Clay minerals origin and paleoclimate implications in quaternary deposits of the Saïs Plain, Fez-Morocco

The city of Fez in northern Morocco is situated on northeastern parts of the Saïs Plain, an alluvial basin. It is spreading southwesterly onto Pleistocene sediments consisting primarily of alluvial, channel-fill conglomerates and overbank deposits, as well as Holocene tufas. As urban development expands, human activities are increasingly eradicating these deposits, about which little is known. This study focuses on the clay-mineral composition of these Pleistocene overbank deposits, informally known as the “Red” and “Yellow” units, as well as of the Holocene tufas. In contrast to previous research that has emphasized the industrial uses of nearby Miocene marls. The objective of this study is to examine these Pleistocene and Holocene rocks to determine the likely origins, either inherited or neoformed, of the included clay minerals by comparing their compositions with those of nearby substrata. The clay-mineral analysis indicates a prevalence of smectites with varying amounts of illite, kaolinite, sepiolite, and chlorite. The presence of smectite and sepiolite likely reflects neoformation in areas of alkaline sedimentation during Quaternary semi-arid Mediterranean climates, when alkaline terrestrial environments rich in calcrete were widespread. Conversely, illite and chlorite appear to be predominantly inherited minerals within the studied units, transported by wind and water. Illite is largely sourced from Jurassic carbonates, whereas chlorite derives primarily from Miocene marine marls. Radiocarbon dating of two tufa samples indicated an age range between 6.7 and 5.6 ka Cal. B.P., aligning with the African Humid Period. The presence of significant quantities of kaolinite in the surface soils suggests that these soils formed during this humid period and are likely equivalent to the Holocene tufa deposits.