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Ataxitic

Pseudoschlieren (predominately)

Horizontally, vertically laminated

Micro-, frequent laminated

Fine-, micro-schlieren

significant role is played by the initial structure and texture of soils. Cavities and fissures in fine-grained soils allow infiltration and other types of ice; they also exert substantial influence over spatial arrangement and orientation of layers of migration-segregation ice formed in the vicinity.

The influence of lamination and inhomogeneities, i.e. contacts between soils differing in thermal-physical and physical properties, lies in the fact that with different shrinkage and deformation characteristics the processes of heat and moisture transfer are modified. A specific setting is created in the zones of contact between layers and inclusions which experience to a great extent the difference in shrinkage deformation values. This leads to concentrated tensile stresses of shrinkage sufficient to overcome the strength of structural bonds of the soil and, consequently, results in the formation of ice layers that inherit the primary lithological inhomogeneity.

Ice formation in rudaceous uncemented materials is mainly conditioned by freezing of meltwater and rainfall that penetrate during infiltration and carry fine-grained material. Freezing of this water in rudaceous rocks with negative temperature often leads to heaving of blocks and formation of basal ice-cement (golets ice). With reduction of depth of seasonal thawing such ice may sometimes become buried in the permafrost sequence. Closed air-filled cavities in which ablimation ice is growing, may occur at contacts of soil with individual blocks.

Rudaceous material with fine-grained infilling often has a massive, crus-tal or composite lens-like laminated cryogenic texture in which ice inter-layers are curved, not persistent in strike and often following the shape of the blocks. In the heterogeneous uncemented materials the inherited cryogenic textures are formed in discontinuities, fractures and cavities. The mechanism of ice formation is the same as that in rocks.

There are cryogenic textures of the seasonally thawing and seasonally freezing layers, existing only in the winter period and there are cryogenic textures of permafrost. The temperature regime during freezing and the chemical-mineral composition of soils have importance for their formation. The mechanism of structure generation in the layers of seasonal thawing and freezing is predominantly that of migration-segregation.

A dual (two-part) cryogenic structure is typical of the soils of the seasonally frozen layer following from variations of the freezing regime with depth. In soils of the seasonally thawing layer a second horizon of cryogenic textures is observed which arises due to freezing from beneath, because of the adjacent permafrost.

In the upper part of the permafrost the epigenetic cryogenic textures are formed at smaller gradients of temperatures and slower rates of freezing compared to the layer of seasonal thawing. Low rates of freezing and small temperature gradients lead to less intense migration flows of moisture, which however persist for a long time. Shrinkage processes and a thick zone of drying caused by long-term dewatering lead to smaller gradients of shrinkage tensions and, accordingly, to the formation of dispersed-laminated, big-latticed and block-like cryogenic textures (Fig. 7.10).

In the permafrost below the depth of zero annual amplitude generation of cryogenic texture took place at lower gradients of temperature and rates of freezing, under freezing of the epigenetic type. The pressure-migration and injection mechanisms of ice segregation are key factors in the formation of cryogenic structure determining its texture (size of segregational layers of ice, spacing between them). The horizontal-laminated cryogenic structure that forms in conditions of free inflow of moisture from below, grades into the block-type cryogenic structure with vertical ice schlieren oriented in the direction of moisture injection when hydrodynamic pressure is greater (Fig. 7.11).

Cryogenic textures of syngenetically freezing deposits form with (from the geological point of view) simultaneous freezing and diagenetic transformation of newly deposited sediments. The process of their transition into the permafrost state is associated with a cyclic (year to year) upward movement of the base of the seasonally thawing layer by the thickness of new sediments, as the freezing of the layer occurs from below. The depth of seasonal thawing is in this case constant. Thus, the cryogenic structure of permafrost soils frozen syngenetically is formed by the lower part of the seasonally thawing layer changing into the permafrost state. Cryogenic structures are formed in this layer only very rarely by freezing from the top (at high mean annual temperatures). In natural conditions there can arise different combinations of structure-generating mechanisms depending on the relation of the rates of accumulation of sediments to the rates of freezing and thawing. In one case soils may have a relatively homogeneous cryogenic structure following syngenetic freezing, while in another, cryogenic textures are repeated at a certain interval, i.e. cyclic cryogenic structure is typical (Fig. 7.12). The formation of cyclic horizons of cryogenic textures is associated with growth of thicker ice layers because of slower freezing from below or segregation ice formation on thawing from the top. Between the horizons of cryogenic textures formed by freezing from below, cryogenic structures may be found that have been formed in the course of seasonal thawing, i.e. in general, this type of cryogenic structure of the frozen soil has a mixed nature with respect to the mechanism and formation conditions. Cryogenic texture that has been formed in the frozen soil during thawing most often grades smoothly into the texture that has been formed on freezing from below. In this case ice is segregated in the permafrost in significant thickness (up to several tens of centimetres thicker than the accumulating sediment). Annually repeated seasonal cycles of freezing-thawing give rise to ice-rich horizons in the upper

Fig. 7.10. Cryogenic structure of epigenetically frozen marine Kazantsev clays. The Yamal, the Seyakha river valley. Block cryogenic texture; horizontal ice schlieren inherit the primary lamination of the clays (photo by G.I. Dubikov).
Fig. 7.11. Block cryogenic texture in silty clay. Vertical ice schlieren of injection origin can be seen. At the base there is tabular ice (photo G.I. Dubikov).

part of perennially frozen soils and ataxic cryogenic textures (Fig. 7.12). Such horizons of textures are typical of fine-grained soils in which the process of ice segregation is rather intense at thawing. On freezing from below the more homogeneous cryogenic structure is created by the relatively slow freezing. The thickness of ice layers in such structures diminishes from below upwards since the upper soils are more compacted.

The cryogenic structure of syngenetic soils is much dependent on the regime of freezing from below and thawing from the top. For soils having high mean annual temperatures (i.e. for the southern type of syngenetically frozen deposits) accumulation of small amounts of ice is typical under both winter freezing from below and summer thawing from the top. This is mainly associated with the low gradients of temperature in the upper layer of perennially frozen materials during both winter and summer. On the contrary, a high content of ice of the perennially frozen soils below the layer of seasonal thawing is typical for soils having low mean annual temperature (i.e. for the northern type of syngenetically frozen deposits).

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