Introduction

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The largest mass extinction in the whole earth history took place at the Permian/Triassic boundary. Possible reasons are gigantic volcanic eruptions in what is today Siberia, leading to a climatic change. 95% of all marine species and 70% of the continental vertebrates were extinct during a short period of only 10 Ma. During the Triassic - 251 to 205 Ma ago - all landmasses were combined to the super continent Pangaea. Globally high temperatures in the oceanic margins were responsible for a highly diversified fauna while the arid climate of the hinterland of the super continent resembled more to the dry regions of today's Africa.

These conditions - arid climate with a super continent stretching from the North to the South Pole, comparably small tropical shallow marine areas as places of highest biological activity in the oceans as well as the freeing of numerous ecological niches caused by the Permian/Triassic mass extinction - supported the development of a fauna and flora that was so different from all preceding periods that the Triassic is regarded as the beginning of the Mesozoic: Last remains of ancient animal groups like the giant amphibians met the first dinosaurs and early mammals, the snails and shells took the place of brachiopods in the seas, and reptiles conquered the sky.


Fig. 2: Paleogeography of the earth during the Triassic (from PALEOMAP Project 2002, www.scotese.com)


The name Triassic derives from the division of the sediments into three typical stages: Bunter, Muschelkalk and Keuper. The basin containing these sediments, reaching from the North Sea area in the north to the Iberian Penninsula in the south and from the Paris Basin in the west to Poland in the east is called Germanic Basin.

The Germanic Basin was a shallow, flat epicontinental sea. Its basement had only little subsidence, so even in central areas only shallow water sediments can be found, and on several occasions clastic sediments from the hinterland were able to spread deeply into the basin. Connections to the Tethys ocean in the south were only temporarily, as well as a connection to the northern ocean in the region of today's North Sea. Together with the dry climate the lack of water exchange led to the frequent formation of gypsum respectively anhydrite and halite as well as red sediments.

Bad environmental and preservation conditions are the reason for a general scarceness of fossils, while many layers and beds can be observed throughout the basin, being the perfect markers for lithostratigraphic divisions over large distances. Only in the Upper Muschelkalk a biostratigraphic division according to ceratids and conodonts is possible.

Fig. 3: Divisions of the Triassic in the Germanic Basin (after BACHMANN et al. 1999, simplified)


Bunter (251 - 240 Ma)

Despite its name, the Bunter ("the colored one") of southern Germany is mainly a reddish colored sandstone with a high content of feldspars typical for a formation under dry continental conditions. Conglomerates and clay can also be found but are less abundant.

Rivers transported sediments from the erosional areas of the crystalline massifs of the southern Black Forest and the southern Vosges into the nearly flat basin, creating a characteristic cross-stratification with N and N-E direction. Mudcracks are also evidence for temporary flooding and arid climate. Aeolic sediment transport is proven by aeolic shaped rocks and pebbles but is of marginal importance. Dolomite nodules and dolostone beds are evidence for temporary marine influx which is most obvious in the Upper Bunter (Plattensandstein ("slab sandstone") and Röttone ("Röt clays"), with wave ripples).

Fossils are very rare, there are mostly remains of conifers (Voltzia) and ferns (Anomopteris). Clays often contain conchostrachs (Euestheria). Burrows of primitive animals are also abundant. Among the vertebrate remains, the tracks of the "handed animal" Chirotherium are most common. Amphibians like Heptasaurus ("Mastodonsaurus") and Cyclotosaurus can also be found.


Fig. 4: Paleogegography of the Germanic Basin during the Bunter (from GEYER & GWINNER 1991)


Muschelkalk (240 - 232 Ma)

With the formation of seaways (gates) in the area of today's western Alps and in the Paris Basin towards the Tethys ocean the Germanic Basin turned into a marine environment. Due to the shallow gates and reefs an exchange of water between basin and ocean was limited and strongly depended on sea level changes.

The Muschelkalk mainly consists of limestones, dolostones and evaporites, alternating with clastic sediments (clays, marls). Especially the Middle Muschelkalk mainly consists of evaporites (gypsum or anhydrite, halite and dolomite). The climate often caused high concentrations of salt and lack of oxygen in water layers near the sea floor, leading to the formation of micritic lime and marl muds that contain few fossils. When storm events brought oxygen to the water near the sea floor shellfish rapidly spread. The resulting bioclastic beds then became the base for a more diverse benthic fauna that was a source of food for vertebrates.


Fig. 5: Limestones of the Upper Muschelkalk near Künzelsau.


High water temperatures and salt content restricted life to a few genera, but those showed a good adaption and reached a high number of individuals. Very small forms of usually larger animals are typical for such restrictive environments.

Fossils are comparably abundant in the Muschelkalk. In the Lower Muschelkalk, they can mainly be found in bioclastic beds, containing remains of echinoderms, vertebrates (Nothosaurus), shellfish and brachiopods (Spiriferina). The ammonoid Beneckeia buchi can also be found. While the evaporitic Middle Muschelkalk yields no fossils, the Upper Muschelkalk contains a comparably rich fauna with locally well preserved echinoderms (Encrinus liliiformis), ceratids (useful index fossils), brachiopods (Coenothyris, Punctospirella) and bivalves (Myophoria, Pleuronectites, Enantiostreon, Plagiostoma, Placunopsis, Hoernesia) that are ususally concentrated in lumachelles. There are also crustaceans (Pemphix), remains of aquatic reptiles (Nothosaurus, Simosaurus, Placodus, Tanystrophaeus) and fish (Colobodus, Palaobates, Saurichthys, Hybodus, Polyacrodus), but they are most often only fragmentary. Abundant trace fossils (Rhizocorallium, Thalassinoides) and tempestites show a temporarily better supply of oxygen to the water layers near the sea floor.


Fig. 6: Paleogegography of the Germanic Basin during the Muschelkalk (from GEYER & GWINNER 1991)


Keuper (232 - 205 Ma)

At this time, Europe was located within Pangea on 15° northern latitude, within the tropics. The climate was continental and arid, but changed to semi-humid conditions until the end of the Keuper. Various changing sedimentation environments led to a general lack of fossils.

At the end of the Muschelkalk, sedimentation nearly had met the rate of subsidence within the Germanic Basin so the Keuper sediments were deposited in wide, flat areas. The clastic dominated ramp received sediments from sea currents, rivers and deltas, wind, processes of evaporation and flash floods.


Fig. 7: Paleogegography of the Germanic Basin during the Keuper (from GEYER & GWINNER 1991)


The Lower Keuper still has some resemblance to the marine environment of the preceding Muschelkalk, with brackish and evaporitic sediments. It's a characteristic sequence of clays, carbonatic (mainly dolomitic) beds and sandstones. Many of these beds can be observed throughout south west Germany. The Lower Keuper is of comparably small thickness, with an increasing thickness towards the north (the center of subsidence), and vanishes towards the Alps.

The Lower Keuper is divided into two complexes by the Lettenkeuper Hauptsandstein ("Main Sandstone"), consisting of fine grained sands of baltic-scandinavian origin.

A greatly reduced fauna indicates an increase in salinity compared to the Muschelkalk.

The usually gray or green sediments of the lower and upper Lower Keuper show that the decay of organic matter created a reducing environment, preventing the formation of red iron III oxides. This indicates marine conditions with at least slightly oxygenated water, although remains of land vertebrates in the upper Lower Keuper also prove for terrestric influence.

Generally, a cyclicity is observable: A sequence starts with a marine transgression and the formation of carbonate beds. The following clays are either of marine or brackish to limnic origin. Temporary exposure to the air is indicated by teeth of lung fishes. Complete cycles are finished with small coal beds.

The Lower Keuper sandstones represent strong sea regressions. The shallow water allowed the formation of horsetail populations, which is shown by root beds within the sandstones. Erosional channels also show that rivers cut into the sediments.

The Hauptsandstein represents the maximum regression. It was sedimented in long, sometimes narrow channels that were the branches of a delta. It prograded from NE to SW, forming two different facies types: The flood facies can be found in the center of the deeply eroded channels, reaching a thickness of up to 12 m. The regular facies, on the other hand, is of much smaller thickness or totally missing.

The Lower Keuper is the most fossiliferous Keuper stage. The Grenzbonebed which is the Muschelkalk/Keuper boundary, represents a sedimentary hiatus, leading to the concentration of massive vertebrate remains. Apart from marine species (reptiles: Nothosaurus, Neusticosaurus, Simosaurus, Mixosaurus, Placodus; Fische: Colobodus, Saurichthys, Hybodus, Palaeobates, Polyacrodus) also remains of inhabitants of the land and river systems can be found (amphibians: Mastodonsaurus, Plagiosuchus, Plagiosternum; fishes: Ceratodus). Numerous further bone beds occur in the basal Lower Keuper. Clays formed in brackish environments sometimes yield complete speciments of small fishes of the Peltopleuridae family, brachiopods (Lingula) are very abundant and can cover whole bedding planes. Well-preserved plant fossils (Equisetites, Neocalamites, Schizoneura, Danaeopsis) are mainly limited to the Hauptsandstein, pyritized or coaled log fragments of Dadoxylon are locally abundant in limnic to brackish clays. Swamp sediments that occur in the upper Lower Keuper can contain nearly complete skeletons of amphibians (Mastodonsaurus, Plagiosuchus, Plagiosternum, Gerrothorax) and lung fish (Ceratodus).


Fig. 8: Lower Keuper, Zwingelhausen.


Fig. 8a: Profile of the Lower Lettenkeuper (ku 1) with lithostratigraphic units.

Estherienschichten 2 and 3 as well as the interbedded Dolomites 2 and 3 were eroded during the formation of the Hauptsandstein.

Rielingshausen


The Middle Keuper is a sequence of fine clastics (clays and silty to dolomitic marls) showing a wide range of colours. Red to violett sediments indicate for an oxidizing environment while gray to greenish sediments prove for a reducing environment. Evaporation of the salinar sea water led to the formation of gypsum and halite crystal pseudomorphs. Rivers and flash floods brought coarser clastics into the basin (sandstones) that spread from S to N and NW. Sea transgressions left numerous carbonate beds that can be followed for more than 100 km (e. g. Lehrbergschichten). The fossils they contain represent a decayed brackish-marine fauna.

The thickness of the Middle Keuper increases in northward direction.

Grabfeld Formation (Gipskeuper - "Gypsum Keuper")

The Gipskeuper in South West Germany shows an constant profile. It can be divided into the Grundgipsschichten ("basal gypsum layers"), Bochinger Horizont ("Bochinger bed"), Dunkelrote Mergel ("dark red marls"), Bleiglanzbank ("galenite bed"), Mittlerer Gipshorizont ("middle gypsum bed"), Engelhofer Platte ("Engelhofer bed") and Estherienschichten ("Estheria beds"). It's a document for the final change from a marginal marine to a fully continental red bed environment.

The Grundgipsschichten are a sequence of sulfate rocks alternating with clay and dolostone layers. The lower part is dominated by dolostone beds, followed by sulfatic sediments and finally clays. Gypsum from this fomation is Baden-Württemberg's most important natural resource. Due to the excellent availability of outcrops it was possible to identify 12 shallowing upward cycles within the Bochinger Hoizont and the Dunkelrote Mergel. (AIGNER & BACHMANN 1989).

The widespread carbonate beds have a sharp boundary towards the underlying sediment or even have cut into them. Internal erosion planes, intraclasts and wave ripples on top of the beds are evidence for a possibly basin-wide marine transgression, interrupting the prevailing evaporitic conditions.

The first four cycles (base carbonate cycles) start with a basal carbonate bed or, seldom, with a thin sandstome layer, followed by massive sulfatic sediments of a shallow marine environment (subtidal evaporites). The continued shallowing of the basin is indicated by gypsum arenite, thinnly bedded gypsum alternating with clays. The upper boundary of each cycle is indicated by gypsum porphyroblasts. Along with this cycles, the clay's colours change from grayish green to red, which means that the environment changes from reducing to oxydizing.

The cycles 5 to 7 (top disturbed cycles) only have thin basal carbonate beds or none at all. A reason for this are either smaller transgressions or closeness to the land. The basal beds are followed by alternating thin gypsum and clay beds. The top is formed by the so-called Gekrösegipse, consisting of diapiric gypsum structures (tepees) that formed under atmospheric conditions. So these cycles show a continuing shallowing until the basin finally dried out.

The last 5 cycles (argillaceous cycles) start with a base that has a sharp boundary towrads the underlying sediment. Sometimes, several centimeters thick sandstones formed whose sand was transported by flash floods from the hinterland and that was partially reworked during marine transgressions. It is followed by a up to 1 m thick layer of thinnly-bedded gypsum showing subaerial structures. The following grayish or reddish clays containing gypsum nodules formed in dryer mud plains. The whole sequence is finished by a clay bed with mudcracks. Gypsum nodules and mudcracks indicate sabkha-like conditions.

These 12 cycles (paracycles") can be correlated over large distances. Siliciclastic sediments replace the carbonates towards the basin margins.

This shows that, despite the numerous transgressional events, the Gipskeuper time is dominated by superimposed regression cycle (3rd order sequence) which led to the formation of thinner and thinner carbonates and thicker and thicker clays.

Marker beds are Bleiglanzbank and Engelhofer Platte. Both formed in shallow water as proven by ripple marks.


Fig. 9: Grundgipsschichten, Bochinger Horizont, Bleiglanzbank, Roter Gipshorizont. Vellberg.


Stuttgart Formation

The Stuttgart Formation comprises Schilfsandstein (reed sandstone), Dunkle Mergel (dark marls) and Hauptsteinmergel (main stone marl).

Schilfsandstein

It's named after the abundant horsetail remains that were misinterpreted as reed by quarry workers in earlier times. Concerning colour (greenish to brownish red), mineral composition, even grain size distribution with usually small grains (0.06 to 0.2 mm), origin and the way it formed, it's different from all other Keuper sandstones. It can be found nearly throughout the entire Germanic Basin and varies greatly in thickness.

This variation results from sedimentation in string-shaped structures. Due to the preceeding phase of fluviatile erosion they have cut into the underlying sediments. Apart from marginal influx from the Vindelician-Bohemian Land the main amount of sand came from the Baltic-Scandinavian area. Cross stratification and current ripples indicate a NNE-SSW direction of the branched fluviatile system. The nearly flat Keuper Basin allowed a fast progression within 400.000 to 500.000 years. Paleomagnetic investigations have proven frequent changes in the polarity of earth's magnetic field during this time. This is documented by three polarity changes that were preserved by the heavy minerals of the Schilfsandstein. Bivalve faunas, a high content of boron and glauconite indicate a marine depositional environment.

THÜRACH (1888) differentiated between a flood and a regular facies. Instead "regular facies", the expression "calm water facies" should better be used. The flood facies is characterized by well-sorted grains, cross stratification and small ripples. It can reach a thickness of up to 30 m. Root horizons are evidence for temporarily very shallow water. In between the sandstones a calm, marine environment existed where dark brown to red, thinnly bedded, sandy to silty clays formed, locally alternating with dolostones or gypsum beds.

WURSTER (1964) regards the Germanic Basin as a large river delta crossed by channels, comparable to the Mississippi delta. The channels were filled with sand. The sandy to silty clays of the calm water facies formed when the river bank dams were flooded and clay and sand got into the calm water areas. For WURSTER, the Schilfsandstein's cutting into the underlying Estherienschichten was caused by sediment weight instead of erosional cutting.

LINCK (1968, 1970) sees the Schilfsandstein as a marine formation. According to him, it prograded from the north into the basin, thereby cutting channels and basins into the underlying sediment. A clear separation of flood and calm water facies would not be possible. This theory is supported by the occurence of boron and glauconite as well as the abundance of marine bivalve faunas and marine trace fossils (Cylindricum, Isopodichnus). According to LINCK, the morphological Schilfsandstein ridges are not channel infills but are merely a product of selective erosion.

However, paleomagnetic investigations (see above) support WURSTER's theory as they assume a quickly prograding delta for the short Schilfsandstein period.

The Dunkle Mergel that follow the Schilfsandstein are dark reddish-brown silty clays with gypsum and sand layers that are assumed to have formed in a flood plain on top of the Schilfsandstein system (FRANK 1929). Their top is formed by the dolomitic beds of the Hauptsteinmergel.

Bunte Mergel (coloured marls)

The Bunte Mergel can be divided into three divisions: Lower Bunte Mergel (Ansbach Group), Lower and Upper Kieselsandstein ("silicified sandstone") and Upper Bunte Mergel.

The Lower Bunte Mergel formed in an aquatic salinar environment as shown by the red to reddish-brown colour and the occurence of gypsum.

Below the Kieselsandstein, the grayish-white Lehrbergschichten ("Lehrberg beds") can be found, consisting of baryte-, galenite- and malachite-containing marls. Lowering salinity and an increasing amount of organic matter are expressed by a change from red to gray colours.

The Lower and Middle Lehrbergschichten locally contain many fossils of gastropods, bivalves and fish.

The Kieselsandstein consists of alternating siliciously or clayish cemented sandstones and colourful marls and clays. The marls in the middle allow a division into Lower and Upper Kieselsandstein. The Upper Kieselsandstein is finely grained and siliciously cemented.

The Kieselsandstein is rich of feldspar (whose weathering created the silicious cement). In combination with poor rounding of the grains, this shows that the sediment was transported only over a short distance. It prograded from SE.

Löwenstein Formation (Stubensandstein)

The name "Stubensandstein" derives from the sand's use as a polish for the wooden floor in living rooms ("Stube") in former times.

The Stubensandstein is quite different from the Kieselsandstein, regarding visible aspects like the cement (clay and carbonate), bad grain sorting, greater thicknesser and wider spreading. It consists of sands from the Bohemian Massif in the E and the Vindelician Land in the SE that were deposited during four periods of deltaic progradation. The thickness of the Stubensandstein can reach up to 120 m. The four sandstones are separated by pelitic layers with hard marl beds, the so-called Hangendletten ("topping clays").

The environment can be reconstructed as a large, delta-like branched alluvial fan, with erosive river channels forming in it's proximal areas during times of little running water. When water flow and sediment freight increased, masses of sand and clay lumps (with a diameter of up to 1 m) spread over the fan's surface when the rivers left their beds. The distal areas are dominated by clays alternating with stony marl beds that formed under fresh water influence during the flash floods (the so-called Steinmergelkeuper of the High and Upper Rhine area). Fresh water seeping from the delta front allowed the settlement of fauna and flora.

Fossils can be found only in few locations, but there more often. Apart from remains of the flora - petrified araucarian logs (Dadoxylon) and horsetail steinkerns (Equisetites) - amphibians (Metoposaurus), fish (Semionotus) and reptiles (Aetosaurus, Nicrosaurus, Proganochelys) can occur.


Fig. 10: Stubensandstein, tectonically disturbed, Stuttgart.


Knollenmergel ("Nodule marl")

Reddish-brown to violet carbonatic clays contain the name-giving carbonatic stony marl nodules. Missing layering down to a microscopic scale led to an interpretation as an aeloian sediment, although the carbonate nodules and some micritic limestone beds indicate an aquatic formation.

Remarkable is the occurence of well-preserved skeletons or parts of the sauropod Plateosaurus.

The Upper Keuper (Rhät) finally again formed under marine conditions. The transgression prograded from N and NE and locally left a thin sediment cover in a deltaic environment. Remarkable is a fine grained sandstone with silicious or clayish cement desposited in elongated bodies that are N to NNE oriented. This configuration already resembles the hettangian (Lower Jurassic) sandstones. The Rhätsandstein is sometimes followed by light gray and green-grayish clays.

Important fossil deposits are the bonebeds within the sandstone that contain (among others) teeth, bones and scales of dinosaurs (Plateosaurus) and fish (Hybodus) as well as teeth of the first mammals. Sometimes, brittle star resting and feeding traces (Asteriacites) can cover entire bedding planes.


Fig. 11: Rhätsandstein (bottom), above are clays and sandstones of the Lower Jurassic (Hettangian und basal Sinemurian), Tübingen.