Ordovician system (period). Ordovician and Silurian periods Ordovician period characteristics

ORDOVICAN PERIOD
505-430 million liters n.

General characteristics, stratigraphic divisions and stratotypes.

The Ordovician system got its name from the Ordovician tribe, who inhabited Wales (Great Britain) in antiquity. Initially, the Ordovician deposits were included in the previously identified Silurian system. For a long time the Ordovician was considered as the lower section of the Silrian system, and its upper section was the Gotlandian (along the island of Gotland in the Baltic Sea). The name "Ordovician system" was first proposed by C. Lapworth in 1879. In the domestic geological literature, A.F. Lesnikova and D.V. Nalivkin in the 30s advocated the independence of the Ordovician and Silurian systems (the latter in the volume gotland). In 1951, the Ordovician system was officially identified on the state geological maps of the USSR. However, only in 1960 the independence and the name of the Ordovician and Silurian systems were approved at the XXI session of the International Geological Congress in Copenhagen. The Ordovician was established from typical sections in the Arenig Bala region in northern Wales. Initially, the boundaries of the Ordovician and its subdivisions were determined by changes in shell fauna complexes, and much later, rapidly evolving graptolites began to be used for these purposes. Therefore, to date, there are two stratigraphic scales that have not yet been finally linked to each other, and this makes it difficult to correlate sections of Wales with sections of other regions, dissected by the remains of graptolites.

In Great Britain, the lower boundary of the Ordovician is drawn along the foot of the Arenig, since the trema-dock here is closely related to the Cambrian. In other European countries and in Russia, the Tremadocian is considered the lower stage of the Ordovician. The upper boundary of the system formally coincides with the top of the Ashgilian Stage. However, it should be noted that the position of both the lower and upper boundaries of the Ordovician is debatable and has not received official international recognition.

The Ordovician tiered and zonal scales are based on graptolites, and the beginning of the Ordovician division was laid down by the works of C. Lapworth and G. Elles. C. Lapworth proposed a three-term division of the Ordo-Vic, but more often used a two-term one. This is associated with certain difficulties of dismemberment and is the subject of discussion. In Russia, a three-term division of the Ordovician is accepted, although the boundaries between the divisions are in some cases not well defined.

The stratotype of the Tremadocian Stage is located in Caernarvonshire. Its volume was established by A. Sedgwick, who attributed this stage to the Cambrian. The stratotype section of the Arenigian Stage is located in the Arenig Mountains of North Wales. It is also installed by A. Sedgwick. The stratotype is incomplete and poorly characterized by fauna.

The Llanvirn Stage has been described from Pembrokeshire in West Wales. Shales with numerous graptolites are common here. Among them, the genus Didymograptus is the most characteristic. The Llandale Stage of the Middle Ordovician in Caernarvonshire, Wales, is composed of platy limestones with Glyptograptus and Nemagraptus.

In the West of England, in Shropshire, there is a stratotype of the Caradocian Stage. Quartz sandstones and quartzites containing Dicranagraptus, Climacograptus are developed here.

The Ashgill Stage takes its name from the Ash Gill Creek in Lancashire in Northern England. Here a series of slates with Dicellagraptus is exposed. Characteristic sections of the Ordovician and Silurian are shown in Scheme III, col. incl.

organic world

In contrast to the Cambrian, life in the Ordovician was much more diverse. AT flora algae dominated, including green ones. A representative of green algae (or cyanobionts?) - the genus Gloeocapsomorpha played an important role in the formation of kukersite oil shales.

Of great importance for the zonal stratigraphy of the Ordovician are graptolites belonging to the Hemichordata type (semi-chordate). Graptolites in the Ordovician evolved rapidly, had significant ranges and are therefore guide fossils. The early Ordovician is characterized by axisless forms (Phyllograptus, Didymograptus), while the middle and late Ordovician are characterized by autumn two-row graptolites (Diplograptus, Climacograptus).

Conodonts, which appeared in the middle Cambrian, were very widespread in the Ordovician. Conodonts belong to the primitive chordates and represent a semblance of the jaw apparatus of these animals in the form of teeth of microscopic size and various shapes: simple ("fangs"), rod-shaped and platform. Conodonts lived in a wide variety of marine environments, from deep water (preferably) to shallow water.

The fauna of the seas is represented by invertebrates, as well as jawless fish-like organisms (thelodonts). Especially widespread were trilobites, sea bladders, brachiopods, cephalopods from the subclasses of endoceratoids and nautiloids, coral polyps from the subclasses of four-beams (rugoses) and tabulatomorphs.

Trilobites are represented mainly by new genera. The most important of them are Asaphus, Trinucleus, Megistaspis, Illaenus and others. Trilobites acquired the ability to fold due to the fact that predators appeared - cephalopods. As a consequence of this - the development of equal-sized and close in shape head and tail shields. Brachiopods are represented by both hingeless forms with chitin-phosphate and hinge forms with a calcareous shell. The genus Obolus is known from the non-locking ones (but other species than in the Cambrian); from castle-Porambonites.

Of the endoceratoids, orthoceratoids, and other similar straight-shelled cephalopods that lived in seas with normal salinity, the Ordovician is especially characterized by large forms of the genus Endoceras, as well as Orthoceras, Actinoceras, and representatives of various orders of nautiloids. They led a near-bottom, active lifestyle. The shells of these predators reached a length of 2-3 m (Endoceras). In the Ordovician, the development of coelenterates - tabulate (genus Syringopora) and four-beam corals (rugos) began, which, together with hydroid polyps- stromatoporates (Stromatoporata) - were not only guiding, but also rock-forming organisms. Together with bryozoans and corals, they built reefs. From the echinoderms in the bottom biocenoses, sea bladders (cystoids) developed, which were joined by crinoids (crinoids) from the Middle Ordovician.

These are the main groups of invertebrates. In addition to them, other groups of fauna existed in the seas of the Ordovician, which did not enjoy such a wide development. These include fora minifers, radiolarians, ostracods, sponges, worms, bivalves, gastropods, bryozoans, etc.

structures earth's crust and paleogeography

The same platforms and geosynclinal belts existed in the Ordovician as at the end of the Cambrian period. In geosynclinal troughs, intensive subsidence continued, which favored the accumulation of many kilometers of strata of predominantly terrigenous marine sediments and effusives.

At the end of the Ordovician, in a number of geosynclinal regions, the second phase of the Caledonian epoch of tectogenesis, the Taconian, began. It appeared in approximately the same areas. northern hemisphere, where the Salair phase of folding took place. In connection with the Taconian phase of folding, some parts of the geosynclinal regions turned into high-elevated mountain structures, some of which existed for a very long time (the Northern Appalachians, the northern ranges of the Tien Shan), and others at the beginning of the Silurian again sank below sea level (Wales in Great Britain).

The regression of the seas of the end of the Cambrian with the onset of the Ordovician was replaced by a new general transgression. The area of ​​the epicontinental seas expanded so much that the Ordovician transgression on the platforms turned out to be the largest in the entire history of the Paleozoic (thalassocratic epoch). However, this transgression was not the same on all ancient platforms. If the Ordovician transgression on the North American platform exceeded the Cambrian by many times and almost covered the entire territory, then on the Siberian and East European it was weaker than the Cambrian. The expansion of the epicontinental seas also occurred in Gondwana.

By the end of the period, due to mountain building in a number of geosynclinal systems, and especially those adjacent to platforms, both geosynclinal and epicontinental seas are shrinking.

For the Ordovician, according to the study of paleomagnetism of rocks, the same plan for the location of the poles is preserved and, accordingly, climatic zones, as in the Cambrian. Obviously, the wide development of transgressions in the Northern Hemisphere softened here climatic conditions. Tropical wet zone was located in a strip stretching from southern Greenland through Novaya Zemlya to Western Siberia. It is characteristic that all the warm zones at that time were shifted far to the north in comparison with the modern position of the equator.

The position of the continents in the Late Ordovician, according to the concept of a new global tectonics, is shown in Scheme XVI, col. incl.

History of platform development

East European (Russian) platform

The Ordovician deposits are distributed in the same place as the Cambrian, that is, in the Baltic, Dniester and Moscow syneclise, and are represented by all three divisions. They occur with stratigraphic unconformity in the Cambrian. These are horizontally lying strata of marine shallow-water sediments of small thickness (~300 m), containing rich marine fauna of the Western European type, which indicates a transgression coming from the west (see diagram III, col. inc.). It begins. The section of Estonia in coastal cliffs and ledges with obol sandstones. A huge amount of phosphate valves of the genus Obolus turned this sequence into a phosphorus-bearing horizon, developed in Estonia and the Leningrad region. Above - black graptolitic mudstones with dicyonems. Both sequences make up the Tremadocian Stage. The upper part of the Lower Ordovician - the Arenigian Stage - is composed of glauconite sandstones and limestones with numerous remains of brachiopods and trilobites (Asaphus, Megistaspis). The Middle Ordovician (up to 160 m thick) is represented by limestones with a rich fauna of brachiopods, trilobites, graptolites, and ostracods. There are strata of combustible shale - kukersites. They testify to temporary uplifts and shallowing of the warm Ordovician sea, its overgrowing with blue-green algae (cyanobionts), from which combustible shale - kukersites were formed. The Upper Ordovician is again composed of limestones with fauna. Ordovician limestones are widely used for a variety of building purposes. The Estonian section is typically platform-like, composed of marine shallow-water sediments, which are good building materials (the Old City Hall and other buildings in Tallinn).

Siberian platform

The Ordovician occupies the western part of the platform and is exposed along the margins of the Tunguska syneclise and in the southwest of the platform. The sections differ in lithology and paleontological characteristics. There is a dominance of carbonate rocks with the remains of a diverse marine fauna, especially brachiopods. On the margins of the basin, shallow-water sediments were deposited: dolomite silts, variegated sands and clays, sometimes with interlayers of gypsum. The sections show a regional hiatus before the Middle Ordovician. The thickness of the deposits is several hundred meters.

Chinese platform

Lower and Middle Ordovician sandy-argillaceous and carbonate deposits several hundred meters thick with remains of brachiopods, gastropods, and nautiloids are widespread here.

North American Platform

At the beginning of the Ordovician, the greatest transgression occurred here, during which carbonate sediments accumulated. At the beginning of the Middle Ordovician there was a short-term regression and islands appeared. In the late Ordovician, the platform began to sink again, and limestone and dolomite oozes were deposited. In the east, clastic material began to enter the sea - the products of destruction of the Taconian uplifts in the Appalachian geosyncline. The thickness of the Ordovician is the first hundreds of meters.

gondwana

In the South American part of Gondwana, uplifts dominated in the Ordovician. Marine clastic deposits are found in the extreme west along the border with the East Pacific geosynclinal region. Sandy-argillaceous deposits of small thickness are known in the Amazon Basin. The African part of Gondwana began to sink in the north at the end of the Cambrian. In the Ordovician, marine quartz sands with interlayers of pebbles and clays were deposited on the territory of the Sahara. They lie directly on the Precambrian basement. The thickness of the strata is 500-800 m, in avla-cogens 2-2.5 km. On the Arabian Peninsula, the Ordovician is represented by sandy-clayey formations of considerable thickness. In the Australian part of Gondwana, a large area of ​​the sea occupied little in the Ordovician. It flooded the central region and spread in the latitudinal direction. Sands were deposited here, less often calcareous silts.

North Atlantic geosynclinal belt

Grampian geosynclinal region. Grampian geosyncline. Within this geosyncline, thick strata of sedimentary and volcanogenic rocks accumulated. The Ordovician section of Wales is stratotypical and is exposed in many parts of this area (see Scheme III, col. inc). The lowest, Tremadocian, schistose mudstones with Dictyonema and trilobites, are overlain by Areniga rocks with distinct unconformity. Therefore, English geologists attribute the tremadoc to the Cambrian. The Arenig Stage is composed of volcanic rocks with intercalations of limestones with trilobites and brachiopods (the thickness of the Lower Ordovician is 1.2 km).

Llanvirn consists of shales with remains of trilobites, brachiopods, and graptolites. Shales are sometimes replaced by effusives along the strike. The Llandale Stage is the most carbonate part of the Ordovician section - platy limestones with brachiopod and trilobite shells. The Caradoksian Stage - carbonate-argillaceous deposits with brachiopods and graptolites or effusives (Middle Ordovician thickness - 2 km). At the end of the Ordovician, volcanic activity ceased and Ashgill is represented by polymictic sandstones, cross-bedded, with signs of ripples and shale (thickness - 1 km).

Altai-Sayan geosynclinal region. The Salair cycle of tectogenesis, which manifested itself in this area in the Middle Cambrian, did not fully stabilize it. Geosynclinal conditions in the Ordovician are restored in the West Sayan and Gorno-Altai troughs, which are separated by the Gorno-Shor uplift. But flysch formations are already accumulating in the Ordovician troughs (thickness - 7-8 km).

On the uplifts, there is a different type of sections: less thickness, sediments - carbonate silts, sands with an abundance of shallow-water fauna. In the Ordovician deposits, breaks in sedimentation are known (these are manifestations of Caledonian movements). It should be noted that in the Altai-Sayan region there is a sharp angular unconformity between the Cambrian and Ordovician deposits. This is the result of the Salair phase of folding.

In general, not the entire territory of the Ural-Mongolian geosynclinal belt in the Ordovician was occupied by the sea. Within ere there were Early Caledonian uplifts and islands that supplied terrigenous material to depression zones. Such uplifts were in Central Kazakhstan and in the east of the belt - in the Altai-Sayan region and Mongolia. At the end of the Ordovician period, in the west of the Central Asian part of the Ural-Mongolian belt, the Taconian phase of the Caledonian folding was actively manifested. Its consequence was the formation of extensive uplifts in Kazakhstan, traced from Kokchetav south to the Tien Shan and were the area of ​​removal of terrigenous material in the Silurian. Caledonian folding in the southern and eastern parts of the Ural-Mongolian geosynclinal belt was accompanied by intrusive magmatism. The Takonian phase is associated with the intrusion of large granitoid intrusions over a vast territory from the Northern Tien Shan to Petropavlovsk and Omsk.

In the European geosynclinal region, the Ordovician deposits are more widespread than the Cambrian ones. They are known in the north of Europe, where they are represented by marine sandstones, shales with intercalations of limestones or effusive horizons. The Franco-Czech massif (Moldanub block) in the Ordovician was a huge island, and marine terrigenous deposits with interlayers of siliceous and effusive rocks accumulated on its eastern margin in the Czech Republic. The section of these strata became classic in the 19th century thanks to the works of I. Barrand. The section begins with conglomerates, Diccyonema schists, and sandstones unconformably overlying Cambrian rocks (Scheme III, col. inc). Above, there are sandstones and shales with trilobites, graptolites, and quartzites with brachiopod shells. In southern Europe, the Ordovician conformably overlies the Cambrian and is represented by marine terrigenous facies of a typical geosynclinal appearance, but there are no effusives. In the Asian region, geosynclinal conditions are also observed with a set of corresponding facies.

Pacific geosynclinal belt

The entire Early Paleozoic is represented by marine facies. In the Ordovician Verkhoyansk geosyncline, there was a vast marine basin with an archipelago of islands, where terrigenous deposits accumulated. The maximum transgression falls on the Middle Ordovician. In the Cordillera Coy and Andean geosynclines, sea ​​conditions with accumulation of terrigenous rocks. This is the early geosynclinal stage of development of this territory.

Minerals

In the Ordovician, the productive horizons of the US Midcontinent (Kansas and Oklahoma) are known, which provide a third of the annual oil production. A large oil field was discovered in the Algerian Sahara in the Cambrian and Ordovician. There are signs of oil on the Siberian platform. Sedimentary uranium is known in the shales of the Lower Ordovician in Sweden. The middle Ordovician includes oil shales - kukersites of the Baltic states (Estonia) and the Leningrad region. In the Ordovician, deposits of oolitic iron ore on the island of Newfoundland in Canada, as well as in Argentina and a number of Western European countries. Ordovician magmatism is associated with copper and cobalt deposits in Norway, polymetals of the Salair Ridge and gold in Kazakhstan. The tremadok (obol horizon) of the Baltic states includes actively developed phosphorite deposits.

History of the development of geosynclinal belts

Grampian geosynclinal region. Grampian geosyncline. A section of the Silurian of Wales, the stratotype locality where the Silurian system was identified, can be seen in Scheme III, col. incl.

The Silurian rests on the Ordovician with a structural unconformity caused by the Taconian orogeny. At the base of the Llandovery lie conglomerates and sandstones, above which are replaced by a sandy-clayey stratum with shell rocks; Pentamerides are numerous (the thickness of Llandovery reaches 1.5 km). Wenlock is lithologically diverse: in some areas there are calcareous-argillaceous rocks and limestones with remains of brachiopods and corals (300-400 m), in others there is a thick layer of sandstones and siltstones (thickness -1.2 km). The Ludlov deposits are predominantly carbonate: limestone, calcareous shales, calcareous siltstones. There are numerous stromatoporates, corals, brachiopods (thickness - 0.5 km). There are fossil banks with Conchidium knighti. In the upper part of the stage, there is a layer of the so-called bone-bearing breccia, which consists of parts and fragments of the bone cover of armored fish.

The described section of three tiers refers to "shell" formations - shallow-water deposits of considerable thickness containing the indicated fauna.

Another type of section of the same stages is also known - in the form of a thin stratum of graptolitic shales. Clay material in this case was deposited in the deep sea areas. The third type of incision is mixed. It contains rocks of the first and second types.

The uppermost part of the Silurian section in England is distinguished as the Downton Stage (thickness -0.6-0.9 km). These are red and variegated sandy-argillaceous rocks with interlayers of red marls. They contain shells of ostracods and ichthyofauna. Downton is gradually replaced by the lower red-colored Devonian. All this is overlapped with structural unconformity by Middle Devonian conglomerates.

In Wales, the total thickness of the Silurian is 3 km. The deposits are folded and metamorphosed. Caledonian folding manifested itself repeatedly and was accompanied by magmatism.

In the Scandinavian part of the Grampian geosyncline, thick clastic strata accumulated, at first typically marine, and towards the end of the Silurian - continental.

Ural-Mongolian geosynclinal belt

The Ural-Tien Shan geosynclinal region extends from Novaya Zemlya to the southern Tien Shan.

Ural geosyncline. Silurian deposits are widely developed in the Urals. On the western slope of the Urals, carbonate and terrigenous sediments (up to 2 km) accumulated quietly under miogeosynclinal conditions. On the eastern slope, in the eugeosyncline, lavas and tuffs, siliceous shales and limestones accumulate (thickness - 5 km). In the Silurian in the Urals, the main geotectonic structures were laid, which later turned into the existing anticlinoria and synclinoria. The Silurian of the Urals of the western and eastern slopes contains the same fauna, which indicates a single geosynclinal Ural basin in the Silurian. On the territory of the western slope of the Urals and on Novaya Zemlya, miogeosynclinal conditions prevailed, so carbonate and carbonate-argillaceous deposits (500-1500 m) with a diverse complex of organic remains accumulated here. Shallow coastal sand and pebble rocks are known on the western outskirts of the Northern Urals (Polyudov Ridge). In the west of the central part of the Urals, on Pai-Khoi and in places on Novaya Zemlya, black clayey graptolite shales are exposed.

Caledonian folding, in contrast to other geosynclines of the Ural-Mongolian belt, is not typical for the Urals; it did not cause structural unconformities, but the ultrabasic and basic intrusions of the central zone are considered Caledonian.

Silurian deposits are widespread in the Kazakh part of the Ural-Mongolian belt. They are represented by typical geosynclinal formations of considerable thickness with the remains of a rich fauna. Horizons of brachiopod and coral limestones are characteristic.

In the context of Mt. Chingiztau Silur is represented only by the lower section. Silurian deposits (up to 2.5 km) accumulated in eugeosynclinal marine conditions with strong volcanism. Actively manifested Caledonian folding. The most pronounced is the last - Late Caledonian - phase of folding, which led to the retreat of the sea from the territory of the Chingiztau Ridge, to the completion of the first, actually geosynclinal, stage of its development. The gently dipping Lower and Middle Devonian effusive rocks and felsic tuffs crowning the section accumulated already in terrestrial conditions. They are usually identified as volcanogenic molasses of the orogenic stage of development. The repeated intrusion of large granitoid intrusions is associated with folding.

Altai-Sayan folded region. Silurian deposits are known in the same place as the Ordovician, but in the west limestones and terrigenous rocks with rich fauna predominate, in the east (Western Sayan, Tuva) the role of coarse clastic rocks with depleted fauna increases. The thickness of the Silurian deposits in the west is 4.5 km, in the east - up to 7.5 km.

In the Silurian section of Western Tuva (see Scheme III, color incl.), the Silurian deposits (Chergak Group) overlie the Ordovician. They have a large thickness (2.5-3 km), consist of sandy-argillaceous rocks with interlayers, packs and lenses of limestones. The highest carbonate content is confined to the middle part of the section. The fauna is rich and varied. These are stromatoporates, tabulates, heliolithids, rugoses, crinoids, bryozoans, brachiopods, trilobites. Many local (endemic) forms. Evidently, in the Silurian there existed a shallow marine basin with small reefs, coral and crinoid thickets, and banks of brachiopods. The endemism of the fauna speaks of the difficult communication with other seas. By the end of the Silurian, the basin gradually shrank, became shallow, its salinity changed, and only euryhaline organisms survived in it.

In the Ordovician, Silurian and early Devonian in Western Tuva, a single huge (10 km) transgressive-regressive Tuva complex was formed with marine deposits in the middle part and red-colored continental rocks in the sole and roof. The deposits of the Tuva complex are folded and intruded by small basic and felsic intrusions. The upper part of the section under consideration is composed of thick Lower Devonian terrestrial effusive rocks and red clastic rocks of the Middle Devonian. These are continental deposits of intermountain depressions formed during the regression caused by the Caledonian orogeny. In the section of Western Tuva, three structural stages sharply differing from each other are clearly distinguished: the first is the Lower Cambrian; the second - Ordovician, Silurian, lower Devonian; the third is the upper part of the Lower Devonian and the Middle Devonian. The floors record different stages of geological development: the first is eugeosynclinal, the third is orogenic, and the second is intermediate (transitional). At the second stage, the subsidence developed on the already consolidated basement, the regime was reminiscent of miogeosynclinal. Ore deposits of iron and copper are associated with acid intrusions.

Thus, the Caledonian epoch of tectogenesis covered the regions of northwestern Kazakhstan, partly Gorny Altai, northern Tien Shan and eastern part Altai-Sayan folded region - Western Sayan and Tuva, where the Caledonides arose.

Mediterranean geosynclinal belt

In the European part of this belt, conditions are preserved that are close to those previously described in the Ordovician. This is still the insular land of the Franco-Czech massif (Moldanub block) and marine conditions to the north and south of it (Prague synclinorium, see scheme III, color incl.). In northern Europe, sandstones, black shales, bituminous limestones (0.5 km thick) accumulate, siliceous shales appear, due to manifestations of underwater volcanic activity. In southern Europe, between the Franco-Bohemian Massif and the Atlas Mountains in Africa, the Silurian is represented by monotonous facies: black shales with graptolites, which are replaced by limestones at the top of the section.

In the Asian geosynclinal region, the Silurian is known in Turkey, the Caucasus, in the mountain structures of Iran, Afghanistan, and the Pamirs. Here, under eugeosynclinal conditions, thick strata of terrigenous rocks and volcanics of basic and felsic composition, or small terrigenous-carbonate facies in miogeosynclinal zones (Zagros Himalayas, etc.) accumulated.

Minerals

Rock salt deposits, commercial oil and gas deposits are known on the North American (Canadian) and Siberian platforms. In the Silurian, deposits of oolitic iron ores Clinton (USA) and a number of small ones in Africa were formed. The Caledonian acid intrusions are associated with the gold deposits of Northern Kazakhstan, Kuznetsk Alatau and Gornaya Shoria.

In late Caledonian intrusions in the Scandinavian mountains, iron, copper, and chromite were found: Nickel, platinum, asbestos, and jasper are known in the Urals. Deposits of rare metals in the Appalachians and Eastern Siberia are associated with pegmatites. Silurian limestones are a building material and a good ceramic raw material.

This period was a time when invertebrates were still the undisputed masters of the ocean floor. Some of them were able to move, others lived alone or in groups, being tied to the bottom. These sedentary or immobile animals collected food that was within reach and did not need a developed brain. But life made more severe and harsh demands on mobile animals. In search of food, they relied on their senses and quick reactions to avoid being attacked by other predators.
Found in South Africa in the early 1990s, a promissum specimen was a giant conodont bearer that reached 40 cm in length. Its bulging eyes indicate that it actively hunted its prey.

armed arthropods.

When the first arthropods appeared (at the beginning), their bodies were very small, and their shells (external skeleton) were no thicker than a sheet of paper. But by the beginning of the Ordovician period, in some arthropods, the shell developed, turning into real armor to protect against enemies. One of the groups of arthropods that possessed such shells and was numerous in the Ordovician period was the "horseshoe crabs", or horseshoe crabs.
Despite the name, these animals were not actually crabs. They belonged to the chelicerae, which include spiders and scorpions. The front part of their body was protected by a domed shield, which completely hid the mouth opening and paws of these animals. The back of the body was protected by a second, smaller shield and ended in a long, sharp spike. Their shells are well preserved in sedimentary rocks, but there is a much easier way to see these animals, because they have survived to this day. These are not the same species that existed in the Ordovician period, but over 400 million years these animals have changed very little.
They fed on small animals, using their limbs ending in claws to capture prey. These claws were hidden deep under the front body shield, which limited their size. Some close relatives of horseshoe crabs - eurypterids, or crustaceans, had claws that were exposed forward. During the Ordovician, most crustaceans were relatively small in size, but in the subsequent Silurian period, they became the largest arthropods.
Arandaspis belonged to heterostracans, or fish-like fish-like fish that did not have jaws. He moved in the water by moving his tail. He didn't have fins.

The first horseshoe crabs moved along the seabed on five pairs of paws. Today, five species of these "living fossils" exist on the east coast of North America and Asia.

Mysterious conodonts.

For more than a century, scientists have collected and systematized many fossils that look like tiny teeth, which date back to Ordovician period or even considered even more ancient. They are known under the name conodonts because they are often cone shaped. These formations obviously belonged to some animals. Over time, the shape of the conodonts changed. Almost every type of conodont corresponds to a certain time, so geologists can determine the age of rocks by the appearance of conodonts. Despite many years of searching, it was not possible to find the animals that belonged to these conical miniature teeth.
But in 1993, fossilized animal carcasses with conical teeth were found in Scotland. Then the same fossils were found in North America and South Africa. One of the found species is promissum. The mysterious animal had a thin, serpentine body and well-developed eyes. In some fossils, traces of V-shaped muscles and a notochord were found. This is already a feature of vertebrates and related vertebrates.
Many scientists think that conodont bearers were among the first vertebrates in evolution. However, unlike other vertebrates from which tetrapods evolved, conodont bearers did not survive.

During the Ordovician period, something very unusual happened: a completely new group of animals arose, one of the very few that appeared after the Cambrian extinction. These animals, named bryozoans, were tiny invertebrates protected by a skeleton composed of cells. They lived in colonies next to each other and often resembled plants in their shape. bryozoans turned out to be a very successful addition to the animal world and not only survived to this day, but are also widespread.
Sea bottom Ordovician period was home to many larger plant-like animals known as crinoids, or sea ​​lilies. They belong to the same type of animal as the starfish, the sea urchin; sea ​​lily has a long stem, consisting of calcareous segments, and a "crown" of fragile branching tentacles that grab food. Later, some crinoids moved from a static existence to a mobile lifestyle in the sea, where food is not just expected, but sought and fought for. At present, sea lilies attached to the bottom still exist in nature.

This Ordovician reef has been reconstructed from nearly 500 million year old fossils found on Newfoundland. Two nautiloids scour the seafloor while trilobites and gastropods, or gastropods, crawl along the seafloor below them. 1. Nautiloids with straight shells; 2. Nautiloids with spirally twisted shells; 3. Trilobites; 4. Gastropods; 5. Corals; 6. Sea lilies.

Ordovician system (period), or for short Ordovician, is the second from the bottom system of the Paleozoic group, corresponding to the second period of the Paleozoic era of the geological history of the Earth. It is underlain by deposits of the Cambrian and overlain by deposits of the Silurian systems. The beginning of the Ordovician period by radiological methods is determined by the age of 488 million years from the present; the total duration of the period is about 45 million years.

Story

The name was proposed by the English geologist C. Lapworth (1879), who indicated the section in the area of ​​Arenig and Bala in Wales as a type. It is named after the ancient Ordovician tribe that lived in Wales. 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 on the territory of the USSR is associated with the names of F. B. Schmidt, V. V. Lamansky, V. N. Weber, B. S. Sokolov, T. N. Alikhova, O. I. Nikiforova, A. M. Obut , R. M. Myannil, A. K. Ryymusoks 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. Havlicek), American (J. Hall, G. A. Cooper, M. Kei), Swedish (V. Jaanusson), Japanese (T. Kobayashi) and other scientists.

Subdivision of the Ordovician system

The Ordovician system is subdivided into 3 divisions:

With this subdivision, the lower and middle substages of the Caradocian Stage usually belong to the middle section, and the upper substage to the upper one. With a two-term division of the Ordovician system, the boundary of the divisions is drawn between the Llanvirnian and Llandale stages. In Great Britain, the lower boundary of the Ordovician system is drawn at the base of the Arenigian, while the Tremadocian is Cambrian. The most subdivisions used in the subdivision and correlation of Ordovician deposits are the graptolitic zones.

general characteristics

The Ordovician system is distinguished on all continents and on many islands. It participates in the structure of the platform cover of the East European, Siberian, North American and Chinese platforms, is exposed along the western, northern and eastern framing of the ancient Gondwana platform - in Bolivia and Argentina, in northern and southern Africa, in eastern Australia, in Antarctica, and is widespread in all folded systems located between these platforms. In most areas, the Ordovician sediments are closely related to the Cambrian ones, but in some places on the border of the Cambrian and Ordovician there are breaks in sedimentation due to short-term regression of the sea. The maximum expansion of marine spaces - the transgression of the sea - falls on the Middle Ordovician. Then comes the regressive stage. In relatively shallow epicontinental seas spreading on platforms, mainly thin (on average up to 500 m) calcareous, less often sandy-argillaceous sediments accumulated. In the transitional areas between platforms and geosynclines - in the miogeosynclinal zones of the Appalachians, the western slope of the Urals, the Altai-Sayan region, etc., the thickness of the sediments increases to 3,500 m; clastic sediments are widespread along with carbonate deposits.

Sources:

• Mining encyclopedia, in 5 t. M., publishing house " Soviet Encyclopedia", 1987, editor-in-chief E.A. Kozlovsky

; in the stratigraphic (geochronological) scale, it follows (period) and precedes the Silurian system (period). The beginning of the Ordovician period is dated by radiological methods at 490 ± 15, and the end at 435 ± 10 million years from the present; the total duration of the period is about 65 million years.

The Ordovician system was established by the English geologist C. Lapworth in 1879 in Great Britain; as a typical section, deposits of the Areniga and Bala region in Wales. The question of the independence of the Ordovician system was finally resolved only 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. On the territory of the Ordovician system deposits were studied by F. B. Schmidt, V. V. Lamansky, B. S. Sokolov, V. N. Weber, T. N. Alikhova, O. I. Nikiforova, B. M. Keller, A. M. Obut, R. M. Myannil, A. K. Ryymusoks and many others. Other works of English geologists C. Lapworth, R. Murchison, H. B. Whittington, A. Williams, Czech geologists - J. Barrand and V. Havlicek, American - J. Hall, G. Cooper, M. Kay are known from foreign studies. , Swedish - V. Jaanusson, S. Bergström, Japanese - T. Kobayashi and others.

Subdivisions. A generally accepted division of the Ordovician system into divisions and stages still does not exist. The most common in the CCCP and other countries is a division into 3 departments and 6 tiers (table). In the two-term division of the Ordovician system, the boundaries of the divisions are drawn between the Llanvirnian and Llandale stages. In Great Britain, the lower boundary of the Ordovician system is drawn at the base of the Arenigian, while the Tremadocian is Cambrian. The most subdivisions used in the subdivision and correlation of Ordovician deposits are the graptolitic and conodont zones.

general characteristics. The Ordovician system is distinguished on all continents and on many islands. It participates in the structure of the platform cover, and is exposed along the western, northern and eastern framing of the ancient Gondwana platform - in Bolivia and Argentina, in northern and southern Africa, in eastern Australia, in Antarctica, and is widespread in all fold systems located between these platforms. In most areas, the Ordovician sediments are closely related to the Cambrian ones, but in some places on the border of the Cambrian and Ordovician there are breaks in sedimentation due to short-term regression of the sea. The maximum expansion of maritime spaces—the transgression of the sea—occurs in the Middle Ordovician. Then comes the regressive stage. In relatively shallow epicontinental seas spreading on platforms, mainly thin (on average up to 500 m) calcareous, less often sandy-argillaceous sediments accumulated. In the transitional areas between platforms and geosynclines - in the mirgeosynclinal zones of the Appalachians, the western slope of the Urals, the Altai-Sayan region, etc., the thickness of the sediments increases to 3500 m; clastic sediments are widespread along with carbonate deposits.

In the USSR, the Ordovician system is widespread on the East European and Siberian platforms, on Taimyr, in the fold systems of the Urals, Pai-Khoi, Novaya Zemlya, on the islands of Severnaya Zemlya and on the New Siberian Islands, in the Middle, Altai-Sayan region and in the north east of the CCCP.

On the East European Platform, the Ordovician system is best exposed and studied in Northern Estonia and the Leningrad region, i.e. along the klint (classical sections of the Ordovician system).

organic world. Representatives of almost all types and most classes of marine invertebrates are found in the Ordovician system, and bacteria and algae are widespread among plants, the first vertebrates and land plants appear. Radiolarians and foraminifers, graptolites, chitinozoans and conodonts lived in the water column of the Ordovician oceans and seas. Numerous and diverse trilobites, brachiopods, sea lilies, bryozoans, sponges, laminabranchs, gastropods, and cephalopods, as well as calcareous green and red algae settled at the bottom of shallow seas, in coastal zones and on shallows. In shallow seas, in zones of reefs and organogenic structures, in shallow water, solitary and colonial corals lived. The most important groups of fauna for the stratigraphic division of the Ordovician system are graptolites, conodonts, trilobites, brachiopods, and colonial corals.

Minerals. Among the mineral deposits occurring in the Ordovician system, the most important are deposits of oil and gas (especially in North America), oil shale, phosphorites, and ores, the formation of which is due to Ordovician magmatism. Industrial accumulations and manifestations of oil and gas are mainly associated with platforms and with their folded framing: on the East European platform (in the CCCP - Baltic and), on the North American platform (in the arched rises of Kansas, Seminole, Chottokwa, Cincinnati, Bend, Forest depressions -City, Salina, Dodge City, Michigan, Illinois, Perm; in Canada - the Eastern Oil and Gas Province) and in its frame (folded structures of the Rocky Mountains, Ouachita - Ouachita), in the north of the African Platform (Libya, Algeria, Morocco) and on Australian platform (depressions of the central part of the Australian plate). Of industrial importance are the Middle Ordovician