Clay illuviation and mechanical clay infiltration — Is there a difference?

Abstract

Soil scientists attribute the presence of coatings of oriented clay in soils and sediments to the process of clay illuviation , which is a process that occurs at the Earth's surface and may penetrate to a depth of several metres. Geologists attribute coatings of oriented clay in coarse sediments to a process of mechanical infiltration of muddy river water or muddy overland flow. Clay illuviation is a climate-dependent process (seasonal climate) which is restricted to the vadose zone. The source of the translocated material is internal, i.e., material from upper soil layers is transported to deeper layers. The resulting accumulation horizons are up to 2 m thick and continuous in clayey deposits, but in sandy deposits they are discontinuous and may reach a depth of several metres. The process of mechanical infiltration is not climate-related, and its effect is not restricted to the vadose zone but may reach far below the ground-water table. The source of the transported material is external: it is provided by muddy water entering the soil. The accumulations are supposed to be deep and continuous. The environment of muddy waters, with higher electrolyte concentrations than water that percolates the soil upon first rain, and a gradient of particles coarser than clay, should, however, preclude mechanical infiltration because: (1) clays will tend to flocculate; (2) both coarser suspended material and flocculated clay will tend to clog the pore system; and (3) below-ground-water transport of suspended material will be virtually impossible because of low percolation velocity. In addition, flocculated clay will not produce oriented coatings upon desiccation. The model of mechanical infiltration is not supported by theory or observations. We suggest that the process of mechanical infiltration of clay needs further study to prove whether it is different from clay illuviation in soils. The distinction between the two is of utmost importance for the interpretation of paleosols.