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.
Especially in the Westphalian area one can recognize a quick alternating of both facies in one series and so do the properties of the slates.

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.
One of the most important fault zones is the 'Taunuskammüberschiebung' on which the whole of the southern Taunus and the Hunsrück thrusted like a nappe about 19 mi. to NW over the 'Hunsrückschieferkomplex' ('Hunsrück slate complex'). Together with the 'Hunsrückschieferkomplex' all units were thrusted more then 2.5 mi. over the 'Mosel syncline' to the 'Bopparder thrust' (Oncken, 1989 in Nierhoff, 1994).

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 economically most important slates can be found at Mayen which are also known as Mosel slate. The term 'Mosel slate' steams from the Rhenish area and developed because of the shipping of the roofing slate from Mayen on the Mosel river. The roofing slates are mined subsurface in the Lower-Devonian (Siegenian) 'Mayener Dachschieferfolge' and they are situated at the NW-flank of the 'Mosel syncline'. The following description follows mainly Wagner (1991):

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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  Fig. 1 Slate facade in Bundenbach (Hunsrück)

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  Fig. 2 Slate facade in Bundenbach (Hunsrück, detail of Fig. 1)

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  Fig. 3 Slate facade in Bundenbach (Hunsrück)

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  Fig. 4 Slate facade in Bundenbach (Hunsrück, detail of Fig. 3)

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  Fig. 5 Slate masonry in Bundenbach (Hunsrück)

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  Fig. 6 Inactive slate quarry "Eschenbach II" in Bundenbach (Hunsrück)

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  Fig. 7 Houses at the market of Mayen

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  Fig. 8 Houses at the market of Mayen (detail of Fig. 7)

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  Fig. 9 St.Clemens church in Mayen

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  Fig. 10 Mosel slate (St.Clemens church in Mayen)

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).
Slates of the Fredeburger and Raumländer district are deposited in Hercynian basin facies. The Nuttlar district stretches from Brilon and Meschede and the slate mining was mainly carried out in the vicinity of Bestwig, Nuttlar and Antfeld. With the beginning of the 90ies of the 20th century the slatemining was stopped.

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 roofing slates were mined within the multiple folded 'Flinzschichten with Nuttlaer- and Wallener Schiefer' ('Flinz beds with Nuttlar and Wallen slate'). The thickness of the slate layers varies between 10-49 ft.

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.
The Fredeburg slates are deposited in Hercynian facies and are characterized by a black to black-grey colour with a lamination caused by intercalated silty layers. Three of this 0.12-1.02 in. silty layers are used as key beds in the mine 'Gomer' and are called 'Eisennähte' ('iron seams'). The thickness of the slate layers itself ranges from 98 ft. in the northern beds to 66 ft. in the southern beds (Kirchner & Zallmanzig, 1995).
The mine 'Gomer' is situated within a 80° south dipping overturned southern limb of the NW-SE striking 'Hömberg syncline'. The fanning fracture cleavage dips here about 35-40° to SW-SE and results in an acute angle between SS-S1. In the abandoned mine 'Felicitas' the roofing slate lie within the 66 ft. slate layer in the folded northern flank of the 'Ostsauerländer Sattel'.

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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  Fig. 1 Slate roof with slate from the Sauerland in Brilon (Sauerland)

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  Fig. 2 Slate roof with slate from the Sauerland in Brilon (detail of Fig. 1, Sauerland)

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  Fig. 3 Slate roof with slate from the Sauerland in Küstelberg (Sauerland)

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  Fig. 4 Slate roof with slate from the Sauerland in Küstelberg (detail of Fig. 3). Due to the replacement by new slates the typical patina is obvious(Sauerland).

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  Fig. 5 Slate roof with slate from the Sauerland in Schmallenberg (Sauerland) after a rain shower.

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  Fig. 6 Ornamented slate facade with slate from the Sauerland in Schmallenberg (detail of Fig. 5, Sauerland).

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  Fig. 7 Slate facade in Kaub

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  Fig. 8 Slate facade in Kaub with rustic slate from Kaub (detail of Fig. 7)

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  Fig. 9 Slate facade in Sauerthal close to Kaub

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  Fig. 10 Slate facade in Sauerthal close to Kaub (detail).