Sediment formation in Lake Baikal

Vera A. Belova, B. F. Lut, L. P. Loginova & G. K. KhursevichLimnology Institute, Listvenichnoye at Baikal, Irkutsk Region, P. O. 666016, U.S.S.R.

Keywords: paleolimnology, Lake Baikal, Pliocene, Pleistocene, pollen, diatoms

Abstract

The following stages in the formation of sediments in Lake Baikal have been distinguished. In the MiddleMiocene lacustrine deposits accumulated in the southern depression. In Early and Middle Pliocene, as well asin Early-Middle Pleistocene the bottom of the southern and middle Baikal depressions was occupied by largelakes, but the northern basin was mainly dry with a system of small lakes, streams and rivers. This accountsfor the great facies variety and the genetic heterogeneity of sediments in the northern depression. At the end ofMiddle and in the Late Pleistocene an abrupt warping of the depression took place, and a shore-line similar tothe present one was formed. Sands in the upper part of the sediments are considered to be the product ofintensive erosion which probably occurred in the pluvial epoch in the Late Pleistocene period.

Introduction

The sediments of Lake Baikal comprise a com-plex heterogenous and heterochronous formation.The sequence of geological events is not distinctbecause of the mixed facies composition of thesediments, their varying thickness, and the complexstructure of the crystalline basement which influ-ences the distribution of sediments within the lake.

An integrated study of the lake bottom, includingseismographic analysis showed that the crystallinebasement consisted of three grabens. Along withsome smaller structures these grabens constitute theSouthern-Baikal, Middle-Baikal and Northern-Baikal depressions respectively. In all these depres-sions unconsolidated sediments are distributed ex-tremely irregularly. For example, from the seismicdata (Lut 1964; Florensov et al. 1978) the thicknessof sedimentary strata at the southern end of LakeBaikal is 200-300 m, whilst at the Angara Riveroutlet it is 1 500 m and in the Selenga River delta it is3 000 m. Tectonic processes are of great significancein the distribution of the sediments. Irregular dip-

ping of different blocks of the depression results instratigraphic discordances between the underlyingNeogene. Pleistocene and Holocene sediments andthe present-day silts. In some parts of the depres-sions, particularly on western slopes, outcrops ofcrystalline basement are superposed by a thin layerof recent mud.

Of particular importance for the formation ofsediments is the character of material supply. Ex-tremely rapid movements of debris cones, mud-flows, landslide processes, and turbidity flowscreate a mixed facies composition and an incom-plete geological record.

Stratigraphy

The oldest sediments occur on the south-easternsubmarine slope of the southern edge of the lake.They are represented by dense clays of grey, blue-grey, yellow and brown colour. As a rule the clayspossess insignificant intercalations of sand. In someplaces outcrops of carbonaceous clay schists are

Hydrobiologia 103, 281-285 (1983).© Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

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observed. The total thickness of these sediments isabout 10 m (Goldyrev et al. 1975). A palynologicalanalysis of clays and carbonaceous sediments foundat the base of this sequence (Fig. 1, zone a) showedsimilarities with lacustrine and telmatic sedimentsof Middle-Miocene age (K-Ar dating) from thesouth-eastern shore of Baikal (Belova 1975).

The next part of the stratigraphic sequence con-sists of dense bluish-grey clays which contain dia-tomite intercalations. Their thickness ranges from 0to 5 metres. The total thickness of sediments asdetermined by G. S. Goldyrev is more than 12 m.They occur in the northern segment of MaloyeMore, along the submerged Academic Range andin several sections in the middle and southern de-pressions. Data presented by L. P. Loginova &G. K. Khursevich show that the main diatoms in thediatomites belong to Stephanodiscus: Stephano-discus niagarae var. magnifica, St. carconensis andvarieties especially var. pusilla. these species arecompletely absent in present day water bodies in theUSSR.

The abundance of these taxa in the sedimentssuggests that the climate was warm at that time andconsequently the sediments are probably of Plio-cene age. Within the period, Early, Middle and LatePliocene sediments can be identified.

Early Pliocene deposits are found along thesouth-eastern end of the South-Baikal depression.They occur with a stratigraphic discontinuity onsediments of the Middle Miocene period. The dia-tom and pollen content of this material is shown inFig. 1, zone b. A similar flora was found in lakesediments of the upper horizon of the Kluevskyfacies complex and the 'blue' Anosovsky clays isdated as Early Pliocene (Belova 1975).

The thickest sediments (4-5 m) of this age arefound in the region of the Academic Range betweenthe northern and middle depressions. They are rep--resented by an alternation of clays and diatoms.The most common diatom is Stephanodiscus nia-garae var. magnifica (Fig. Ic).

Similar diatom assemblages were found in blueclays in parts of the middle of Olkhon Island. Theseclays are covered by subaerial sediments with theremains of rodents of Middle Pliocene age (Ada-menko 1975; Mats & Pokatilov 1976). Bottomsediments and coastal exposures are characterizedby lacustrine deposits which contain pollen andspores with the following composition: dominants:

Picea sp., subdominants: Abies sp., Ulmus sp., andassociates: Tsuga sp., Pinus sp., Corylus sp., Caryasp. Tilia sp., Juglans sp., Salix sp. (Belova &Endrikhinsky 1980).

The distribution of clays of great thickness andtheir associated diatom assemblage make it possi-ble to reconstruct the contours of the Early Plio-cene basin of the lake. They embrace the middle andnorthern parts of Maloe More and extend nearly tothe Barguzin Bay. An extensive lake existed inEarly Pliocene time on the south-eastern coast ofBaikal. The contours of this water body do notcoincide with those of the present-day lake which ismore to the south. The climate of the Early Pliocenewas warm and humid.

A sharp change to more arid conditions accountsfor the absence of diatoms in the beds of the densegrey-bluish clays, 1-1.5 m thick (Fig. 1, zone d).

Sediments contain the following pollen andspores: dominants - Artemisia sp., subdominants -Betula sec. Albae, Ulmus sp., associates: Ephedrasp., Chenopodiaceae, Caryophyllaceae, Fabaceae,Quercus, Ilex sp. This assemblage is similar to thatfrom the Middle Pliocene of Priolkhonye in itsfloristic composition (Volkova & Belova 1981).Diatoms with this pollen flora have been foundtogether only on the Academic Range. The MiddlePliocene desiccation was likely to cause a decreasein the area of the lake basin and a reduction in thediversity of the diatom flora. Lake deposits withinthe Maloe More area are disturbed by subaerialerosion and the development of gorges, slopes etc.

A marked change in the composition of the sub-dominant diatoms is observed (2-3 m thick) on theAcademic Range (Fig. , zone e). The palaeofloris-tic assemblage represents a Late Pliocene stage ofthe lake at the boundary between the northern andmiddle Baikal depressions. The area of this basin ismuch smaller than that of the Early Pliocene andevolved under conditions of dry climate, steppe andwood-steppe landscape.

Analogous sediments of the same age have notbeen found in the southern and northern basins ofthe Baikal. They were probably buried beneathyounger sediments of considerable thickness.

The following section of the lithostratigraphy ofLake Baikal is represented by very dense gray clays.They include an admixture of inwashed sands andgravel. In places sand and gravel make up smallintercalations. In the upper segment of the section

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there is a distinct alternation of sands and clays witha thickness close to one metre. According to Gol-dyrev (Goldyrev et al. 1975) dense clays possessevidence of washout on the contact in which sandsoccur. Sediments of such a composition are widelydistributed in all depressions of the lake. The totalthickness of these sediments is more than 10 m.

A palynological study of sediments in this part ofthe section has been performed (Fig. , zone g) andas a result the second and the third pollen assem-blage zones have been recognized (Belova 1975).They are dated as Late Pliocene and Early Pleisto-cene sediments. The entire stratum is characterizedby the dominance of the Melosira baicalensis, in thelower part of the section, and Cyclotella baicalensisin the upper part. There is also a constant presenceof Stephanodiscus carconensis.

Of particular importance to the general strati-graphy of the Lake Baikal sediments are sands,which are widespread in the Baikal depressions.They are of polymictic character, with componentsof lighter material (quartz, feldspars) predominant.In the sand strata Goldyrev found cross-bedding,essentially rounded grains and a good grading. Allthese factors are evidence for moving water and acurrent system existing during the period of forma-tion of the sandy stratum (Goldyrev et al. 1975).

Diatoms are confined to the fine-aleurite sedi-ments and have quite a different composition thanin the underlying sediments (Fig, 1, zone h). Thereis a general predominance of Melosira baicalensisand Cyclotella baicalensis with its varieties in theupper part. A constant component of the assem-blage is Stephanodiscus astraea and varieties,which occurs quite infrequently in the underlyinghorizons. This species has not yet been observed inpresent-day Baikal sediments. Stephanodiscus as-traea var. intermedia is specific to this zone; it hasbeen found neither in older sediments nor in recentones. From experience so far this species, and varie-ties, may be considered as a representative one forsand beds.

The composition of pollen and spores is verycharacteristic for these sediments. Over the entirewater area of Baikal the sand beds are characterizedby a predominance of pollen of Pinus sibirica andPicea obovata. Among the shrubs Betula midden-dorfii and Betula exilis are of special interest. Thegrasses show that mountain-tundra and mountain-steppe elements are widespread. All these data

show that a comparatively large amount of precipi-tation took place over Baikal. It is likely that sandysediments in Baikal are the products of intensivewater erosion, which may have taken place in apluvial epoch of the Late Pleistocene. This epoch isassociated with the culmination of mountain-valleyglaciation in the region studied (Belova 1973, 1975).

Conclusion

In the developmental history of lacustrine basinswithin Lake Baikal the following stages are known.Middle-Pliocene time was characterized by thepresence of a rather deep lake in the south-easternpart of the basin. Its deposits are only partly buriedunder the sediments of the present lake. They arerepresented by marginal lacustrine swampy forma-tions.

In the Early Pliocene period a comparativelydeep and extensive water body existed in the south-ern and middle parts of the basin. The diatom as-semblage resembles those in the rift-valley lakes ofAfrica, Japan and USA, and indicates the similarityof climatic conditions that occurred in the earlyBaikal stage of rift genesis.

The formation of a single lake in the Lake Baikalbasin probably occurred in the Late Pliocene-EarlyPleistocene. The complexity of the stratigraphicstructure of the sediments of this time is associatedwith sedimentation processes during glaciation.

With regard to the history of the Baikal depres-sion two stages are distinguished. A slow subsi-dence is typical of the first (Middle Miocene-EarlyPliocene) stage. It was balanced by the accumula-tion of lacustrine sediments in the southern depres-sion. During the second (Neo-Baikal) stage twoperiods are known. In the early period (Early-Mid-dle Pliocene) the bottom of the southern and mid-dle parts were occupied by rather big lakes. Thenorthern one was dry land with a system of smalllakes, streams and rivers. In the second period (Ear-ly-Middle Pleistocene), due to an abrupt warpingof the depression the present shore-line formed, thewestern side of the depression was elevated andalong the eastern shore a lacustrine-alluvial plainformed that became the source of terrigenous mate-rial that was carried to the bottom of the lakethrough a system of submarine canyons.

285

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