Silurian System Period

the third system from the base of the Paleozoic erathem, corresponding to the third period of the Paleozoic era of the earth’s geologic history. The system was named after the ancient Celtic tribe of the Silures, who inhabited the Welsh borderland in Great Britain. According to radiometric data, the Silurian period began 440 million years ago and lasted approximately 30 million years. It follows the Ordovician period and precedes the Devonian period.

The Silurian system was established in 1835 by R. Murchison in Great Britain. The type area is the Welsh borderland. As first defined, the system included not only Silurian (Gotlandian) deposits proper but those now termed Ordovician. After the Ordovician was split off as an independent system, the name “Silurian” was applied to only the upper part of Murchison’s system. These boundaries of the Silurian system were officially accepted in 1960 by the 21st session of the International Geological Congress in Copenhagen.

The original division of the Silurian system (late 19th and early 20th centuries) into series, stages, and smaller biostrati-graphic subdivisions was based on brachiopods and trilobites, and was established on the basis of type sections of shelf facies in Wales. The first zonal scale based on graptolites was worked out by the British geologists C. Lapworth, G. Elles, and E. Wood in southern Scotland and in Wales.

In Russia study of the Silurian system was begun in the 1840’s by Murchison and the Russian geologist A. Kaizerling. The first investigations of the Silurian in the European part of Russia, in Siberia, and in Middle Asia were made by the Russian geologists F. B. Shmidt, E. Eikhval’d, I. V. Mushketov, and N. I. Lebedev between 1850 and 1890. In the early 20th century the Russian geologists V. N. Veber and A. N. Riabinin made substantial contributions to the study of the Silurian. Important Soviet investigators of the Silurian include D. V. Nalivkin, B. N. Aver’ianov, V. P. Nekhoroshev, V. I. Iavorskii, B. B. Cherny-shev, B. S. Sokolov, O. I. Nikirofova, A. N. Khodalevich, and A. M. Obut. Valuable studies in Western Europe were made by Lapworth, T. Davidson, J. Barrande, O. M. Bulman, and R. Kozłowski. In North America, important investigations were carried out by J. Hall, C. Schuchert, G. A. Cooper, A. Boucot, and W. Berry.

Subdivisions. The Silurian system is divided into two series (see Figure 1)—the Lower and Upper. The Lower series includes the Llandovery (Llandoverian) and Wenlock (Wenloc-kian) stages, and the Upper includes the Ludlow (Ludlovian) and Přídolí stages—the Přídolí corresponds to the Downton (Downtonian) stage. A geostratigraphic scale of graptolitic zones exists for the entire Silurian system. For the three lower stages the scale is based on the zones of the stratotype regions of Great Britain; in the case of the uppermost stage the zones are from the Přídolí stage of the Barrandian in Czechoslovakia.

General characteristics. Silurian deposits are known on all continents except Antarctica. Classic sections have been studied in Great Britain, Czechoslovakia, Sweden, North Africa (Morocco), North America, and the USSR, including the European part, the Urals, Siberia, Middle Asia, the Altai, and Tuva.

The structural plan of the earth’s crust during the Silurian was generally inherited from the Cambrian and Ordovician. The large and relatively stable Eastern European, African, Indian, Chinese, Siberian, and North American cratons continued to exist. Their smallest dimensions were of the order of thousands of kilometers. Such mobile zones, or geosynclines, as the Grampian, Mediterranean, Ural-Siberian, Pacific, and Appalachian, occupied more limited areas.

The relative position of the continents and seas remained approximately the same as during the Ordovician. The largest landmass, the continent of Gondwanaland, included Arabia, India, Antarctica, Brazil, and a large part of modern Africa (south of the Sahara). Smaller landmasses were located in what is now North America and Eurasia; examples are the Greenlan-dian, Balto-Sarmatian (in northeastern Europe), and Angaran (in the Asian part of the USSR) masses. The configurations of these masses changed during the Silurian as a result of regressions and transgressions of the sea. According to one hypothesis, two other continents, Euramerica and Asia, existed in addition to Gondwanaland during the Silurian. The continents were characterized by low, weakly dissected relief. Mountain massifs and chains were apparently absent.

The beginning of the Silurian saw an attenuation of the geo-cratic regime. The extensive subsidence of the earth’s surface was accompanied by a global marine transgression, which reached its peak in the Llandovery. The subsidence of the continental platforms occurred unevenly; the North American and Siberian platforms experienced the greatest subsidence.

Figure 1. Schema of the stratigraphy of the Silurian system

On the whole, the tectonic regime of the Silurian was relatively stable. The géosynclinal zones were characterized by differential tectonic movements; individual segments of greater stability and regions of island uplifts existed against a background of general downwarping. Fine-grained terrigenous deposits accumulated as a result of erosion of the land in the Urals, Central Europe, North Africa, and the Andes, and elsewhere. In areas where geosynclinal development was supplanted by orogenic development, coarse deposits of molasses predominated—for example, in Kazakhstan, the Altai-Saian region, and the Appalachians. As a result of submarine volcanic eruptions, the floor of the geosynclinal seas was covered with lavas, volcanic breccias, and tuffs primarily of basaltic and andesitic composition. Closely connected with the eruptions of submarine volcanoes was the accumulation of siliceous-argillaceous silts, which, together with effusives, gave rise to sedimentary-volcanic formations in the Ural-Tien-Shan, Kazakhstan, Appalachian, and Mediterranean geosynclines. Shelly and reef carbonate facies characterize only the narrow shelf zones.

The end of the Silurian saw the completion of the Caledonian tectonic cycle, orogeny, and folding in a number of geosynclinal systems in Northwestern Europe, northern Greenland, Alaska, and elsewhere (seeCALEDONIAN FOLDING). The classic example is the Grampian geosyncline, located in what is now Scotland; it was eliminated by the end of the Ludlow. Throughout Norway and Sweden, Silurian and more ancient deposits were compressed into folds and broken by faults with horizontal displacements. In Scotland, Northern England, and Ireland, markedly dislocated deposits are overlapped by continental red sandstones. In the Mediterranean and Ural-Siberian geosynclinal regions, regressions of the epicontinental seas occurred at this time. Saline incursions in isolated late Silurian basins led to the formation of red beds, salts, and gypsums (North American and Siberian platforms).

Few data are available on the climate of the Silurian period. The broad distribution of rich invertebrate faunas and the abundance of reefs in the Silurian seas support the hypothesis that the faunas developed in a warm, mild climate. Paleomag-netic data indicate that the equator during the Silurian passed through North America diagonally from southwest to northeast, crossed the Atlantic Ocean, went from northwest to southeast in Western Europe, ran parallel to the Red Sea southward through the Indian Ocean, and then continued in a southwesterly direction from Australia. Thus, most of the known outcrops of Silurian deposits are within the tropical zone. The red beds, gypsums, and salts indicate the existence of arid zones, for example, in North America and in some areas of the Asian part of the USSR. The investigation of fluid inclusions in salt crystals has shown that the average temperatures were between 30° and 50°C. There is evidence that polar regions existed in Gond-wanaland. The presence of rocks similar to tillites in the Lower Silurian sections of northern Argentina, Bolivia, and Mauritania indicates possible glaciation in these areas.

Organic world. By the beginning of the Silurian, all the principal classes of invertebrates had formed, and the first primitive vertebrates had appeared. The large-scale Llandovery transgression created favorable conditions for widespread migration of fauna. This fact accounts for the abundance and diversity of fauna during the early Silurian.

Rich communities of shelled and coral fauna were characteristic of the shallow epicontinental seas and shelf portions of the geosynclinal basins with carbonate sedimentation. The most numerous shelled organisms were the brachiopods, which formed banks or settled in separate groups among stands of cri-noids, anthozoans, and hydroids. The brachiopods burrowed into the soft substrate, lay on top of it, floated above it, or attached themselves to more solid portions of the bottom. Abundant brachiopods in the Silurian were the orthids, strophomenids, rhynchonellids, and forms with spiralia (atrypids and spiri-ferids). The predominant group among the brachiopods was the pentamerids, which migrated extensively and rapidly began to flourish.

In the shelf biocenoses the gastropods and bivalve mollusks were more diverse than during the Ordovician, and tentaculites and numerous nautiloids appeared.

The arthropods (ostracods and trilobites), which were mobile and inhabited soft substrates, played an important role in the composition of the fauna communities. With respect to the ostracods, the difference between the Silurian and the Ordovician is exemplified by the appearance and expansion of the typical Silurian families Primitiopsidae and Beyrichiidae. Trilobites became more uniform, and many Ordovician families disappeared. The Silurian is characterized by representatives of the genus Illaenus and certain forms with elaborate shells (Lichas and Acidaspis). The genus Homalonotus appeared; it lacked a clear-cut three-lobed shell structure. A new group of arthropods, the eurypterids, were found along with ostracods and lin-guloids in shallow basins in association with saline incursions.

The shallow areas of the shelf were inhabited by various an-thozoans, hydroids, crinoids, and algae. The tabulate corals, heliolitids, and tetracorals reached their maximum development. Their various polyparies formed underwater stands and created the numerous growths and banks found in the Wenlock and Ludlow. At the boundary between the Ordovician and Silurian, a considerable change occurred in the representation of the tabulate corals and heliolitids. The Silurian tetracorals, especially the Wenlock tetracorals, became numerous and diverse, with more than 40 genera. Among the echinoderms, crinoids and cystoids were most common; blastoids appeared for the first time. The calcium carbonate secretions of blue-green algae—for example, Girvanella—contributed to the building of such calcareous structures as bioherms and stromatolites. The abundant evidence of vital activity testifies to the existence of large numbers of skeletonless burrowing organisms in the biocenoses of the shallow-water shelf.

The pelagic zones of the Silurian basins, rich in zooplankton and phytoplankton, were inhabited by graptolites. The numerous remains of the graptolites were buried primarily in fine argillaceous silts, which produced the Silurian black siliceous-argillaceous shales typical of deposits in folded regions throughout the world. Among the graptolites existing at the boundary between the Ordovician and Silurian, representatives of the order Axonolipa disappeared completely, and representatives of the order Axonophora continued to develop.

During the Silurian, various uniserial simple and many-branched colonies of graptolites (suborder Monograptinae) appeared, flourished, and began to decline. Biserial graptolites (suborder Diplograptinae) were numerous in the early Silurian and rare in the late. The Monograptinae evolved rapidly; a number of forms appeared that quickly became extinct. The following are examples of such forms: Rastrites, with isolated thecae, during the Llandovery; Cyrtograptus, with many-branched colonies, during the Wenlock; and Saetograptus, with spinelike theca processes, during the early Ludlow.

Two groups of vertebrates were present during the Silurian period: jawless vertebrates (Agnatha) and fishes. The jawless vertebrates included types with and without bony armor. Among the fishes were acanthodians.

During the late Silurian the first terrestrial flora developed —psilophytes (Cooksonia). These were herbaceous vascular plants that inhabited the coastal regions. Imprints have been found in Upper Silurian deposits in Great Britain, Czechoslovakia, and the USSR—for example, Podolia and Kazakhstan.

The most important groups for a detailed stratigraphic subdivision of Silurian deposits are the graptolites, brachiopods, and corals. The ostracods are useful for subdividing and correlating sediments of different facies. Conodonts, chitinozoans, and acritarchs have been successfully used in detailed stratigraphy in the second half of the 20th century.

Biogeographic regionalization. The vast epicontinental seas that arose as the result of the Llandovery transgression gave rise to global paths of migration and were responsible for the cosmopolitan nature of the Silurian fauna. The geographic distribution of, for example, brachiopods, trilobites, nautiloids, and tabulate corals indicates the absence of marked provincial differences at the genus level. In the USSR, however, the European, Siberian, and Central Asian provinces can be identified on the basis of characteristic distributions of species and, sometimes, complexes of brachiopods and tabulates. In Upper Silurian deposits of South America, the endemic brachiopod fauna Clarkeia has been found (the Malvinokaffrian Province). Species complexes of Silurian graptolites featured an identical composition throughout the world. This fact indicates that the Ordovician Atlantic and Pacific provinces had disappeared.

Deposits in the USSR. Silurian deposits are widespread in the European and, especially, the Asian parts of the USSR. All the Silurian stages of the international stratigraphic scale can be identified in the country. Regional zonal schemata have been worked out on the basis of graptolites in sections of geosynclinal and platform regions. These schemata are correlated with the unified standard graptolite scale. The unbroken sections of the boundary deposits of the Ordovician and Silurian have been characterized by paleontological means in Kazakhstan, Tien-Shan, and the southeastern USSR. The boundary between the Silurian and Devonian is substantiated by fauna in sections in Podolia, the Urals, and Kazakhstan.

Silurian deposits have been found by numerous boreholes in the western part of the Eastern European platform in a broad belt stretching from the Baltic region to Podolia. In the northeastern part of the platform, Silurian deposits are known in the Timan Range and on the Kanin Peninsula. Findings from deep drilling suggest that such deposits are absent in the central part. Within the bounds of the Siberian Platform, deposits have been found along the margins of the Viliui and Tunguska syneclises. Silurian deposits are equally widespread in the Ural-Tien-Shan, Kazakhstan, Taimyr, and Verkhoiano-Chukotka geosynclinal regions. Outcrops are known in the Caucasian and Mongolian-Okhotsk folded systems. The Silurian system is represented in the USSR primarily by marine facies of the platform and geosynclinal types. The most thoroughly and carefully studied sections of platform formations are in Podolia and the Baltic region. The Podolian section of the Silurian is the type section for the USSR and a world parastratotype for the boundary of the Silurian and Devonian. Platform deposits are usually represented by continuous series of carbonate and argillaceous rocks containing remains of varied benthic and, less often, planktonic fauna. In the platform sections, breaks are observed between the Ordovician and Silurian; they are particularly significant in deposits of the Llandovery.

In the folded regions, the most complete sections are found in the Urals, the Pai-Khoi, the Tien-Shan, Kazakhstan, and Tuva, on the Taimyr Peninsula, and in the northeastern USSR. The geosynclinal formations are characterized by the development of thick volcanic and pyroclastic basic and acidic deposits, argillaceous rocks, siliceous rocks, sandstones, and, less often, conglomerates. Carbonate sediments are represented by laminated and reef limestones. Carbonaceous-siliceous-argillaceous shales containing graptolites are found in all folded regions of the USSR.

Minerals. Silurian deposits contain a number of important useful minerals. The sedimentary-volcanic rocks of the Urals and Norway contain chalcopyrite ores. Deposits of manganese and phosphorites are associated with the siliceous beds of the Southern Urals and Middle Asia. In the USA, iron-ore deposits found in coarse-grained Llandovery rocks are worked in the states of New York and Alabama. Gypsum and salt deposits are worked in the central part of New York.