Keuper

Keuper, in geology the third or uppermost subdivision of the Triassic system. The name is a local miners’ term of German origin; it corresponds to the French marnes irisées. The formation is well exposed in Swabia, Franconia, Alsace and Lorraine and Luxemburg; it extends from Basel on the east side of the Rhine into Hanover, and northwards it spreads into Sweden and through England into Scotland and north-east Ireland; it appears flanking the central plateau of France and in the Pyrenees and Sardinia. In the German region it is usual to divide the Keuper into three groups, the Rhaetic or upper Keuper, the middle, Hauptkeuper or gypskeuper, and the lower, Kohlenkeuper or Lettenkohle. In Germany the lower division consists mainly of grey clays and schieferletten with white, grey and brightly coloured sandstone and dolomitic limestone. The upper part of this division is often a grey dolomite known as the Grenz dolomite; the impure coal beds—Lettenkohle—are aggregated towards the base. 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. laxitesta); the Schelfsandstein, used as a building-stone; the Lehrberg and Berg-gyps beds; Semionotus beds (S. Bergeri) with building-stone of Coburg; and the Burgand Stubensandstein. The salt, which is associated with gypsum, is exploited in south Germany at Dreuze, Pettoncourt, Vie in Lorraine and Wimpfen on the Neckar. A ½-metre coal is found on this horizon in the Erzgebirge, and another, 2 metres thick, has been mined in Upper Silesia. 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 Triglyptus 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 S. Tirol and the Judicarian Mountains the Rhaetic is represented by the Kössener beds. In the Alpine region the presence of coral beds gives rise to the so-called “Lithodendron Kalk.”

In Great Britain the Keuper contains the following sub-divisions: Rhaetic or Penarth beds, grey, red and green marls, black shales and so-called “white lias” (10-150 ft.). Upper Keuper marl, red and grey marls and shales with gypsum and rock salt (800-3000 ft.). 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 (150-250 ft.). The basal or “dolomitic conglomerate” is a shore or scree breccia derived from local materials; it is well developed in the Mendip district. The rock-salt 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 north-westward into north Ireland and south Scotland. As in Germany, there are one or more “bone beds” in the English Rhaetic with a similar assemblage of fossils. In the “white lias” the upper hard limestone is known as the “sun bed” or “Jew stone”; at the base is the Cotham or landscape marble.

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.