Keuper

The Keuper is the highest of the three lithostratigraphic groups of the Germanic Triassic . This tripartite division, which gave the chronostratigraphic system Trias its name , only applies to the area north of the Alps (Germanic Trias supergroup). The Keuper is roughly dated from 235 to 199.6 million years ago. It follows the lithostratigraphic group of the shell limestone and is overlaid by the lithostratigraphic group of the Lias (Northern Germany) and the Black Jura (Southern Germany).

history

Red sandstone and shell limestone were firmly established rock units as early as the end of the 18th century. In contrast, it was not until the 1820s that the Keuper was recognized as an independent rock unit. Up to this point in time it was mostly regarded as part of the Buntsandstein . The term Keuper was introduced into geoscientific literature by Leopold von Buch . However, he used the term in the sense of a rock name, not in the sense of a lithostratigraphic unit. The name "Keuper" is derived from the Franconian dialect name Kiefer , Kieber or Keiper , Keuper for colorful, crumbly clay stones. The transfer of this term to the rock unit goes back to Friedrich Hoffmann , who used it orally in this sense in 1823. Presumably independently of him, Christian Keferstein also did this and so these two geologists must be considered to be the namesake of the Keuper rock unit.

The Keuper was earlier and is still partly regarded in popular scientific literature as a time interval in the history of the earth ("Keuperzeit", "Keuperzeitlich"). If the term Keuper period is used in the sense of "at the time of the deposition of the Keuper sediments of the Germanic Basin", this is still linguistically correct. If, on the other hand, the term is transferred to other areas (e.g. "the Keuper period deposits in the Alps"), this is not correct, because Keuper is not a chronostratigraphically defined time interval, but a rock unit (or a unit of lithostratigraphy ) whose sub- and upper limit can be somewhat different from area to area in relative and absolute time.

definition

The Keuperweg near Heilbronn - a geological educational trail with 13 stations

The lower limit of the Keuper is defined in northern Germany with the lower edge of the so-called Lower Lettenkohlensandstein . In southern Germany the lower limit is the basis of the “ Grenzbonebed ”. The upper limit is the base of the Lias in northern Germany or the Black Jura in southern Germany. The type region is Franconia . In the North German Basin average are thicknesses between 300 and 500 meters, achieved locally to 1000 meters. In the Glückstadt Graben, seismic studies indicate a thickness of up to 5000 meters. These are marine- influenced limestone and claystones, lacustrine limestone and claystones , fluvial and littoral sandstones and siltstones , fossil soils and root horizons and powerful saline successions , with rock salt in the basin center in northern Germany , anhydrite (CaSO ₄ ) and in the more marginal areas Gypsum (Ca [SO ₄ ] · 2 H ₂ O) were deposited.

Chronostratigraphic Correlation

The lithostratigraphic units of the Keuper are difficult to correlate with the international chronostratigraphic levels , since the Keuper sediments are often extremely low in fossils or contain no fossils that can be used for biostratigraphic dating. The base of the Keuper can be dated to the lower Ladinium . The top of the keuper reaches just below the base of the hettangium . However, it must be taken into account that there are larger gaps in the layer within the Keuper ; Larger parts of the time of the levels Carnium and Norium are not documented by deposits, but are stuck in the shift gaps.

Lithostratigraphic breakdown

The Keuper has been divided into twelve lithostratigraphic formations since 1997 . The previous division into lower, middle and upper Keuper (also Rhätkeuper) is only used informally or in the sense of the lithostratigraphic unit subgroup .

Lower Keuper

The Derdinger Horn geotope shows the transition zone between gypsum keuper and reed sandstone

The Untere Keuper (also Unterkeuper) includes the Erfurt Formation and the upper parts of the Grafenwoehr Formation. The western and northern peripheral areas of the Germanic Triassic Basin have not yet been further subdivided into lithostratigraphic units (formations, etc.).

Erfurt formation , this roughly corresponds to the terms Lettenkeuper, Kohlenkeuper or Lettenkohlenkeuper; However, these terms were used differently depending on the author; in some cases, upper sections of the Muschelkalk group were also included.
Grafenwoehr Formation , this formation is only formed on the eastern edge of the basin to the Vindelizisches Land and interlocks in the upper part with the Erfurt Formation.
"Edge area to the Ardennes threshold" and "Edge facies in the Baltic Sea area". Both deposit areas are not yet subdivided into lithostratigraphic units.

The Lower Keuper is characterized by a cyclical alternation of sandy-clayey and clayey-carbonate sediments. There are also thin coal seams that are not exploitable (name Lettenkohlenkeuper!). They document a rapid change from limnic-fluvial and lacustrine deposit conditions to marine and brackish deposit conditions. The lower limit of the Lower Keuper (and the Erfurt Formation) is the so-called Grenzbonebed in southern Germany and at the base of the "Lower Lettenkohlensandstein" in central and northern Germany.

Middle Keuper

Ripple marks in the reed sandstone of the Hassberge mountains

The range of sediments of the Middle Keuper ranges from sandstones, claystones, marlstones, saline rocks and carbonate banks. The sediments come from the fluvial and limnic milieu, in which some marine banks are integrated in the lower part. The Mittlere Keuper (rarely Mittelkeuper) is divided into different formations in northern and southern Germany, which interlock laterally and partially correspond to one another.

Upper Keuper

In the Upper Keuper (Oberkeuper) only one formation has been distinguished so far. Further lithostratigraphic units have yet to be defined.

External formation , inside the pelvis, encompasses the entire upper keuper down to the base of the first Lias layers

The Obere Keuper contains dark clay and sandstones that were deposited in a deltaic environment. This is followed by marine and again delta-shaped sandstones. Limnic-delta sandstones were deposited at the top of the unit. It is overlaid by the Psilonotenton Formation (Lower Jurassic)

Transition from Unter- to Mittelkeuper: The Unterkeuper ends with the yellow border dolomite of the Erfurt formation, above the Mittelkeuper begins with the basic plaster of the grave field formation

Allostratigraphic breakdown

In allostratigraphy (guide surface stratigraphy) the Keuper is subdivided into six sequences, which are numbered from k1 to k6. The boundaries of sequences and formations often coincide.

Use of geothermal energy in the North German Plain

The Keuper and the Rätkeuper are the strata of the earth that are visited in Neustadt-Glewe at a depth of 2,455 meters, from which the thermal water is extracted for the Neustadt-Glewe geothermal power plant , Germany's first geothermal power plant . The salt content of the brine is 220 grams per liter.

Individual evidence

↑ See Edgar Nitsch, 2005, p. 7.

↑ Geothermal power plant Neustadt-Glewe ( Memento of the original from November 26, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2

literature

Johannes Baier: The geological nature trail on the Kirnberg (Schönbuch) - The Keuper's past. In: fossils. 31 (5), 2014, pp. 36-40.
Johannes Baier: The geological nature trail on Kirnberg (Keuper; SW Germany). In: Jber. Mitt. Oberrhein. geol. Ver. NF 93, 2011, pp. 9-26.
Johannes Baier: The new geological nature trail in Kirnbachtal (Keuper, Schönbuch). In: Aufschluss 71 (2), 2020, pp. 81–89.
Gerhard Beutler: Lithostratigraphy. In: German Stratigraphic Commission (Ed.): Stratigraphie von Deutschland IV - Keuper. (= Courier Research Institute Senckenberg. 253). Stuttgart 2005, ISBN 3-510-61376-7 , pp. 65-84.
Klaus-Peter Kelber : The preservation and palaeobiological significance of the fossil wood from the southern German Keuper (Triassic, Ladinian to Rhätian). In: H. Schüßler, T. Simon (Hrsg.): Wood becomes stone - pebble wood from the Keuper Franconia. Eppe, Bergatreute-Aulendorf 2007, ISBN 978-3-89089-091-3 , pp. 37-100.
Edgar Nitsch: Keuper 1820–34: Birth of a stratigraphic term. In: Annals of Science. Volume 53, 1996, pp. 489-500. (Abstract)
Edgar Nitsch: The Keuper in the Stratigraphic Table of Germany 2002: Formations and consequences. In: Newsletters on Stratigraphy. 41 (1-3), Stuttgart 2005, pp. 159-171. ISSN 0078-0421
Edgar Nitsch: On the history of Keuperstratigraphy. In: Gerhard Beutler (Coord.): Stratigraphy of Germany IV Keuper. (= Courier Research Institute Senckenberg. 253). Frankfurt am Main 2005, ISBN 3-510-61376-7 , pp. 6-14.
Edgar Nitsch, Dieter Seegis, Ullrich Vath, Norbert Hauschke: Sediments and sedimentation pauses in the German Keuper: How complete is the tradition of the late Triassic period? In: Newsletters on Stratigraphy. 41 (1-3), Stuttgart 2005, pp. 225-251. ISSN 0078-0421

Web links

[1] See Edgar Nitsch, 2005, p. 7.