The Late Triassic (also known as Upper Triassic, or Keuper) is the third and final of three epochs of the Triassic period. It spans the time between 228 ± 2 Ma and 199.6 ± 0.6 Ma (million years ago). The Late Triassic is divided into the Carnian, Norian and Rhaetian faunal stages. The name is a local miners' term of German origin; it corresponds to the French marnes irises.
The formation is well exposed in Swabia, Franconia, Alsace and Lorraine and Luxembourg; it extends from Basel on the east side of the Rhine into Hanover, and through England into Scotland and north-east Ireland; it appears flanking the central plateau of France and in the Pyrenees and Sardinia.
Representatives of the Rhaetic are found in south Sweden, where the lower portion contains workable coals, in the Himalayas, Japan, Tibet, Burma, eastern Siberia and in Spitzbergen. The upper portion of the Karroo beds of South Africa and part of the Otapiri series of New Zealand are probably of Rhaetic age.
In the German region it is usual to divide the Keuper into three groups;
- Rhaetic or upper Keuper
- The upper part of this division is often a grey dolomite known as the Grenz dolomite; the impure coal beds Lettenko/ileare aggregated towards the base. The upper Keuper, Rhaetic or Avicula contorta zone in Germany is mainly sandy with dark grey shales and marls; it is seldom more than 25 metres thick. The sandstones are used for building purposes at Bayreuth, Culmbach and Bamberg. In Swabia and the Wesergebirge are several bone-beds, thicker than those in the middle Keuper, which contain a rich assemblage of fossil remains of fish, reptiles and the mammalian teeth of Microlestes antiquus and Triglyptzas Fraasi. The name "Rhaetic" is derived from the Rhaetic Alps where the beds are well developed; they occur also in central France, the Pyrenees and England. In South Tirol and the Judic. arian Mountains the Rhaetic is represented by the Kossenei beds. In the Alpine region the presence of coral beds gives rise to the so-called Lithodendron Kalk.
- Hauptkeuper or gypskeuper, the middle
- The middle division is thicker than either of the others (at Göttingen, 450 metres); it consists of a marly series below, grey, red and green marls, with gypsum and dolomite—this is the gypskeuper in its restricted sense. The higher part of the series is sandy, hence called the Steinmergel; it is comparatively free from gypsum. To this division belong the Myophoria beds (M. Raibliana) with galena in places; the Estheria beds (E. laxilesta); the Schelfsandstein, used as a building-stone; the Lehrherg and Berg-gyps beds; Semionotus beds (S. Bergen) with building-stone of Coburg; and the Burgand Stubensandstein.
- Kohlenkeuper or Lettenko/jie, the lower
- The lower division consists mainly of grey clays and schieferletten with white, grey and brightly colored sandstone and dolomitic limestone.
The salt, which is associated with gypsum, is exploited in south Germany at Dreuze, Pettoncourt, and Bad Wimpfen on the Neckar, as well as in Vie in the Lorraine region of France. A 4-metre coal is found on this horizon in the Erzgebirge on the border between Germany and the Czech Republic, and another, 2 metres thick, has been mined in Upper Silesia, now in Poland.
In Great Britain the Keuper contains the following sub divisions:
- Rhaetic or Penarth
- Grey, red and green marls, black shales, and so-called white has (10150 ft.). Upper Keuper marl, red and grey marls and shales with rock salt (800300o ft.). As in Germany, there are one or more bone beds in the English Rhaetic with a similar assemblage of fossils.
- Lower Keuper
- Sandstone, marls and thin sandstones at the top, red and white sandstones (including the so-called waterstones) below, with breccias and conglomerates at the base (15o250 ft.).
- Basal conglomerate
- A shore or scree breccia derived from local materials; it is well developed in the Mendip district. The rocksalt beds vary from 1 in. to 100 ft. in thickness; they are extensively worked (mined and pumped) in Cheshire, Middlesbrough and Antrim.
The Keuper covers a large area in the Midlands and around the flanks of the Pennine range; it reaches southward to the Devonshire coast, eastward into Yorkshire and northwestward into north Ireland and south Scotland. In the white has the upper hard limestone is known as the sun bed or Jew stone; at the base is the Cotham or landscape marble.
The Keuper is not rich in fossils; the principal plants are cypresslike conifers (Walchia, Voltzia) and a few calamites with such forms as Equisetum arenaceum and Pterophyllum jaegeri, Avicula contorta, Protocardium rhaeticum, Terebratula gregaria, Myophoria costata, M. goldfassi and Lingula tenuessima, Anoplophoria lettica may be mentioned among the invertebrates. Fishes include Ceratodus, Hybodus and Lepidotus.
Labyrinthodonts represented by the footprints of Cheirotherium and the bones of Labyrinthodon, Mastodonsaurus and Capitosaurus. Among the reptiles are Hyperodapedon, Palaeosaurus, Zanclodon, Nothosaurus and Belodon. The first fossil mammals also make their appearance at this time.
This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
|Lower/Early Triassic||Middle Triassic||Upper/Late Triassic|
|Induan | Olenekian||Anisian | Ladinian||Carnian | Norian|