Geographical position
The West Siberian Plain is one of the few physical-geographical countries whose borders are clearly expressed in relief. Its borders in the west are the eastern foothills of the Urals. In the east the plain is limited by a ledge Yenisei Ridge and Central Siberian Plateau, along which the valley of the Yenisei River was formed, in the north it is washed by waters Kara Sea. The southern part of the plain goes beyond Russia into Kazakhstan and only in the extreme southeast borders Altai.
From north to south, Western Siberia stretches for almost 2500 km: from 73°30" (northern outskirts of Yamal) to 51° N (extreme southeast). In plan, its territory has the shape of a trapezoid with the greatest extent from west to east at the latitude of Krasnoyarsk (about 1900 km).The area of Western Siberia is about 3 million km 2.
The specific features of the nature of Western Siberia, which determine its originality and uniqueness among other physical-geographical countries, are a rather monotonous, weakly rugged topography with low absolute and relative altitudes, exceptional swampiness and pronounced latitudinal zoning of natural conditions.
The southern part of the plain is the most developed and inhabited areas of Siberia, where nature has been significantly changed by human economic activity.
The geological structure of the West Siberian Plain is a consequence of its position on the young plate of the same name Ural-Siberian(Central Eurasian, Ural-Tien Shan) epipaleozoic platform(Figure 4).
The foundation of the slab is a huge depression with steep eastern and northeastern sides and gentle southern and western sides. It consists of pre-Paleozoic, Baikal, Caledonian and Hercynian blocks. The most ancient is the Irtysh-Nadym middle massif. The foundation is broken by deep faults of different ages. The largest are the East Trans-Ural and Omsk-Pur (Koltogorsk-Urengoy) submeridional faults. The surface of the slab foundation is divided into the Outer Edge Belt and the Inner Region, which are complicated by a system of depressions and uplifts reflecting its lateral structure.
Outer belt represented by slopes of a mountain-folded frame, gently or more steeply sloping towards the central part of the depression. The foundation within its boundaries lies shallow (less than 2.5 km). It comes closest to the surface in the extreme southwest of the Kustanai saddle (300-400 m). Inner area divided into two stages. South step(Middle Ob meganteclise) is characterized by a basement depth of 2.5 to 4.0 km. Most omitted north step The slab is the Yamalo-Taz megasyneclise (8-12 km). The Yamalo-Taz megasyneclise is apparently separated from the Middle Ob meganteclise by a sublatitudinal deep fault (Trans-Siberian), north of which the depth of the basement sharply increases from 4 to 6 km.
Between the foundation and the sedimentary cover of the plate lies a transitional complex of Triassic-Lower Jurassic age. Its formation is associated with arch-like uplift and stretching of the basement, which resulted in the formation of an intracontinental rift zone with a system of graben-like depressions. In these depressions, sedimentary-volcanogenic and sedimentary coal-bearing continental strata up to 3-5 km thick accumulated. Igneous rocks of the transitional complex are represented mainly by basaltic lavas and tuffs. The development of the West Siberian intracontinental rift zone did not lead to the formation of a new ocean.
The general subsidence of the plate and the accumulation of the sedimentary platform cover began in the deepest northern part from the Upper Triassic, and in the rest of the territory - from the Middle Jurassic and was of a differentiated nature. The formation of the cover in Meso-Cenozoic time proceeded virtually continuously under conditions of long-term stable subsidence.
The cover is represented by interlayered sandy-siltstone coastal-continental deposits and marine clayey and sandy-clayey strata with a thickness of 3-4 km in the southern part and over 7-8 km in the northern part. Marine deposits predominate in the lower part of the section (up to and including the Lower Oligocene) and are associated with boreal transgressions. The maximum transgressions, which covered almost the entire territory of the plate, took place at the end of the Jurassic and the beginning of the Late Cretaceous and Paleogene.
The activation of tectonic movements at the platform stage of plate development is associated with the emergence of numerous local structures, expressed only in the sedimentary cover. It has been established that in near-fault zones the number of local uplifts, which are the main reservoirs of oil and gas, increases by 3-4 times compared to the rest of the territory.
The tectonic movements of the Oligocene are associated with the uplift of the northern block of the plate, which separated the West Siberian Sea from the Arctic basin. The marine regime still persists for a short time in the central and southern parts of the plain, but already in the middle of the Oligocene the sea finally leaves Western Siberia through the Turgai depression. In this regard, the upper part of the sedimentary cover is composed of continental strata, reaching great thickness in the southern, sagging part of the plate, in places up to 1-2 km. Among them, lacustrine-alluvial sandy-clayey and lacustrine, predominantly clayey, sediments predominate.
In the Neogene, a zone of sublatitudinal Ob-Yenisei uplifts, located above the Trans-Siberian Fault and corresponding to the modern Siberian Uvals, is clearly distinguished.
By the end of the Neogene, the general orographic features of Western Siberia had already formed. The low areas coincided with tectonic troughs, in which river valleys were probably located. The sea level at that time was 200-250 m lower than today, and most of the bottom of the Kara Sea, together with the northern regions of the plain, was dry land, deeply dissected by river valleys.
The general cooling of the climate that occurred in the Neogene especially intensified towards the end of the period, which led to the development of Quaternary glaciation.
Figure 4 – Main orographic elements of Western Siberia
During periods of glaciation in Western Siberia, deep freezing of soils and the formation of permafrost occurred in ice-free areas. In non-glacial areas, loess-like loams formed, overlying more and more ancient sediments and in some places reaching a thickness of 2-2.5 m.
During the Pleistocene, repeated changes in the sign and speed of tectonic movements were observed. At the end of the last glaciation, the northern coastal areas again subsided, were flooded by sea waters, and accumulated strata that made up the Holocene marine terraces.
The general warming of the climate in the Holocene led to a shift to the north of the boundaries of natural zones, to the replacement of tundra-steppes and cold forest-steppes that existed near the border of glaciers with forest vegetation. In the southern part of the plain, forest-steppes and steppes are preserved. Warming reached its maximum during the xerothermal period (boreal xerothermic maximum 8-9 thousand years ago), when woody vegetation spread 3°-4° north of the modern border. This is evidenced by the presence of tree trunks and stumps in the tundra deposits of Yamal and Gydan.
The beginning of widespread swamping in Western Siberia is associated with the xerothermal period. Intense evaporation from the surface led to the drying out of numerous lakes, reducing their depths and becoming overgrown. In place of overgrown lakes, multiple pockets of swamping arose. Closely located foci merged, and the area of the swamps increased. This happened especially intensely during periods of cold weather. During the Holocene there are several periods of warming and cooling. Currently, there is a cooling of the climate and an associated slow shift of the boundaries of natural zones to the south. This process is quite clearly visible in the northern part of the plain, where tundras displace woody vegetation near the northern limit of the distribution of sparse forests. In the south, the encroachment of forests into the forest-steppe is prevented by human economic activity. By cutting down forests, people interfere with the natural process and contribute to the expansion of the area of the steppe zone.
Relief
The modern relief of Western Siberia is determined by geological development, tectonic structure and the influence of various exogenous relief-forming processes. The main orographic elements are closely dependent on the structural-tectonic plan of the plate, although long-term Meso-Cenozoic subsidence and the accumulation of a thick layer of loose sediments largely leveled out the unevenness of the foundation. The low amplitude of neotectonic movements is due to the low hypsometric position of the plain. The maximum amplitudes of uplifts reach 100-150 m in the peripheral parts of the plain, and in the center and in the north they are replaced by lowerings of up to 100-150 m. However, within the plain there are a number of lowlands and hills, comparable in area to the lowlands and hills of the Russian Plain.
Western Siberia has the shape of a stepped amphitheater, open to the north, towards the coast of the Kara Sea. Three altitude levels are clearly visible within its boundaries. The first level, occupying almost half of the territory, has a height of less than 100 m. The second hypsometric level is located at heights of 100-150 m, the third - mainly in the range of 150-200 m with small areas up to 250-300 m.
The highest level is confined to the marginal parts of the plain, to the Outer Tectonic Belt. It is presented Severo-Sosvinskaya, Verkhnetazovskaya And The Lower Yisei Uplands, the Ob Plateau, the Turin, Ishim, Kulunda, and Kets-Tym plains.
Among morphostructures dominated by those formed on monoclises, gently inclined to the inner part reservoir(oblique) plains And plateau. In the marginal parts they predominate strata-denudation plains. As one moves away from the outskirts, the amplitude of the newest uplifts decreases, the thickness of Quaternary deposits increases, and stratal-denudation plains are replaced reservoir-accumulative.
The lowest areas (below 100 m) are located in the northern and central parts of Western Siberia, in its Inner tectonic region. This Nizhneobskaya, Nadymskaya, Purskaya, Tazovskaya, Kondinskaya, Sredneobskaya And Vakh lowland, having elevations of less than 50 m. Towards the periphery the surface gradually rises. Only Sibirskie Uvaly- clearly defined strip of hills (Lyulimvor, Belogorsky Continent, Numto ridge, Verkhnetazovskaya upland)- cross the interior regions of the plain near 63° N, from the Urals to the Yenisei. The central part of the Siberian Uvals belongs to the average hypsometric level (100-150 m), and the western and eastern peripheral parts, extending into the Outer Tectonic Belt, belong to the highest, third level. Thus, the descent of the surface of the slab towards the center and its elevation at the edges are orographically well expressed.
In the Inner Region, characterized by the development of a thick cover of Mesozoic sediments, the clarity of expression in the relief of large basement structures is lost. The modern relief primarily reflects the Meso-Cenozoic structural plan, in which a large number of cover structures appear, a number of which are reflected in the relief. The number of inversion structures is increasing. For example, the Vasyugan Plain is a hill - an anteclise, attenuating with depth, located within the syneclise.
Under the conditions of the latest subsidence, accumulative and strata-accumulative plains, composed of loose Neogene-Quaternary strata. There is a clear pattern in the distribution of morphostructures across the territory: wide low-lying accumulative plains are separated by relatively narrow stratified-accumulative low elevations (100-150, rarely up to 180 m) - Vasyugan, Siberian Uvaly, Yamal and Gydan plains
In placement on the plain of types morphosculptures, created by the activity of exogenous relief-forming processes in the Neogene-Quaternary time, a natural change in the direction from north to south is clearly visible. In the north, the shores of the Kara Sea and its bays are adjacent to sea plains, which in late and post-glacial times rose above sea level. To the south are moraine and fluvio-glacial plains, the main relief features of which are associated with Quaternary glaciation. They are adjacent to glacial lacustrine-alluvial plains, contoured to the south by non-glacial structural-denudation plains.
Marine accumulative plains differ in flat relief. There are many small shallow lakes occupying wide, flat-bottomed depressions. The surface rises towards the inner parts of the Yamal, Gydansky and Tazovsky peninsulas, forming a series of terraces (up to four to six levels), the width of which is measured in tens of kilometers. In some places, terrace ledges 10-20 m high are clearly visible. Modern processes that transform the relief of the sea plains are primarily permafrost-solifluction.
Glacial and water-glacial plains are characterized by more rugged terrain. Gently hilly terrain predominates. Height fluctuations in the interfluves are 10-15 m, rarely more. Only in the marginal parts of the plain adjacent to the Urals and the Central Siberian Plateau do relative heights increase and moraine hills and ridges, eskers, kamas and basins that arose when blocks of dead ice melted are relatively well defined. In the southern part of the region, flat water-glacial plains are widespread. The main factor in the modern transformation of relief is the activity of flowing waters. An erosional relief is formed, especially pronounced at higher elevations.
Alluvial-lacustrine plains differ in flat relief. For a long time, processes of river and lake accumulation dominated here. When people talk about Western Siberia as a giant alluvial plain, they usually mean this part of it.
In late and post-glacial times, these plains entered a stage of erosional dissection. The depth of erosional incisions of valleys usually does not exceed 20 m. Only the largest transit rivers (Ob, Irtysh, Yenisei) are incised to 60-70 m. Many small rivers do not have morphologically pronounced valleys. Over vast areas, the relief is characterized by very weak vertical dissection.
Denudation plains occupy the southern part of Western Siberia. Accumulation processes were replaced by erosion processes here in pre-Quaternary times. However, the great distance of the territory from the ocean and the dry climate determine the poor development of the river network. Only the southeastern part of the plain, where there are many transit rivers flowing from the mountainous regions of the Altai-Sayan region, is distinguished by a well-developed erosional topography with gently convex interfluves and a dense network of river valleys. In the rest of the territory, the interfluve spaces are poorly developed by an erosion network and are characterized by a flat, slightly undulating topography. On the surface there is a huge number of suffusion-subsidence depressions, usually occupied by lakes, and a mass of small flat swampy depressions. Near the Ob, Yenisei, Chulym, Irtysh, and Tobol, the dissection becomes deeper and the slopes become steeper. Young growing ravines appear.
A characteristic feature of the Priob Plateau and the western part of the Chulym-Yenisei Plain is ridge-hollow relief. Extremely rectilinear hollows parallel to each other break the surface into a system of flat-topped ridges and ridges, rising 100-160 m above the bottom of the hollows. The ridges and hollows extend from southwest to northeast. Towards the eastern part, their strike gradually changes to latitudinal (Figure 5).
The origin of the ridge-hollow relief has not yet been definitively established. There are several hypotheses: tectonic, aeolian, erosion.
Figure 5 – Geomorphological zoning of Western Siberia
