwest and lies within the 'Ostsauerländer anticline'. The roofing slates of Willingen are within the northern-northeastern flank of the 'Ostsauerländer Sattel' and were mined underground within the Middle-Devonian 'Asten Schichten'. The thickness of the slate beds is between 7-66 ft.
Today, slates are mined at Heminghausen, about 2 mi. west of Fredeburg in the mine 'Gomer' close to Fredeburg. This slates are within the upper part of the 'Fredeburg Schichten' of the Eifel-Stage.
In the Raumland district slate mining was mainly carried out at Bad Berleburg, Raumland and Dotzlar an der Eder within the 'Raumländer beds' of the Eifel-Stage. The 'Raumländer beds' represent a series of sandstones, siltstones, mudstones, calcareous thin beds and volcanic rocks and the thickness of the roofing slate beds varies between 20-26 ft. The fracture cleavage dips 35-45° to SE.
The properties of the slates are very different. Some slates show a very good splitability and thus makes the production of roofing slate possible. In other parts only very thick slates can be produced and these slates are used for tables and stairs etc. Today no mining of roofing slates exist in the Raumland district.
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