Rhaetic - Encyclopedia

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RHAETIC (Fr. Rhetien or Rhcetien; Ger. Rhdt or Rhatisch; It. Retico), in geology, the assemblage of rocks classed by most English and German authorities in the Triassic system, and by most French geologists placed at the base of the Lias, in the Jurassic system. It has been called the Infra-Lias. This diversity of opinion is due to the fact that the Rhaetic formation presents the characters of a group of passage-beds, uniting certain features of the Trias with others of the Jurassic system; none the less, it has sufficient individuality to be recognized with tolerable certainty over a wide area in Europe and beyond. The name Rhaetic was first applied by C. W. Gumbel to the strata of this horizon in the Rhaetic Alps, where they are thickly developed and in parts fossiliferous. The labours of E. V. Mojsisovic and E. Suess have demonstrated that in the Alpine Rhaetic several distinct facies may be recognized, viz. a Swabian facies: shore and lagoon deposits with a pelecypod fauna, poor in species but rich in individuals; a Carpathian facies with corals, algae, Terebratula gregaria and Plicatula intusstriata, exemplified in the upper part of the Dachstein limestone; a Kussener facies: black limestones and marls, with a brachiopod fauna in which Spirigera oxycolpos is very noticeable; and a Salzburg facies, characterized by pelagic pelecypods and some ammonites (see table in Triassic System). The whole of the Rhaetic falls within Mojsisovic's zone of Avicula contorta. This epoch is marked off from the earlier Triassic period by a very general marine transgression which proceeded with minor irregularities and retrogressions over the whole area, until at its close it was followed by the more decided transgression which indicates the commencement of the Lias.

Among the marine fossils of the Rhaetic, Avicula contorta, the principal zone form, is very characteristic and has a wide range; Myophoria inflata, Modiola iiinuta, Protocardium rhaeticum and Terebratula gregaria are common species. True belemonites make their first appearance. Corals, Thecosmilia, &c., are common in some districts. Plant remains are abundant in certain areas, and in places give rise to beds of lignite and coal. The flora is more nearly akin to that of the Trias than to that of the Jurassic rocks. Vertebrate remains are fairly abundant in the form of teeth, isolated bones, scales and coprolites in what are known as "Bone Beds" (q.v.). These beds are a very characteristic feature; they occur on several horizons in many tracts of the European Rhaetic, and recur in beds of this age in America. In England there is usually a bone bed about the base of the formation; in Germany one occupies a similar position; a second occurs less constantly about the middle, and in the Wurttemberg district a third bed separates the Rhaetic and Lias, and constitutes the well-known manure bed of Bebenhausen. In these beds are found the bones of Ichthyosaurus and Pliosaurus, anticipating their great development in the Lias, while the remains of Belodon and Mystriosuchus serve to link this epoch with Triassic stego cephalian reptiles. Several coleopterous insects have been found in the same beds, but the most interesting feature of the bone-bed fauna is the first appearance in the northern hemisphere of true mammals: Microlestes in England and Wurttemberg, Triglyphus in Wurttemberg, Dromatherium and Microconodon in America.

In England the Rhaetic formation occurs as a thin but constant series of beds at the base of the Lias and above the Keuper marls. The upper part, often called the "White Lias," is a series of thinbedded shales, limestone and marls, i to 25 ft. thick; the lower portion consists mainly of dark shales, sometimes with very perfect lamination - "paper shales." Below there are beds of grey and "tea-green" marls which are now usually regarded as the topmost Keuper beds, but they have often been included in the Rhaetic formation (see Keuper). The best exposures in Britain are those between Penarth Head and Cavernock Point, Aust Cliff and Garden Cliff near Westbury-on-Severn, and Wainlode Cliff between Tewkesbury and Gloucester. From their excellent development near Penarth the Rhaetic beds have long been known in England as the Penarth Beds (H. W. Bristow, .1864). The more prominent beds in the White Lias of the west of England and Glamorganshire are the Estheria beds and the insect limestone or Pseudomonotis-bed, and on both of these horizons the limestone may assume the peculiar characters of landscape marble, sometimes called Cotham marble, from Cotham House near Bristol. A hard fine-grained limestone, known locally as the Sun-bed, occurs at the top of the series near Bath and Radstock; at Street, Wedmore and south of the Mendips generally it is called Jew stone. Wedmore stone is a tough, shelly and sandy limestone in the black shales at Wedmore, near Wells; it is employed in the neighbourhood as a building stone. Nofth of Somersetshire the White Lias is poorly represented; in Glamorganshire it appears between Cardiff and Pyle, west of Bridgend and at Sutton and Southerndown. Rhaetic beds have been traced at Market Drayton, Salop; near Audlem, Cheshire; Rugby and Stratford-on-Avon in Warwickshire; Wigston in Leicestershire; Needham Forest in Staffordshire, and in Nottinghamshire and Yorkshire as far as the coast. The y have not yet been proved beneath the Lias of Cumberland. Rhaetic fossils have been found in great numbers in fissures in the Carboniferous limestone of the Mendips. On the western side of Scotland Rhaetic rocks occur at Applecross, Ardnamurchan, Morven, Mull, Raasay and Skye. In Sutherlandshire sandstone and conglomerate and large transported masses occur; one of them, at Linksfield, carries a bone bed. Here the black shales of the English type fail; sandstones with coaly layers and yellowish-grey crystalline and oolitic limestones take their place. In Antrim a small outcrop of black shales with Avicula contorta occurs near Port Rush.

On the European continent the Rhaetic rocks are most thickly developed in the Alpine regions; and, as in the case of the older Triassic formations, calcareous and dolomitic strata predominate here and in the Mediterranean province. In the Alpine district the main divisions are the Rhaetic Dachstein limestone and the Kossener beds; shales, marls and limestones. In tile northern tract the following subdivisions have been recognized in descending order: beds with Choristeceros Marschi; Starhem passage beds; Rhynchonella fissicostata beds; Lithodendron limestone; beds with Terebratula gregaria; beds with Avicula contorta; " Platten Kalk" with Rhynchonella alpina. In the southern tract the subdivisions are: Conchodus dolomite (Conchodus infraliassicus = Lycodius cor.), Lithodendron limestone, Azzarola beds, Contorta marls, "Plattenkalk." Much limestone is of the "reef" type. In Germany the rocks are mainly fine, clean yellow sands, suggesting littoral or dune conditions, with bituminous clays and marls. The formation is often missing in south-west Germany. Similar beds occur in Lorraine and Luxembourg (gres de Vic, grey de Kedange, gres de Mortinsart). In Cotentin are dolomitic sandstones and marl; round the central plateau of France the rocks are coarse sands, arkoses, and conglomerates; while in the south of France the sandy and calcareous facies occur intermixed. In Spain limestones and. dolomites occur up to too metres in thickness; in Portugal sandy beds recur. The Rhaetic of Scania, south Sweden, consists mainly of sandstone and shales with beds of coal up to one metre thick. Only the upper beds contain marine fossils; the bulk of the formation is of lacustrine or estuarine origin, with plant remains and insects. In Italy the formation is well developed in the north and at Rotzo, Spezzia and Carrara; and yields the famous statuary marble and the black variety known as portor. Rhaetic beds have been recognized in Sardinia, Corsica, Sicily, in the Balkan Peninsula and Greece; in Asia Minor, Afghanistan, Turkistan, Persia, Siberia and India (limestones and dolomites of Niti and the Mahaveda beds, sandstones and conglomerates, nearly to,000 feet thick in Satpura); in China, Japan and Tongking (with coal beds). In Australasia the Wianamatta beds of New South Wales, the Bellarine beds of Victoria, the Ipswich and Tivoli beds of Queensland, and the J erusalem beds of Tasmania, and beds on a similar horizon in New Zealand, have been regarded as equivalents of the Rhaetic. In Africa the Stormberg beds of the Karoo series and the Molten() beds of the Cape have been assigned to this epoch. In America Rhaetic rocks are recognized in N. Carolina, Connecticut, California, Mexico, Bolivia and Chile; the formation is also recorded from Spitzbergen. Franz Joseph Land and elsewhere in the Arctic regions.

For the English Rhaetic see L. Richardson, "The Rhaetic Rocks of North-west Gloucestershire," Proc. Cotteswold Club, xiv. p. 127 (Glos. 1901-1903). U. A. H.)

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