Textural, mineralogical and chemical parameters of clays belonging to the Upper Cretaceous "Argilas de Aveiro" formation were determined and correlated, using geostatistical analysis, with some relevant geotechnical parameters. The data obtained made it possible to subdivide the region under study into distinctive sectors on the basis of geotechnical behaviour. Clay fraction content and composition control geotechnical parameters such as the expandability and the Atterberg plastic and liquid limits. A positive correlation was found between both expandability and the Atterberg plasticity index values and the total phyllosilicates, smectite, MgO and Al 2 O 3 . However, Atterberg plasticity index values were relatively independent of the diverse clay mineral assemblages.