Main tectonic element North America- North Amer and Kan platform with Canadian crystal shield within the platform, a number of large tectonic elements are distinguished that control the location of oil and gas provinces and regions (Fig. 54)

In the inner part of the platform, Paleozoic oil and gas bearing provinces are distinguished, within which oil and gas bearing areas associated with tectonic elements are isolated: with the arched uplifts of Cincinnati, Central Kansas, etc.; with intraplatform depressions Illinois, Michigan, Permian basin. In the junction zones of the platform with folded belts, the Paleozoic Appalachian in the east and the Paleozoic-Mesozoic of the Rocky Mountains in the west stand out. In the southwest of the continent, the Gulf Coast (Gulf of Mexico) province is distinguished, which is a passive continental margin of the North American continent that has been developing since the Lower Mesozoic. On the Pacific coast

arctic ocean

Rice. 54. Scheme of tectonic and petrogeological zoning of North America (according to Yu.N. Uspenskaya).

1 - outcrops of the Precambrian crystalline basement, 2 - area of ​​development of the Caledonian folding, 3 - area of ​​development of the Hercynian folding, 4 - areas of development of the Mesozoic-Cenozoic folding of the Cordillera, 5 - oil and gas areas of the North American platform, 6 - intermountain depressions of the Cordillera folded belt.

Oil and gas bearing provinces and regions: 1 - Colville trough; 2 - Beaufort-Mackenzie basin; 3 - Albert depression; 4 - syneclise Williston; 5 - intermontane depressions of the Rocky Mountains; 6 - Inner Western Province; 7 - Perm depression; 8 - bend arch; 9 - ledge Azark; 10 - Illinois depression; 11 - Michigan depression; 12 - the arch of Cincinnati; 13 - Pre-Appalachian trough; 14 - the province of the Gulf of Mexico; 15 - Atlantic province; 16 - California; 17 - Cook's Bay.

the California province of Alpine age stands out. On the Alaska Peninsula, two provinces are distinguished - the Paleozoic-Mesozoic of the Arctic slope (Colville Basin) and the Cenozoic of Cook Inlet on the Pacific coast of the peninsula.

The North American continent is distinguished by the highest degree of exploration.

Deposits are known here in deposits from the Cambrian to the Pliocene, confined to a wide variety of traps within large intra-platform depressions and uplifts, in zones of articulation of the platform with folded areas, various intermountain depressions and modern passive and active continental margins. As an example of a large vault, we can cite the Tsincinnati vault, which is 1000 km long and up to 400 km wide. The deposits are confined to local brachianticlines and zones of wedging out of sandstones. The main productive horizons are concentrated in the Ordovician and Silurian parts of the section. One of the richest intraplatform structures is the Permian depression. Its area is 365 thousand km 2. The deposits are confined to local structures and traps of stratigraphic and lithological types. The main productive horizons are concentrated in the Permian and Carboniferous parts of the section. In total, more than 5.5 thousand deposits have been discovered here. The Western Canadian Petroleum Province is a typical example of a junction zone structure ancient platform with a folded area. Here, the deposits are confined to local structures, pinchout zones, and reef structures; in the junction zone of the trough with the folded zone, deposits associated with thrust dislocations are widely developed; on the eastern side of the trough, the world's largest deposits of heavy oils and malts are known (fields Athabasca, Vabasca, etc.), with reserves of 120 billion tons. The oil and gas province of the Gulf of Mexico is an example of the oil and gas potential of a passive continental margin that continues its development. Its evolution begins from the Permo-Triassic period. The stratigraphic range of oil and gas potential is from Upper Jurassic to Quaternary deposits. The number of productive horizons exceeds 100. The deposits are confined to local structures, diapiric domes, to traps of stratigraphic and lithological types. A large number of deposits have been discovered in the waters of the Gulf of Mexico (about 500). Among the largest fields in this province is the East Texas oil field, the second largest in the United States (initial recoverable reserves of about 800 million tons). It is assumed that this field will be developed for almost 100 years; by the beginning of the 1990s, more than 600 million tons were produced at the field. oil (beginning of production in 1933).

On the western coast of the continent there are numerous Cenozoic intermountain depressions, the productive horizons in which are confined to Miocene and Pliocene sediments. In the south of the Alaska Peninsula, there is the oil and gas region of Cook Inlet, which is genetically related to the active continental margin that continues to develop. Oil and gas fields are discovered here both on the mainland and in the waters of the bay.

In the United States, the largest oil field in the northern hemisphere, Prudhoe Bay (province of the Arctic slope of Alaska), was discovered. The deposit is confined to an anticline cut by an unconformity (Fig. SS). Three deposits were found at the field in the depth interval of 2050-3200 m in the Permian-Carboniferous, Triassic and Lower Cretaceous deposits. The recoverable oil reserves at the field are estimated at 1.3 billion m 3 .

Rme. 55 Schematic section of the Prue do Bay field (Gabrieliants, 1984). 1 - oil; 2 - gas; 3 - water; 4 - surface of stratigraphic unconformity.

This platform experienced a short-term uplift at the beginning of the Silurian as a result of the manifestation of the Taconian phase of folding in the Appalachian geosyncline. Regression has been replaced by transgression with wide distribution of carbonate deposits and reef formations.

Silurian deposits are represented by limestones and dolomites. There are many reef structures in the Lower Silurian sections, and halogen rocks appear in the Upper Silurian, especially in the east of the platform - anhydrites, gypsum and rock salt.

At the very end of the Silurian, huge salt pools arose in North America. The thickness of the Silurian is measured in several hundred meters. In depressions, it increases, for example, in the Michigan depression - up to 1.5 km.

gondwana

southern continents in the Silurian they are still above sea level, and the Silurian sediments are insignificant, but where they are present (along the periphery of Gondwana), they are represented by terrigenous formations.

In the South American part of Gondwana, a restructuring took place at the end of the Ordovician - the beginning of the Silurian, probably caused by the influence of the Caledonian folding. In the Silurian, the area of ​​the sea increased. Depressions of the meridional direction appeared. They accumulated significant thickness (up to 800-1200 m) clastic sediments with subordinate carbonate layers. In the Amazon Basin (latitudinal direction), marine sandy-argillaceous sediments 100 m thick are observed. In the Late Silurian and at the very beginning of the Devonian, uplifts again occurred as a result of Late Caledonian movements.

In the African part of Gondwana, sandy strata at the end of the Ordovician and in the Silurian were replaced by dark clays with graptolites. Carbonate muds appeared in the northern part of the basin. Along the margins of the area of ​​marine accumulation were deposited coastal sands. The thickness of the Silurian rocks is usually small. On the Arabian Peninsula The Silurian is represented by a continuous section of sandy-argillaceous formations of considerable thickness. At the end of the Silurian, a regression began everywhere in Africa, which was especially clearly manifested in Arabia.

The Australian part of Gondwana in the Silurian was mostly land.

History of the development of geosynclinal belts North Atlantic geosynclinal belt

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 of calcareous-argillaceous rocks and

limestones with remains of brachiopods and corals (300-400 m), in others - 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

Ural-Tien Shan geosynclinal region stretches 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. Silurian of the Western Urals 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 dominated, so carbonate and carbonate-argillaceous deposits (500-1500 m) with a diverse complex of organic remains accumulated here. Shallow coastal sand-pebble rocks are known on the western margin 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 Kazakhstani 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 (see Fig. scheme III, col. on). 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.

tiya. The gently dipping Lower and Middle Devonian volcanic rocks and tuffs crowning the section acid composition already accumulated on the ground. 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 base 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 floors are clearly distinguished from each other: the first is the Lower Cambrian; the second is the 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 - eugeosynclinal, the third - orogenic, and the second - intermediate (transitional).At the second stage, the subsidence developed on an already consolidated basement, the regime resembled miogeosynclinal.Ore deposits of iron and copper are associated with acidic intrusions.

Thus, the Caledonian epoch of tectogenesis covered the regions of northwestern Kazakhstan, partly the Altai Mountains, the northern Tien Shan and the eastern part of the Altai-Sayan folded region - the 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 diagram III, color incl.). AT northern Europe sandstones, black shales, bituminous limestones (thickness - 0.5 km) accumulate, siliceous shales appear, due to manifestations of underwater volcanic activity. AT 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.

AT Asian geosynclinal region the Silurian is known in Turkey, the Caucasus, in the mountainous structures of Iran, Afghanistan, and the Pamirs.

Here, under eugeosynclinal conditions, thick strata of terrigenous rocks and volcanic rocks of basic and felsic composition, or small terrigenous-carbonate facies accumulated in miogeosynclinal zones (Zagros Himalayas, etc.).

Minerals

deposits rock salt, industrial deposits oil and gas known on the North American (Canadian) and Siberian platforms. In the Silurian, deposits of oolitic iron ore Clinton (USA) and a number of smaller ones in Africa. Deposits associated with the Caledonian felsic intrusions gold Northern Kazakhstan, Kuznetsk Alatau and Mountain Shoria.

In late Caledonian intrusions in the Scandinavian mountains, iron, copper, chromite: Known in the Urals nickel, platinum, asbestos, jasper. Deposits associated with pegmatites rare metals in the Appalachians and Eastern Siberia.

Silurian limestones are building material and good ceramic raw materials.

DEVONIAN PERIOD - D

General characteristic, stratigraphic divisions and stratotypes

The Devonian system was established in 1839 by the famous English geologists A. Sedgwick and R. Murchison in England in Devonshire, after whom it was named.

The duration of the Devonian period is 48 million years, its beginning is 408 million years, and its end is 360 million years ago.

"The sections of the Devonian of Great Britain are composed of continental facies and willows can be used as stratotypes to distinguish stages. Therefore, the division of the Devonian system was carried out in the Ardennes on the territory of Belgium, France and in the Rhine Slate Mountains on the territory of Germany. The Devonian system is divided into three sections (Table 8).

Table 8 General stratigraphic units of the Devonian system

The boundary between the Silurian and the Devonian, as mentioned above, is drawn at the base of the graptolite zone. Monograptus uniformis(Barrandien, Czech Republic). At present, this boundary is the only one officially adopted by the Stratigraphic Commission of the International Geological Congress. The upper limit has not been officially approved. In view of the fact that at the beginning of the Devonian period, an extensive regression continued, which began as early as the Silurian, a variety of facies settings arose with the corresponding fauna. This greatly complicates the division and comparison of sections and was the reason for the creation of a "combined" scale, consisting of tiers established in different regions. Stage division of the Lower Devonian of Barrandien, Rhineland is based on marine life, and the sediments of England corresponding in age - on the remains of fish found in lagoonal-continental sediments.

Zhedinsky stage, named by A. Dumont in 1848 along the river. Zhedin in the Ardennes, unites the lower layers of the Devonian of the Ardenno-Rhine region. They are represented by coastal facies and lie transgressively on Cambrian deposits (hence the difficulties in determining the exact boundary with the Silurian). In the stratotype Bottom part It is represented by the Fepan conglomerates 10–40 m thick, the Ebb arkoses 30 m thick, and the Mondrechon shales with sandstone interbeds. Sandstones and shales contain rich assemblages of brachiopods. In the upper part there are red and maroon shales with small calcareous concretions, interlayers of red

and green sandstones and quartzites. They are characterized by fish remains. The total thickness is lo 750 m.

The name "Siegen Stage" was first used by E. Kaiser, designating the greywackes in the Rhine Slate Mountains. Siegen greywackes are most fully represented in the Siegerland region, where lagoonal and coastal-marine facies with remains of fish, bivalves and brachiopods are developed. The thickness of the deposits in the stratotype section is 4 km.

The Emsian stage was established by C. Dorlodo in 1900 in the Ems town near Koblenz in the Rhineland. The deposits of this stage are represented by a sequence of sandstones, quartzites, and shales with interlayers of volcanic rocks. The thickness reaches 2 km. In the layers there are accumulations of brachiopods, bivalves, and occasionally corals (Fig. 51).

Previously, the Siegen and Ems stages were combined into one stage, which was called the Koblenz-Kim. However, according to the decision of the International Stratigraphic Commission, the Lower Devonian is now accepted in the volume of three stages.

The Eifelian Stage was named by A. Dumont in 1848 after the Eifel Mountains, where the stratotype section is located. The volume of the stage was modified and, after the work of M. Düsseldorf in 1937, it was accepted as the volume of calceol and upper cultivated Laukh layers with a stratotype in the Wetteldorf section of the Eifel Mountains. Here, a sequence of marls, platy limestones, calcareous sandstones and coral-stromatoporous limestones (about 450 m thick) is exposed. In the thickness of a large number of corals of the genera Favosites, Calceola, Damophyllum, remains of cephalopods and conodonts.

The Givetian stage was identified in the Ardennes by J. Gosselet in 1879. The name comes from the city of Givet, located in Northern France. This stage combines deposits characterized by stringocephalic brachiopods, the presence of conodonts, corals, and, less commonly, trilobites. The stage is composed of limestones and calcareous shales, organogenic and organogenic-detrital limestones.

The Frasnian Stage was established in 1879 by J. Gosselet in Belgium. The name was received from the village. Fran near the city of Couvin. In the stratotype section, it is composed of shales and reef coral-stromato-porous limestones (about 500 m thick). Characterized by brachiopods, conodonts, corals and bivalves.

* The Famennian stage was first identified in the Ardennes by A. Dumont in 1855. It got its name from the Famenn area in Belgium. Sandstones, shales with intercalations of limestones are developed here. In stratotic terrain, it is characterized by great variability. Marine sediments contain conodonts, corals, and brachiopods, while lagoon sediments contain fish remains and plant imprints.

In the 1960s, Czechoslovak researchers suggested that instead of the Zhedino and Siegen stages, the Lochkovian and Pragian stages should be distinguished, which were established in the marine sections of the Barrandov trough in the Bohemian massif not far from Prague, which are well characterized by fauna. There is also a recognized border between the Silurian and the Devonian, drawn between the Przhidolsky and Lochkovian stages. In 1985, the International Subcommittee on Devonian Stratigraphy recommended the Lochkovian and Pragian stages of the Czech Republic as typical for the Lower Devonian. Since then, geologists have been using precisely these stages, although the former Zhedinsky and Siegen stages roughly corresponding to them have not been formally abolished. This explains the "dual power" in the lower part of the tiered scale of the Devonian system.

Characteristic sections of the Devonian system are shown in schemes IV and V, col. incl.

organic world

The organic world of the Devonian period was rich and varied. Terrestrial vegetation has made significant progress. The beginning of the Devonian period was characterized by the wide distribution of "psilaphites" (rhinophytes), which reached their peak at that time.

Rice. 51. Characteristic fossil remains of Devonian organisms

Brachiopods:/ - Euryspirifer(early and middle Devonian), 2a, 6 - Stringocephalus(average Devonian), 3-Karpinskia(Early Devonian), 4 - Cyrtospirifer(mostly Late Devonian), 5a, b - Hypothyridina(Middle and Late Devonian); cephalopods:6 - Clymenia(Late Devonian), 7 - Timanites(Late Devonian) 8-Tornoceras(Late Devonian); crinoids:9 - Cupressocrinites(Middle Devonian); rugosa corals:10-Calceola(Early - Middle Devonian), // - Hexagonaria(Middle - Late Devonian); conodonts:12-Palmatolepis(Late Devonian) 13 - Polygnathus(Devonian), 14 - Icriodus(Devonian); lungfish:15 - Dipterus(Middle - Late Devonian); lobe-finned fish:16 - Holoptychius(Late Devonian); amphibians:17 - Ichthyostega(Late Devonian); rhinophytes:18-Rhynia(Early Devonian) 19, 20 - Sawdonia(Early Devonian)

(Fig. 52, color on). Their dominance is observed in swampy landscapes. At the beginning of the Middle Devonian, rhinophytes died out, they were replaced by great ferns, in which leaf-like forms began to form. In the middle Devonian, all the main groups of spore plants already existed. These are clubs, arthropods and ferns, and at the end of the Devonian the first representatives of gymnosperms appeared; many of the shrubs turned into trees and gave rise to the first coal seams (Svalbard, Barzas). The Late Devonian flora was called Archeopteris, after the widespread heterosporous fern. Archaeopteris(Fig. 53, color on). At the end of the Devonian, forests already existed on the planet, consisting of the plants listed above.

Conodonts have the greatest biostratigraphic importance in the Devonian. These representatives of the primitive chordates, which appeared in the middle Cambrian, already gained a dominant position in the Ordovician. In the late Devonian, the second peak of their heyday is observed. The conodonts changed so rapidly in the Devonian that they make it possible to distinguish more than 50 standard zones in the Devonian deposits with a duration of the Devonian period of about 50 million years. This is a prime example of using the remains of rapidly evolving organisms to create ultra-detailed stratigraphy. w Graptolites survive in the Devonian (one rare genus in the Lower Devonian Monograptus) and cystoids; the variety of forms of trilobites and nautiloids is sharply reduced. Widespread castle brachiopods (brachiopods) from the family Spiriferidae with the main genus Spirifer and pentamerides (genus Pentamerus), four-rayed corals, tabulates.

Cephalopods (Fig. 51) are significant in their significance: the orders of goniatites, agonyatites and climenia. They have a simple septal line with solid pointed lobes and solid rounded saddles (goniatite), or with rounded lobes and saddles (agoniatite). Clymenia is a specific group of ancient ammonoids, in which the siphon was located closer to the dorsal side, and not to the ventral side, as in most representatives of the ammonoid subclass. Clymenia were characteristic only of the Late Devonian.

For the first time in the history of the earth big role bivalves and some lower crustaceans began to play, which is associated with the existence of numerous basins of abnormal salinity in the Devonian. It should be noted the abundance of the smallest crustaceans - ostracods and phyllopods.

For the stratigraphy of marine sediments, the most important are conodonts, ammonoids, brachiopods, corals, tentaculites, and ostracods. Vertebrates began to acquire ever-increasing importance. Jawless and especially fish are widespread: lungfish, armored, lobe-finned, cartilaginous (sharks, rays) (Fig. 51). In freshwater and brackish-water basins, fish, apparently, were already numerous. Since the Devonian, the first amphibians are known - stegocephals.

The development of land by plants and animals continued. Among the latter, there are scorpions and centipedes, which appeared in the Silurian, as well as wingless insects.

Crustal structures and paleogeography v

Does not occur during the Devonian significant changes in the distribution and outline of the main structural elements of the earth's crust, created by the beginning of the Devonian (platforms, geosynclinal belts and Caledonides). This is explained poor development in the Devonian fold processes, which are distinguished by low intensity. Only at the end of the period in some geosynclinal areas did the Breton folding phase - beginning Hertzin era of tectonogenesis. The Breton phase of folding is established in the northwest of the Mediterranean (European) geosynclinal region (Brittany Peninsula) and in the South Appalachian geosynclinal region. The Caledonian folding led to uplifts not only of the Caledonian regions, but also of many platforms. reached its maximum in the early Devonian regression, which began at the end of the Silurian. The areas of destruction and demolition were the Caledonides and extensive pro-.

platform spaces. Sedimentation on the platforms was sharply reduced, it continued only in areas bordering the Caledonides. This stage is characterized by inland water bodies with abnormal salinity. The marine regime has been preserved in geosynclines.

From the middle of the Devonian, in many parts of the world, ascending movements gave way to subsidence, and a new transgression developed. The sea advanced on the platforms and penetrated the limits of the Caledonides (see Diagram IV, col. inc.).

At the end of the Late Devonian, in the Famennian, the rise of the platforms began again (the Breton phase) and, in connection with this, some regression of the sea.

; A characteristic feature of the Devonian is the formation of intermountain depressions, in which continental terrigenous, predominantly red-colored deposits and volcanic rocks with a thickness of several thousand meters accumulated. The deposits of intermountain depressions are collected in folds or lie flat. In some depressions, they are cut through by intrusions and metamorphosed to varying degrees. The appearance of depressions is associated with the emergence and activation of faults, with block movements characteristic of the Devonian. The formation of such depressions occurred during the final - orogenic- the stage of development of geosynclines.

The beginning of the Devonian period (Early Devonian epoch) is quite deserving of the name geocratic epochs in the life of the Earth, that is, epochs with a predominance of the continental regime. Since the Middle Devonian, the areas occupied by the seas have increased both on platforms and in geosynclinal areas. The land area is shrinking. At the same time there is a general alignment, gradual peneplanization continents, as well as insular land areas scattered over the area of ​​geosynclinal regions. This is evidenced by the almost ubiquitous change of terrigenous sedimentation characteristic of the Early Devonian to carbonate. Until the end of the Devonian period, the mountainous relief remained most stable in the areas of the Caledonides, but even there, by the end of the period, it turned out to be significantly smoothed in places, as evidenced by the relative fine-grained upper layers of the "ancient red sandstone" british isles, Minusinsk depressions, etc. (Fig. 54).

The late Devonian epoch, in contrast to the early Devonian, especially its first half (the Frasnian age), was a time of widespread development of marine transgressions, a time of predominant dominance of the sea over land. Similar epochs in the life of the Earth are called thalassocratic.

Restoring the position of the climatic zones of the Devonian is difficult, since the ground vegetation is sparse. Only the characteristic features of a number of continental and lagoonal facies of the Devonian allow us to draw some paleoclimatic conclusions, which, however, are insufficient for reconstructing the general picture of climatic zonality in the Devonian period.

When considering the conditions for the formation of "ancient red sandstone", many facts point to the arid climate of the intermountain depressions in which these sediments accumulated. Dry and hot climate was characterized, apparently, in the Devonian middle part Russian plate, as evidenced by the widespread development of lagoonal chemogenic sediments here (dolomites, gypsum, etc.). The same precipitation outlines within Europe a zone of arid climate, stretching from the northwest to the southeast. Other evidence of the Devonian climate is the tillites of the Cape Mountains of South Africa (30 m thick), 500 km long. It is not clear whether the moraine accumulations associated with this glaciation have continental or mountain genesis. Other manifestations of glacial activity in the Devonian are unknown.

The most characteristic facies of the Devonian is the "ancient red sandstone" facies. (Old Red sandstone) widespread in all countries northern hemisphere(Fig. 54). It is assumed that this is a continental facies of sandy deserts. However, finds of organic remains in red sandstone (armored fishes, phyllopods) force us to consider this facies as mixed.

Rice. 54. schematic map continent of ancient red sandstone and the zone bordering it / - the main modern outlets ancient red sandstone; 2 - Hercynian massifs (Marine Devonian); S-S- northern border marine transgressions to the ancient red sandstone continent; Yu Yu- the southern boundary of the distribution of layers of ancient red sandstone in the marine Devonian of Central Europe (Ginyu, 1952)

shanna lagoon-continental and lagoon-marine. In addition to the "ancient red sandstone" lagoonal facies are often represented by the facies of closed brackish water basins. They formed the oil-bearing facies of the cypridine shales and the peculiar Domanik facies of the European part of Russia.

History of platform development

Page 1

The North American platform is surrounded by folded or folded-block systems of various ages. The longest and most extensive of them is the Cordillera folded-block system, framing the platform in the west. AT cross section Cordillera from east to west (in Alaska from north to south) the following tectonic zones are distinguished.

The North American platform stretches from north to south for 4000 km, and in the latitudinal direction for 2500 km. In the eastern and northern parts of the platform, there is a predominant accumulation of Paleozoic, and in the southwestern and southern regions platform - Meso-Cenozoic deposits.

On the North American platform, mainly Paleozoic deposits are oil and gas bearing, while predominantly gas-bearing areas are located in the western part of the platform in the zone of its junction with the folded structures of the Rocky Mountains and in the deep intra-platform depression of Anadarko. Mesozoic and Cenozoic rocks are oil and gas bearing on the epi-Hercynian platform (Gulf Coet Province), as well as in the intermountain basins of California.

The northern frame of the N American Platform is the Caledonian-Early Hercynian Innuit fold system, largely overlain by the Sverdrup syneclise. The latter is composed of thick sedimentary sequences of the Carboniferous, Permian, Mesozoic and Cenozoic.

Most of the N American Platform south of the Canadian crystal shield is located in the USA.

Within the North American platform, a number of large tectonic elements are distinguished (Fig. 240): ledges of the basement of the plate and the sedimentary complex - the Ozarks, Adirondacks, etc.; domed uplifts - Cincinnati, Bend, etc.; intraplatform depressions - Michigan, Illinois, Perm, etc.; Mexican marginal depression.

The southern and southwestern parts of the N American Platform form its plate. Part of the platform, located south of the Canadian-Greenland shield, stands out under the name of the Midcontinent or Midland plate. Almost over its entire area, the sedimentary cover is composed of Paleozoic rocks. The western margin of the N American Platform represents the Great Plains plate.

The core of the North American continent is the Precambrian North American Platform, in the northeast of which the Canadian Shield stands out.

The Cincinnati dome is the largest geostructural element of the North American Platform, its length is 1000 km and its width is 400 km. It is located within the states of Ohio, Indiana, Kentucky, and Tennessee. Sediments from the Cambrian to the Carboniferous take part in the structure of the sedimentary cover. Oil deposits are associated with gently sloping structures or with zones of wedging out of sandstones on the slopes of the arch. The main accumulations of oil are known in the Lyme-Indiana region.

The Permian Basin is located on the southwestern margin of the N American Platform. Its structural frame in the west is the marginal western elements of the platform, involved in the uplift of the epiplatform orogen of the Rocky Mountains, in the northeast - the Wichita-Amarillo system and the Munster swell. In the east and south, the basin is bordered by the Hercynian Washita-Marathon fold belt. This boundary is buried under a cover of gently dipping Mesozoic sediments. The metamorphic rocks of the frontal part of the Washita Belt are exposed here by erosion on the Marathon Rise.

The Cincinnati dome is the largest geostructural element of the North American Platform, its length is 1000 km and its width is 400 km. It is located within the states of Ohio, Indiana, Kentucky and Tennessee. Sediments from the Cambrian to the Carboniferous take part in the structure of the sedimentary cover. Oil deposits are associated with gently sloping structures or with zones of wedging out of sandstones on the slopes of the arch. The main accumulations of oil are known in the Lyme-Indiana region.

The United States is located in various geotectonic conditions, on the North American platform, in the Mexican depression, intermountain and foothill depressions and troughs of the Cordilleras and Appalachians, on the shelf.

According to N. Yu. Uspenskaya (1952), there is not a single large oil and gas bearing horizon in limestones on the North American platform that would not be associated with the erosion surface. About 95% of all production from carbonate reservoirs in the United States comes from horizons that lie below unconformity surfaces. An example of a direct relationship between the productivity of carbonate reservoirs and unconformities are oil and gas deposits in the limestone-dolomite strata of the Ordovician of the Lima Indiana region, the Devonian limestones of the Michigan and Eastern Inner Basins, in the Devonian, Mississippian and Ordovician limestones of the Western Inner Basin, as well as in the Permian limestones and dolomites of the Permian basin.

America occupies the position of a watershed between the expanses of the Atlantic and Pacific Ocean.

From the west, it is bounded by folded mountain structures that rise steeply above the deeply submerged bed of the Pacific Ocean. In the east, the continents have abrasive shores. The continental slope is sharply defined and steep, rising at some distance from the coast above the great depths of the Atlantic Ocean.

The vast land masses of the Western Hemisphere - North and South America - are independent, historically unrelated continental structures. However, both continents have much in common. Their wedge-shaped outlines have a southerly direction. The extended part of the land is facing north. The western shores of the continents are bordered by high mountain ranges, and plains predominate in their eastern part. North America in relation to South is located much to the west. The continents are separated by a latitudinal mobile zone, in which the island arcs of the Antilles and the mountain structures of Central America, which have already articulated with the continents, are located. The Antilles-Mexican region, as we noted (Bondarchuk, 1946), is a structural analogue of Indonesia, located between the continents of Asia and Australia.

North American Platform. Most of North America has a crystalline Precambrian basement. Precambrian rocks are found in the area Canadian shield. Separate Precambrian blocks protrude in Colorado, the Rocky Mountains, in the provinces of basins and ranges. Most of the N American Platform is covered by a thick sedimentary platform cover. In the north, in some islands of the Arctic archipelago and Greenland, the crystalline basement lies under a thick ice sheet.

The model of the structure of the North American platform, in the light of the data of K. K. Stockwell (1967) and F. B. King (1967), is characterized by such features. The oldest part of the crystalline basement in the Hudson Bay Basin, the central part of the United States and the Arctic Islands is covered by a platform cover. The Canadian shield has a zonal structure of Precambrian folded zones, gradually increasing its boundaries. Paleozoic and subsequent folded structures, building up the platform in the same way, determined the modern features of the tectoorogeny of the North American continent.

Within the territory under consideration, Precambrian folding is distinguished (King, 1967): Kenoran, Hudson, Elson, and Grenville. They deform the thick Precambrian strata, which have complex composition. The oldest formations of the shield are volcanogenic and sedimentary formations located among gneiss fields and other metamorphic rocks. These formations, as well as the gneisses surrounding them, host numerous gabbro and granite intrusions of different ages. Precambrian fold zones characterize individual provinces.

Kenoran folding is located in the southeast of the shield in the provinces of Upper and Slane, as well as in its northwestern part, bordering younger structures. Its age is 2390 million years.

The leveled surface of the Kenoran folding is overlain by undisturbed strata of the platform cover of the Proterozoic age. The Huronian folding includes Proterozoic deposits and older undivided gneisses and granites. It occupies the northeastern part of the shield, where it is adjacent to the Kenoran folding. In the northwestern part of the Canadian Shield, the Huronian folding is located between the areas of Kenoran folding. In Labrador and the southern edge of the Rocky Mountains, Nain Province, but according to F. B. King, these structures are reworked by later, Olson, folding.

The Huronian folding on the Canadian Shield is expressed in the provinces of Churchill, Bor, and South. Its age is determined by the early and middle Proterozoic about 1640 million years ago. The Elson folding is considered to be Middle-Late Proterozoic. It ended 1280 million years ago.

Late Proterozoic deposits lie horizontally on the Huronian folded basement.

In the southeast of the Canadian Shield, there is an area of ​​Grenville folding, concentrated mainly in the Grenville province. In the era of the Grenville folding, older structures were reworked. This folding belongs to the Late Proterozoic. It ended about 800 million years ago. On the Huronian folded basement, in some places, a platform cover of the Late Proterozoic age has been preserved.

Intrusions of mafic rocks, mainly gabbro and anorthosites, as well as alkaline syenites, play an important role in the structure of the Canadian Shield. These rocks are considered older than granites. Latest different ages and are associated with the corresponding folding phases. The largest intrusions are concentrated in the strata of the Kenoran structural stage. Among the post-orogenic formations, “circular structures” are distinguished, which are considered cryptovolcanic formations. They are rings of highly deformed rocks of the platform cover, and some of them belong to the Precambrian formations. Separate circular structures cut through the Kenoran and Grenville deposits. They contain igneous rocks and volcanic breccias of post-Ordovician age. Gabbro and diabase dikes are also known among the platform formations. Where the crystalline basement is exposed, all these rocks can be traced in relief.

The Precambrian basement of the N American Platform is perfectly aligned. It is strongly dissected by faults into blocks, the different positions of which create a series of depressions and elevations (Nalivkin, Gostintsev, Grossgeim, 1969).

The platform cover of the Canadian Shield is composed of sedimentary and volcanic rocks, their occurrence is horizontal or slightly disturbed. The age of the cover deposits is not the same. In the Upper Lake area, the Keninavan series of platform cover forms a wide syncline. Its layers are broken by normal faults and contain numerous bedded gabbro intrusions. In the western part of the shield and up to the Cordillera, the belt sedimentary series, also of Precambrian age, forms the platform cover. Its anchorage has not been broken.

In the Hudson Bay region, between the shield and the Appalachians, Paleozoic deposits take part in the structure of the shield. They form the lowlands south of the Canadian Shield, the plains of Western Canada, and extend into the Arctic archipelago. Further to the west, the platform cover is composed of Mesozoic and Cenozoic rocks.

In the southwestern part, the N American Platform stretches to the Rocky Mountains. Here it is broken by faults into separate blocks, one of which forms the Colorado Plateau. However, it is possible that this block is an independent island massif, one of the island system of the Cordillera fold zone. The Colorado Plateau is bounded on all sides by the structures of the Rocky Mountains. Only in the southwest does it break off with a steep ledge to the Khila valley.

The surface of the plateau rises to 1800-2600 m above sea level. Highest point- Mount San Francisco (3840 m) - an extinct volcano. The surface of the plateau is strongly denuded. Table ootans mountains and individual laccoliths rise above it. River valleys form grandiose canyons up to 1800 m deep.

The basement of the Colorado Plateau is composed of Precambrian crystalline rocks. They are overlain by a horizontally layered sequence of sedimentary rocks from Paleozoic to Quaternary age.

Of great importance are intrusions of igneous rocks and volcanogenic deposits, and on the outskirts of the plateau - lava flows. Extinct volcanoes and laccoliths are characteristic features of the plateau landscapes.

The Precambrian of the Greenland crystalline shield, according to BF King (1967), has much in common with the structure of the Canadian Shield. It composes several islands covered with a common ice cover.

The Precambrian basement of the North American Platform is bordered by fold systems of different ages located between the craton and the oceans surrounding the mainland. The oldest of the Innuit (Caledonian) systems is located along the Northern Arctic Ocean in Northern Greenland and in the north of the Arctic archipelago. The formations of the East Greenland fold zone are considered syntectonic with Innuitic. In the northeast of Greenland, both branches of the Caledonian are articulated. From here, the East Greenland Fold Zone extends south across Scoresby Bay. Deposits of the Cambrian, Ordovician, very thick Silurian and, in places, Devonian age take part in the structure of the Early Naleozoic folded structure. On the leveling surface of the Caledonides lies a platform cover of Carboniferous, Permian and Mesozoic deposits. In some places, the occurrence of these deposits is disturbed by faults.

The southeastern part of the North American Platform is bordered by the Appalachian (Hercynian) folded zone. The formation of this zone was completed in the early Mesozoic. Both sedimentary and igneous formations take part in the structure of the Appalachians. They form a mountainous terrain.

In the southwest, the Ouachita fold region is a continuation of the Appalachians. Its strongly leveled structures are buried over a large area under younger formations. They stretch towards the Pacific Ocean, into Mexico, and can be traced under the Cordillera, lying across the strike of their structures.

From the west, the N American Platform is framed by the Cordillera fold system, stretching from Alaska north to South America, where they are continued by the Andes of Venezuela and Colombia. The Cordilleras were formed on the site of several island arcs, they consist of parts of different ages and structures.

The inner zone of the Cordillera includes older formations, dislocated and penetrated by intrusions in the middle Mesozoic (Nevada orogeny). On the outer margins of the zone, structure formation developed later - in the Late Cretaceous and Paleogene (Laramian folding, orogeny of the Rocky Mountains and British Columbia). In the Tertiary period in the Cordillera mobile zone, folding developed in local basins. At that time, fault tectonics and related volcanism played an important role.

As a result of the outpouring of plateau basalts, large volcanic plateaus arose in the states of Oregon, Washington, British Columbia and Greenland. Their outpouring continued also in the Quaternary period. At that time, volcanic fields were formed in the state of Idaho of southern Mexico, etc., as well as volcanic ridges parallel to the general strike of folding in the Cascade Range, structures stretching along the Pacific coast in Central America from Guatemala to Costa Rica.

Along the Pacific coast and in the western part of the Cordillera, the Pacific Fold Zone stands out. The structures of the Antilles island system are considered to be synchronous to it. Deformations in this zone continue to this day.

The structure of the North American Platform is characterized by the same features as other Precambrian parts of the continental crust. Its formation took place around the centers - constituent parts island arcs. The process of structure formation in North America naturally developed throughout the entire geological history. Its structures are spatially fixed and do not have drift layers.

The relief of the platform is characterized by a significant smoothness, large areas of accumulative plains, combined with highland countries. The brilliance of the country's landscapes is enriched by extremely diverse forms of denudation, presented on large areas and often huge size. Their features reflect the influence of climate on the physical geography of the steppe plains, semi-deserts, snow-covered Arctic islands, mountainous countries and subtropics covered with forests.

South American platform. The Precambrian crystalline basement of South America is exposed in the northern half of the mainland. Separate protrusions of it are known in the south in Argentina and Chile. In the northwest and west, the platform is framed by the folded mountain zone of the Andes. The mountains and basement projections are separated by the forward trough. Toward the Atlantic Ocean, the platform forms a steep continental slope and has abrasion shores. The general configuration of the coast of South America fully reflects the configuration of the adjacent part of the Mid-Atlantic Ridge.

In structure South American Platform the Guinean, Central, or West Brazilian, Coastal, or East Brazilian shields stand out. Isolated protrusions of the Precambrian in the southern part of the mainland are the Apa, Tebikuari, Uruguayan, Northern Hills of Buenos Aires, the Pampa Blocky Country, the South Mendossa Massif, the Sovero-Patagonian and South Patagonian shields. They are separated by the Amazonian, Parnaibsky, San Franonsky, Paranskaya troughs and the Serra Geral plateau basalts associated with them, the La Plata depressions, or the Chaco-Pampasskaya, Rio Negro, Chubutskaya and Santa Cruz. Thick strata of platform cover occur within their limits.

The Guianan shield lies in the north of South America between the Orinoco and Amazonian depressions. Its distribution generally corresponds to the Guiana Highlands. The surface of the shield is located within 500-1000 m in the west and 200-500 m above sea level in the east. The highest point - the top of Roranma - 2771 m. The highlands in the south are limited by steep slopes, and in the east - by rocky ridges. At the foot of the slopes is rolling plain, gradually decreasing to the Amazonian lowland.

In the structure of the shield, sediments of the Middle and Late Pre-Cambrian age are distinguished. Hornblende and other gneisses, mica schists and granite gneisses are considered the oldest. It is associated with gabbro intrusions, as well as deposits of diabases and andesites. The younger formations of Guiana include ferruginous quartzites, a volcanogenic series of predominantly basaltic and andesitic tuffs. In the British part of Guiana, the volcanic series is composed of layered tuffs, agglomerates, lavas, quartzites, shales, and phyllites. This series is cut through by dolerite and gabbro intrusions. It contains large batholiths of granite.

The most complete Precambrian section has been described in French Guiana (Tugarinov and Voitkevich, 1966). The Cayenne system, composed of amphibolites, quartzites, hornfelses, gneisses, and migmatites with interlayers of crystalline limestones, belongs to the Lower Precambrian. These deposits are highly dislocated. The strike of their structures is variable, most often latitudinal. The Middle Precambrian is represented by the Paramaka system. It includes only intensely metamorphosed strata of chlorite, micaceous and talc schists interbedded with lavas, including peridotites and granite intrusions. Paramak deposits are folded. The Upper Precambrian of French Guiana is divided into two parts: the lower Bonidoro Series and the upper Oranou Series. The first is dominated by detrital rocks, shales, lavas, and volcanic tuffs, including granite intrusions; the second begins with strata of conglomerates, quartzites and shales lie above. It is also cut by granite intrusions, its folded structures extend in a west-north-west direction. The Oranu Series is intruded by rhyolites, on which the sedimentary-volcanogenic Roranma Series of post-Cambrian age overlies.

Three orogenic belts are distinguished in the structure of the coastal part of the Guiana Shield (Shubert, 1956). The oldest - Gilea - covers the Cayenne system. The sedimentary and igneous rocks composing it are highly metamorphosed. The middle belt - the Guianan - includes the strata of the Paramaca system and the youngest Caribbean - deposits of the Bonidoro and Oranu series.

Thus, the Guiana Shield can be considered as an independent center of formation of the continental crust in the Precambrian. As on other shields, the expansion of the land occurred here sequentially, joining the core, composed of sedimentary-volcanogenic strata of new structural floors of folded zones.

After consolidation, the surface of the Guiana Shield was completely leveled. In the late Mesozoic, mainly in the Cretaceous, a cover of sandstones of continental origin formed on it. The remnants of this sandstone, which survived from denudation, form table uplands and play a significant role in the landscapes of the Guiana Highlands.

In the south, the Guianan shield separates the Amazonian trough from the Brazilian one. It stretches in a latitudinal direction from the Atlantic to the Pacific Ocean, from which it is separated by the folded zone of the Andes. Along the trough flows the greatest rock of the world, the Amazon, which has a tectonic valley (a very convincing example of the unity of the structure and topography of the earth's crust). The Amazonian trough is filled with Paleozoic and younger sediments. This is an inter-island accumulation basin. Its development continues in modern conditions.

The Brazilian shield is the central part of the mainland of South America south of the Amazonian trough. The Paramba-San Francisco meridional depression divides the shield into western, central and eastern, Atlantic, parts. Opies are considered as independent shields. The Paramba-San Frapsis Basin separating them is a relic of the interisland basin. The tectonic valleys of Paramba, San Francisco and the upper Parana are associated with it. In the south, the Parana and Chaco-Pampas depressions adjoin the Brazilian Shield.

The surface of the shield is very uneven and considerably raised. Throughout the shield corresponds to the Brazilian Highlands. This is an undulating plain, located on average at an altitude of 600-800 m above sea level. The crystalline foundation of the shield is broken by numerous faults into blocks that are significantly displaced relative to each other. The position of the blocks creates the orographic appearance of the highlands.

The most elevated part of the Brazilian Highlands is made up of blocky massifs of Pico di Bandeira - 2884 m and the city of Itatnaya - 2821 m above sea level. In the central part of Brazil, the watershed of the Paranaiba - Tacantins rivers rises to 1678 m. Atlantic Ocean. Along the right bank of São Francisco, blocky ridges of the Serra do Espinhaço (up to 1800 m) stretch from the northeast to the southwest. In the south of the highlands lies the vast Serra Geral lava plateau, rising to a height of up to 1018 m.

The structure of the Brazilian shield is very complex and has not yet been sufficiently studied. The stratigraphic subdivision of the sedimentary-metamorphic complexes composing it includes an extremely large number of series and systems, the relationship of which is not unified. Conditionally, in the structure of the crystalline basement, the Precambrian is lower, middle and upper. The most ancient are the Bakoa gneisses, whose age is 2400-2500 million years. Younger formations of the Middle and Upper Precambrian are distinguished in the Minae and Itakolomi series.

The composition of the Minae series is quite variable. In the Barbacena area, it is represented by strata of gneisses and shales; north of Lafayette, the Middle Precambrian includes conglomerates, quartzites, dolomites, iron formations, graphite phyllites, lava flows and volcanic tuffs. The thickness of the series exceeds 3000 m. It includes intrusions of ultramafic rocks and diorites. The ultramafic rocks are locally transformed into serpentinite and talc schists. The entire sequence has a northeast strike. In its southern part, isoclinal folding is well expressed. Numerous faults are known. The formation of this series is correlated with the Grenville formations of North America.

The Itacolomi Series of the Upper Precambrian of Brazil is composed of sedimentary-metamorphic strata, which include phyllites, itabirites (thin-layered, flyschoid, ferruginous quartzites), dolomites, detrital rocks, talc schists, etc. The thickness of the series is about 3000 m.

The general section of the ancient deposits of the Brazilian Shield ends with clastic sedimentary rocks of the Lavras and Bambum series, the age of which is considered Late Precambrian - Early Paleozoic. Some deposits of the Lavras Series are considered as tillites.

The structure of the Brazilian Shield is not well understood. So far, there are four stages in the history of its structure formation: 2400–2510, 1000–1100, 720–760, and 460–600 Ma (Tugarinov and Voitkevich, 1966). Structural relationships of parts of the shield of different ages are most fully displayed in the state of Mipas Gerais. The central part of the massif here is made up of Bakao gneisses (2400, 2510 Ma), they are bordered by formations aged 1350 Ma, further - sedimentary-metamorphic strata of Rio das Veyjas. From the east and west they are bordered by the formations of the Minae series, and from the south by the massifs of the Itakolomi series.

Thus, the general plan of the structure of the Brazilian Shield is a consistent expansion of the ancient structural centers due to the attachment of folded regions, which is also characteristic of the South American platform. The consolidation of the Brazilian Shield ended in the Late Precambrian. Subsequently, its surface was leveled for a long time and was the arena for the formation of a platform cover. The submeridional depression separating the shield is filled with Paleozoic and Mesozoic deposits. In some places, the platform cover on the shield is composed of Triassic continental formations, marine layers of Turonian and Paleocene age in the northern part and in the center - continental horizontally occurring Eocene strata.

The relief of the Brazilian Shield, as well as other Precambrian massifs, is characterized primarily by the position of the leveling surface deformed by faults and the position of blocks. In exposed places, the surface of the Precambrian basement has the appearance of a hilly or undulating plain, the features of which vary significantly depending on the composition of the exposed rocks. The surface dissected by erosion is characterized by rocky relief. The rivers here are rapids, mountainous.

In places covered by platform cover, the Brazilian Shield has a two-story structure. The lower floor is a crystalline plinth, the upper one is a platform cover. It is characterized by a flat surface of plateaus and plateaus, mesas, remnant elevations, limited steep or gentle slopes, the features of which in each individual case are due to the nature of the deposits exposed by depudation and many climatic factors.

In the southern part of the mainland of South America, the Precambrian formations act as separate, unrelated massifs, which in the past were independent islands. Their structure has been studied very little.

In the structure of the Uruguay crystalline shield, the Lower, Middle, and Upper Precambrian are distinguished. The Lower Precambrian deposits are extended along the La Plata valley and have a sublatitudinal strike. Their composition includes various gneisses and mica schists hosting granite intrusions. The Middle Precambrian - the Minae Formation of Uruguay - includes massive quartzites, lenses of crystalline limestones, talc schists and volcanogenic deposits. The intrusions are represented by alkaline rocks and granitoids. Upper Precambrian rocks are combined into the Otgua series. The latter includes volcanic breccias and folded quartzites. Their structures extend in the meridional and northeast directions.

Between Uruguay and the Brazilian Shield, a vast territory is occupied by the Serra Geral volcanic plateau, structurally connected with the La Plata depression. The plateau has a flat, slightly dissected surface.

Crystalline massifs in the central part of South America stand out along Paraguay - the Ana and Tebikuari horsts. In the south of the mainland, the Precambrian protrusions are concentrated in the west and are adjacent to the mobile Pacific zone. In Patagonia, they form separate shields separated by large depressions. In the Precambrian of the central part of Argentina, phyllites and greywackes are known, crumpled into folds. Their age is considered late Precambrian. In the ridges of Catamarca, La Rioya, San Luis, metamorphic strata contain granite batholiths. The gneisses of the hills of Buenos Aires host diorite intrusions.

There is still very little data on the features of the relief of the Precambrian massifs in the southern part of the South American Platform.

From the west, South America is bordered by a grandiose shaft of the South American Cordillera, which separates the platform from the Pacific Ocean. Between platform and folded mountain system a piedmont trough extends, filled mainly with Cenozoic deposits. The structure of the Cordillera is complex and combines parts of different ages. The model of the cross section of the Cordillera folded zone from east to west consists of the following structural elements:

1) a platform steeply plunging to the west;

2) the forward trough of the Andes;

3) Eastern Cordillera, composed of sedimentary deposits of Paleozoic age, crumpled into folds. At the outer edge, this fold system contains isolated massifs of Precambrian schist, including granite intrusions;

4) Western Cordillera, composed of marine sediments of the Mesozoic age and younger volcanogenic formations. Their volcanic cones form the highest peaks - Chimborazo 6310 m, Cotopaxi 5943 m. In the structure of the mountains, a batholith elongated along the strike of the mountains stands out;

5) remains, or, more precisely, islands, mainly of Hercynian structures. The entire mountain range rises steeply above the adjacent deep depressions of the Pacific Ocean floor.

There are four phases in the formation of the structure of the South American Cordillera. The main folds and faults were laid in the chalk. Thrusts were formed, volcanic activity became more active. Structural formation reached its greatest strength in the early Oligocene, when the Eastern Cordillera was formed. Volcanic activity began in the Andes and continues to this day. A new intensification of movements occurred in the Miocene. Then there were many faults and normal faults, accompanied by numerous intrusions. Intrusive rocks of this age are especially common in the foothills of the Andes. Later, a leveling surface was developed in the Andes. The last phase of mountain building occurred in the Pleistocene. As a result of the general arched uplift, the modern Andes were formed. The uplift was accompanied by grandiose faults and block movements that created modern relief mountains (King, 1967).

The structure of the South American Cordillera, as rightly stated by W. Oppenheim (Oppenheim, 1948), is final result development of the Late Mesozoic island arc composed of igneous rocks. The islands were separated from the mainland by a geosynclinal trough, and from the ocean by a deep depression. This structure emerged in the Cretaceous, during the first phase of orogeny in the Andes. Since then, the western structural boundary of the mainland has changed little. At the beginning of the Cenozoic, the islands, in the structure of which volcanic rocks took part, gradually united into one mountain shaft. The adjacent geosyncline was filled with terrigenous masses and limestones of marine origin. Accumulation continued until the Middle Oligocene. In the middle of the Cenozoic, the Eastern Cordillera took shape. The succession of mountain uplift is reflected in leveling surfaces and river terraces, indicating periodic rejuvenation of valley erosion.

Structural and geomorphological analysis shows that the mainland of South America has a heterogeneous structure. Its main components - the Guiana and Brazilian shields and the Amazonian trough that separates them - are the most ancient parts of the mainland. They are characterized by a sublatitudinal extent. The southern part of the mainland unites structures of different ages, the main elements of which are paleotectonic island systems, in the east - crystalline massifs of the southern part of the East Brazilian, Coastal and Uruguayan shields, in the west - the blocky country of the Pampa, the North and South Patagonian shields, etc. Between the eastern and the western systems, the La Plata depression in the south has the same importance in the structure of the mainland as the Amazonian depression in the north. With the formation in the Cenozoic of the complex folded system of the South American Cordillera at the site of the island arcs, the final configuration and orography of South America was determined.