JOURNAL OF QUATERNARY SCIENCE (2002) 17(4) 361–382Copyright 2002 John Wiley & Sons, Ltd.Published online 30 May 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.677

Depositional history of the North Taymyrice-marginal zone, Siberia—a landsystemapproachHELENA ALEXANDERSON,1* LENA ADRIELSSON,1 CHRISTIAN HJORT,1 PER MOLLER,1 OLEG ANTONOV,2

SASKIA ERIKSSON1† and MAKSIM PAVLOV3

1 Department of Geology, Lund University, Solvegatan 13, SE-223 62 Lund, Sweden2 State Research Institute of Mining Geomechanics and Mine Surveying (VNIMI), 82 Sredny pr., 199026 St. Petersburg, Russia3 Arctic and Antarctic Research Institute (AARI), 38 Bering Street, 199397 St. Petersburg, Russia

Alexanderson, H., Adrielsson, L., Hjort, C., Moller, P., Antonov, O., Eriksson, S. and Pavlov, M. 2002. Depositional history of the North Taymyr ice-marginal zone,Siberia—a landsystem approach. J. Quaternary Sci., Vol. 17 pp. 361–382. ISSN 0267-8179.

Received 2 December 2001; Revised 18 February 2002; Accepted 28 February 2002

ABSTRACT: The sediment–landform associations of the northern Taymyr Peninsula in ArcticSiberia tell a tale of ice sheets advancing from the Kara Sea shelf and inundating the peninsula,probably three times during the Weichselian. In each case the ice sheet had a margin frozen toits bed and an interior moving over a deforming bed. The North Taymyr ice-marginal zone (NTZ)comprises ice-marginal and supraglacial landsystems dominated by thrust-block moraines 2–3 kmwide and large-scale deformation of sediments and ice. Large areas are still underlain by remnantglacier ice and a supraglacial landscape with numerous ice-walled lakes and kames is formingeven today. The proglacial landsystem is characterised by subaqueous (e.g. deltas) or terrestrial (e.g.sandar) environments, depending on location/altitude and time of formation. Dating results (OSL,14C) indicate that the NTZ was initiated ca. 80 kyr BP during the retreat of the Early Weichselianice sheet and that it records the maximum limit of a Middle Weichselian glaciation (ca. 65 kyrBP). During both these events, proglacial lakes were dammed by the ice sheets. Part of the NTZwas occupied by a thin Late Weichselian ice sheet (20–12 kyr BP), resulting in subaerial proglacialdrainage. Copyright 2002 John Wiley & Sons, Ltd.

Journal of Quaternary Science

KEYWORDS: ice-marginal zone; landsystem; glaciation; Weichselian; Siberia.

Introduction

This paper concerns the depositional history of the NorthTaymyr ice-marginal zone (NTZ), the northernmost of severalice-marginal zones on the Taymyr Peninsula in central ArcticSiberia (Fig. 1). It is a complex, palimpsest feature consistingof glacial, glaciofluvial and glaciolacustrine deposits formedduring the Weichselian glaciations (Alexanderson et al., 2001).This paper aims (i) to describe the NTZ in some detail, and(ii) to recognise the processes responsible for its formationand the nature of the ice sheets involved. To provide apicture of the history of the NTZ a landsystem approach has

* Correspondence to: Helena Alexanderson, Department of Geology, LundUniversity, Solvegatan 13, SE-223 62 Lund, Sweden.E-mail: helena.alexanderson@geol.lu.se†Presently at Sweco VBB VIAK AB, Geijersgatan 8, SE-216 18 Malmo, Sweden.

Contract/grant sponsor: European Union ‘Eurasian Ice Sheets’; Contract/grantnumber: ENV4-CT97-0563.Contract/grant sponsor: European Science Foundation’s QUEEN.Contract/grant sponsor: Royal Swedish Academy of Sciences.Contract/grant sponsor: Swedish Environmental Agency.Contract/grant sponsor: Swedish Polar Research Secretariat.

been adopted, combining information from satellite imagery,geomorphological mapping and sedimentological logging.

A landsystem is ‘a recurrent pattern of genetically linkedland facets’ (Eyles, 1983, p. 3) and can be used as amodel for the expected occurrence of sediment–landformassociations in an environment (Eyles, 1983; Benn and Evans,1998). Sediment–landform associations, in turn, are ‘faciesassociations that have surface expression in a landform whichis genetically related to the underlying facies’ (Benn and Evans,1998, p. 381). In this paper, they are classified according totheir depositional environment into ice-marginal, supraglacial,proglacial and paraglacial associations (cf. Benn and Evans,1998).

The concept of landsystems and sediment–landform asso-ciations thus emphasises the genesis of a landscape and theprocesses responsible for its formation. It is a way to learn moreabout the geographical limits, sedimentary processes, glacialdynamics, hydrology, age and evolution of former ice sheets.Similar approaches have been used successfully in other areasby, for example, Clayton and Moran (1974; USA), Sharpe(1988; Canada), Benn and Clapperton (2000; Patagonia) andBennett et al. (2000; Iceland).

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Figure 1 (A) Map of northern Eurasia showing the location of theTaymyr Peninsula and the late Weichselian (LGM) ice extent,modified from Svendsen et al. (1999) and Svendsen (2001). From adynamic point of view, it seems more likely that the LGM iceinundating the northwestern fringe of the Taymyr Peninsula wasconnected to the ice sheet covering Novaya Zemlya, than that itderived from an independent ice cap on the northeasternmost KaraSea shelf. (B) Ice-marginal zones on the Taymyr Peninsula. The redline and part of the yellow line correspond to the NTZ. Dashed linesdenote uncertain extent: U. Taymyr is the Upper Taymyr ice-marginalzone, correlated with the Mokoritto ridge further west, DSK stands forthe Dzhangoda–Syntabul–North Kokora ice-marginal zone. (C) Thecentral area of the NTZ with important geographical names andfieldwork areas (base map ‘Russia and the Conterminous States,’Russia’s Geodetic and Cartographic Federal Office, Moscow, 1996).The squares show the locations of Figs 2–5

Glacial history background

Several ice-marginal zones have been mapped on the TaymyrPeninsula (Fig. 1b) and they have been described and dated

to various extents (Kind and Leonov, 1982; Isayeva, 1984;Arkhipov et al., 1986a; Alexanderson et al., 2001; Hjort et al.,2002). The occurrence of remnant glacier ice in some areashas been taken as an indication of a Weichselian age forthose features, because it was assumed that the ice would nothave survived the warm Eemian interglacial (Astakhov, 1998).However, large amounts of Weichselian ice have survived theHolocene ‘interglacial’ in the Eurasian Arctic (e.g. Astakhovand Isaeva, 1988) and it is indeed quite possible that some ofthis remnant ice is of Saalian age.

Studies of glacial geomorphology, glaciotectonics, till fabric,mineralogy and the provenance of erratic clasts (Grosswald,1980; Andreeva and Isaeva, 1982; Andreeva et al., 1982;Arkhipov et al., 1986b; Hjort et al., 2002) indicate that the icesheets on the Taymyr Peninsula advanced from the Kara Seashelf to the north. Ice sheets advancing from this direction musthave blocked the northward drainage on the peninsula and asa result, large ice-dammed lakes were formed in front of theice sheets. These lakes drained southwards into the TaymyrLake basin and then further on into the Kara or Laptev seas(Alexanderson et al., 2001).

A recent view is that the maximum glaciation of the TaymyrPeninsula, when the entire region was ice covered, took placeduring the Saalian glacial period, which ended ca. 125 ka(Svendsen et al., 1999; Svendsen, 2001; Hjort et al., 2002).This huge ice sheet caused significant isostatic depressionand during the following post-glacial period, including theEemian interglacial, the sea inundated large parts of theTaymyr Peninsula and marine sediments were deposited atcurrent altitudes of up to 130 m a.s.l. on northern Taymyr(the Boreal Transgression; Gudina et al., 1983; Molodkov andRaukas, 1988).

The Weichselian maximum glaciation (≥100 ka) reachedsouth of the Byrranga Mountains, either to the Urdachand Sambesin ice-marginal zones (Andreeva et al., 1982;Andreeva and Isaeva, 1982; C. Siegert et al., 1999) or tothe Dzhangoda–Syntabul–North Kokora (DSK) ice-marginalzone (Astakhov, 1998); see Fig. 1 and Hjort et al. (2002).The retreat of the Early Weichselian ice sheet seems to havehappened step-wise. The Mokoritto–Upper Taymyr Ridge andlarge glaciomarine deltas deposited at ca. 100 m a.s.l. in thevalleys of the Byrranga Mountains during the Ledyanaya GravelEvent (Moller et al., 1999a, 1999b) represent stillstands. Northof the Byrranga Mountains, some 100 km from the present-day Kara Sea coastline, lies the North Taymyr ice-marginalzone (NTZ), which seems to have been initiated during areadvance or a temporary stillstand in the retreat stage of theEarly Weichselian glaciation (Alexanderson et al., 2001).

More recent glaciations have been less extensive andhave not reached south of the Byrranga Mountains, or tothe Cape Chelyuskin area in the northeast (as summarisedin Alexanderson et al., 2001). There is, however, evidenceof both Middle and Late Weichselian ice advances on thenorthwesternmost Taymyr Peninsula, up to some 100 kminland of the present coast (Alexanderson et al., 2001).

North Taymyr ice-marginal zone

The North Taymyr ice-marginal zone (Severotaimyrskayaridge; NTZ) was first mentioned by Andreeva et al. (1982)and by Isayeva (1984), who mapped the area from satelliteradar images and aerial photographs. They believed it hadbeen formed during a halt in the ice recession during the LateWeichselian deglaciation, but did not exclude the possibilitythat it represented the Late Weichselian maximum limit.

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The NTZ runs largely parallel to the present coastline, fromthe Michailova Peninsula in the west (75°10′ N, 87°30′ E)to near the Tessema River mouth in the north (77°20′ N,102°15′ E); see Fig. 1b (and Andreeva et al., 1982; Isayeva,1984; Arkhipov et al., 1986a). Although not continuous allthe way, it can be tracked for 700–750 km. The NTZ is bestdeveloped in the lowlands, e.g. in the central parts of theShrenk- and Lower Taymyr river basins, and is less prominentin higher terrain.

Much new data on the NTZ have been collected duringrecent Swedish and Russian work on the Taymyr Peninsula, in1998 and 1999. These investigations have shown that the NTZis a large and complex feature, containing different generationsof glacial, glaciofluvial and glaciolacustrine deposits, as wellas older reworked or deformed sediments (Alexanderson et al.,2001).

Methods

A suite of satellite images of the northern Taymyr Peninsulawas studied to obtain an initial overview of the area (Table 1).Both visual and digital analyses were performed, the latter byusing the GIS programs IDRISI (Eastman, 1997) and ArcView(Environmental Systems Research Institute); cf. Alexanderson(2000). For more detailed studies of the fieldwork areas weused aerial photographs.

Fieldwork was conducted from three base camps duringtwo seasons: the Barometric Lake (Ozero Barometricheskoe)area and the White Lake (Ozero Beloe) area in 1998 and theAstronomical Lakes (Ozera Astronomicheskie) area in 1999(Fig. 1c). Short stops were also made along the Lower TaymyrRiver, e.g. where that river cuts through the NTZ. In the field,the geomorphological features were mapped and documented.Sediments were investigated by detailed logging of shore andriver sections and palaeocurrent and fabric analyses weremade. Samples were taken for radiocarbon- and opticallystimulated luminescence (OSL) dating, as well as for laboratoryanalyses and documentation.

Some of the sites in the study areas are without names,but to simplify descriptions they have been given unofficialnames, indicated by quotation marks. Russian names arein general translated into English followed by the originalin parentheses. More sites than are presented here wereinvestigated, but original site numbers have been keptto aid correlation with the public PANGAEA data base(http://www.pangaea.de), from which all sedimentary logs canbe retrieved. Latitude–longitude coordinates were measuredwith a Magellan GPS Pioneer. Altitudes are either taken directly

from Russian topographical maps (1 : 100 000), or measuredby a Thommen HM30 digital altimeter, calibrated at fix-pointson the topographical maps.

Geological setting

The bedrock on the northwestern Taymyr Peninsula belongsto the Taymyr fold belt, which can be divided into three zonesby mainly SW–NE stretching lineaments (Vernikovsky, 1997).The Byrranga Mountains (<1150 m high) and the area southof them (e.g. the Taymyr Lake basin; Fig. 1) consist mainly ofLate Palaeozoic sedimentary rocks penetrated by dolerites. Thebasin occupied by the Lower Taymyr, Shrenk, Trautfetter andLeningradskaya rivers north of the Byrranga Mountains is madeup of Neoproterozoic–Palaeozoic carbonates, sandstones,shales, volcanics and some granites, which to a largeextent are covered by younger (Mesozoic–Cenozoic), partlyunconsolidated deposits. It is enclosed to the west and east bybedrock hills 300–400 m high, dominated by flysch deposits(Neoproterozoic–Cambrian) and granitoids (Late Palaeozoic).The seas surrounding the peninsula (Kara and Laptev) arelarge, shallow shelf areas (<100 m deep) with a cover ofpartly permafrozen Quaternary sediments (Kim et al., 1999;Musatov, 1999; Kleiber et al., 2001).

The landscape on northern Taymyr Peninsula has a clearglacial imprint with, e.g., several U-shaped valleys crossingthe Byrranga Mountains. In higher terrain, there is no oronly a thin cover of loose deposits (Figs 2–5) and patches offrost-shattered bedrock and low tors are found (Fig. 6A). Atlow altitudes bedrock is exposed in canyons along someof the rivers in the Astronomical Lakes and White Lakeareas. The thickest cover of unconsolidated sediments onnorthernmost Taymyr is found in the basin between theByrrangas and the coastal hills. The thickness is in the order of50–100 m, but may be up to 150 m (G. Schneider, CAGRE,Norilsk, personal communication 1998). It consists mainlyof Quaternary sediments but there are also older deposits,e.g. Cretaceous coal-bearing sands and Pliocene marine andalluvial clays to gravels. The main glacial deposition occurredin ice-marginal positions, and marine, lacustrine and alluvialsediments dominate the surrounding areas.

The North Taymyr ice-marginal zone (NTZ) is located mainlywithin the large basin, but partly in the coastal hills as well.The Astronomical Lakes and Barometric Lake study areas aresituated in similar positions on the northwestern side of theShrenk River valley, just south of the coastal bedrock hills(Figs 1–3). White Lake is in an elevated terrain, close to aseries of Vendian Limestone cuestas (Figs 1 and 4).

Table 1 Satellite images used in this study

Satellite/system Image identifier Acquisition date Area

Landsat 5 MSS XE8E15300592221 8 August 1992 Cape Chelyuskin—Leningradskaya RiverLandsat 5 MSS XE8E15300692221 8 August 1992 Leningradskaya River—Shrenk RiverLandsat 5 MSS XE8E15300792221 8 August 1992 Shrenk River—Byrranga MtsCorona KH-4 DS009041005DA008 2 August 1962 Astronomical Lakes—Lower Taymyr RiverCorona KH-4 DS009043005DA038 18 September 1962 Astronomical Lakes—Barometric LakeCorona KH-4A DS1022-1037DA013 22 July 1965 Astronomical Lakes—Leningradskaya RiverCorona KH-4A DS1022-1037DF007 22 July 1965 Astronomical Lakes—Leningradskaya RiverCorona KH-4A DS1022-2069DA029 24 July 1965 Cape Michailova—Mammoth RiverCorona KH-4A DS1022-2069DA030 24 July 1965 Cape Michailova—Mammoth RiverCorona KH-4A DS1022-2069DA031 24 July 1965 Cape Michailova—Mammoth RiverResurs-F1/KATE-200 00443 1475 1979 Lower Taymyr River—Tessema River

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Figure 2 Geomorphological map of the Barometric Lake area. The legend covers Figs 2–5

The low-lying areas north of the Byrranga Mountains aretoday covered by arctic and high-arctic tundra. The presentannual average temperature on central Taymyr is ca. −15 °Cand precipitation <300 mm yr−1 (Taymyr Lake meteorological

station; Siegert and Bolshiyanov, 1995). In the central Kara Seathe mean annual temperature is −22 °C (Siegert and Marsiat,2001). The permafrost is more than 500 m thick and the activelayer generally <1 m.

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Figure 4 Geomorphological map of the White Lake area. For legend, see Fig. 2

Figure 5 Geomorphological map of the NTZ–Lower Taymyr River junction. For legend, see Fig. 2

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Figure 6 (A) View over a tor landscape just south of (outside) the NTZ at the Lower Taymyr River. (B) Thaw slump exposing remnant glacier ice inthe White Lake area. The headwall is ca. 1.5 m high. (C) A ca. 2 km long, low and narrow moraine ridge (<2 m high) that winds its way alongthree lakes on the Chukcha Moraine, cf. Fig. 2. (D) Glaciofluvial terraces on top of exposed bedrock along the Mammoth River distal to theAstronomical Lakes Moraine. (E) Subglacial meltwater erosion in the Mammoth River canyon. (F) Thaw (thermokarst) lake on top of theAstronomical Lakes Moraine. All photographs by the authors

Moraine mega-geomorphology

West of the Astronomical Lakes and north of the Leningrad-skaya River, the North Taymyr ice-marginal zone is indistinctand can be followed mainly as discontinuous hummocks. Inits more central parts, on which this paper concentrates, it con-sists partly of large, continuous moraine ridges and partly ofwider, yet morphologically conspicuous, areas of large-scalehummocky terrain.

The ice-marginal landsystems include thrust-block moraines2–3 km wide, small push moraines and large-scale deforma-tion of sediments and ice. They are closely associated withthe supraglacial landsystems, which are underlain by rem-nant glacier ice and contain numerous ice-walled lakes andkames. In some areas the ice is actively backwasting, thus

showing that the supraglacial landscape is still developing.The proglacial landsystems are either subaqueous or terres-trial, depending on location/altitude and time of formation,and those sediment–landform associations range from glacio-lacustrine sediments and deltas to glaciofluvial sandur plains.Since the active ice retreated from the NTZ, paraglacial asso-ciations have formed: fluvial and aeolian sediments, lakes andpatterned ground. The subglacial landsystems on present landproximal of the NTZ have not been studied in any detail, butseems mostly to be represented by non-deposition or erosionin relatively high-altitude areas. In the lowlands, diamictonsinterpreted as deformation tills (Moller et al., in preparation)have been found, and large boulders scattered on the surface(Moller et al., in preparation; S. Funder, personal communica-tion 2001) may be the only remnants of the most recent till,mainly owing to extensive gelifluction.

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Sediment–landform associations

Ice-marginal associations

Large moraines and glaciotectonics

The large moraines that constitute the base of the NTZ inthe Astronomical Lakes and Barometric Lake areas (Figs 2 and3) formed where debris was concentrated at the ice-sheetmargin. A relatively large moraine ridge is also found at theNTZ and Taymyr River junction (Fig. 5), whereas at WhiteLake, there is no distinct ridge but rather small push morainesand a proximal, ice-cored landscape extensively affected byslumping and gelifluction processes (Fig. 4).

The Astronomical Lakes Moraine (ALM) is ca. 70 km longand stretches SW–NE in the Mammoth River basin (RekaMamonta; Fig. 3). It has an overall arcuate shape and consistsof several sublobes. The width varies significantly (2–15 km)and it is 50–80 m high. The 15-km-long Chukcha Moraine(CM) stretches northeastwards from the bedrock scarp in thewest until cut off by the Barometric Lake Moraine in theeast (Fig. 2). The ridge is ca. 80 m high and has an arcuateplanform (2–4 km wide), convex towards the south. It isslightly steeper on its distal (south) than on its proximal (north)side. The Barometric Lake Moraine (BLM) has a distinct margin,which is ca. 45 km in circumference and convex towards thewest-southwest (Fig. 2). Lake basins and hummocks indicatesublobes and in the north, its outer margin is connected toan adjacent lobe (cf. Fig. 6 in Alexanderson et al., 2001).The landscape generally is highest in the southwest part. Themoraine at the NTZ and Taymyr River junction (Fig. 5) isca. 8 km long and 200–600 m wide, with frequent hummocksand kettle holes.

Associated with the glacial sublobes within the ALM areslightly overdeepened basins, some of which can be followedbackwards (up-ice) into the coastal bedrock hills, over whichthe ice advanced. These basins are 1–3 km wide and have arelative relief of 80–100 m. We suggest that the basins wereformed during an active stage of the ice advance.

Large bodies of ice under a 0.5–1 m cover of silty diamictonare visible in shallow slumps within the BLM and in the WhiteLake area (Figs 2 and 4). The ice contains folds, dirt bands,shear planes, erratic clasts, striated rocks, marine shells anddisplaced Cretaceous sand and coal. The contact with theoverlying diamicton is sharp but irregular. Based on thesecriteria the ice is interpreted as remnant glacier ice (Fig. 6B; cf.French and Harry, 1988, 1990; Worsley, 1999). At a few sitesnear White Lake, younger ice wedges that extend from thebase of the overlying diamicton penetrate the glacier ice. Theobserved maximum ice thickness is ca. 8 m, at a small lakenorth of Bird Lake (Ozero Ptichye; Fig. 2) within the BLM. Theorientation of shear planes in ice south of Central Lake (OzeroTsentral’noe, BLM; Fig. 2) indicate ice movement towards thewest, which is in agreement with the configuration of theformer ice margin here. North of White Lake shear planes andfolds in the ice show an ice movement from NNW–NE.

No ice-slumps were observed on the CM or the ALM butmassive ground ice, possibly glacier ice, was observed in agully in the distal front of the ALM (Fig. 3). If the ALM andthe CM are still ice-cored, then this ice lies below the presentactive layer. The high bedrock level at the ALM margin mayindicate that it also has a bedrock core.

Hummocks containing deformed sorted sediments are foundat the intersection between the CM and the BLM. At ‘57 mLake’ 3 (80–100 m a.s.l.), circular to elongated hummocks<5 m high form a reticulate pattern (Figs 2 and 7A). Exposures

Figure 7 Hummock and lake complexes. (A) The intersectionbetween the CM and BLM in the Barometric Lake area. The Vetkadelta partly surrounds ‘57 m Lake’, which is interpreted as a kettlehole (cf. Fig. 2). North of ‘57 m Lake’ there are small, deformedhummocks in a reticulate pattern (cf. Fig. 8). The ‘151 m delta’ (at thetop right-hand corner) may be related to the concentric rows ofsubaqueous fans to the south. (B) Hummock complexes are situatedalong the distal margin of the ALM in the Astronomical Lakes area:DG, ‘Dolgan Gate’, ML, ‘Mushroom Lake’. From Corona satelliteimage DS009043005DA038

show steeply (40–90°) dipping sandy and gravelly beds(Fig. 8A) with various dip directions, mainly towards W–NW.A few kilometres to the east, tilted and folded silty sedimentsare exposed in a small gully at ca. 85 m a.s.l. (‘57 m Lake’ 4;Fig. 8B). The fold axes here strike approximately E–W and onefold is overturned towards the north-northeast.

The sorted sediments of the hummocks mainly weredeposited from traction, with current strengths varying fromlow to moderate. Suspension load fall-out and occasionaldebris flows occurred intermittently. The deposition tookplace in a subaqueous environment in front of the ice sheetand the reticulate pattern of the hummocks indicates somestructural control during deposition, possibly an ice-frontcrevasse pattern.

Compression, typical of ice-marginal and proglacial defor-mation (van der Wateren, 1995), is evidenced at ‘57 m Lake’ 4,whereas signs of subglacial deformation (e.g. shearing) or sub-sidence (e.g. normal faults) are lacking. Thus, we believe thatthe sediments were deformed proglacially or marginally by theice sheet that deposited the sediments, or by a later one. Thedifference in deformation style between ‘57 m Lake’ 3 (rigid)and ‘57 m Lake’ 4 (ductile) probably reflects their lithology

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Figure 8 Deformed hummocks. (A) ‘57 m Lake’ 3: beds of massive sand or gravelly sand, trough cross-bedded and ripple-laminated sand andlaminated silt have been rigidly tilted and dip 43–56° towards the west. The beds are slightly wavy but no subsidence structures (such as normalfaults) are present. (B) ‘57 m Lake’ 4: The sediments consist of laminated, graded and massive silts with some thin clay and sand beds. Thethickness of individual beds range from 0.5 to 10 cm and thicker beds alternate with thin ones. Large folds of different character are seen in thesection. On the ground behind the section, at ca. 85 m a.s.l., the tilted beds show up as parallel stripes striking E–W on the surface

(cf. Aber et al., 1989; Astakhov et al., 1996). Rigidness dur-ing deformation is usually taken as an indication of frozensediments (Benn and Evans, 1998), although it is not alwaysthe explanation (van der Wateren, 1995; Etzelmuller et al.,1996; Astakhov et al., 1996). If the sediments were frozen atdeformation, freezing must have occurred after a drop in waterlevel to below 80–100 m a.s.l.

Deformed sediments and remnant glacier ice are foundin close association within the large moraines and hum-mocky landscapes. The deformation thus seems to have beenice-marginal rather than purely proglacial and the morainesresemble the thrust-block push moraine with compression-from-within detailed in Bennett (2001). The widths of themoraines probably correspond to the width of the compres-sional zone within the former ice sheet. This varied in timeand space along the ice-margin, from a few kilometres to>10 km, owing to, for example, differences in slope, pres-ence of weak layers in the substratum, the nature of theice–sediment contact, or the sub/proglacial drainage (cf. Aberet al., 1989).

Small moraines

Small ridges, stretching more or less normally to the inferredice-movement direction, are in some places superimposedon the large moraines (Figs 2–4). On their surfaces there iscoarse, rather angular material (<0.5 m diameter). The internalstructure and content of the ridges are unknown, but basedsolely on morphology two types are distinguished: narrow andbroad ridges.

The narrow ridges are 0.3–2 km long, ≤5 m high and2–10 m wide. These are found alongside some lakes on thedistal side of the Chukcha Moraine, along the lateral edges ofdeltas in the Barometric Lake area (Fig. 2) and east of the Moun-tain Lakes (Ozera Gornye, ALM; Fig. 3). Most of them haverather gentle crests, but at least one is sharp-crested (Fig. 6C).

The three ridges southwest and east of White Lake (Fig. 4)belong to the broad ridge type; they are 1–1.5 km long,ca. 10 m high and 100–150 m wide. Their relief is irregular,with smooth crests and some cobbles and boulders on thesurface. In cross-section they are almost symmetrical, but thetwo eastern ridges have slightly steeper distal than proximalsides. The ridges form the proximal edges of glaciofluvialdeposits and one is situated distal to a small basin.

The ridges are interpreted as push moraines correspondingto the small and large push-moraine types detailed in Bennett(2001). The small push moraines are considered by Bennett(2001) as moraines formed as a result of seasonal ice-marginalfluctuations, whereas the large ones are considered to beproduced during a prolonged advance. These moraines wouldthus represent the outermost margin of the ice sheet.

One of the moraines at White Lake is situated a few hundredmetres north of a limestone cuesta (Fig. 4) and the ice sheettherefore seems to have halted before reaching that majortopographic obstacle. The northern side of the cuesta is todayoccupied by perennial snowfields and it is likely that theseexpanded during colder periods. Thus, the position of themoraine may in this case reflect where the ice sheet and thesnowfield interacted, and the moraine formation can be similarto the ‘snow-bank push’ mechanism suggested by Birnie (1977)for annual moraines, although on a slightly larger scale.

Kames/subaqueous fans

Along the distal margin and between sublobes of the largemoraines there are several hummocks or hummock complexesof sorted sediments (cf. zone 1 mentioned below; Figs 2–4).They are especially evident along the ALM and the southernmargin of the BLM but also occur, although more scattered,on the CM and at White Lake. On the BLM, the top altitudesof the hummocks range from 80 to >120 m a.s.l., but most arejust above 100 m a.s.l.

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The hummocks vary in appearance, but are in general5–20 m high and have asymmetric cross-profiles, with theproximal slopes steeper than the distal ones. Although somehummocks appear solitary, they usually form complexes wherethey are arranged in concentric rows or in reticulate patterns(Fig. 7). At White Lake, hummocks commonly occur in smallgroups, but generally are not arranged geometrically. Theirsurfaces are covered by a thin lag of gravel and pebbles, butscattered larger clasts (up to boulder size) occur. Some of thehummocks are deeply dissected by erosion along ice-wedgepolygon patterns.

The internal composition is predominantly sand but also siltand gravel in horizontal beds or beds dipping gently (<10° ina distal direction (southwards). Diamict material also occurs,preferential in the lower parts of the successions. Gravel-sizedclasts are mostly subrounded. Shell and coal fragments arecommon. Some of the hummocks, e.g. north of White Lake,show signs of deformation (folded and tilted beds).

‘Dolgan Gate’ (Figs 3 and 7B) is a row of hummocks,conforming to the distal side of a minor ‘closed’ basin withinthe ALM. The hummocks are up to ca. 20 m high, reachingat most ca. 150 m a.s.l. A section (Fig. 10) was investigatedwhere the hummocks are cut by drainage from the basin.Beds of massive gravelly sand with scattered clasts dominatein the lower half of the sequence. In the upper half the bedsare thinner and the massive sand frequently is interbeddedwith ripple-laminated sand and silt and with planar parallel-laminated sand and gravelly sand. The palaeocurrent direction,as indicated by ripple lamination, is towards the southwest.The sediments at the ‘Dolgan Gate’ were deposited as debrisflows (massive facies) and from fluvial traction (ripples, planarlamination). The environment was mainly subaerial and thesehummocks are interpreted as kames.

‘Maxi-section’ is a flat-topped hummock at the margin of theBLM with its top surface at ca. 140 m a.s.l. (Fig. 2). The loggedsuccession shows a coarsening upward trend and consistsmainly of alternating beds of planar-parallel laminated fineto medium sand and massive or planar parallel-laminated(sandy) gravel (Fig. 10) dipping up to 15–20°towards S–SW.Some of the beds include diamict intraclasts and shell andcoal fragments are found throughout the succession. Sedimentsamples reveal a very poor, probably reworked foraminiferalfauna. Clasts are rounded to well-rounded. The deposits havebeen dated by OSL to 78 and 81 ka (Figs 9 and 10).

Figure 9 Dates from the North Taymyr ice-marginal zone (cf.table 1 in Alexanderson et al., 2001): � OSL-dates, • calibratedradiocarbon dates. The LGM deglaciation date refers to the ice-frontarea only; an older (13.8 cal ka) date from the Oscar Peninsula showsthat more central parts of the ice sheet disappeared earlier

At ‘Maxi-section’, the massive facies probably weredeposited subaqueously by hyperconcentrated density flowsor debris flows just in front of the ice margin. The ripple-and planar parallel-laminated sediments show that intermit-tent currents occurred, probably related to density underflows,and sometimes succeeded by fall-out from suspension. The‘Maxi-section’ and similar hummocks are interpreted as ice-contact subaqueous fans, some of which were built up to alake level to become deltas (see below).

Both the kames and the subaqueous fans have a controlleddistribution along the margin of the moraines (especially theBLM). Some of the hummocks correlate with the position ofthe ice margin, whereas others may be related to crevasse orthrust-plane patterns of the ice-sheet front.

Supraglacial associations

Ablation terrain

The ice-marginal areas are topographically and morpholog-ically different from their proximal and distal surroundings(Figs 2–5; Alexanderson, 2000; Alexanderson et al., 2001).The terrain also differs between the study areas and withinthe moraines three morphological zones can be distinguishedfrom the ice margin and upstream. This zonation is most evi-dent on the southern margin of the BLM but the general trendis true also for the other areas (Figs 2–5). Zone 1, closest tothe former ice margin, is dominated by sandy, gravelly hum-mocks (described above) in a rather narrow zone. This zoneis not found along the entire former ice margin. A higheraltitude landscape (zone 2), rich in large-scale, mainly neg-ative landforms, replaces zone 1 upstream and where zone1 is absent, it reaches the former margin. Lakes (see below),post-glacial peat or organic-rich fine-grained sediments usu-ally occupy the depressions and thaw slumps are frequent. Yetfarther upstream is a flat, low-lying landscape (zone 3) withrelatively few lakes. The surface here generally is dissected bya characteristic dendritic drainage pattern.

This zonation reflects the relative abundance of ice anddebris in the former ice-sheet margin. The outermost zone(1) is debris-dominated and sediments were largely reworkedby meltwater. The next zone (2) is dominated by buried iceand only a thin till layer is found on top of the ice. Theinnermost zone (3) corresponds to the clean (debris-poor) ice,which melted relatively quickly as deglaciation set in and didnot leave many traces in the present landscape.

The surfaces of the large thrust-block moraines and the ice-marginal landscapes mainly are covered by silty diamicton,which in places contains whole and fragmented mollusc shells(e.g. Hiatella arctica and Astarte borealis). The diamictongenerally is less than a metre thick and has scattered boulderson the surface, most frequently within the BLM. Most of theseboulders are crystalline erratics and many are striated. Thestriae are short and with a variety of orientations (Fig. 11),indicating that the clasts rotated during transport, which islikely in a subglacial deforming bed. The diamicton is probablymade of former marine sediments that have been deformedduring basal transport by the advancing ice sheets and isinterpreted as a supraglacial meltout till. The marine sedimentsderive from the Kara Sea shelf, and from the coastal lowlandsthat were inundated by the sea during the Eemian. Shells inthe diamicton from the ALM have given infinite radiocarbonages, whereas at White Lake two shells from the same sitehave been dated independently to ca. 20 kyr BP (Fig. 9).

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Figure 10 Sedimentary logs representing kames (Dolgan Gate, Astronomical Lakes area) and subaqueous fans (Maxi-section, Barometric Lakearea)

The landscape-forming processes include downwasting asa result of thermokarst subsidence and backwasting throughfrequent retrogressive thaw slumps (or ground-ice slumps;

French, 1996). The thaw slumps have been observed withinthe BLM and at White Lake (Figs 2 and 4). The aerial densityof slumps varies widely and in some places entire valley sides

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Figure 11 Striated rocks found within the silty, supraglacial till atWhite Lake (upper) and within the ALM (lower). The striae are shortwith a variety of orientations

may be covered with slumps. The slump scars are 20–100 min size, usually with headwalls 1–2 m high showing siltydiamicton 0.5–1 m thick on top of buried glacier ice (Fig. 6B,also cf. above). At some sites, the diamicton can be divided intoa lower, stratified and frozen zone and an upper, structurelesspart, which is within the present active layer. The lowerzone has probably been refrozen after a warmer period (theHolocene climatic optimum?) with deeper active layer (cf.Worsley, 1999).

These ice-marginal zones are thus landscapes under devel-opment, most obviously so in the BLM and at White Lake.Similar situations have been described from other parts ofSiberia (Astakhov, 1997) and from Arctic Canada, where broadzones of ice-cored hummocky moraine are still active (Sharpe,1988; Dyke and Savelle, 2000).

Glaciokarst lakes

The large moraines are rich in lakes, in contrast to surroundingareas (Figs 2–5). The glaciokarst (ice-walled) lakes haveirregular shapes, are relatively large (>0.5 km in diameter)and deep. The lakes usually are situated in basins much largerthan themselves and the surrounding slopes are rather lowand dominated by slow gelifluction processes. Small streamssupply sorted sediments that may form deltas and around manylakes there are also thaw slumps producing debris flows. Ataltitudes above the present lake-level there are hummocks orterraces (Fig. 2) that most likely are shoreline features reflectinga once higher lake-level (generally ca. 10 m higher). Theabsolute levels vary between lakes and are therefore local;they thus do not correspond to any regional lake-level. Thereason for the lowered lake-levels probably is thermokarstsubsidence (French, 1996), but also could be partly the resultof a reduced water balance. For some lakes, this lowered levelhas meant a large reduction in size, or total disappearance.

The lake infilling and the lowering of the ice-coredsurroundings as a result of ablation eventually will leadto topographic inversion of the landscape (Clayton, 1964;Boulton, 1972; Clayton and Moran, 1974). The lake deposits

will then form ice-walled lake plains, analogous to those foundin Scandinavia (e.g. Westergard, 1906; Lagerback, 1988) andNorth America (e.g. Parizek, 1969; Johnson et al., 1995; Hamand Attig, 1996).

Proglacial associations

Ice-contact deltas

There is a gradual transition from subaqueous fans to deltasand it is tempting to think that most flat-topped sorted-sedimentmounds along the moraine margins represent delta surfaces,graded to a specific water level. In many cases, however,no good sections were available and their internal large-scalestructures and, thus, their genesis could not be determined.Nonetheless, some plateaux are certainly deltas, as on thedistal sides of the Chukcha and Barometric Lake moraines.Generally, the deltas are irregularly shaped, relatively large(>1 km2) but not very thick (10–25 m). Their altitudes seemto reflect two different regional lake levels: ca. 130 m and70–80 m a.s.l. The deltas are situated partly in proglacial andpartly in supraglacial positions and the steep proximal sides,fringing moraines and the lack of preserved feeding systemsindicate that they are ice-contact deltas.

The ‘Pozdnyaya delta’ is a lateral delta between the bedrockscarp and the western end of the CM (Fig. 2). Its top altitude is130–140 m a.s.l. and the flat surface is covered by gravel. Thedelta occupies at least 1.5–2 km2 and is more than 12 m thick.It has a well-delimited steep front with several lobes directedtowards the south. The delta contains beds of laminated sand,silt and clay, occasionally interrupted by massive sand orgravelly sand, dipping towards the southwest and capped bymassive, matrix-supported gravel and cross-bedded sand.

The ‘151 m delta’ (ca. 1 km2) consists today of two adjacentplateaux reaching ca. 150 m a.s.l. (Figs 2 and 7A). Towardsits proximal side it merges with a ‘chaotic’ kame and kettlelandscape. There is a small elongated lake on the delta plain,the origin of which is uncertain. The surface consists ofrelatively poorly sorted sand and gravel, but boulders arealso present. In places there seems to be a thin diamict cover,which is thicker towards the southeastern margin of the delta,where there is contact with a lateral moraine. The generaldrainage direction was towards the southwest.

The ‘Vetka delta’ is a large (ca. 5 km2) feature, not allof which may have been built up to the water surface atca. 70–80 m a.s.l. (Figs 2 and 7A). Sections, cutting into theplateau along the ‘57 m Lake’ and the Vetka River, have beeninvestigated at two sites. The site Vetka 2 is situated along theVetka River on the southern side of the plateau (Figs 2, 7Aand 13). About 50 m from the logged site, the succession isoverlain by aeolian sediments. The ‘57 m Lake’ 1 and 2 sitesare sections on the northern side of the plateau (Figs 2 and 7A).The sediment facies are in general similar to those in Vetka 2but are slightly coarser (Fig. 13).

The lowermost part of the sequence (unit A) is dominatedby ripple-laminated sandy silt/silty sand, planar parallel-laminated (gravelly) sand and normally graded massive sand.Several erosional scours occur, filled with normally gradedmassive sand/gravelly sand or deformed sediments. There arealso some dewatering structures in the sand. The palaeocurrentis towards the southwest and east.

The predominating facies of unit B is delta-slope planarlaminated sand dipping 21–30° towards the south-southeast,but massive, ripple-laminated and planar cross-laminatedsands also occur. Some of the massive beds are normally

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Figure 12 Sedimentary log of glaciolacustrine sediments in theBarometric Lake area. The section comprises ca. 2000 couplets. Forlegend, see Fig. 10

graded and there is one bed with stacked sets of inverselygraded planar parallel-laminated sand. Occasional shellfragments were found.

The uppermost unit C is ca. 1.5 m thick and consists ofplanar parallel-laminated gravelly sand, massive gravel, troughcross-laminated sandy gravel and gravelly sand, overlyingan erosional surface. On top, there is also a massivesilty–sandy diamicton, corresponding to the soil layer. Atboth sites, a few of the gravelly beds are clast-supported, withimbricated particles that indicate a palaeocurrent directionfrom N–NE.

The major part of unit A seems to have been deposited byunderflows. During occasional calm periods, deposition fromsuspension dominated. The erosional scours represent slumpsor chutes (the paths of major turbidity currents or debrisflows). These processes may have taken place on a lowerdelta slope and the diverging palaeocurrents can be explainedby switching active delta lobes. The dipping beds in unit Brepresent delta-slope deposits, in which the sediments havebeen deposited mainly by traction and occasional sedimentgravity flows on a relatively low-angle upper delta slope. UnitC represents a sand-and-gravel braided river, with shallowchannel and bar deposition, probably on a delta-plain. The‘Vetka delta’ is interpreted as a Hjulstrom-type delta, which is

characteristic of steep feeder systems terminating in relativelyshallow water (Postma, 1990). The maximum-particle size,which is 10–11 cm, indicates that discharges were not verylarge.

Two OSL dates give an approximate age of ca. 65 ka (63 kaand 66 ka; Figs 9 and 13) for the ‘Vetka delta’.

Glacial-lake shorelines

Distal to the central parts of the NTZ the land generally islow-lying with few places where former glacial lake-levelscould have left traces. However, one beach ridge has beenobserved on a hill adjacent to the Vetka River, at ca. 80 ma.s.l. (Fig. 2; see also Fig. 7c in Alexanderson et al., 2001).This ridge is 1 m high and 5–10 m wide and consists of gravelwith some pebbles. The exposed bedrock at the southeastcorner of the BLM (Fig. 2) lies at the level of the higher glaciallake (ca. 130 m a.s.l.) and may have been washed bare at thattime. A moraine ridge at the CM–BLM intersection flattens outto a terrace at 135–140 m a.s.l., which also may represent aformer lake-level.

Glaciolacustrine sediments

Fine-grained glaciolacustrine deposits, with observed thick-nesses up to 12 m, are found distal to the large moraines in theAstronomical Lakes and Barometric Lake areas. Their exactmaximum altitude is not known, but is at least 65 m a.s.l. inthe Barometric Lake area and 80–100 m a.s.l. outside the ALM(Figs 2 and 3). Also, the surface cover in the enclosed ChukchaRiver basin, proximal to the CM but distal to the BLM, hasroughly the same spectral signature on satellite images as thearea south of CM and NTZ (Alexanderson, 2000) and thereforealso may be covered by glaciolacustrine silts. This area hasnot, however, been ground-truthed.

Laminated glaciolacustrine sediments were observed in riversections, e.g. along the Vetka and Mammoth rivers. In surficialtest pits, however, the silt appears massive, which most likelyis the result of secondary disturbance by soil and permafrostprocesses within the active layer. In contrast to the localtill, the silt does not contain any gravel or shell fragments, norany foraminifers (M.-S. Seidenkrantz, personal communication2001). A few boulders have, however, been found on thesurface.

Vetka 4 (Fig. 12) is situated ca. 3.5 km outside the formerNTZ ice-margin (Fig. 2). The lower part of the sectionconsists predominantly of rhythmically laminated clayey–siltysediments, whereas in the upper ca. 5 m, the laminated clayeysilt is frequently interbedded with ripple-laminated sand or silt.

For this logged succession, the number of couplets isestimated to ca. 2000. The couplets are normally graded andhave sharp bases. In the lower part the mean couplet thicknessis 6–7 mm, whereas in the uppermost 5 m it is 4–5 mm. Thereseems to be a proportional relationship between thickness ofthe coarse and fine part, respectively, of individual couplets,indicating rapid but continuous deposition. No coarse particles(≥ gravel) have been found in the sediments, nor any signs ofbioturbation. The sediments commonly are cracked and showpronounced centimetre-scale fissility.

These sediments have been deposited by intermittentunderflows (low-density turbidity currents) and the massiveclay beds represent gradual settling during interruptions inthe sediment supply (Smith and Ashley, 1985). It is not likelythat the couplets represent annual varves, but this cannot

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Figure 13 Sedimentary logs from the Vetka delta in the Barometric Lake area. Vetka 2 is situated on the southern side of the plateau, ‘57 m Lake’1 and 2 on the northern side (Fig. 2). For legend, see Fig. 10

be excluded. The change in facies at ca. 60 m a.s.l. withincreasing frequency of ripple-laminated sets might representvariations in sediment load or a shift towards a more proximalenvironment, reflecting switching delta lobes, a change inlake-level or fluctuations at the former ice margin. The lack ofcoarse particles indicates that there was no, or very limited,deposition of ice-rafted debris.

Sandar and valley fills

Glaciofluvial sediments are found just outside the former icemargins in the Astronomical Lakes and the White Lake areasand at the NTZ–Taymyr River junction. Downstream of theALM the Mammoth River is flanked by terraces of glaciofluvialand alluvial fan sediments (Figs 3 and 6D). They begin some

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100 m outside the ALM, at an altitude of ca. 100 m a.s.l.,and can be followed downstream, more or less continuously,for ca. 25 km. Close to the Mammoth–Shrenk junction, atca. 80 m a.s.l., the terraces pass into flat-lying, fine-graineddeposits. They mostly lie directly on top of the bedrock andthe sediment thickness is about 10 m.

At the NTZ–Taymyr River junction (Fig. 5) and in the WhiteLake area (Fig. 4), small sandur plains extend in distal directionfrom the NTZ margin. In the former area the flat surfaces at120–125 m a.s.l. are cut by small gullies and consist mainlyof erosional remnants in their distal parts. South of WhiteLake, two sandar flank the cuesta ridge and between them, aglaciofluvial terrace has been built up to about 130 m a.s.l.between the former ice margin and the cuesta ridge (Fig. 4). Inthe eastern part of the terrace system adjacent to White Lake,there are flights of successively lower terraces reaching downto the present river level. The glaciofluvial terrace is up to500 m wide and the sediment thickness at least 10 m.

Mammoth River 3 (Fig. 3) lies just upstream of wherethe Yellow Stream joins the Mammoth River and ca. 2 kmdownstream from the former ice front. A section in the terracewas investigated but, due to frequent sediment failures, notall of it could be logged in detail (Fig. 14). At the bottomof the exposure is a clayey silty diamicton (0.5 m thick),which here and at nearby sites contains whole shells and shellfragments of Mya truncata, Hiatella arctica, Astarte borealis,Macoma calcarea and Portlandia arctica. Of the latter species,there was also one paired shell. This blue-grey diamicton issucceeded by massive (gravelly) sand, which contains shellfragments and is interbedded with clast-supported gravel andthin massive silt beds. A brown silty–sandy diamicton is foundat 95 m a.s.l. Within the terrace there is a horizon with manylarge boulders, which seems to be at the same stratigraphicallevel as the lower silty diamicton.

The exposures of the silty diamicton were too limitedto draw any certain conclusions as to its genesis. It maybe a glaciomarine diamicton with large dropstones or atill consisting of redeposited marine sediments. The twointerpretations have widely different implications, neitherof which presently can be ruled out as a possibility: aglaciomarine incursion to more than 90 m a.s.l. or an iceadvance over the area, both before 50–70 ka (cf. Alexandersonet al., 2001). The facies within the sandy sequence areinterpreted as debris and hyperconcentrated flow depositsand glaciofluvial sediments, probably alluvial fan deposits.

Shells from the diamicton have infinite 14C ages (>40 kyrBP) and a U–Th age of 47 ka. These are all regarded asminimum ages. The OSL dating on the glaciofluvial sand hasgiven 54 ± 3 and 70 ± 9 ka (Figs 9 and 14).

Sections in the sandar at Site 1 and 3 at the NTZ and TaymyrRiver junction (Fig. 5) expose sandy–gravelly successionsdominated by alternating planar-parallel laminated and troughcross-laminated beds (Fig. 14). Some scours, ripple-laminatedand massive beds also occur. These sediments representdeposition in and between channels on an intermediate-type sandur. The palaeocurrent direction generally is towardsS–SW.

At White Lake 1 a section was logged in the glaciofluvialterrace at the former ice-front south of White Lake (Fig. 4).The sequence shows a coarsening upward trend startingwith ripple-laminated and planar parallel-laminated silty sandthat passes into cross-laminated sand, topped by matrix- andclast-supported gravel (Fig. 14). Shell fragments are frequentlyfound. The sand in the lower part is yellowish and rich incoal fragments, which indicates a provenance from nearbyCretaceous sediments. An OSL date gives the sediments an ageof 65 ± 7 ka (Fig. 9). The palaeoflow was towards W–NW,

along the cuesta ridge and the ice front. The change inlithofacies from sandy to gravelly deposits represents a changetowards a more proximal or high-energy environment.

A few canyons are found where the bedrock is exposed closeto the former ice margin; e.g. at White Lake where the cuestasouth of the NTZ is cut by two such canyons (Fig. 4). Theirbottoms are today mainly covered by local, rather angularmaterial. The present water flow is very limited outside themelt season and the canyons probably derive from the timewhen the ice-front abutted against the cuesta ridge and muchmore water than today drained southwards. Another well-developed canyon is found where the Mammoth River flowsthrough the ALM margin (Fig. 3). At its bottom there are well-abraded bedrock knobs, many of which exhibit longitudinalP-forms (furrows; Fig. 6E), whereas the steep walls are moreweathered. The northern part of the Mammoth River canyonthus likely served as a subglacial drainage path.

Paraglacial associations

Recent fluvial and floodplain sediments are found both insideand outside the former ice margin (Figs 2–4). Since the onsetof deglaciation many rivers have eroded down to a lower level,possibly owing to a lowered base level or a change in sedimentsupply/erosional capacity. A succession interpreted to recordbar deposition in a low-sinuosity river at ‘Upper White River’2 (Fig. 4) east of White Lake dates the most recent deglaciationin that very area to 9.5 kyr (14C) BP (twig; Fig. 9).

Massive silty sand with scattered coarse sand and gravel-sized particles make up the surface along part of the VetkaRiver (Barometric Lake area; Fig. 2) and east of Steep Lake(Ozero Kritoye, ALM; Fig. 3). These deposits have beeninterpreted as aeolian coversands and sand–loess intergrades(Lea, 1990) including coarse particles transported by surfacecreep (McKenna Neuman, 1993; Wilson, 1989). At some sitesaeolian deposits are closely associated with fluvial and nivalsediments, e.g. at Vetka 1 (Fig. 2) and at White Lake (Fig. 4).

Thaw lakes (thermokarst lakes; French, 1996) are typicalthermokarst phenomena and are found on floodplains, but theyalso are found on the crests of the large moraines, where theymark the position of the watershed. They usually are circular,<100 m in diameter and shallow (Fig. 6F). In addition to thawlakes, there are also oxbow lakes on floodplains adjacent tomeandering rivers.

Patterned ground, mainly non-sorted stripes and high-centred, ca. 50 m wide tundra polygons, has developed onthe crests of the large moraines, especially on the ALMand the CM. The constantly changing landscapes within theBLM and in the White Lake area make patterned ground lesscommon there. In large depressions and along some streams,low-centred orthogonal polygons (ca. 25 m) occur, some ofwhich are orientated in radial-concentric spider-web patternsindicating recently slowly drained lakes (Mackay, 1958). Onthe smaller scale, irregular frost-sorted polygons or frost-boilsoccur on many surfaces, both within the moraine areas andoutside them. Palsa swarms were observed at several places(Figs 2–4), with palsas usually being 1.5–2 m high and ca. 5 min diameter.

Discussion

The temperature of an ice sheet is dependent on the heatexchange with the atmosphere, the geothermal heat flux from

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Figure 14 Sedimentary logs representing proglacial sedimentation in sandar and alluvial fans, from Mammoth River 3 (Astronomical Lakes area),White Lake 1 (White Lake area) and sites 1 and 3 in the NTZ and Lower Taymyr River junction area. Dates within parentheses are not from thesection shown in the log but from an adjacent, correlated section. For legend, see Fig. 10

below and the heat generated by the ice flow itself (Sugdenand John, 1976; Paterson, 1994). An ice sheet forming ina high-arctic area will initially consist of cold ice, as thetemperature in the upper parts of the ice will be roughlysimilar to the annual mean temperature (Benn and Evans,1998). During the Last Glacial Maximum (LGM) this may

have been as low as ca. −30 °C in the Kara Sea area (Siegertand Marsiat, 2001). As the thickness of the Late Weichselianice sheet on the Kara Sea shelf was probably never more thanca. 500 m (M. Siegert et al., 1999), the pressure melting pointwould not have been reached as a result of ice thicknessalone. Moreover, although the deeper parts of the Kara Sea

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shelf were probably thawed, most of the ice overlay severalhundred metres of permafrozen ground, which meant that notmuch geothermal heat was available. However, the fact thatthis ice has eroded and transported mollusc-bearing marinesediments from the Kara Sea shelf far on to land and the natureof striated clasts within the NTZ suggest that the ice sheet waspartly or at some time wet-based and moved by deformingits bed and sliding over it (cf. Astakhov et al., 1996). Theonly heat source available to form a warm, deformable layerwould then be strain heating and friction (cf. Dyke, 1993)and the deforming basal layer would thus only exist for aslong as the ice moved rapidly enough to produce sufficientheat to keep it at the pressure melting point. An estimate,without considering the lowered effective friction resultingfrom high porewater pressures, suggests necessary velocitiesof several hundred metres per year, which is similar to that offast-flowing glaciers and ice streams (Benn and Evans, 1998).Such a change from cold-based to at least partly wet-based icemust have been initiated by some distinct change in ice-sheetdynamics, maybe related to damming of subglacial water or todecoupling of ice from a thawed bed (as theoretically modelledby, for example, Grosswald and Hughes, 2002).

The thickness of the subglacial deforming bed need not havebeen great, with a silty–clayey composition perhaps less thana few metres (Boulton, 1996). In the landscape proximal tothe NTZ, e.g. in the lowlands around the Lower Taymyr River,not much of such resulting deposits seem to remain today.This could be the result of a non-constructive deformingbed or to extensive gelifluction and erosion subsequent todeposition. The large boulders found on top of older sedimentsuccessions in coastal areas north of the Lower Taymyr Rivermouth (S. Funder, personal communication) thus may be theonly indication of such a youngest glacial deposit. Olderdeformation tills have, however, been recognized in the coastalareas of the northernmost Taymyr Peninsula (Moller et al., inpreparation) and south of the NTZ, indicating that earlier icesheets had similar subglacial conditions. These till beds aresometimes up to 2–3 m thick, but also may be representedonly by boulder lags, as in the Cape Chelyuskin area (Molleret al., in preparation). No landforms related to any subglacialice movement, such as drumlins or flutes, have been observed.

The subglacial drainage probably happened largely throughdistributed drainage systems such as linked cavities, braidedcanals or porewater flow, which are commonly associatedwith deformable beds (e.g. Clark and Walder, 1994; Walderand Fowler, 1994). Distributed drainage systems also wouldhave been favoured by the presence of relatively deep lakes infront of the Early and Middle Weichselian ice sheets (cf. Fyfe,1990). Negative evidence for this type of subglacial drainagesystem is the absence of eskers (cf. Clark and Walder, 1994).However, during the early stages of the NTZ build-up in theBarometric Lake and Astronomical Lakes areas, the majordrainage seems to have been concentrated to lateral and inter-lobate positions. Considering the steep ice-marginal hydraulicgradient and the instability of a distributed drainage system(Clark and Walder, 1994), a switch to a channelised system atthe ice margin could occur as drainage concentrated to a fewconduits that were connected to the ice front (Kamb, 1987;Benn and Evans, 1998).

A number of factors determined the final position of theice margin during the different Middle- and Late Weichselianstages. Some factors may have interacted, such as topographicobstacles, changes in the hydrological conditions in thesubstratum, diminished downstream transfer of ice and therebydriving force from the main ice sheet, or exhaustion of thedeforming layer. Topography may have been important atWhite Lake and in the western part of the ALM, where the ice

sheets advanced towards higher bedrock terrain, and in theBarometric Lake area the younger ice-sheet advances may haverun into older moraine ridges. According to geological maps(Pogrebitsky, 1998), the substrate does not change significantlyat the position of the ice-marginal zones, with the possibleexception of a bedrock threshold at the ALM. The slowdownand halt also resulted in a change in basal thermal conditionsand, subsequently, the margin froze to its base.

A margin frozen to its bed obstructs fast-moving icefrom behind and, as a result, compressional flow takesplace, elevating subglacial debris to the surface. Extensivecompressional glaciotectonism at ice-sheet margins is alsofavoured by topographic obstacles and weak layers in thesubstratum (Aber et al., 1989), both of which occurred at theNTZ. The relative importance of these factors has, however,varied in time and space. Generally, porewater pressuresprobably were high and there were beds of older silty tillsand fine-grained marine and lacustrine sediments that couldact as decollement planes.

The extent of the moraines and associated supraglaciallandscapes reflects the width of the compressional zones andthe amount of debris within the ice sheets. The most prominentmoraine ridges (ALM, CM) were formed in a piedmont-likefashion by ice sheets that advanced over the coastal bedrockhills, whereas the wider moraines and landscapes at BLM andWhite Lake derive from ice-sheet lobes advancing over openlowland areas. In these lowlands, there were fine-grained,mainly marine, Quaternary deposits and poorly consolidatedCretaceous sands, which, although permafrozen, constituted acompletely different substrate than the crystalline bedrock hills.This resulted in more debris-rich lowland ice-lobe margins,which, however, were relatively clean up-glacier from thecompressional zone. They could thus melt away leaving fewtraces behind their frontal zones.

The compressional zones should correspond to the frozentoe of an ice sheet, although compression also may occurin non-marginal situations owing to changes in subglacialtopography, lithology or temperature (Astakhov et al., 1996).For the ice sheets terminating at the NTZ, this cold-based zonewould have been 2–15 km wide. This can be compared withthe width of the frozen zone at the southern margin of theLaurentide Ice Sheet, which ranged from 60 to 200 km at itsmaximum position (Cutler et al., 2000).

The material now constituting the silty diamicton (supragla-cial melt-out till) on top of the remnant glacier ice was broughtto the surface from the base of the ice sheet only at theice margin, where compressive flow occurred. Melted-outsupraglacial debris efficiently insulates the underlying glacierice and thus reduces the ablation rate, so that once the debriscover has become thicker than the active layer (ca. 1 m),the ice is protected from further melting and may survivefor very long periods, gradually becoming incorporated intothe permafrost zone. Strictly speaking, such a landscape isnot yet deglaciated—deglaciation has only been retarded(Astakhov and Isaeva, 1988). The NTZ is thus an excellent‘snap-shot’ of an ice margin still under deglaciation, whichcan be used to understand, for example, the formation ofPleistocene hummocky moraines in more southerly latitudes(cf. Dyke and Savelle, 2000).

The arcuate shapes of the moraines indicate that theice-sheet front that formed them was active, advancing orreadvancing, and the three moraines discussed representdifferent ice-sheet lobes. The ‘Pozdnyaya delta’ (CM) and the‘151 m delta’ (BLM) both formed at a 130–140-m glacial lake-level and are probably contemporaneous. The ‘Maxi-section’and the rows of subaqueous fans (BLM) also seem to belongto this period. This implies that at one time an ice front stood

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along the distal margin of both the CM and the southern BLM.However, because the BLM is younger than the CM, the olderice inside the CM must have melted away or at least havebecome stagnant when the ice advance that created the neatcircular form of the westernmost part of the BLM (Fig. 2) tookplace. The timing of these events is not yet known.

A schematic model for the dynamics of the final (LateWeichselian) ice advance on to the Taymyr Peninsula is shownin Fig. 15. It has been divided into four phases, from build-up to maximum extension. During phase 1 cold ice wasformed on the Kara Sea shelf, the deeper parts of which werethawed. As the ice advanced on to the shallower, permafrozenparts of the shelf (phase 2), much debris was frozen on tothe base and the subglacial drainage towards the ice marginbecame blocked by impermeable permafrost. As a result of thisobstruction, the porewater pressure in the closed subglacialaquifer was raised (phase 3) and consequently the ice liftedand started to slide over subglacial water and a deforming bed.This movement continued southwards until the driving forcefrom behind became reduced and the ice movement sloweddown. Eventually, the ice margin froze to its base (phase 4)causing development of a compressional-flow regime alongthe margin.

The distribution of sediment–landform associations alongthe NTZ reflects the spatial and temporal variations inenvironments at the former ice margins. The NTZ landsystem(Figs 2–5) has many similarities to subpolar ice-marginallandsystems (e.g. Sharpe, 1988; Benn and Clapperton, 2000),but there are differences, for example, subglacial landformssuch as drumlins are lacking. It thus seems as if at least the finalice sheet—during the LGM—when reaching its maximumat the NTZ just stagnated, and thereafter possibly retreated(collapsed) too fast for any large recessional moraines to form(cf. Dyke and Savelle, 2000). In most models the temporalaspect is not clear—at what stage does the snout becomecold-based? In this case, the ice-sheet dynamics requires thatthe front was not cold-based during the advance phase overpermafrozen land. The snout froze, owing to successive loss offriction heat, only as the ice reached its maximum extension.

The depositional history of the NTZ

Our interpretation of the formation of the North Taymyr ice-marginal zone comprises three successively less extensive

glaciations during the Weichselian (Fig. 16; cf. also Alexan-derson et al., 2001). The foundation of the NTZ was laid downca. 80 ka, during the deglaciation of an Early Weichselianice sheet, and probably represents a readvance phase duringthe general retreat from a maximum position well south ofthe Byrranga Mountains. At the NTZ, part of the ice-sheetfront ended in an ice-dammed lake, which was about 100 mdeep (maximum level 120–130 m a.s.l.) in the present-dayShrenk, Trautfetter and Lower Taymyr river valleys, and whichextended into the Taymyr Lake basin (Fig. 16A). The outlet ofthis large lake was probably westwards, through the UpperTaymyr and Pyasina river valleys, to the Kara Sea. The icefront had probably calved northwards from the Byrrangas andreached a grounding-line position at the NTZ.

New deposits and landform segments were added to the NTZduring the Middle Weichselian (ca. 65 ka), when an ice sheetreached approximately the same position as during the EarlyWeichselian retreat stage (Fig. 16B). Glaciolacustrine depositsat the present Kara Sea coast, dated to 70–80 ka (OSL; Funderet al., in preparation), indicate that the Early Weichselian icehad retreated all the way back to the shelf (and perhapstotally disappeared?) and that a new or reactivated ice sheetlater advanced on to the present land. At this time anotherproglacial lake, dammed to only ca. 80 m a.s.l., came intoexistence in the areas around the Lower Taymyr River valley,and like its deeper predecessor also draining southwards intothe Taymyr Lake basin. The ‘Vetka delta’ in the BarometricLake area was deposited into this lake, whereas glaciofluvialmaterial was deposited in subaerial positions in higher areas,e.g. along the Mammoth River in front of the ALM and atWhite Lake.

During the Late Weichselian, the last global glacial maxi-mum (LGM) ca. 20 ka, only a small, thin ice sheet advancedon to the coast of the Taymyr Peninsula (Fig. 16C). It mayhave originated in the present-day very shallow, and atthat time dry, northeasternmost part of the Kara Sea shelf(including Severnaya Zemlya), but also could have been theresult of an ‘ice-lobe surge’ from the Novaya Zemlya centredice cap. Its extent on present land was limited to lowlandareas around the Lower Taymyr River valley, but nonethe-less reached ca. 100 km inland. No ice-dammed lake seemsto have formed north of the Byrranga Mountains during thisperiod, but the drainage in the Lower Taymyr River valley wasalso now reversed towards the south and the drainage from theice led to an elevated water level and increased sedimentationrate in the Taymyr Lake basin (Moller et al., 1999a; Niessenet al., 1999). This glacial event seems to have been of a

Figure 15 Model of the ice-dynamics during the late Weichselian ice advance on to northwesternmost Taymyr. A cold-based ice sheet formed onthe Kara Sea shelf (phase 1) and during its advance on to permafrozen shallow shelf areas and present land large amounts of debris were frozen toits base (phase 2). Owing to a change in ice dynamics, possibly related to damming of subglacial water, the ice sheet started to move rapidly over adeforming bed (phase 3). When the driving force from behind became too weak for the ice-sheet front to advance further it froze to its bed,resulting in compressive flow at the margin (phase 4). Arrows represent geothermal heat, ‘+’ temperatures at or above the pressure melting pointand ‘−’ freezing temperatures/permafrost

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Figure 16 Reconstruction of the history of the North Taymyr ice-marginal zone (NTZ). After retreating from its maximum position south of theByrranga Mountains, the early Weichselian ice sheet stood still at or readvanced to the NTZ on the northwesternmost Taymyr Peninsula (A). Alater, middle Weichselian ice advance reached its maximum at the NTZ (B). During both these events, large glacial lakes were dammed in front ofthe ice sheets, the older one having a higher level (ca. 130 m) than the younger one (ca. 80 m). During the last glacial maximum (LGM) a smallerand thinner ice sheet inundated the coastal lowlands (C). This ice melted away rather quickly, but left large amounts of debris-rich dead-ice at itsformer margin. Proglacial drainage during all these stages was directed towards the south, opposite to the present northwards-draining system.Map D is for orientation and is a flat version of the area shown in the three-dimensional figures

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rather short duration—post-dating 20 ka and pre-dating 12 ka(Alexanderson et al., 2001). The collapse of the debris-rich icefront occurred around 9.5 14C kyr BP (11 cal. kyr), as shownby dated organic material (twigs) in river sediments from theWhite Lake area, dating the change of drainage from south tonorth. However, the more central, debris-poor parts of the icesheet had melted away much earlier, as indicated by terrestrialpeat on the Oscar Peninsula (Poluostrov Oskara) dated to 11.814C kyr BP (ca. 13.8 cal. kyr; Bolshiyanov et al., 2000).

Conclusions

According to presently available data, our conclusion is thatthe ice advances responsible for the build-up of the complexNorth Taymyr ice-marginal zone (NTZ) emanated from theKara Sea shelf, which during long periods of the Weichselianwas partly dry and permafrozen. The first phase of NTZformation occurred during a stillstand or oscillation duringgeneral ice retreat from the Weichselian maximum ice-frontposition well south of the Byrranga Mountains, and is dated tothe Early Weichselian. Later stages in NTZ build-up occurredduring Middle and Late Weichselian ice advances, which bothterminated at the NTZ.

The ice sheets centred on the Kara Sea were probablyinitially mainly cold-based but later developed warm-basedconditions, through melting of the permafrost base bygeothermal heating and thickening of the ice and throughfrictional heating due to increased speed initiated by hydrauliclifting of the ice. The occurrence of deforming-bed tills indicatethat the ice sheets moved by sliding over subglacial water andby deforming its own bed. The ice velocity thus had to be sohigh that it generated enough frictional heat to keep this thinlayer from freezing once the ice had advanced on to the thickpermafrost on the shallow shelf and present-day land areas.Thus, the existence of thick permafrost does not necessarilyimply cold-based ice-sheet conditions. During the Middleand Late Weichselian ice advances towards the NTZ, the iceeventually became too thin and the velocity too low near themargin to maintain the deforming bed, which subsequentlyfroze. Thus, the ice margin lost its ability to move and frozeto its bed. As ice from behind still pushed on, compressionalflow developed at the margin, resulting in deformation ofice and sediment. Large thrust-block moraines were formedand much debris was brought from the base to the surfaceof the ice sheet at its front. Some excavational erosion alsooccurred close behind the ice margin, as demonstrated byice-marginal basins with moraine ridges on their distal fringes.The subglacial drainage system was probably a distributedone, but in some marginal areas it changed to a discretesystem, partly caused by steep hydraulic gradients and therelease of trapped subglacial water during deceleration at themargin (Kamb, 1987). Sediments carried by meltwater duringthe different build-up stages of the NTZ were deposited assubaqueous fans, deltas, kames and sandar, all depending onthe influx position with respect to proglacial lake-levels at thetime of deposition.

The landsystems of the NTZ generally correspond to thosedescribed for a subpolar ice-sheet margin, but there aresome differences, mainly the lack of subglacial landforms.In addition, the cold-based margin seems to have developedonly when the ice reached its maximum extent, not during itsadvance. Compressional flow against an ice margin frozen toits bed is indicated by the large-scale deformation of sedimentand ice within the 2–3 km wide thrust-block moraines. In

the former ice-marginal areas there are still large amounts ofremnant glacier ice under a thin cover of diamicton and thissupraglacial landscape is slowly forming and will eventually,if the processes continue, develop a hummocky terrain withnumerous ice-walled lake plains and kames. The adaptation toa non-glacial climate and hydrology has included changesin fluvial character, lake formation, deposition of aeoliansediments and the development of patterned ground.

Acknowledgements This work has been carried out as a cooperativeventure between Lund University (Sweden) and the Arctic and Antarc-tic Research Institute (St Petersburg, Russia), within the EuropeanUnion financed ‘Eurasian Ice Sheets’ programme (contract no. ENV4-CT97-0563) and under the European Science Foundation’s QUEEN(Quaternary Environment of the Eurasian North) umbrella. Financialsupport also has been received from the Royal Swedish Academy ofSciences and the Swedish Environmental Agency (SNV). The logis-tics have been handled and largely financed by the Swedish PolarResearch Secretariat via the St Petersburg based INTAARI company.Ulrik Jansson and Charlotte Jonsson are appreciated for their valuablefield assistance, Laila Johannesson for her useful suggestions duringmanuscript writing and Ingela Bergenrud for checking the language.The reviewers Jan Mangerud and Martin Siegert provided constructivecomments on the manuscript. We also wish to thank Dmitry Bol-shiyanov (AARI), Svend Funder (Geological Museum, Copenhagen),Gennady Schneider (CAGRE, Norilsk) and Marit-Solveig Seidenkrantz(Aarhus University) for providing access to unpublished data. Thesedimentary logs from this investigation are included in the PANGAEAdata base at www.pangaea.de.

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