Turan plate

In the northwest it borders on the Caspian basin of the East European platform. In the north, the border passes through the Hercynian ridge of Mugodzhary, crosses the Turgai trough between the sources of the Turgay and Ubagan rivers.

In the northeast, the plate is limited by the caledonoid outcrops of the Kazakh small hills. The southeastern border of the plate extends from the extreme western point of the lake. Balkhash to the Tashkent region, crossing the western spurs of the Tien Shan , which together with the ridge. Karatau are included within the limits of the plate.

The Ferghana Depression is also part of the plate. South of Tashkent, the border of the plate can be traced along the edge of young mountain ranges, through Samarkand and further, going around the spurs of the Gissar Range, to the Amu Darya , it goes south, to the borders of Afghanistan , where it occupies its northernmost part.

Starting at p. Murghab and to the west the border of the plate passes on the territory of Turkmenistan, continuing directly north of the foot of the ridge. Kopetdag, composed of alpine folded complexes, facing the Caspian coast directly south of the gulf (since 1980, the lagoon) Kara-Bogaz-Gol . The most conventional western border of the plate.

With the Scythian Epigercin Plate located to the west of the Caspian, the Turanskaya Plate is one, and the term “Scythian-Turan Plate” is increasingly used in the latest literature.

However, for convenience of presentation of the material, we will consider them separately, and draw a purely conditional border between them from south to north along the middle part of the Caspian to the mouth of the Urals .

The Turanian plate has been studied in great detail. Here, geological and regional geophysical surveys were carried out, a large amount of seismic research and hydrogeological work was carried out.

Like all plates, Turanskaya is formed by three structural floors: the lower is a geosynclinal, forming a folded foundation, the quasiplatform or intermediate structural floor is located above, the sediments of which, unlike the West Siberian plate, most researchers do not include in the foundation, and, finally, platform cover.

The foundation is composed of Precambrian and Paleozoic rocks of the geosynclinal type, intensively deployed and penetrated by numerous bodies of igneous rocks.

In the northeast of the plate, almost to the right bank of the river. Syrdarya, to the line of deep fault of sub-latitudinal strike, the foundation is formed by Caledonian folding.

Judging by the outcrops of rocks, in the northern part of the ridge. In Karatau and in the adjacent territory northeast and east of the plate, the foundation is composed of intensely deployed geosynclinal strata of the Precambrian, Lower Paleozoic and Silurian, broken by numerous intrusions. The foundation is predominantly Hercynian throughout the rest of the plate.

The articulation of these blocks of different ages occurs along a deep fault, which can be traced in the gravitational field in the form of a sharp gradient step, and in the magnetic field - a narrow and very intense zone of linear anomalies.

In the Hercynian basement, also based on the tracking of gravimagnetic anomalies, two folded zones are clearly distinguished. One of them, which is part of the Ural-Mongolian belt, stretches from north to south from the Urals , Mugodzhar and Turgai troughs up to the Aral Sea and then, gradually curving to the southeast, articulates with the Tien Shan Hercynides.

She is apparently of Early Hercynian age. The second zone, probably late Hercynian, extends almost in a latitudinal direction from the northern Black Sea region (Scythian plate) through the Ciscaucasia and further east, right up to Ferghana. Both of these folded zones are articulated along a deep fault passing from the spurs of southwestern Gissar through the middle course of the river. Amu Darya ( Bukhara ) to the area directly south of the Buzachi Peninsula - the edge of the Russian Platform. This is the so-called "Bukhara fault." There is a gravitational step of great amplitude and a fairly clear sub-latitudinal linear magnetic anomaly. The fault, which separates this sublatitudinal zone of the late Hercynides from the alpine folded region of Kopetdag, is also clearly reflected in the gravimagnetic fields. According to the data of gravimagnetic studies, a number of Caledonian or, possibly, Baikal hard massifs streamlined by the Hercynian folded structures are identified as part of the marked Hercynian zones. The areas are confined to them, in which in most cases there is no precipitation of the intermediate floor and large and largest positive structures are located.

Hercynian folded foundation is exposed on the ridge. Karatau and on the hills north of the middle course of the Amu Darya river, in the Sultanuizdag mountains (lower reaches of the Amu Darya river). In addition, in a number of sections of the plate, it was opened by deep wells.

In the regions adjacent to the Mugodzhary and Turgay, quartzites, crystalline schists, marbles, sandstones (Lower Paleozoic or Riphean), thick flyschoid strata of the Silurian, limestone and volcanic-sedimentary strata of the Devonian - Middle Carboniferous form the foundation. The Middle Paleozoic main ( spilite- keratophyre and diabase) and acidic (liparite-dacitic) volcanic formations, as well as Late Carboniferous-Early Permian granitoids are widely developed. Especially characteristic are the effusive tour (up to 4000 meters), stretching along the zone of articulation of the caledonides of Kazakhstan and Hercynides of the Urals. The total thickness of the Paleozoic sediments that make up the foundation is 8,000 meters.

To the south of the deep “Bukhara Fault”, within the latitudinal branch of the Hercynides, the foundation reaches the surface on the hills in the region of Mount Krasnovodsk (Tuarkyr and others). In addition, it is opened by wells at the Central Karakum and Karabogaz arches. It is represented by granite gneisses, effusive rocks of the Lower and Middle Paleozoic, Middle Paleozoic siliceous schists and quartzites, broken through by gabbroids. The youngest strata that make up the foundation are carbonate-terrigenous deposits and liparitic-dacitic tuffs of middle and upper carbon.

The rocks of this structural floor with sharp angular disagreement lie on the rocks of the foundation and are overlain by the rocks of the cover lying on them disagreeably. Within the development area of ​​the Caledonian basement, this floor is opened by boreholes in the western half of the Chu-Sarysuyskaya Depression and is exposed in a number of sections along the northern edge of the plate. At its base lie the volcanic-sedimentary sequence of the Lower Devonian and lower Middle Devonian (andesites, dacites, liparites, packs of red-colored sandstones and conglomerates), overlapped by Middle and Upper Devonian sandstones and conglomerates. Cut power up to 7 km. Above it lies saline (gypsum, anhydrite, rock salt) or coal-bearing strata, the age of which is from the top of the Famennian to the tour, inclusive. Its power is up to 4 km. In a number of sections ~ within this stratum, manifestations of salt tectonics are noted. Industrial areas of oil and gas are associated with these areas. The tops of the complex under consideration are composed of a variegated sandy conglomerate thickness up to 3.5 km thick, covering the age interval from Middle Carboniferous to the lower three Aces inclusive. This complex is usually weakly dislocated (angles of incidence up to 15–20 °).

Within the area of ​​development of the Hercynian basement, the intermediate complex is of Early Permian - Late Triassic age. It is distributed almost everywhere and is absent only on the ledges of the foundation. The thickness of the complex varies from zero — the first hundred meters to 2–4 km within Central Ustyurt and 5 km on the Buzachi Peninsula and south of it ( Kaydak sor), and on the Mangyshlak Peninsula and in the Pre-Kopet Dag Trough, according to seismic, 11 km. This complex has been studied in detail on the Mangyshlak Peninsula, where, being intensely deployed, it is exposed on the surface in the near-axis zone of the Karatau rampart (Karatau complex). Within its boundaries, suites of marine and continental genesis are distinguished. At the base of the exposed part lies a layer of highly compacted sandstones, siltstones and mudstones, with intercalations of limestone, less often tuffs and effusive rocks. This part of the section, according to the definition of fauna and flora, belongs to the Permian, Lower and Middle Triassic. The tops of the complex are composed of black limestone, carbonaceous, shale and mudstone with a rare fauna of ammonites of the Upper Triassic.

In the northwestern part of the plate ( Ustyurt plateau) and on the Buzachi Peninsula, this complex is represented mainly by a thick and very weakly dislocated layer of red-colored sandstones, siltstones and mudstones, and carbonate-terrigenous packs of marine genesis sometimes appear only in the top of the section, in the Triassic . Permian-Triassic deposits are one of the most important oil and gas complexes of the Turan Plate.

The rocks forming the cover lie either horizontally or form structures with tilt angles that usually do not exceed 1–5 °. Only in the fault zones and in the zones of alpine activation are more abrupt angles observed.

The cover consists of the following five main complexes, each of which forms an independent sub-floor: Lower Jurassic, Middle and Upper Jurassic, Cretaceous - Lower Miocene, and also reflecting the neotectonic stage of development of the plate - Middle Miocene-Upper Pliocene and Upper Pliocene-Anthropogenous sub-floors. Three ancient sub-floors are oil and gas industrial.

The Lower Jurassic complex lies with sharp angular disagreement on the dislocated rocks of the basement, more often on the intermediate floor. Usually developed only in deflection zones. In large arches, it is usually absent. It is composed of gray-colored sandy clay deposits, usually weakly carbonaceous. Its power ranges from zero to 1000 m or more. The Middle and Upper Jurassic complexes in some areas of the plate lie with weak angular disagreement on the Lower Jurassic deposits or more often - sharply disagree directly on the Permian-Triassic complex or foundation. The Middle Jurassic is represented in the northern and northeastern parts of the plate by coal-bearing sandy-clay deposits, and in the southwestern and southern parts by gray-colored terrigenous strata with a rare marine fauna. The thickness of the Middle Jurassic varies from zero to 1000 m. The Upper Jurassic is represented in different parts of the plate of the marine, terrigenous, terrigenous-carbonate or carbonate sequence. The “seainess” of the section increases from the northwest to the southeast. In the extreme north of the plate, in the section of the Upper Jurassic (starting from Kimeridge), salts and anhydrites appear. The thickness of the deposits of the Upper Jurassic varies from several hundred to 1000 m or more.

Cretaceous - the Lower Miocene complex in some areas of the plate (Caspian coast, north of Mangyshlak ) lies transgressively on the Jurassic and Permian-Triassic deposits. The Cretaceous bottoms in the northwestern and western parts of the plate are represented by shallow-water gray and green-colored sandy-clay strata with a rare marine fauna. On the rest of the territory, the plates are continental variegated and red-colored strata, in places saline and gypsum-bearing.

The upper Cretaceous, especially in the southern part of the plate adjacent to the Alpine zone, is a marine with a rich fauna stratum of interbedded clays, glauconite sandstones, limestones and marls. The thickness of chalk sediments within the plate reaches 1300 m or more. Upper Cretaceous deposits were formed in the era of the last extensive transgression, covering almost the entire plate, except for the extreme eastern part of the Ferghana Depression. They are represented in the lower parts by glauconite sandstones with phosphorite nodules, marls and clays with rich fauna (sea urchins, inocerams, etc.). At the top, it is mainly a carbonate stratum. The thickness of the Upper Cretaceous reaches 3 km.

In the east of the plate, the Upper Cretaceous complex consists of continental mottled and red-colored clays, sandstones and conglomerates with fragments of dinosaur bones and pieces of wood, usually lying directly on the foundation or on Permian-Triassic deposits. Their thickness rarely exceeds 50–100 m. Paleogene deposits on several plate sections, on the tops of the largest uplifts — anteclises and arches — are located directly on the Permian – Triassic or basement rocks. However, according to its main part, they lie on the Upper Cretaceous deposits. In the west and north-west, plates of Paleocene and Eocene deposits are formed by marine strata - limestones, marls and clays. Their thickness usually does not exceed 200-250 m. In the eastern part of the Ferghana Depression, marine deposits are replaced by lagoon and continental.

Oligocene-Lower Miocene deposits, (analogues of the Maikop series of the Ciscaucasia) are dark brown calcareous clays with characteristic remains of the ichthyofauna. In the direction to the east, especially to the northeast (in the Tashkent region and northeast of it), marine sediments of the Paleogene and Lower Miocene are gradually replaced by terrigenous coastal-marine strata. In the extreme east of the plate, almost the entire section of the considered interval is continental sediments: sands, sandstones, variegated clays and conglomerates. In the southeast of the plate, in the Eocene sediments, covers of basalts, andesites, their tuffs and tuff breccias are noted. Interbeds of ash tuffs are recorded among deposits of the Upper Eocene up to the Aral Sea .

The Middle Miocene - Upper Pliocene complex of deposits is especially widely developed in the western part of the Turan Plate. These are marine terrigenous and carbonate sediments with a thickness of 100-200 m in areas of young uplifts to 1000 m and more - in areas of young subsidence (Predkopetdag trough). Towards the east (east of the Aral Sea meridian) they are gradually replaced by a thin continental terrigenous sequence. With angular disagreement, this complex lies at different horizons of the Paleogene or Lower Miocene, reflecting the stage of intense neotectonic movements that took place in the Neogene both in Western Siberia and the Urals . In the early and at the beginning of the Middle Pliocene, throughout the entire south of the USSR , a major regression occurred, which was replaced at the very end of the Middle and especially at the beginning of the late Pliocene by an extensive Akchagyl transgression: the sea again spreads eastward, covering the space up to the Urals and the lower reaches of the river Amu Darya .

The Upper Pliocene-Quaternary complex is represented in the west and north of the plate by both marine (Baku, Khazar and Khvalyn transgressions) and continental deposits - sand and gravel, clay, siltstone. The thickness of these deposits does not exceed several tens of meters. In the Pre-Kopetdag trough and throughout the east, the plates are alluvial, lacustrine, and mainly aeolian formations, the thickness of which varies from tens and hundreds of meters to 1-3 km in the Pre-Kopetdag trough.

• Kröner, A. (2015) The Central Asian Orogenic Belt (Central Asian Orogenic belt, English) ISBN 978-3-443-11033-8