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GEOLOGIC DEVELOPMENT OF THE BAIKAL RIFT

Source:  Geologic Development of the Baikal Rift // Lake Baikal: the Past, the Present and the Future: Atlas. – Irkutsk, 2005. – p. 16-20.

Geologic history and Lake Baikal origin have been studied well by now. It is possible to find out the main stages and peculiarities of the Baikal rift development and origin on the ground of research and many other modern sources.

The earth’s crust territory on which the present Baikal cavity is located has not been covered with sea waters since the Paleozoic period of the Earth history (more than 500 mln years ago), e.g. it has been a piece of land raised over the sea level for almost half milliard years. Thus, from 500 mln years to 70 mln years ago there was the longest pre-rift period which determined the tectonic preparation for Lake Baikal origin on the territory of Southern Siberia.

The next rift megastage embracing the period from 70 mln years ago to the present, is subdivided into two stages: the pre-rift one and the rift one proper.

The pre-rift stage (the upper Cretaceous-Eocene; 70-35 mln years).

At that time the Baikal land was plain and plain-hilly. Within the present contours of Lake Baikal there were low hilly mountains up to 100 m high among which the first not very deep lake cavities (Lake Baikal prototypes) formed in the south, in the lower parts of the relief. According to some data (Salop, 1967) the Praselenga river crossed the arch (vault) raise from the north-east to the south-west and continued in the canyon-like graben which there is in the present valley of the Angara river. In the place of the present Ushkanyi islands an opening of alkaline-basaltic volcanism is found, by the only paleovolcano on the territory of the present-day Baikal.

The climate of the period can be defined as warm and damp one. The average year temperatures were about +15-20°C which is proved by finds in the Eocene and Oligocene sediments of the pollen of the coniferous and deciduous forests as well as tropic and subtropic forms. They are myrtles, sumacs, araucarias, miricas, nissas, magnolias, palms and cedars. There is also the pollen of tsugas, water pines, ketelirias, elm-trees, hazels, palms, chestnut-trees, oak-trees, beeches, magnolias and others (Belova, 1975).

The rift stage (3.5-0 mln years) is subdivided into the Protobaikalian (35-3.5 mln years) and Neobaikalian stages (3.5-0 mln years). (Mats, 1990).

During the Protobaikalian stage deepening and widening of the lake basins preceding Lake Baikal was going on. The lakes were several hundred metres deep. Shallow lake flanks occupied the territories of the south-eastern and north-western shores of the present-day Baikal. The gulfs of the lakes occupied the lands on the north of Svyatoi Nos cape and came to the part of the present-day Maloye Sea. In the Pliocene the lake basin in the north of the Baikal cavity started forming, the Barguzin river valley had formed.

Since Oligocene the endemic molluscs of Baikaliiodae and Benedictiidae had settled in the lakes of the Prabaikal (fore Baikal, the original Baikal) (they are found on the south-eastern shore of Lake Baikal). And the family of Lubomirskiidae molluscs are found in the Tunka cavity which provides evidence that these two cavities are hydrologically connected. More than 50 species of molluscs of these families have been found. In the sediments of the Pre-Baikalian sagging the molluscs of Opeas and Gastrocopta were found which gives evidence about warm subtropical climate (similar to the climate of the present-day China) during the Miocene-early Pliocene period. The average year temperatures are +10-12°C. Since the Pliocene period fish of the Cottidae family (bull-head fishes and golomyanka) had spread. Since the middle Miocene (about 15 mln years ago) mass development of diatome water-plants had started; there are more than 700 species of them in the present-day Baikal.

Throughout the Pliocene period coniferous and deciduous forests were developing, and herbaceous steppe forms added to them; fir-trees, pines, birches, alder-trees, elm-trees, lime-trees were also developing. All these give evidence about transition from subtropical to moderate climate in the late Pliocene.

In the Miocene-Pliocene sediments  remains of rodents are found – jerboas, hamster-morphous, root-dental hares, vole-mice, gophers, there were also musk-deer and gluttons. (Mats, 1990).

The Neobaikalian stage can be subdivided into three sub-stages: Upper-Pliocene-Eopleistocene, Pleistocene and Holocene, or modern one.

The Upper-Pliocene-Eopleistocene sub-stage (3.5-0.8 mln years) was marked by the significant general raise of the Baikal vault though the rivers were flowing primarily into the Lena at that time, not into Lake Baikal, e.g. the Pragoloustnaya, the Prabuguldeika, possibly the Prairkut or the Praangara rivers were flowing into the Lena basin. The deepened lakes of the southern and the central hollows joined together, and the united lake basin of the southern and the central hollows was formed. The northern hollow was also increased almost up to the present-day size including the eastern coast of Lake Baikal (the Davsha hollow), but the land crosspiece between the southern and northern Prabaikal remained, at the place of the Malomorsky gulf existent in the previous stage there was also land.

For Eopleistocene the species of Planorbis and Heli-corbis are characteristic, as well as the Corbicula genus which is known in the Tunka hollow and in the Pre-Baikal sagging as well as in Middle Asia, in the Caucasus, in the Primorye (the Far East of Russia) and in Sakhalin island.

It is probable that in the late Pliocene and Eopleistocene when there were deep lakes with low deep-water temperatures, the Baikal seal (nerpa) got into the Baikal and the golomyanka fish took its place in the ecosystem.

Deep-water species of diatome water-plants appeared in the late Pliocene-Eopleistocene Baikal, such as Cockneys, Navicula which also inhabit the present-day Baikal. The characteristic kinds of vegetation around Lake Baikal are pine-trees, birches, abies (silver firs), oak-trees, cedar-trees, hornbeams, ash-trees and alder-trees. Cedar creepers appear too. The herbaceous are represented by the wormwood, the lycopodium and the goose-foot.

The average year temperatures of that period are about +5°C.

The Pleistocene sub-stage (0.8-0.01 mln years). In the Pleistocene period the most significant reconstruction of the vault raise and the Prabaikal cavity relief took place. In the early Pleistocene period the mountains of the western coast of the Baikal hollows were rising intensely, as a result of which the Lena river flow was broken, it also influenced sharp raise of the level in both Baikal cavities. The waters from the northern cavity got into the Maloye Sea and the northern and the southern cavities joined into the united Baikal lake. The high level of the Baikal waters caused the formation of the Irkut flow, but in the mid-Pleistocene period there was no Angara river yet, flowing out of Lake Baikal.

The most significant event of the mid-Pleistocene (starting about 300 thousand years ago) was freezing of the Baikal mountain region. A great ice shield, with the area of more than 400,000 sq. km (Salop, 1967; Bukharov, 1996 and others) took the space of the Baikal-Patom plateau and the Baikal and Khamar-Daban mountain ranges. The ice tongues together with icebergs broken off them went down the valleys into Lake Baikal where they were melting filling the Baikal cavity with glacial waters.

The mountain-valley freezing was going on with some intervals until the late Pleistocene. In the late Pleistocene the Baikal cavity was formed in about the present-day contours. Also in the late Pleistocene the Lisvennichny gulf and the Angara river-head in its present-day shape formed as a result of a tectonic, evidently catastrophic block sinking. The Irkut flow closed.

The characteristic forms of molluscs in the early Pleistocene were Baicalia angarensis which were widespread in the South and North Baikal.

The average year temperatures of the early Pleistocene were not more than +2-3°C.

In the late Pleistocene abies (silver firs), fir-trees, cedar-trees, larches, alder-trees, birches and cedar creepers were growing. The herbaceous were the wormwood, the ephedra, the lycopodium, but in the mid-Pleistocene the steppe vegetation appeared. The deciduous forests were present in the south of the Baikal land only (oak-trees, elms, maple-trees). In the glacial periods the average year temperatures were some degrees below zero (-3-5°C and lower).

The Eopleistocene and Pleistocene periods were rich in big mammals. There are finds of hipparions, gazelles (antelopes), horses, reindeer, rhinos, cave hyenas, spiral-horned antelopes, beavers, cave lions and tigers (which had become extinct by the end of Pleistocene). In the upper Angara polar foxes and foxes lived, and in Transbaikal land there were bisons, primitive aurochs, Khazar horses, kulans and wooly rhinos. Mammoths had died out by the end of the lower Pleistocene.

Speaking about the history of Lake Baikal it is necessary to say about volcanic activity which accompanied the formation of the Baikal rift hollow. In the cavity itself there are no volcanic displays known. Intense eruptions of the basaltic lavas and formation of the volcanic cones of the central type are known in the Vitim river basin, in the Tunka valley, in the mountains of Udokan and Khamar-Daban and in the East Sayan. Apparently the structural disposition of such kind separating the volcanic territories and the Baikal cavity is connected with the fact that Lake Baikal was formed not in the centre of the vault raise where the processes of tectonic extension were the most significant but it was a little shifted towards the western slope of the vault. The volcanic processes were going on in the period between 14 mln years ago (the late Miocene) to 2 thousand years ago (Holocene), i.e. before the modern period when primitive people could see volcanic eruptions. The volcanic activity is probable to be manifested in the foreseeable future which is proved by high seismic activity in the Baikal region and numerous hydrothermal springs which are located both around the Baikal cavity and on the bottom of the lake itself.

The Holocene (modern) sub-stage (0.01-0 mln years). Thus, as a result of interaction of many processes, such as mountain raises formation, inter-mountain sinking, filling the cavity with waters, freezing, climate changes, sediments accumulation, in the Holocene period a unique geological structure appeared on the Earth –  Lake Baikal rift cavity. In the Holocene period the mountain formation and the Baikal cavity sagging processes displayed in most contrasting way. The maximum depths of Lake Baikal were formed.

The Baikal region is a model for studying the geological development of our planet from the earliest periods of its development (Archaic) until modern times. All this determines the special condition of Lake Baikal region and distinguishes it from other known geological structures known on the Earth.

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