Structure of the permafrost and spatial variability of its thickness

The permafrost is defined as the part of the Earth's crust in which the earth materials have negative temperature, regardless of the presence or phase composition of water in it. The structure of the permafrost, its thickness and distribution reflect the combined result of the whole history of its formation and the dynamics of perennial ground freezing from the end of the Neogene up to the present time. The permafrost thickness within the territory of the former USSR varies greatly, from the first metres to 1500 m and more depth, and is distinguished by continuous and discontinuous (interrupted) distribution over the area and in section. The continuity of the permafrost over the area (or its discontinuity) depends on the conditions of the ground temperature regime established in the layer of annual temperature fluctuations, on the distribution of open taliks, and on its thickness and structure. In accordance with these features it is subdivided into the northern and southern geocryological zones of continuous and discontinuous (sporadic to interrupted) permafrost. The discontinuity of the permafrost over the area (see Table 15.1) is associated with the development of open and closed taliks of different genetic types (see Chapter 13).

The distribution of the permafrost across the region depends on the geological tectonic structure, on the conditions of perennial ground freezing and on the interaction of the ground with ground waters and essentially on the history of geocryological development in the Late Cenozoic. As a result, continuously frozen strata (through the whole depth) as well as two frozen layers separated (vertically) by a layer of thawed material, have been formed in various geologic-structural, hydrogeological, topographic and landscape-climatic conditions. With respect to geological structure the permafrost zone can be represented by loose deposits of Cenozoic (mainly Quaternary) age and by geologic formations of the PreCenozoic age.

With respect to the type of cryogenesis and structure of the permafrost, syncryogenic and epicryogenic sediments can be characterized as having low ice content, average ice content or as being ice-rich. With respect to cryogenic age (the onset of perennial freezing) the strata of the permafrost region are Pliocene-Pleistocene (not differentiated), Late Pleistocene, Late Holocene and recent materials.

In the cryogenic structure of the permafrost profile there may be several

Central Siberian Plane Permafrost

Fig. 15.3. The structure of the West Siberian permafrost zone along a meridional profile (after G.I. Dubikov): 1 - permafrost; 2 - ground below 0°C with cryopegs; 3 - stratigraphic boundaries between: Quaternary sand-clay layer Q, Palaeogene clay P, Cretaceous clay and sand-aleurolite C, and Palaeozoic crystalline materials Pz.

Fig. 15.3. The structure of the West Siberian permafrost zone along a meridional profile (after G.I. Dubikov): 1 - permafrost; 2 - ground below 0°C with cryopegs; 3 - stratigraphic boundaries between: Quaternary sand-clay layer Q, Palaeogene clay P, Cretaceous clay and sand-aleurolite C, and Palaeozoic crystalline materials Pz.

strata modified by the presence and content of ice, salt waters, brines and gases in them. These strata are as follows: 1) frozen and cryotic, i.e. with and without ice; 2) cooled below 0°C, containing salt groundwater and brines (cryopegs) and gas hydrates; 3) relict (Pleistocene) permafrost, occurring beneath the surface and overlain by thawed and perennially frozen (Late Holocene) soil and recently frozen ground.

Strata with negative temperatures containing ice, salt water and brines are termed cryogenic strata after S.M. Fotiyev. The variations in permafrost thickness depend mainly on the existing geocryological latitudinal variations (or the altitudinal zonation in mountains) and on the peculiarities of the development of the permafrost zone in the Late Cenozoic.

The formation of the permafrost is considered to have occurred at various times from many hundred thousand years ago (about 2 million or more years ago) to some years ago. Thus the first formation of permafrost on the north of Eurasia was Late Pliocene and Early Pleistocene. With respect to the occurrence and conditions of formation in the upper part of the Earth's

Fig. 15.2. (opposite) The map of the thickness of the permafrost of the former USSR. Southern, subaerial and subglacial, geocryological zone: 1 - 0-15 m; 2 - 0-5 m; 3 - 0-50 m; 4 - 0-100 m. Northern, subaerial and subglacial, geocryological zone: 5 - 100-200 m; 6 - 100-300 m; 7 - 200-400 m; 8 - 300-500 m; 9 - 300-700m; 10 - 400-600m; 11 - 400-700m; 12 - 500-900m; 13 - 700-1100m; 14-900-1500m; 15 - 100-1000m; 16-relict permafrost of 100-200m thickness occurring at depths of up to 100 m (a); 100-200 m {b); > 200 m (c); from the surface. Submarine part of the permafrost: 17 - 0-100 m; 18 - 100-300m; 19 - 200-400 m. Limits of permafrost: 20 - limits of subaerial and subglacial parts; 21 - of submarine permafrost; 22 - of relict permafrost; 23 - of present day permafrost; 24 - of possible distribution of the permafrost in the Pleistocene.

crust the permafrost region is subdivided into the subaerial part of the continental permafrost, the submarine permafrost part, situated under the Arctic basin and basins of adjacent seas, and the subglacial part of the permafrost under glaciers. Permafrost thickness and the types of its cryogenic structure within the territory of the former USSR are shown on the map drawn at the 1:25 million scale (Fig. 15.2).

The subaerial permafrost occupies the continental part of the territory of the former USSR and is divided into southern and northern geocryological zones with respect to areal distribution, thickness, cryogenic structure and cryogenic age.

Within the southern zone the cryogenic thickness from the surface is represented by one layer only of perennially frozen strata commonly up to 100 m in thickness. Changes in thickness depend on the latitudinal variations of heat exchange on plains and highlands and on the altitudinal variations of heat exchange in mountains. The permafrost strata are underlain by unfrozen strata with hydrological conditions in accordance with the hydrogeological structure.

The thickness (varying from 0 to 1500 m and more) and the structure of the cryogenic rocks in the northern geocryological zone depend to a lesser degree on the present conditions of heat exchange, being dictated instead by the history of geocryological development and depending essentially on the type of tectonic and hydrogeological structure and on topography. Thus the permafrost is represented within the mountain and ancient shield regions mainly by one permafrost layer, that is, by perennially frozen materials underlain by unfrozen water-bearing strata.

The permafrost layer (from 25 to 500 m in thickness) within ancient platforms, young plates and on the coast of the Polar basin is underlain by cooled rocks, with cryopegs. There are two permafrost layers within the Western Siberian plate, on the north-east of the Russian Platform and in the intermontane depressions of Pribaykal'ye, these regions having been subjected to deep thawing from the surface during the climatic optimum of the Holocene (Fig. 15.3). The upper layer of the permafrost is of Late Holocene age, and the lower one is of Late Pleistocene age. The lower layer of relict frozen strata is usually associated with sand-clay and clayey-silty sand materials showing large phase transitions of water.

The cryogenic development of the Eurasian continent in the Pleistocene and Holocene retained the zonal distribution of thickness of permafrost on young plates and partly on ancient platforms, and was responsible for the more complex pattern of its distribution over the vast areas of the mountain regions of the Russian North-East.

Fig. 15.4. Geocryological section of Gydan peninsula (after G.I. Dubikov): 1 - syncryogenic unit of Upper Quaternary proluvial-marine (pmQ1,,, and pmQ2,,,), recent and Upper Quaternary alluvial (aQ]v, aQ4,,,) deposits; 2 -epicryogenic unit of Upper Quaternary marine (mQ1,,,) and Middle Quaternary marine and glacial-marine (m, gm Q,2-4) deposits; 3-4 - ice wedges in syncryogenic and sheet ice in epicryogenic deposits, respectively.

Fig. 15.4. Geocryological section of Gydan peninsula (after G.I. Dubikov): 1 - syncryogenic unit of Upper Quaternary proluvial-marine (pmQ1,,, and pmQ2,,,), recent and Upper Quaternary alluvial (aQ]v, aQ4,,,) deposits; 2 -epicryogenic unit of Upper Quaternary marine (mQ1,,,) and Middle Quaternary marine and glacial-marine (m, gm Q,2-4) deposits; 3-4 - ice wedges in syncryogenic and sheet ice in epicryogenic deposits, respectively.

The latitudinal zonation of the permafrost thickness is best-defined in the southern geocryological zone which includes sporadic permafrost (up to 15-25 m in thickness), massive islands (up to 25-50 m in thickness) and discontinuous permafrost (up to 100 m in thickness). The permafrost may extend over limited areas (less than 25% of the total area) and may be more or less than the thicknesses given by a factor of 0.5-1.5 or even 2. Such a decreased permafrost thickness can occur only under favourable conditions azonal for the given area, while an increase of 50-100 m in thickness can occur in the areas where joining of Late Holocene permafrost is possible (as far as its cryogenic age is concerned) with frozen layers surviving from the Late Pleistocene. The permafrost within the southern geocryological zone as a whole is mainly of Late Holocene and recent cryogenic age correlated with the perennial freezing of the ground after the Holocene climatic optimum (see Fig. 14.8). The layer of relict (Pleistocene) permafrost occurring at some depth (70-200 m and more) from the surface has been retained within the southern geocryological zone owing to dynamics of climate in the Pleistocene and Holocene, sea transgressions and regressions and glaciations under particularly favourable conditions (unconsolidated ice-rich Mesozoic and Cenozoic material). Such relict permafrost is found at the present time even to the south of the Arctic circle on the north-east of the Russian plain, from the latitude 60° N to the Arctic circle within the Western Siberian plain (see Fig. 15.3) with the thickness varying, characteristically, from 300 to 100 m and less. In the places where the permafrost of Holocene cryogenic age is developed above it the 'two-layered' permafrost occurs. On the north of the Russian plain and within the Western Siberian plate, cryopegs cooled below 0°C are developed below the relict permafrost in marine deposits filling depressions between dome structures. This cooled layer is 50 m and more in thickness. Relict permafrost, not very thick (to 100 m), is developed also at depths of 40-80 m within the Upper Angara and Barguzin depressions of Pribaikal'ye.

Within the northern geocryological zone the permafrost of the Late Holo-cene and of Pleistocene cryogenic age is mainly joined, forming cryogenic units of great thickness: to 300-500 m in Western Siberia, to 1000-1500-m within the Siberian Platform and to 300-800 m and more on the mountain ridges and to 200-400 m in depressions of the North-East of Russia. Within this cryogenic unit open and closed taliks of the underwater and hydro-geogenic types are developed due to the heat effect of aquifers, streams and ground water. Radiation-thermal and infiltration taliks situated within sand masses, for the most part of the closed type, are developed locally.

Two types of permafrost are developed from the surface within low plains on platforms and young plates. These are: 1) ice-rich epicryogenic (marine, glacial) loose deposits with occurrences of sheet ice (Fig. 15.4); 2) ice-rich syncryogenic (lacustrine, alluvial, colluvial, biogenic, etc.) loose deposits, with thick syncryogenic ice veins up to 40-80 m - the 'ice' and 'yedoma' complexes. Syn- and epicryogenic permafrost of the loose cover is underlain by a layer of epicryogenic hard and semihard bedrocks at various depths.

There exists rather clear-cut geocryological zonation in the distribution of permafrost thickness within the Western Siberian plate, on the NorthEast of European Russia and within the plains part of the Siberian Platform. It is disturbed only by altitudinal geocryological zonation in mountainous regions. At the same time the subzone with the permafrost layer of 300500 m in thickness (300 m in valleys; 500 m on watershed areas) is typical of highland surfaces having an absolute altitude of about 400 m within the Siberian Platform.

Such thickness of the permafrost layer is typical of the hyperzone extending from the left bank of the latitudinal flow of the Lena river to the latitudinal flow of the Anabar river. The permafrost, being 300-400 m in thickness, is mainly the continuation of this zone in Western Siberia. Within the more ancient high altitude topography the permafrost increases in thickness to 500 m. The permafrost thickness on the north of the Siberian Platform and on Taymyr peninsula reaches 400-600 m. So the latitudinal zonation of the level of heat exchange during the Pleistocene and Holocene manifests itself in this way.

The essential disruption of the zonal permafrost distribution is the layer of cooled rocks with cryopegs. This is seen most clearly within the Siberian

Fig. 15.5. Profile along meridian of the Central Siberia and Zabaykal'ye permafrost zone: 1-3 - epicryogenic units of hard and semihard rocks (1 - with ice in fractures freezing in the period N, - Q,3V-4; 2 - with ice in fractures and with saline waters at negative temperature freezing in the period Q, - Qm; 3 - with negative temperature brines, freezing in the period Q, - Qni); 4 - bottom of permafrost.

Fig. 15.5. Profile along meridian of the Central Siberia and Zabaykal'ye permafrost zone: 1-3 - epicryogenic units of hard and semihard rocks (1 - with ice in fractures freezing in the period N, - Q,3V-4; 2 - with ice in fractures and with saline waters at negative temperature freezing in the period Q, - Qm; 3 - with negative temperature brines, freezing in the period Q, - Qni); 4 - bottom of permafrost.

Fig. 15.6. Schematic map of heat flux of the North-East of Russia (after V.T. Balobayev): Heat flux, W m~2; 1 - < 0.02; 2 - 0.02-0.04; 3 - 0.04-0.06; 4 - 0.06-0.08; 5 - > 0.08.

Platform. The permafrost layer and the layer of cooled rocks (below 0°C) with cryopegs has been revealed in bore-holes in the upper reaches of the Markha river (central part of the Siberian Platform) extending to a depth of 1500 m (Fig. 15.5). Such deep freezing is associated with directional cooling of the Earth's crust in the Pliocene (see Fig. 14.2), with the increase of Arctic

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