This platform experienced a short-term uplift at the beginning of the Silurian as a result of the manifestation of the Taconic phase of folding in the Appalachian geosyncline. Regression gave way to transgression Withwidespread carbonate sediments and reef formations.
Silurian deposits are represented by limestones and dolomites. In the Lower Silurian sections there are many reef structures; in the Upper Silurian, halogen rocks appear, especially in the east of the platform - anhydrites, gypsum and rock salt.
At the very end of the Silurian, huge salt basins arose in North America. The thickness of the Silurian is measured at several hundred meters. In depressions it increases, for example, in the Michigan Trench - up to 1.5 km.
Gondwana
Southern continents in the Silurian they still stand above sea level, and Silurian sediments are insignificant, but where they exist (on the periphery of Gondwana), they are represented by terrigenous formations.
In the South American part of Gondwana, at the end of the Ordovician - the beginning of the Silurian, a restructuring occurred, probably caused by the influence of Caledonian folding. In the Silurian, the area of the sea increased. Depressions of a meridional direction appeared. They accumulated significant thickness (up to 800-1200 m) of clastic sediments with subordinate carbonate layers. In the Amazon basin (latitudinal direction) marine sandy-clayey sediments with a thickness of 100 m are observed. In the late Silurian and the very beginning of the Devonian, uplifts occurred again as a consequence of the Late Caledonian movements.
In the African part of Gondwana, sandy strata at the end of the Ordovician and Silurian were replaced by dark clays with graptolites. Carbonate silts appeared in the northern part of the basin. Coastal sands were deposited along the margins of the marine accumulation area. The thickness of Silurian rocks is usually small. On Arabian Peninsula The Silurian is represented by a continuous section of sandy-clayey formations of considerable thickness. At the end of the Silurian, regression began everywhere in Africa, especially clearly manifested in Arabia.
The Australian part of Gondwana in the Silurian was predominantly land.
History of the development of geosynclinal belts North Atlantic geosynclinal belt
Grampian geosynclinal region. Grampian geosyncline. A cross-section of the Silurian of Wales, the stratotype area where the Silurian system was identified, can be seen in diagram III, color. on
The Silurian overlies the Ordovician with a structural unconformity caused by the Taconic folding. At the base of the Llandovery lie conglomerates and sandstones, which are replaced higher up by sandy-clayey strata with shell rocks; Pentamerids are numerous (the thickness of Llandovery reaches 1.5 km). Wenlock is lithologically diverse: V In some areas, calcareous-clayey rocks and
limestones with remains of brachiopods and corals (300-400 m), in others there is a thick sequence of sandstones and siltstones (thickness -1.2 km). Ludlovsky deposits are predominantly carbonate: limestones, calcareous shales, calcareous siltstones. Stromatoporates, corals, and brachiopods are numerous (thickness - 0.5 km). There are fossil banks with Conchidium knighti. In the upper part of the tier there is a layer of so-called bone-bearing breccia, consisting 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 sequence of graptolite shales. In this case, clayey material was deposited in deep-sea areas. The third type of cut is mixed. It contains breeds of the first and second types.
The uppermost part of the Silurian section in England is distinguished as the Downtonian stage (thickness -0.6-0.9 km). These are red and variegated sandy-clayey rocks with interlayers of red marls. They contain shells of ostracods and ichthyofauna. Gradually, the Downtonian is 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 sediments are folded and metamorphosed. The Caledonian folding manifested itself repeatedly and was accompanied by magmatism.
In the Scandinavian part of the Grampian geosyncline, thick clastic strata accumulated, initially typically marine, and by 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 there was a quiet accumulation of carbonate and terrigenous sediments (up to 2 km) in 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 down, which later turned into the existing anticlinoria and synclinorium. The Silurian of the Urals on 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-clay deposits (500-1500 m) with a diverse complex of organic remains accumulated here. Shallow coastal sand and pebble rocks are known on the western edge 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.
The 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 Kazakhstan part of the Ural-Mongolian belt. They are represented by typical geosynclinal formations of considerable thickness with the remains of a rich fauna. Characteristic horizons are brachiopod and coral limestones.
In the context of the ridge. Chingiztau Silurian is represented only by the lower section (see diagram III, color on). Silurian sediments (up to 2.5 km) accumulated in eugeosynclinal marine environments with strong volcanism. The Caledonian folding was actively manifested. 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.
Tia. The shallow-lying Lower and Middle Devonian effusives and felsic tuffs crowning the section accumulated already in terrestrial conditions. They are usually isolated into volcanogenic molasse 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 a rich fauna predominate, in the east (Western Sayan, Tuva) the role of coarse clastic rocks with a depleted fauna increases. The thickness of 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 diagram III, color incl.), Silurian deposits (Chergak series) lie conformably on Ordovician ones. They are thick (2.5-3 km) and consist of sandy-clayey rocks with interlayers, packs and lenses of limestone. The highest carbonate content is confined to the middle part of the section. The fauna is rich and diverse. These are stromatoporates, tabulates, heliolitids, rugosas, crinoids, bryozoans, brachiopods, trilobites. Many local (endemic) forms. Obviously, in the Silurian there existed a shallow sea basin with small reefs, coral and crinoid thickets, and banks of brachiopods. The endemism of the fauna indicates 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 Tuvan complex was formed with marine sediments in the middle part and red continental rocks in the base and roof. The deposits of the Tuvan complex are folded and intruded by small basic and acidic intrusions. The upper part of the section under consideration is composed of thick terrestrial effusives of the Lower Devonian and red clastic rocks of the Middle Devonian. These are continental deposits of intermountain basins, formed during regression caused by the Caledonian folding. - "In the section of Western Tuva, three structural floors, sharply different from each other, are clearly distinguished: 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 are recorded different stages geological development: the first is eugeosynclinal, the third is orogenic, and the second is intermediate (transitional). At the second stage, subsidence developed on an already consolidated foundation; the regime resembled a miogeosynclinal one. Associated with acid intrusions ore deposits iron and copper.
Thus, the Caledonian era of tectogenesis covered the areas 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 close to those previously described in the Ordovician are preserved. This is still the island land of the Franco-Bohemian massif (Moldanuba block) and the marine conditions to the north and south of it (Prague synclinorium, see diagram III, color on). IN northern Europe sandstones, black clay shales, bituminous limestones accumulate (thickness - 0.5 km), 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, giving way to limestones at the tops of the section.
IN Asian geosynclinal region 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 acidic composition accumulated, or low-thickness 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. Oolitic deposits formed in the Silurian iron ores Clinton (USA) and a number of small ones in Africa. Deposits associated with Caledonian acid intrusions gold Northern Kazakhstan, Kuznetsk Alatau and Mountain Shoria.
Found 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 a building material and a good ceramic raw material.
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 the county of Devonshire, after which it was named.
The duration of the Devonian period is 48 million years, its beginning is 408 million years ago, and its end is 360 million years ago.
"The Devonian sections of Great Britain are composed of continental facies and can be combined with stratotypes to distinguish stages. Therefore, the division of the Devonian system was carried out in the Ardennes in Belgium, France and in the Rhine Slate Mountains in 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 Devonian, as mentioned above, is drawn at the base of the graptolite zone Monograptus uniformis(Barrandien, Czech Republic). Currently, this boundary is the only one officially accepted by the Stratigraphic Commission of the International Geological Congress. The upper limit has not been officially approved. Due to the fact that at the beginning of the Devonian period the extensive regression that began in the Silurian continued, many different facial settings with corresponding fauna arose. This greatly complicates the division and comparison of sections and was the reason for the creation of a “composite” scale, consisting of tiers installed in different regions. The stage division of the Lower Devonian of Barrandien, Rhineland is based on marine fauna, and sediments of England corresponding in age - on the remains of fish found in lagoonal-continental sediments.
Zhedino stage, named by A. Dumont in 1848 after 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 overlie Cambrian deposits transgressively (hence the difficulty in determining the exact boundary with the Silurian). In the stratotype Bottom part It is represented by Fepan conglomerates with a thickness of 10-40 m, Ebb arkoses with a thickness of 30 m and Mondrechon shales with sandstone interlayers. Sandstones and shales contain rich brachiopod assemblages. In the upper part there are red and burgundy shales with small calcareous concretions; interlayers of red
and green sandstones and quartzites. They are characterized by fish remains. The total thickness is 750 m.
The name “Siegenian Stage” was first used by E. Kaiser, denoting the greywackes in the Rhine Slate Mountains. The 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 K. Dorlodo in 1900 in the town of Ems 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. The layers contain accumulations of brachiopods, bivalves, and occasionally corals (Fig. 51).
Previously, the Siegen and Emsian stages were combined into one stage, which was called the Koblenzian. However, according to the decision of the International Stratigraphic Commission, the Lower Devonian is now accepted as 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 calceolic and upper culture-jugate Lauch 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 large quantities there are corals of genera Favosites, Calceola, Damophyllum, remains of cephalopods and conodonts.
The Givetian Stage was identified in the Ardennes by J. Gossel in 1879. The name comes from the city of Givet, located in Northern France. This stage unites 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-clastic limestones.
The Frasnian stage was established in 1879 by J. Gossel in Belgium. Got its name from the village. Fran near the city of Kouven. 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 received its name from the Famennian area in Belgium. Sandstones and shale with interlayers of limestone are developed here. In stratospheric 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 60s, Czechoslovak researchers proposed to distinguish the Lochkovian and Prague stages instead of the Zedino and Siegen, established in the marine sections of the Barrandova Trough in the Bohemian Massif, not far from Prague, which were perfectly characterized by fauna. Here is also the recognized boundary of the Silurian and Devonian, drawn between the Przydolian and Lochkovian stages. In 1985, the International Subcommission on Devonian Stratigraphy recommended the Lochkovian and Prague stages of the Czech Republic as type stages for the lower Devonian. Since then, geologists have used precisely these stages, although the former Zhedino and Siegen stages approximately corresponding to them have not been formally abolished. This explains the “dual power” at the bottom of the tier scale of the Devonian system.
Typical sections of the Devonian system are presented in diagrams IV and V, color. on
The organic world of the Devonian period was rich and diverse. Terrestrial vegetation has made significant progress. The beginning of the Devonian period was characterized by a wide distribution of “psilaphytes” (rhiniophytes), which reached their greatest prosperity 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); rhyniophytes:18 - Rhynia(Early Devonian), 19, 20 - Sawdonia(Early Devonian)
(Fig. 52, color included). Their dominance is observed in wetland landscapes. At the beginning of the Middle Devonian, rhyniophytes died out and were replaced by proto-ferns, which began to develop leaf-like forms. In the Middle Devonian, all the main groups already existed spore plants. These are lycophytes, arthropods and ferns, and at the end of the Devonian the first representatives of gymnosperms appeared; many of the shrubby ones turned into tree-like ones and gave rise to the first layers of coal (Spitsbergen, Barzas). The Late Devonian flora was called Archaeopteris, after the widespread heterosporous fern Archaeopteris(Fig. 53, color included). At the end of the Devonian, forests consisting of the plants listed above already existed on the planet.
Conodonts are of greatest biostratigraphic importance in the Devonian. These representatives of 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 flowering is observed. Conodonts changed so quickly in the Devonian that they make it possible to distinguish more than 50 standard zones in Devonian deposits with a duration of the Devonian period of about 50 million years. This is a striking example of using the remains of rapidly evolving organisms to create ultra-detailed stratigraphy. w Graptolites survive in the Devonian (one genus rarely found in the Lower Devonian Monograptus) and cystoids; The diversity of forms of trilobites and nautiloids is sharply reduced. Castle brachiopods (brachiopods) from the family Spiriferiidae with the main genus are widespread Spirifer and pentamerides (genus Pentamerus), four-rayed corals, tabulates.
Of significant importance are the cephalopods (Fig. 51): the orders Goniatita, Agonyatita and Clymenia. They have a simple septal line with solid pointed lobes and solid rounded saddles (goniatite), or with rounded lobes and saddles (agoniatite). Clymenia are 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 exclusively of the Late Devonian.
For the first time in the history of the Earth, bivalves and some lower crustaceans began to play a major role, 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 have conodonts, ammonoids, brachiopods, corals, tentaculites and ostracods. Vertebrates began to acquire increasing importance. Jawless fish and especially fish are widespread: lungfish, armored fish, lobe-finned fish, cartilaginous fish (sharks, rays) (Fig. 51). In freshwater and brackish water basins, fish were apparently already numerous. The first amphibians, stegocephalians, are known from the Devonian.
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.
Structures earth's crust and paleogeography v
During the Devonian period there are no significant changes in the distribution and outline of the main structural elements the earth's crust created by the beginning of the Devonian (platforms, geosynclinal belts and Caledonides). This is explained by the weak development of folding processes in the Devonian, which are characterized by low intensity. Only at the end of the period did it appear in some geosynclinal areas Breton folding phase - beginning Her-cyn era of tectonogenesis. The Breton folding phase is established in the north-west of the Mediterranean (European) geosynclinal region (Brittany Peninsula) and in the South Appalachian geosynclinal region. The Caledonian folding led to uplifts of not only the Caledonides regions, but also 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 wanderings. Sedimentation on the platforms decreased sharply; 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 mid-Devonian, in many areas of the world, ascending movements gave way to subsidence, and a new transgression developed. The sea advanced on the platforms and penetrated into the Caledonides (see diagram IV, color on).
At the end of the Late Devonian, in the Famennian Age, the uplift of the platforms began again (Breton phase) and, in connection with this, some regression of the sea.
; Characteristic feature The Devonian is the formation of intermountain depressions in which continental terrigenous, predominantly red-colored sediments and volcanics 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 broken 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- stage of development of geosynclines.
The beginning of the Devonian period (Early Devonian era) deserves the name geocratic eras in the life of the Earth, that is, eras with a predominance of the continental regime. Since the Middle Devonian era, the areas occupied by seas have increased, both on platforms and in geosynclinal areas. The land area is decreasing. At the same time, a general leveling occurs, a gradual peneplanation continents, as well as island land areas scattered across geosynclinal areas. This is evidenced by the almost universal change from terrigenous sedimentation, characteristic of the Early Devonian, to carbonate. Until the end of the Devonian period, the mountainous relief remained most stable in the Caledonian regions, 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 era, in contrast to the Early Devonian, especially its first half (Frasnian age) was a time of widespread development of marine transgressions, a time of predominant dominance of the sea over the land. Similar eras in the life of the Earth are called thalassocratic.
Restoring the position of the Devonian climatic zones is difficult, since terrestrial 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 to restore the general picture of climatic zonation in the Devonian period.
When considering the conditions for the formation of the “ancient red sandstone,” many facts point to the arid climate of the intermountain depressions in which these sediments accumulated. Apparently, the middle part of the Russian Plate was characterized by a dry and hot climate in the Devonian, as evidenced by the widespread development of lagoonal chemogenic sediments (dolomite, gypsum, etc.) here. The same precipitation marks a zone of arid climate within Europe, stretching from northwest to southeast. From other evidence of the Devonian climate - the tillies of the Cape Mountains South Africa(thickness 30 m), length 500 km. It is unclear whether the moraine accumulations associated with this glaciation are of continental or mountain origin. No other manifestations of glacial activity in the Devonian are known.
The most characteristic Devonian facies is the "Old Red Sandstone" facies. (Old Red sandstone) widespread in all countries of the Northern Hemisphere (Fig. 54). It is assumed to be a continental sandy desert facies. However, finds of organic remains in red sandstone (shelled fishes, phyllopods) suggest that this facies is mixed
Rice. 54. Schematic map of the continent of ancient red sandstone and its bordering zone / - the main modern outcrops of ancient red sandstone; 2 - Hercynian massifs (marine Devonian); S-S- northern border marine transgressions onto the continent of ancient red sandstone; Yu-Yu- southern boundary of the distribution of ancient red sandstone layers in the marine Devonian Central Europe(Ginou, 1952)
lagoon-continental and lagoon-sea. In addition to "ancient red sandstone", lagoonal facies are often represented by the facies of closed brackish basins. They formed the oil-bearing facies of cypridine shales and the peculiar Domanik facies of the European part of Russia.
History of platform development
The relief of South America is varied. Based on the nature of the geological structure and the features of the modern relief, South America is divided into two heterogeneous parts. The eastern part of the continent is the ancient South American Plate; Western - actively developing pleated belt Andes. The raised sections of the platform - the shields - correspond in relief to the Brazilian and Guiana plateaus. The troughs of the South American platform correspond to gigantic lowland plains - the Amazonian, Orinoco, a system of internal plains (Gran Chaco plain, Laplata lowland), and the young Patagonian platform - the plains of Patagonia.
The Amazonian lowland is filled with marine and continental sediments. It was formed as a result of the activity of the Amazon River, as a result of the accumulation of sediment brought by the current. In the west, the lowland is very flat, the river valleys are weakly incised, the heights barely reach 150 m. Its northern and southern outskirts, underlain by crystalline shield rocks, are elevated and gradually turn into plateaus.
The Brazilian plateau is located in the east of the mainland. It represents protrusions of the crystalline foundation of the platform, between which there are troughs filled with sedimentary rocks and volcanic lavas. This is the largest rise within the platform. The Brazilian plateau has altitudes from 250-300 m in the north to 800-900 m in the southeast. The relief of the plateau is a relatively level surface, above which blocky massifs and plateaus rise.
In the north of the continent, the Guiana Plateau (300-400 m) is confined to the vast protrusion of the folded base of the platform. Its relief is dominated by stepped plateaus.
The vast plains and large areas of the plateaus of South America are convenient for the life and economic activities of the population. (Show the largest lowlands and plateaus on the map and determine their maximum heights.)
The Andes are the longest Mountain chain on land 9000 km long. The Andes are one of the highest mountain systems in the world. In height it is second only to the Tibetan-Himalayan mountainous country. Twenty peaks of the Andes rise to a height of more than 6 thousand m. The highest of them is the city of Aconcagua (6960 m).
The formation of the Andes is the result of the interaction of two lithospheric plates, when oceanic plate Nazca "drew" under the continental South American. At the same time, the edge of the continental plate folded into folds, forming mountains. Currently, mountain building continues. This is evidenced by the eruptions of numerous volcanoes and severe catastrophic earthquakes. Among the large volcanoes we can note such as Chimborazo (6267 m), Cotopaxi (5897 m). West Coast occupied by the Andes belongs to the Pacific “Ring of Fire”.
The strongest in the world recorded at 11-12 points occurred in 1960 in Chile. In 2010, an earthquake in Chile claimed several hundred lives. Serious disasters occur in the Andes every 10-15 years.
The Andes mountain system consists of several meridianally elongated mountain ranges. Between the ranges lie internal plateaus and plateaus, ranging in height from 3500 to 4500 m.
Minerals of South America
The continent is rich in minerals. The richest deposits of iron and manganese ores are confined to the ancient shields of the South American Platform: the center and outskirts of the Brazilian Plateau, as well as the north of the Guiana Plateau. Largest district iron ore mining is Carajas. In the northern part, on the outskirts of both plateaus, there are very large deposits bauxite, raw material for the aluminum industry. Bauxite occurs at shallow depths and is mined by open-pit mining.
In the Andes, ores of copper (Peru, Chile), tin (Bolivia), lead and zinc (Peru) have been explored. The foothills of the Andes, especially Venezuela and Colombia, are rich in oil and natural gas. Place of Birth coal less significant (Ecuador, Argentina). Many Andean countries are famous for their mining of precious stones. This primarily applies to emerald mining in Colombia. From noble metals In South America, the largest reserves of silver are in Peru. The Andes belt is also famous for some non-metallic minerals. Among them, saltpeter takes first place. The famous Chilean saltpeter and iodine are mined in the dried-up reservoirs of the Atacama.
The relief of South America is more diverse compared to Africa and Australia. The high Andes in the west separate the mainland from Pacific Ocean. South America is characterized by active seismicity. South America is called the "storehouse of the world." The mainland is rich natural resources necessary for the development of many sectors of the economy.
This platform experienced a short-term uplift at the beginning of the Silurian as a result of the manifestation of the Taconic phase of folding in the Appalachian geosyncline. Regression gave way to transgression With wide distribution of carbonate sediments and reef formations.
Silurian deposits are represented by limestones and dolomites. In the Lower Silurian sections there are many reef structures; in the Upper Silurian, halogen rocks appear, especially in the east of the platform - anhydrites, gypsum and rock salt.
At the very end of the Silurian, huge salt basins arose in North America. The thickness of the Silurian is measured at several hundred meters. In depressions it increases, for example, in the Michigan Trench - up to 1.5 km.
Gondwana
The southern continents in the Silurian are still above sea level, and Silurian sediments are insignificant, but where they exist (on the periphery of Gondwana), they are represented by terrigenous formations.
In the South American part of Gondwana, at the end of the Ordovician - the beginning of the Silurian, a restructuring occurred, probably caused by the influence of Caledonian folding. In the Silurian, the area of the sea increased. Depressions of a meridional direction appeared. They accumulated significant thickness (up to 800-1200 m) of clastic sediments with subordinate carbonate layers. In the Amazon basin (latitudinal direction) marine sandy-clayey sediments with a thickness of 100 m are observed. In the late Silurian and the very beginning of the Devonian, uplifts occurred again as a consequence of the Late Caledonian movements.
In the African part of Gondwana, sandy strata at the end of the Ordovician and Silurian were replaced by dark clays with graptolites. Carbonate silts appeared in the northern part of the basin. Coastal sands were deposited along the margins of the marine accumulation area. The thickness of Silurian rocks is usually small. On the Arabian Peninsula, the Silurian is represented by a continuous section of sandy-clayey formations of considerable thickness. At the end of the Silurian, regression began everywhere in Africa, especially clearly manifested in Arabia.
The Australian part of Gondwana in the Silurian was predominantly land.
History of the development of geosynclinal belts North Atlantic geosynclinal belt
Grampian geosynclinal region. Grampian geosyncline. A cross-section of the Silurian of Wales, the stratotype area where the Silurian system was identified, can be seen in diagram III, color. on
The Silurian overlies the Ordovician with a structural unconformity caused by the Taconic folding. At the base of the Llandovery lie conglomerates and sandstones, which are replaced higher up by sandy-clayey strata with shell rocks; Pentamerids are numerous (the thickness of Llandovery reaches 1.5 km). Wenlock is lithologically diverse: V In some areas, calcareous-clayey rocks and
limestones with remains of brachiopods and corals (300-400 m), in others there is a thick sequence of sandstones and siltstones (thickness -1.2 km). Ludlovsky deposits are predominantly carbonate: limestones, calcareous shales, calcareous siltstones. Stromatoporates, corals, and brachiopods are numerous (thickness - 0.5 km). There are fossil banks with Conchidium knighti. In the upper part of the tier there is a layer of so-called bone-bearing breccia, consisting 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 sequence of graptolite shales. In this case, clayey material was deposited in deep-sea areas. The third type of cut is mixed. It contains breeds of the first and second types.
The uppermost part of the Silurian section in England is distinguished as the Downtonian stage (thickness -0.6-0.9 km). These are red and variegated sandy-clayey rocks with interlayers of red marls. They contain shells of ostracods and ichthyofauna. Gradually, the Downtonian is 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 sediments are folded and metamorphosed. The Caledonian folding manifested itself repeatedly and was accompanied by magmatism.
In the Scandinavian part of the Grampian geosyncline, thick clastic strata accumulated, initially typically marine, and by 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 there was a quiet accumulation of carbonate and terrigenous sediments (up to 2 km) in 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 down, which later turned into the existing anticlinoria and synclinorium. The Silurian of the Urals on 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-clay deposits (500-1500 m) with a diverse complex of organic remains accumulated here. Shallow coastal sand and pebble rocks are known on the western edge 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.
The 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 Kazakhstan part of the Ural-Mongolian belt. They are represented by typical geosynclinal formations of considerable thickness with the remains of a rich fauna. Characteristic horizons are brachiopod and coral limestones.
In the context of the ridge. Chingiztau Silurian is represented only by the lower section (see diagram III, color on). Silurian sediments (up to 2.5 km) accumulated in eugeosynclinal marine environments with strong volcanism. The Caledonian folding was actively manifested. 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.
Tia. The shallow-lying Lower and Middle Devonian effusives and felsic tuffs crowning the section accumulated already in terrestrial conditions. They are usually isolated into volcanogenic molasse 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 a rich fauna predominate, in the east (Western Sayan, Tuva) the role of coarse clastic rocks with a depleted fauna increases. The thickness of 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 diagram III, color incl.), Silurian deposits (Chergak series) lie conformably on Ordovician ones. They are thick (2.5-3 km) and consist of sandy-clayey rocks with interlayers, packs and lenses of limestone. The highest carbonate content is confined to the middle part of the section. The fauna is rich and diverse. These are stromatoporates, tabulates, heliolitids, rugosas, crinoids, bryozoans, brachiopods, trilobites. Many local (endemic) forms. Obviously, in the Silurian there existed a shallow sea basin with small reefs, coral and crinoid thickets, and banks of brachiopods. The endemism of the fauna indicates 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 Tuvan complex was formed with marine sediments in the middle part and red continental rocks in the base and roof. The deposits of the Tuvan complex are folded and intruded by small basic and acidic intrusions. The upper part of the section under consideration is composed of thick terrestrial effusives of the Lower Devonian and red clastic rocks of the Middle Devonian. These are continental deposits of intermountain basins, formed during regression caused by the Caledonian folding. - "In the section of Western Tuva, three structural floors, sharply different from each other, are clearly distinguished: 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, 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 era of tectogenesis covered the areas 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 close to those previously described in the Ordovician are preserved. This is still the island land of the Franco-Bohemian massif (Moldanuba block) and the marine conditions to the north and south of it (Prague synclinorium, see diagram III, color on). In northern Europe, sandstones, black shales, bituminous limestones (thickness - 0.5 km) accumulate, and 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, giving way to limestones in the upper sections.
IN Asian geosynclinal region 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 acidic composition accumulated, or low-thickness terrigenous-carbonate facies accumulated in miogeosynclinal zones (Zagros Himalayas, etc.).
America occupies the position of a watershed between the expanses of the Atlantic and Pacific oceans.
On 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 banks. The continental slope is sharply defined and steep, rising some distance from the coast above the great depths of the Atlantic Ocean.
Huge land masses Western Hemisphere- North and South America are independent, historically unconnected continental structures. At the same time, both continents have a lot in common. Their wedge-shaped outlines have a southern direction. The extended part of the land faces north. The western shores of the continents are bordered by high mountain ranges, in their eastern part plains predominate. North America is located much further west in relation to South America. The continents are separated by a latitudinal mobile zone, in which the island arcs of the Antilles and the mountain structures of Central America, already articulated with the continents, are located. The Antillean-Mexican region, as we noted (Bondarchuk, 1946), is a structural analogue of Indonesia, located between the continents of Asia and Australia.
North American Platform . In most of the territory North America a crystalline Precambrian basement can be traced. Rocks of Precambrian age are found in the area Canadian Shield. Individual blocks of the Precambrian appear in Colorado, the Rocky Mountains, and in the basin and ridge provinces. Most of the North 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 data from K. K. Stockwell (1967) and F. B. King (1967), is characterized by such features. The oldest part The crystalline basement in the Hudson Bay basin, the central part of the USA and the Arctic islands is covered with a platform cover. The Canadian Shield has a zonal structure of folded zones of Precambrian age, gradually increasing its boundaries. Paleozoic and subsequent folded structures, in the same way building up the platform, determined modern features tecto-orogeny of the North American continent.
Within the territory under consideration, Precambrian folds are distinguished (King, 1967): Kenoran, Hudson, Elson and Grenville. They deform thick Precambrian strata, which have a complex composition. The most ancient formations The shield is considered to be volcanic and sedimentary formations located among gneiss fields and other metamorphic rocks. These formations, like the surrounding gneisses, host numerous gabbro and granite intrusions of different ages. Precambrian folded zones characterize individual provinces.
The Kenoran fold is located in the southeast of the shield in the Upper and Slane provinces, as well as in the northwestern part of it, bordering younger structures. Its age is 2390 million years.
Undisturbed strata of the platform cover of Proterozoic age lie on the leveled surface of the Kenoran folding. Huronian folding covers Proterozoic deposits and older undivided gneisses and granites. It occupies the northeastern part of the shield, where it is adjacent to the Kenoran fold. In the northwestern part of the Canadian Shield, the Huronian fold is located between the areas of the Kenoran fold. in Labrador and southern outskirts Rocky Mountains, Nain Province, but according to F.B. King, these structures were reworked by later, Olsonian folding.
The Huronian folding on the Canadian Shield is expressed in the Churchill, Bor and South provinces. 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.
On the Huronian folded basement, Late Proterozoic sediments lie horizontally.
In the southeast of the Canadian Shield there is an area of the Grenville fold, concentrated mainly in the Grenville province. During the Grenville folding era, older structures were reworked. This folding dates back to the Late Proterozoic. It ended about 800 million years ago. On the Huronian folded foundation, in some places, a platform cover of Late Proterozoic age has been preserved.
Intrusions of basic rocks, mainly gabbros 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 of different ages and are associated with the corresponding folding phases. The largest intrusions are concentrated in the strata of the Kenoran structural floor. Among the post-orogenic formations, “circular structures” are distinguished, which are considered cryptovolcanic formations. They represent rings of highly dislocated rocks of the platform cover and some of them belong to Precambrian formations. Individual circular structures cut through the Kenoran and Grenville deposits. They contain igneous rocks and volcanic breccias of post-Ordovician age. Among the platform formations, gabbro and diabase dikes are also known. Where the crystalline basement is exposed, all these rocks can be traced in relief.
The Precambrian basement of the North American platform is completely leveled. It is strongly dissected by faults into blocks, the different positions of which create a number of depressions and hills (Nalivkin, Gostintsev, Grossheim, 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 varies. In the Lake Superior region, the Keninawan Series of platform cover forms a broad syncline. Its layers are broken by faults and contain numerous sheet intrusions of gabbro. In the western part of the shield and up to the Cordillera, the platform cover is formed by the Belt sedimentary series, also of Precambrian age. Its position is not disturbed.
In the Hudson Bay region, between the shield and the Appalachians, sediments of Paleozoic age take part in the structure of the shield. They make up 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 rocks of Mesozoic and Cenozoic age.
In the southwestern part, the North American Plate extends to the Rocky Mountains. Here it is divided 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 system of islands of the Cordillera fold zone. The Colorado Plateau is bordered on all sides by structures of the Rocky Mountains. Only in the southwest does it end with a steep ledge towards the Gila Valley.
The surface of the plateau rises to 1800-2600 m above sea level. The highest point is Mount San Francisco (3840 m) - an extinct volcano. The surface of the plateau is heavily denuded. Above it rise table ootans mountains and individual laccoliths. River valleys form grandiose canyons up to 1800 m deep.
The base of the Colorado Plateau is composed of crystalline rocks of Precambrian age. They are overlain by a horizontally layered sequence of sedimentary rocks from Paleozoic to Quaternary age.
Intrusions of igneous rocks and volcanic deposits are of great importance, and lava flows on the outskirts of the plateau. Extinct volcanoes and laccoliths constitute characteristic features of the plateau landscapes.
Precambrian Greenlandic crystal shield, according to B.F. King (1967), has much in common with the structure of the Canadian Shield. It consists of several islands covered with a common ice cover.
The Precambrian foundation of the North American platform is bordered by folded systems of different ages located between the craton and the oceans washing the continent. The oldest of the Innuit (Caledonian) systems is located along the Arctic Ocean in Northern Greenland and in the north of the Arctic archipelago. The formations of the East Greenland folded zone are considered syntectonic with Inuit ones. In northeast Greenland, both branches of the Caledonpd are articulated. From here the East Greenland fold zone extends south across Scoresby Sound. The structure of the Early Naleozoic folded structure includes sediments of Cambrian, Ordovician, very thick Silurian and, in places, Devonian age. On the surface of the Caledonides alignment lies a platform cover of Carboniferous, Permian and Mesozoic sediments. 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, a continuation of the Appalachians is the Ouachita folded region. Its highly aligned structures are buried over a significant area under younger formations. They extend towards the Pacific Ocean, to Mexico, and can be traced under the Cordillera, lying across the strike of their structures.
From the west, the North American platform is framed by the folded system of the Cordillera, 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 and consist of parts of different ages and different 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 (Laramie folding, orogeny of the Rocky Mountains and British Columbia). In the Tertiary period in the Cordilleran mobile zone, folding developed in local basins. At this time, fault tectonics and associated volcanism played a major 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 also continued in the Quaternary period. At this time, volcanic fields were formed in the state of Idaho in 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 folded zone is distinguished. The structures of the Antillean island system are considered synchronous with 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 occurred around centers - components of island arcs. The process of structure formation in North America naturally developed throughout geological history. Its structures are spatially fixed and have no drift layers.
The relief of the platform is characterized by significant smoothness, large areas of accumulative plains combined with high mountainous countries. The beauty of the country's landscapes is enriched extremely various forms denudations, present over large areas and often of enormous size. Their features reflect the influence of climate on the physical geography of steppe plains, semi-deserts, snow-covered Arctic islands, mountainous countries and forested subtropics.
South American Platform. The Precambrian crystalline basement of South America is exposed in the northern half of the continent. Its individual projections 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 the projections of the foundation are separated by a foredeep. Towards the Atlantic Ocean, the platform forms a steep continental slope and has abrasive shores. The general configuration of the coast of South America completely reflects the configuration of the adjacent part of the Mid-Atlantic Ridge.
In the structure of the South American platform, the Guinean, Central, or West Brazilian, Coastal, or East Brazilian shields are distinguished. Isolated Precambrian protrusions in the southern part of the continent are the Apa, Tebicuari, Uruguayan, Northern Buenos Aires Hills, Pampa Block Country, Southern Mendosa Massif, Severo-Patagonian and Southern Patagonian shields. They are separated by the Amazonian, Parnaibian, San Françon, Paraná troughs and the associated Serra Geral plateau basalts, the La Plata basins, or Chaco-Pampas, Rio Negro, Chubut and Santa Cruz. Within their boundaries lie strata of thick platform cover.
The Guiana Shield lies in northern South America between the Orinoco and Amazon basins. Its distribution generally corresponds to the Guiana Highlands. The shield surface is located within 500-1000 m in the west and 200-500 m above sea level in the east. The highest point is the peak of Roranma - 2771 m. The highlands are limited in the south by steep slopes, and in the east by rocky ridges. At the foot of the slopes there is a hilly plain, gradually descending towards the Amazonian lowland.
The structure of the shield contains sediments of Middle and Late Cambrian age. The oldest are considered to be hornblende and other gneisses, mica schists and granite gneisses. Associated with it are gabbro intrusions, as well as deposits of diabase and andesite. Younger formations of Guiana include ferruginous quartzites and 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 granite batholiths.
The most complete Precambrian section is described in French Guiana (Tugarinov and Voitkevich, 1966). The Lower Precambrian here includes the Cayenne system, composed of amphibolites, quartzites, hornfels, gneisses and migmatites with interlayers of crystalline limestones. 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, mica and talc schists interbedded with lavas, including peridotites and granite intrusions. The Paramak deposits are folded. The Upper Precambrian of French Guiana is divided into two parts: the lower - Bonidoro Series and the upper - Oranu 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 higher. It is also cut by granite intrusions, its folded structures extend in a west-northwest direction. The Oranu Series is intruded by rhyolites, which overlie the sedimentary-volcanogenic Roranma Series of post-Cambrian age.
In the structure of the coastal part of the Guiana Shield, three orogenic belts are distinguished (Shubert, 1956). The oldest - Hylean - covers the Cayenne system. The sedimentary and igneous rocks composing it are highly metamorphosed. The middle belt - Guiana - includes strata of the Paramaka system and the youngest Caribbean - deposits of the Bonidoro and Oranu series.
Thus, the Guiana Shield can be considered as an independent center for the formation of the continental crust in the Precambrian. As on other shields, the expansion of the land occurred here sequentially, joining to a core composed of sedimentary-volcanogenic strata of new structural levels 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 remains of this sandstone that survived denudation form tabletop hills and play a significant role in the landscapes of the Guiana Highlands.
In the south, the Guiana Shield is separated from the Brazilian Shield by the Amazon Trough. 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 world's greatest rock, the Amazon, which has a tectonic valley (a very convincing example of the unity of the structure and relief 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 makes up the central part of the South American continent south of the Amazon Trough. The Paramba-São Francisco meridional depression divides the shield into western, central and eastern, sub-Atlantic parts. Opi are considered as independent shields. The Paramba-San Frapsis depression separating them is a relict of the interisland basin. The tectonic valleys of Paramba, San Francisco and upper Paraná are associated with it. In the south, the Paran and Chaco-Pampas depressions adjoin the Brazilian shield.
The surface of the shield is very uneven and significantly raised. The entire extent of the shield corresponds to the Brazilian Highlands. It 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 consists of the blocky massifs of Pico de 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. In the east, the highlands form the Great Escarpment - 1000-1500 m above sea level and ends with a fault ledge to the Atlantic Ocean. Along the right bank of São Francisco, the blocky ridges of the Serra do Espinhaço (up to 1800 m) stretch from northeast to southwest. In the south of the highlands lies the vast lava plateau of the Serra Geral, 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 extremely many series and systems, the relationship of which there is no single idea. Conventionally, the structure of the crystalline basement is divided into Lower, Middle and Upper Precambrian. The most ancient are the Bacoa gneisses, whose age is 2400-2500 million years. Younger formations of the Middle and Upper Precambrian are recognized in the Minae and Itakolomi series.
The composition of the Minae series is quite variable. In the Barbacena region it is represented by strata of gneisses and shales; north of Lafayette, the Middle Precambrian includes conglomerates, quartzites, dolomites, ferruginous formations, graphite phyllites, lava flows and volcanic tuffs. The thickness of the series exceeds 3000 m. It includes intrusions of ultramafic rocks and diorites. Ultrabasites are locally transformed into serpentinite and talc shales. 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 comparable to the Grenville formations of North America.
The Itakolomi 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 ancient deposits of the Brazilian shield ends with clastic sedimentary rocks of the Lavras and Bambum series, the age of which is considered to be late Precambrian - early Paleozoic. Some of the sediments of the Lavras series are considered tillites.
The structure of the Brazilian Shield is not well understood. So far, four stages have been distinguished in the history of its structure formation: 2400-2510, 1000-1100, 720-760, and 460-600 million years (Tugarinov, Voitkevich, 1966). The structural relationships of different-aged parts of the shield are most fully evident in the state of Mipas Gerais. The central part of the massif here is made up of the Bacao gneisses (2400, 2510 million years old), they are bordered by formations 1350 million years old, then the sedimentary-metamorphic strata of Rio das Veijas. From the east and west they are bordered by formations of the Minae series, and from the south by massifs of the Itakolomi series.
Thus, overall plan the structures of the Brazilian shield are a consistent expansion of ancient structural centers due to the attachment of folded regions, which is also characteristic of the South American platform. Consolidation of the Brazilian Shield was completed in the Late Precambrian. Subsequently, its surface was leveled over a long period of time and became an arena for the formation of platform cover. The submeridional depression dividing the shield is filled with sediments of Paleozoic and Mesozoic age. In some places, the platform cover on the shield is composed of continental formations of the Triassic, marine layers of Turonian and Paleocene age in the northern part and in the center - continental horizontal strata of the Eocene.
The relief of the Brazilian Shield, like other Precambrian massifs, is characterized primarily by the position of the planation surface, deformed by faults and the position of blocks. In exposed areas, the surface of the Precambrian basement looks like 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, mountain type.
In places covered by platform cover, the Brazilian Shield has a two-story structure. The lower floor is a crystalline plinth, the upper floor is a platform cover. It is characterized by a flat surface of plateaus and plateaus, mesas, outlier hills, limited steep or gentle slopes, the features of which in each individual case are determined by the nature of the deposits exposed by depudation and many climatic factors.
In the southern part of the South American continent, Precambrian formations appear as separate, unconnected massifs, representing in the past independent islands. Their structure has been studied very poorly.
The structure of the Uruguayan crystalline shield is divided into Lower, Middle and Upper Precambrian. Lower Precambrian deposits extend along the La Plata valley and have a sublatitudinal strike. They include various gneisses and mica schists that host granite intrusions. The Middle Precambrian - Minae Formation of Uruguay - includes massive quartzites, lenses of crystalline limestones, talc shales and volcanic deposits. The intrusions are represented by alkaline rocks and granitoids. Upper Precambrian rocks are grouped into the Otgua series. The latter includes volcanic breccias and quartzites, crushed into folds. Their structures extend in the meridional and northeastern directions.
Between Uruguay and the Brazilian shield, a vast territory is occupied by the Serra Geral volcanic plateau, structurally connected to the La Plata basin. The plateau has a flat, slightly dissected surface.
Crystalline massifs in the central part of South America stand out in isolation along Paraguay - the Ana and Tebicuari horsts. In the south of the continent, the Precambrian protrusions are concentrated in the west and are adjacent to the mobile Pacific zone. In Patagonia they form separate shields, separated large depressions. In the Precambrian of central Argentina, phyllites and greywackes, folded into folds, are known. Their age is considered to be Late Precambrian. In the ridges of Catamarca, La Rioya, and San Luis, metamorphic strata accommodate granite batholiths. The gneisses of the Buenos Aires hills host diorite intrusions.
There is still very little data on the features of the relief of the Precambrian massifs of the southern part of the South American platform.
From the west, South America is bordered by the grandiose rampart of the South American Cordillera, separating the platform from the Pacific Ocean. Between the platform and the folded mountain system there is a foothill trough, made mainly of Cenozoic deposits. The structure of the Cordillera is complex and unites parts of different ages. The model of the cross-section of the Cordilleran folded zone from east to west consists of the following structural elements:
1) a platform plunging steeply to the west;
2) the Andes foredeep;
3) Eastern Cordillera, composed of sedimentary deposits of Paleozoic age, folded. At its outer edge this folded system contains isolated masses of Precambrian schist, including granite intrusions;
4) Western Cordillera, composed of marine sediments of Mesozoic age and younger volcanic 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) remnants, or, more precisely, islands, mainly Hercynian structures. The entire mountain ridge 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 chalk contained major folds and faults. Thrusts formed and volcanic activity intensified. Greatest strength structure formation reached in the early Oligocene, when the Eastern Cordillera was formed. Volcanic activity began in the Andes and continues to this day. A new increase in movements occurred in the Miocene. Then many faults and faults arose, accompanied by numerous intrusions. Intrusive rocks of this age are especially common in the Andean foothills. Later, a leveling surface was developed in the Andes. The last phase of mountain building occurred in the Pleistocene. As a result of a general arching uplift, the modern Andes were formed. The uplift was accompanied by enormous faults and block movements, which created the modern mountain topography (King, 1967).
The structure of the South American Cordillera, as W. Oppenheim rightly states (Oppenheim, 1948), is the final result of the 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 orogenesis in the Andes. Since then, the western structural boundary of the continent 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 sequence of mountain uplift is reflected in planation surfaces and river terraces, indicating periodic rejuvenation of valley erosion.
Structural and geomorphological analysis shows that the continent of South America has a heterogeneous structure. Its main components are the Guiana and Brazilian shields and the Amazonian trough separating them - the most ancient parts of the continent. They are characterized by a sublatitudinal extent. South part 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 Pampa, the North and South Patagonian shields, etc. Between the eastern and Western systems the La Plata depression in the south is of the same importance in the structure of the continent as the Amazon basin in the north. With the formation in the Cenozoic of the island arcs of the complex folded system of the South American Cordillera, the final configuration and orography of South America was determined.