Distribution and origin of clay minerals in the Lower Cretaceous of the Alava Block (Basque-Cantabrian Basin, Spain)

Lower Cretaceous clay minerals suites of the Alava Block, analysed by XRD, an dominated either by illite or kaolinite. These assemblages are mainly in herited in origin, although textural evidence (SEM) indicates some diagenet ic clay. Precipitation of blocky kaolinite rook place in sandstones from th e southern domain of the Block, characterized by low sedimentation and subs idence rates and by low vitrinite reflectance values (%Rr <0.8). In the nor thern domain, where sedimentation and subsidence rates were high (%Rr >3.3) , illitization of kaolinite and smectitic clays and precipitation of authig enic chlorite occurred. Some chemical aspects of these reactions are discus sed.