FINE-GRAINED LAMINATED QUATERNARY SEDIMENTS IN THE EBRO VALLEY (SPAIN) - CHARACTERISTICS AND FORMATION

Fine-grained sediments are a major parent material in Ebro Valley soil s. During the 1960s, the area was transformed by irrigation and soils underwent serious degradation processes. The sediments have a fine tex ture, high CaCO3 contents and high salinity-sodicity levels, and conta in illite as the most important phyllosilicate in the fine fractions. Two groups of finely laminated organization have been recognized in te rms of layer thickness and grain-size distribution. Based on texture, the microstructure of the silty and clayey layers is different, irresp ective of their mode of formation; porosity characteristics also diffe r between layers, leading exclusively to a capillary evaporation flow that results in salinization of the plough horizons. The sediments are formed by two different mechanisms, each layer resulting from uninter rupted sedimentation of a suspension or from deposition of suspended p articles by water discharge.