Fig. 15.7. Schematic profile along the Olyenyok-Zhigansk line (after V.S. Yakupov):l - topography; 2 - lower limit of the permafrost; 3 - vertical electric sounding; 4 - geothermal data.

Fig. 15.7. Schematic profile along the Olyenyok-Zhigansk line (after V.S. Yakupov):l - topography; 2 - lower limit of the permafrost; 3 - vertical electric sounding; 4 - geothermal data.

Ocean ice cover throughout the Pleistocene, with the increase of climatic continentality in connection with this and the sharp decrease of atmospheric precipitation. Periodic climatic warming probably did not cause the permafrost to thaw while the low mean annual temperatures (more than 10 °C lower than the present ones) contributed only to a limited temperature increase within their negative values, in the opinion of T.N. Kaplin. As a result the great permafrost thickness formed on the platforms of the Earth's continents was comparable with the permafrost thickness found only within the highest mountain ridges today.

The inertia of negative temperatures in the interior of the Siberian Platform is associated with the weak tectonic activity of this platform in the Cenozoic with a consequent small intensity of geothermal flux (Fig. 15.6). As a result at the present time small gradients of rock temperature of the order of 0.5-1 °C per 100 m (0.1-0.5°C per 100 m in the zone of the greatest thickness) are typical of this platform. Such gradients in the permafrost contribute to the development of rather high mean annual ground temperatures also. The analysis of these temperatures with due regard to information on the nature of the permafrost layer shows the discrepancy between the present conditions of heat exchange and the thickness of the permafrost. This fact most likely reflects the ancient Pliocene-Pleistocene cryogenic age of this unit.

Within the areas where the Precambrian basement elevation is the aquic-lude for the water-bearing horizons of the strata of the Siberian platform, the permafrost zone is mainly represented by one layer of epicryogenic Palaeozoic and Mesozoic rocks. The permafrost is 700-800 m in thickness according to the data from a bore-hole situated on the Anadyr-Olenyok interstream area. At the same time great variations in permafrost thickness, as much as 300-500 m (Fig. 15.7), are noted in the eastern part of the Siberian Platform from geophysical data from the cross-section Olenek-Zhigansk. Such variations in thickness are associated with the effect of tectonic structures of different order and with the presence of fractured zones with various rock structures and composition, with their gas and water permeability, etc., in the opinion of V.S. Yakupov.

By and large the essential distinctive geocryological feature of the Siberian Platform, which is part of the Eurasian continent of low mobility, is that its northern half has the most continuous permafrost in area and the thickest in section of the continental subaerial type and, as opposed to the other great megastructures, it does not have open taliks with positive temperatures. These taliks are developed only under the bed of the Vilyi river, in the region of the Central Yakutiya depression filled with thick Cenozoic and Mesozoic rocks and under some sections of the Lena river bed within the eastern margin of the platform.

The enhancing of geocryological altitudinal zonation by geocryological latitudinal zonation is seen most clearly on the high lavas of the Putorana plateau, in the Anabar highlands and within the Byrranga mountains in Taymyr. As a result the thickness of the permafrost in the upper belt of the Putorana plateau at altitudes of 1500 m and higher and on the summits of the Anabarskoye highlands at altitudes of 900-1000 m, reaches 1000 m and more. The epicryogenic perennially frozen Archean, Proterozoic, Palaeozoic and Mesozoic rocks contain ice along the tectonic dislocations, the number of which decreases with depth. Within the lower structures the permafrost is underlain by cooled rocks, containing cryopegs, cooled below 0°C (see Fig. 15.3). Open taliks are developed in the Western part of the platform under large lakes of glacial-tectonic origin (Lama, Keta, Khan-tayskoye, Dyupkun, Vivi, etc.).

The permafrost has a multi-layered structure (see Fig. 15.3) on the low lacustrine-alluvial, fluvioglacial and marine plains of the North Siberian lowland, including the northern part of the Central Siberian highland and the Yenisey-Khatanga depression.

The upper layer consisting of ice-rich syncryogenic Late Pleistocene lacustrine-alluvial deposits up to 50-70 m in thickness and originally frozen glacial drift, is developed patchily in this region on account of the particular topographic and sedimentation conditions. It is underlain by epicryogenic deposits of fluvioglacial, glacial, marine, eolian and other origins, and having a Pleistocene cryogenic age, varying from a few tens to 200 m in thickness. The third layer is composed of epicryogenic Jurassic and Cretaceous sedimentary deposits of a cryogenic age varying from Early to Late

Pleistocene. The fourth layer is represented by rocks cooled below 0°C with cryopegs and with gas hydrates in dome structures. The cryogenic structure is defined merely by the layers of epicryogenic frozen unconsolidated bedrock within the area of surficial marine, littoral-marine and fluvioglacial surficial deposits. Unfrozen layers from 20-80 m in thickness have been found at depths of 30-60 m within the areas of marine transgression along the valleys (for example along Pyasina river) where there are epicryogenic sand masses interbedded with sandy-silt and clays.

The layered structure of the permafrost in mountain regions depends on geological-structural features, topographic variability and intensity of water-exchange in the stratum. The upper part of the profile (to 100-300 m) in mountain regions usually consists of permafrost containing ice along fractures. Ice does not fill the structures completely and the separate blocks of rock are frosted below this upper part. The thickness of the permafrost in mountain regions increases with altitude and dissection because of better drainage and lower temperatures.

With the same height, degree of dissection and general orientation toward the Sun, the thickness of the permafrost within the mountain regions is subject to geocryological latitudinal zonation, that is, it increases from south northward. The geocryological latitudinal zonation becomes more important with increase in altitude. Thus for example the base of the permafrost in the Pamirs and Tien Shan is situated at an altitude of 3500-4000 m while in the ridges of the northern part of Russia it goes down to 300-500 m below sea-level.

The discontinuity of the permafrost in the valleys which follow tectonic dislocations, is the distinctive characteristic of the orogenic region of the Russian North-East.

The permafrost from 300-500 m in thickness, constituting the so-called hyperzone of the low mountain ridges which extend to the east of the Lena river between the Arctic and Pacific oceans, is the most widespread within this part of the territory. The extensive formation of this permafrost is caused by a combination of increase of mountain relief and height from north southward, while the Arctic cooling effect weakens in the same direction, with the surface radiation balance increasing at the same altitudes. At the same time the thickness of the permafrost in the region of intensive neotectonic movements is more variable than that within low-movement regions because of the deep permafrost of mountain ridges and shallow freezing within river valleys with water-bearing taliks typical of the former.

The permafrost is represented mainly by three layers in the large inter-

montane basins in the Russian North-East. The upper layer consists of Late Pleistocene syncryogenic ice-rich sediments of variable, mainly lacustrine alluvial origin with thick ice wedges. The middle layer is represented by Neogene-Pleistocene epicryogenic materials of fluvioglacial, glacial, lacustrine alluvial and other origins and has less ice content; the lower layer is composed of epicryogenic hard and semihard Mesozoic and Palaeozoic rocks. Loose syncryogenic deposits can occur immediately above the epicryogenic bedrock within shallow depressions.

Northern coastal lowlands such as Yana-Kolymskaya, Srednekolym-skaya and Abyiskaya consist mainly of polygenetic multi-layered permafrost (as far as wedge ice is concerned) occurring on epicryogenic Neogene-Pleistocene loose materials or on strata cooled below 0°C, with cryopegs and with gas hydrates in individual structures. The upper permafrost layer gives topographic expression and is represented by a complex of ice-rich syncryogenic, polygenetic deposits of Late Pleistocene age with thick ice wedges. Ice-rich syncryogenic alas and epicryogenic lacustrine deposits of various cryogenic ages (from late Pleistocene to recent) are included in the upper part of this unit. The middle layer is composed mainly of epicryogenic Pliocene-Early Pleistocene loose deposits with moderate ice content. The lower layer of the permafrost is represented by epicryogenic hard and semihard rocks of the Mesozoic cover with ice inclusions along tectonic dislocations. The lowest layer of the permafrost consists of rocks cooled below 0°C, with cryopegs.

The Zabaykal'ye folded mountain system occupying a mid-latitude part of the region of the former USSR between latitude 50 and 60° N is distinguished by the broadest range of geological conditions. This territory, situated within the region of sharply continental climate, lacking sea basins, with latitudinally oriented mountain ridges and peaks of high absolute altitudes (from 2500 to 3000 m) and a long history of geocryological development including numerous glaciations in the colder periods and thawing during the shorter warm periods, has led to a complex combination of geocryological latitudinal and altitudinal zonation. At the present time the thickness of permafrost on ridges and peaks reaches 1000 m, sharply decreasing toward deep valleys and intermontane basins and often thinning in their bottoms because of the warming effect of ground water. The permafrost is one-layered in mountainous areas. It is composed of epicryogenic bedrock of Pleistocene and Late Holocene cryogenic age with ice along tectonically fractured zones overlain by a 5m layer of coarse boulder deposits on watersheds, and rock streams on slopes cemented with 'golets' ice. The permafrost is overlain by thawed water-rich rocks in which water exchange is very intensive in connection with the high seismicity of the region. This circumstance contributes to a sharp decrease of the permafrost thickness in valleys formed along tectonic fractures. The cryogenic section in valleys is mainly two-layered. The upper layer is represented by alluvial, fluvioglacial, slope wash and other ice-poor epicryogenic or thawed, relatively thin layers (to 25 m, more seldom 50 m). Taliks situated under large river beds and along numerous fractures with thermal mineralized ground water outlets are typical of the valleys. The lower layer is represented by epicryogenic hard and semihard frozen rocks to 50-100 m in thickness.

A two-layered permafrost structure is typical of the Verkhneangarskaya and Barguzinskaya depressions.

The low mountain and highland regions of Zabaykal'ye are characterized by temperature inversions accompanied by decrease of snow cover in valleys, development of marshes, fine-grained deposits, development of sparse forest landscapes, etc. Therefore the thinnest permafrost (about 50-150 m) is typical of the lower watersheds. The thickest is typical of valleys (100-200 m) and high watersheds (200-300 m). Within the low mountain belt and in the south-eastern highland area of Zabaykal'ye with steppe landscapes, the permafrost is developed in depressions and in valleys and is characterized by a one-layered structure. It is represented by epicryogenic loose deposits of various origin, of Late Holocene cryogenic age and not more than 50-100 m in thickness decreasing to 50-25 m southward.

The thickness of the permafrost of epicryogenic hard and semihard rocks is in agreement with the altitudinal geocryological subzones within mountain systems to the south of the former USSR (Sayany, Altay, Tien Shan, the Pamirs,the Caucasus). The hypsometric level of these zones rises from the east westward. The greatest permafrost thicknesses are typical of the perig-lacial area and range from the first metres near the foothills (Sayany, Altay) and, at altitudes of 2000 m and higher (Tien Shan, the Pamirs), to 1000 m and more under the summits of the high ridges and the highest peaks.

The thickness of the permafrost follows a general pattern within the territory of the former USSR as a whole, which appears throughout the history of geocryological development: a continuity in distribution with area and depth increasing from the south northward, from the regions with oceanic climates to the regions with continental climates, from low mountains to high ones, from tectonically movable seismically active structures to structures of low mobility in recent time, and from young structures to more ancient ones as far as their cryogenic age is concerned.

Submarine permafrost

This is situated within the Polar basin and includes oceanic and shelf areas. The oceanic permafrost includes the greater part of the Arctic basin depression but is absent within the areas affected by branches of the North Atlantic current. It is represented only by cooled sea water-saturated materials in the upper ten metres, with temperatures to — 0.7 °C. The shelf permafrost was formed as a result of the submersion of the permafrost below sea-level in the Holocene. This permafrost was formed on the continent during the Late Pleistocene epoch by regression of the sea and possibly as a result of depression of the ground surface under a glacier. The upper ice-rich horizons of epi- and syncryogenic deposits were subject to marine action and the temperature of bottom deposits which ranged from —9 to — 13°C and lower, increased to —1.9 to — 0.7 °C; the ground ice was partly melted and replaced by salt water on account of the marine transgression. As a result the cooled materials include relict layers and lenses of frozen material degrading from the top and from the bottom (because of the Earth's heat) and which were formed under the sea water at the isobaths of 60-10 m. The discontinuity of relict frozen layers increases and the thickness decreases from the coast towards the sea basin. Seasonal freezing and thawing occurs on shoals and on the shelf in shallow waters (with isobaths of less than 10 m); littoral-marine syncryogenic deposits are formed locally. Cryopegs are developed in the lower layers within shallow water parts of the shelf, in deltas and on the elevated bottom area. Near the mouths of large rivers the permafrost is sporadic or is absent entirely.

Subglacial permafrost

This is situated under cold glaciers, the bottom temperature of which is below 0°C. In the case of the base of the glacier lying above sea-level the permafrost is represented mainly by frozen ground; however, it is represented by cooled material with cryopegs when situated under ice shelves. The thickness of the subglacial permafrost varies from a few metres to 500 m and more depending on the glacier basal temperature, its thickness and the geothermal flux. Under warm glaciers with an ice-firn mass that has existed for a long time there is a possibility of unfrozen materials with positive temperatures (but close to 0°C) holding fresh and slightly saline waters, giving way at depth to rocks below 0°C with cryopegs.

Within the high mountains of the Caucasus, the Pamirs and Tien Shan, the glaciers exert a cooling effect where firn masses are absent in the near-surface portion of the glacier, as well as having a warming effect when firn is formed on icesheds and ice caps. The latter occurs in glacier beds

Fig. 15.8. Map of types of seasonal ground thawing and freezing in the former USSR. Types of seasonal thawing with respect to mean annual temperature,0 C:

I - arctic and polar (below - 10°C); 2 - stable (-5 to - 10°C); 3 - stable for a long time and semitransitional (— 1 to — 5°C); 4 - transitional and semitransitional (+ 2 to — 2°C); 5 - type of seasonal thawing and freezing -semi-transitional and stable for a long time (1 - 5°C). Types of seasonal freezing with respect to the mean annual temperature, °C: 6 - stable (5-10°C); 7 - southern (10—15°C); 8 - subtropical (15-20°C and higher). Types of seasonal freezing and thawing with respect to amplitude of temperature fluctuations on the surface,°C: M -marine (less than 7.5 °C); TM - temperate-marine (7.5-11 °C); TC -temperate-continental (11-13.5°C); C - continental (13.5-17°C); RSC - rather sharply continental (17-21 °C); SC - sharply continental (21-24°C); ESC -extremely sharply continental (higher than 24 °C); 9 - boundary of seasonal ground thawing and freezing with respect to the mean annual temperature; 10 -the same with respect to the amplitude of temperature fluctuations on the surface;

II - southern limit of permafrost.

when the ice is flowing down the slope and when it is thawing from the bottom in the course of sliding. Therefore the mean annual ground temperatures under the moving glaciers can be positive or close to 0°C (negative) while the temperatures below glaciers of low mobility and temperatures of rocks adjacent to glaciers can decrease to — 10°C, — 15°C and lower. Taliks of the closed type occur under small thawing glaciers surrounded by low temperature ground with great thickness of permafrost.

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