Fig. 9.1. Types of cryogenic structure of epigenetically frozen strata of fine-grained basin deposits: (a - of homogeneous composition; b - containing as a basal layer a previously aquiferous layer of sandy composition; c - containing two previously aquiferous layers of sandy composition): 1 - cryogenic textures of fine-grained deposits; 2 - previously aquiferous sands.

tude. Short-term temperature variations dictate the high ice content of the near-surface portion of the frozen strata to a depth of 5-10 m. Temperature variations of a longer period determine the higher ice content at a greater depth (from 15 to 30 m). Temperature variations of still longer period have an impact for still greater depths, etc. In summary, this leads to the situation where a third of the frozen series acquires a high ice content in fine-grained materials where cellular and laminated-cellular cryotextures begin to form. Since heat exchanges are drastically reduced with depth, the ice content cannot be high over the whole frozen series. Besides, fine-grained soils are usually more compact and dried with depth because of the normal dia-genetic processes and migration of moisture towards the freezing front. Consequently, ice content diminishes downwards and networks of subhorizontal and subvertical ice schlieren become rare; still lower, the series is not rich in ice, and the cryogenic texture becomes massive.

In the second case several horizons are observed in the section of a series, having a higher ice content with cellular and laminated-cellular cryogenic textures. G.F. Gravis assumes this to be associated with development of cryogenic pressures, on freezing of homogeneous materials, in the lower (still unfrozen) portion of the series. As a result, repeated migration of moisture under pressure,towards the freezing front, often induces intense ice segregation at certain levels in the frozen series.

Epigenetic freezing of loose fine-grained deposits in the type of 'open'

system when there are interlayers of aquiferous sands and pebbly gravel leads to a highly complicated picture of cryogenic structure (see Fig. 9.1b,c). Ice-rich layers characterized by fine-laminated and laminated-cellular cryostructure are created over the aquifers. Often, excess moisture is associated with the formation of ice strata and lenses of segregational origin. Nonuniformity of the freezing processes in different parts of the frozen series leads to cryogenic pressures in the aquifers and formation of intrusive ice lenses and ground ice which is often associated with fold dislocations. The zones of higher ice content with net and cellular cryostructures often immediately overlie lenses of intrusive ice and ground ice; creation of these cryotextures is assumed by G.I. Dubikov to be due to pressurized water injection into fine-grained materials.

Results of drilling in recent years show that in the north of the West Siberian Plate, within the limits of the Siberian Platform in the coastal lowlands of Siberia, below the epicryogenic frozen strata, cryopegs occur with highly saline water (up to 200 g F1 and over), the freezing point of which is much lower than 0°C. The thickness of the zone with highly saline water having negative temperature reaches several hundreds of metres. After the zone is formed, it is believed easily soluble salts are pushed out of pore solutions of the freezing strata (chlorides of sodium, calcium, magnesium, etc.) and they concentrate below the beds of ice-rich frozen and cryotic rocks. Cryopegs are also observed inside the frozen strata of loose deposits.

Formation of epigenetic ice wedges in the near-surface part occurs in epicryogenic strata. Vertically, they extend no more than 3-5 m, although 6-8 m high wedges are known, and theoretically, they can be as high as 12-15m. Their width at the top usually does not exceed 1.5-2m. The ice inside the wedges has a distinct vertical banding due to the arrangement of air bubbles and organic-mineral admixtures.

In the uppermost part of epicryogenic strata below the seasonally thawing layer the so-called transitional layer with higher ice content, 1-2 m thick, stands out in many cases. In this layer the degree of saturation with ice can reach 70-80%, cryotexture is mainly laminated, and often ribboned. The layer is formed either by transition of the lower part of the saturated seasonally thawed layer into permafrost, or by migration of moisture from that layer into the upper part of the permafrost.

In a number of cases the upper part of epicryogenic strata are represented by palaeocryoeluvial continental deposits formed outside the area of permafrost development but having been subject to multiple and deep seasonal freezing-thawing in the course of their accumulation (10). In the case of general climate cooling and one-time freezing from the top of these continental formations, epicryogenic palaeocryoeluvial continental deposits (see §6.2) are formed.

Syncryogenic deposits

These are always formed above a frozen substratum, i.e. above epicryogenic perennially frozen strata, by simultaneous, synchronous, accumulation and freezing of subaqueous and subaerial deposits. The extension upwards of the frozen series involves saturated soil; the bottom of the seasonally thawing layer freezes onto the top of the frozen stratum and ice schlieren are formed from the moisture from this layer. It is the manner of increase in thickness of the perennially frozen ground that distinguishes syn-and epicryogenic strata. The thickness of the latter increases by deepening of the permafrost while that of the former increases by the rise of the upper surface of the permafrost. Thus, with a constant thickness of the seasonally thawing layer, the upper surface of permafrost rises annually by the thickness of annually accumulated terrestrial deposits (with allowance for their heaving when transferring into the frozen state). It is also clear that sediments accumulating from the top grade into the permafrost state gradually, not suddenly, but undergoing a rather great number of freeze-thaw cycles until they come to lie below the layer of seasonal thawing. Tentative estimates show the maximum number of freeze-thaw cycles can be as high as 10000(10).

The process of syngenetic freezing is most apparent in the accumulating deposits of flood-plains, deltas, laida, alases, mainly of sandy-silty composition enriched in organic matter, fine-grained slope deposits - talus and solifluction as well as eolian. Much less developed are syncryogenic deposits characterized by a small thickness (the first metres), forming in shallow parts of fresh water bodies on drying shelf areas, the freezing of which both laterally and from below (when the frozen stratum is at — 3°C and lower) can occur with simultaneous accumulation of sediments.

Syncryogenic strata are most widely developed in such areas where a continental regime of sediment accumulation existed during the Quaternary period. Thus, the thickest (80 m and over) syncryogenic strata are developed in areas which were subject to gradual tectonic sinking during the Pleistocene compensated by the accumulation of continental deposits. Such areas are littoral lowlands and the Mesozoic-Cenozoic superimposed basins of the North-East of Russia, Novosibirskiye Ostrova [New Siberian Islands] and the plains of Central Yakutia. In addition, syncryogenic rocks are developed within river valleys and lacustrine-marshy basins in the continuous permafrost zone and at sites of low temperature, where their thickness is not more than some tens of metres.

Among syncryogenic strata there are 'southern' and 'northern' variants as regards their cryogenic structure. In the southern one, for each cycle, there are very small thicknesses of a frozen icy layer of soil, which have both icy soil frozen from below and from the side of the frozen strata and which overly a dewatered layer with low ice content. Uniform distribution of thin horizontal ice layers along the section is typical of such syncryogenic strata. This is conditioned by the predominant flow of cold from the top during autumn-winter freezing of the seasonally thawing layer, while the temperature of the frozen ground remains relatively high (not below — 3°C). As a result, the lower part of the seasonally thawing layer is drained and thin ice layers arise at its base. Sometimes, they can be absent altogether with formation of low-ice content massive cryogenic texture.

In the northern variant freezing of the seasonally thawing layer from below is considerable; there are syngenetically frozen strata for which a high ice content over the whole section is typical as well as the presence of bigger ice schlieren up to several centimetres thick on a background of thin laminated and lamellar-reticulate cryotexture, called by Ye.M. Katasonov 'belts'. Depending on the ground surface topography and, accordingly, on the outlines of the base of the seasonally thawing layer, these belts occur either horizontally or have a concave shape inheriting the shape of the thaw basin inside the dike polygons over the surface of flood-plains, laida, and bottoms of kettle lacustrine basins.

Formation of 'belts' of ice-rich cryogenic structure is, in the opinion of many investigators, associated with a continuous and rather uniform uplift of the surface of accumulating deposits, on the background of which there are cyclic variations of seasonal thawing depth induced by climate fluctuations. Climate cycles of 11, 40, 100 and 300 years, superimposed on each other, lead to the nonuniform occurrence of belts over the section.

Syncryogenic strata are also formed with accumulation of deluvial talus, solifluction, eolian deposits and their freezing from beneath. Syncryogenic fine-grained slope deposits which are formed in conditions of accumulation exceeding wash (with a constant thickness of the seasonally thawing layer) are mainly represented by peaty, sandy-silty and silty-clay materials and with inclusions of stone and sod material. Indistinct lamination, oriented in general along the slope, is characteristic of these. Layers are grouped together into bundles. Ice is irregularly distributed over the section of each bundle.

In the lower and upper parts of a bundle, laminated, reticulate and block or lens-like cryogenic structures are observed. The middle part is often characterized by massive cryotexture with infrequent ice schlieren. Syngeneic ice wedges are typical of the deposits; the depth they reach corresponds to the thickness of the stratum, and their distinctive feature is curvature along the slope gradient.

In general, syncryogenic strata have a number of attributes making it possible to distinguish them from epicryogenic strata. They predominantly have a sandy-silty composition, high content of ice, leaf-by-leaf enrichment in peaty and vegetation residues and diffused organic matter with different degrees of degradation, and the whole thickness is penetrated by vertical ice wedges. The shape of ice wedges is tongue-like with constrictions. Their lateral fringes are not smooth, typical is the presence of'shoulders' to which are 'welded' thick schlieren - belts of ice-bearing strata. The latter are deformed irregularly near contacts with ice wedges. Elementary ice wedges inside the big wedges appear on their lateral contacts.

Diacryogenic frozen strata

These are formed when basin sediments (coastal-marine, tidal marsh, lacustrine, lagoon, estuarine) freeze at the bottom of a water body, at shallow places and on shelf areas as these basin formations emerge from beneath the water level. Sediments are poorly compacted and saturated with water being at the initial stage of diagenetic transformation. Diacryogenic materials are exemplified by the upper part of basin deposits of taliks, frozen for several metres (below the seasonal thawing layer) and underlain by unfrozen materials which further undergo epigenetic freezing. When the diacryogenic type of freezing takes place immediately after emerging from beneath water level, the sediments freeze as synchronous-epicryogenic, and if freezing occurs after a period in the thawed state - as asynchronous-epicryogenic (see §6.2). For materials frozen diacryogenically a high ice content is typical, with basal, reticulate and grating structure.

Polycryogenic strata are distinguished based on two approaches: 1) in a single frozen series the attributes of syn-, dia-, and epigenetic freezing are combined; 2) the frozen strata have a two-, three- and more member structure (stage-type): the upper part is syncryogenic and the lower one epicryogenic. There are many examples of different types of freezing of a single series of sediments at different stages of their formation. In particular, relatively deep-set lacustrine deposits freeze epigenetically and with shallowing of a water body and grading into a lacustrine-marshy regime the sediments can freeze and accumulate synchronously. Alluvial deposits can serve as a typical example of complicated cryogenic structure: their lower part (river bed facies) freezing epigenetically, the upper part (flood-plain), syngenetically, and crescent lakes (bottom), diagenetically. A composite polycryogenic structure is typical of coastal-marine deposits, tidal marshes, laida and beaches. In other words, the frozen strata may be genetically one, yet have the attributes of both syn- and epigenesis.

Among the most sophisticated problems of the so-called'glacial complex' or 'yedoma series' is the genesis of deposits developed on plains of the North-East of Russia. The 'glacial or 'yedoma' complex of loess-like syn-cryogenic frozen sediments composes almost wholly (beyond the limits of river valleys) the Aldan-Olenek, Yana-Indigirka, Kolyma coastal lowlands, Novosibirskiye Ostrova [New Siberian Islands], the Mesozoic-Cenozoic superimposed basins of the North-East of Russia, and the plains of Central Yakutia. The ice-rich loess-like deposits are 80-100 m thick. The formation of the biggest syngenetic ice wedges is associated with these deposits (Fig. 9.2).

Loess-like ice-rich aleurites and ice wedges form an interrelated complex. 'Yedoma' is a smooth uplifted surface composed of high ice content sediments easily subject to wash-out. There are different points of view concerning their origin. Thus, there is the concept of their eolian genesis, i.e. accumulation of sediments under the influence of wind. This idea is mainly applicable to loess-like materials of similar type within the permafrost zone of North America and Alaska. It is applicable also to the thick loess series within the permafrost zone of Eurasia, both ancient and modern. Among the geocryologists in the former USSR, the view is that a thick series of ice-rich loess-like deposits are continental aqueous sediments that have undergone thawing and freezing in the course of their formation, mainly under conditions of flood-plains. The investigators supporting the flood-plain hypothesis describe the conditions of formation as follows: rivers issuing from mountains into the littoral plains are distinguished by slower flow, tending to meander or, on the contrary, remain steady, branching into channels and creeks ('forking') and forming vast deltas. In the opinion of Y.A. Lavrushin, the same deposits are alluvial facies of shallow bars in river beds and river laida. In the Lena river basin in Central Yakutia, a series of loess-like deposits resulted from carrying of silt from firn fields by glacial meltwater floods (A.A. Grigor'yev) with inundation of the lower reaches of the Lena river valley or from a combination of this with tectonic uplift at its mouth (G.F. Lungersgauzen) and purely tectonic factors and uplift of positive structures and sinking of depressions in which vast inundated basins arise. It is supposed that in the inundated basins fine elutriated sediments of predominantly aleurite composition are accumulating. The formation of a

Fig. 9.2. Syngenetic ice wedges in the series of aleurites of the yedoma in the North-East of Russia (photo by S.F. Khrutskiy).

loess-like aleurite series is considered to be associated with activity of glacial meltwater. G.F. Gravis and M.N. Alekseyev are of the opinion that their origin lies in talus-solifluction.

In the late 1940s and early 1950s the idea prevailed that the vast accumulations of ice in loess-like series in the North-East of Russia were buried remains of the former glacier cover supposed to have existed during the Pleistocene on the littoral lowlands of the Siberian North (E.V. Toll', K.A. Vollosovich, V.A. Obruchev, A.A. Grigor'yev et al.). Subsequently, a polygonal-wedge pattern of the principal monolithic ice bodies was found in strata of yedoma deposits (B.N. Dostovalov, P.A. Shumskiy, et al); the answer was also obtained to the question of the big vertical dimension of ice wedges - they have grown upwards synchronously with accumulation of the flood-plain deposits (E.K. Leffingwell, A.I. Popov, Ye.M. Katasonov et al.). However, many questions still remain unanswered. Thus, alluvial strata where flood-plain facies have been accumulated to a height of 50-60 m and even 100 m above water level are not known in nature. Therefore, the concept of a flood-plain origin of the deposits that bear ice wedges is criticized by specialists in the structure of the alluvial deposits. The eolian hypothesis is developed by some investigators (S.V. Tomirdiaro et al.) as an alternative to the alluvial flood-plain hypothesis. It was found that in the late Pleistocene about 20000 years ago, in the north-east of Eurasia vast areas of Arctic shelf emerged. America joined to Asia, represented a single continent and, with the growing land area, the climate became very severe and continental. This also led to desert conditions. Freezing winters with little snow and dry hot summers promoted intensified wind-borne transfer of fine sand and aleurite grains off the surface of the emerged Arctic shelf. By settling on various topographical elements mineral particles of fine sand and aleurite became mixed with peaty residues of the tundra grassy-sod-vegetation and these began to accumulate and freeze with a general uplift of the surface, thereby resulting in the possibility of parallel growth of ice wedges. The original concept of a lacustrine-thermokarst origin for the thick ice-rich aleurite strata with big ice wedges was suggested by N.A. Shilo (21).

Thus, at the present level of scientific progress, only one question has been solved concerning the polygonal ice wedge origin of big ice bodies in the beds of yedoma deposits. The question of what were the conditions of their accumulation remains without answer. The strata under consideration, ice-rich loess-like deposits with large vertical ice wedges, are characterized by homogeneity of granulometric composition with a continuously high content (to 60-80%) of coarse-aleurite fraction (0.01-0.05 mm) and high porosity which becomes evident in the thawing of ground and evaporation of ice in the walls of collapsing blocks and baydzherakh ('cemetery mounds'). However, what makes them distinct from typical loess is the low carbonate content, as a rule not exceeding 1.5-2% and only seldom reaching 4.5-5%, and the absence in the walls of prismatic cleavage breaks, columns, and sometimes a number of other attributes of loess.

A fine horizontal lamination is characteristic of the yedoma deposits. There are bands of relatively 'pure' aleurite composition and bands of highly peaty material containing much redeposited (allochthonous) peat alternating in the section, sometimes grading into 'leaves' of autochthonous peat, i.e. buried peats. In the beds of ice-rich aleurites there are horizons of fossil soil characterized by aggregation (granular texture), dark colour (humus horizon) and higher content of grass vegetation roots as well as the presence of shrub and underwood roots. Aleurite deposits of the yedoma complex are distinguished as those rich in ice, which makes up 70-75% and often

70-80% of their volume. The prevailing type of cryogenic texture is thinly laminated, horizontally oriented schlieren varying in thickness (1-15 mm), alternating along the section. 'Belt-type' cryotexture is characteristic. Between thickened interlayers there are bands of fine ice schlieren arranged either parallel to them, or forming a fine-lens network. Belts themselves can be arranged horizontally or inclined, often they are concave following the former shape of the bottom of the seasonally thawing layer in polygons between ice wedges.

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