The abundance of specific clay minerals have been used to reconstruct the origin of the Kilwa Group clays, and to determine the relative changes in diagenesis, weathering and climate conditions. Late Cretaceous to Early Oligocene fine-grained sediments from the Kilwa Group in southern coastal Tanzania have been analysed. The Kilwa Group is composed of four formations; the Nagurukuru, Kivinje, Masoko and Pande Formations. This study comprises detailed petrographical, mineralogical (thin section, XRD and SEM) and geochemical (trace and REE) analyses to infer the nature of the adjacent source rocks and provide insight regarding to the depositional environment. Two main lithologies were observed in the Kilwa Group, calcareous rich marine-clays are dominant, and limestones. The Kilwa Group clays variation variations in the clay mineral assemblages, which were based on abundant changes of nine clay minerals determined by fitting simulated with experimental diffraction patterns. The deposition of these clays took place across a passive margin, in a mid to outer shelf environment, with periodically influx of shallow shelf detritus transported by density currents. The broadly homogeneous appearance of the Kilwa Group clays indicate that the sediments were repeatedly homogenized before the final deposition. The uppermost Pande Formation, however, reflects a shift from deep- to more shallow-marine facies compare to the underlying formations. This could be attributed by a substantial regional uplift or eustatic sea level drop during Early Oligocene. The trace element data of the Kilwa Group clays were compared with North American Shale Composite (NASC), Post Archean average Australian shales (PAAS), and the upper continental crust (UCC). Chondrite normalized REE patterns are comparable to the UCC, and indicate a felsic average provenance and a relatively severe weathering regime. Various dicrimination diagrams and ratios revealed minor contribution from mafic volcanic rocks. However, these signals may have been smoothed by transitional storage before the final deposition. The XRD-bulk and trace elements results of the four Statoil offshore clay samples (Cenomanian and Paleocene) were compared with Kilwa Groups clays of same age, and the data suggest comparable depositional conditions and source areas.