Features of the cryogenic types of frozen strata

Currently the strata of perennially frozen materials are subdivided into two basic types with respect to conditions of their freezing, namely, epicryogenic and syncryogenic. Under certain conditions (locally) a third type can be distinguished - diacryogenic perennially frozen materials.

In the majority of cases the frozen strata represent different combinations of these types and are then called polycryogenic (polygenetic) types.

Epicryogenic frozen strata are formed by freezing (usually from the top downwards) of lithified materials in which complex diagenetic physical-chemical processes have already taken place. With respect to their cryogenic structure they are subdivided into epicryogenic Pre-Quaternary bedrock (with rigid bonds, monolithic and others) and epicryogenic Neogene-Quat-ernary materials of the loose mantle.

Syncryogenic strata are formed by processes of sediment accumulation and freezing occurring simultaneously (synchronous in a geological sense). Therefore, frozen strata of this kind can be represented only by loose, Quaternary deposits. Their accumulation and freezing take place from below upwards. At the commencement of syngenetic freezing the base of the accumulating series would be composed of epigenetically frozen strata.

Diacryogenic strata are formed by the freezing from the top, from below and laterally of oversaturated non-lithified materials - newly deposited sediments and silts in which complex diagenetic physical-chemical processes have either just begun or are far from completion, being stopped by the processes of freezing.

The structure of epicryogenic strata

All the mountain territories of the permafrost zone are the areas of predominant development of epicryogenic strata of hard and semi-hard rocks. The cryogenic structure of epigenetically frozen bedrock is dictated by composition, structure, moisture content and Assuring at the commencement of freezing, mean surface temperature, variations of climatic conditions, the neotectonic and glacial setting, etc. Cryogenic textures, being inherited, are determined by the pattern of primary cavities in the rocks and peculiarities of their freezing.

The greatest depths of epicryogenic bedrock strata with negative temperatures that have been observed, exceed 1000-1500 m. Two or three zones can be distinguished from the top downwards depending on the conditions of formation of the cryogenic structure. The near-surface zone of highly fissured rocks is that of rather active cryogenic weathering, corresponding to the layer of annual fluctuations of negative temperatures where phase transitions of moisture, causing expansion or reduction of ice volume in fractures, are expressed. Below, the frozen rocks of the ancient weathering crust can occur, underlain by low ice content, often cryotic rocks which extend hundreds of metres vertically. Finally, at the bottom of the epicryogenic permafrost strata, in the fissured bedrock, strata can occur with saline water having negative temperature - cryopegs.

The cryogenic structure of epigenetically frozen loose deposits is to a significant extent determined by their lithogenetic type, moisture content before freezing, availability or absence of aquifers, degree of lithification and landscape-climatic setting, varying in accordance with the natural environment in the Cenozoic. Thick strata of basin deposits have been frozen epigenetically: marine, glacial-marine, lagoon and lacustrine deposits mainly having a fine-grained composition (clays, silty clays, sandy silty materials). The same type of freezing occurs in glacial accumulations of block-boulder/rock waste/fine grain composition as well as river-bed alluvial sands, pebble gravel, partially eolian deposits, peats, and ancient weathering crusts.

The strata of basin deposits have the most typical cryogenic structure. These freeze according to the 'closed' or 'open' type of system depending on the availability or absence of aquifers that provide the inflow of moisture on freezing.

The deposits of coarse composition - sands and pebbles - freeze under conditions of a 'closed' system with pushing of moisture ahead of the freezing front ('piston effect'). Therefore, the ice content of the frozen material is generally small (up to 10-20 %). Among the dominant types of cryogenic texture are the massive and the crustal. The availability of clayey layers serving as aquicludes gives rise to cryogenic pressure of ground water resulting in the formation of high ice content materials with basal cryogenic texture as well as layers and lense-like deposits of ice. The same effect is typical of coarse-grained deposits freezing in an 'open' system when there is inflow of pressurized or unpressurized ground water. In this case, series and horizons of ice-rich deposits often arise, with layered and lense-like deposits of ground ice usually containing particles of local rock material.

The series of basin deposits of homogeneous, predominantly fine-grained composition, can be 200-300 m thick, for instance in West Siberia or in the European North-East of Russia. These are composed of poorly sorted silty-clays with inclusions of gravel, pebbles, boulders or, rarely, relatively well-sorted clays and aleurites. The thickest strata are of glacial-marine origin and have the largest areal extent. A less thick series of lagoon, estuarine and lacustrine deposits typical of vast and deep water bodies have a smaller areal extent.

Typically, the strata of epicryogenic fine-grained deposits of homogeneous composition, having been frozen under a 'closed' system with upward migration of moisture and formation of segregated ice, have the structure shown in Fig. 9.1a. The horizon 5 to 10 m deep from the surface, rarely 15 m, has the highest ice content. In general, this horizon corresponds to the layer of annual temperature variation of the frozen stratum in which small transitions of unfrozen water into ice and vice versa occur. Grain size and chemical-mineral composition of this upper part of the epicryogenic strata are determined by the conditions of sediment genesis, diagenesis and weathering existing before freezing as well as by the conditions of freezing and the geochemical setting subsequently (10). Volumetric ice content here reaches 40-50 %, and the soils are often heaved. Cryogenic texture is predominantly fine-schlieren, often cellular and laminated-cellular, and in peaty deposits it is basal. The spacing between ice schlieren increases with depth while they become thicker. At a depth of 20-30 m from the surface total ice content of the frozen ground diminishes to about 20-30 %, with mostly big-cell and block-type cryogenic structures. The cross-section of the blocks is 0.5-0.7 m, sometimes 1-2 m, ice schlieren are 2-3 cm thick, sometimes 5-7 cm. At the depth of 30-40 m big-block cryotextures with open nets are developed, while farther down (sometimes about 100 m) only isolated broken schlieren are encountered, cryogenic texture being predominantly massive. Volumetric ice content diminishes from the top downwards from 20-10% within the interval considered above.

With a high ice content in the upper part of an epicryogenic series, structures may have either a homogeneous nature or several horizons with a high ice content. In the former case, according to V.A. Kudryavtsev, the higher ice contents of the upper third of the frozen epicryogenic strata are caused by long-term temperature variations of various periods and ampli-

a b c

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