Paleozoic Erathem Era
Paleozoic Erathem (Era)
one of the erathems, or groups, in the stratigraphic scale of strata of the earth’s crust; the name is also applied to the corresponding era of the earth’s geologic history. According to radiological data, the total length of the Paleozoic erathem is estimated to be 340–350 million years. The erathem began 570 million years ago and ended 230–220 million years ago.
The Paleozoic erathem was identified in 1837 by the British geologist A. Sedgwick, who included in it the Silurian and the Devonian systems. Subsequently, the Paleozoic was considerably expanded and was broken down in greater detail. Six geologic systems are now distinguished in the Paleozoic: Cambrian, Or-dovician, Silurian, Devonian, Carboniferous, and Permian.
General description. The large-scale folding movements that preceded the Upper Precambrian were followed by the formation of platforms and geosynclinal regions whose outlines were preserved, with small changes, throughout the entire Paleozoic era. The most important platforms were the Eastern European (Russian), Siberian, Sino-Korean, South China, North American, South American, African, Australian, and Indian. These vast regions of the earth’s crust were tectonically stable. The platforms were periodically flooded by shallow seas, in which various kinds of sediments were deposited in thin layers that formed a sedimentary mantle. Over enormous areas the rocks of this mantle occur in even, almost horizontal strata. The thickness of the sediments increases in the marginal parts of the platforms. The accumulation of sediments was particularly intensive in the marginal deeps during the epochs when mountain ranges carrying large amounts of detritus, which had collected in low-lying areas, rose around the platforms. In some of these areas—the Donbas, the Pechora Coal Basin, and the Appalachians—thick coal seams formed. In others—such as the Cisural foredeep—saline and red detrital deposits were laid down.
The conditions of sediment accumulation in the geosynclinal regions were different. Such regions were distinguished by high mobility and very rugged relief, where deep troughs corresponding to sections of the initial ocean crust alternated with uplifted ridges. The earth’s crust was cut by a network of faults along which individual blocks shifted; lava flowed out through fissures, and the products of volcanic eruptions were ejected. The geosynclinal sediment accumulations are distinguished by thick layers of volcanogenic and siliceous rocks and by various types of detrital layers.
The Paleozoic era had two principal episodes of folding. One, the Caledonian folding, manifested itself with greatest intensity at the beginning and particularly in the middle of the Paleozoic. Its main phases were between the Ordovician and Silurian and in the beginning of the Devonian, after which the formation of mountain chains and accumulation of red detrital molasse-type deposits began over large areas. The Caledonian folding was particularly clearly manifested in Great Britain, the Scandinavian Peninsula, Spitsbergen, Kazakhstan, the Zapadnyi Saian, Newfoundland, and the Appalachians. The immense Her-cynian folding encompassed the end of the Paleozoic; its most intensive manifestations fell in the second half of the Carboniferous period and in the Permian period. After the mountain building at the boundary of these two periods, red molasse formations became particularly diverse. Such mountain structures as those of Central Europe, the Urals, and the Appalachians were a result of the Hercynian folding.
In the first half of the Paleozoic era there probably already existed enormous landmasses that determined the paleogeographic situation not only of the Paleozoic but also of subsequent geologic history. The enormous continent of Gondwanaland encompassed the platforms of the southern hemisphere. In the Paleozoic the shallow sea basins flooded only the marginal parts of Gondwanaland. Various kinds of continental deposits usually formed on the continents, as well as glacial deposits in some epochs. Traces of ancient glaciation can be seen among the rocks of the Upper Ordovician in Africa, but they are most extensively represented in strata of Carboniferous and Permian age. Tillites, or fossil glacial formations, of this age are known in South America, South Africa, Australia, and India. During the second half of the Paleozoic, the vast continent of Angaraland, which included the Siberian Platform and the adjacent mountain structures, existed in the northern hemisphere.
Organic world. At the very beginning of the Paleozoic, forms of life with hard skeletons—forms that had not been seen earlier —suddenly appeared and spread rapidly. They included hyoli-thids, chiolitelminths, gastropods, brachiopods, and archaeocya-thids; the archaeocyathids, the most ancient reef-building organisms, became extinct by the end of the early Cambrian. The most ancient arthropods, the trilobites, became widespread in the Lower Paleozoic. They made up a considerable part of the organic world of the Cambrian and Ordovician seas but were less numerous in the Silurian and Devonian and became extinct at the end of the Paleozoic era. At the very beginning of the Cambrian period, hyolithids and true mollusks appeared in considerable numbers. The brachiopods, which are important for distinguishing strata of the Middle and Upper Paleozoic, attained great diversity. Together with the gastropods and bivalves (pelecypods), they lived at various depths in the sea. During the Ordovician many solitary and colonial corals, bryozoans, and stromatoporoids appeared.
Paleozoic invertebrates that swam freely on the surface of the sea included graptolites, which were generally restricted to the Ordovician and Silurian, and cephalopods of the nautiloid group, which were especially richly represented in the Ordovician. During the Devonian they declined in importance, but the goniatites, with more intricately constructed shells, developed rapidly. Finally, in the Upper Paleozoic, the foraminifers, a group of single-cell animals, became widespread. Particularly important among them were the fusulinids, which had especially intricate tests. These tests underwent changes in comparatively short intervals of time, a fact that permits highly detailed comparisons of coeval deposits containing fusulinid remains in different regions.
Among the vertebrates, fishes appeared during the Paleozoic. Primitive jawless fishes were present in the Cambrian and Ordovician. In the Silurian and particularly the Devonian, dipnoan and crossopterygian fishes were widespread. The amphibians, the first animals to emerge on land, developed from the crossop-terygians at the end of the Devonian. The ancient amphibians of the Paleozoic era belonged to the extinct group of the stegoce-phalians. In the Carboniferous and particularly the Permian, herbivorous and carnivorous reptiles existed along with them.
The plants of the Paleozoic developed just as rapidly. During the Cambrian and Ordovician they were represented primarily by algae. The existence of higher land plants at that time is still a matter of controversy. Remains of spores have been found in Silurian deposits, and the rocks of the Lower Devonian everywhere exhibit imprints of psilophytes, primitive plants that apparently grew in coastal regions. In the Middle and Upper Devonian the vegetation became much more diverse; arborescent lycopods, the first joint-stemmed plants (arthrophytes, including sphenopsids), primitive ferns (primofilicales), pro-togymnosperms, and the first gymnosperms were common. Particularly important is the varied flora of the Carboniferous period, where we find calamites; arborescent lycopods, such as lepidodendrids and sigillarids; various ferns; fernlike seed plants (pteridosperms); and cordaites. The dense forest vegetation of the time provided the material for the formation of numerous seams of hard coal. Beginning with the Carboniferous, paleoflo-ristic regions can be delineated. Glossopteris flora, which was particularly characteristic of the next period (the Permian), apparently already existed in Gondwanaland at this time.
Paleozoic deposits in the USSR. Both platform and geosynclinal Paleozoic beds are common in the USSR. The complex of sedimentary deposits of the Eastern European and Siberian platforms is composed primarily of Paleozoic rocks. Cambrian marine clays and sandstones and Ordovician and Silurian limestones are well developed in the northwestern part of the Eastern European Platform. Devonian and Carboniferous marine deposits are widespread. They are represented primarily by limestones with separate interlayers and bands of sandy-argillaceous rocks, with which oil pools and deposits of coal, bauxite, and refractory clays are associated. Continental Paleozoic deposits are present in the thick coal-bearing series of the Donbas and the Pechora Basin and are widespread among the red beds that formed in the eastern part of the Eastern European Platform between the Volga and Urals during the Permian period.
On the Siberian Platform, the marine deposits of the Cambrian—limestones, dolomites, and in places rock salt, gypsums, and anhydrites—are especially well developed among the Paleozoic rocks. Limestones and dolomites alternate with sandy-argillaceous, partially continental formations in the Upper Cambrian and Ordovician. Silurian and Devonian deposits, represented by calcareous-shale and sandy-argillaceous rocks, respectively, have a limited distribution here. Carboniferous and Permian deposits are represented primarily by continental coal-bearing deposits of great thickness.
Paleozoic geosynclinal formations consist of sedimentary and effusive deposits penetrated by various types of intrusive rocks. They are extensively developed in the Urals, Kazakhstan, Middle Asia, the mountain structures bordering the Siberian Platform, the Far East, and the entire northeastern part of the USSR. Paleozoic deposits are extremely limited in distribution in the recent folded structures of the Pamir, Caucasus, and Carpathians, as well as on Sakhalin and Kamchatka, where they outcrop in the arched portions of anticlinal uplifts among younger rocks.
Minerals. Rich ore deposits are associated with the Caledonian and Hercynian intrusive rocks of the Paleozoic in the Urals, Kazakhstan, the Altai, Western Europe, and North America. The oil pools of the Volga-Ural Oil-Gas Region of the USSR, the central part of North America, and the province of Alberta in Canada are in Paleozoic sedimentary rocks, as are the hard coal deposits of the Donets, Moscow, Pechora, Karaganda, and Kuznetsk basins, of the coal basins of Western Europe, and of the Appalachians in North America. The combustible shale deposits of Estonia and the copper sandstone deposits of the Cisural Region and Kazakhstan are also found in Paleozoic sedimentary rocks. Also of Paleozoic age are large deposits of phosphorites in the Karatau Basin in the USSR and the Rocky Mountains in the United States; of bauxites in the Urals, the Salair, and other areas; and of rock and potassium salts near Solikamsk, in the Ilek basin, and in the Irkutsk group of deposits in the USSR and near Stassfurt in the German Democratic Republic. Many Paleozoic rocks make excellent building materials, for example, the Ordovician limestones in the vicinity of Leningrad, the Carboniferous limestones of the Moscow area, and the marble of the Urals.
REFERENCESStrakhov, N. M. Osnovy istoricheskoi geologii, 3rd ed., parts 1–2. Moscow-Leningrad, 1948.
Gignoux, M. Stratigraficheskaia geologiia. Moscow, 1952. (Translated from French.)
Laz’ko, E. M. Osnovy regional’noi geologii SSSR, vols. 1–3. L’vov-Mos-cow, 1962–71.
B. M. KELLER