Geologicgenetic types of sedimentary materials in the permafrost regions

Owing to the specific conditions and factors of lithogenetic processes in the permafrost regions there are a number of distinctive features in the composition, structure and properties of the frozen soils with development of essentially new genetic types of sedimentary formations, found only in the areas of permafrost development. A number of terms appeared in the literature, on the one hand specifying the genesis of sedimentary formations in the permafrost regions, such as 'cryogenic eluvium', 'cryogenic alluvium', 'cold and permafrost soil' and on the other hand, pertaining to formation of the frozen sedimentary layers proper, such as 'syngenetic' and 'epigenetic' strata, deposits of a 'yedoma' or 'glacial' complex, buried ice and others. These questions to a greater extent refer to the loose Cenozoic rocks than they do to Pre-Cenozoic bedrock as the cryogenic structure of the latter is determined by the availability of cavities and wetting before freezing. As a result, under a certain combination of conditions ice fills fractures, pores, caverns, small karst cavities, mainly in the near-surface (weathered) zone of bedrock masses.

Eluvial formations of the permafrost regions, representing accumulated residual products of hypergene bedrock transformation and found within the limits of recent or ancient crustal weathering, are characterized in general by modest thickness, controlled by the penetration depth of annual temperature fluctuations. Owing to predominance of physical weathering in the permafrost regions, the cryogenic crust of weathering on hard rocks is characteristically rudaceous accumulations (large blocks, block-rock waste, rock waste and debris-gravel) with fine earth admixture. At a mature stage of eluvium development, owing to the active removal of fine particles by water seepage and freezing out of coarser debris upwards, there is differentiation of weathered material in the vertical profile. The upper part of the weathering crust in high mountains and polar landscapes is practically devoid of fine particle infilling and acquires the appearance of a purely fragmental horizon, while blocky-rock waste material on the surface often forms stone rings and polygons. As a result, the lower part of the eluvium is enriched in fine material. Lower in the section sharp enrichment in rudaceous material is observed with gradual transformation into the horizon of 'disintegrated rock' - bedrock. Crustal, basal and branched-fracture cryostructures are typical of eluvial formations in the permafrost regions.

The cryogenic crust of weathering on soils is characterized by a high content of fine-grained material and, owing to this, by predominance of crustal, massive and lense-like cryostructures.

The cryogenic crust of weathering on loose sedimentary material is enriched in the silt fraction (0.01-0.05 mm), and eluvial formations have a distinctly silty composition. The overlying loess-like silty-clay materials here have a high homogeneity of granulometric composition, high porosity and little easily soluble salts. The basic rock-forming mineral of the sandy fraction is usually quartz, while those of the clay fraction are hydromicas and montmorillonite. It is because of this specific composition and structure of eluvial formations in the permafrost regions, compared with the eluvium of humid zones, that some investigators refer these deposits to a specific type of eluvium - 'cryoeluvium'.

Among deposits of the slope series along with slide, creep, collapse and talus formations, within the limits of the permafrost regions there are the frozen rock stream and solifluction deposits which are typical representatives of cryogenic formations proper.

Collapse and talus accumulations around the steep and precipitous slopes are mainly of rudaceous composition (blocks, rock waste, gravel) and enriched with fine-grained material in different amounts. The cryogenic textures of such rudaceous formations are predominantly crustal in poorly wetted soil and basal in highly moistened soil - as for talus formations, the crustal and contact cryotextures are typical of coarse-grained materials; in individual cases, in conditions of excess wetting before freezing (usually locally in the foothills of slopes) basal cryotextures arise. Rudaceous formations on relatively gently sloping or moderately steep slopes being mobile (cryogenic and thermogenic creep) are called the deserptium type (rock stream). Their thickness varies depending on geological-geographical conditions from several centimetres to 3-6 m. A typical attribute of rock streams is the near-surface rudaceous mantle without fine-grained infilling material, underlain by a horizon of fragmental material with silty-clay material and sandy silty clay, sand and gravel infilling, grading directly into fissured underlying bedrock. The cryogenic structure of the upper part of the rock streams is characterized by open cavities between blocks; in the lower part, with accumulation of infilling - textures become massive and crustal and still lower, there is an alternation of ice and soil layers and, as a rule, there is 'golets' ice filling the cavities between fragments and forming separate pockets, with bedding and lenses up to 1 m thick and more, sometimes several tens of metres long.

Solifluction deposits are characterized by consistent composition and structure, with typically a laminated pattern due to the buried turf layers broken and folded inside near the external margin of the solifluction formation. In the opinion of G.F. Gravis the cryogenic structure of these deposits is subdivided into two geographical variants, Arctic and subarctic. The Arctic variant is characterized by a high ice content and small inclusions of ice, and the subarctic by high ice content but with the presence of rather thick closely spaced indistinct ice schlieren and small lenses. Formation of cryogenic textures in such deposits can occur in freezing both from the top and from below.

Alluvial deposits of the permafrost regions are first of all distinguished by the type of their freezing. Their characteristic and distinctive features pertain to cryogenic structure and wide development of large accumulations of ice wedges. Thus, for the epigenetically frozen sands and pebble gravel of river-bed facies, crustal and massive cryogenic textures prevail. In the case of freezing of river-bed deposits with moisture inflow due to injection, the deposits are much enriched in ice with formation of lens-like and bedded ice deposits. Old lake deposits in the permafrost state are characterized by a high ice content and presence of broken lens-like bands of ice, which according to E.M. Katasonov, are usually inclined in the shore area and form oblique lens-like or cross-bedded cryogenic textures of the inherited type.

The flood-plain alluvium differs in the case of internal flood-plains compared to that adjacent to rivers, both in composition and cryogenic structure. The flood-plain adjacent to a river has a ridge-hollow topography, and is composed of sediments characterized by non-consistent, but in general coarse-grained composition (sands, sandy silty-clay materials) and sparse peat content. These formations when frozen syngenetically with low moisture availability show intermittent horizontal schlieren and lenses of ice 2-3 mm thick, and with abundant moisture - small reticulate, plexiform and fine net cryotextures and syngenetic ice wedges up to 1 m thick. The cryogenic structure of deposits of the internal flood-plain zone is closely connected with the polygon-ridge topography of its surface and is characterized by a high ice content owing to numerous fine lenses up to 1 mm thick, on the background of which thicker and persistent layers (belts) of ice

0.5-2.5 cm thick are observed. The most distinctive feature of the cryogenic structure of the flood-plain alluvial deposits with polygon-ridge micro-relief is the formation of the ice wedges (with width at the top of several metres and depth of over 10 m).

Composition and structure of glacial and glacio-fluvial deposits vary greatly. Thus, unsorted moraine is typical of glacial deposits - boulders, rock debris, pebble, gravel with sandy, silty and clay-rich materials, with different types of cryogenic structure and high ice content (30-50%). Fluvioglacial deposits are basically more homogeneous, coarse-grained, and typically with lower ice content (10-20%) and massive,crustal and lense-like cryostructures. Fine- and medium-grained silty sands have fine schlieren, lens-like and reticulate-laminated cryotextures. Their ice content is greater, up to 30-40%.

Marine, lagoon and lacustrine deposits in the permafrost regions are also characterized by a number of specific features distinguishing them from the southern geographic variants of these formations. The main factors that determine the specific nature of the accumulation of sediments in the northern water bodies are the low temperatures of the aqueous medium and the surface ice. Thus, bottom water temperatures in polar seas are negative over most of the area ranging between —1.5 and — 1.8°C. Owing to this, biogenic carbonaceous and siliceous accumulations are practically absent in polar seas, and the deposits are terrigenous almost everywhere. The influence of surface marine ice on the sedimentation process is to enrich marine deposits with fragmentai material brought in fast ice and icebergs from the coast. The cryogenic structure of the marine deposits is connected with epigenetic freezing. Prevailing cryogenic textures of clayey-mixed deposits are superimposed reticulate and net or block-type, and in the upper near-surface portions of the sections, laminated-reticulate and lens-like. In general, thick homogeneous marine strata of clays and silty-clays that have been frozen under a 'closed' system, i.e. without water inflow from outside, typically have an ice content decreasing with depth and wider spacing of the ice schlieren network. The upper zone of maximum saturation with ice is 10-15 m thick. Below lies the ice-poor zone of'drying' with big net and block cryogenic textures; at depth they are massive, on the back-ground of which only fine oblique fractures filled with ice occur. The zone of segregation ice formation has, as a rule, 50-60 m thickness.

The strata of marine shelf deposits that had included aquifers before freezing, i.e. they were frozen under conditions of an 'open' system, typically show several icy horizons, mainly with laminated and bedded-reticulate cryostructures; sometimes they are highly saturated with ice, up to sheet ice formation. A feature of the cryogenic structure of marine shelf deposits is ice deposits having plate-like and lens-like shapes.

Continental basin fresh water (lacustrine) and mineralized (lagoon) deposits are mainly represented by silty-sandy and silty-clay materials with different amounts of organic matter. Specifically, there are banded-laminated clays and aleurites with alternating sandy or sandy-silty layers formed during spring-summer periods, and clayey layers, formed in winter while the lakes are covered with ice.

Thus, their formation is associated with cyclicity (winter-summer) in the accumulation of sedimentary material in the final water bodies under conditions of severe continental climate typical of the permafrost regions. In other words, the banded-laminated formations are typical deposits of the permafrost regions.

The cryogenic structure of lacustrine and lagoon deposits is determined by the type of their freezing and their specific lithologic composition. The deposits of large and deep water bodies with through taliks that have been frozen epigenetically (from the top) are similar to marine deposits in cryos-tructure. Near the banks of shallow lakes steeply inclined lens-like reticulate cryostructures arise, associated with lateral freezing. The ice-rich laminated-reticulate, reticulate, sometimes basal (ataxite) cryostructures arise far from the banks, when freezing is from below. In the uppermost part of lacustrine and lagoon deposits saturated with organic matter, there are mainly ice-rich, schlieren, laminated-reticulate, and reticulate (plexiform) cryotextures.

Among frozen deposits there is the specific genetic formation which is called the 'glacial' or lyedoma complex of the frozen loess materials'. The latter compose the northern plains of littoral lowlands, deposits of the Mesozoic-Cenozoic superimposed depressions of the North-East of Russia, the plains of Central Yakutia and the like. The thickness of ice-rich loess-like deposits of predominantly aleurite composition can be as high as 80-100 m, and formation of the biggest syngenetic ice wedges is associated with them.

The strata of deposits of the ice complex are characterized by homogeneity of granulometric composition with a consistently high content (60 to 80%) of the coarse-aleurite fraction (0.01 to 0.05 mm), high porosity and often by the presence of fine horizontal lamination created by bands of relatively 'pure' aleurite and peaty layers. High ice content is typical of the glacial complex deposits (70-80% of their volume).

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