1911 Encyclopædia Britannica/Bunter
|←Bunsen, Robert Wilhelm von||1911 Encyclopædia Britannica, Volume 4
|See also Bunter (geology) on Wikipedia, and our 1911 Encyclopædia Britannica disclaimer.|
BUNTER, the name applied by English geologists to the lower stage or subdivision of the Triassic rocks in the United Kingdom. The name has been adapted from the German Buntsandstein, Der bunte Sandstein, for it was in Germany that this continental type of Triassic deposit was first carefully studied. In France, the Bunter is known as the Grès bigarré. In northern and central Germany, in the Harz, Thuringia and Hesse, the Bunter is usually conformable with the underlying Permian formation; in the south-west and west, however, it transgresses on to older rocks, on to Coal Measures near Saarbruck, and upon the crystalline schists of Odenwald and the Black Forest.
The German subdivisions of the Bunter are as follows:—(1) Upper Buntsandstein, or Röt, mottled red and green marls and clays with occasional beds of shale, sandstone, gypsum, rocksalt and dolomite. In Hesse and Thuringia, a quartzitic sandstone prevails in the lower part. The "Rhizocorallium Dolomite" (R. Jenense, probably a sponge) of the latter district contains the only Bunter fauna of any importance. In Lorraine and the Eifel and Saar districts there are micaceous clays and sandstones with plant remains—the Voltzia sandstone. The lower beds in the Black Forest, Vosges, Odenwald and Lorraine very generally contain strings of dolomite and carnelian—the so-called "Carneol bank." (2) Middle Buntsandstein-Hauptbuntsandstein (900 ft.), the bulk of this subdivision is made up of weakly-cemented, coarse-grained sandstones, oblique lamination is very prevalent, and occasional conglomeratic beds make their appearance. The uppermost bed is usually fine-grained and bears the footprints of Cheirotherium. In the Vosges district, this subdivision of the Bunter is called the Grès des Vosges, or the Grès principal, which comprises: (i.) red micaceous and argillaceous sandstone; (ii.) the conglomérat principal; and (iii.) Grès bigarré principal (=grès des Vosges, properly so-called). (3) Lower Buntsandstein, fine-grained clayey and micaceous sandstones, red-grey, yellow, white and mottled. The cement of the sandstones is often felspathic; for this reason they yield useful porcelain clays in the Thuringerwald. Clay galls are common in the sandstones of some districts, and in the neighbourhood of the Harz an oolitic calcareous sandstone, Rogenstein, occurs. In eastern Hesse, the lowest beds are crumbly, shaly clays, Brockelschiefern.
The following are the subdivisions usually adopted in England:—(1) Upper Mottled Sandstone, red variegated sandstones, soft and generally free from pebbles. (2) Bunter Pebble Beds, harder red and brown sandstones with quartzose pebbles, very abundant in some places. (3) Lower Mottled Sandstone, very similar to the upper division. The Bunter beds occupy a large area in the midland counties where they form dry, healthy ground of moderate elevation (Cannock Chase, Trentham, Sherwood Forest, Sutton Coldfield, &c.). Southward they may be followed through west Somerset to the cliffs of Budleigh Salterton in Devon; while northward they pass through north Staffordshire, Cheshire and Lancashire to the Vale of Eden and St Bees, reappearing in Elgin and Arran. A deposit of these rocks lies in the Vale of Clwyd and probably flanks the eastern side of the Pennine Hills, although here it is not so readily differentiated from the Keuper beds. The English Bunter rests with a slight unconformity upon the older formations. It is generally absent in the south-eastern counties, but thickens rapidly in the opposite direction, as is shown by the following table:—
|500 ft.||50-200 ft.||Absent|
|500-750 ft.||100-300 ft.||0-100 ft.|
|200-500 ft.||0-100 ft.||Absent|
The material forming the Bunter beds of England came probably from the north-west, but in Devonshire there are indications which point to an additional source.
In the Alpine region, most of the Trias differs markedly from that of England and northern Germany, being of distinctly marine origin; here the Bunter is represented by the Werfen beds (from Werfen in Salzburg) in the northern Alps, a series of red and greenish-grey micaceous shales with gypsum, rock salt and limestones in the upper part; while in the southern Alps (S. Tirol) there is an upper series of red clays, the Campil beds, and a lower series of thin sandstones, the Seis beds. Mojsisovics von Mojsvar has pointed out that the Alpine Bunter belongs to the single zone of Natica costata and Tirolites cassianus.
Fossils in the Bunter are very scarce; in addition to the footprints of Cheirotherium, direct evidence of amphibians is found in such forms as Trematosaurus and Mastodonsaurus. Myophoria costata and Gervillea Murchisoni are characteristic fossils. Plants are represented by Voltzia and by equisetums and ferns.
In England, the Bunter sandstones frequently act as valuable reservoirs of underground water; sometimes they are used for building stone or for foundry sand. In Germany some of the harder beds have yielded building stones, which were much used in the middle ages in the construction of cathedrals and castles in southern Germany and on the Rhine. In the northern Eifel region, at Mechernich and elsewhere, this formation contains lead ore in the form of spots and patches (Knotenerz) in the sandstone; some of the lead ore was worked by the Romans.
For a consideration of the relationship of the Bunter beds to formations of the like age in other parts of the world, see TRIASSIC SYSTEM.
- (J. A. H.)