Triassic System Period

Triassic System (Period)

 

the first (lowest) system of the Mesozoic erathem, corresponding to the first period of the Mesozoic era of earth history. The Triassic system lies above the Permian and below the Jurassic. According to radiometric data, the Triassic period began 230 million years ago and ended 195 million years ago; it lasted about 35 million years. The name “Triassic, ” which is derived from the Greek word trias (triad), reflects the circumstance that the Triassic system was established by combining three sequences of strata identified in cross sections in Central Europe: Bunter, Muschelkalk, and Keuper. In 1831 the Belgian scientist J. d’Omalius d’Halloy joined the three under the name “Keuper deposits, ” and in 1834 the German scientist F. von Al-berti proposed the name “Trias, ” which was subsequently modified to “Triassic.”

In what is now the USSR, Triassic deposits were first established by L. von Buch on the basis of identifications made in 1831 of Triassic ceratites in the Caspian region (from the collection of the 18th-century Russian geologist P. S. Pallas). In the mid–19th century the Triassic fauna of the northeast was described by the Russian scientist A. Kaizerling (Keyserling). In the late 19th and early 20th centuries continental Triassic deposits were studied by such Russian geologists as S. N. Nikitin, N. N. Iakolev, N. G. Kassin, and A. N. Mazarovich. Among Soviet geologists, important contributions have been made to the subdivisions of the marine Triassic by V. N. Robinson in the Caucasus; by E. M. Liut-kevich, E. I. Sokolova, and others on the Eastern European Platform; by L. D. Kiparisova, Iu. N. Popov, and 1.1. Tuchkov in the northeast USSR and the Far East; by V. I. Slavin in the Carpathians; and by V. I. Dronov in the Pamirs. Foreign geologists who have worked on the stratigraphy of the Triassic include E. Suess, E. Mojsisovics von Mojsvar, W. Waagen, C. Diener, A. Bittner, and D. Stur.

Subdivisions. Triassic deposits were first studied in the mid–18th century in Central Europe—that is, in Germany and northeastern France. Because of their uniqueness and the scarcity of organic remains, however, the subdivisions established in this area are difficult to trace elsewhere. In the second half of the 19th century the sequence of marine deposits of the Alps came to be used as the standard for the Middle and Upper Triassic. The standard sequences for the Lower Triassic are in the Himalayas and in the Salt Range of Pakistan. The original division of the Triassic system into three units has been retained, although the series of the Alpine Triassic are not exact equivalents of the Bunter, Muschelkalk, and Keuper of the Central European Basin (see Figure 1). The geological structure of the northern calcareous Alps, where the stratotypes of the Anisian, Carnian, Norian, and Rhaetian stages are located (the stratotype for the Ladinian stage is located in the southern Alps), is distinguished by a complicated structure and an extreme diversity of facies. As a result, it is difficult to establish the true sequence of stratification and paleontológica! zones. In the 1960’s the sequence of paleontological zones of North America, where the geologic sections of Triassic deposits are more complete and the structure is simpler, was proposed as the standard for Triassic time.

Figure 1. Correlation of the Alpine and Central European Triassic

The stages of the Middle and Upper Triassic established in the Alps have been universally recognized; Figure 2 shows the correlation of the zones of the most thoroughly studied Upper Triassic in Europe (the Alps), Asia (the northeast USSR), and North America. General agreement has not yet been reached as to the stages of the Lower Triassic. In the Alps one (the Scythian, or Werfenian) or two (the Seisian and Campilian) stages are identified; in the USSR two are distinguished (the Induan and Olene-kian), and in North America four have been identified (the Griesbachian, Dienerian, Smithian, and Spathian).

General description. By the beginning of the Triassic the Hercynian geosynclinal structures of Western Europe, Asia (the Urals-Tien-Shan and Mongol-Okhotsk belts), North America, and eastern Australia had become young cratons. In the northern hemisphere they united Laurasia into a single whole; in the southern hemisphere they were joined to Gondwanaland. The greatly narrowed Mediterranean geosynclincal belt separated these two cratons, and the Cordilleran and East Asian geosynclinal regions bounded them on the west and east. In the Triassic, continental areas that roughly coincided with the cratons predominated over the marine basins, which, with minor exceptions, corresponded to the geosynclines. The Tethys basin occupied the Mediterranean geosynclinal belt. Although it occasionally spread somewhat beyond the geosynclinal belt, Tethys in the early Triassic did not even completely cover the belt. The Boreal, or Northern, Basin occupied the northern parts of the Cordilleran and East Asian geosynclines and the cratonic regions of the Arctic islands. The Pacific Ocean basin corresponded to the larger part of the Cordilleran and East Asian geosynclines. Since marine Triassic deposits are absent along the shores of the Atlantic Ocean, it may be assumed that the ocean did not yet exist in the Triassic. The presence of marine deposits in East Africa and Madagascar testifies to the beginning of the formation of the Indian Ocean.

Orogenic movements were weak in the Triassic and only of local importance. The early Cimmerian (Kimmerian) orogeny late in the Triassic created folded structures primarily in East and Southeast Asia.

In the Triassic, as a result of erosion of the Hercynian mountains, continental deposits (proluvial, fluvial, lacustrine, lagoon-al, and eolian deposits) accumulated in certain isolated depressions on the cratons. In the first half of the Triassic continental deposits also formed in the western part of the Mediterranean belt and the eastern part of the Cordilleran geosyncline. Arenaceous-argillaceous strata accumulated in Central Europe, on the Eastern European platform, and in the intermontane basins of the Urals, Northern Siberia, Southern Siberia, northern Kazakhstan, and Middle Asia, where effusives and tuffs make up a considerable part of these strata. The trap formation continued to develop on the Siberian Platform, as did coal-bearing deposits in eastern Australia and varicolored strata in South Africa and India. Marine deposits within the cratonic areas are known in the northern part of the Siberian Platform, in the Arctic islands, in Central Europe, in the Caspian region, in eastern Africa, and on Madagascar.

In the second half of the Triassic gypsum-bearing deposits formed in Central Europe. Coal accumulated in the Cisuralic region, the eastern part of the Urals, Northern Siberia, Southern Siberia, Middle Asia, eastern Indochina, the southern Appalachians, South America, southern Africa, eastern Australia, and India. In addition, volcanic activity occurred in the Appalachians. Marine terrigenous deposits are known in the Canadian Arctic Archipelago and in Svalbard.

Carbonate sediment accumulation prevailed in the Mediterranean geosynclinal belt, where reef facies formed by lime-excreting algae, hexacorals, thick-shelled mollusks, echinoderms, and other bottom-dwelling organisms were widespread. In the deeper parts of the sea, red cephalopod-bearing limestones formed. Terrigenous and carbonate sediments were deposited in the Cordilleran geosyncline, and thick deposits of shales, sandstones, and conglomerates formed in the East Asian geosyncline; carbonate and siliceous rocks played a subordinate role. During the late Triassic continental deposits containing coal accumulated in what is now Primor’e Krai and Japan. During the middle and late Triassic there was considerable basic volcanism in the Cordilleran geosyncline, the Hercynian mountains of the Appalachians, and, to a lesser extent, the East Asian geosyncline. Minor manifestations of volcanism occurred in the Mediterranean geosynclinal belt.

The climate during the Triassic was quite hot; it was drier in the first half and more humid in the second half. The tropical belt coincided with Tethys and the southern part of North America and reached rather high latitudes (up to 60° north and south of the equator).

Organic world. In the second half of the Permian many Paleozoic groups of plants and animals became extinct; this episode of extinction continued until the end of the Triassic. New (Mesozoic) species of plants and animals appeared at the end of the Permian and, especially, at the beginning of the Triassic. The new forms coexisted with the ancient forms throughout the Triassic and became predominant only in the late Triassic and early Jurassic.

The land flora of the first half of the, Triassic greatly resembled that of the end of the Permian: the arborescent Lycopodiophyta, Calamitales, Cordaitales, and Primofilices that had predominated in the Paleozoic had disappeared, along with many of the ancient conifers. These extinctions caused a fundamental change in flora, even though new groups of plants had not yet appeared: in the end of the Permian and in the early Triassic ancient conifers and pteridosperms (seed ferns) predominated. The primary mesophytic plant groups (dipterid ferns, Cycadopsida, Bennetti-tales, ginkgoes, and mesophytic conifers) appeared in considerable numbers in the second half of the Triassic. Until the very end

Figure 2. Correlation of the Upper Triassic in Europe, Asia, and North America

of the Triassic, however, the ancient groups (seed ferns, true ferns and conifers) and those groups that reached their peak during the Triassic (two families of seed ferns and the Equisetaceae) were prominent. In the Triassic seas reef-forming algae played an important role, as evidenced, for example, in the Dolomite Alps.

During the Triassic there flourished reptiles that were adapted to life on land, in the sea, in bodies of fresh water, and in the air; some groups of amphibians also became sea dwellers. Alongside the labyrinthodonts and cotylosaurs that had existed during the Permian, new groups of reptiles characteristic of the Mesozoic appeared: archosaurs, ichthyosaurs, several groups of synapto-saurs and lepidosaurs, and turtles. In the late Triassic true bony fish and the first mammals also appeared. In addition, bodies of fresh water were populated by small crustaceans, the ostracods and phyllopods.

In the Triassic seas the dominant invertebrates were the cera-tites, which appeared in the late Permian and became extinct at the end of the Triassic. The pelecypods, gastropods, and nauti-loids were also common. There were small numbers of true ammonites and belemnites. The brachiopods, crinoids, and foramin-ifers played a considerably less important role than in the Paleozoic. Tetracorals were supplanted by hexacorals.

Biogeographic regionalization. The Triassic seas of the basins of Tethys (from the Alps to Indonesia) and southern North America were characterized by an abundant and diverse fauna of invertebrates along with extensive development of carbonate reef fa-cies (the tropical region). The other biogeographic region, which had a temperate climate, was located in the Boreal Basin and the basins of New Zealand and New Caledonia, where tropical forms of invertebrates and reef facies were absent. From the end of the early Triassic the differences between these provinces was pronounced. In the course of the Triassic the boundary between them shifted slightly to the north in the western hemisphere and to the south in the eastern hemisphere. The flora of the first half of the Triassic was not diverse.

Beginning in the mid-Triassic, a tropical flora existed along the northern shore of Tethys, in the Appalachians, and on the Colorado Plateau. A more temperate flora existed in the region between Tethys and the Boreal Basin and in the continents of the southern hemisphere, including Antarctica.

Triassic deposits in the USSR. Marine deposits of the Triassic are known in the southern and eastern border regions of the USSR and in the northeastern part of the country. In the Carpathians primarily limestones and marls are found. In the Crimea deposits of sandstone and shale occur. The lower and upper parts of the Triassic in the Northern Caucasus are composed of limestones, and the middle part consists of terrigenous rocks. In Transcaucasia carbonate strata, which reach a total thickness of more than 1,000 m, predominate. Marine terrigenous deposits in which carbonates play a subordinate role are known in Mangyshlak, where the thickness of the strata reaches 6,000 m, and in Tuarkyr. In the Pamirs carbonate sediments predominate in the lower half of the Triassic strata, and terrigenous sediments prevail in the upper half. Thick marine terrigenous deposits are found in the northeast USSR, in Transbaikalia, and in Primor’e Krai; the thickness reaches 3,500 m. In the uppermost part of the Triassic deposits of Primor’e Krai, marine and continental deposits alternate.

The continental deposits of the European part of the USSR (central regions, Donbas, Ural Region, and Caspian region) are composed of arenaceous-argillaceous rocks with remains of vertebrates, land plants, charophytes, ostracods, and phyllopods. In the Caspian region, where the thickness of the Triassic system exceeds 2,000 m, marine carbonate deposits are known in the middle part of the geologic section. In the isolated depressions of the eastern Urals, Siberia, and Middle Asia, the Lower Triassic (thicknesses of up to 1,400 m) is generally composed of effusive-sedimentary strata (including the traps of the Siberian Platform and Taimyr), and the Upper Triassic (thicknesses of up to 2,700 m) is made up of coal-bearing beds with plant remains.

Minerals. Deposits of hard and brown coal are associated with Triassic strata in the USSR (eastern Urals and southern Primor’e Krai), Japan, Vietnam, the USA (southern Appalachians), southern Africa, and eastern Australia. Petroleum and gas of Triassic origin are found in the USSR (Caspian region and Far East), Great Britain, the USA, Algeria, and Libya. Other minerals associated with Triassic deposits include diamonds in the USSR (Yakutia); uranium in the USA (Colorado Plateau); salt in Central Europe; copper and copper-nickel ores in the USSR (Noril’sk), Poland, the USA, and Canada; lead-zinc ores in the USSR (Far East), Poland, and Austria; and silver-lead ores in Yugoslavia. Triassic deposits are a source of valuable building materials, for example, the famous Carrara marble of Italy.

REFERENCES

Gignoux, M. Stratigraficheskaia geologiia. Moscow, 1952. (Translated from French.)
Leonov, G. P. Istoricheskaia geologiia. Moscow, 1956.
Leonov, G. P. Osnovy stratigrafii, vol. 1. Moscow, 1973.
Stratigrafiia SSSR: Triasovaia sistema. Moscow, 1973.
Strakhov, N. M. Osnovy istoricheskoi geologii, part 2. Moscow-Leningrad, 1948.
Pia, J. Grundbegriffe der Stratigraphie. Vienna, 1930.
Schmidt, M. Die Lebewelt unserer Trias. Óhringen, 1928.
Tozer, E. T. A Standard for Triassic Time. Ottawa, 1967.
Kozur, H. “Probleme der Triasgliederung und Parallelisierung der germanischen und tethyalen Trias.” Freiberger Forschungshefte, 1974, no. 298.

I. A. DOBRUSKINA

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