Upper Cenomanian–Turonian clay mineral assemblages of sediments cropping out in the Western High Atlas basin are studied in four sections. Smectite and mixed-layer illite-smectite (I-S) have been identified as major constituents of the deposits. The composition of clay associations in black shales and associated sediments varies considerably according to age, but usually depends either on the general lithology, the abundance of organic matter, or the depth of burial. A distinct correlation is evident between clay mineral distribution and sea-level. Smectite and mixed-layer I-S with greater percentages of smectite layers increase in sediments deposited during transgressive periods, whereas they decrease progressively in the shallower facies deposited during regression in favour of illite and mixed-layer I-S with a greater percentage of illite. The vertical evolution and lateral distribution of clay assemblages and their relationships with sea-level as well as the palaeogeographic conditions prevailing during the Late Cenomanian–Turonian period (flattened topography and arid climate), indicate a detrital origin of the smectite minerals and a distribution pattern controlled by differential settling processes.