Planet

The evolution of the Earth extends over almost 5 billion years. Its history ean be divided into three significant stages which are basically responsible for the present state of the lithosphere. These are the initial or Archaean (2 x 109 years), Proterozoic (on the order of 2 x 109 years) and Phanerozoic (0.6 x 109 years) stages of the development of the geosphere and lithogenesis. During the Archaean stage of the Earth's development the vulcanogenic type of lithogenesis, characterized by accumulation of lava and loose ash material on the sea bottom with a low content of weathered terrigenous sediments, was dominant. The differentiation of the lithogenetic types is likely to have begun only at the end of the Archaean stage, being most apparent in the Proterozoic-Riphean stage with the formation of humid, arid, volcanogenic-sedimentary and glacial or cryogenic types of sedimentary rock. The first certain glacial drift of the continental type is referred to this stage. The expansion of the area of the continents caused not only glaciation and formation of specific deposits of the cryogenic type but also considerable increase of deposition of detrital and dissolved material in oceans (especially of carbonates) with the predominance of exogenous lithogenesis over the volcanogenic-sedimentary.

The cryogenic type of lithogenesis in the Proterozoic is the latest and it becomes more and more important as the recent epoch is approached. An increase in climatic severity and frequency of glaciation supports this. We can see the process clearly beginning from the end of the Mesozoic era (Fig. 14.1), that must have led to the regular and more frequent occurrence of the cryogenic type of lithogenesis. In this manner the irreversibility of the evolution of the type of lithogenesis shows itself in progressive replacement of the volcanogenic-sedimentary type at first by the humid, then by the arid type, and finally we can see a trend toward prevalence of the cryogenic type

Archaean Expasion
Fig. 14.1. The repeating intense climatic warming-cooling rhythms (glacial epochs) in the post-Archaean history of the Earth (tectono-cryogenic periodicity): 1-2 - glacial epochs (1 - recognized; 2 - supposed).

(10). In this connection we can follow a change to accumulation of more and more polygenetic, polydispersed detrital poorly sorted moisture-rich (ice-rich) deposits of the permafrost type for the most part of mesomictic (quartz, feldspar, hydromica, montmorillonite, etc.) and sand-aleurite (sand, coarse silt) composition.

General trends in planetary development (such as Earth's radiogenic heat decrease, expansion of continental areas and, consequently, climatic severity and increase of continental sedimentation rate) may cause not only cryogenic freezing of the stratigraphic units but also, possibly, the formation of syncryogenic deposits as early as the end of the Proterozoic (Fig. 14.2).

As the cryogenic type of lithogenesis has not been adequately studied yet (with the exception of the glacial drift and syncryogenic rock units) there exist to date only few data for the occurrence of permafrost in the ancient epochs. Ancient continental glaciations are recognized from moraine deposits, from the established phenomena of rocks broken away from the glacier bed and of detritus transportation and reworked deposits from glacier thawing. Consolidated and cemented ancient moraines with a density close to that of rocks of the sandstone type are termed tillites. Discovery of such kinds of formations of Precambrian, Ordovician, Silurian, Permian and Carboniferous age in different regions of the Earth indicates in a unique manner the repeated appearance, existence and disappearance of glacial covers and, consequently, of the permafrost.

The relations between the areas of the glacial ice cover and the permafrost can be various and depend on the glacier type, sizes and thickness, climatic severity and continentality, atmospheric moisture content and amount of solid precipitation, etc. Large glacial covers, for example, of Antarctica and Greenland, cause an essential decrease of mean annual temperatures not only over the glacier itself (up to —30 to — 60°C), but also over the large h ,m

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