Part II: Mesozoic Era

Triassic (252,2 – 201,3 million years ago)

In plate tectonic terms the Triassic development is determined by the beginning of the collapse of the Pangaea supercontinent and the progressive opening of the Neotethys Ocean. The northwestern peripheral regions of the Neotethys Ocean were formed by extensive shelf areas on top of which several kilometre-thick sequences of marine carbonate rocks were deposited. These pelagic sedimentary sequences of the Alpine-Mediterranean Triassic can be found today in the folded orogens of the Alps and Carpathians as well as in the Dinaric Alps.

Plate tectonic development from Triassic to Jurassic / from: Meschede (2015)

Separate from this is the area of Central Europe where the sediments of the Germanic Trias were deposited. The sedimentation basin extended over around 1800 km from England in the west to Belarus in the east and over around 600 km in the north-south direction from Scandinavia to the southern peripheral region, reaching as far as Eastern France and into Switzerland. The Central European Basin and therefore the main supply regions were separated by the surrounding substrata. A Proto-Atlantic expansion system led to the formation of north-south oriented trench structures.

Buntsandstein (252,2 – 246,5 million years ago)

The Buntsandstein is characterized by dominance of solid land. The horizontal border to the Zechstein/Permian system is lithologically unsharp within sandy-conglomerate stratigraphic sequences. On the other hand, the occurrence of epicontinental dolomitic limestone beds marks a significant border to the overlying sediments of the Muschelkalk/Middle Trias system.

Palaeogeography at the time of the Buntsandstein / from Röhling & Lepper (2013)

At the time of the Buntsandstein, Central Europe lay in the east of the Pangaea supercontinent, approximately between 25 and 30 degrees north. Periodically, monsoons from the Neotethys Ocean in the southeast spread to the basin and led to extensive flooding, which then alternated with relatively long dry periods. Dominant are red sediment colours caused by finely distributed haematite/goethite, which are regarded as indicators of an arid climate and oxidizing environment.

At the time of the Buntsandstein Central Europe passed through the desert belt/Wadi Rum (2019)

Monsoonal flash floods caused kilometre-wide dry valleys, so called wadis (2019)

Deep canyons and bizarre forms from wind and water erosion (2019)

Typical for the peripheral regions are alluvial fans and sandstones, while for the interior of the basin, brackish to saline argillaceous rocks with rock salt deposits are found. Vertically the sequence of strata is grouped into a series of small-cycle “fining upwards” (Sohlbank) sequences, each cycle beginning with clastic material. On top follow sandstone-argillaceous rock in alternation, which then go over into the clayey top of the respective small cycle. The typical small cycle is around 10 to 25 m thick in the basin facies and reflects the climatic change from a humid to a dryer climate. The duration of a humid-dry cycle is estimated at around 100,000 years.

Argillaceous rock from the Lower Buntsandstein (type 9)

In initially still arid climatic conditions, the Calvörde formation forms the conclusion of the evaporation epoch of the Zechstein/Permian. In temporary, sometimes salinar, landlocked lakes, silts and clays were deposited, often together with finely dispersed dolomite and sulphates. The Calvörde formation (s1-sequence) begins with an oolite-bearing basic sandstone before going into a sandstoneclaystone alternation. The fine-grained top of the Calvörde formation,
consists of thick, usually sandy-oolitic argillaceous rock strata. The thickness of the s1-sequence measures a total of around 135 to 190 m.

With the Bernburg formation (s2-sequence), a long-term change to a semi-arid climate with periodic water supply can be clearly recognized. In the sediment, this is reflected especially by the wave ripple marks. Drying cracks, on the other hand, evidence the temporary-cyclic drying of the sedimentary basin. Extremely fine-layered clay and mica deposits represent temporary still water sediments.

The s2-sequence: legendary motorway clay at the A44 near Marsberg/Hesse (2016)

Sporadic drying indicated by fossil dry cracks (2016)

Clay fragments as resediments at the base of Sohlbank sequences (2016)

The numerous small cycles of the Bernburg formation can be described in simplified terms as fine sandstone-siltstone-argillaceous rock alternations with intercalated oolite zones, which can also be calcareous and then referred to as roestone. The basis of a new cycle sequence is indicated by clay fragments in the Sohlbank sandstones. The thickness of the Bernburg formation measures up to over 210 m in the subsidence zones and on crests inside the basin around 80 m.

s2 sequence: The Peißen claystone quarry near Bernburg/Saxony-Anhalt (2016)

Over-consolidated clay stone disintegrates at the earth’s surface (2016)

Classic wave ripples as indicator for ancient moving water (2016)

The Bernburg formation is used as a basic raw material in big brickworks. Processing is conventional, which means semi-wet/plastic processing in high-powered pan mills and roller mills. For the production of tiles in some specific cases dry processing can be necessary, to reduce spalling due to lime. The material generally has low pit moisture. This correlates with advantages during feeding and drying.

The mica dominance of the mineral paragenesis causes a low drying and preheating sensitivity. During the firing process, the potassium in the mica acts as a moderate to strong flux. In the high temperature range locally increased Na-feldspar (albite) contents lead to abrupt softening. The sedimentation in desert-like conditions reflects in a low content of organic carbon.

Argillaceous rock from the Middle Buntsandstein (type 10)

As a result of tectonic movements, in the Middle Buntsandstein there was an increase in the relief energy so that the transport of relatively coarse sediments became increasingly possible. Gravel and coarser sands were thrust in broad fans from the high ground into the North German Basin. At the southern edge of the Basin, high-feldspar arkoses were deposited, which were discharged from the crystalline areas of the Vindelician Crest and the Bohemian Massif from the southern direction.

The Middle Buntsandstein begins with the significantly marked basic sandstone of the Volpriehausen formation (s3-sequence). On top follow fluviatile and limnic depositions of the Volpriehausen alternated bedding, consisting of clay, silt and fine sandstones. The s3-sequence is terminated by the clayeysilty Avicula strata, which were deposited via the North Sea Strait as part of a marine ingression. The thickness of the Volpriehausen formation in the Hessian Basin measures up to 230 m.

In principle, the strata of the Detfurth formation (s4-sequence) are structured in the same way. Here too the sequence begins with a basic sandstone, then it changes over to sandstone-argillaceous rock alternations and ends with the Detfurth clay. The primary thicknesses of the Detfurth formation vary within the North German Basin between 50 and 120 m, the maximum values being found in the Reinhardswald.

In the Hardegsen formation (s5-sequence), sandy river deposits alternate cyclically with clayey-silty sediments, which settled in relatively small, standing waters. The moister climate is indicated by increased plant remains. In the North German Basin, the thickness of the s5-sequence is generally around 130 m. In subsidence centres like in the Reinhardswald Trough, maximum thicknesses of up to 230 m are reached.

In the Lower Solling formation (s6-sequence), cyclic, monsoon precipitation led to the formation of an inland lake that reached from the North Sea to southern Lower Saxony. The cyclical structure of the strata sequence is only evident regionally or relictically. In simplified terms it is possible to assume a division into fine-grained building sandstone and clayey boundary layers. The thickness in the North German Basin measures up to 110 m.

Open pit mining territory Hirschau-Schnaittenbach in the Oberpfalz region (with friendly permission)

In every white clinker mass is kaoline/Westerwald (2017)

Type 10 with extremely low thermal conductivity/Egge hills

Interesting for the brick industry are kaolinized arkoses, which are formed by in-depth feldspar weathering, for example in the Halle/Saale and Hirschau- Schnaittenbach/Upper Palatinate districts, but also at locations around Kahla/Jena and Fürstenberg in the Solling. They are used predominantly in the manufacture of white bricks and brick slips, as well as for special high thermal insulation bricks. Walpernhainer clay from the Volpriehausen alternating sequence is especially well suited as a tile brick raw material.

Argillaceous rock from the Upper Buntsandstein/Röt (type 11)

The Upper Buntsandstein begins with the onset of the Röt salinar or its peripheral facies equivalent. With the beginning of the Röt formation (s7-sequence), the climate became arid again, which is shown by thick evaporites. A block tilt closed the North Sea strait in the Röt. Instead, in the south with the Carpathian and Upper Silesian Gaps, new connections to the Neotethys Ocean opened up.

Paleogeography in the Upper Buntsandstein with Röt salinar / from Meschede (2015)

The sediments are deposits of a large playa lake, into which, however, multiple marine transgressions broke. Whereas the deeper part of the Röt formation in large areas of the Basin is characterized by thick evaporites (anhydrite, gypsum, rock salt), in the higher part, more clayey-marly sediments dominate, which already lead over to the overlying rocks of the Muschelkalk group. Here especially dolomitic clay marl was deposited. In the centre of the basin with intercalated evaporites, thicknesses of around 350 m are reached, without evaporites around 200 m.

s7-sequence: saline clay sequence with gypsum/former brickyard Hölle/Eichsfeld (1988)

s7-sequence: Röt clay pit with boundary to Muschelkalk/Egge hills (2013)

s7-sequence: exploratory drilling at a new Röt site/Egge hills (2017)

Carbonate type and amount determine the specific use of Röt clay in the brick industry. Largely carbonate-free deposits are used as basic components in the roofing tile industry, although a low content of finely distributed carbonates can be accepted. Dolomitic-marly clays with finely distributed carbonate are used in the production of backing bricks.

Keuper (235.0 – 201.3 million years ago)

After the Buntsandstein (Lower Triassic) and Muschelkalk (Middle Triassic), the Keuper (Upper Triassic) represents the third lithostratigraphic group of the German Triassic. The name is derived from the Franconian dialect word “Keiper/Kieber”, originally referring to variegated, crumbly argillaceous rocks. In the year 1822, Leopold von Buch introduced the term Keuper into geoscientific literature. With more than 30 million years, the Keuper covers the longest period in the Triassic.

In plate tectonic terms, the development was determined by rifting processes. The position and differences of the Central European Basis still corresponded largely to that of the Buntsandstein. However, the morphological relief had become wider and flatter. This favoured marine ingressions and fine clastic sedimentation. The Central European Keuper Basin extends over around 1 500 km from England in the west as far as Poland in the east and over around 1 000 km in the northsouth direction from Southern Sweden to Northern Switzerland. For a time, it was connected to the Neotethys Ocean via the Belfort Gap.

Paleogeography in the keuper/modified modell according Beutler & Nitsch (2005)

At the time of the Keuper, Central Europe lay on the northern hemisphere in the sub-tropical zone. The polar caps were ice-free and the global mean temperature of the atmosphere was higher than today. As already in the Buntsandstein, the initially still desert-like climate brought forth typical playa sequences. These include primarily pelitic red beds and evaporites. Later, as a result of prolonged periods of rain, fluviatile sands were deposited, which interlock with fine clastic and chemical sediments of the basin facies. The delivery areas were primarily Scandinavia and the Vindelician-Bohemian Massif.

Argillaceous rock from the Lower Keuper (type 12)

The Lower Keuper was deposited in a short time of just around 2.5 mill. years in limnic-brackish to shallow marine conditions. It comprises the Erfurt formation (k1-sequence, Lettenkohlen Keuper), which, on account of impure coal and plentiful plant chaff, is also referred to as Lettenkohlenkeuper. In the Keuper, the Muschelkalk Sea was successively replaced by a wide delta landscape with depressions without any outlets. As a result of short-term marine ingressions, the clayey-sandy base beds were repeatedly interrupted by dolomitic rock layers. The ingressions came through the Belfort Gap in the southwest.

k1-sequence: Former brickworks pit Schöningen/Subhercynian Basin

Lower ‚Lettenkohlen‘ sandstone with main dolomite and variegated marl (1987)

Flooding of the pit with artesian groundwater from Muschelkalk layers (1987)

The Lower Lettenkohlen sandstone consists of dark grey and red-variegated clay/siltstones, which are characterized by cyclically washed-in light-coloured fine sand layers/lenses and mica soaps. Massive dolomite beds alternating with dark grey clay marl stone and bilious green clay stones characterize the rock formations of the main dolomite. This is covered by clayeymarly beds of the Lower Bunter Marl and clayey-sandy beds of the Upper Lettenkohlen sandstone. The Upper Bunter Marl introduce the transition to the Middle Keuper. The Erfurt formation is around 60 to 80 m thick in basin facies. The top of the sequence is marked by the shallow marine peripheral dolomites.

The clayey-marly complex strata of the Keuper are characterized by lively material changes and numerous petrographic inhomogeneities. Significantly increased product requirements, have led to the fact that the Lower Keuper (k1-sequence) is no longer used at all. The last active sites included the ELM-Poroton Brick Plant in Schöningen.

Argillaceous rock from the Middle Keuper (type 13)

The Middle Keuper constitutes in increasingly arid climates a time period with a stronger continental influence within the German Triassic. Shallow waters and temporary lakes take up large areas. Variegated siltstone and dolomitic clay marlstone form the main mass of the sediment. During a period of around 29 million years, a total of four formations were deposited.

By the beginning of the Grabfeld formation (k2-sequence, Lower Gypsum Keuper), the shallow inland sea that had formed at the end of the Lower Keuper had largely evaporated. In an evaporite-playa environment, alternate strata of dolomitic clay marls and gypsum were deposited. In the lower part, the complex strata contain numerous bands of gypsum/gypsum residues, from which the name Gypsum Keuper and banded marl are derived. In some places, up to 15-m-thick basinal gypsum beds were formed. On top follow thick dolomitic clay/marlstone series with nodular sulphate intrusions. The thickness of the Grabfeld formation varies in the basin facies between 150 and 220 m.

Evaporite-Playa environment in the Middle Keuper / source: Meschede (2015)

Then, in extensive river and trench systems, the 40 to 60 m thick Stuttgart formation was deposited (k3-sequence, Reed Sandstone). The reason was a more humid climate and persisting precipitation. The Reed Sandstone gets its name from fossil horsetail grass residue, which was erroneously identified as reeds. In river channels/flood facies, fluviatile sand banks from Scandinavian delivery areas were sedimented. On flood plains/stagnant water facies, primarily silts and clays settled.Then, in extensive river and trench systems, the 40 to 60 m thick Stuttgart formation was deposited (k3-sequence, Reed Sandstone). The reason was a more humid climate and persisting precipitation. The Reed Sandstone gets its name from fossil horsetail grass residue, which was erroneously identified as reeds. In river channels/flood facies, fluviatile sand banks from Scandinavian delivery areas were sedimented. On flood plains/stagnant water facies, primarily silts and clays settled.

In the Thuringian Basin, the Reed Sandstone is widely distributed in stagnant water facies as silt/clay stone to friable fine sandstone. Characteristic are accessory impurities of carbonates and sulphates. The sequence consists mostly of a lower grey part (0 – 25 m) and an upper reddish-brown part (8–35 m). The lower limit of the Reed Sandstone lies at the basis of the D2 discordance. The upper limit is not sharp and is defined by the end of the silty-sandy sedimentation and onset of the gypsum content.

k3-sequence: pit with roofing tile mono-clay batch in the Reed Sandstone/Thuringian Basin (2017)

Stagnant water facies, alternating layers of clay silt and fine sandstone (2017)

The homogenization begins in the pit (2017)

At the large-scale pit area at the Red Hill south of Mühlhausen different pressed clay roofing tile plants are located. With clever control of the extraction process, granulometrically ideal roofing tile bodies can be produced without the addition of external raw materials, which is very rare these days. One limitation is the content of sulphates/gypsum, which must be compensated for with correspondingly high additions of barium carbonate. Lime spalling can be an issue and must be prevented by means of suitable measures. Locally, the Reed Sandstone is prepared very fine and dry for this reason.

With the beginning of the Weser formation (k4-sequence, Upper Gypsum Keuper), the climate became hotter and dryer again. In wide shallow salt flats, red dolomitic marly clays and marl were sedimented. As a result of short-term flooding and evaporation, as in the Grabfeld formation, cyclic deposition of gypsum ensued. Because of the characteristic red colouring, the bottom part of the Weser formation is also referred to the Red Wall. The upper part of the Weser formation is formed by hard dolomitic marl. The Weser formation is 100 to 150 m thick and is bordered at the cap of rock by the D4 discordance.

k4-sequence: brittle tectonic deformation in the clay pit Friedland/Leinetal Trench (2008)

Upper Gypsum Keuper with white gypsum residues and swellable clay minerals (2008)

External raw materials are delivered for the modern brick production (2011)

Especially for the area of the Red Wall, a high content of innercrystalline swelling clay minerals is characteristic. Worth mentioning is the expansive clay mineral corrensite, but also smectites and illite-smectite interbedded minerals. The resulting drying and preheating sensitivity limits the batch content of the plant’s own clay raw materials often severely. The ultimate buying in of external raw materials was already the last-resort solution for a string of clay block production plants.

In Franconia and the Upper Palatinate, the Weser formation goes over into its marginal facies equivalent and is laterally interlocked with the Steigerwald, Hassberge and Mainhardt formation. The Steigerwald formation comprises the Lehrberg beds, an around 30-m-thick sequence of dolomitic-marly claystone with intercalated marl Beds/Lehrberg Beds. The deposition of the Lehrberg Beds was completed in a salinar playa clay pan with frequent drying up.

k4-sequence in marginal facies: one of the last big pits in the area around Nürnberg (2018)

Selective extraction of the Lehrberg beds for roofing tile production (2014)

Inhomogeneities in the layered beds cause big spoil heaps (2014)

The Lehrberg Beds are covered by bubble sandstone of the Hassberge formation. Unlike the Nordic Reed Sandstone, the sand was deposited from the Vindelician-Bohemian Massif. The Middle Keuper closes with the 100 to 150-m-thick Arnstadt formation (k5-sequence, Marl Keuper). Because of the dominance of carbonates, this formation is largely unimportant for the clay brick and tile industry.

Argillaceous rock from the Upper Keuper (type 14)

At the beginning of the Upper Keuper came the change from largely continental to brackish-marine sedimentation. Associated with this, the intensive variegation of the sediments is lost. The mostly dark grey to subtle violet clay stones/shale clays and acid green sandstones of the Exter formation (k6-sequence, Rhaetic Keuper) are deposits of a shallow sea that advanced from the Northern to Southern Germany.

Characteristic are bone beds – that is bone remains from land and marine animals, which formed as boulder layers in the moving shallow water. According to the respective fossil community, the k6-sequence is subdivided into the sub-units of the postera beds (marine bivalve faunas Unionites postera), the contorta beds (Rhaetavicula contorta bivalves) and the Triletes beds (Triletes spores). The k6-sequence covers a time span of around 3.5 million years and is up to 150 m thick.

k6-sequence in marginal facies: Extremely plastic Rheatic clay/Upper Franconia (2013)

k6-sequence: Rheatic clay pit with changeover to the overlaying Lias/Egge Hills (2014)

Word famous location due to the find of the Plesiosaur in the Contorta layers (2014)

As a result of their brackish-marine origin, the beds of the Upper Keuper (k6-sequence) are often more favourable than the underlying red-variegated playa sequences. Marl beds and evaporites are absent, instead organically bonded carbon can be present in a relatively high concentration. In basal facies, mostly the contorta strata have a great similarity with Lias clays. Illite dominates clearly before kaolinite and chlorite. Expansive clay mineral content is mostly low and can therefore be controlled. The overlying Triletes strata are with lower clay mineral contents more sandy and silty and often coloured light-green because of the increased content of chlorite. The preferred use of the contorta and Triletes strata is currently in the production of masonry bricks.

Very different from this is the formation of the Rhaetic Keuper in the south-eastern marginal facies, that is in Upper Franconia and the Upper Palatinate. With much lesser diagenetic consolidation, disordered kaolinite is the clay mineral characterizing the properties here. Pronounced plasticity, light fired colours and the associated ceramtechnological properties are the essential characteristics of these raw materials. Associated with this are often higher-quality applications, like, for example refractories or in the tile industry.

k6-sequence in marginal facies: Rheatic clay pit with overlying Lias/Upper Franconia (2012)

One of the very few yellow-firing slate clays in Germany (2012)

All possibilities for storage, processing and clay grinding (2012)

At the site of Grossheirath south of Coburg in Upper Franconia, beds from the Rhaetic Keuper to the overlying Lias are extracted selectively and prepared to ceramic bodies. With the selective use of external raw materials, raw and ground clays with a fired colour spectrum from creamy white through buff to red are produced. In addition, in modern preparation and firing plants, tailored chamottes are produced. The main buyers are producers of building ceramics
and refractories.

Jurassic (201.3-145.0 million years ago)

In plate tectonic terms the Jurassic is characterized by North America splitting off from Europe, South America as well as Africa. Especially the opening of the Central Atlantic and the Pennine Ocean influenced development in Central Europe. As the crust lowered, the North Sea advanced as far as Central Europe. In the south, a wide corridor to the Neotethys Ocean opened up.

Following the terrestrial-dominated sediments of the Germanic Triassic, the Jurassic is characterized especially by shelf sea sediments. At the beginning of the Jurassic, a shallow epicontinental sea with deposited archipelagos developed. The climate was tropical with lush vegetation and lateritic weathering. In the Lower and Middle Jurassic, thick beds of dark clay were deposited. In the Upper Jurassic came a general change in the direction of chemically precipitated sediment, especially to lightcoloured marl- and limestone.

Brickwork clay pit Wellersen in the Arieten formation/Lower Saxony (2007)

Upper part of the clay pit Grossheirath in the Psilonoten formation/Bavaria (2014)

Former clay brickwork pit Goslar overlooking the Rammelsberg/Lower Saxony (2016)

According to the international stratigraphic chart, the Jurassic
system is divided into eleven stages. In mining, a lithostratigraphic division is common. In Southern Germany, a differentiation is made according to the prevailing colour between Black, Brown and White Jurassic. In the Northwest German Basin, a differentiation is made between the Lias, Dogger and Malm. English quarry workers used these names for thinly bedded shaly clays (layers/lias), for high-iron sandstone (dogger) and for light-coloured brick-like solid rock (malm brick).

Jurassic-Park: ambitious young palaeontologist in the Herford Lias basin

Many ammonites and belemnites can be found in the Amaltheen formation (2017)

Shells as indicators for shallow marine environments in coastal waters (2017)

The Jurassic is characterized by a wealth of fossils. The most important index fossils for the biostratigraphic division are ammonites, an extinct type of cephalopod, which also includes cuttlefish and nautiloids living today. Insignificant for the stratigraphy, on the other hand, are dinosaur finds.

Argillaceous rock from the Liassic (type 15)

The Lower Jurassic/Lias was deposited in shallow-marine conditions and water depths of around 150 m. In the North German Basin, the Lias emerges more and less continuously from the inland-sea-like sedimentation of the Rhaetian. The lower limit of the Lias is marked by transgressed broken tempestites. In the Lias, mainly bituminous clays and clay marls were deposited, which were formed in euxinic conditions.

Paleogeography of the Lower Jurassic/source: Meschede (2015)

At water temperatures above 20° C phytoplankton, ammonites, crocodiles and ichthyosaurs developed in abundance. On the sea floor, large quantities of dead organisms collected, which, because of the anaerobic milieu, could not decompose. In the sludge, hydrogen sulphide was formed, which was reduced by bacteria to sulphate and elementary sulphur. In combination with reactive iron, pyrite/marcasite was formed from this.

Pit of the last HMZ factory in Lower Saxony/Markoldendorfer Basin (2017)

Extraction of geode containing slate clay of the Arieten formation (2017)

Lower part of the weathering zone with thin ferric coverings on separation planes (2017)

As a result of diagenesis, the sediments were consolidated to parallel stratified mudstone with flaky to thick-shaly structure For this reason, they are also referred to as shaly clays in the brick industry. Depending on the degree of weathering, the colours vary from greyish black through greyish brown to yellowish brown. In the upper part, the clay is stained light brown by trivalent iron complexes. Red sediment colours and hematite are absent.

One of 5 slate clay pits of the company STORK Tongruben GmbH (2017)

Since more than 50 years STORK extracts slate clay from the Liassic and Dogger (2017)

Organic and mobilized iron ions dictate the colour of the clay (2017)

Traditionally the black mudstones/shaly clays of the Lias and Dogger are used as basic raw materials in the production of backing bricks, where they stabilize the grain size ranges and ensure high compressive strength. Limitations can result from lumpy limestone, which can be present in the form of limestone benches or geode beds. Concentrated fossil accumulations/battlefields (ammonites, belemnites, shells) can lead to lime spalling. A well-known example is the clay pit Mistelgau, formerly DEHN-Ziegel in Bavaria.

Argillaceous rock from the Dogger (type 16)

During the Dogger, the sea expanded further to the east. Expanses of Scandinavia, however, remained as uplands. The London-Brabant Massif, the Rhenish Massif and the Bohemian Massif were preserved as mainland pockets. With flooding of the Vindelician mainland, in the south a connection to the Pennine Ocean was opened over a wide front.

Paleogeography of the Middle Jurassic/source: Meschede (2015)

The stratigraphic sequence of the Dogger goes from the facies from the Lias and is dominated by thick dark mudstones/shaly clays of the Opalinus clay formation. In the South German Basin, coarser sediments led to the formation of the Dogger and iron sandstone with typical iron-oolite horizons. One special case with the formation of the iron sandstone are the red haematite layers/coloured earths in the Northern Franconian Alps, also known as Troschenreuth ruddle or red bolus.

In the North German Basin, sedimentation was fine-grained for a relatively long time. The change to iron oolitic beds/Cornbrash only began in the Bathonian and was triggered in the North Sea-Denmark-Baltic Sea region by temporary lifting of the Cimbric mainland. The uppermost part of the Dogger is dominated by clayey deposits. It is formed by the mudstones/shaly clays of the Ornatenton formation.

Slate clay from the Dogger: long live the roof/North Rhine-Westphalia (2010)

Tradition and modern age: clay arrives for processing (2010)

Typical weathering profile of slate clay of the Liassic and Dogger (2016)

Severely weathered replastified Jurassic clay are choice materials for the northwest german brick and tile industry. They are found in thicknesses usually between 1 and 5 m. The distribution of the weathered horizons is not geologically charted. Only by means of exploratory drilling and, if appropriate, test pits can these beds be found. In addition comes the fact that the weathered clays are often covered by Quaternary overburden. the course of geological history, they can also have been partially or completely eroded. The exploration risk is high. Weathered clay is hard to find.

Plastic filter cakes from the Dogger (type 17)

In the Freihung/Grossschönbrunn region in the Upper Palatinate/Bavaria, for more than 135 years, the company STROBEL Quarzsand GmbH has extracted kaolin-containing Dogger sandstone from the Aalenian stage and processes this for use in the glass industry and construction chemicals. The deposit developed because of the deposition of deltaic sediments about 170 million years ago.
Starting from crystalline supply areas from the region of the Bohemian Massif, the fine-grained quartz sands were deposited with a thickness of about 100 m. The supply of the deposit is recently estimated at 150 million tons.

A traditional company in Freihung/Bavaria (2007)

Kaolin-containing Dogger sandstone near Großschönbrunn/Bayern (2010)

Small membrane filter press in a heated press facility (2013)

Big membrane filter press in a heated press facility (2013)

Genuine service: filter cake production ensured all-year round without winter break (2010)

Handed on a silver platter: up to 120.000 tons a year (2010)

During silica sand washing the kaolinite-rich fines < 40 µm are deslimed and dewatered on membrane filter presses. German backing brick and roofing tile factories have been supplied with these filter cakes for more than 18 years. Until now only a small share of the available material is used. This is a so far rarely recognized potential for Bavarian brick factories.

Argillaceous rock from the Malm (type 18)

The London-Brabant Massif, the Rhenish Massif and the Bohemian Massif became preserved mainland pockets by regression and separated the North German and South German Basin. The South German Basin developed into a carbonate platform open to the Pennine Ocean. A wide range of corallian and platy (lithographic) limestones were deposited. World famous for fossil finds (e.g. the Archaeopteryx) are the Solnhofen limestones from the Tithonian.

Paleogeography of the Upper Jurassic/source: Meschede (2015)

At the end of the Malm, the sea receded, leaving large parts of the South German Basin to go dry. In the north, the sea receded to the central Lower Saxony Basin. Here thick beds of limestone and marl were deposited. Evaporation led to the formation of sulphates and salts. Only in peripheral regions of the Basin clayey-clastic and largely lime-free sediments were deposited.

Bramsche intrusive body in the in the deep underground of the Ueffeln open pit / after DSK

From the Ueffeln open-cast mine, HOLLWEG, KÜMPERS & COMP. extracts quartzitic sandstones and pyrophyllitic shaly clays from the Oxford stage.
They are thick sequences deposited from the northwestern supply areas as delta sequences. With the rise of the Bramsche Intrusion, the strata from the Upper Cretaceous were imprinted during contact metamorphosis. With circulation of around 400° C hot solutions over a long time, kaolinite was secondarily transformed in pyrophyllite. A local phenomenon that can only be observed in this open mine. The pyrophillitic shaly clay is the only one in Europe.

The open mine Ueffeln near Bramsche aerial view/Lower Saxony (2003)

Planned hot-air-balloon ride before the time of modern drones (2005)

Beginning of pre-homogenising with combined sieve/milling machine (2005)

Wide-spaced alternating sequence of hard sandstone packages and shaly clays (2004)

Pre-homogenized shaly clay mixture 0/30 mm for clinker plants (2017)

Since 2003 big large quantities of shaly clay are delivered (2005)

The stratigraphic sequence of the Oxford stage can be described in simplified terms as a wide-spaced alternating sequence of hard sandstone packages and soft, but still strongly consolidated mudstones/shaly clays. In the covering layers of the Kimmeridgian, the alternating layers are more narrowly spaced, which makes extraction of pure mudstones/shaly clays difficult to impossible. Traditionally, the raw materials are processed in brick plants, where they have a grogging effect and improve the degassing behaviour of high-carbon bodies.

Cretaceous (145.0 – 66.0 million years)

The major part of the Cretaceous was characterized by strong global warming and deep-reaching kaolinization of the bedrock. The end of the Cretaceous is correlated with worldwide mass extinction that affected numerous plant groups and almost all animal groups including dinosaurs. Extremely strong volcanic activity, presumably in combination with a meteorite impact are assumed to be the most probable causes.

Plate tectonic development from Triassic to Cretaceous/source: Meschede (2015)

In terms of plate tectonics, the development was affected by rifting. The Atlantic and the Indian Ocean opened up. With the global expansion of the Mid-Oceanic ridge systems, a considerable volume of sea water was displaced. Therefore, the sea level was at times around 170 to 250 m higher than today. The polar caps were free of ice. The surface area of the epicontinental shallow seas had doubled worldwide within a short time, reaching its maximum expansion at the transition from the Lower to the Upper Cretaceous.

Geological profile section through the Northern Harz Boundary Fault/source: Meschede (2015)

In the Upper Cretaceous, the plate tectonic conditions changed fundamentally. In Central Europe, pronounced compression tectonics resulted, as documented impressively today in the area of the North Harz Boundary fault. The Harz was lifted out around 6 000 to 7 000 m as a half-horst. In this process, its Jurassic surface layers were removed. In the adjacent foreland, the strata of the Mesozoic bedrock were set up at steep angles. Today the vertically standing hogbacks of the Upper Cretaceous like the Teufelsmauer (Devil’s Wall) are some of the most striking features of the Northern Harz foreland. Other examples of compression tectonics are the Osning overthrust fault and the Externsteine rock columns in Teutoburger Forest.

The Teufelsmauer (Devil’s Wall) near Neinstedt: product of tectonic and erosion/Saxony-Anhalt (2019)

Further west: the Königstein (King’s Stone) near Westerhausen/Saxony-Anhalt (2016)

Cretaceous Cliffs with erosion pattern from sandstone weathering (2016)

According to the international stratigraphic table, the Cretaceous is divided into the series of Lower Cretaceous and Upper Cretaceous as well as in twelve stages. The series differ based on their petrography: Lower Cretaceous primarily clayey-sandy and dark, Upper Cretaceous mostly marly-limey and light-coloured. For both series, intercalations of silicified quartz sandstones are characteristic. The subdivisions and stages are based on palaeontological aspects. The main index fossils are ammonites, belemnites, inoceramidae and foraminifera.

The most important areas for the clay brick and tile industry are in the Lower Saxony Basin with the Hanover region, the Wiehen Hills as well as the Weser and Leine Uplands, and further in the region of the Subhercynian Basin with the Northern Harz foreland. The largest connected surface exposure area is the Münsterland Cretaceous Basin with the adjacent Teutoburger Forest. The outcrop of the Southern German Cretaceous is confined to a relatively small area of the South-Eastern Franconian Jura and on the northern boundary of the Alps.

Argillaceous rock from the Lower Cretaceous (type 19)

Central Europe was at first dominated by tectonic extension movements. Along a lateral displacement zone, a graduated arrangement of depressions was formed that served as sediment basins for the deposition of thick clay layers in the Lower Cretaceous. As a result of the continued regression, first an extensive mainland area developed, which stretched from Scotland over the Ardennes-Rhenish-Bohemian Massif to the East Sudetes Basin. This resulted in facies separation of the northern and southern marine areas. While in the northern marine areas, clastic sedimentation dominated, large carbonate platforms were formed on the Helvetian Shelf.

Paleogeography of the Lower Cretaceous/source: Meschede (2015)

The Lower Saxony Basin was a big inland lake, which was connected with the open boreal North Sea by narrow, shallow passages. The basin was bounded to the north by the Pompeckj-rise. In a subtropical climate cypress-like trees, conifers, ferns and palm trees grew in widespread swamp forests. Local swamp areas at the Basin’s margin formed and turned into economically significant coal seams.

In the Berassium predominantly brakish-lagoonal Wealden clays were deposited. Occasionally sandy delta cones timbered with subtropical marshland forests advanced in the brackish water basin. Special attention should be given to the natural monument Münchehagen near Rehburg-Loccum, where numerous dinosaur tracks were found in sandy delta sediments.

Worldwide important tracks of sauropods/Münchehagen (2015)

Iguanodon left tracks with over 22 footprints (2015)

2009 tracks of predatory dinosaurs were found (2015)

Compared to the sands Lower Cretaceous clays are weakly digenetically consolidated. On account of their lamellar to platey joint areas, depending on the stratigraphic position, they are also called Wealden shales, fish shales or black shales. Already on the introduction of small amounts of water, they become plastic and are characterized by pronounced plasticity. Up until the late 19th century, they were sought-after pottery clays as they proved water-tight and resistant to acid even without glaze. After the era as pottery clays, the focus was on the production of stoneware and clay pipes, floor tiles and facing bricks.

Clay pit Duingen near Alfeld in the Hils Basin/Lower Saxony (around 1950)

Conveyor lines are dominant on the premises (around 1975)

Stone ware pipe production until the end of the 1970s (around 1975)

On the north-eastern flank of the Hils Basin the Norddeutsche Steinzeugwerke extract Wealden shale from the Deister formation. The clay area extending over several hundred hectares will permit extraction for generations to come. Following the shutdown of the stoneware pipe production, today the site is operated specifically for the supply of external brickworks.

Extraction of quaternary surface layers and 40m Wealden clay (2020)

Selection and processing of the clay in the pit (2020)

Mixed bed homogenization and clay storage facility (2020)

In the Valanginian and Hauterivian, transgressions from the north began. Through the Danish-Polish Trough there is a connection to the Pennine Ocean/Tethys. In the central part of the Lower Saxony Basin, dark clays and marl clays were deposited with a thickness up to 600 m. In the Barremian, as a result of the regressive tendency, the strait to the Tethys was interrupted. In a calm and poorly aerated sea basin, up to 500-m-thick, often bituminous clay strata with inclusions of lamellar clay packages were deposited. On the seabed, euxinic conditions were increasingly created. In the Aptian, clay thicknesses up to 350 m were reached. Besides finely layered lamellar clays and fish shales, monotonic mudstone sequences were formed.

Characteristic is a high clay mineral content, which is derived from disordered kaolinite as well as alternating beds of innercrystalline swelling illite-smectite.
They contribute to a high binding capacity and excellent plasticity.With the absence of effective stabilizing grain scaffolds, the fine particles of the Lower Cretaceous clays lead to a correspondingly difficult drying and outgassing behaviour. Due to intercalations of thin coal seams and finely dispersed bituminous amounts this phenomenon is enhanced. No wonder various brickworks closed in times of continually higher demands on productivity.

Argillaceous rock from the Upper Cretaceous (type 20)

Already from the Upper Albian, the northern flank of the Ardennes-Rhenish-Bohemian Massif fell below the sea level and was covered by more than 2000-m-thick sediments of the Upper Cretaceous. The sea had spread across the whole of Northern Germany. The transition was completed from clayeysandy to predominantly marly-limey sedimentation.

Regionally, strata of the Upper Cretaceous are found mainly in the area of the Münsterland and the Subhercynian Basin. All clayey deposits are characterized by a more or less high level of carbonate constituents and can therefore be classified as marly clays, clay marls or marls.

Owing to the only low to completely absent clay mineral content, large parts of the Upper Cretaceous profile are not relevant for use in the clay brick and tile industry. Only in the Campanian, there are clayey-marly raw materials that are used today in the clay brick and tile industry. Modern brickworks in the Münsterland generally prefer to manage largely without the use of local Upper Cretaceous clays.