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Pereletki and the initiation of glaciation in Siberia
~
Quaternary International, Vols 41/42, pp. 27-32, 1997.
) Pergamon PIh S1040-6182(96)00033-X
PERELETKI
AND
THE
INITIATION
OF GLACIATION
© 1997INQUA/ElsevierScienceLtd All rights reserved. Printed in Great Britain. 10v,0~182/97 $32.00
IN SIBERIA
R.O. G a l a b a l a Modern glaciers present in northern Siberia, adjacent to the Arctic Ocean littoral, develop through the gradual accumulation of windblown snow and firn in river valleys and other depressions. Such snow glaciers ("pereletki") are largely ephemeral in nature, and produce landscapes dominated by nivation hollows and other snow-related landforms. Deposits associated with pereletki include nival, nivalsolifluctionary, and fluvio-nivalunits. The modern pereletki are considered to represent an initial form of glaciation, and their development can be regarded as an analogue for the formation of nival deposits commonly attributed to a more extensive Sartanian glacial event. Late Quaternary glacial events in Siberia involved both alpine and piedmont glaciation, and widespread development of pereletki in lowland areas. Distinguishing between features and sediments producedby glacial ice, and those attributable to ephemeral pereletki, is necessary for accurate assessment of Late Quaternary glaciation in Siberia. © 1997 INQUA/ElsevierScience Ltd
S N O W - G L A C I E R S AS AN I C E - A G E R E L I E F FORMING FACTOR
INTRODUCTION The currently completed geological and geomorphologic 1:200,000 surveying in Siberia resulted in the establishment of detailed limits of the maximal extension of Late Quaternary ice-covers and mountain glaciers, providing a large amount of analytical data relevant to the age of glacial deposits and the dynamics of glacial phenomena in time (Kind, 1971; Kolpakov and Belova, 1980; Galabala, 1982). In the light of this evidence, one cannot fail to wonder at continuously appearing articles and monographs arguing for a huge-scale glaciation of the Arctic shelf and the neighbouring Siberian land-mass (Grosswald, 1980; Grosswald and Spektor, 1990). These publications seem to be completely ignoring the evidence obtained by time-consuming field studies of established geologic features, and base their conclusions on intuitive probabilistic scenarios, and often, wishful thinking. For example, among the features interpreted as glacigenic are large structural-denudational ridges of pre-Quaternary age on the Kola Peninsula, a chain of Cretaceous intrusives in the Indigirka basin, snow glaciers in the vicinity of the Sevastian Lake in the Tiksi area, and many other landforms. These publications arguing for large scale glaciation in Late Cenozoic Siberia are particularly embarrassing, due to the fact that they discuss active glaciers, the limited distribution of which is well established by reliable evidence. On the other hand, one has to acknowledge that the hypotheses relating to major glaciations of Siberia in the past have the right to exist, the peculiarities of the socalled periglacial environment being, as yet, not sufficiently clear. If the evidence pertaining to mountain glaciations, ice sheets and glaciations in Siberia is abundant and varied, the geological effects of stable or quasi-stable firn and snow glaciers is relatively poorly known. The latter being denied by the majority of present-day scholars, were viewed by Grigor'ev (1930) as the evidence of an "embryonal glaciation".
It has been well established that firn and snow-glaciers form one of the typical features of the present-day landscape in the northern periphery of Eurasia. They are particularly common along the northern littoral, from the Kola Peninsula to the Okhotsk Sea, both in t h e ' m e d i u m high mountains and on the lowland, reaching sea-level in the north of Siberia. They are formed by snow, blown by strong winds from the interfluves to the river valleys, in spite of a relatively low solid precipitation in this northern part of Siberia (100-120 m m annually). Snowdrifts occur here throughout the winter, and are particularly common between November and February. Thus by the end of the winter, all major depressions, including river valleys, are filled in with snow, ranging in depth from 5-8 to 1015 m. In contrast, the thickness of snow on watersheds is negligible, its cover completely disappearing by June. Considerable accumulations of snow forming temporary glaciers ("pereletki") emerge on the northern slopes of valleys, hollows and cliffs (Fig. 1). In places they develop into firn glaciers, which may be seen in the Northern Khauralakh, the Pronishchev Massif, the Chekanovski
FIG. 1. Snow glaciers accreted by the end of the summer in the upper Soguru Tigie river, ChekanovskyMassif. 27
28
R.O. Galabala
Massif, and on the Taimyr Peninsula, as well as on the littoral of the Laptev and East-Siberian Seas. For example, a small-size firn glacier on the left bank of the Lower Lena River (72°N, at the altitude of 50 m a.s.1.) which was discovered in 1961, has remained in existence with insignificant fluctuations over the last 30 years. This type of firn glacier, however, is relatively rare, the greater part of "pereletki" disappearing during the warmer summers. Sufficiently common snow glaciers and their seasonal erosional and depositional activities result in the emergence of typical relief features and the accumulation of a specific sedimentary assemblage on the lowland. While in the mountains, where erosion clearly prevails over the glacial deposition, such snow-related landforms as nivation hollows, cirques, troughs, pseudo-terraces and other features are sufficiently known (Shchukin, 1960; Osokin, 1981), this was not the case for accumulative features of a considerable magnitude on the lowland. Consequently the classification of these features became necessary, particularly for the practical purposes of largescale geological mapping. Genetic classification suggested below, related to the evolutionary dynamics and the degradation of snow glaciers, is based on 10 years experience of geologic 1:50,000 mapping in northern Yakutia with the wide use of aerogeologic imagery. This classification includes: nival, nival-solifluctionary and fluvio-nival deposits; grading laterally into alluvial, alluvial-lacustrine and, rarely, into deltaic ones. Nival deposits are viewed as the products of nivation, or snow weathering, forming in the contact area between the snow-glaciers and the bedrock. The intensive weathering results from the considerable fluctuations of diurnal temperature, leading to the phase transition of fracture filling water, and, consequently, expansion and contraction of the rock. The frost weathering is particularly intensive at the upper edge of the snow-glacier, where the diurnal fluctuation of temperature is particularly great. In this case, snow-glaciers form snouts or cliffs, thus making up the bulk of nival deposits. There are considerable differences in the structure and composition of nival deposits mostly depending on the character of the bedrock, yet they commonly feature a lack of stratification and sorting and high concentrations of coarse and angular clasts, with mineral composition usually similar to that of the bedrock. The high content of coarse and angular clasts is equally typical of the areas where the bedrock predominantly consists of clay and silt. In this case the clasts result either from the weathering of the concretions or lenses of calcareous sandstone or limestone, or from the influx of the debris carried by meltwater and solifluction (Fig. 2). Nival deposits usually form the basal units in nivation hollows, cirques, troughs, as well as on the northern slopes of major river valleys and sea cliffs. Their thickness varies between 1-3 and 5-7 m, rarely reaching 10 m. The maximal thickness, in excess of 10 m, has been established in the north of the Chekanovski Massif, in the deeply eroded sub-latitudinal valleys. In that area, snow
FIG. 2. Nival deposits covering the cirque floor.
glaciers which form elongated ridges consisting of finegrained debris with fragments of sandstone, are separated from the bedrock by a narrow hollow formed by a recently melted part of the snow glacier. In rare cases one may see small (0.5-1.0 m long) till-like ridges formed by cobbles of local origin, at the outer edge of the firn glacier, indicative of minor oscillations of the firn ice. Nival-solifluctionarv deposits are seen as the products of nivation displaced down the slope (also along the snow-glacier) by the action of solifluction and meltwater. They are usually made up of clay and silt with rare inclusions of gravel of local origin. They equally include numerous organic remains brought in from the watershed, as well as layers of moss, and slightly decomposed peat, both formed and buried in the course of deposition. Remains of timber, mostly larch, are often encountered at the bottom of nival-solifluctionary deposits in the presentday tundra. Their radiocarbon ages range between 6000 and 8000 years BP, rarely reaching 9000 years. Irregularly stratified deposits are locally disrupted by landslides. Polygonal ice-wedges are often included at the foot of the slopes and the slope-adjacent floor of the valleys, their thickness varying from 0.5-1.0 m on the slopes to 12-16 m at the foot (Fig. 3). Fluvio-nival deposits consist, predominantly, of silt and sand particles and, rarely, gravel, washed out by meltwater from snow-glaciers and deposited in the stream valleys resulting from the melting of snow-glaciers. Within the uplands and massifs temporary meltwater streams often form sandy-pebbly deposits indistinguishable from the river alluvium. One should note that the
FIG. 3. The floor of a trough covered by nival-solifluctionary deposits.
Pereletki and the initiation of glaciation in Siberia present-day environment is unsuitable for the formation of fluvio-nival deposits, the prevailing erosion of water channels resulting in the washout of these sediments. They may be encountered predominantly in the valleys of smaller streams fed by snow-glaciers, which normally dry out after the disappearance of the latter. These streams often end up in small lakes and inner deltas, formed at the points of discharge of the streams into major river valleys of major lakes. Such deltas consist of fine-grained sand and silt, resulting from the washout of nival or solifluctionary-nival deposits. The normal thickness of fluvio-nival deposits is 2-5 m, exceeding 10 m in the areas of present depressions or naturally dammed lakes. The above-cited data on the snow-glaciers and related sediments are seemingly of theoretical interest only, their present-day distribution being insignificant. Yet their study is of paramount importance for the reconstruction of past environments, for the interpretation of several enigmatic deposits in the extraglacial areas, and for the understanding of the genesis of certain landforms. One should take into account that the present-day snow glaciers actually being formed are on the Siberian lowland, while the ice sheets are found exclusively at high altitude, ranging from 700-850 m (Putoran Plateau, Byrranga Mountains) to 1500-1900 (the Upper Yana area). Consequently, during the Ice Age when, in the opinion of the majority of scholars, the snow-line was approximately 1000 m below the present-day position, snow glaciers should be formed in a wide area and on a larger scale. The creation of a data-base for the present-day snow glaciers, and the landforms either formed or modelled by them (valleys, lake depressions, hollows, cirques etc.) and their sediments, enabled identification of similar landforms and sediments of pre-Holocene age both in the course of field studies and in the processing of air and satellite imagery. Such features have been identified in extraglacial Siberia, particularly in the periphery of the central Yakut Lowland, on the Lena Plateau, in the Kharaulakh Mountains (the North Upper Yana area), in the greater part of the Yana basin, in the Kular and Poluosny ridges, on the slopes of Yana-Indigirka Lowland, in the Anyui ridge, in the Kolyma Massif, the Oloi Ridge, the Yukagir Plateau and other areas. Judging from the wide occurrence of the nival landforms and sediments, the scale of snow glacier activity in the preHolocene epoch was enormous. Jointly with mountain glaciers, they were the main source of the terrigenous material, which formed gigantic accumulative plains. Ancient nival deposits on the plains, and particularly in the mountains, were often washed away by the powerful Holocene erosion which included snow glaciers. As exceptions one may mention the areas either immune to or only slightly affected by the later erosion. Thus, in the Chekanovski Massif, nival sediments up to 10 m thick were protected by the deposits of Holocene snow glaciers. For the same reason, the old nival-solifluctionary deposits are rarely intact, usually being eroded in the upper and middle parts of the slopes. These deposits, reaching the
29
FIG. 4. Pre-Holocene fluvio-nival deposits on an air-photograph: (a) coalescent with ice-dammeddeposits on the southern slope of the YanaIndigirka Lowland; (b) cirque-like widening at the head of the tributaries. thickness of 10-15 m, are often encountered in the lower part of the slopes, both on the lowlands and in the lower mountains. Thus, in the Chekanovsky Massif, sand and loam of nival-solifluctionary origin, their thicknesses ranging between 1-2 and 6-8 m, form wide plains 250300 m high at the foot of large cuestas. In contrast to the Holocene ones, the older fluvio-nival deposits are widely spread both in the mountains and on the plains, mostly due to numerous dams, enhancing the accumulation of these deposits in the valleys of the streams fed by snow glaciers. This resulted in the annual formation of stratified icy deposits of loam and sand reaching a considerable thickness (usually 30-60 m, locally, extending up to 90-100 m). These deposits are totally preserved in several valleys, reaching the upper stretches. Beyond the valleys of the streams fed by snow glaciers, the fluvio-nival deposits coalesced with those of major dammed streams, formed by the melting of both glaciers and snow glaciers, as well as other sources. They are indistinguishable on the terrain, their altitudes varying in relation to the local sedimentary environment (Fig. 4).
DISCUSSION Based on the data relative to the present-day and past nival processes one is in a position to interpret, more coherently, the palaeogeography of Ice Ages and, particularly, the formation of icy deposits. The first
30
R.O. Galabala
attempt at this has been made by the present writer, based on the evidence of geologic mapping and the special investigations of 1985. At that time a detailed mapping of the areas of glacial deposits, as well as that of icy deposits of problematic origin, was carried out. We have also collected vast amounts of evidence pertinent to their mineralogical composition and age. Based on this evidence, it has been suggested (Galabala, 1987a) that the icy deposits in question were formed in an environment similar to that currently occurring in the deltas of the Lena and Yana, synchronously to the MuruktinskSartan Glaciation, under conditions of notable damming. Apart from ice dams, there occurred a considerable accumulation of ice in the north, which led to the elevation of the base level of erosion in major rivers, and the accumulation of the alluvium of a constructive type in the fiver valleys and off-shore deltas. The rapid transition of icy deposits with polygonal ice wedges into the massive fresh-water ice with the inclusion of mineral columns, viewed by Toll (1895) as the remains of glaciers, may be interpreted as resulting from the accumulation of transported suspension particles, rapidly diminishing in volume with distance from dry land. It has been suggested that the annually frozen firm ice with mineral columns coalesced with the shelf ice. The latter, during the Karginsk age, formed a barrier preventing the Karginsk sea from penetrating east of the Khatanga river mouth. The new evidence, resulting from the large-scale mapping of the Kharaulakh low mountains, the Pronchishchev and Chekanivsky massifs, the Olenek uplift, the North Siberian lowland, the Lena Delta and other regions, is in no way contradictory to this reconstruction. The investigations of present-day snow glaciers eliminate the last arguments for the interpretation of icy deposits as aeolian features, the hypothesis still supported by several scholars (Shilo, 1964; Tomirdiaro and Cernen'ky, 1987). As had been mentioned, the altitude of icy deposits may fluctuate considerably depending on the sedimentary environment. Thus, in the north Siberian Lowland, where these deposits in consolidated areas are usually found at 70-90 m, they may reach 120-140 m on the slopes of the lowland. Locally, along fiver valleys, they may extend to 180-200 m. The same pattern is recognizable in the area of Kular ridge: situated in the consolidated area at 7080 m, these deposits may reach 160 m along the valleys and on the slopes of the Ulakhan-Sis (Kular) ridge and Khabdji-Tas Mountain. In the area of Olenek Uplift, the icy sand and loam ascend from 80-100 m up to 300 m. In these circumstances, the aeolian hypothesis may seem to be plausible, if one fails to take into account the local icedamming and snow glaciers. In this respect it is worth mentioning the case of the Lena Delta, which may be seen as a transparent model for the formation of icy deposits. In that area, apart from annually increased flood deposits accompanied by the increasing polygonal ice wedges, one may equally visualise the' aeolian activity. Strong winds prevailing through the summer considerably affect the thermal abrasion of the shores, forming aeolian terrace rim walls
and watershed covers. Thus, the rim wall on the surface of the first terrace near Trofimovskoye settlement reached the altitude of 0.7~0.9 m over the last 50 years. Further afield, the thickness of aeolian sediments rapidly diminishes, completely thinning out at a distance of 3540 m. Aeolian sediments formed through the Holocene are widely spread on a major deltaic island of AgraMuora-Sise. They consist of silty sand with vertical plant stems and roots. Lenses of peat (0.5-1.5 m in thickness) are found at the bottom, indicative of a mire occurring on the surface prior to the aeolian accumulation. Peat layers are presently forming, resulting from the interfluve grasssedge mire being buried under sand and dust carried by the wind from near-by lake shores, fiver valleys and cliffs. The thickness of aeolian sediments varies between 0.7 and 2.0 m (Galabala, 1987b). It is important to stress that the aeolian deltaic deposits have no direct analogies amongst icy deposits anywhere in Siberia, neither in structure nor composition. Nevertheless, aeolian activity took part in the formation of icy deposits, transporting sand and finer sediments from barren water channel shores and surrounding areas; the wind transported particles forming a notable admixture to predominantly aquatic deposits. In certain areas, the icy deposits may include lenses of typical aeolian sediments, the later being of secondary importance in relation to the bulk of icy deposits.
CONCLUSIONS Numerous recently published articles and monographs discuss various aspects of Late Quaternary events both in the Siberian mainland and the adjacent northern seas. However, if the evidence related to the Siberian mainland is sufficiently rich, this is not the case for the seas, particularly, the Laptev and East Siberian seas. This is the main reason why palaeogeographic interpretations related to these areas are often contradictory, being not well in accord with the evidence obtained for the adjacent mainland, deltas and islands. The author's suggestion about the occurrence of a shelf ice dwells entirely on the established fact that the rivers emptying into the Arctic Ocean: the Anabyr, Olenek, Lena, Yana, Indigirka and Kolyma, which underwent a downcutting lasting until the Kazantsevian age, started to form thick constructive alluvial deposits during the Post-Kazantsevian period, often resulting in the total alluviation of considerable valley stretches as well as low-lying interfluves. The increased formation of shelf ice sheets, in the writer's opinion, may plausibly explain the gradual accumulation of icy deposits on the present-day littoral, in the deltas and in the New Siberian Islands. It should be noted that several scholars (Mercer, 1970; Vozovik, 1982; Grosswald, 1980) had mentioned the probability of the existence of this scenario to various degrees. However, there is no positive evidence concerning the limit between the solid terrestrial ice and the shelf ice sheet. No distinct oscillations of the shelf ice sheet are acknowledgeable either on the littoral, or in the deltas, or
Pereletki and the initiation of glaciation in Siberia the New Siberian Islands. If this limit ever existed, it was located further north of the present-day coastal area, including the deltas and the islands. The problem of the dynamics of solid fresh-water shelf ice emerges: was this process constant or interruptive? Taking into account the occasional occurrence of stratigraphic disconformities in the sequence of icy deposits, the occurrence of lenses of sand, pebbles and large-size driftwood, which are common features in the Lena Delta, it is apparent that ice dams were at least partly destroyed during the course of the Karginsk age. Hence, based on present knowledge, one may ascertain that the past glaciations in Siberia (ice sheets, networked, valley or piedmont, Malaspina type glaciers) were of limited extent. Yet, if one takes into account the embryonal glaciation, as well as the subterranean ice, and the accumulation of stable fresh-water ice on the shelf, the scale of Siberian glaciation proves to be much greater than is currently believed (Fig. 5).
Macwtas
In conclusion, it should be stressed that a detailed analysis of landforms and related sediments is essential in the areas of potential Ice Age snow glaciers. Amongst the former, one may mention abnormally wide troughs with cirque-like widening in the upper stretches, as well as large niches, steep escarpments on the northern slopes, and similar features. In these cases, the floor and slopes of the valleys are usually covered with sediments of uncertain origin, indistinguishable by their mineralogical content from the bedrock. Judging from the existing records, researchers tend to ignore the role of snow glaciers in the formation of these sediments, which are often viewed as talus, solifluction deposits, colluvium or alluvium. Apart from the theoretical importance, the study of snow glaciers has also a practical significance in clarifying the origin of various landforms. Thus, among the nival landforms in the northern Yakutia were reported valleys with widening in their upper stretches, basically
1:12000
_
31
000
I"I ' t i
II
FIG. 5. Palaeogeographical sketch-map of the Muruktinsk-Sartan Ice Age in the northem Siberia. Key: l - - m a x i m u m extension of oscillating ice sheets on land; 2--areas of wide spread stable firn ice and snow glaciers; 3--accumulative plains resulting from erosional activities of firn and snow glaciers; (a) formed by icy loam, sand-loam and sand with polygonal ice-wedges; (b) formed by accumulated fresh-water ice with mineral columns; 4--probable area of shelf ice sheet and its southern limit.
32
R.O. Galabala
similar to the dry valleys ("dols") in the Trans-Volga Syrt area. Similar valleys with flattened floors and shallow hollows in the head parts also occur in the southern Russia and Ukraine. One may suggest that these landforms were at least partially moulded by snow glaciers.
REFERENCES Galabala, R.O. (1982). Verhnechetvertichnye oledenenija Vostochnoi Yakutii po dannym deshifrirovanija kosmicheskih aerosnimkov. In: I.V.N. Bzuhanov et al. (eds), Kosmogeologicheskie Metody v lzuchenii Chetvertichnogo perioda, pp. 110-122. Leningrad. Galabala, R.O. (1987a). Uslovija formirovanija pozdnechetvertichnyh akkumuljativnyh ravnin severa Sibiri. In: N.A. Logachiov (ed.), Processy Formirivanija Rel'efa Sibiri, pp. 63-68. Nauka, Novosibirsk. Galabala, R.O. (1987b). Novye dannye o stroenii del'ty Leny. In: V.P. Pohnalaynen (ed.), Chetvertichnyi Period Severo-vostoka Azii, pp. 152-171. Magadan. Grigor'ev, A.A. (1930). Vechnaja merzlota i drevnee oledenenie. In: L.S. Bezg and B.L. Lichkor (eds), Vechnaja Merzlota, pp. 43-104. Academy of Sciences of the USSR Publishers, Moscow. Grosswald, M.G. (1980). Poslednee Velikoe Oledenenie SSSR. Znanie, Moscow.
Grosswald, M.G. and Spektor, V.B. (1990). Sledy pokr0vnogo oledenenija na zapadnom poberezh'e guby Buor-Khaja. In: Transactions of the International Symposium: Quarternary Events and the Stratigraphy of Eurasia and Pacific Area, Vol. 2, pp. 89-90, Yakutsk. Kind, N.V. (1971). Izmenenie klimata i oledenenie v verhnem antropogene. DSc Dissertation Abstract, Moscow. Kolpakov, V.V. and Belova, A.P. (1980). Radiokarbonovoe datirovanie v lednikovoi oblasti Verhojan'ja i ego obramlenija. In: J.K. Ivanova and N.V. Kind (eds), Geohronologija Chetvertichnogo Perioda. Nauka, Moscow. Mercer, J.H. (1970). A firm ice sheet in the Arctic Ocean?. Palaeogeogr. Palaeoclimatol. Palaeoecol., 8, 1 19-27. Osokin, N.I. (1981). Snezhniki i Snezhnikovye Sistemy Nizko--i Srednegornyh Rajonov SSSR. Nauka, Moscow. Shchukin, I.S. (1960). Obshchaja Geomorfologija, Vol.1. Moscow University Press, Moscow. Shilo, N.A. (1964). K istorii razvitija nizmennostei subarkticheskogo pojasa severovostoka Azii Trydy. SVKNIISOANSSSR, 11, 154-168. Toll, E. (1895). Wissenschaftliche Resultate zur Erforschung dae Jamallandes und der Neusibierischen lnseln, Vol 13. MAJS, St Petersburg. Tomirdiaro, S.V. and Cernen'ky, B.I. (1987). Kriogenno-eolovye Otlozhenija Vostochnoi Arktiki i Subarktiki. Nauka, Moscow. Vozovik, Yu.I. (1982). Paleogljaciologicheskie i paleoklimaticheskie aspekty razvitija poslednego lednikivogo pokrova. In: A.A. Aksionov (ed.), Problem)' Geomorfologii, Litologii i Litodinamiki Shel'fa, pp. 143-147. Moscow.
Quaternary International, Vols 41/42, pp. 27-32, 1997.
) Pergamon PIh S1040-6182(96)00033-X
PERELETKI
AND
THE
INITIATION
OF GLACIATION
© 1997INQUA/ElsevierScienceLtd All rights reserved. Printed in Great Britain. 10v,0~182/97 $32.00
IN SIBERIA
R.O. G a l a b a l a Modern glaciers present in northern Siberia, adjacent to the Arctic Ocean littoral, develop through the gradual accumulation of windblown snow and firn in river valleys and other depressions. Such snow glaciers ("pereletki") are largely ephemeral in nature, and produce landscapes dominated by nivation hollows and other snow-related landforms. Deposits associated with pereletki include nival, nivalsolifluctionary, and fluvio-nivalunits. The modern pereletki are considered to represent an initial form of glaciation, and their development can be regarded as an analogue for the formation of nival deposits commonly attributed to a more extensive Sartanian glacial event. Late Quaternary glacial events in Siberia involved both alpine and piedmont glaciation, and widespread development of pereletki in lowland areas. Distinguishing between features and sediments producedby glacial ice, and those attributable to ephemeral pereletki, is necessary for accurate assessment of Late Quaternary glaciation in Siberia. © 1997 INQUA/ElsevierScience Ltd
S N O W - G L A C I E R S AS AN I C E - A G E R E L I E F FORMING FACTOR
INTRODUCTION The currently completed geological and geomorphologic 1:200,000 surveying in Siberia resulted in the establishment of detailed limits of the maximal extension of Late Quaternary ice-covers and mountain glaciers, providing a large amount of analytical data relevant to the age of glacial deposits and the dynamics of glacial phenomena in time (Kind, 1971; Kolpakov and Belova, 1980; Galabala, 1982). In the light of this evidence, one cannot fail to wonder at continuously appearing articles and monographs arguing for a huge-scale glaciation of the Arctic shelf and the neighbouring Siberian land-mass (Grosswald, 1980; Grosswald and Spektor, 1990). These publications seem to be completely ignoring the evidence obtained by time-consuming field studies of established geologic features, and base their conclusions on intuitive probabilistic scenarios, and often, wishful thinking. For example, among the features interpreted as glacigenic are large structural-denudational ridges of pre-Quaternary age on the Kola Peninsula, a chain of Cretaceous intrusives in the Indigirka basin, snow glaciers in the vicinity of the Sevastian Lake in the Tiksi area, and many other landforms. These publications arguing for large scale glaciation in Late Cenozoic Siberia are particularly embarrassing, due to the fact that they discuss active glaciers, the limited distribution of which is well established by reliable evidence. On the other hand, one has to acknowledge that the hypotheses relating to major glaciations of Siberia in the past have the right to exist, the peculiarities of the socalled periglacial environment being, as yet, not sufficiently clear. If the evidence pertaining to mountain glaciations, ice sheets and glaciations in Siberia is abundant and varied, the geological effects of stable or quasi-stable firn and snow glaciers is relatively poorly known. The latter being denied by the majority of present-day scholars, were viewed by Grigor'ev (1930) as the evidence of an "embryonal glaciation".
It has been well established that firn and snow-glaciers form one of the typical features of the present-day landscape in the northern periphery of Eurasia. They are particularly common along the northern littoral, from the Kola Peninsula to the Okhotsk Sea, both in t h e ' m e d i u m high mountains and on the lowland, reaching sea-level in the north of Siberia. They are formed by snow, blown by strong winds from the interfluves to the river valleys, in spite of a relatively low solid precipitation in this northern part of Siberia (100-120 m m annually). Snowdrifts occur here throughout the winter, and are particularly common between November and February. Thus by the end of the winter, all major depressions, including river valleys, are filled in with snow, ranging in depth from 5-8 to 1015 m. In contrast, the thickness of snow on watersheds is negligible, its cover completely disappearing by June. Considerable accumulations of snow forming temporary glaciers ("pereletki") emerge on the northern slopes of valleys, hollows and cliffs (Fig. 1). In places they develop into firn glaciers, which may be seen in the Northern Khauralakh, the Pronishchev Massif, the Chekanovski
FIG. 1. Snow glaciers accreted by the end of the summer in the upper Soguru Tigie river, ChekanovskyMassif. 27
28
R.O. Galabala
Massif, and on the Taimyr Peninsula, as well as on the littoral of the Laptev and East-Siberian Seas. For example, a small-size firn glacier on the left bank of the Lower Lena River (72°N, at the altitude of 50 m a.s.1.) which was discovered in 1961, has remained in existence with insignificant fluctuations over the last 30 years. This type of firn glacier, however, is relatively rare, the greater part of "pereletki" disappearing during the warmer summers. Sufficiently common snow glaciers and their seasonal erosional and depositional activities result in the emergence of typical relief features and the accumulation of a specific sedimentary assemblage on the lowland. While in the mountains, where erosion clearly prevails over the glacial deposition, such snow-related landforms as nivation hollows, cirques, troughs, pseudo-terraces and other features are sufficiently known (Shchukin, 1960; Osokin, 1981), this was not the case for accumulative features of a considerable magnitude on the lowland. Consequently the classification of these features became necessary, particularly for the practical purposes of largescale geological mapping. Genetic classification suggested below, related to the evolutionary dynamics and the degradation of snow glaciers, is based on 10 years experience of geologic 1:50,000 mapping in northern Yakutia with the wide use of aerogeologic imagery. This classification includes: nival, nival-solifluctionary and fluvio-nival deposits; grading laterally into alluvial, alluvial-lacustrine and, rarely, into deltaic ones. Nival deposits are viewed as the products of nivation, or snow weathering, forming in the contact area between the snow-glaciers and the bedrock. The intensive weathering results from the considerable fluctuations of diurnal temperature, leading to the phase transition of fracture filling water, and, consequently, expansion and contraction of the rock. The frost weathering is particularly intensive at the upper edge of the snow-glacier, where the diurnal fluctuation of temperature is particularly great. In this case, snow-glaciers form snouts or cliffs, thus making up the bulk of nival deposits. There are considerable differences in the structure and composition of nival deposits mostly depending on the character of the bedrock, yet they commonly feature a lack of stratification and sorting and high concentrations of coarse and angular clasts, with mineral composition usually similar to that of the bedrock. The high content of coarse and angular clasts is equally typical of the areas where the bedrock predominantly consists of clay and silt. In this case the clasts result either from the weathering of the concretions or lenses of calcareous sandstone or limestone, or from the influx of the debris carried by meltwater and solifluction (Fig. 2). Nival deposits usually form the basal units in nivation hollows, cirques, troughs, as well as on the northern slopes of major river valleys and sea cliffs. Their thickness varies between 1-3 and 5-7 m, rarely reaching 10 m. The maximal thickness, in excess of 10 m, has been established in the north of the Chekanovski Massif, in the deeply eroded sub-latitudinal valleys. In that area, snow
FIG. 2. Nival deposits covering the cirque floor.
glaciers which form elongated ridges consisting of finegrained debris with fragments of sandstone, are separated from the bedrock by a narrow hollow formed by a recently melted part of the snow glacier. In rare cases one may see small (0.5-1.0 m long) till-like ridges formed by cobbles of local origin, at the outer edge of the firn glacier, indicative of minor oscillations of the firn ice. Nival-solifluctionarv deposits are seen as the products of nivation displaced down the slope (also along the snow-glacier) by the action of solifluction and meltwater. They are usually made up of clay and silt with rare inclusions of gravel of local origin. They equally include numerous organic remains brought in from the watershed, as well as layers of moss, and slightly decomposed peat, both formed and buried in the course of deposition. Remains of timber, mostly larch, are often encountered at the bottom of nival-solifluctionary deposits in the presentday tundra. Their radiocarbon ages range between 6000 and 8000 years BP, rarely reaching 9000 years. Irregularly stratified deposits are locally disrupted by landslides. Polygonal ice-wedges are often included at the foot of the slopes and the slope-adjacent floor of the valleys, their thickness varying from 0.5-1.0 m on the slopes to 12-16 m at the foot (Fig. 3). Fluvio-nival deposits consist, predominantly, of silt and sand particles and, rarely, gravel, washed out by meltwater from snow-glaciers and deposited in the stream valleys resulting from the melting of snow-glaciers. Within the uplands and massifs temporary meltwater streams often form sandy-pebbly deposits indistinguishable from the river alluvium. One should note that the
FIG. 3. The floor of a trough covered by nival-solifluctionary deposits.
Pereletki and the initiation of glaciation in Siberia present-day environment is unsuitable for the formation of fluvio-nival deposits, the prevailing erosion of water channels resulting in the washout of these sediments. They may be encountered predominantly in the valleys of smaller streams fed by snow-glaciers, which normally dry out after the disappearance of the latter. These streams often end up in small lakes and inner deltas, formed at the points of discharge of the streams into major river valleys of major lakes. Such deltas consist of fine-grained sand and silt, resulting from the washout of nival or solifluctionary-nival deposits. The normal thickness of fluvio-nival deposits is 2-5 m, exceeding 10 m in the areas of present depressions or naturally dammed lakes. The above-cited data on the snow-glaciers and related sediments are seemingly of theoretical interest only, their present-day distribution being insignificant. Yet their study is of paramount importance for the reconstruction of past environments, for the interpretation of several enigmatic deposits in the extraglacial areas, and for the understanding of the genesis of certain landforms. One should take into account that the present-day snow glaciers actually being formed are on the Siberian lowland, while the ice sheets are found exclusively at high altitude, ranging from 700-850 m (Putoran Plateau, Byrranga Mountains) to 1500-1900 (the Upper Yana area). Consequently, during the Ice Age when, in the opinion of the majority of scholars, the snow-line was approximately 1000 m below the present-day position, snow glaciers should be formed in a wide area and on a larger scale. The creation of a data-base for the present-day snow glaciers, and the landforms either formed or modelled by them (valleys, lake depressions, hollows, cirques etc.) and their sediments, enabled identification of similar landforms and sediments of pre-Holocene age both in the course of field studies and in the processing of air and satellite imagery. Such features have been identified in extraglacial Siberia, particularly in the periphery of the central Yakut Lowland, on the Lena Plateau, in the Kharaulakh Mountains (the North Upper Yana area), in the greater part of the Yana basin, in the Kular and Poluosny ridges, on the slopes of Yana-Indigirka Lowland, in the Anyui ridge, in the Kolyma Massif, the Oloi Ridge, the Yukagir Plateau and other areas. Judging from the wide occurrence of the nival landforms and sediments, the scale of snow glacier activity in the preHolocene epoch was enormous. Jointly with mountain glaciers, they were the main source of the terrigenous material, which formed gigantic accumulative plains. Ancient nival deposits on the plains, and particularly in the mountains, were often washed away by the powerful Holocene erosion which included snow glaciers. As exceptions one may mention the areas either immune to or only slightly affected by the later erosion. Thus, in the Chekanovski Massif, nival sediments up to 10 m thick were protected by the deposits of Holocene snow glaciers. For the same reason, the old nival-solifluctionary deposits are rarely intact, usually being eroded in the upper and middle parts of the slopes. These deposits, reaching the
29
FIG. 4. Pre-Holocene fluvio-nival deposits on an air-photograph: (a) coalescent with ice-dammeddeposits on the southern slope of the YanaIndigirka Lowland; (b) cirque-like widening at the head of the tributaries. thickness of 10-15 m, are often encountered in the lower part of the slopes, both on the lowlands and in the lower mountains. Thus, in the Chekanovsky Massif, sand and loam of nival-solifluctionary origin, their thicknesses ranging between 1-2 and 6-8 m, form wide plains 250300 m high at the foot of large cuestas. In contrast to the Holocene ones, the older fluvio-nival deposits are widely spread both in the mountains and on the plains, mostly due to numerous dams, enhancing the accumulation of these deposits in the valleys of the streams fed by snow glaciers. This resulted in the annual formation of stratified icy deposits of loam and sand reaching a considerable thickness (usually 30-60 m, locally, extending up to 90-100 m). These deposits are totally preserved in several valleys, reaching the upper stretches. Beyond the valleys of the streams fed by snow glaciers, the fluvio-nival deposits coalesced with those of major dammed streams, formed by the melting of both glaciers and snow glaciers, as well as other sources. They are indistinguishable on the terrain, their altitudes varying in relation to the local sedimentary environment (Fig. 4).
DISCUSSION Based on the data relative to the present-day and past nival processes one is in a position to interpret, more coherently, the palaeogeography of Ice Ages and, particularly, the formation of icy deposits. The first
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attempt at this has been made by the present writer, based on the evidence of geologic mapping and the special investigations of 1985. At that time a detailed mapping of the areas of glacial deposits, as well as that of icy deposits of problematic origin, was carried out. We have also collected vast amounts of evidence pertinent to their mineralogical composition and age. Based on this evidence, it has been suggested (Galabala, 1987a) that the icy deposits in question were formed in an environment similar to that currently occurring in the deltas of the Lena and Yana, synchronously to the MuruktinskSartan Glaciation, under conditions of notable damming. Apart from ice dams, there occurred a considerable accumulation of ice in the north, which led to the elevation of the base level of erosion in major rivers, and the accumulation of the alluvium of a constructive type in the fiver valleys and off-shore deltas. The rapid transition of icy deposits with polygonal ice wedges into the massive fresh-water ice with the inclusion of mineral columns, viewed by Toll (1895) as the remains of glaciers, may be interpreted as resulting from the accumulation of transported suspension particles, rapidly diminishing in volume with distance from dry land. It has been suggested that the annually frozen firm ice with mineral columns coalesced with the shelf ice. The latter, during the Karginsk age, formed a barrier preventing the Karginsk sea from penetrating east of the Khatanga river mouth. The new evidence, resulting from the large-scale mapping of the Kharaulakh low mountains, the Pronchishchev and Chekanivsky massifs, the Olenek uplift, the North Siberian lowland, the Lena Delta and other regions, is in no way contradictory to this reconstruction. The investigations of present-day snow glaciers eliminate the last arguments for the interpretation of icy deposits as aeolian features, the hypothesis still supported by several scholars (Shilo, 1964; Tomirdiaro and Cernen'ky, 1987). As had been mentioned, the altitude of icy deposits may fluctuate considerably depending on the sedimentary environment. Thus, in the north Siberian Lowland, where these deposits in consolidated areas are usually found at 70-90 m, they may reach 120-140 m on the slopes of the lowland. Locally, along fiver valleys, they may extend to 180-200 m. The same pattern is recognizable in the area of Kular ridge: situated in the consolidated area at 7080 m, these deposits may reach 160 m along the valleys and on the slopes of the Ulakhan-Sis (Kular) ridge and Khabdji-Tas Mountain. In the area of Olenek Uplift, the icy sand and loam ascend from 80-100 m up to 300 m. In these circumstances, the aeolian hypothesis may seem to be plausible, if one fails to take into account the local icedamming and snow glaciers. In this respect it is worth mentioning the case of the Lena Delta, which may be seen as a transparent model for the formation of icy deposits. In that area, apart from annually increased flood deposits accompanied by the increasing polygonal ice wedges, one may equally visualise the' aeolian activity. Strong winds prevailing through the summer considerably affect the thermal abrasion of the shores, forming aeolian terrace rim walls
and watershed covers. Thus, the rim wall on the surface of the first terrace near Trofimovskoye settlement reached the altitude of 0.7~0.9 m over the last 50 years. Further afield, the thickness of aeolian sediments rapidly diminishes, completely thinning out at a distance of 3540 m. Aeolian sediments formed through the Holocene are widely spread on a major deltaic island of AgraMuora-Sise. They consist of silty sand with vertical plant stems and roots. Lenses of peat (0.5-1.5 m in thickness) are found at the bottom, indicative of a mire occurring on the surface prior to the aeolian accumulation. Peat layers are presently forming, resulting from the interfluve grasssedge mire being buried under sand and dust carried by the wind from near-by lake shores, fiver valleys and cliffs. The thickness of aeolian sediments varies between 0.7 and 2.0 m (Galabala, 1987b). It is important to stress that the aeolian deltaic deposits have no direct analogies amongst icy deposits anywhere in Siberia, neither in structure nor composition. Nevertheless, aeolian activity took part in the formation of icy deposits, transporting sand and finer sediments from barren water channel shores and surrounding areas; the wind transported particles forming a notable admixture to predominantly aquatic deposits. In certain areas, the icy deposits may include lenses of typical aeolian sediments, the later being of secondary importance in relation to the bulk of icy deposits.
CONCLUSIONS Numerous recently published articles and monographs discuss various aspects of Late Quaternary events both in the Siberian mainland and the adjacent northern seas. However, if the evidence related to the Siberian mainland is sufficiently rich, this is not the case for the seas, particularly, the Laptev and East Siberian seas. This is the main reason why palaeogeographic interpretations related to these areas are often contradictory, being not well in accord with the evidence obtained for the adjacent mainland, deltas and islands. The author's suggestion about the occurrence of a shelf ice dwells entirely on the established fact that the rivers emptying into the Arctic Ocean: the Anabyr, Olenek, Lena, Yana, Indigirka and Kolyma, which underwent a downcutting lasting until the Kazantsevian age, started to form thick constructive alluvial deposits during the Post-Kazantsevian period, often resulting in the total alluviation of considerable valley stretches as well as low-lying interfluves. The increased formation of shelf ice sheets, in the writer's opinion, may plausibly explain the gradual accumulation of icy deposits on the present-day littoral, in the deltas and in the New Siberian Islands. It should be noted that several scholars (Mercer, 1970; Vozovik, 1982; Grosswald, 1980) had mentioned the probability of the existence of this scenario to various degrees. However, there is no positive evidence concerning the limit between the solid terrestrial ice and the shelf ice sheet. No distinct oscillations of the shelf ice sheet are acknowledgeable either on the littoral, or in the deltas, or
Pereletki and the initiation of glaciation in Siberia the New Siberian Islands. If this limit ever existed, it was located further north of the present-day coastal area, including the deltas and the islands. The problem of the dynamics of solid fresh-water shelf ice emerges: was this process constant or interruptive? Taking into account the occasional occurrence of stratigraphic disconformities in the sequence of icy deposits, the occurrence of lenses of sand, pebbles and large-size driftwood, which are common features in the Lena Delta, it is apparent that ice dams were at least partly destroyed during the course of the Karginsk age. Hence, based on present knowledge, one may ascertain that the past glaciations in Siberia (ice sheets, networked, valley or piedmont, Malaspina type glaciers) were of limited extent. Yet, if one takes into account the embryonal glaciation, as well as the subterranean ice, and the accumulation of stable fresh-water ice on the shelf, the scale of Siberian glaciation proves to be much greater than is currently believed (Fig. 5).
Macwtas
In conclusion, it should be stressed that a detailed analysis of landforms and related sediments is essential in the areas of potential Ice Age snow glaciers. Amongst the former, one may mention abnormally wide troughs with cirque-like widening in the upper stretches, as well as large niches, steep escarpments on the northern slopes, and similar features. In these cases, the floor and slopes of the valleys are usually covered with sediments of uncertain origin, indistinguishable by their mineralogical content from the bedrock. Judging from the existing records, researchers tend to ignore the role of snow glaciers in the formation of these sediments, which are often viewed as talus, solifluction deposits, colluvium or alluvium. Apart from the theoretical importance, the study of snow glaciers has also a practical significance in clarifying the origin of various landforms. Thus, among the nival landforms in the northern Yakutia were reported valleys with widening in their upper stretches, basically
1:12000
_
31
000
I"I ' t i
II
FIG. 5. Palaeogeographical sketch-map of the Muruktinsk-Sartan Ice Age in the northem Siberia. Key: l - - m a x i m u m extension of oscillating ice sheets on land; 2--areas of wide spread stable firn ice and snow glaciers; 3--accumulative plains resulting from erosional activities of firn and snow glaciers; (a) formed by icy loam, sand-loam and sand with polygonal ice-wedges; (b) formed by accumulated fresh-water ice with mineral columns; 4--probable area of shelf ice sheet and its southern limit.
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similar to the dry valleys ("dols") in the Trans-Volga Syrt area. Similar valleys with flattened floors and shallow hollows in the head parts also occur in the southern Russia and Ukraine. One may suggest that these landforms were at least partially moulded by snow glaciers.
REFERENCES Galabala, R.O. (1982). Verhnechetvertichnye oledenenija Vostochnoi Yakutii po dannym deshifrirovanija kosmicheskih aerosnimkov. In: I.V.N. Bzuhanov et al. (eds), Kosmogeologicheskie Metody v lzuchenii Chetvertichnogo perioda, pp. 110-122. Leningrad. Galabala, R.O. (1987a). Uslovija formirovanija pozdnechetvertichnyh akkumuljativnyh ravnin severa Sibiri. In: N.A. Logachiov (ed.), Processy Formirivanija Rel'efa Sibiri, pp. 63-68. Nauka, Novosibirsk. Galabala, R.O. (1987b). Novye dannye o stroenii del'ty Leny. In: V.P. Pohnalaynen (ed.), Chetvertichnyi Period Severo-vostoka Azii, pp. 152-171. Magadan. Grigor'ev, A.A. (1930). Vechnaja merzlota i drevnee oledenenie. In: L.S. Bezg and B.L. Lichkor (eds), Vechnaja Merzlota, pp. 43-104. Academy of Sciences of the USSR Publishers, Moscow. Grosswald, M.G. (1980). Poslednee Velikoe Oledenenie SSSR. Znanie, Moscow.
Grosswald, M.G. and Spektor, V.B. (1990). Sledy pokr0vnogo oledenenija na zapadnom poberezh'e guby Buor-Khaja. In: Transactions of the International Symposium: Quarternary Events and the Stratigraphy of Eurasia and Pacific Area, Vol. 2, pp. 89-90, Yakutsk. Kind, N.V. (1971). Izmenenie klimata i oledenenie v verhnem antropogene. DSc Dissertation Abstract, Moscow. Kolpakov, V.V. and Belova, A.P. (1980). Radiokarbonovoe datirovanie v lednikovoi oblasti Verhojan'ja i ego obramlenija. In: J.K. Ivanova and N.V. Kind (eds), Geohronologija Chetvertichnogo Perioda. Nauka, Moscow. Mercer, J.H. (1970). A firm ice sheet in the Arctic Ocean?. Palaeogeogr. Palaeoclimatol. Palaeoecol., 8, 1 19-27. Osokin, N.I. (1981). Snezhniki i Snezhnikovye Sistemy Nizko--i Srednegornyh Rajonov SSSR. Nauka, Moscow. Shchukin, I.S. (1960). Obshchaja Geomorfologija, Vol.1. Moscow University Press, Moscow. Shilo, N.A. (1964). K istorii razvitija nizmennostei subarkticheskogo pojasa severovostoka Azii Trydy. SVKNIISOANSSSR, 11, 154-168. Toll, E. (1895). Wissenschaftliche Resultate zur Erforschung dae Jamallandes und der Neusibierischen lnseln, Vol 13. MAJS, St Petersburg. Tomirdiaro, S.V. and Cernen'ky, B.I. (1987). Kriogenno-eolovye Otlozhenija Vostochnoi Arktiki i Subarktiki. Nauka, Moscow. Vozovik, Yu.I. (1982). Paleogljaciologicheskie i paleoklimaticheskie aspekty razvitija poslednego lednikivogo pokrova. In: A.A. Aksionov (ed.), Problem)' Geomorfologii, Litologii i Litodinamiki Shel'fa, pp. 143-147. Moscow.