1911 Encyclopædia Britannica/Triassic System

TRIASSIC SYSTEM, in geology, the lowest or youngest system of the Mesozoic era; it occupies a position above the Permian and below the Jurassic system of rocks. The principal formations of the type region, Germany, are the Bunter, Muschelkalk and Keuper; these were for the first time grouped together under the systematic name “Trias” by F. von Alberti (1834).

Triassic Period Hypothetical distribution of Land & Sea
Triassic Period Hypothetical distribution of Land & Sea

A description of the rocks in these formations will be found under their respective headings. For a long time this German development of the strata was regarded as typical of the period; later, however, the discovery of another more fossiliferous phase in the Alps and Mediterranean region, and subsequently in Asia and elsewhere, led geologists to take a different view of the system as a whole. It was clearly seen that there existed two distinct phases of Triassic rock-building, the one continental (terrestrial and lagoonal), the other marine (pelagic). The original Trias of the “Germanic” area (including Great Britain) must be understood as a special local expression of the continental Trias, while the thoroughly marine type represents the normal aspect of sedimentation. Similarly, the fauna of the marine Trias is the standard for comparison with the life of other geological systems. The term Trias-indicative of the threefold grouping in Germany-thus loses its original significance when applied to the world-wide deposits of the period; its use, however, is continued by general consent.

Continental Trias.—The records of the terrestrial and lagoonal conditions during this period are to be found in the coarse conglomerates, red and mottled sandstones, marls and clays with their accompanying beds of dolomite and limestone, and layers of gypsum, anhydrite, rock-salt and coal. The coarser breccias and conglomerates appear to represent ancient screes and shore deposits, and in part at least their formation may have been due to torrential action. The remarkable oblique bedding in many of the sandstones, coupled with the fact that the sand grains are often very perfectly rounded, points to the transporting action of wind. Even the pebbles occasionally exhibit the dreikanter form, familiar in our modern deserts. But the marls, muds and many sandy beds were certainly deposited in sheets of water, which were evidently shallow and subject to frequent periods of desiccation. Of this we have evidence in the great abundance of reptilian foot-prints, of rain pits, ripple marks, and sun cracks upon what were once surface muds and sands. That the drying up of the water sheets repeatedly produced a highly saline condition is shown by the common occurrence of rock-salt, gypsum and anhydrite. In short, the physical conditions under which the continental Trias was formed appear to have been similar to those obtaining at the present day in the Caspian region.

In Europe the earlier deposits of the continental Trias occupy a compact area covering nearly the whole of Germany, whence they may be followed into central and northern England, Heligoland, Upper Silesia and the Vosges. Another tract lay over what are now the western Alps and south-east France; also in the Pyrenees, Balearic Islands, Sardinia, Sicily and southern Spain, and on to the north coast of Africa. In the Carpathians the same rocks appear, and they cover a large area in north-east Russia (Tartarian), and north-west Siberia. Later, the Muschelkalk limestones point to a temporary influx of the sea involving most of the above regions except Britain and Russia. Three encroachments of the sea are indicated, each followed by a period of excessive evaporation and contraction; these happened in the time of the Roth, the Lower and the Upper Muschelkalk. Finally the last influx, that of the Rhaetic Sea, not only spread much beyond the limits of the earlier incursions but remained as the forerunner of the succeeding Jurassic waters. In North America the continental Trias appears with a close resemblance to that of western Europe along the Atlantic coastal strip from Prince Edward's Island, through New Brunswick, Nova Scotia, Connecticut, New York, Pennsylvania, ~Virginia, to North Carolina. These are the rocks of the Newark series. Southwards it may be traced in Honduras, the Andes, Brazil, Argentina and Chile. Another large area in the western interior, Wyoming and New Mexico, is occupied by “red beds" (600–2000 ft., in part Permian) with gypsum and rock-salt. In southern Africa the upper part of the Karoo formation appears to represent Triassic time—the Stormberg beds (Permo-Trias) and the Beaufort beds (Rhaetic). In India the Panchet beds of the Gondwana system and in New South 'Wales the Hawkesbury series (Wianametta shales with coals and iron-stone, Hawkesbury sandstone, and at the base the Narraburra beds) belong to about the same horizon. In New Zealand the Otapiri, Wairoa and Oreti series appear to contain fossils indicating a transition from Permian to Rhaetic.

The Marine or Open-sea Trias.—This type of Triassic deposit is frequently referred to under the titles “Alpine,” “Mediterranean” or “Pelagic.” It first came into notice through the discovery of fossils in the neighbourhood of Recoaro and St Cassian on the southern side of the Alps, and these rocks were subsequently correlated with those at Hallstatt on the northern side. On both sides of the Alps rocks of this age flank the central core, but they are better developed, thicker and less altered towards the east than towards the west. In the western Alps Triassic beds can be only dimly recognized amongst the masses of schists called the Schistes-luslrés and Bundnerschiefer. In the eastern Alps, however, although there are sandy and conglomeratic members, such as the Werfen beds and Lunz sandstone, yet the most striking feature, in contrast with the continental Trias, is the prevalence of calcareous and dolomitic strata, to which must be added the enormously greater abundance of organic remains. The Alpine Trias varies in lithological character so rapidly from point to point, and has furthermore been subjected to so much dislocation, that great difficulty has been experienced in correlating the beds in different areas and in placing them in their proper order of sequence. The result of this difficulty has been the production of a nomenclature so unwieldy that no attempt at a detailed exposition is possible in the space here available. The principal members of the Alpine Trias will be found in their correct relative positions in the table. One of the most striking aspects of the Alpine Trias, on both the northern and southern sides, is the great development of dolomite which is so prominent a feature in the scenery of southern Tirol (Drei Zinnern, &c.). Some of these rocks contain the remains of corals, still more bear the fossils of calcareous algae, and although the view originally advanced by F. v. Richthofen that they represent Triassic coral reefs has been strongly opposed, it still seems to be the most reasonable explanation of their origin. The rocks of the marine Trias generally are argillaceous beds and dark limestones; in the Alpine regions many of the latter have been marmorized. The well-known white marble of Carrara in the Apuan Mountains is a metamorphosed Triassic limestone. The same type of Trias occurs also in south Italy (Longobardian), in Sicily, Barcelona, Balearic Islands, Crete, Bosnia, East Hungary, and the Carpathian Mountains by Bukovina and Dobrudja.

The Alpine-Mediterranean Trias sea evidently had a prolongation into Western Asia, for in Asia Minor, Armenia and Bokhara rocks with closely related fossils have been found. In Central Asia Triassic rocks are known in Afghanistan (sandstones with coal), Russian Turkestan, and in the Pamir. In India the lower Trias of the Salt Range presents the most typical example of the marine deposits of this stage. The Himalayan Trias more perfectly represents the upper portion of the system. Triassic limestones are found also in Kashmir and Hazara, and shales in Baluchistan. The marine Trias is known in Burma, Tongking, China and north-east Tibet; also in ]apan, Siberia and in the arctic regions of Spitsbergen and Bear Island. In the Australasiatic region the marine Trias is found in the Sunda Islands, Sumatra, Roth and Timor and in New Caledonia.

Climate, Vulcanism.—There seems little room for doubt that the climate of Triassic times was, over large tracts of the northern continental region, dry and arid in character, certain features in the flora tending to support this view. On the other hand, the southern continental deposits, with Glossopteris and its allies, is more suggestive of a moist climate. There is no evidence of the glacial condition of the preceding Permian period. The Triassic period was one of rest so far as crustal movements were concerned. Volcanic activity, however, was exhibited on a large scale in the north-western part of North America, the great batholith of the Coast Range being nearly 1000 m. long; in British Columbia and Alaska large bodies of igneous rock are supposed to belong to this period. On the eastern side of the continent the diabase and dolerite lava flows, veins and sills of the famous Palisades of the Hudson valley belong to the Newark system. In Europe and Asia igneous rocks are scarce, but tuffs, porphyrites, &c., occur in the Schlern district (Upper Cassian age) and at Falzarego Strasse, Trarenanzes (Wengen horizon), in the Alpine region.

Life of the Triassic Period.—The plant life of this period exhibits on the whole a closer relationship with the Jurassic than with the preceding Palaeozoic formations. Flowering plants are unknown in the Triassic deposits and the dominant forms are all gymnosperms, the prevailing types being ferns and fern-like plants, cycadeans, conifers and equisetums. The Palaeozoic calamities, sigillarias and lepidodendrons became extinct early in this period; but in the southern hemisphere the Glossopteris flora still held on in considerable force. Amongst the ferns were Lepidopteris, Sagenopteris, Danaeopteris, with the Carboniferous genera Sphenopteris, Pecopteris and others. Equisetites and Schizoneura became common. Characteristic conifers were Voltzia, Araucarites, Brachyphyllum. The Cycadeans were represented by Pterophyllum, Cycadites, Podozamfites, &c. Baiera was the representative of the ginkgos. Calcareous algae were important rock builders in some of the Triassic seas (Gyroporella, Diplopora). Fish remains are not generally common in the Trias; teeth and scales are crowded together in the “bone beds ” in the Rhaetic and between the Keuper and Muschelkalk; in the marine Trias of the Alpine region skeletons are much more common. They are abundant also in the bituminous shales of the Connecticut Valley and in the Hawkesbury series of New South Wales. Selachians are represented by species of Hybodus, Acrodus and Palaeobates; dipnoids by Ceratodus and Gasfordia. The ganoids, with Palaeozoic as well as younger forms, include Gyrolepis, Semianotus, Dictyopyge, Graphiums, Belonorhynchus and Pholidopleuras. Bony fish were very feebly represented. The amphibian 'labyrinthodonts (Stegocephalia) were numerous, their bones being found in the “bone beds ” and in the Bunter and Keuper sandstones and their equivalents in North America, South Africa and India (Labyrinthodont, Mastodonsaurus, Trevnatosaufus, Capitosaurus). Their footprints are often very abundant, e.g. Cheirotherium. The reptiles of the Triassic deposits, unlike the amphibians, which are Permian in character, show a closer relationship with Jurassic forms; one of the most interesting facts in the life-history of the group is the development during this period of sea-going forms such as at a later geological period played so prominent a part. Early crocodilian reptiles are represented by Belodon, Mystriosuchus, Stagonolepis, Parasuchus; and Rhyncocephalia by Telcrpeton and Hyperodapedon. Ichthyopterygians were represented by Mixosaurus, Nothasaurus, Cymatosaurus; early dinosaurs (carnivorous) by Zvnclodon, Anchisaurus, Thecodontosaurus, Palaeosaurus; the remarkable theromorphs (anomodonts), by Elginia, Dicynodon, Geikia, Gardenia. Turtles became well established during this period (Psammochelys, Chelyzoon). Of great interest is the discovery of the earliest traces of mammals in the Trias of Europe, South Africa and North America. The imperfect remains (teeth and jaw-bones) do not admit of any certainty in deciphering their relationships. Microlestes from the Rhaetic of England and Württemberg and Dromatherium from North America are perhaps the best known; Tritylodon from South Africa may also be added. Among the lower forms of marine life foraminifera and sponges play a subordinate part. Corals, which with the calcareous algae built considerable reefs in some regions, at this time began to assume a modern aspect, and henceforth the Hexacorallids took the place of the Palaeozoic Tetracorallid forms (Stylophyllum, Pinacophyllum, Thecosmilial. Crinoids were locally very numerous individually (Encrlnus lilizformis, Dadocrinus gracilis). Urchins were not very common, but an important change from the Palaeozoic to the Mesozoic type of shell took place about thisftime. Brachiopods were important; rostrate forms like Terebratula and Rhynohonella from this time onward became more prevalent than broad hinged genera. Pelecypods were abundant, Myophoria, Halobia, Daonella, Pseudomonolis, Avicula, Gervillia and many others." Gasteropods also were numerous; at the beginning of the period, as in other groups, many Palaeozoic forms lingered on, but one of the main changes about this time was the development and expansion of siphonostomous forms with canaliculate shells. Quite the most important Mollusca were the Cephalopods. In the early Trias there still remained a few of the Palaeozoic genera, Orthoceras, Hungariles, and forms which linked up the goniatites with the ammonites, which henceforth took the lead in numbers and variety. Prionolobus, Aspidites, Celliles, Meekoceras, Tiroliles, Plychites, Tropiles, Ceraliles, Arcesles, Psiloceras and Flemingiles are a few of the prominent Triassic enera. The nautiloids were fairly well represented, but they exhibit no such marked development from Palaeozoic to Mesozoic types as is shown among the ammonoids. In the tabulated synopsis of the Triassic system given below it has been impossible to include many of the names of groups and subordinate divisions. Some of these, such as the term “ Noric ” (Norian), have been used in a variety of ways. A clear account of the history of the study of the Trias will be found in K. A. von Zitteys History of Geology and Palaeontology (Eng. trans., London 1901 .

REFERENCES.-The literature of the Trias is very voluminous. A full account, with full references as to date of publication, in Lellzaea Geognostica, ed. by F. Frech, Theil II.; Das Mesozoicum, Bd. i. “ Einleitung des Mesozoicum und der Trias" (F. Frech); “Continentale Trias” (E, Philippi and ]. Wysogorski), 1903; 2nd Lieferung, “Die asiatische Trias ” (F. Noetlingly 1905; 3rd Lieferung, “Die Alpine Trias des Mediterran-Gebietes ” (G. von Hathaber), Stuttgart, 1905. (J. A. H.)