INFLUENCE OF GEOGENESIS AND PEDOGENESIS ON CLAY MINERAL DISTRIBUTION IN NORTHERN VIETNAM SOILS

This is a study of a sequence of five soils developed in the Red River valley of Vietnam along a transect extending from the foothills to old terraces. Most of these soils are chemically depleted, having, in particular, a very low CEC. The mineralogical study shows that the <2-mu m fraction is made up of a mixture of vermiculite, illite, kaolinite, quartz, and iron oxyhydroxides. Other minerals, such as pyrophyllite in the foothills and chlorite and gibbsite in the old terraces, are present in small quantities. The major mineralogical differences among these soils lie in the relative abundance of quartz in the <2-mu m fraction, the vermiculite/illite ratio, and the degree of the intergrade character of vermiculites, which is variable not only within each profile but also from one region to another. The amount of quartz generally increases toward the foothills, indicating the greater weathering of foothill materials relative to old terraces, which have been less weathered during geogenesis. The vermiculite/illite ratio, which also increases toward the foothills, indicates an increasing transformation of 2:1 minerals. The effects of pedogenesis are characterized by the difference in vermiculite/illite ratios between the surface and 1-m-deep horizon. Nevertheless, the intensity of pedogenesis cannot be evaluated in profiles developed on highly weathered sediments because illite of the parent material has been completely transformed into vermiculite during a preweathering stage. Alternate wet and dry conditions apparently lead to an increase in illite content by the collapse of vermiculite sheets in surface horizons. The creation of a plow pan in these hydromorphic conditions may have favored the dissolution of clay by ferrolysis and the lateral leaching of fine clay particles out of the profile, increasing quartz content in the <2-mu m fraction of the plow pan.