Germanic Triassic

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Germanic Triassic
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The Germanic Triassic is a geological supergroup of the older Mesozoic Era of Central Europe north of the Alps, consisting of the lithostratigraphic units Buntsandstein , Muschelkalk and Keuper . The sequence of their layers was researched relatively early - not least because of their often characteristic coloring.

In the past, the individual subdivisions of the Germanic Triassic were also treated as terms of time, sometimes even equated with the Lower , Middle and Upper Triassic . However, based on recent research, this is not tenable, since the beginning and the end of the individual lithostratigraphic units are sometimes quite different from region to region and do not coincide with the boundaries of the Triassic series . The lithostratigraphic group of the Germanic Triassic is limited to the Central European region north of the Alps. In the Alps (and south of it) the Triassic is developed as an Alpine-Mediterranean Triassic.

history

The Triassic system was founded in 1834 by Friedrich von Alberti . He orientated himself on the typical threefold structure of the deposits (red sandstone, shell limestone and keuper) in central and southern Germany. It is the only geological system that was set up in Germany. The names red sandstone, shell limestone and keuper partly go back to the 18th century. For a long time they were understood as concepts of time. However, recent research has shown that they are of different ages depending on the region. At the edge areas, for example, the shell limestone can almost completely fail. The red sandstone is only thinly developed locally. The Keuper has also developed very differently in Central Europe. More recently there has been a move to define red sandstone, shell limestone and keuper as exclusively lithostratigraphic units in the hierarchical-lithostratigraphic rank of a group. All three units are combined in a lithostratigraphic supergroup Germanic Triassic.

definition

The supergroup Germanic Triassic is on average around 800 m thick. The thicknesses, however, vary greatly from region to region. Regionally, for example, the red sandstone in Lower Saxony alone can be up to 1400 m thick. It is made up of the three lithostratigraphic groups Buntsandstein, Muschelkalk and Keuper. Only the shell limestone represents a predominantly marine formation, red sandstone and keuper are predominantly continental. The lower limit (and thus the Zechstein / Buntsandstein limit) is set in the center of the basin with the onset of the Calvörde sequence . In the Spessart and Odenwald, the lower limit of the red sandstone is the lower limit of the Heigenbrücken sandstone. In the Black Forest, the border is placed at the base of the so-called Eck'schen conglomerate. The type region of the Germanic Triassic is Central Europe, where the unity is most fully developed. It is the center of the Germanic Basin , which in the Triassic comprised large parts of Central and Western Europe as far as the North and Baltic Seas.

According to more recent biostratigraphic studies, the lower limit of the red sandstone group and thus the Germanic Triassic as a supergroup can be found in the highest part of the Changhsingium , the highest chronostratigraphic level of the Permian . The Germanic Triassic is overlaid by the Jura, but there is a gap between the layers. After biostratigraphic data, the upper limit of the Keuper and thus the Germanic Trias as a super group is the chronostratigraphic Rhaetian stage, but still well ahead of the chronostratigraphic Triassic / Jurassic boundary. In the Keuper in particular, there are also larger layer gaps between the individual formations, i.e. This means that larger sections of the Upper Triassic are not documented by sediments. According to the current correlation between chronostratigraphy and geochronology, the Germanic Triassic ranges from around 252 to around 200.5 million years. The boundaries of the lithostratigraphic unit Germanic Triassic do not differ very much from the chronostratigraphic boundaries of the Triassic, but are slightly shifted back in time compared to the international chronostratigraphic boundaries.

Transition from the red sandstone to the shell limestone, which begins with the striking yellow border limestone; Exposure at Kallmuth near Homburg am Main

Breakdown

The well-known tripartite division of the Germanic Triassic, from which the name is derived, has already been mentioned. Accordingly, the lithostratigraphic unit Germanic Triassic as a supergroup is divided into three lithostratigraphic groups:

  • Keuper (further subdivided into Unter-, Mittel- and Oberkeuper; these units are understood as subgroups)
  • Muschelkalk (further subdivided into Lower, Middle and Upper Muschelkalk: are understood as subgroups in the hierarchy of lithographic units)
  • Buntsandstein (further subdivided into lower, middle and upper red sandstone: are understood as subgroups in the hierarchy of lithographic units)

The groups of the Germanic Triassic are very difficult to integrate into the international chronostratigraphic levels. This is due to the almost complete lack of fossils that can be used for biostratigraphic dating. The ammonites, for example, went through their own development in Muschelkalk. Today one would say they were endemic to the Germanic Basin. The Zechstein / Buntsandstein border is already in the central parts of the basin in the Upper Changhsingium . The Buntsandstein / Muschelkalk border is currently dated in the lower anise , the Muschelkalk / Keuper border around the Middle Ladin . The Keuper reaches from the Mittelladin to the Rhaetium and thus represents by far the longest part of the three groups of the Germanic Triassic.

The Alpine-Mediterranean Triassic

Opposite the Germanic Triassic is the Alpine Triassic or, more generally, the Alpine-Mediterranean Triassic. It is also called the “pelagic Triassic” and was deposited on the edge of the Tethys Ocean. It is usually much more powerful and mostly marine. There are shallow sea reef complexes in addition to layered deposits of clays, limes and marls that were formed in deeper water. The names of the chronostratigraphic levels of the Triassic were mainly coined in the Alps, only the names of the levels of the Lower Triassic were set up in other areas: Indus , Olenek , Anis , Ladin , Karn , Nor and Rät .

Individual evidence

  1. ↑ Lepper et al., P. 131

literature

  • Friedrich von Alberti: Contribution to a monograph of the colored sandstone, shell limestone and keupers, and the connection of these structures to a formation. Cotta publishing house, Stuttgart 1834.
  • Jochen Lepper, Dietrich Rambow, Heinz-Gerd Röhling: The red sandstone in the Stratigraphic Table of Germany 2002. Newsletters of Stratigraphy, 41 (1-3): 129-142, 2005, ISSN  0078-0421 .
  • Otto Grunert : The Scaphopods and Gastropods of the German Triassic . Erlangen: A. Vollrath, 1898.

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