Abstract: The sedimentary subbasins of Oulad Abdoun and Timahdit (Morocco) were part of a single marine gulf that extended across Morocco for more than 300 km (west to east) from Late Cretaceous through the Eocene. The differential structural development of this basin resulted in the presence of a stable shelf in Oulad Abdoun and a subsident orogenic zone in Timahdit. This structural asymmetry explains the sedimentary asymmetry of the Maastrichtian sequence that consists of a thin, 5-m thick, phosphorite sequence interbedded with dolomitic limestone, marl, and shale near Oulad Abdoun, and an oil shale, 200-m thick, in Timahdit. Widespread reducing conditions that prevailed in the basin resulted in preservation of large amounts of organic matter. Quantitative and qualitative analysis shows an identical phytoplanktonic origin for all of the organic matter in both of the two subbasins, and the organic matter still remains in an immature state. The subsequent history of this organic matter, however, reflects the specific environments of the two related but disparate depositional settings. In each part of the basin, the organic material associated with marls, limestones, and shales contains kerogen, whereas in the phosphate-rich strata the organic matter is in two forms: humic compounds inside the phosphatic grains and kerogen in the matrix. Humic compounds from different lithologies have a similar composition, whereas the kerogen in samples from the phosphate-rich strata shows the effect of oxidation (during synsesimentary reworking), increasing with the amounts of P 2 O 5 of the total rock. These changes in the phosphate-rich strata have allowed the crystallization of apatite and the genesis and preservation of humic compounds trapped inside these phosphatic grains. In the oil shale, large amounts of associated clay resulted in a rapid and total unsolubilization of the organic matter and its evolution toward kerogen. The study of the organic matter, which is associated with a wide variety of facies from both parts of the basin, shows the extreme sensitivity of organic compounds to processes that have affected them from their deposition through all phases of diagenesis. The fate of the organic matter during early diagenesis is related to the presence or absence of clays: the presence of clays led to the formation of black shales and the absence of clays led to the formation of phosphorites.