Inorganic geochemistry of Miocene sediments from the Lower Chelif Basin (NW Algeria) for approaching weathering and palaeoclimatic conditions

Geochemical studies of major and trace elements were conducted on Miocene sediments from three sections of the Lower Chelif Basin, Northern Algeria. Geochemical proxy records demonstrate that the Lower Chelif Basin has experienced weak to moderate weathering and the Index of Compositional Variability (ICV) values suggest that these sediments were, in general, enriched in rocks forming minerals. Al₂O₃ versus (vs.) K₂O diagrams indicate that, in Miocene sediments, the minerals containing Al₂O₃ and K₂O are primarily illite; likely derived from K-feldspar decomposition. The application of the Al₂O₃ vs TiO₂ binary plot, as a provenance indicator, indicates that all the samples fall along the basalt + rhyolite/granite line indicating that the sediments derived from mixed source sediments the composition of which ranges from mafic and felsic rocks. Compared to Upper Continental Crust (UCC) composition, Miocene sediments depict strong depletion in SiO₂, Al₂O₃, MnO, Na₂O, K₂O, Zr and Sr during the weathering process as well as an enrichment in Cr and Cu. Calculated percentage variation plotted against the Chemical Index of Alteration (CIA) diagrams provides a basis for assessing the chemical mobility during weathering in the Chelif Basin. For the study stratigraphical range, Si, Al, Mn, Na and K show depletion in relation to Ti. Sodium decreases more rapidly than K, suggesting a Na-plagioclase alteration higher than that of K-feldspar. During the Tortonian, the chemical motilities of Rb and K are tightly correlated (r = 0.72), but the former decreases lesser. Calculated values of C-proxy suggest a roughly semi-arid to semi-moist climate during the Burdigalian-Langhian, arid to semi-arid during the Tortonian and more humid conditions during the Messinian. Sr/Ba ratio ranging from 0.44 to 6.48 indicates a palaeoenvironment with variable salinity during the Miocene.