Sandstones and mudstones of the Skagerrak Formation from the Danish Sub-basin have been studied to assess the origin of the clay minerals, their diagenesis and their transformation/conversion temperatures (compared to modelled maximum burial temperatures). Detrital smectite and vermiculite occur only at shallow levels. Conversion of smectite into random S-I took place at >47 degrees C and of random S-I into ordered I-S at >74 degrees C. The omnipresent illite increases in amount with burial. The amount of authigenic kaolinite (kaolin) decreases with increasing temperature (>75 degrees C), possibly due to the growth of chlorite. Random S-C is converted into ordered S-C between 70 degrees C and 105 degrees C and chlorite is most abundant at maximum burial temperatures (150 degrees C). This investigation suggests that diagenesis of clay minerals begins at relatively low temperatures, and emphasizes the importance of the availability of necessary ions (K (super +) , Mg (super 2+) Fe (super 2+) ) for the reactions, besides time and temperature, as controlling parameters.