Soil Mineral Diversity in the Sedimentary Basins of Southwestern Amazon, Brazil.

Soils in the Brazilian Amazon jungle are far more diverse than the perceived simple Oxisols concept. The objective of this study was to characterize the mineralogical and chemical variations in soils derived from two typical sedimentary deposits in the Amazonian environment and their variations with landscape and age.

Nine soils in the southwest part of Brazilian Amazon were analyzed.  The first chronosequence consist of soils derived from Madeira River sediments with ages of 1.8 Ma (soil P2) and 0.01 Ma (soils P1 and P3), respectively. The dominant pedogenic process is clay accumulation in the B horizons of the soils. The contents of exchange bases in these three soils are similar but the exchange Al increased from 0.77 cmol_c kg^-1 in the youngest soil P3 to 7.10 cmol_c kg^-1 in soil P2.   Vermiculite, mica, kaolinite and quartz occur in the clay fractions of the three soils. The oldest soil P2 is dominated by kaolinite while the youngest soil P3 contains the greatest amount of mica. Soil P1 has the greatest amount of vermiculite.With decreasing abundance, mica, kaolinite, rutile, and anatase also occur in all soils.

The second chronosequence soils P4 and P5 derived from the sediment of the Lower  Solimões Formation (20-5 Ma, Miocene) and soil P7 from the upper Solimões Formation(5-1.7 Ma, Pliocene).Soil P4 had the highest degree of weathering and formed an Oxisol that is dominated by kaolinite and iron oxides. This soil had a lower exchangeable aluminum between 0.42-1.86 cmol_ckg^-1, presumably due to the absence of 2:1 layer silicates in the Oxisol. Soil P5 is clayey with exchangeable aluminum between 6.4-12.82 cmol_ckg^-1 and high values of calcium (11.53-37.66 cmol_ckg^-1). Soil P7 is also clayey with high exchangeable aluminum (2.76-13.01 cmol_ckg^-1) but low exchangeable calcium (0.44-1.67 cmol_ckg^-1). Kaolinite is the predominate clay mineral in soil P1, smectites in soil P5, and vermiculite in soil P7.