Lower Cretaceous clay minerals suites of the Alava Block, analysed by XRD, are dominated either by illite or kaolinite. These assemblages are mainly inherited in origin, although textural evidence (SEM) indicates some diagenetic clay. Precipitation of blocky kaolinite took place in sandstones from the southern domain of the Block, characterized by low sedimentation and subsidence rates and by low vitrinite reflectance values (%Rr <0.8). In the northern domain, where sedimentation and subsidence rates were high (%Rr >3.3), illitization of kaolinite and smectitic clays and precipitation of authigenic chlorite occurred. Some chemical aspects of these reactions are discussed.