Devonian System Period

Devonian (from Devonshire, a county in Great Britain), the fourth system of the Paleozoic group of layers of the earth’s crust, which corresponds to the fourth period of the Paleozoic era in the geologic history of the earth. It follows the Silurian system (period) and precedes the Carboniferous system (period). The time period during which the rocks of the Devonian system were formed has been determined by radiological methods to be from 410 million to 350 million years ago; thus the duration of the Devonian period was approximately 60 million years.

The Devonian system was first identified in 1839 by the English geologists R. Murchison and A. Sedgwick in Great Britain. The first division of the Devonian system into series and then stages was conducted in the Rhenish Slate Mountains and in the Ardennes by the German geologists the Sandberger brothers (1845), the Frenchman Gosselet (1875-90), and others. The initial boundaries and dimensions of the stages distinguished underwent some changes during subsequent analyses of the Devonian system. Nevertheless, the initial stratigraphic subdivisions are considered as type sections today.

Deposits of the Devonian system in the USSR were first noted by the Russian investigator E. I. Eikhval’d in what is today Novgorod Oblast (1839-40) and by the German geologist L. von Buch; more thorough descriptions were given in the works of the English geologist R. Murchison, the French paleontologist P. E. Verneuil, and the Russian paleontologist A. A. Keizerling (1845). Important contributions to the study of the Devonian system were made later by the Russian geologists F. N. Chernyshev and P. N. Veniukov, who identified the most important stratigraphic subdivisions of the Devonian system. Among Soviet specialists, significant contributions to the study of the Devonian system have been made by D. V. Nalivkin, B. P. Markovskii, D. V. Obruchev, and R. F. Gekker.

Subdivisions. The Devonian system is divided into three series and seven stages, of which the three lower stages cannot as yet be considered firmly established. The boundaries of the Devonian system and of its lower and middle series are to this day the subject of discussion. The lower boundary is now drawn at the base of the graptolitic zone Monograptus uniformis; the upper boundary, at the top of the Wocklumeria zone and the Belgian Etroeungt layers.

General characteristics. The history of the development of the continents in the Devonian period is conditioned by their structural plan, which was inherited from preceding periods. At the beginning of the Paleozoic, in the northern hemisphere, old Eastern European, Siberian, Chinese, and North American platforms existed, which constituted the single continent of Laurasia; the Hindustan, African, South American, and Antarctic platforms were part of the enormous southern continent of Gondwanaland. Within the platforms, the larger portion of which consisted of dry land, there were clearly discernible elevations (shields, anteclises) and depressions (syneclises), usually containing shallow epicontinental seas.

Between the platforms were mobile geosynclinal zones, the separate parts of which were in various stages of development. At the end of the Silurian and the beginning of the Devonian, the Caledonian tectonic cycle of the geological history of the earth was completed; for a number of geosynclinal zones adjoining the platforms, it culminated in folding and mountain building. Thus, the Caledonian mountain formations arose. During the Devonian they were intensively eroded, and at its end they had acquired the form of a platform. The geosynclines that underwent such development included the Grampian, Altai-Saian, and Cape geosynclines. The orogen of Central Kazakhstan and northern Tien-Shan, which lay in the central part of a mobile zone, remained in an isolated position.

A significantly large area, in comparison with geosynclines adjacent to the platforms, was occupied by the Hercynian and future alpine portions of the geosynclinal zones, which continued to subside; these portions were covered by seas. In the Devonian period they underwent the initial stage of the next, Hercynian, tectonic cycle. The sea basins were characterized by considerable variations in depth; on dry land a dissected topography predominated. The most distinctive mountain topography existed in the early Devonian in regions of completed Caledonian folding. Abundant terrestrial volcanism (of the porphyritic or andesite-rhyolite formations) in fractures and thick series of terrestrial, detrital, usually red, sedimentary deposits—Old Red Sandstone (the “Old Red” of the British Isles) and others—testify to such topography. In younger geosynclines geanticlinal zones were uplifted and chainlets of mountainous islands were formed. Detrital (in external downwarps) and carbonaceous marine sediments were deposited in these islands, while in the internal deep-water downwarps there occurred lava eruptions of keratophyre-spilite-diabase formation of the initial stages of geosynclinal development. At the beginning of the Devonian period there was also considerable geomorphic contrast on platforms that were high peneplains.

Climatic conditions of the Devonian period may be judged from lithological and paleomagnetic data. The latter are scarce and do not entirely agree with the lithological data. The greatest amount of material is available for the northern part of Eurasia. Paleomagnetic data on other continents, as well as on the southern part of Asia, do not agree, unless one assumes that there was a relative spatial continental displacement (drift). In the Devonian period the equator was located at an angle of 55°-65° to the present one and passed approximately through the Caucasus, the Eastern European Platform, and southern Scandinavia or northern Europe. The North Pole was in the Pacific Ocean between 0°-30° N lat. and 120°-150° E long. On other continents the subtropical latitudes of the southern hemisphere have been established on the basis of paleomagnetic data for southeastern Canada, Brazil, southeastern Australia, and other regions. In general terms, only two climatic zones may be distinguished—the tropical (humid) and the northern (arid). In addition, in South Africa (in the Cape Mountains) in the Table Mountain suite (S₂-D₂), the Congo basin, and southern Brazil there are ice-laid deposits (tillites), which are evidence of a cold, possibly circumpolar, climate. The tropical belt, which at various times of the Devonian period ran from the present-day Western Siberian Plain in the north to Central Europe in the south, is marked by iron ores, bauxites, kaolin weathering mantles, coal, and other indicators of a humid climate. Indicators of a humid climate—iron ores—are also present in southwestern Asia (Turkey), North Africa (Sahara), and in eastern North America (Appalachians). The arid belt embraced Angarida and the folded structures adjacent to it in the south and east. Here there are deposits of salt and gypsum and extensive areas occupied by red deposits of saline basins. The arid zone may also be traced in the greater northeastern part of North America. In all, the arid climate prevailed on the Angara, Kazakh, Baltic, and North American continents. During the Devonian, climates were apparently more dependent on land and sea distribution than in later periods.

The Lower Devonian was a geocratic epoch—an epoch of the predominance of dry land, extensive elevations of continents, and retreat of the sea. The sea (according to A. B. Ronov and V. E. Khain, 1954) occupied only 30 percent of the contemporary continents. In the geosynclines the area of marine sediments did not exceed 50 percent on the average, and on the platforms 17 percent. The seas almost entirely disappeared from the Cordilleran geosyncline and decreased in the Ural-Tien-Shan and Tasmanian geosynclines. The Eastern European Platform, Central Kazakhstan, and Zapadnyi Saian became dry land. The seas in the Verkhoiansk-Chukchi and Andean (South America) geosynclines retained their transgressive character. Intensive subsidence of downwarps, which became filled with the products of land erosion, was characteristic. On the whole, the climate of the Lower Devonian was marked by continentality and aridization.

The Middle Devonian was marked by some rearrangement of the structural schemes, the growth of marine transgression in a number of geosynclines and platforms, decrease in the amplitude of elevations and, associated with this, a general decrease in the distribution of detrital formations and an increase in saline and marine carbonaceous-detrital formations. Activization of volcanism was due to intensification of subsidence. Seas occupied more than 40 percent of the continental areas. Transgression in the geosynclines of the Cordillera, Australia, and eastern Asia attained broad development. The Ural-Tien-Shan geosyncline became broader. The sea inundated large parts of the Eastern European and Siberian platforms and began to inundate the North American and Australian platforms. At the same time, areas of sedimentation on the platforms of South America, and to a smaller degree on those of Africa, continued to decrease. At the end of the epoch volcanism in the Caledonides had considerably lessened. The climate became milder, and vegetation developed. A mantle of soil appeared on dry land.

In the Upper Devonian redistribution of land and sea continued. The different directions of the fluctuating movements of the northern, Laurasian, and southern, Gondwandian, groups of continents continued. The increase of areas of sedimentation and transgression was less considerable in northern Eurasia and North America, and at the end of the epoch (in the Famennian stage) was replaced by regression on the platforms. The Siberian platform rid itself of the sea. The sea of the Russian Platform was transformed into lagoons. In the south, in Australia, the sea area did not change, in Africa it decreased, and in South America the sea almost entirely receded from the continent. The elevations of the Hindustan Platform persisted throughout the entire Devonian. The total area of seas on the platforms and in the geosynclines changed but little. There remained a similarity in the formational composition of deposits, among which marine carbonaceous-detrital formations predominated. Areas of accumulation of carbonaceous and submarine volcanogenic spilite-keratophyre formation increased owing to the increase in volcanism in Hercynian geosynclines, especially those close to the Pacific Ocean. The epoch culminated in some places in folding and uplifts, accompanied by acid and basic intrusions.

The organic world. Changes in the organic world corresponded to the great changes in the paleogeographic situation produced by the Caledonian uplifts and draining. Draining fostered the development of terrestrial animals and plants. The developing bodies of fresh water on the continents became populated with fish. The first amphibians, Stegocephalia, sprang from crossopterygian fish in the Upper Devonian. Plants that first appeared on dry land in the Silurian began to conquer the continents in the Devonian. Their remains appear with growing frequency in lake, deltaic, lagoonal, and marine-coastal deposits. Small beds of coal appeared. In the Lower Devonian many groups of higher terrestrial plants already existed. The psilophytes and lycopsids, which had already begun to appear in the Silurian, became considerably more diverse. The first proto-ferns appeared. In the Middle Devonian progymnosperms (Progymnospermae) appeared, and, possibly, arthrophytes. The abundance of terrestrial vegetation increased even more during the Upper Devonian.

Progymnosperms became the principal group of plants and at the end of the period gave rise to true gymnosperms. The proto-ferns became more diverse, and arthrophytes appeared. The enrichment of the flora is reflected in the increased diversity of complexes of fossil spores and pollen.

Marine fauna underwent great changes. During D₁ the number of cystoideans decreased greatly, graptolites became extinct, nautiloids continued to exist, and ammonites appeared and developed abundantly. Armored fish were widely distributed; the Devonian period is often called the age of fishes. During the Devonian period Foraminifera gradually developed, mainly with calcareous shells. Many new genera and families of Radiolaria appeared. The development of Coniconchia (Tentaculoidea) continued: tentaculites, novakia, and styliolinans. Brachiopoda, Ostracoda, Tabulata, and Tetracoralla flourished.

Fish and Agnatha yield valuable material for dividing the continental sand-aleurites, in particular the red masses that were widely distributed during the Devonian. They are also present in coastal and marine deposits. Representatives of 120 genera, of which eight had appeared in the Silurian and two survived in the Carboniferous, have been described from the Devonian in the USSR. Most significant are the Heterostraci, Arthrodira, Antiarcha, and Crossopterigii.

Biogeographic regionalization. Distribution of the principal groups of marine fauna (chiefly corals) allows one to distinguish the following zoogeographic provinces in the Lower Devonian: Mediterranean, which includes Western and Central Europe, Asia Minor, Iran, and the Himalayas; Magrib (North Africa); Ural-Tien-Shan; Dzungaria-Balkhash; Altai-Saian; Indigirka-Kolyma (Taimyr, northeastern USSR); Mongolo-Okhotsk (eastern Transbaikalia and Amur basin); Indo-Chinese (Indochina, southwestern China); East Australian; California-Canadian; Appalachian; and Malvinas-Kaffrarian (southern parts of South America and Africa). In the Middle Devonian the connections between these provinces became greater and their differences gradually became regional. In the Upper Devonian only the North American and Australo-Eurasian provinces are discernible. Apparently phytogeographic provinces also became differentiated during the Devonian period.

Devonian deposits in the USSR. Devonian deposits are well developed over extensive areas from the western borders of the USSR to the Shantar Islands and the Chukchi PeninsuIa. There are platform, miogeosynclinal, eugeosynclinal, and orogenic formations. On the Eastern European Platform coastal-marine clayey-aleurite sediments give way to marine clayey-carbonaceous and carbonaceous sediments. Bituminous sediments of domanic facies are characteristic; gypsum and salt are present. Brightly colored clay and marl sediments of salinized and salt lagoon-type bodies of water with sparse fauna predominate on the Siberian Platform. In the Hercynian and future alpine geosynclines, along with widely distributed limestone formations, there are massive slate (the Zilair suite of the Urals) and terrigenous flysch formations of the Altai, Transbaikalia, and the Far East (2-3 km thick). In the eugeosynclinal zones of the eastern Urals, southern Tien-Shan, the Rudnyi Altai, and the Far East, a substantial role is played by volcanites: basalt-andesite, spilitekeratophyric, and volcanogenic-siliceous formations (4-6 km). Foremost in the orogenic zones of Central Kazakhstan, the Altai-Saian region, and on the Kolyma central massif are multikilometer terrestrial formations: rhyolite (porphyrite), basalt-andesite, and andesite, which are accompanied by the development of coarse-grained and red continental Molasses in intermontane depressions. Granitoid and basic intrusions are widely developed.

Minerals. Vast reserves of petroleum and gas, which are concentrated in local elevations of interior and marginal parts of the depressions of the Eastern European and North American platforms (USSR, USA, Canada), are associated with deposits of the Devonian system. There are blanket de-posits of rock salt and sulfates in the depressions of the Eastern European, Siberian, North American, and Australian platforms, in Central Kazakhstan, and in Tuva. The rich concentrations of potassium salts in the Williston basin (USA) and in the Pripiat’ downwarp are associated with these blanket deposits. In Central Kazakhstan there are iron-manganese and lead ores, apparently of sedimentary origin. In the same place and in the Dnestr region there are cuprous sandstones. Stratified iron oolitic ores are widely distributed (Eastern European Platform and the Ural-Mongolian, Mediterranean, Western Atlantic, and Cordilleran geosynclinal zones). There are bauxites in the Northern Urals and near Salair owing to the weathering crusts of effusive and sedimentary rocks.

Endogenic ore manifestation, associated with intrusions of the Devonian period and also confined to the Devonian system but younger in age, is well developed in Paleozoic geosynclinal orogenic regions, including the Altai, the Urals, the Sudetes, and Central Kazakhstan. It is represented by pyritic polymetallic, essentially copper, and ironore deposits of hydrothermal, contact-metamorphic, and volcanogenic-sedimentary origin. Arsenic-copper-nickel-cobalt, gold and baritic, and mercuric mineralizations are also found.

The manganese deposits in the Ural jaspers and deposits of andalusite, corundum, molybdenum, and precious metals are due to the solfataric activity of the Devonian period. The largest deposit of cryolite is in southern Greenland. There are primary deposits of diamonds in the Viliui basin (USSR).