The Accumulation of Organic Matter in Arid Zones
In humid regions the deposition of organic matter is widespread, both on the continents as coal, and in marine basins as oil shales. By contrast, in arid regions forests give way to steppes, and farther within the arid region even the continuous grass cover disappears, leaving only a poor, thin flora, or even in large areas no flora at all. (see Vol. 2, Fig. 134). Under such conditions no coal will accumulate, since it forms only in places of forested growth. Along the margins of deserts only small oases survive locally, under favorable hydrogeological conditions, and these may give rise to insignificant coal beds, a few centimeters thick, and negligible in area. In seas the situation is different. Since increased generation of organic matter, enriching the sediments and converting them into marine sapropels, has been associated not with the supply of biogenic elements from the continent but only with more active vertical mixing of the water, which has raised these elements from deep levels to the zone of photosynthesis, there is a real possibility of oil shales forming in the seas of arid regions. Large bituminous and oil shale sequences are actually formed within arid zones.
KeywordsArid Region Arid Zone Lower Cambrian Clastic Material Humid Region
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