Transformation of loose deposits of the permafrost regions into rock

Transformation of the basin and continental sediments into rock is considered to be the most specific stage of lithogenesis in the permafrost regions. Sedimentary formations are subject to single or multiple freezing-thawing and adapt to the changing thermodynamic and physico-chemical conditions of the environment (as they progress deeper). As a result of tectonic movements and denudation processes these formations may approach the Earth's surface many times (and even be found within the weathering zone) and then sink again, passing once more through the stages of diagenesis and epigenesis.

In the freezing and thawing of the sedimentary strata, processes associated with phase transition of water into ice are fundamental in the cryogenic transformation of composition, structure and properties of rocks. During freezing, the development of crystallization and structural relationships, the 'petrification' of soils takes place (according to P.F.Shvetsov). Simultaneously concentrations of dissolved matter become redistributed in the freezing strata (cryogenic desalination, and concentration, sulphatiz-ation and carbonization, formation of cryopegs, etc.); moisture migration towards the front of freezing and ice separation also takes place by segregation, injection and ablimation with shrinkage, swelling and heaving of the soils subject to freezing, etc. Of specific importance for the process of lithogenesis in the permafrost regions are volumetric gradient tensions that arise in the freezing and frozen soils causing substantial transformation of the initial structure and texture of the mineral portion of the unfrozen soil, its density and strength. The process of perennial freezing for a rather short-term period of time is sufficient to make such transformation of structure and texture of the soil's mineral portion as would take tens and hundreds of millenia with diagenesis of the normal type. Therefore, a specific type of diagenesis can be distinguished, namely, the cryogenic one that gives rise to a qualitatively new rock - cryogenic rock.

Transformation of the basin sediments of the permafrost regions by diagenesis and epigenesis occurs progressively, as in the warm humid areas, and is associated with a cycle of physical and physico-chemical transformations: dewatering and compaction of sediments, formation of typical fault and plastic deformations, alteration of chemical-mineral composition and the like. In this case there is no drastic transformation of clay minerals.

A quite different transformation of the basin deposits is typical of regression diagenesis and epigenesis in the permafrost regions, when deposits influenced by the conventional diagenetic physico-chemical processes move upwards from beneath the water level thus approaching the ground surface. They can freeze immediately after coming from beneath the water level (.synchronous-epicryogenic) or exist in the unfrozen state for a long time, becoming frozen later if the climate becomes colder (asynchronous-epi-cryogenic).

A synchronous-epicryogenic strata of the basin deposits

These usually develop on sedimentary formations that have undergone the stages of sedimentation, diagenesis and epigenesis outside the extent of permafrost development. Accordingly, it is typical of them to have oligo- or mesomictic composition and a high content of minerals of the kaolinite group, while their structural-textural features and properties are typical of the sedimentary formations in the warm humid or arid regions. Coming from beneath the sea, the weathering crust of these sediments is not yet influenced by the cryogenic factor. Then, owing to general or regional cooling these strata are subject to unilateral freezing from the top in the course of which grain size and chemical-mineral composition change. In the upper section of deposits which have not undergone deep lithification, high ice content will develop, characterized by a variety of types of superimposed and inherited segregation-migration cryogenic structures. Lower in the section with more dense and dewatered deposits, as well as lower freezing rate and lower temperature gradients, superimposed relatively thin horizontal layers will be formed, occasionally together with inclined ice schlieren, as well as injection and ablimation varieties of ice. In general, asynchronous-epicryogenic strata of the basin deposits are characterized by a three-member structure (Fig. 6.2).

Synchronous-epicryogenic strata

These can be formed over the basin sedimentary formations which have undergone the stages of sediment genesis and progressive diagenesis and epigenesis both within and beyond the area of development of perma-

Fig. 6.2. Structure of asynchronous-epicryogenic strata of basin deposits: 1-3 - frozen materials (1-2 respectively, of recent and ancient crusts of weathering, 3 - of the zone of regression cryodiagenesis); 4 - unfrozen; 5-10 - cryogenic textures (5 - fine and frequent-banded, 6 - thick-and rare-banded, 7 - thick- and big netted, blocky, 8 - inherited, 9 - fine- and small-netted, 10 - massive); Han, and Hfr - respectively, thicknesses of the seasonally thawing layer, annual temperature fluctuation layer, perennially frozen layer.

frost. The latter is of importance since it influences chemical-mineral composition and grain size of materials which will be further subjected to cryogenic transformation at a stage of regression diagenesis. Beyond the permafrost regions these rocks will be frozen only from the top, whereas in the permafrost regions (as they are coming from beneath the water level) they will be frozen both from the top and laterally, giving rise to specific obliquely laminated cryogenic structure. In general, such synchronous-epicryogenic strata are characterized by the following features.

Prior to continental (subaerial) freezing of the basin deposits on shallow and shelf sites they are subject to epigenetic submarine (subaquatic) freezing without having been much transformed diagenetically. Formation of cryopegs is possible in this case.

Above these epicryogenic (subaquatically frozen) materials a comparatively thin (a few metres) layer of syngenetic subaquatically frozen materials can be formed as shelf sites are drained and their freezing beneath the water takes place simultaneously with accumulation of sediments. Subaerial freezing from the top, accordingly, takes place through the layer of the subaquatically frozen materials uplifted from beneath the sea level. Their thickness is, as a rule, small (a few tens of metres) and is greatly dependent on the value of the negative temperature of the bottom water layer, geothermal gradient, composition and moisture content of the bottom sediments and other factors. On shallow parts of fresh-water bodies the formation of syn-and epigenetic subaquatically frozen sediments is also possible. In general, the composition and structure of synchronous-epicryogenic strata of the basin deposits differ greatly from those of asynchronous-epicryogenic strata. Four specific layers of the frozen materials are distinguished in the profile (Fig. 6.3).

Grading of continental deposits into rocks occurs differently in subaerial and subaqueous conditions.

Subaerial and subaqueous continental deposits

The two groups of deposits differ substantially in the pattern and intensity of sedimentation which brings about a wide range of chemical-mineral composition and structural-textural features. These differences exert a noticeable impact on the process of cryogenic transformation of sediments into rocks with formation of epi- or syncryogenic strata.

Subaqueous deposits (of flood-plains, mud flats, dead lakes, bogs and the like) are characterized by a stable regime of sediment accumulation, the rate of which is measured by fractions of or a few millimetres per year. In the majority of cases these deposits are silty, highly porous (owing to the presence of fine pores), supersaturated, well graded, with well-defined orientation, dark-grey colour and contain a great amount of poorly decomposed vegetation residues and organic matter. Usually, these sediments are poorly drained owing to supersaturation and the high content of fine particles and there are traces of gleying. The latter is evidence of a predominantly reducing environment as well as of the presence increasingly of proto-oxides of iron, hydrogen sulphide, methane (in marshlands) and high carbon dioxide content. Among new mineral formations are typically hydromicas, vivianite and siderite.

The formation of subaerial continental deposits occurs irregularly with time. Thus, for example, accumulation of solifluction deposits can occur either annually, or with 2-3-year intervals. However, despite such a nonstable nature of accumulation, the thickness of subaerial sediments can much exceed that of subaqueous sediments for a series of years, since the accumulation rate for a season (cycle) can reach centimetres and tens of centimetres (and even metres). These deposits are mainly represented by poorly graded mixtures of fine-grained and coarse material with poorly defined orientation and inclusions of buried peat. Their porosity is characterized by big and inter-aggregate pores. Subaerial deposits are, as a rule, little saturated and of

Fig. 6.3. Structure of synchronous-epicryogenic strata of the basin deposits:

1-4 - frozen materials (1 - epicryogenic of the recent weathering crust,

2-3 - respectively, syngenetically and epigenetically frozen under water, 4 - epigenetically frozen during regression cryodiagenesis); 5 - unfrozen; 6-13 - cryogenic textures (6 - fine- and frequent bands, 7 - thick- and sparse bands, 8 - fine- and small-netted, 9 - thick- and big-netted, block,

10 - underdeveloped netted, 11 - inclined bedding and scaly, 12 - inherited, 13 - massive).

low humus content which gives rise to a predominantly oxidizing-neutral medium. Typically, in connection with this is the presence of iron compounds as oxides, quartz fragments and new formation of montmorillonite minerals.

Continental deposits can undergo diagenetic transformations either simultaneously with seasonal or perennial freezing, or prior to the action of the cryogenic factor. Based on the above, three types of frozen strata can be distinguished which reflect three different transformations of continental deposits; 1) asynchronous-epicryogenic; 2) asynchronous-epicryogenic palaeocryoeluvial; 3) syncryogenic.

The structure of asynchronous-epicryogenic strata of continental deposits is rather simple. Their section presents a recent and comparatively thin weathering crust of cryogenic type that grades into the poorly defined ancient (noncryogenic) weathering crust which has undergone a single freezing. The lower, thickest part of the sequence under consideration is represented by the epigenetic frozen continental deposits. The chemical-mineral composition of these rocks reflects cryogenic transformations caused by freezing and by changes in geochemical setting.

When accumulation and diagenesis of continental deposits occurs beyond the limits of permafrost but with a well-defined process of seasonal freezing, these deposits undergo multiple (cyclic) seasonal freezings. Practically the whole sequence of such continental deposits at the stage of diagenesis should experience the multiple impact of the cryogenic factor on the background of general weathering. There is a correlation between the depth of the deposits and the length of time of the cryogenic eluviation of the unfrozen rocks. This gives the palaeocryoeluvial sequence of unfrozen continental deposits.

The process of cryoeluviation in the given case is characterized by a number of specific features differing from those typical of frozen ground. Thus, as the duration of the warm period is longer than that of the cold one, more heat and moisture is obtained by eluvial rocks within a year cycle; they are characterized by better drainage with the absence of underlying permafrost, and a predominantly oxidizing medium which leads to the formation of non-gley soil. The medium is usually neutral and even alkaline, with intense leaching of iron and aluminium elements. As a rule, these palaeocryoeluvial materials are slightly salinized with a high humus content (mainly, humic acids), contain great amounts of silty particles and carbonates and have light colours. Among the newly formed clay deposits minerals of the montmorillonite group and hydromicas prevail. The so-called asyn-chronous-epicryogenic palaeocryoeluvial units are formed under global cooling and freezing of palaeocryoeluvial unfrozen continental deposits.

Finally, accumulation of continental deposits in the permafrost areas promotes formation of syncryogenic strata. These are formed in cyclic layers, varying in thickness (mainly, during the warm season of a year) and with seasonal freezing (during the cold season). Accordingly, with a constant thickness of the seasonally thawing layer the upper surface of the permafrost rises annually, by an amount equal to the thickness of annually accumulated continental sediments (taking into consideration their heave while in the frozen state).

In the given case there is an apparent distinction in the formation of syn-and epicryogenic strata. The epicryogenic strata grow due to the deepening of the lower boundary of the permafrost while the syncryogenic strata increase due to the rising of the upper boundary of the permafrost. The problem and mechanism of syncryogenic formations were thoroughly studied by E.M. Katasonov, A.I. Popov and I.D. Danilov et al.

In the course of formation the syncryogenic strata are somewhat influenced by the usual processes of diagenesis. Thus, for example, as shown by Yu.A. Lavrushin, there is a specific type of diagenesis traceable in the alluvial deposits of the Arctic and sub-Arctic areas, namely permafrost diagenesis associated with multiple freezing and thawing and with accom panying physico-chemical reactions, phenomena and processes. The absence of diagenesis of the usual type is confirmed by the uniform colour of the sediment: there is no authigenic formation of minerals (excepting the mineral ice), and the processes of degradation of organic and vegetation residues are slow. The most drastic transformations of syncryogenic rocks are found in changes in structural-textural aspects. Consequently even before their transition into the permafrost state (at depths that do not exceed some metres) continental formations undergo the stage of specific frozen diagenesis and grade into materials of cryogenic type in respect to their mechanical and structural-textural properties. Accordingly, transformation of continental sediments and deposits into syncryogenic deposits takes place at the stage of cryogenic diagenesis, occurring generally within the horizon of intense phase transitions of moisture which corresponds roughly to the layer of seasonal thawing.

The intensity of the transformations of continental deposits of syncryogenic type differs substantially depending on whether these are subaqueous or subaerial sediments and is determined by tectonic regime, climatic conditions, the pattern of terrain, the rate of sedimentation, depth of seasonal thawing and the like. Thus, subaqueous sediments that are accumulating at a steady slow rate grade into the permafrost state undergoing a number of freezing cycles within a layer of seasonal thawing. The above leads to greater transformations of syncryogenic strata of continental-subaqueous origin by the processes of cryoeluviation as compared with continental-subaerial deposits.

In general, syncryogenic strata, unlike the epicryogenic ones, are characterized by a specific appearance, by composition, structure and properties. They are, as a rule, more silty and have a higher humus content which restricts the possibilities of absorption. Their composition mainly comprises monoxides of iron, hydrous micas and montmorillonite. Higher porosity, plasticity and thixotropy are typical of these deposits with uniform distribution of ice in the section, whereas epicryogenic strata are characterized by ice content diminishing with depth. Furthermore, the occurrence of syncryogenic strata in the profile indicates two important features: 1) syncryogenic strata, as a rule, cannot exist independently and are underlain by epicryogenic rocks, i.e. if syncryogenic rocks occur in the profile, the permafrost sequence is in general polygenetic; 2) in practice, it is unlikely that syncryogenic strata will extend deeper than the first few hundred metres. Usually, the thickness of syncryogenic materials of continental subaqueous origin does not exceed several tens of metres. Thus, if the maximum rate of accumulation of subaqueous-continental deposits is assumed to reach

1 mm yr 1, then with the long-term conditions of severe climate necessary for the formation of syncryogenic sediments their thickness can reach about 100 m.

The features of the transformations on freezing of basin and continental deposits differ substantially between elevated sites and depressions. Thus, in the European North of Russia and in West Siberia the percentage of the frozen strata is much greater in the region of tectonic depression than in regions of uplift. The soils that underwent numerous cycles of freezing and thawing are loess-like in composition and texture and, as shown by experimental studies, it is possible to determine the relationship between loess formation and the number of cycles. The greater the number of cycles undergone by the initial sediments the more closely they resemble the typical loess-like, loess cover formations. As regards epicryogenic strata, the thickness of such a layer is limited by the layer of seasonal thawing. The formation of loess-like soils down the whole profile is possible in the syncryogenic and palaeocryoeluvial strata of fine-grained soils that have undergone multiple freezing-thawing. Their formation is assumed to take place in the course of syngenetic freezing of continental sedimentary deposits and, especially, subaqueous continental sediments.

At negative mean annual temperatures syncryogenic strata are formed with distinct attributes of loessification, while at positive temperatures palaeocryoeluvial strata of loess soils are formed. In both cases the highest degree of cryogenic transformation is observed in the range of mean annual temperatures near to 0°C given that the deepest layers of seasonal thawing and freezing then occur. The degree of loessification of soils resulting from their alternating freezing and thawing is in inverse proportion to the rate of accumulation of the sediments. At Facc > £ the process of loessification comes to a full stop. The process is most intense at Facc < £ when the newly deposited layer of sediments is undergoing some tens of thousand cycles of freezing-thawing. Therefore, there is a complicated and interrelated dependence of cryogenic transformation of sediments depending on the values of Facc and £

Formation of loess and loess-like soils of cryogenic origin is promoted by low rates of sediment accumulation with maximum depths of seasonal thawing or freezing of silty-sandy and silty-clay subaqueous-continental soils.

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