Quaternary fluvial sediments in the Russian Arctic and Subarctic:Late Cenozoic development of the Lena River system,

northeastern Siberia

M. N. Alekseev & V. A. Drouchits

AL EKSEEV, M. N. & DR OUCHrTS, V. A. 2004. Qu aterna ry fluvial sedime nts in the RussianArctic and Suba rctic: Late Cenozo ic development of the Lena River system, northeasternSiberia . Proceedings of the Geologists' Associatio n, 115, 339-3 46. Thi s study summar izes theresults of research spanning man y years on the Late Cenozoic fluvial sequence of the LenaRiver in the NE Siberian Arctic, illustrating the present sta te of kn owledge regarding how itsdeposits correlate with those in other parts of the world. The Lena is a very ancient river system;Upper Cretaceous fluvial sedimen ts deposited by this river a re recorded in parts of its course.However, the present course of the lowermost Lena (the so-called Lena 'pipe' or 'truba') is veryyoung, having developed during the Pleistocene. Reas on s for the observed course changes,including glaciations and crusta l deformation, are discu ssed .

Key words: Ru ssian Arctic, Siberia, fluvial sediments, Qu atern ary, dr ain age evolution, LenaRiver

Geological Inst itute of the Russian Academy of Sciences. Pyzhevsky Lane 7, Moscow 119017,Russia (email: drouchitsiiiginras.ru)

1. INTRODUCTION

The Lena is one of the largest rivers draining fromSiberia into the Arctic Ocean , with a length of 4700 kmand a drainage basin area of 2 419 000 km2 (Fig. 1).Alth ough it is in the High Arctic, the aridity of thisarea has prevented the formation of major lowland icesheets, so long-time-scale Quaternary fluvial recordsexist.

In the Russian Arctic and Subarctic, fluvialdepositional environments are influenced by climate,which is the main factor determining sediment charac­teristics and distribution. In these periglacial andglacial regions, sedimentary compl exes can be dividedinto various types of fluvial, deltaic, alluvial fan,fluvioglacial, solifluction, lacust rine and coastal facies.A characteristic feature of all these is their formation,in this area, in association with the development ordegradation of permafrost. In the mountain areas ofthe North ern Ura ls and in the western part of theRussian Arctic Plain, the main sedimenta ry processesduring the Quaternary are known to have been drivenby the advance, degeneration and retreat of glaciers. Inthe less-familiar northeastern part of country - thearea with which this paper is pr imar ily concerned - thesituation is comparable. Thi s paper will, thus, explorethe record from the Lena , with brief reference to otherArc tic river systems.

The Lena River system is vast (Fig. I); although itflows into the Arct ic Ocean , its headwaters and thoseof its principal right-bank tributar ies, the Vitim and

Proceedings of the Geologists ' Association, 115, 339-346.

Aldan, rise at the southern margin of Siberia. TheVilyuy, the principal left-bank tributary, drains thecentr al part of the Siberian Craton, which is amajor source of diamonds brou ght to the surface inkimberlite pipes. Pro specting for diamond s (fromkimberlite pipes over a wider area than the Vilyuydrainage basin) and other valuable minerals is a majorreason for detailed study of the gravels of the Lenasystem. The Siberian Traps flood basalt province ofPermo-Triassic age occupies a vast region, with dimen­sions of c. 1000 krn, extending westward from thevicinity of Mirnyy into the Angara and Tun guskadrainage basins (Fig. I). Its eastern part, drained bythe Upp er Lena and Vilyuy rivers, presumably pro­vided the source for the igneous rocks observed insediments of the Lower Lena (see below).

The Verkhoyansk Mountain system, which runsgenerally southward from the vicinity of the LowerLena, forms the interfluve between the Lena andYana/Ind igirka river systems, furthe r east (Fig. I).This moun tain system accommodates convergencebetween the Eurasian plate (to the west) and the NorthAmerican plate (to the east) by reverse fault ing. Thishas produced a foreland basin to the west known asthe Verhkoyansk foredeep, along which the Lena flowson its reach downstream of Yakutsk. Th e (Euler) polefor relative rotation between the Eurasian and No rthAmerican plates is located somewhere near the presentLaptev Sea coastline; further north the required com­ponent of extension between these plates leads tooceanic spreading in the Arctic Ocean basin.

0016-7878/04 S15.00 © 2004 Geologists' Associa tion

340 M. A. ALEKSEEV & V. A. DROUCHITS

Major diamond field in Siberian Craton

Fig. 1. Map of the study region showing localities mentioned in the text. Most of the Lena drainage basin is in the Russianprovince of Yakutia, exceptions being the headwater regions such as upstream of Vitim, as well as the Arctic coastline betweenthe Kolyma and Khatanga rivers. The Verkhoyansk Mountain system runs generally southward from the vicinity of the LowerLena near the Arctic coastline to c. 650 N, c. 1200 E, then generally ESE to c. 63 0 N c. 1420 E, near the Sea of Okhotskcoastline. Its northern part, trending N-S, is known as the Orulgan Range.

2. FLUVIAL SEDIMENTS OF THE SIBERIANARCTIC AND SUBARCTIC

In the Siberian Arctic part of Russia, fluvial terracesediments usually consist of different sub-facies: stream

sediments, shore shoal sediments (defined below) andfloodplain deposits. There is also a characteristic typeof deposit consisting of unsorted piles and large lensesof mixed boulders, pebbles, sand and silt depositedfrom floating ice during seasonal flooding by large

QUATERNARY FLUV IAL SEDIMENTS IN T HE R USSI AN ARCTI C 34 1

Fig. 2. Late Cenozoic development of the lower catchment ofthe Lena River. The earliest Lena course is thought to be theMotorchuna- Ajakit palaeo-valley, of supposed Neogene age,located west of the modern course. The Linde-Khoryongskapalaeo-valley is thought to be younger, of Pliocene-earlyMiddle Pleistocene age, and is also located west of the presentcourse of the Lena. The Ajakit -Kelimyar palaeo-valley fur­ther downstream, linking the Middle Lena to the Olenek, isinferred to have existed at about the same time, The Sobopol­Siktyach palaeo-valley is younger still, being inferred to belate Middle Pleistocene; it pre-dates the main glacial event inthe Verkhoyansk Mountain s (thought equivalent to the LateSaalian Moscow glaciation in Europe; Tab le I) and is locatedeast of the modern course of the Lena. Sa, St and A denoteSardakh (Sardakh Khaya) Island, Stolb Island and AmericaKhaya mountain in the Lena Delta. America Khaya isso-named because its summit is the site of a memorial to theAmerican explorer George Washington de Long, who per­ished nearby with most of his crew after his ship becameicebound during an expedition in 1879-1 881 (cf. Guttridge,1986). Note that stages of time mentioned here follow theinternat ional usage; in standard Russian usage, the 'EarlyPleistocene' is equivalent to the international early MiddlePleistocene, the Russian 'Middle Pleistocene' being equivalentto the international late Middle Pleistocene. The Russian'Eopleistocene' is equivalent to the international EarlyPleistocene (i.e. the span of time from c. 1.8 Ma to c. 0.8 Ma).

km

200,Vtlyu y

Lapte v Sea

Sobopol-5iktyachpalaeo-valley:pIe-dales main

l~~~ Verkhoyansk Mts.....x glac iatIOns;

lasted to tateMiddle Pleistocene

Ajak it-Kelimyarpalaeo-valley,

~ linking the Lena~ to the lower

Olenek;abandoned inMiddJePleistocene

~Motorchuna­

~JAiakit palaeo­val ley; (7) Neogene

mrecuees ct1,'1 glacial a dva nce(from Verkho y­ansk Mts. to E)

Location of cross­sect ion where thepresent Menkere

r-;-l tributary valley~ cuts through the

depo sits 01 the lenain its Sobopol­S ,klyach palaeo­valley (Fig. 4)

rivers. During summer large blocks of ice ca n be seenin these piles, ha ving been worked into the sed imentsduring seas ona l flood s. A peculiarity of all these fluvialsedime nts is the freezing en vir onment and th e forma­tion of segregationa l ice and seco ndary ice veins.Somet imes 'macro -ice' re aches 60-70% of th esedimen t-bo dy volume because of seco nda ry ice veins.During th e short summers, the surface layer th aws toins ufficient depth to melt such ice.

The erosional bases of both strath and aggradationalterraces a re sometimes underlain by old er Qu at ernaryalluvi um o r th e muddy-cla yey dep osit s o f more a ncientva lley fills. Ages ca n be determined on th e basis ofmammal ian fauna l remains or geochro no log ica lmethod s such as radiocarbon dating, There a reimportant o utcrops in the Vilyuy, Ald an and Ady chasystems (F ig. I), as well as in the tributa ries and lowerreaches of the Kolyma (Patyk-Kara & Postol en ko ,2004) . Pebble and boulder gravel (lag gravel) and sandtypically overlie th e bedrock surface, with wash ou ttraces. Cha nnel deposit s are divid ed into two sub­facie s: 'near rac e' (deposited in the central pa rt of ariver cha nne l) and 's ho re shoal' (deposi ted in sha llowparts of a river , suc h as in point ba rs). These sedi­me nt s, deposited under a va ria ble hyd rol ogical regim e,co nsist typ icall y of coarse sa nds, co bbles and bould ers.The sho re-shoal fac ies is formed in th at part of theriver cha nnel with the gen tlest hydrol ogical regime.typified by low turbulence and low flow. Lavrush in(1963) d ivided shore-shoal sub-facies into three types:th e low , middle and upper parts of th e shore shoal.Sometimes th e shore shoa l sub-facies is represented bysandy sedi me nts, with in terbedded loams and sa ndyloams in rip ple lam inati on s, lying d irectly above th ebedro ck surface. These lamination s ind icate gentlestre am cu rre nts. Sectio ns a long north ern Sibe rianriver s ca n a lso revea l chaotic m ixtures of largeboulders, roc k debris and sa nd . Such fo rma tio ns canbe observed in the Len a upstream of its co nstrictio n ina narrow ca nyo n-like reach c. 150-1 80 km so uth ofthe delta (Fi g. I) . In the lower reach es of th e Lena ,floods can be 20-25 m above low water level. Springfloods com monly carry floating ice, with block s some­times 3 m across . Thick floating ice blocks ca use bankerosion, result ing in the deposition o f un sorted sedi­ments such as typify the Late Ple istocene andHolocene river terra ces of the Lena.

Ter race deposit s in a reas adjacent to upland glacialcen tres incl ude glacigenic and fluvioglacial o utwashfan mat eri a l. Th is situa tion ha s been observed in theLen a river terrace sections of the Verkhoyan sk fore­deep (F ig. 2). In the Zhigansk dist rict (Fig. I), o n theleft bank of the Lena , the terrace all uviu m co n tai nsblock s of till deri ved from a glacial lobe locat ed o n th eright bank of th e rive r (o riginating fro m the glac iatedVerkhoyansk mountain s to th e eas t) and carried totheir presen t locati on by floating ice.

In Arctic enviro nme nts ce rta in highl y cha racteristicfluvial sediments occur within flood pla in st ra ta(Lavrushin, 1963, 1966). Man y sma ll meandering

342 M. A. ALEKSEEV & V. A. DROUCHITS

channels develop wihin floodplains, along with oxbowlakes where channels become cut off after reachingthe limit of their curvature. This morphology is typicalin the coastal plain of the lower Indigirka and Yanaand in other regions of NE Siberia. Y. A. Lavrushinhas studied these processes near the mouth of theIndigirka. The same situation has been observed byour team in the lower reaches of rivers on the NewSiberian Islands, where the main rivers are divided intomany small meandering streams and lakes. In this casealluvial plains can reach very considerable widths,many times the width of the river itself. In the lowerreaches of the River Indigirka, 'laida' sediments can bedistinguished. This name is given to systems of flood­plain depressions, typically several kilometres in lengthand up to 2 km in width, found in coastal areas.During floods they form networks of lake basins,connected by channels and creeks, in which lacustrinesedimentation occurs. A characteristic fluvial 'laida'facies is thus created, typified by specific types ofArctic sediments.

Crustal deformation has been very important inthe topographical evolution of wide alluvial plains,especially near the northern margins of the Russianand Siberian Platforms and also in the folded areaof the Russian northeast. They are also expressedvery clearly by the geological and geomorphologicaldisplacement of some abandoned valley fragments.

Solifluction is also an important sedimentary processin Arctic and Subarctic areas. It usually occurs duringseasonal thawing of frozen ground and ice veins. Thisprocess is accompanied by the formation of cryogenictextural features (involution, cracks and layer displace­ment) and the initiation of mudflows. Sometimes theresultant mudflow deposits, with characteristic texturesbrought about by secondary freezing (Katasonov &Ivanov, 1973), become incorporated into fluvial terracesequences. Sometimes this occurs as a result of destruc­tion oftalik, which is a layer of unfrozen ground betweenthe seasonal frozen ground and the permafrost or anunfrozen layer within the permafrost (Muller, 1947).

Floodplain sediments are present in all the northernriver valleys. At the base of each sequence a pebble andsand layer is usually found. This is overlain by inter­bedded sand and silt layers, above which are beds ofloam and muddy formations with traces of poorly­developed soils. Other parts of the floodplain are com­posed of sand beds. The composition of these floodplainsediments depends on the character of the underlyingdeposits. The floodplains oflarge rivers like the Lena are15-20 m above normal river level; during major floodsnot only the floodplain, but also the first terrace isinundated. Catastrophic events related to exceptionallyhigh flood levels occurred in Yakutia in 2001.

3. GEOMORPHOLOGICAL FEATURES OF THEARCTIC COASTAL REGION

Most Quaternary sediments of the Russian Arcticcoast have been formed in a cryogenic environment,

especially in the eastern Arctic. Here, in addition tomassive development of segregated ice over a widearea, fossil and present-day ice wedges occur, penetrat­ing through cover deposits into fluvial terrace sedi­ments. In the western Russian Arctic, fossil cryogenicstructures are represented by pseudomorphs.

During the Late Pleistocene regression of the ArcticOcean, valley systems extended across the subaerialcontinental shelf. Fragmented remnants of fluvialsediments related to this marine lowstand can beobserved in what are now offshore islands. A numberof terrace sections, with alluvial and lacustrine layers,were discovered in the Balyktakh and Dragotcennayaand other rivers of Kotelnyy Island, in the LaptevSea (Makeev et al., 1989). These are typical islandrivers at present, but fragments of their depositsrelate to the older river system originating from theSiberian mainland. There are many lakes in the marinelowlands of the Arctic islands, joined by smallcreeks, representing environments of Holocene tidalsedimentation and redeposition. All of them formdistinctive fluvial systems, along the courses ofwhich are found outcrops of Pleistocene and Holocenesediments.

Deltaic sediments of the larger rivers form importantcomponents of fluvial sequences. For example, theLena River delta occupies an area of c. 20000 km2

(Fig. 2). The exposed part of this delta sequenceconsists of Late Pleistocene and Holocene sediments.Pleistocene sand beds, distributed in the western partof the delta, consist of alternations of cross-beddedand horizontally bedded channel sands, together withfine sands deposited by subsequent aeolian activity.These beds are covered by grey sandy loams, some­times with interbedded gyttja. The sequence is com­pleted by up to 2.5 m of peat. The most importantoutcrops have been recorded in the Olenek and Bulkurdistributaries (Fig. 2). Remains of Late Pleistocenefossil mammals are preserved in the frozen silty, loamyor sandy sediments of the Lena delta, depositedfluvially or by slope processes. The eastern part of thisdelta, where the main distributary is now located,consists predominantly of Holocene sands, sandyloams and loams. The boundary between the westernand eastern parts of the delta is formed by a series ofstructural highs of Palaeozoic basement, which riseabove the low delta islands. These Stolb, AmericaKhaya and other local highs follow a N-S trend andmark an active fault zone that bisects the Lena delta(Fig. 2). Beyond its delta, the Lena forms a hugesubmarine fan, the study of which remains incomplete.The main sources of clastic material for this deltaicsedimentary complex are local rocks and also clastsof basalt from the Late Permian Siberian Trapsflood basalt province, transported from the southwest(Fig. I). Strongly weathered boulders and smallerclasts of basalt are observed in the Miocene deposits ofSardakh island, in the right bank of the Trofimovchannel within the southern part of the Lena delta(Fig. 2).

QUATERNARY F LU V I A L SED IMENTS I N TH E RUSSIAN A RCTIC 343

km

Ie) Present-day

100

Ib) late MiddlePleistocene

Fig. 3. Evolution scheme of the Olenek and Lena river systems to the north of latitude 70° N. Areas of river confluence ordrain age diversion (after Zhu ravlev, 1960) are marked with diagonal shading. (a) In the early Middle Pleistocene. the Lenaflowed west of Cheka novsky Ridge, where the Olenek joined it as a left-bank tributary, reaching the (present-day) coastlinec. 250 krn west of the present Lena delta . The present cour se of the lowermost Lena car ried only local drainage. ThePalaeo-Kengdey was a separate parallel river furt her east. (b) In the late Middle Pleistocene, the Lena was captured orotherwise diverted at the southern end of Chekano vsky Ridge into its modern course east of the ridge. A local north -flowingdrainage system known as the Palaeo-Kelimyar was initiated in the former Lena course along the west side of ChekanovskyRidge, forming a right-bank tributary of the Olenek. The Palaeo-Kengdey was diverted into the lowermo st Lena and its formerseparate course further north was abandoned . (c) Subsequently, the Ajakit cut through the southern part of ChekanovskyRidge and captured the headwaters of the Palaeo-Kelimyar. The drainage divide west of Chekanovsky Ridge moved northfrom its southern end to its mid-point, where the present headwaters of the Kelimyar are located. Selected spot heights areadded to (c) to convey an impression of the relief of this region and to indicate amounts of fluvial incision that have occurred.At present the Lena is no more than a few tens of metres above sea-level along this reach.

D uring marine lowstands, the large rivers of theSiberia n Arctic crossed the subaerial con tinentalshelf to mouths located near the continental margin,hundreds of kilometres north of the modern coastline(Patyk-Kara & Postolenko, 2004). Traces of theseancient river valleys arc revealed on bat hymetric mapsby linear depressions, filled with younger sediments,which can be followed down to the 50-70 m isoba th inthe Laptev and East Siberian seas. Further north,canyons cut into the con tinental slope have provided

routeways for the transport of clastic sediment ofterrestrial, fluvial origin into the Arctic Ocean basin.

The distribution of fluvial sequences (includ ingfluvial, fluvioglacia l and lacustrine components) inno rthern Russian Arctic coastal regions is relateddirect ly to Pliocene and Quaternary crustal deforma­tion . An example is the history of the Palaeo-Lena andthe evolution of the Lena valley in the area of theLena-Olenek interfluve to the south of ChekanovskyRidge (Strelkov et al., 1959; Zh uravlev, 1960; Fig. 3).

344 M. A. ALEKS EEV & V . A . DROUCHITS

Ob servations in the Ajakit river va lley (Figs 3b , c)indicate extensive distribution of sandy allu vium (theresult of significant post-depositional winnowing) inthe wide depression to the east of the lowermostOlenek valley. Geological evidence gathered byZhuravlev (1960) testifies to the existence of thePalaeo-Lena in areas occupied today by the Ajakit andKelimyar. These observations suggest that the Palaeo­Lena joined the OIenek and flowed into the LaptevSea to the west of its more recent mouth and delta(Fig. 3a). In the late Middle Pleistocene-Late Pleisto­cene , as a result of active crustal deformation, rivercapture occurred between Tuora Sis Ridge (Fig. 3) andChekanovsky Ridge, resulting in the diver sion of theLena along its modern-day canyon, the so-called Lena'pipe' or ' truba' (Figs 3b , c). This gorge is up to400 m deep and c. 2 km wide, much narrower thanelsewhere on the Lower Lena where this river is upto 10 km wide . Study of the terrace sequence in thelower reaches of the Lena has shown that, in thesection near the confluence with the Ajakit, onl y LatePleistocene terrace levels can be traced. This alsosuggests the existence of a Palaeo-Lena channel,located further west, during earlier stages of thePleistocene,

Research in the lowermost Lena has revealed evi­dence of sign ificant modification of parts of thi s valley.It was discovered by the USSR Geological Survey(Kolpakov, 1966) that realignment of this valley belowthe Vilyuy confluence (Fig. 2) was the result ofQuaternary glaciation in the Verkhoyansk Mountainsand active deformation along the boundary betweenthe Verkhoyansk fold belt and the Siberian craton.This valley realignment took place in the region ofthe Verkhoyan sk foredeep and the folded are a of thecraton margin . The most significant result of thisprocess ha s been the evolution of the Sobopol­Sikt yach palaeo-valle y of the Lena (Figs 2, 4). Thewestward displacement of the Lena valley to its currentposition ha s been established by investigation of ter­race and glacigenic sediments in this ancient valley(Kolpakov, 1966). Interbedded glacial sediments in theLena fluvial sequence show that this displacement tookplace at the end of Middle Pleistocene, during anextensive glaciation of the Verkhoyansk Mountains.Remnants of morphologically expressed surfacedepression s filled with fluvial deposits (palaeo-valleysof the Lena), as well as blo cks of glacial diam icton,f1uvioglacial and lacustrine sediments, sur vive inpre sent right-bank tributary valleys, notabl y those ofthe Sobopol, Menkere and Dzhardzhan (Fig. 2) .

The ancient Linde-Khoryongska valley (Fig. 2), onthe west (left) side of the Len a, is marked by a widebody of sand several tens of kilometres wide. It loopsacross the Zhigansk area, where blocks of till ofVerkhoyansk origin have been mapped in terracesections. Observations show that this ancient Lenapalaeo-valley pre-dates the late Middle Pleistoceneand ma y date from the Pliocene to the early MiddlePleistocene.

EastWest

•• , :1I1t1> • • •. . ..1--=-:='="'''''''0 ' do'0 0 <> 0 0- --Fig. 4. Generalized stratigraphic sequence of the Sobopol­Siktyach palaeo-valley of the Lena (see Fig. 2 for location),based on Ministry of Geology of the USSR (1986), not toscale. Subdivisions of the Pleistocene used in this figure andits caption follow international usage, with Russian terms inbrackets (cf. Table I). Note the staircase of tributary terracesinset into an older staircase of Lena terraces. Stratigraphicsubdivisions (in stratigraphical order):

alV, Holocene alluvium;alii - IV, Upper Pleistocene-Holocene alluvium;fIll sr, Upper Pleistocene fluvioglacial deposits, Sartanianhorizon;gIIlsr, Upper Pleistocene glacial deposits, Sartanianhorizon;aIJIkr, Upper Pleistocene alluvium, Karginian suite;aIll mr, Upper Pleistocene alluvium, Murutkanian suite;lall -III, lacustrine alluvium, late Middle Pleistocene­Upper Pleistocene deposits (the Russian Middle-UpperPleistocene);glls and Iglls, late Middle Pleistocene (the RussianMiddle Pleistocene) glacial and limnoglacial deposits ofthe Samarovian horizon (of probable Early Saalian age;Table I);alltb, early Middle Pleistocene (the Russian EarlyPleistocene) alluvium, Tobolian suite (deposited by theLena);alp, early Middle Pleistocene (the Russian EarlyPleistocene) alluvium of the Peleduy horizon (depositedby the Lena).

The Motorchuno-Ajakit palaeo-valley (Fig. 2) hasalso been mapped northwards from this region, typi­cally sepa rated from the modern Lena valley by a200 m high interfluve. Geomorphological observati onsin this region rai se the possibil ity that at the beginningof the Pleistocene a t least one Lena channel wasorientated towards the present Olenek valley (F ig. 3a)and its mouth. G eomorphological stud ies combinedwith research int o Quat ernary crustal deformation andstratigraphy have established that the Lena valley hasmigrated along th is reach . The ob served migration ofthe Lena river valley complexes may relate to westwardoverthrusting of the Orulgan mountains onto theeast ern margin of the Siberian platform. The north­trending course of the Lena downstream of its Ald anconfluence directly a buts the front of the Orulganmountain range, as evidenced by the asymmetry of itsvalley, with a cliff line along its right bank and a broad

QUATERNARY FLUVIAL SEDIMENTS IN THE RUSSIAN ARCTIC 345

Table 1. Late Cenozoic stratigraphy of Yakutia.

Stage Climate Notes

HoloceneHolocene Interglacial Since II ka

Glacial and periglacial environments. Cold stage maximum at 20-18 ka

InterglacialGlacialInterglacial

Upper PleistoceneSartanian GlacialKarginian InterglacialMurutkinian Glacial Glacial and periglacial environmentsKazantcevian Interglacial The Mikulin interglacial of western Russia; Eemian of western Europe

late Middle Pleistocene (Middle Pleistocene in Russian terminology)Tazovian Glacial The Moscovian (Moscow) glacial of western Russia; Late Saalian of W Europe.

Main glacial stage of Verkhoyansk MountainsEquivalent to the Middle Saalian interglacial of western EuropeThe Early Saalian glacial of western EuropeThe Likhvin interglacial of western Russia, the Holsteinian of western Europe

ShirtinskSamarovianTobolian

Distinguished only on geomorphology

early Middle Pleistocene (Lower Pleistocene in Russian terminology)Pokrovian Periglacial Possible equivalent of the Elster glaciation of western EuropePeleduyan Interglacial Correlated with the Tiraspol stage in Europe

Early Pleistocene (Eopleistocene in Russian terminology)Tustakh Probably cold Distinguished only on geomorphologyTchekendey Distinguished only on geomorphology

PlioceneTabaginian

This table summarizes results from the Lena and its tributaries (including the Vilyuy, Aldan and Linde; Figs I, 2), including data frompalynology, mammalian biostratigraphy and magnetostratigraphy (notably on the lower Aldan), with radiocarbon dating for the LatePleistocene and Holocene. A key locality is on the right bank of the Lena upstream of Yakutsk.

shallow left-bank valley side. The most recent west­ward migration of this reach of the Lena, from theSobopol-Siktyach palaeo-channel to the modernchannel, may thus reflect continuing growth of thismountain range by active overthrusting. The largelowland around the Vilyuy-Lena confluence (Fig. 2)marks the Vilyuy Basin (i.e. sedimentary basin, notdrainage basin), the part of the Verkhoyansk foredeepthat has subsided the most as a result of the westwardoverthrusting of the Orulgan Mountains and theassociated crustal loading.

4. WIDER COMPARISON

Comparison can be made between the Lena and otherriver systems in the Russian Arctic. Evidence in theYenisey (Fig. I), from Krasnoyarsk to its mouth, hasrevealed a complex system of interrelated Pleistocenefluvial and glacial formations (Kind, 1967). It wasnoted that downcutting and aggradation near themouth of the Yenisey have been controlled directly bythe level of the Kara Sea (the regional base level).An important incision event affected the Yeniseybetween 21 ka and 18 ka, during the late Zyryan Stage.This incision was coincident with major marine regres­sion and glaciation both on the continent and thearctic islands. Like the Lena, this offshore Yeniseypalaeo-valley can also be traced in the bathymetry ofthe Kara Sea.

In the European Russian Arctic, further west, studyof the complex of fluvial sediments in the SevernayaDvina river basin has been concentrated in its tributaryvalleys, the Pasva and Vaga, where Late Pleistocenefluvial, lacustrine and marine sediments are exposed.These interglacial sediments of have yielded pollenspectra of typical Mikulin affinity (Devyatova et al.,1981), corresponding to the Eemian of western Europe(=the Ipswichian of the UK). On the left bank of theSevernaya Dvina, outcrops of the post-Mikulin Veldglacial complex occur.

In Russia the study of fluvial sediments follows astratigraphical approach, which allows classificationon the basis of age and genetic features, the recognitionof marker beds and levels and widespread correlationof sediments, processes and events. Subdivision of LateCenozoic fluvial strata is based on climatic, litho­stratigraphic, biostratigraphic and sedimentologicalbases, adopting a clear system of hierarchical andtaxonomic units and stratigraphic boundaries. Thestratigraphical scale used is that established by theInterdepartmental Stratigraphic Committee in 1995(ISC, 1996). The following stratigraphic units wereintroduced in this scale: system, super-subdivision(series), subdivision (subseries), link and step. A 'series'is roughly equivalent to the international stratigraphicterm 'group' and a sub-series to the international term'formation' and the older Russian term 'suite'. Thefiner subdivisions of this system thus correspond

346 M. A. ALEKSEEV & V. A. DROUCHITS

approximately to the international stratigraphic terms'member' and 'bed'. A 'horizon' is another Russianterm for a bed that has only local extent. The definitionand stratigraphic relations of Quaternary units aremostly based on climatostratigraphic and bio­stratigraphical characteristics. This new scale enablescorrelation of key Russian sequences with those ofother regions.

Table I illustrates the stratigraphic units that havebeen defined for Late Cenozoic deposits of Yakutia(Fig. I) and have been used for many years forstratigraphic subdivision and geological mapping ofthis region. In the lower Lena only late MiddlePleistocene-Late Pleistocene and Holocene terraces aredescribed, from several good exposures (using theinternational definition for the Middle Pleistocene; seeTable I). Older sediments are thought to have beendestroyed as a result of crustal deformation. However,further upstream, early Middle Pleistocene (='EarlyPleistocene' of the Russian definition) and EarlyPleistocene (='Eopleistocene') fluvial deposits areobserved in the part of the Lena valley between theVitim confluence and Yakutsk (Fig. I).

5. CONCLUSIONS

This study has summarized the results of many years ofresearch on the Late Cenozoic fluvial sequence of theLena River in the NE Siberian Arctic, illustrating thepresent state of knowledge regarding how its depositscorrelate with those in other parts of the world. TheLena is an ancient river system, the evolution of thelower reach of its course having been connected closelywith the long-term geological evolution of theVerkhoyansk Foredeep. However, the modernlowermost course of the Lena (the so-called Lena'pipe' or 'truba') is very young, having only developedduring the Pleistocene. Reasons for its observedcourse changes, including glaciations and crustaldeformation, have been discussed.

ACKNOWLEDGEMENTS

This paper contributes to IGCP 449 (GlobalCorrelation of Late Cenozoic Fluvial Deposits).

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Manuscript received 12 June 2003; revised typescript accepted 16 July 2004