SEDIMENTATION AND PEDOGENESIS IN A CENTRAL AMAZONIAN BLACK WATER BASIN

Sedimentation rates were estimated in a Central Amazonian Black-water inundation forest. Sediment deposition on the forest ground, remote fr om the river bed, during an annual flood period, is of the order of 1 to 10 tons per hectare, depending on water depth and duration of flood ing. The sediments consisted of fine organic matter, kaolinite, quartz sands and biogenic particles of silica. Their genesis and deposition depend on the interplay between pedogenic, limnological and biological processes. Sediments derive primarily from the materials leached from the soils. Clay soils are the main source of dissolved silica, and th e sandy soils are the main sources of organic coumpounds and mineral p articles. The physical sedimentation of particles as quartz sand grain s only occurs in the upper reaches of the studied river. In the flood plain, the sedimentation is due to the coagulation and deposition of c ombined mineral particles and humic substances, and to the biological precipitation of the silica leached from the soil by sponges.