Arid processes in the SE-Brazilian relief evolution during the last glacial

M. Lichte


The general index-plain "Sulamericana" can be found in vast areas of SE-Brazil. It is mainly conserved with the help of a duricrust and covered by laterite. The next younger element of relief-evolution, a level 40 to 60 m below the Sulamericana, is completely different: the weathered rock is covered by a stoneline and a fine-textured sediment. This structure was examined with the help of granulometry, analyses of heavy minerals, thermoluminescence datings, pollen analyses and 14C-datings. Different conditions for the development of stonelines may occur according to the climatic conditions at the time of their origin; in this case it must be assumed that the stonelines were formed by slope-wash due to episodic precipitation under the condition of nearly complete absence of vegetation (geologic or climatic reasons). They are a clear post-Sulamericana development. A fine-textured sediment was deposited on top of the stonelines. The material of the fine-textured sediment was, according to the then prevailing conditions, transported by as well slope-wash as eolian processes; especially for the "demi-oranges" we must assume eolian transport. The material was probably derived partly from the local material which covers the remnants of the Sulamericana, but may be seen as an eolian sediment transported for long distances as well. The time of the deposition of the fine-textured sediment on the stonelines coincides with the climax of the last Glacial; the 14C-dating indicates 19,700 years B.P., the thermoluminescence-dating about 20,000 years B.P.. The climatic conditions were probably semi-arid. These results are supported by the findings of other researches of the Southamerican climate during the last Glacial.

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