Ordovician period

The name of the period was proposed by the English geologist in 1879. He indicated as a typical section in the area of ​​Arenig and Ball in Wales .

It comes from the name of the ancient Ordovician tribe that lived in Wales.

Ordovician adopted as an independent system in 1960, at the 21st session of the International Geological Congress . Prior to this, in many countries the Ordovician system was considered as the lower (Ordovician) division of the Silurian system .

The study of the Ordovician system in the USSR is connected with the names of F. B. Schmidt , V. V. Lamansky , V. N. Weber , B. S. Sokolov , T. N. Alikhova , O. I. Nikiforova, A. M. Obuta , R. M. Myannil, A. K. Ryymusoksa and many others. The works of foreign researchers are known: English geologists (C. Lapworth, R. Murchison , H. B. Whittington, A. Williams), Czech ( J. Barrand , V. Gavlichek), American (J. Hall, G. A. Cooper, M. Kay), Swedish (V. Yaanusson), Japanese (T. Kobayashi) and other scientists.

The Ordovician system is divided into 3 departments:

With this subdivision, the lower and middle sub-tiers of the Karadok tier usually belong to the middle section, and the upper sub-tier - to the upper. In the binomial division of the Ordovician system, the border of the departments is drawn between the Llanwyrn and Llandailian tiers. In Great Britain, the lower boundary of the Ordovician system is drawn at the base of the Arenigian stage, and the Tremadoc layer belongs to the Cambrian. The most fractional units used in the dismemberment and correlation of the Ordovician deposits are graptolite zones.

A general characteristic of the Ordovician system is highlighted on all continents . She is involved in the construction of the cover of most platforms and is widespread in folded structures. In places on the border of the Cambrian and Ordovician, breaks in sedimentation are established due to short-term sea ​​regression . The maximum expansion of marine spaces - transgression of the sea on platforms - falls on the Middle Ordovician. In the future, the regression stage begins again. In relatively shallow epicontinental seas, which covered significant areas of the platforms of the Northern Hemisphere in the Ordovician, mainly thin (on average up to 500 m) calcareous, rarely sandy-clay sediments accumulated. In the transitional areas between platforms and geosynclines (in the myogeosynclinal zones of the Appalachian Mountains , the Urals, and others), the sedimentary powers of the Ordovician system increase (in places up to 3500 m); along with limestone clastic deposits are widespread. In the inner parts of the geosynclinal belts (the eugenosynclinal zones of Magog and Fraser of North America , the caledonides of Great Britain and Kazakhstan , etc.), the thickness of the deposits of the Ordovician system reaches 10 thousand meters. Numerous volcanoes existed in these zones and, along with clastic sediments, thick strata of lava and tuff accumulated, as well as siliceous rocks. Both shallow and deep sea sediments are common here. As a result of the manifestation of the taconic phase of tectonic movements in the Caledonian geosynclines, folded structures formed and mountain constructions arose towards the end of the Ordovician. According to the theory of plate tectonics in the Paleozoic era, including the Ordovician system, the continents of North and South America were close to Europe and Africa , and Australia adjoined Africa and southern Asia . One of the poles, apparently, was located in the northern sector of the Pacific Ocean , and the second - in North Africa or in the adjacent part of the Atlantic Ocean .

In the Ordovician period, as in the Cambrian, bacteria dominated. Blue -green algae continued to develop. Lush green and red algae , living in warm seas at a depth of up to 50 m, are lushly developed. The existence of terrestrial vegetation in the Ordovician period is indicated by the remains of spores and rare finds of fingerprints of stems, probably belonging to vascular plants.

Of the animals of the Ordovician period, only the inhabitants of the seas , oceans , and also some representatives of fresh and brackish waters are well known. Terrestrial fauna was probably absent (although it is possible that some arthropods could temporarily go to land). There were representatives of almost all types and most classes of marine invertebrates. Then lived the first well-studied jawless vertebrates ( arandaspids ). Planktonic radiolarians and foraminifers lived in the thickness of the waters of the oceans and seas. In warm-water seas, corals and other coelenterates inhabited. Of the echinoderms , sea ​​buds , sea ​​bubbles , sea ​​lilies , starfish , and edrioasteroids have flourished . Mollusks were widespread - bivalves , gastropods, and cephalopods , which included, among others, genera such as orthoceras , endoceras , oncoceras, and ascoceras . Among snails, in particular, there were many wing-shell and bellerophons . In addition, crustaceans , trilobites , brachiopods , bryozoans , sponges , graptolites , horseshoe crabs and many other animals were common in Ordovician.

Ordovician ends a major stage in the development of the ancient Paleozoic organic world. By the beginning of the Silurian, many families were dying out among graptolites, brachiopods, corals, cephalopods, and trilobites, as well as a number of peculiar echinoderms that were characteristic only of the Ordovician period.

In the Ordovician period, two zones are outlined according to the distribution characteristics of various groups of the organic world. The first of them united North America together with the Arctic archipelago, Greenland, Scotland, Scandinavia and the Baltic states, the Urals, almost the entire Asian part of the former USSR (with the exception of the Pamirs) and, apparently, China. This belt covered the Ordovician equatorial regions and was distinguished by a hot and warm climate, a great variety of the organic world. A number of paleobiogeographic regions are distinguished within the belt (in the USSR, the Baltic, Kazakhstan, including the Tien Shan, Siberian, Kolyma). The second belt combined the Ordovician circumpolar regions with a cold climate. It covered Southern Europe, Africa, southern Asia (in particular, the Pamirs), apparently, Australia and South America. The organic world of this belt was distinguished by a poor composition. In Africa, southern Europe and South America, signs of Ordovician glaciation were found within this belt.

Ordovician deposits on the territory of the former USSR are widespread within the East European and Siberian platforms, in the folded systems of the Urals, Pai-Khoi and Novaya Zemlya, on the islands of Severnaya Zemlya and Novosibirsk, in Kazakhstan, Central Asia, Altai-Sayan region and in the north- east of Russia.

Cross sections of the southern coast of the Baltic Sea are considered classic. With disagreement, Cambrian sands and sandstones are covered by sandstone sandstones, and then shale with graptolites ( dictionem shale ), belonging to the Tremadoc tier of the Lower Ordovician, are overlain by low-power glauconite sands and sandstones, gradually turning into marl and limestone with marl. The rocks of the Upper and Middle Ordovician are represented by limestone and dolomites. At the level of the Llandailian tier there are packs of oil shale - kukersitov. The thickness of the Ordovician system on the platform does not exceed 200-250 m. On the western slope of the Urals, on Pai Khoi and on Novaya Zemlya (in the western myogeosynclinal zone of the Ural-Siberian geosynclinal belt), the deposits of the Ordovician system consist of marine terrigenous sediments and limestones. In places, their thickness reaches 3800 m. In the inner, eugeosynclinal, part of the Ural-Siberian belt (on the eastern slope of the Urals, in the eastern half of Kazakhstan, in the Middle and Northern Tien Shan), a number of zones with different composition of marine sediments with a thickness of up to 10 thousand are established. m. In the Ural-Siberian belt, in the Altai-Sayan myogeosynclinal region, green-colored terrigenous sediments with a thickness of about 3500–4500 m prevail. Powerful strata appear in the Middle and Upper Ordovician in Gorny Altai, Salair, Gornaya Shoriya and Kuznetsk Alatau Vestniks, in Tuva, red-colored coarse clastic sediments are known. In the Urals, in the Altai-Sayan region, in Kazakhstan and the Tien Shan, intrusive Ordovician rocks are widespread. On the Siberian platform, the Ordovician system is characterized by variability and diversity of rock composition. Along with limestones and dolomites, red-colored and variegated sandy-clay sediments, with places with gypsum-bearing and saline interlayers, are widespread here. The thickness of the sediments usually does not exceed 500 m, but in some places along the periphery of the platform they reach 1,500-1,700 m. In the Verkhoyansk-Chukotka geosynclinal, the Ordovician system is exposed mainly in the Kolyma massif and is represented by three types of marine deposits - limestone (up to 500 m thick), terrigenous ( with a capacity of not more than 1000 m) and volcanic sedimentary (with a capacity of up to 2500 m).

In platform sediments in Estonia and in the Leningrad Region, oil shale (Kukersites) are developed; phosphorites are also known in the same place, as well as on the Siberian platform and in Kazakhstan. Small geosynclinal-siliceous-siliceous deposits are associated with small deposits of iron and manganese ores in North America, Western Europe, Kazakhstan, China and others. Gold and other metals are associated with Ordovician intrusions in Kazakhstan. In North America, oil deposits are known in the Ordovician deposits.

• Ordovician-Silurian extinction

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• Ordovician system (period)
• Ordovician system : Geological dictionary VSEGEI