| Late Cenozoic development history of the arctic seas sedimentary cover |
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TARASOV G. A. Department of geology and geodynamics MMBIKSC RAS, Russia Investigations on bottom relief nature and underlying sediments in Arctic seas have a long history (Klenova. I960).Starting since the 70- ies qualitatively new data have been obtained and new literature devoted to lithology, stratigraphy, thickness and age of sediments in the upper sedimentary cover, geological- geophysical structure, geomorphology of the bottom in the region appeared in connection with the studies of oil- gas bearing perspectives in the Arctic shelf. Late Cenozoic glacial shelf of the West Arctic Seas is a large sedimenthological province developed in the conditions of global and deepest climate change. Glaciers of the Quaternary glaciations especially during last 20 thousand years played a decisive role in the processes and way of Pleistocene sediments formation. Analysis of the fact materials available proves that in the limits of the West- Arctic continental margins there widely developed sediments complexes referring to the interval Late Pleistocene- Holocene sediments accumulations- that is deposited during last glacial maximum (20-18 thousand years. Late Vislinian, Late Valdai, Viscontian, Late Vermurian, Sartanian). It is considered that sediments of Middle and Early Pleistocene age are removed by later glaciers, at least in the northern shelf areas. Fragmentary they might be preserved in the depressions and in the relief cuttings of the before Quaternary surface. Early Pleistocene sediments have been recently marked by different scientists in the west of the Barents Sea as Quaternary formations (Solheini, Kristoffersem, 1984). Pliocene- Quaternary and marine sediments on the Kola shelf (Samoilovich, 1982; Samoilovich, Skorobogatko, 1986), Neogene - Quaternary glacial- marine and marine sediments on the Kanin- Pechora shelf (Krapivner et aL 1986; Gritsenko, 1986). Comparing heterogeneous geological- geophysical material with the data obtained during studies of column and core sections of loose sediments, estimations of absolute age of the coastal terraces on the Spitzbergen, Franz Josef Land and Fennoscandia and with bottom relief the following picture of contours of the last glacial maximum can be drawn in the limits of the Barents- Kara region. It agrees well with the concept of limited distribution of glaciers and glacial caps in the Barents Sea and its surrounding forwarded by G. G. Matishov already in the 80-ies (1984). During last regressive stage with falling of sea level by 120-140 m in the north of region Spitzbergen and Franz Josef Land archipelagoes, Bear island, Hope island were covered by glacier under the unified cap which center was located in the frames of the submarine Perseus and Hope elevations, that is to the east of the Edge island and Northern-East Land. Almost analogous picture is given by Yu. A .Lavrushin(1989) and S. Fomian and colleagues (Fomian et al 1995). Southern glacier limit on the west of the Barents Sea approached approximately 73°N, occupying completely Bear elevation and partially Bear trench. Then it passed along 75° N to the east till the Central elevation touching its north-west slope and stretched further to the north -east. On the west and north of Spitzbergen and in the north of the Franz Josef Land glacial front did not leave the shelf limits. Deep water graben-trenches Franz Victoria and Saint Anna were filled with the glacier, hi the center of dome the glacier thickness was of the order 2000-2500 m3 that in the marginal areas- up to 500m. In the initial stage of glacial shield formation the center was located in the archipelago limits, during the process of glacier growth it center shifted beyond the limits of islands into the shelf area took place. Correspondingly, glacier lay densely in the shelf filling in all relief depressions. The major directions of glacial diffluence were southern and southeastern, which depended completely on the atmospheric circulation in the Arctic in general and in its turn on the input of sediments for the nourishment of glaciers. It should be borne in mind that at the start of Late Glacial period warm North-Atlantic waters penetrated through the Greenland Sea far to the north like modem West Spitzbegen current Gulf Stream. The major arguments to confirm this location model of glacial cap of the last placation maximum on the north of the Barents Sea are as follows: 1). significantly small thickness of Quaternary sediments (not more than 5-15m) in this region in comparison to the southern areas of the Barents Sea (Gurevich, Musatov. 1992; Musatov, 1996): 2) availability on the submarine elevations and uplifts of the final moraines laid most frequently directly on the bedrocks (Mesozoic) rocks (Dibner, 1978; Elverhoi, Solheim, 1983; Matishov, 1984; Solheim et al., 1988); 3). the character of the elevated coastal terraces and relative velocities of glacial-isostatic elevations of archipelagoes (Forman et al, 1995). The Novaya Zemlya glacial shield had significant areas on the Northern (Sevemy) island though as far as its dimensions and thickness are concerned it yields significantly to glaciation of the Barents Sea north. The glacier flew to the greatest degree to he west but not more than 100-120 km away from the archipelago. In the Novaya Zemlya north glaciers stepped down to the eastern board the Eastern Novaya Zemlya trench. It should be noted the more acceptable reconstruction of the Novaya Zemlya glacial cover is presented in the papers by G. G .Matishov (1984), V. Yu. Biryukov (Biryukov et al., 1988); Yu. A. Lavrashin (1989); Yu. A. Pavlidis et al. (1990); Yu. A. Pavlidis (1992) though morphological parameters have been most probably overstated. Distribution of final moraines of the Novaya Zemlya glacial cover on the insignificant distances from the coast testify to it (Spiridonov et al., 1986; Lavrashin, 1989; Pavlidis, 1992 and others). Fennoscandinavian glacial cover during Late Pleistocene period (maximum of the last glaciation) is accepted in the reconstructions by O. Holtedal, 1964; A. Aseev, 1974; A. A. Velicliko, 1979 etc. Thickness of the Scandinavian covering glacier during maximum of the last glaciation is estimated as 2650- 3750 m, approximately 3000 m in average with the diminishing in the east direction. The most insignificant and little active glacial lobes were located in the eastern part of the Kola Peninsula. They stepped down to the to the Barents Sea not farther than 50-100 km whereas the glacial edge of the Scandinavian glacial cover on the Norwegian Sea shelf approached the upper brow of the continental slope. During maximum of the last glaciation Barents Sea region glaciers widened in the southern direction. Free of glaciers marine corridor passed along the Scandinavian coast through which Atlantic waters penetrated into the north -eastern areas of the Barents Sea -into the Central depression which was not occupied by glaciers Atlantic waters inflow did not cease but was much weaker than nowadays. During transfer from the glacial to the inter-glacial period as the result global wanning of climate the whole periglacial zone was subjected to the enforced transformation. This the major and decisive role in the morpho and lithogenesis is given to the melted subglacial waters as the deglaciation passes «geologically» quickly, throwing an enormous amount of melted water into the surrounding territory. Geological role of this water increases mutifold together with and with participation of other glacial and periglacial relief forming agents. Up to now investigators studying glacial and periglacial phenomena and processes in the scales of seas and oceans paid little attention to this problem, especially domestic scientists thinking that glacial role in the marine conditions is insignificant, hi this problem G. G .Matishov (1984) is the first he earned out analyses of decoding of the large number of echograms for the first time grounded and showed glacio-exogenous role of melted waters in the formation of the ran- off- channels on the West Arctic seas shelf Lithological - facial analysis of loose strata sections, obtained by ground samplers and during sea drilling in different parts of the Barents, Norwegian, Pechora and Kara Seas showed that under a thin layer of modem sediments more massive formations are widely distributed. According to the set of properties determining the strata structure these deposits might be considered as shelf periglacial formation. Correspondingly, accumulation of such large amount of sediment material over a large shelf area cannot be considered outside the glaciers positions as this stratum was formed in the polar latitudes in the extremely severe climatic conditions affected directly by glaciers covers of the Last Pleistocene glaciation. Upon the whole formations of two generations are revealed on the Arctic seas shelf, these are Late Pleistocene glaciers sediment strata and younger glaciers marine sediments.References |