Alluvial facies complex
Among the continental sediments, gravel-sand formations overlain by clayey floodplains are of the greatest interest in the search for oil and gas deposits.
The distribution of precipitation in the riverbed is due to the fact that the products of destruction formed in the erosion zone are unevenly distributed along the bottom of the river, depending on the flow velocity, size and weight of the debris. The largest of them, which the river can move, remain in place or move for short distances, the smaller ones are carried to the opposite shore by a transverse current and, according to the weakening of the water jet, settle. Even smaller ones are carried away by the flow downstream of the river for long distances, and the clay particles become suspended and remain in this state for a long time.
Each river system, including the rivers of the distant past, passes through three stages of its development: youth, maturity, old age (Figure 62).
In the period of youth, the river conducts an intensive channel deepening and transports the products of destruction downstream into the sedimentation zone. The flow rate of the water flow in this period is the highest, and the channel is relatively straight. Only a small amount of terrigenous precipitation accumulates in the river bed.
In the period of maturity, the river, having deepened the channel and reaching the equilibrium profile, leads, mainly, its erosion due to lateral erosion, forming numerous meandering bends. As a result of these bends, the length of the channel increases, although its slope remains the same. This leads to a decrease in the speed of the water flow.
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The destruction products can no longer be taken to the sedimentation area and begin to be deposited within the channel in the form of channel banks.
The old age of the river is characterized by a further increase in the meander and an increase in the length of the channel: channel banks reach the largest size. The flow of water is so slow that the river in some of its sections is unable to transport the products of destruction. There is a clogging of the riverbed, and the river breaks up into a series of isolated reservoirs of the olden.
Thus, alluvial formations always consist of two parts. The lower part is predominantly gravel-sand sediments of the channels, while the upper part consists of clayey sediments of the floodplains.
Modern rivers are subdivided into three types according to the nature of the river beds: rectified, branched (furring) and meandering (meandering).Rectified channels of rectified rivers. Rectified channels form in the initial stages of development of river systems, or during periods of rejuvenation of meandering lowland rivers in connection with the lowering of the basis of erosion and the development of a new, more direct channel corresponding to the changed paleohydrodynamic conditions . The development of rivers of rectified type begins in narrow ravines or gorges, where in the first stages only channel sediments are deposited, passing on the sides to slope colluvial-deluvial deposits (Figure 63).
Fig. 62. The structure of the river system
Fig. 63. Modern rivers of rectified type: a - Mountain Altai; b - Pamir
Thus, for rivers of rectified type, characteristic are: increased flow rates; limited development of channel banks, which are composed of poorly sorted sand-gravel-pebble material and form lenticularly curved sand bodies with sharply defined lower and upper boundaries; large oblique unidirectional stratification.
Faction of channel banks, branching rivers. Branchy rivers are typical for mountainous terrain. The conditions of sedimentation in them are due to high flow velocities, the transfer of predominantly sandy-pebble material, the presence of numerous shallow converging and diverging channels of various orders, and fuzzy boundaries of the floodplains (Fig. 64). These deposits are characterized by the roundness of large pebbles.
The facies of channel banks of meandering rivers. The meanders, or bends, are the smooth curves of a plain river. The jet of water flow blurs the concave shore, it becomes steep, and the bottom of the river deepens. At the opposite bank, the velocity decreases noticeably, since the depth is smaller and, in addition, transverse bottom currents arise. These currents capture particles of detrital material with them and postpone them. On the gentle inner slope of the bank of the river, cross-bedded sediments of the channel bank are deposited (Figure 65).
Meandering or meandering rivers form at the stage of maturity, when the river leads to intensive lateral erosion, and at the stage of old age the river is characterized by the maximum tortuosity.
Fig. 64. Mountain branching (flickering) rivers: a, b - Pamir; in - Central Tien Shan
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Fig. 65. Formation of oblique lamination on the channel bank of the river. Ob
The appearance of the meander leads to the formation of numerous branches, but with which the water flows parallel to the bed, and the emergence of vast alluvial plains (Figure 66).
Faction of the Old Men (Figure 67). Formation of old alluvium occurred under conditions of changing hydrodynamic regimes. In the period of high water, the old people temporarily turned into active side channels and channels, where the accumulation of sand material was going on. With the decline of flood waters and the decrease in the rates of water flow, smaller sediments were deposited in starets. When the connection between the old man and the river was interrupted, it turned into an isolated reservoir in which accumulation of mainly clayey sediments proceeded.
Fig. 66. Alluvial Plains: a - Iceland: 6 - Western Siberia, p. Ob
Fig. 67. Old people - former sections of the river Ob river
The floodplain facies. The floodplain deposits are formed by facies of temporarily flooded areas and floodplain lakes and marshes. Both these facies closely intertwined with each other and are characterized by mutual transitions both in the section and in the area. This is due to the fact that the hollow waters reached the inner, most remote parts of the floodplain, when their velocities were minimal, and the overwhelming mass of coarser detrital material already precipitated. In this regard, flooded waters were able to tolerate only the finest-grained aleurite-clay particles. With the decline of the hollow waters, the most elevated parts of the floodplains were first drained, on which thin-horizontally laminated aleurite-argillaceous sediments accumulated, which were replaced by the precipitation of fine silt. The drained areas were covered with vegetation, subsoils and soils were formed on them.
In the lower parts of the floodplain, the deposit of the aleurite-clay material brought by the hollow waters also began in the beginning. Then, with the decline of the hollow waters, shallow isolated water bodies formed in the lowered areas, in which suspended precipitation continued
in the water of clay particles. These particles were deposited on the bottom of water bodies in the form of thin horizontally layered clay strata.
Limnic (lacustrine-marshy) facies
The formation of this group of facies takes place in inland or coastal-sea lakes and marshes. Common signs of limnic formations are limited distribution, corresponding to the form of a lake or marsh, and relatively small capacity. In this connection, in the section, the complex of lake sediments is a lens with a concave base and a relatively flat roof, which, unlike the alluvial roof, forms not a strip but a relatively isometric zone.
In the climate of the lake, more water is produced than evaporates from their surface. Therefore, these lakes are usually flowing, fresh and characterized, as a rule, by terrigenous composition of sediments. Precipitation is generally characterized by comparatively good sorting, the presence of regular, often thin stratification, sometimes - ripple marks and indistinct oblique and cross-layered stratification.
Often in the lake sediments are found violations caused by the slipping of semiliquid plastic sediments along the inclined bottom of the lake. Such landslides develop even with small inclines of the bottom. As a result, there appear peculiar deformation, including landslide textures.
In the lakes there is often an accumulation of organic matter of the sapropelic type. Shallow-water lakes, which are well warmed in the summer, are rich in nutrients and plankton, and are highly bio-productive. In such lakes there are favorable conditions for the preservation of the formed organic material. In the lake sediments, plant remains of good conservation are noted.
In lakes of cold humid climate with the weakening of the introduction of clastic material, it is possible to form iron bean ores by transporting iron by rivers. If the weathering crust served as a source of precipitation, then bauxites, iron and manganese ores are formed. They are confined mainly to the coastal part.
In an environment of arid climate, when the flow of water is small and often does not compensate for evaporation, drainless lakes with increased salinity are formed. In contrast to the lakes of the humid zone, here, along with terrigenous, there is, and sometimes predominates, chemogenic sedimentation. Accumulated limestones, dolomites, magnesian silicates, as well as soluble salts - gypsum and anhydrite, chlorides.
To the group of limnic facies are also bogs. In the sediments of bogs, the accumulation of peat, passing into coal, prevails. In addition, among
marshy sediments are clay (mainly kaolinite composition), and in certain periods and sandy-silty sediments, usually with abundant plant residues. Peat bogs often lie on lake sediments or fossil soils. Marsh facies are one of the examples of concentrated accumulation and conservation of organic matter. The initial composition, in which the higher vegetation predominates, predetermines predominantly the humus character of the organic material and its subsequent carbonation.
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