Part I: Palaeozoic Era

Ordovician (485.4 – 443.4 million years ago)

The world looked completely different than today. The area of land that is now Germany (D/red) was divided into two, Northern Germany being a part of the Avalonia microcontinent and Southern Germany situated on the Armorica terrane assemblage. As a result of plate tectonics processes, Avalonia drifted north and collided with Baltica, resulting in the closure of the Tornquist Ocean. In this first phase of the Caledonian Orogeny, Avalonia became welded to the Eastern European Platform. The weld seam can be geophysically identified. It stretches from the island of Bornholm through Poland to the Carpathian Arc and is called the Tornquist Line.

Reconstruction of the global plate tectonic development in the Ordovician and Devonian, modified from Meschede (2015)

In the course of the second phase of the Caledonian Orogeny, the Laurentia and Baltica Continents were fused together during the closure of the lapetus Ocean. At the end of the Silurian, a new big continent was therefore born, which is known as Laurussia or the Old Red Continent.

Argillaceous rocks of the Ordovician (type 01)

The around 2000-m-thick Ordovician consists of a sequence of psammitic argillaceous slate series. The stratigraphic sequence begins with the up to 600-m-thick Frauenbach group at the bottom, which is composed of a tripartite alternation of dark clay slate and light-coloured quartzitic sandstones/quartzites.

On top follow the around 900-m-thick layers of the Phycodes group, which formed mainly as quartz-banded argillaceous to silt slate in the bottom and middle sections. The sequence begins with the Phycodes roof slate formation. An easily split light greenish-grey clay to silt slate, that can occur as phyllite and mica schist when subjected to more intensive metamorphism.

The world’s most famous Ordivician schists/Norway (2017)

Mining of Phycodes Slate close to Gera/Thuringia (2018)

Bivalent iron causes the green colour of the clay mineral chlorite (2013)

Benefication in the slate processing plant Tschirma /Gera Thuringia (2014)

Fine Slate Chippings delivered in the grain size 0,2/0,6 mm (2013)

Slate flour delivered in the grain size 0/0,2 mm (2013)

The overlying strata of the Gräfenthal group measure up to 500 m in thickness and are strongly pelitic. It is a sequence of dark grey to black argillaceous slates with intercalations of sandstone.

On account of the strong geological consolidation and deleterious, quartz mineralisation on joint surfaces the unweathered slate has to be very finely dry ground. The raw materials are characterized by very low pit moisture, low plasticity as well as low drying, preheating and cooling crack sensitivity. In the firing process, the potassium from the mica minerals is the cause for good sintering activity and high compressive strength.

Devonian (419.2 – 358.9 million years ago)

The Devonian was dominated by the large continents Laurussia and Gondwana.
The Old-Red land extended from south of the equator to the southern tropics. Gondwana was located in the South Pole region. Northern Germany was part of the Old-Red land and was located south of the equator. The area between the two continents was flooded by the Rheic Ocean, an elongated geosyncline and long-lived sedimentary basin.

In the Devonian, the Armorica terrane assemblage including the area that is now Southern Germany, France and Spain drifted in the direction of the Old Red Continent to the north, leading to closure of the Rheic Ocean. That was the beginning of a development that would not only result in the unification of Northern and Southern Germany, but also in the unification of all land masses to form the Pangaea super continent. It was the beginning of a long-lasting orogeny that was most active in the Carboniferous and is referred to as the Variscan Orogeny.

Geotectonic classification of Europe and the Variscides, from Meschede (2015)

The diverse plate tectonic processes are reflected in the complex structure of Western and Central Europe. The Rhenohercynican zone is the northernmost structural unit of the Variscides. This includes the Ardennes, the Rhenish Slate Mountains and the Harz region. South of this is the Saxothuringicum with the Mid-German Crystalline Zone. The Mid-German Crystalline Zone represents the weld seam between Avalonia and Armorica and is understood to be a former subduction zone. Parts of the Mid German Crystalline Zone outcrop in the Odenwald, Spessart, Thuringian Forest and Kyffhäuser regions. The largely high metamorphic Moldanubian zone includes large parts of the Vosges, the Black Forest and the Bohemian Massif. Joined to the Moldanubian in the south is the geologically much younger Alpidic system.

In the lower Devonian isostatic subsidence led to the sedimentation of 10,000 m thick deposits in the Rhenohercynian Basin. Source area of the sediments was the northern Old-Red land. Its shoreline extended from Belgium to the central part of Poland.

Palaeogeography of the Lower Devonian / from Meschede (2015)

Coarser-grained deltaic sediments were deposited on the coastal continental shelf (Rhenish facies). Deep water and offshore stillwater areas were filled with fine-grained clay sediments of the Herzynian facies. Here, the up to 6,000-m-thick Hunsrück slate was deposited. Further south the clay sediments interfinger with coarser-grained barrier sediments such as the red Hermeskeil beds and the up to 1,200-m-thick white Taunus quartzite.

Facies distribution in the Rhenohercynian zone / from Meschede (2015)

In the Middle Devonian, finer-grained and more calcareous sediments were deposited such as the up to 900-m-thick Wissenbach slates; marls and pelagic limestones were more widespread. In the eastern Sauerland and Thuringian Slate Mountains, the up to 400-m-thick Tentaculite slate was deposited in the Saxothuriangian ocean. In the western Sauerland the 100-m-thick greywacke of the Finnentrop beds was laid down and massive, bluish-grey phyllites of the Pokrzywianka beds were formed in the Sudetes. Meanwhile, the Middle German Crystalline Rise gradually developed into an uplift and erosion area. Thick massive limestones and reefs formed on submarine crests and volcanic stumps. Basinwards, these tropical reef limestones interfinger with the turbiditic Flinz slates.

Palaeogeography of the Middle Devonian / from Meschede (2015)

In the Upper Devonian, the various facies groups became even more differentiated, however, fine-grained and calcareous sedimentation continued to dominate. In deep basin zones, several-hundred metre- thick siliceous schist, red slate and Cypridina slate settled while massive reef limestones and platy cephalopodic limestones continued to form on submarine crests. In the Harz region, one of the biggest compacted limestone deposits built up and reached a thickness of around 500 m. In facies transition zones nodular limestones and clays were sedimented that were later metamorphosed and now form an interbedded sequence of calcareous-nodular slates and clayey slates.

Argillaceous rocks from the Devonian (type 02)

Unweathered Devonian slates are mostly irrelevant for the brick and tile industry. An exception are locations that have the ability to process and dry grind the material. Weathered and partly plasticized slates can be used in the brick industry after pre-crushing into suitable delivery grain sizes.

Unweathered slate from the Middle Devonian/Southern Poland (2017)

Weathered Devonian slate in contact with basalt/Rhineland-Palatinate (2018)

Standardized processing of Devonian slate/Saarland (2007)

Devonian slates weathered during the Cretaceous-Palaeogene are, however, of extraordinary importance. They represent the source rocks of the Westerwald clay and will be described in Part III: Cenozoic.

Plastic filter cakes of the Devonian (type 03)

At the Taben-Rodt quarry, Lower Devonian hard rock is extracted and processed to high-grade chippings, railway ballast and aggregates. The quarry is situated in the Hünsrück-Taunus ridge. In the Lower Devonian a tropical marginal sea of the Rheic Ocean was located at this location of the Rhenohercynian zone. Iron oxide-rich debris was deposited on its seabed, washed into the basin from the southern Avalonian rise.

Quartzite quarry Taben-Rodt near Mettlach/Saarland (2012)

Currently the best membrane filter press in Germany (2018)

Red-firing filter cakes for the brick industry (2018)

Since 2018 the quartzites are wet processed. A filter cake is produced from the wet residues of aggregate washing and supplied to the German and Dutch brick industry.

Taunus quartzite is also produced and processed into construction materials in the Saalburg quarry near Friedrichsdorf/Hesse. During mineral washing large amounts of filter cake are obtained since 2014. Geologically speaking Saalburg is located in the area of the Taunus ridge which represents the southern rim of the Rhenish Slate Mountains in the Hunsrück-Taunus hill range. Middle Devonian stratigraphic sequences are exposed in the region, deposited in the Rhenohercynian shelf sea.

Quartzite quarry Saalburg near Friedrichsdorf/Hesse (2018)

In the background the mineral washing and the filter press house (2016)

Each year more than 50,000 t of filter cake are produced (2016)

The complete Devonian sedimentary sequence has experience metamorphism. The metamorphic overprint is evident by the dominance of muscovitic mineral phases in the fine fraction and the absence of inner-crystalline swellable clay minerals. An outstanding thermal conductivity of 0.22 W/mK was measured in the institute for brick research in Essen (Institut für Ziegelforschung Essen e.V.).

Mineral washing of Middle Devonian reef limestone is carried out at various locations in the Harz and Sauerland region. One example is the location of the largest massive limestone deposit in central Europe near the city Oberharz at the Brocken/Saxony-Anhalt. Here, up to 200,000 tons of filter cake are produced per year. The Grevenbrück quarry is another example of mineral washing of carbonate rocks and supplies the brick industry with clay-rich, dolomitic filter cakes since 2011.

Dolomite quarry Grevenbrück near Lennestadt/Northrhine-Westphalia (2011)

Dolomitic aggregate before mineral washing and chamber filter press (2011)

Pronounced plastic filter cakes with low body density (2020)

Devonian massive limestone is mined at the quarry Hahnstätten south of Limburg/Rhineland Palatinate. It can be used to produce white filler for specialty paper and other applications. During calcium carbonate precipitation a seasonally limited amount of high carbonate PCC filter cakes is produced. It can be used to produce thermally highly insulating bricks.

PCC calcium carbonate slurry in the blue lagoon (2014)

Pressing of slurry in a mobile chamber filter press (2017)

Snow white filter cakes with ceramic body densities of 1.01 g/cm3 (2017)

Rock fillers of the Devonian (type 04)

In the Hunsrück and Sauerland regions rock fillers are offered at a number of locations. Fillers can be moistened in tubular screw conveyors and transported with tipper semi-trailers to guarantee dust free processing at the brickworks. The maximal grain size is usually < 0,2 mm. The rock fillers are predominantly produced from sedimentary rocks such as the Taunus quartzite and the Finnentrop beds (greywacke) but also from volcanic rocks (diabase).

Greywacke quarry Berge near Meschede/Northrhine-Westphalia (2015)

Beneficiation equipment and silo with fully automatic loading conveyor (2015)

Tubular screw conveyor used to moisten the rock filler (2015)

Devonian rock fillers are used as drying agents and as quartz-poor to quartz-free additives in the brick and tile industry.

Carboniferous (358.9 – 298.9 million years ago)

In plate-tectonic terms, the Carboniferous is characterized by a progressive coalescence of all land masses to form the supercontinent Pangaea.
This was accompanied by closure of the Rheic, Saxo-Thuringian and Moldanubian oceans. The Variscan Orogeny beginning in the Upper Devonian climaxed in the early Lower Carboniferous and again in the Upper Carboniferous. In the course of several phases, a global belt of folded strata took shape. Some 500 km in width, it stretched from North America across Central Europe all the way to China. Major hard-coal deposits developed all around the equatorial zone.

Reconstruction of the global plate tectonic development from the Carboniferous to the Triassic, modified from Meschede (2015)

The Carboniferous deposits crop out in the Saxo-Thuringian zone and in the Rheno-Hercynicum. The main regions are the Thuringian-Franconian-Vogtlandian Slate Mountains, the foothills depression of the Slate Mountains, the northern and eastern fringes of the Rhenish Slate Mountains, the Osnabrück Uplands and the Harz.

Ranging up to some 7500 m in thickness, the Carboniferous stratigraphic sequence is dominated by sandstone, argillaceous slates and greywackes, facially interlocked at the bottom with calcareous deposits. The seam-containing Upper Carboniferous is up to about 3 000 m thick and contains only two to three percent hard coal in those strata. Raw material potentials for the brick and tile industry are situated within the bottom to middle Lower Carboniferous and at the top of the Upper Carboniferous.

During the early Lower Carboniferous, complex subduction processes constricted the once wide Rheic Ocean to produce a narrow sea basin. The Central German crystalline zone (MCZ) now bears witness to the magmatic activity that took place within the subduction zone. As a ridge, it separated the Rheno-Hercynian zone from the Saxo-Thuringian Basin.

Cross-sectional plots describing the mid-European segment of the Variscides / from Meschede (2015)

Central Europe, sedimentary rock accrued in thicknesses up to 3 500 m, whereas a distinction is drawn between shallow-marine Carboniferous limestone facies and deep-marine Culm facies. Carboniferous limestone, between 200 and 700 m thick, formed in the euxinically defined shelf zones of the Old Red Continent. In southeasterly direction, the Carboniferous limestone facies is interwedged with the Rheno-Hercynian Culm Facies via calcareous turbidites (submarine slurry slumps) and olistostromes (submarine slide masses).

Palaeogeography at the time of the Lower Carboniferous / source Meschede (2015)

Argillaceous rock from the Carboniferous (Type 05)

Thick orogenic sediments consisting primarily of alternating layers of resedimented argillaceous slates and greywackes accumulated in the deep-marine Culm. In the bottom part of the series, carbonatic argillaceous slate and black sooty slate situated under planar-cleaving roofing slates in the Lehesten Formation.

Above that come some 900 m of sand-banded argillaceous slate/banded slate of the Hasenthal- and Kaulsdorf Formation, which are used in the roofing tile industry after fine grinding. At medium high to high contents of sheet silicates the clay mineral paragenesis consists of illite/mica and chlorite. Intra-crystalline, swellable clay minerals are lacking. Significant amounts of potassium ions of the illite/mica ensure good sinterability in combination with adequate refractoriness.

Banded slate and greywackes at the Kamsdorf Mine/Thuringia (2006)

The Kaulsdorf-formation discordantly overlayed by Zechstein limestone (2012)

Turbidites formed by debris from the Mid-German Crystalline Zone (2018)

During the late Visean Stage, the pelagic conditions broke down. Due to tectonic uplift and falling sea levels, the shelves became narrower, the Carboniferous limestone facies ended, and thick groups of greywacke sedimentated. They represent the detritus of the uplifting mountain range in the vicinity of the Central German crystalline ridge. As Orogenesis proceeded, the orogenic sediments became tectonically folded and steeply inclined. Greywacke fillers are universally applicable in the brick industry as a layer silicate-rich grog.

Greywacke quarry Hüttengrund near Sonneberg/Thuringia (2014)

Saxothuringian flysch series of the Ziegenrück formation (2014)

During crushing and classification 30,000 tons filler a year are produced (2014)

During the Upper Carboniferous, the main distribution province of the narrowing sea basin shifted into the unfolded foreland basin of the Variscan Mountain Range. As conditions became increasingly terrestrial, fluviatile and limnal sediments became predominant. Thus emerged the sub-Variscan Foredeep as a Molasse Basin of the Orogeny. With the onset of the Upper Namurian, a predominantly fluviatile-deltaic regime prevailed, repeatedly interrupted by periodical sea transgressions. The Ruhr Group‘s coal-bearing strata sedimented under these paralic conditions.

Palaeogeography at the time of the Upper Carboniferous / source Meschede (2015)

The uppermost part of the Ruhr Group is more heavily characterized by pelitic sediments. The up to 1 000 m-thick Ibbenbüren beds contain alternations of sand, silt and argillaceous rock. Due to local horst formation/uplift during the Cretaceous, these layers are only exposed in the vicinity of the Osnabrück Uplands.

Upper Carboniferous clay stone/slate clays are extracted at numerous pits located within the Ibbenbüren Carboniferous fault block. They serve as basic raw materials for local clay construction product plants, where they are mixed with such plastic raw materials. In the course of production, they display the generally well-appreciated merits of slate clays containing large proportions of non-expansive three-layer silicates of the illite/mica group. At nearly all quarries, however, a significant concentration of organically bonded carbon appears as a limiting factor.

The quarry Kälberberg near Recke from the air (2003)

Slate clays and sandstones of the Osnabrück formation (2014)

Quarry intern jetty at the Mittelland Canal (2014)

Osnabrück Formation layers representing the top of the Upper Carboniferous, including in part red-coloured sediments marking the point of transition to terrestrial-arid sediments of the Rotliegend, constitute exceptions to the rule. Notably, the Kälberberg quarry near Obersteinbeck/Recke contains the youngest exposed Upper Carboniferous strata in Germany.

Perm (vor 298,9 – 252,2 Millionen Jahren)

Arid climatic conditions led to the formation of red sediments and salt deposits. During the Permian period, there was a fundamental change in tectonics from compressive to expansive. Due to plate breakage and thermal subsidence, crustal movements set in, causing formation of small and large sedimentary basins and numerous trough fault systems. In zones of tectonic weakness, ascending molten mantle material spawned continental volcanism with predominantly rhyolitic lava. In the Upper Permian, marine conditions returned for the first time since the Carboniferous. Via cyclical sea transgressions in conjunction with an arid climate, clays and evaporites aggraded.

Palaeogeography at the time of the Rotliegend / source Meschede (2015)

According to old mining tradition, the Continental Permian of Central Europe is subdivided into Rotliegend and Zechstein. With a span of some 45 million years, the Rotliegend covers a very long space of time, during which almost exclusively continental sediments accumulated in Central Europe. Due to flattening of the Variscan Mountains, the Rotliegend Group was characterized by heavy erosion and coarsely clastic sedimentation. In for the most part separate intramontane basins, red sediments consisting of conglomerates and sand accumulated to thicknesses exceeding 1 000 m.

With arid desert climate the Variscan Mountains were levelled/Wadi Rum (2019)

Enormous dry valleys, dune sands and fanglomerates were typical phenomena/Wadi Rum (2019)

Bizarre erosion forms and desert varnish were also characteristic/Wadi Rum (2019)

As Pangaea gradually drifted northward, the initially humid, tropical climate gave way to an arid desert climate. At the same time, shallow depositional environments appeared, with Playa lakes in which iron oxide-rich clays accrued. Sands and fan-glomerates (alluvial talus fans) dominate the fringes of the shallow basins. Regionally useful raw materials for the clay brick and tile industry are concentrated mainly in the middle and upper parts of the Rotliegend profile.

The Rotliegend is followed by evaporitic sediments from the Zechstein Formation. At the onset of the Zechstein, thawing of the southern hemispheric ice sheet caused an event-like sea transgression into the Central European Basin, which at the time had already sunk to some 250 m below sea level. Once economically significant copper slate deposits collected under euxinic conditions at the base of the Zechstein. During the Zechstein, thick evaporitic sequences consisting respectively of sedimented clay, collected as a result of repeated evaporation of the sea water in combination with cyclical sea transgressions.

Argillaceous rock from the Permian (type 06)

The argillaceous rock of the Rotliegend contains significant amounts of red iron oxide/hematite. Clay mineralogically, the rock has a dominant illite/mica content and, due to the effects of weathering, primarily medium-plastic properties that make it suitable for local use as a basic raw material in the roofing tile industry. Rotliegend clays also find use as additional components in the production of backing bricks. Crystalline quartz veins, carbonate contents and flakes of muscovite can have a mineralogically limiting effect.

New Rotliegend quarry near Lauban/Lower Silesia (2019)

Segregation of hematite geodes with separator shovel (2019)

Near surface replastified Rotliegend claystones (2019)

Within the evaporite sequences of the Zechstein there are comparably view clay potentials for the brick industry. Examples are detritic pelites in salt separation basins from fluvial or wind transport. At the base of the Leine sequence in the Zechstein 3, grey salt clay (T3) and at the base of the Aller sequence in the Zechstein 4, red salt clay (T4) were sedimented.

Salt clay overburden on dolomite (Ca2) near Osterode/Harz foreland (2020)

Geological exploration of the discordant overlying red salt clay (2020)

High plasticity and complex clay mineralogy dominate the scene (2020)

On top of the Zechstein 7 in the Fulda sequence, there accumulated as much as 60 m of saliferous clay as sediment in excessively saline, periodically subsiding lakes. Marking the end of the evaporitic sequences, these clays are referred to as crumbly shale because of their typically polyhedral fracture planes. Above that, light-coloured sandstones and conglomerates introduce the technically controversial transition to variegated sandstone.

Permian plastic filter cakes (type 07)

The Black Forrest is located in the Moldanubian zone of the Variscides and is, also because of its strong metamorphic imprint, one of the most complex orogenic structures in Central Europe. In this area the early Permian was characterized by levelling of basement rocks, crustal extension and continental volcanism. One of the largest extraction sites of volcanic rhyolite is the quarry Ottenhöfen in the Central Black Forrest.

The rhyolite quarry Ottenhöfen in the Black Forrest/ Baden-Wuerttemberg (2019)

Mineral washing, rock classifying and chamber filter press (2019)

Highly sinter-active filter cake for red floor tiles and roofing tiles (2019)

Horizontally bedded crumbly shale conglomerates stemming from the uppermost Zechstein found in the outcrop of the eastern outer plateau zone of the Thuringian Basin are put through wet preparation and relieved of their kaolinitic-illitic clay matrix. The thusly obtained filter cake is characterized by medium-plastic properties, high sinterability and very attractive fired colours situated within the clinker-relevant red/blue spectrum. At present, the range of applications is centred on high quality facing bricks and clinkers.

Mining of clayey to gravelly sandstone in Untschen near Schmölln/Thuringia (2006)

Plastic filter cakes with red to blue firing colour spectrum (2006)

Filter cake stockpile – a dream for every brickwork (2020)

Rock fillers from the Permian (type 08)

In south-west Germany numerous locations produce dry as well as wettened rock fillers from the Rotliegend time. The maximal grain size is usually < 0.2 mm. The rock fillers are mainly from intermediate to acidic volcanic rocks like andesites and rhyolites.

Andesite quarries Kirchheimbolanden – Eisensteiner Kopf/ Rhineland-Palatinate (2016)

Each year more than 500,000 tons of quartz-free rock filler are produced (2019)

Rhyolite quarry Neu-Bamberg near Bad Kreuznach/ Rhineland-Palatinate (2016)

Rock fillers from the Rotliegend are generally used as drying and opening agents in the tile and brick industry. Andesitic fillers are absolutely quartz-free. Rhyolitic fillers are characterized by high potassium contents and high sintering activity.