Palygorskite and sepiolite occurrence in Pliocene lake deposits along the River Nile: evidence of an arid climate

To characterise the Pliocene lacustrine deposits on the east side of the River Nile, a mineralogical and chemical study on material taken from cores was undertaken after a large-scale geological survey. Microscopic (transmitted light and electron microscope), X-ray diffraction, and thermal and chemical data enabled the identification of significant amounts of fibrous clay minerals such as palygorskite and sepiolite in the lacustrine deposits. Smectite, calcite and volcanic rock fragments were identified among detrital components. The mineralogical associations in the three logs are homogeneous from a qualitative point of view, but quantitative variations were recognised, suggesting a rapidly changing regime within the lake. The textural character of the samples suggests an authigenic origin for the fibrous clay minerals, indicating an arid to semi-arid climate. Part of the carbonate components (coeval biogenic) are also authigenic. The origin of smectite was considered to be detrital because of its association with detrital carbonates, recognised by means of selective dissolution of calcite and evaluation of diffractometric data. Part of the terrigenous supply derives from the Ethiopian volcanic province, according to the geochemical signature of the lake sediments, and this is in agreement with previous observations. All samples show positive anomalies of Na, Mg and P with respect to average shale that was used as a reference for upper crustal composition. Such geochemical anomalies reflect the mineralogical occurrence of halite, palygorskite and sepiolite detected by X-ray diffraction analyses, whereas the occurrence of phosphates was hypothesised because they concentrate, to a certain degree, in lacustrine environments