Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 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 combi- nation 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 ϫ 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, some- times 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 ϫ 5 cm). Photo by Y.L. Shur. due to erosional activity during accumulation of sedi- ments. 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 ϫ 1.2 100–180 Reticulate Quasi-syngenesis 7.7 0.1–0.3 (max 0.6) 0.2–0.7/0.7 ϫ 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) Ͻ0.1/Ͻ0.1 40–90 close-layered quasi-syngenesis Microschlieren Syngenesis 3.6 0.1–0.3 Ͻ0.1/0.2–1.5 30–70 layered, reticulate (1.3 ϫ 0.7) Latent-schlieren Syngenesis 8.8 0.2–0.5 (max 0.7) Ͻ0.02/Ͻ0.1 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%.