MICROMORPHOLOGY OF SOME TROPICAL ALLUVIAL CLAY SOILS

The micromorphological properties of some marine and estuarine tropical clays from Holocene coastal plain soils of Surinam and Thailand have been explained against the background of sedimentation and initial soil formation. During geogenesis stratified sediments are formed, above which are sediments with slightly disturbed stratification, with or without matric faecal pellets. Pedogenesis during the sedimentation phase included development of channels in the sediment, formation of channel neostrians, biological homogenization, possibly pyrite accumulation, and partial oxidation and precipitation of iron as ferric hydroxide. During the brackish water phase pyrite accumulates in various amounts, mass illuviation may occur, part of the iron may oxidize and precipitate and part of the pyrite may be oxidized. During and after the swamp phase further oxidation and precipitation of iron as ferric hydroxide occurs, ferric hydroxide crystallizes into goethite. Pyrite becomes oxidized, resulting in the formation of jarosite, gypsum, silica, and ferric hydroxide.