Clays - a tool for tropical geomorphology

摘要

Conventional analyses of granulometry of samples from soils from tropical shield areas revealed low means of clay but c. 50% of sand. This is an effect of pseudosand (i.e. small aggregates cemented by iron oxides and/or silica). The low correlation of granulometry and gradient of slopes hints at a restricted surface flow whereas infiltration and a mainly lateral subterraneous transport are strong. These findings are supported by the analysis of landforms (including pipes) and a far spread occurrence of soils with high volumes of large pores. Kaolinite has been found as dominant clay mineral, gibbsite (c. 20% of our samples) is only formed in the rainforest ecozone whereas smectitc (c. 20% of our samples) is more common in the savanna than in the rain forest. Smectite has been found in different horizons, it shows no close relation to the weathering front. Both observations show, that the Strakhov-model is of very limited use for tropical areas. Kaolinite and gibbsite, which have crystallized in pseudomorphs on the walls of pores and in the matrix reveal, that both minerals have not been products of the weathering of smectite. In cooperation with crystallization of other substances and the formation of pseudosand these formations are the reason for the high stability of regolith covers. The citations are based on the Cologne database (CRD) holding 1,220 samples with 12 field parameters and 20 lab parameters, containing each up to 14 components.