Ordovician System Period

the second system from the bottom of the Paleozoic group, corresponding to the second period of the Paleozoic era in the earth’s geological history. The Ordovician overlies the Cambrian system and underlies the Silurian. It has been determined radiologically that the Ordovician system began 500 million years ago and lasted 60 million years.

The name was proposed in 1879 by the English geologist C. Lapworth, who designated as a typical cross section rocks in the Arenig and Bala regions in Wales, inhabited in ancient times by the Ordovices tribe. The Ordovician was accepted as a separate system in 1960, at the 21st session of the International Geological Congress. Previously it had been frequently classified as the lower (Ordovician) division of the Silurian system.

In the USSR the Ordovician has been studied by F. B. Shmidt, V. V. Lamanskii, V. N. Veber, B. S. Sokolov, T. N. Alikhova, O. I. Nikiforova, A. M. Obut, R. M. Miannil’, and A. K. Ryymusoks. Abroad, important work has been done by C. Lap-worth, R. Murchison, H. B. Whittington, and A. Williams in Great Britain, by J. Barrande and V. Havliček in Czechoslovakia, by J. Hall, G. A. Cooper, and M. Kay in the United States, by V. Jaanusson in Sweden, and by T. Kobayashi in Japan.

Subdivisions. There is still some disagreement as to the subdivision of the Ordovician system into series and stages and the position of the system’s boundaries. In the USSR the Ordovician system is generally divided into three series—the Lower, Middle, and Upper—and six stages—the Tremadoc, Arenig, Llanvirn, Llandeilo, Caradoc, and Ashgill. (See Figure 1 for the the major divisions of Ordovician rocks.) The lower and middle substages of the Caradoc stage are usually classified in the Middle series and the upper substage is assigned to the Upper series. When the Ordovician system is divided into two parts, the boundary between the series is drawn between the Llanvirn and Llandeilo stages. In Great Britain the lower boundary of the Ordovician is drawn at the base of the Arenig stage, and the Tremadoc stage is included in the Cambrian. Graptolitic zones are more detailed subdivisions used in differentiating and correlating Ordovician deposits.

General description. The Ordovician system, identified on all the continents except Antarctica, forms part of the structure of the sedimentary mantle of most platforms and is widely found in folded structures. In some places breaks in sediment accumulation, caused by a short-term regression of the sea, have been established on the boundary between the Cambrian and the Ordovician. The maximum extent of marine areas—the transgression of the sea onto platforms—occurred in the Middle Ordovician and was followed by a period of regression. In the relatively shallow epicontinental seas that covered large parts of the platforms of the northern hemisphere in the Ordovician there accumulated chiefly shallow beds (averaging 500 m) of calcareous and sometimes sandy-argillaceous sediments. In the transitional zones between the platforms and the geosynclines—the miogeosynclines of the Appalachians, Urals, and other areas—the thickness of Ordovician sediments increases to 3,500 m in places. In addition to limestones, detrital beds are also widespread. The thickness of deposits of the Ordovician system reaches 10,000 m in the interior parts of geosynclinal belts, for example, the Magog and Fraser eugeosynclinal belts in North America and the Caledonides of Great Britain and Kazakhstan. There were numerous volcanoes in these zones, and thick layers of lavas, tuffs, and siliceous rocks accumulated in addition to detrital sediments. Both shallow-water and deep-water sediments are found in these places. As a result of Taconian orogeny, folded structures and mountains were formed in the Caledonian geosynclines toward the end of the Ordovician. Many scientists believe that the continents of North and South America were closer to Europe and Africa and that Australia was joined to Africa and southern Asia during the Paleozoic era, including the Ordovician period. One of the poles was probably located in the northern Pacific Ocean, and the other was in North Africa or the adjacent part of the Atlantic Ocean.

Organic world. As in the Cambrian, bacteria predominated during the Ordovician period. Blue-green algae continued to develop, and calcareous green and red algae flourished in warm seas to depths of 50 m. The remains of spores and rare finds of stem imprints, probably belonging to vascular plants, attest to the existence of land vegetation during the Ordovician.

Of the Ordovician faunas, only those living in seas and oceans, as well as a few inhabitants of fresh or brackish waters, are well known. Virtually all phyla and most classes of marine invertebrates were represented. The first vertebrates, fishlike Agnatha, appeared. Planktonic radiolarians and foraminifers inhabited the oceans and seas, and graptolites became very numerous in these waters. Numerous species of trilobites, brachiopods, echinoderms, bryozoans, sponges, pelecypods, gastropods, and cephalopods lived in great numbers on the floors of shallow seas, in coastal waters, and in shallows. Corals and other coelenterates were common in the warm seas. A major stage in the development of the ancient Paleozoic organic world ended in the Ordovician. By the beginning of the Silurian, many families among the graptolites, brachiopods, corals, cephalopods, and trilobites had become extinct, as well as several distinctive groups of echinoderms typical only of the Ordovician period.

Biogeographic regionalization. The Ordovician may be divided into two belts according to the distribution of different groups in the organic world. The first belt encompassed North America, including the arctic archipelago, Greenland, Scotland, Scandinavia, the Baltic region, the Urals, almost all of the Asian part of the USSR (excluding the Pamirs), and probably China. Consisting of the Ordovician equatorial regions, this belt had a hot or warm climate and was inhabited by a great variety of species. The belt comprised a number of paleobiogeographic regions—in the USSR, the Baltic, Kazakhstan (including Tien-Shan), Siberian, and Kolyma regions. The second belt included the Ordovician subpolar regions with cold climates. It encompassed southern Europe, Africa, southern Asia (the Pamirs in the USSR), and probably Australia and South America. The organic world of this belt was meager. Traces of Ordovician glaciation have been found in the second belt in Africa, southern Europe, and South America.

Deposits in the USSR. Ordovician deposits are widespread on the East European and Siberian platforms; in the folded systems of the Urals, Pai-Khoi, and Novaia Zemlia; on the Severnaia Zemlia and Novosibirskie islands; and in Kazakhstan, Central Asia, the Altai-Saian region; and in the northeastern USSR.

Classical cross sections have been found on the southern coast of the Baltic Sea. Here the eroded layers of the Cambrian are overlain by glauconitic sandstones and by shales with Tremadoc graptolites. Above these rocks, to the Silurian boundary, there are various limestones with occasional interlayers of dolomites. Layers of combustible shales, called kuckersite, are found in the Llandeilo stage. The thickness of the Ordovician system on the platform does not exceed 200–250 m.

On the western slope of the Urals, in the Pai-Khoi, and on Novaia Zemlia—the western miogeosynclinal zone of the Ural-Siberian geosynclinal belt—Ordovician deposits consist of marine terrigenous deposits and limestones up to 3,800 m thick in places. In the internal, eugeosynclinal part of the Ural-Siberian belt—the eastern slope of the Urals, the eastern half of Kazakhstan, and the central and northern Tien-Shan—a series of zones has been identified with marine sediments of diverse composition up to 10,000 m thick. Green terrigenous sediments about 3,500–4,500 m thick predominate in the eastern Ural-Siberian belt and in the Altai-Saian miogeosynclinal region. Thick limestone beds occur in the Middle and Upper Ordovician in Gornyi Altai, the Salair, Gornaia Shoria, and the Kuznetskii Alatau, while red-colored, coarse, detrital deposits are found in Tuva. Intrusive rocks of the Ordovician system are widespread in the Urals, the Altai-Saian region, Kazakhstan, and the Tien-Shan.

On the Siberian Platform, Ordovician rocks show variability and variety. Common rocks, in addition to limestones and dolomites, are red and multicolored sandy-argillaceous sediments with gypseous and salt-bearing interlayers in places. Generally not more than 500 m thick, the deposits may reach 1,500–1,700 m on the periphery of the platform. In the Verkhoiano-Chukotka geosyncline the Ordovician system outcrops primarily on the territory of the Kolyma Massif and is represented by three types of marine deposits: calcareous (up to 500 m thick), terrigenous (not more than 1,000 m), and volcanic-sedimentary (up to 2,500 m).

Minerals. Combustible shales (kuckersite) are worked in the platform sediments in the Estonian SSR and in Leningrad Oblast. Phosphorites are also found here, as well as on the Siberian Platform and in Kazakhstan. Small deposits of iron and manganese ores are found in geosynclinal volcanic-siliceous sediments in North America, Western Europe, Kazakhstan, China, and elsewhere. Deposits of gold and other metals are associated with the Ordovician intrusive rocks in Kazakhstan, and petroleum deposits have been discovered in Ordovician deposits in North America.