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Fig. 14.2. Tentative character of the change in the maximum thickness of (1) syncryogenic and (2) epicryogenic sedimentary layers in the development of the Earth's lithogenesis.

adjacent territories (on account of cold and dry glacier winds). Thus, large ice covers have as a rule rather wide adjacent periglacial zones within which the low temperature permafrost exists. At the same time the permafrost can be lacking under ice covers of great thickness because of the pressure of the ice mass or because of the increased geothermal flux. In any case the thickness of frozen ground under glaciers will probably be less than that within the areas free of ice. In accordance with this it can be expected that permafrost degradation (its decrease of thickness) will be observed under the advancing glaciers and that the severity of permafrost conditions will increase on the ground after the recession of the glacier. On this basis we can conclude that the glacial cover and the permafrost will develop asynchronously, i.e. the maxima of areal development of the glaciation and of the permafrost will not be in phase. However in any case the occurrence of large ice covers indicates that permafrost also occurred, developing over larger areas than the glaciers themselves.

On the basis of the greatest glaciations determined with the help of deposits, we can recognize five time intervals (periods) in the Earth's history, when the permafrost should have been widespread on the planet (Fig. 14.3). These are early Proterozoic (2.4-2.1 billion years bp), Late (Riphean) Pro-terozoic (1-0.6 billion years bp), Palaeozoic (460-420 million years bp), Late Palaeozoic (330-320 million years bp) and Late Cenozoic (25-0 million years bp). The places where tillites were found and, consequently, the regions

Guiyang Map Geology
Fig. 14.3. Diagram of the correlation of glacial periods with tectonic epochs and the development of continental and sea areas, 02 content in the atmosphere and evolution of fauna and flora.

of glacial and permafrost development in those periods are scattered and now separated from each other by great distances. Thus, for example, traces of simultaneous glaciation during the Carboniferous-Permian period are found in Central and South Africa, in Australia, Antarctica and on the

Fig. 14.4. The proposed ice sheets and occurrence of permafrost on the continents during the Earth's history: a - Early Proterozoic glaciation (after G. Yang, N.M. Chumakov); b - Early Palaeozoic glaciation (after B. John; S.A. Ushakov and N.A. Yasamanov); c - Late Palaeozoic glaciation (after A. Wegener; B. John, S.A. Ushakov and N.A. Yasamanov); d - Late Cenozoic glaciation (after B. John); 1 - glaciation traces; 2 - areas of the supposed ice sheets and occurrence of permafrost.

Fig. 14.4. The proposed ice sheets and occurrence of permafrost on the continents during the Earth's history: a - Early Proterozoic glaciation (after G. Yang, N.M. Chumakov); b - Early Palaeozoic glaciation (after B. John; S.A. Ushakov and N.A. Yasamanov); c - Late Palaeozoic glaciation (after A. Wegener; B. John, S.A. Ushakov and N.A. Yasamanov); d - Late Cenozoic glaciation (after B. John); 1 - glaciation traces; 2 - areas of the supposed ice sheets and occurrence of permafrost.

Arabian peninsula, in the Indian mountain regions and in South America. We can explain this taking into account continental drift and the arrangement of continents relative to each other in ancient epochs, reconstructed in the context of plate tectonics over the period of the Earth's development. Fig. 14.4 shows the suggested position of the continents over the different periods of the Earth's geological development and the regions of the assumed development of permafrost, determined on the basis of the traces of the ancient continental glaciation that have been found. The data on the ancient continental glaciations and on their characteristics required for such geological reconstructions and cited below, are taken from different literature sources (E. Derbyshire, B. John, S.A. Ushakov, N.A. Yasamanov, N.M. Chumakov, etc.).

The most ancient - Early Proterozoic - glacial period (2.4-2.1 billion years bp) in the Earth's history is associated with the discovery of sedimentary rocks of great thickness (up to a few hundred metres) which are considered to have a glacial origin. They were found in the region of Lake Huron in Canada, in the eastern part of South Africa and in the northwestern part of Australia. Such stratigraphic units have great similarity to moraine and consist of debris of various shapes and sizes with fine-grained material, and can be characterized as typical tillites. At the same time bedrock surfaces and debris with glacier striations were found in the Huron deposits. The three regions of the Early Proterozoic glaciations mentioned above are widely separated now. According to recent reconstructions of the position of the continents in the past, the giant 'Proterozoic supercontinent' is supposed to have existed with two main centres of glaciation: in North America and in South Africa-Western Australia. Possibly it was precisely those regions with which large areas of the permafrost were associated in Early Proterozoic (see Fig. 14.4a).

The Late Proterozoic glacial period (0.95-0.6 billion years bp) as well as the Early Proterozoic one appear to have comprised three (if not more) individual glacial stages: about 600, 750 and 950-900 million years ago. The formation of the ice sheets took place in this case on the background of the break-up of the 'Proterozoic supercontinent' and of its rapid dispersal. A number of American researchers reason that this glaciation was the most wide-spread in the Earth's history because the ice sheets could be formed in high, middle and even low latitudes. Traces are found within practically all the present continents. The glacial drift sections were studied best of all in Scotland where tillite formations are characteristically up to 870 m in thickness and accommodate more than forty 'mixtite' horizons of undetermined origin, which can be interpreted as tillites. The mixtites are sedimentary rocks having a mixture of fine particles, pebbles and larger-sized debris which usually alternate with sandstones, aleurites and other layered and sedimentary rocks. This fact points to their formation in conditions of a shallow water shelf. Generally speaking, the Late Proterozoic tillite formations are characterized by glacial-marine cycling, indicating possible advances and recessions of ice sheets on large areas of shallow water shelf. Large polygonal fissures, filled with sandstones and conglomerates, have been found in the top of some mixtite horizons. These can be interpreted as fissure-polygonal formations (sand, and earth wedges and possibly ice wedge pseudomorphs) common to the zone of recent permafrost development and periglacial regions.

The Late Proterozoic glacial drift could, obviously, have formed under the effect of mountain glaciers; however, the majority of researchers believe the widespread continental ice sheets made the greatest contribution. At the same time the development of those glaciations in the Late Proterozoic is likely to have been associated with orogenesis followed by the rapid movement of the ancient continents which were fractured as a result. Because of this, these different continents moved towards high and middle latitudes in succession.

The Early Palaeozoic (Late Ordovician) glacial period (460-430 million years ago) has been identified by researchers in the past 15-20 years following the work of petroleum geologists in Western Africa and the Sahara. They have found firm evidence of a large continental glaciation. Sand composition is one of the main features of the Ordovician glacial drift. Sandstones with high porosity are oil reservoir rocks. Tillite formations in this case often contain giant boulders with traces of glacial striation as well as boulders and pebbles in sandstones and more rarely in aleurites. The data for the Sahara suggest the existence of not less than three glacial advances and recessions in the Ordovician. At the same time a number of sea transgressions and regressions are noted in the Late Ordovician all over the Earth. They could be associated with the alternation of interglacial and glacial periods.

By the Late Ordovician time (compared with Late Proterozoic) the fundamental change in the position of the continental plates had taken place (see Fig. 14.4b). The ancient analogues of North America and Europe in the west, were separated from each other and the supercontinent named Gon-dwana included the present South America, Africa, Antarctica and Australia. The South Pole was located in the region of the modern Sahara at that time, therefore the Late Ordovician glacial drift and permafrost were developed on the territory of the present Africa, Saudi Arabia and South America simultaneously (see Fig. 14.4b). There is also scattered evidence of Upper Ordovician boulder horizon outcrops (which are thought to be glacial drift by some researchers) in Scotland, Spain and France.

The joining together of several different ice caps could only take place during the main stages of glaciation, on the large and likely rather plane territory of the Gondwana continent. There probably existed vast areas with sharply continental climate and low negative temperatures between the several ice caps with a consequent formation of a great thickness of frozen ground. The traces of the proposed Silurian and Devonian glacial formations were found in a number of regions on the east of South America and on the west coast of South Africa. However these data need further support and are unlikely to indicate the existence of large glaciations. They most likely are the traces of mountain glaciers that have survived under favourable conditions after thawing of large continental glacial covers on plains.

The Late Palaeozoic (the Great Permian-Carboniferous) glacial period lasted 50 million years (310-260 million years ago). In this period ice sheets, permafrost, shelf glaciers and icebergs were wide-spread. It was the time of the 'assembly' of all the continents joined together in the large superconti-

nent Pangea (see Fig. 14.4c) with a consequent fundamental rearrangement of the ocean currents. Plate tectonics and orogenesis were intensive in Permian-Carboniferous time. When the plate margins collided with each other new mountain ridges were formed (Ural Mountains, Hercynian mountains of Europe, etc.). The formation of large mountain chains was accompanied by widespread volcanic eruptions, lava flows, and thermal ejections into the atmosphere along the plate margins, causing a lowering of atmospheric transmissivity and decrease in the amount of solar energy arriving at the Earth's surface. At the same time the gigantic concentration of continents would have led to snow accumulations and glaciers, causing an important increase of the amount of reflected solar radiation (at least in the region of the South Pole and in high mountain regions even near the Equator). All this would contribute to cooling of the ground, global temperature lowering and development of the greatest glacial period, as far as duration and extent is concerned. The Late Palaeozoic glaciation in the Southern Hemisphere was of continental type. The centre of that glaciation is likely to have been located in East Antarctica-South Africa, with the glaciers moving from there toward Australia, Hindustan and South America. In the Northern Hemisphere, which was for the most part oceanic, traces of the glaciation have been found only in a limited number of regions in the north-east of Siberia.

Within the greater part of Gondwana (southern part of Pangea) glacial and periglacial conditions prevailed for many million years and vast areas of polar desert covered with tundra vegetation extended in front of the ice margin. In the Permian-Carboniferous glacial period the permafrost is likely to have occupied the largest areas in the Earth's history in high as well as in middle latitudes. These areas increased and decreased repeatedly. The maximal glacial extension probably took place about 280 million years ago. The Permian-Carboniferous glacial period is likely to have had several glacial epochs, asynchronous within different continents (see Fig. 14.4c). During the main glaciations the separate thick ice sheets are likely to have joined together forming a single giant cover twice as large as the present Antarctica. Characteristically the ice sheet of Gondwana was partly situated in the sea, since glacial-marine deposits have been found in the regions of the Permian-Carboniferous ice sheets. Aggradation and degradation of the ice sheets was accompanied by sea-level fluctuations of 150-200 m during glacials and interglacials.

The most dramatic evidence for the Permian-Carboniferous glaciation has been found in the southern part of South America. Glacial formations up to 1600 m in thickness and including unstratified tillites, bedrock surfaces with glacial striations, trough valleys, stratified tillites with glacial-marine deposits, cut pebbles, osers, etc. have been preserved in Brazil and Argentina. It is supposed that the glacials covering Brazil and Uruguay were moving from a centre situated in South Africa. Such a supposition is partly based on the fact that the Brazilian tillites contain erratics of quartzites, dolomites and cherts unknown in those countries but typical of the Namibian tillites where they are often found in bed deposits. The glacial drift in South Africa and Madagascar (of up to 1000 m in thickness) is mainly represented by three types: homogeneous tillites of continental origin; stratified tillites with marine fossils; and ribbon-layered formations with sporadic inclusions of rudaceous deposits. Evidence for the glaciations in the Southern Hemisphere has been found also in southern Australia, Antarctica, India and Pakistan.

In the Northern Hemisphere the mixtites of Massachusetts, USA, formed probably under local mountain conditions, as well as the glacial-marine deposits in the north-eastern part of Siberia (for example, in the Omolon river basin at the Kolyma river inflow), are evidence of the existence of the Permian-Carboniferous glaciation.

In Antarctica the Cenozoic glacial period (generally 25-0 million years ago) is likely to have begun more than 30 million years ago, and about 8-12 million years ago the high mountain ridges of Alaska were subject to glaciation. According to S.A. Ushakov, the intensive spreading (drift) of continents took place latitudinally. In the Palaeocene the Greenland plate was separated from Eurasia and from the North-American continent and Australia was split off from eastern Antarctica. In the Oligocene epoch the collision and joining of India and Eurasia took place, causing the formation of a number of large mountain systems in Central Asia. The distribution of the continents in accordance with their present position probably occurred in the Miocene. During the Cenozoic era cooling developed nonuniformly because of the alternation of warmings and coolings which occurred at particular time intervals. It is supposed that in the Eocene the Earth's generalized mean temperature was higher than at present* by 9-10°C and at the end of Oligocene by only 4-5 °C, with the temperature decrease being more intensive in the Southern Hemisphere, and probably caused by the appearance of glaciers in the subpolar regions.

In the period when the deep strait was formed between Antarctica and Australia (in the middle of the Oligocene epoch), the Antarctic circumpolar current developed with the consequent decrease of influx of middle latitude

*According to M.I. Budyko (1), the recent temperature is about + 15°C.

waters and the strengthening of cyclonic activity. Associated with this was the origin of the ice sheet in Antarctica moving slowly toward the South Pole region. The Antarctic glaciation in turn caused a sharp increase of the surface albedo and greater cooling of this continent and, as a consequence, further lowering of the Earth's mean temperature. The glaciation in Antarctica is likely to have originated in the Transantarctic Mountains and Gam-burtsev Mountains as early as in the Oligocene (38-26 million years ago).

The glacial cover existing now is likely to have begun to form in early Miocene (25-20 million years ago).

The earliest evidence for glaciation in the Northern Hemisphere has been found in the high mountains of south Alaska where the glacial drift alternates with lavas. According to potassium-argon dating the ice sheet existed from the middle of the Miocene (about 10 million years ago).

The onset of the ice cover in the Arctic Ocean is probably comparable with the formation of the Antarctic ice sheet as far as the effect on the Earth's climate is concerned. Beginning from that time progressive cooling took place on continents on the background of rhythmic climatic changes. There were several large glaciations in the Middle and Late Pleistocene such as the Oka, Dniepr (Samarov), Valday (in the European North of Russia), Zyryan and Sartan (in Siberia). The widespread development of the permafrost is associated with these rhythmic epochs of cooling.

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