Cryogenic Structure of Mountain Slope Deposits, Northeast Russia

Cryogenic structure of mountain slope deposits, northeast Russia

M. Kanevskiy Earth Cryosphere Institute SB RAS, Tyumen, Russia

ABSTRACT: This paper presents observations of Late Pleistocene-Holocene syngenetic slope deposits in the mountain systems of Northern Yakutia, mostly in the basins of the Omoloy, Yana, and Indigirka rivers. Cryogenic structures, ice contents and other features of slope deposits are discussed and a morphogenetic classification of these cryogenic structures is developed. The paper describes the important role of the quasi-syngenesis process and the characteristic distribution of quasi-syngenetic horizons in cross-sections of the slope deposits.

1 INTRODUCTION thermokarst, thermo-erosion, and thermo-denudation play active roles. Slope deposits, which have mostly slopewash, solifluc- Cross-sections of slope sediments usually show a tion and proluvial origin, are widespread in the moun- high degree of interstratification of deposits belonging tainous parts of northeast Russia. The most prominent to various facies, whose determination is rather diffi- feature of this region is the widespread occurrence of cult. In high- and medium-mountainous areas colluvial, ice-rich syngenetic Quaternary sediments containing proluvial and slopewash facies prevail in the composi- huge ice wedges. In this paper some features of moun- tion of slope sediments; in low-mountainous areas, tain slope deposits are discussed, including their slopewash and solifluction facies are most common. cryogenic structure, ice content, ice wedges and other The accumulation of Quaternary slope sediments characteristics. The paper is based on borehole samples in northeast Russia occurred under conditions of syn- and observations of exposures, pits and mine galleries genetic freezing. At the same time, the formation of in various parts of northern Yakutia. cryogenic structure was influenced by the process of quasi-syngenesis.

2 STUDY AREAS 4 THE PROCESS OF QUASI-SYNGENESIS The analysis of the cryogenic structure of slope sedi- Quasi-syngenesis (a term coined by Y.L. Shur) is ments was carried out during fieldwork in the moun- defined as a process that forms sediments with high tainous areas of northern Yakutia from 1983 to 1991. ice contents in the top layer of permafrost due to The most detailed research was carried out in the Kular freezing from below as a consequence of a gradual gold-bearing region (Yana-Omoloy area) and in the reduction in active layer thickness (Shur 1988). The piedmonts of Selennyakh ridge (Yana-Indigirka area). main cause of this process is the evolution of a vege- tation cover after the end of sedimentation. The oper- ation of this process results in the formation of the 3 GENERAL CHARACTERISTICS OF ice-rich intermediate layer with an average thickness MOUNTAIN SLOPE DEPOSITS 0.5–1.5 m, which is widespread in the cryolithozone. An alternative hypothesis concerning the origin of Late Pleistocene-Holocene slope deposits are wide- this layer is that it is the result of freezing of horizons spread in northeast Russia, especially in mountainous deeply thawed during the Holocene climatic optimum areas. The cryogenic structure of slope sediments (Kaplina 1981, Murton & French 1993, Romanovskiy & in northern Yakutia was described in detail by Lisitsina 1997). However, we believe, that Y.L. Shur’s G.F. Gravis (1969). opinion is more correct. These deposits have a complicated structure caused In addition to the modern intermediate layer, quasi- by the combined action of various geomorphic pro- syngenetic deposits also occur at various depths as cesses including slopewash, solifluction and other buried lenses or horizons inside syngenetic layers. These mass movements. Ephemeral streams also play signif- horizons usually correspond to periods of slowing- icant roles in the erosion and deposition of these down or termination of sedimentation. In northern deposits. In addition, cryogenic processes, such as Yakutia, buried quasi-syngenetic horizons, with thick- frost cracking, the formation of the ice wedges, nesses of 0.3–0.5 to 2–3 m, occur mainly in fluvial,

513 slope and lake sediments, and are composed of silty loam or sandy loam (Kanevskiy 1991). The main type of cryostructure present in quasi- syngenetic sediments is layered (belt-like), in combination with ataxitic, reticulate and partly lens-layered; the total gravimetric moisture content usually exceeds 100% and in many cases reaches 150% and more.

5 CRYOGENIC STRUCTURE OF SLOPE DEPOSITS IN LOW-MOUNTAINOUS AREAS

Detailed investigations of slope sediments in low- mountainous areas were carried out in the northern part of Ulakhan-Sis ridge (Yana-Omoloy area), whose elevation is no more than 300–400 m above sea level. The sediments of slopewash and solifluction origin are brown-grey silty sandy-loams and loams with inclusions of breakstone and can attain thicknesses of 15–20 m and more. The slope sediments by their com- position, cryogenic structure and properties are very similar to the Late Pleistocene polygenetic sediments of the “Ice complex” (ice-rich syncryogenic deposits including huge ice wedges). In many cases it is diffi- cult to define the boundary between the two com- plexes of deposits. Figure 1. Ice wedges in slope sediments (depth 17–18 m; The volumetric ice content of the expanse of ice the labels are 5 5 cm). Photos by Y.L. Shur. wedges in the slope sediments is also similar to that of the “Ice complex” (the values reach 35–40%). However there are considerable differences in the structure of the ice wedges themselves. For instance, in an inclined mine-shaft located in the lower part of the slope of Mount Iekiyees where the slope gradient is about 5°, very complex wedges were observed (Fig. 1.). Wedges running downslope are symmetrical and wide (up to 3–4 m). Wedges running cross-slope are asymmetrical, have a discontinuous character, their widths do not exceed 1–2 m, and the distance between the wedges changes from 2–3 to 7 m. The majority of ice wedges are tilted downslope, sometimes even to a subhorizontal position. In the opinion of G.F. Gravis (1969), this indicates the activity of Figure 2. Ice-rich quasi-syngenetic horizon in slope sedi- solifluction during the accumulation of sediments. ments bedding by syngenetic deposits (depth 9 m; the label Numerous lenses of thermokarst-cavity ice are is 5 5 cm). Photo by Y.L. Shur. due to erosional activity during accumulation of sediments. Sometimes these ice bodies form ice casts at These estimates include determination of the fraction the sites of partly thawed ice wedges, whereas in the of syngenetic and quasi-syngenetic sediments over sediments of the “Ice complex” such formations can various depth intervals, gravimetric moisture content, be observed much less frequently. Some ice bodies description of cryogenic structures, and so on. For this have a zigzag form in plan, repeating the configura- purpose a cross-section of slope sediments in the tion of small streams. Burguaat river valley (basin of the Omoloy river) was Analysis of the cryogenic structure of slope sedi- chosen. About 50 boreholes were drilled along a slope ments shows that quasi-syngenetic horizons are com- with a length more than 5 km and a height of 200 m, to mon at various depths (Fig. 2). depths of 1–2 m in the upper part of the slope and up We attempted to derive an estimate of the character- to 15–17 m in the middle and the lower parts of the istic distribution of these horizons in cross-sections. slope.

514 Table 1. Morphogenetic classification of cryogenic structures in slope sediments (from boreholes in the valley of Burguaat river, Yano-Omoloy area, northern Yakutia).

Thickness of Parameters of Type of Percentage of basic cryostructure cryogenic Gravimetric F Cryogenic structure cryolithogenesis the cross-section horizons, (m) structure, (cm)* moisture content, (%)

Ataxitic Quasi-syngenesis 8.8 0.1–0.3 (max 0.8) –/0.5 1.2 100–180 Reticulate Quasi-syngenesis 7.7 0.1–0.3 (max 0.6) 0.2–0.7/0.7 1.5 60–130 Lens-layered Quasi-syngenesis, 18.3 0.2–0.5 (max 0.8) 0.2–0.5/0.2–0.6 50–120 syngenesis Microschlieren Syngenesis, 47.5 0.3–1.0 (max 3.5) 0.1/0.1 40–90 close-layered quasi-syngenesis Microschlieren Syngenesis 3.6 0.1–0.3 0.1/0.2–1.5 30–70 layered, reticulate (1.3 0.7) Latent-schlieren Syngenesis 8.8 0.2–0.5 (max 0.7) 0.02/0.1 30–60 close-layered Massive Syngenesis 5.3 0.1–0.3 (max 0.5) – 30–50

*In numerator– thickness of ice shliers (lenses), in denominator – distance between shliers (lenses) or size of mineral blocks, cm.

Borehole cores were described in detail and samples close-layered, massive (or almost so) and lens-layered were taken for gravimetric moisture content analysis. cryostructures are observed. Thick ice layers are not During the description of each core, basic horizons with typical: their thickness usually does not exceed 1 cm identical cryogenic structure were recognized. These and the average distance between them varies from horizons were frequently interstratified and their thick- 20 to 50 cm. Lens-layered cryogenic structures have ness at the investigated cross-section usually did not much lower ice contents than in quasi-syngenetic exceed several tens of centimetres. In total we recog- horizons. nized seven basic kinds of cryogenic structures, whose The average gravimetric moisture content of syn- characteristics are given in the Table 1. genetic sediments is 65.4% while it is 87.5% in quasi- The most widespread cryogenic structure observed syngenetic sediments. The latter is an underestimate, was microschlieren close-layered, with a total fraction as traditional sampling methods do not take the thick in the investigated cross-section of nearly 50%, and ice layers into account, and the true value is greater the greatest thickness of basic horizons (in several than 100%. cases up to 3.5 m). From the middle part of the slope The typical thickness of quasi-syngenetic horizons to the lower part, the cryogenic structure of the sedi- in the investigated cross-section is between 0.3 and ments underwent a number of changes. For instance, 1.5 m. The modern intermediate layer was found in the fraction of massive, latent-schlieren close-layered almost 80% of the boreholes and had a thickness of and ataxitic cryogenic structures increased downhill, 0.1 to 1.5–2.0 m. while the proportion of lens-layered cryogenic struc- In order to determine the vertical distribution of quasi- ture decreased. The increase in the percentage of syngenetic horizons, the investigated cross-section was massive and latent-schlieren close-layered cryogenic divided into six depth intervals: 0–0.5 m (the average structures we connect to an increase in sedimentation active layer interval); 0.5–1.5 m (the average intermedi- rates in the lower part of the slope. ate layer interval); 1.5–3 m; 3–5 m; 5–7 m; and 7–10 m. The analysis of the cryogenic structure of the slope For each interval the ratio of total thickness of quasi- sediments allowed two basic sets of cryogenic struc- syngenetic sediments (for all boreholes) to total amount tures to be identified, which correspond, in our opin- of drilling was calculated (Figures 3A and 3B). ion, to quasi-syngenetic and syngenetic conditions of The average percentage of the cross-section made freezing. The first set (quasi-syngenetic) includes up of quasi-syngenetic sediments is 14.8%, while the ataxitic, reticulate, and partially lens-layered cryo- ice wedge volume is 43%. If the ice wedge volume is genic structures. The thin layers with these cryogenic neglected, the proportion of quasi-syngenetic horizons structures are divided by ice layers (belts), whose in the cross-section increases to 25.9%. As shown in thickness usually varies from 2 to 5 cm. Figure 3B, the greatest proportion of quasi-syngenetic For syngenetic horizons, which prevail in the inves- sediments occurs in the 0.5–1.5 m interval (intermedi- tigated cross-section, a completely different set of ate layer), where it exceeds 60%, and from 3 to 5 m cryogenic structures is typical. Most important is the (about 40%). microschlieren close-layered cryogenic structure on The distribution of quasi-syngenetic sediments also which background horizons with latent-schlieren varies between the middle and the lower parts of the

515 clast-rich material. The sediments contain thick ice wedges: in slopewash and solifluction sediments ice wedge widths up to 3 m and heights up to 20–30 m were observed; the widths of ice wedges in proluvial sediments reach 1–2 m with heights up to 10–15 m. We recognized two types of cryogenic structure in slope sediments of slopewash and solifluction origin. The first (quasi-syngenetic) includes a layered cryogenic structure (thickness of ice layers up to 3 cm; distance between them of 5–10 cm), in combination with reticulate (thickness of ice shliers up to 0.3 cm; average size of mineral blocks, 0.5 1.5 cm), ataxitic and crust-like (the thickness of ice crusts around inclusions of breakstone reaches 1.0 cm). The maximum moisture content of these sediments is 150%. The second type of cryogenic structure in these sediments is syngenetic. It is characterized by rarer and thinner ice layers, a prevalence of a lens-layered cryogenic structure (thickness of ice shliers up to 0.2 cm; distance between them 0.2–0.4 cm) in combination with a crust-like cryogenic structure, partly with a microschlieren close-layered cryogenic structure. The gravimetric moisture content of sediments with this Figure 3. Distribution of quasi-syngenetic horizons with cryogenic structure does not exceed 80%. depth in the cross-section of slope sediments (data from The cryogenic structure of proluvial sediments is drilling in the valley of Burguaat river, Yano-Omoloy area, also divided into two main types. The first type is northern Yakutia). A – including ice wedge volume; B – ice-rich and is characterized by the prevalence of a excluding ice wedge volume. layered cryogenic structure (thickness of ice layers 1 – active layer; 2 – quasi-syngenetic sediments; 3 – syngenetic sediments; 4 – ice wedges; 5 – average values. reaches 3–5 cm; layers are discontinuous) in combination with a basal cryogenic structure. The maximum moisture content of these sediments reaches 150%. slope. In the middle part, buried quasi-syngenetic The second type of cryogenic structure in proluvial horizons of significant thickness were observed at sediments is characterized by the prevalence of a depths of 3–8 m in every borehole, while the thickness crust-like cryogenic structure (the average thickness of modern intermediate layer was insignificant. In of ice crusts is 0.1–0.2 cm; maximum of 1.5 cm) with the lower part of the slope, buried quasi-syngenetic individual ice layers, partly with a lens-layered cryo- horizons were not ubiquitous, but the thickness of the genic structure. The gravimetric moisture content of modern intermediate layer was much greater. these sediments varies from 20 to 70%. A cross-section of a fan formed by interstratified slopewash, solifluction and proluvial sediments, con- 6 CRYOGENIC STRUCTURE OF SLOPE structed from pits and boreholes, is shown in Figure 4. DEPOSITS IN MEDIUM-MOUTAINOUS AREAS 7 CONCLUSIONS An analysis of the cryogenic structure of slope sediments in medium-mountainous areas was carried out Ice-rich slope deposits are widespread in northeast in the region of the ore deposit “Tirekhtyakh”, located Russia. Analysis of borehole samples, and observations in the piedmonts of Selennyakh ridge, with maximum of exposures, pits and mine galleries show that the elevations of about 1000 m above sea level. The cross- most common cryogenic structure of syngenetic slope section of slope sediments, whose thickness reaches deposits is microschlieren close-layered; this indicates 30–40 m, is characterized by interstratification of a rapid accumulation of deposits and syngenetic freez- slopewash, solifluction and proluvial sediments. ing. The cryogenic structure of deposits is closely con- The sediments of slopewash and solifluction origin nected with the rate of sedimentation: the lowest ice at this site are represented by ice-rich silty sandy- content corresponds to the fastest sedimentation. loams and loams with inclusions of breakstone; prolu- Ice-rich horizons with a layered cryostructure, in vial sediments are characterized by a large fraction of combination with ataxitic and reticulate cryostructures,

516 Figure 4. Part of cross-section across the fan formed by interstratified slopewash, solifluction and proluvial sediments (“Tirekhtyakh” ore deposit, Northern Yakutia). Horizontal and vertical scales in metres. 1 – the number of pit-hole (24) or borehole (52); 2 – ice wedges. Cryogenic structures: 3 – ataxitic; 4 – reticulate; 5 – lens- layered; 6 – massive; 7 – crust-like. Lithology: 8 – loam; 9 – sandy-loam; 10 – rounded pebble; 11 – breakstone; 12 – weath- ered rocks (Paleogene – Neogene); 13 – sandstones, argillites, aleurolites (Trias). 14 – gravimetric moisture content, (%).

517 were found at various depths. These horizons, whose Western Canadian Arctic. Permafrost and Periglacial thickness varied from 0.3–0.5 to 2.5–3.0 m, are inter- Processes, Vol. 4 No. 3, Jul–Sept 1993: 217–229. preted as “quasi-syngenetic”, because they were formed Gravis G.F. 1969. Slope sediments of Yakutia. Moscow: during interruptions of sedimentation, when the thick- Nauka. (in Russian) ness of active layer diminished. The total gravimetric Kanevskiy M.Z., Maximov V.V. 1990. The features of engineering-geocryological researches in areas of moisture content of such quasi-syngenetic deposits development of the ice-rich syngenetic frozen soils. In can reach 100–130%. In contrast, the moisture content Methods of engineering-geocryological mapping: of syngenetic deposits with a microschlieren close- 94–100. Moscow: VSEGINGEO. (in Russian) layered cryostructure usually is less than 80–90%. Kanevskiy M.Z. 1991. The role of quasi-syngenesis in The interstratification of syngenetic and quasi- formation of Quaternary sediments cryogenic structure syngenetic horizons is typical in mountain slope in Northern Yakutia. In The top horizon of permafrost deposits in northeast Russia. The total thickness of strata: 47–63. Moscow: Nauka. (in Russian) quasi-syngenetic horizons commonly attains 20–30% Kaplina T.N. 1981. History of permafrost strata of the of the total thickness of syngenetic permafrost. The Northern Yakutia in the late Cainozoic period. In History of development of permafrost of the Eurasia presence of quasi-syngenetic horizons usually results (on the example of various regions): 153–181. Moscow: in an increase of the average moisture content of the Nauka. (in Russian) upper layers of permafrost by 10–15% or more (neglect- Romanovskiy N.N., Lisitsina O.M. 1997. Paleo-permafrost ing massive ice in the calculations). map of Russia of the last climatic extreme epoch. In Results of basic researches of the Earth cryosphere in Arctic and Sub-Arctic: materials of International ACKNOWLEDGMENTS Conference, Pushchino, 23–26 April 1996: 30–39. Novosibirsk: Nauka. (in Russian) The author would like to thank the reviewers and edi- Shur Y.L. 1988. The top horizon of permafrost strata and tor of this paper who helped to improve the translation thermokarst. Novosibirsk: Nauka. (in Russian) of the text. Special thanks are expressed to Dr. Yuri Shur for his valuable comments and suggestions.

REFERENCES

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