Geochemistry of Fluvial Sediments from Geregu, Southwest Nigeria

Geochemical analysis of fluvial sediments on the banks of River Ero using inductively coupled plasma mass spectrometry illustrates their maturity, provenance and tectonic setting. The analysed sediment samples show low SiO₂/Al₂O₃ ratios of 2.92-2.99 (units FL_A, FL_B and FL_E) and high SiO₂/Al₂O₃ ratios of 4.064-4.852 (units FL_C, FL_D, FL_F and FL_G). Sediments were geochemically classified as shales (units FL_A, FL_B and FL_E) and greywackes (units FL_C, FL_D, FL_F and FL_G). Variability in sediment maturity (FL_F > FL_G >FL_C >FL_D >FL_A > FL_B > FL_E) parallels a decreasing order in the ratios of SiO₂/Al₂O₃ and K₂O/Al₂O₃, as well as the proportion of quartz grains and matrix components. Evidence from Al₂O₃/TiO₂, K₂O, Rb, La/Co, Th/Co, Cr/ Th, Th/Cr, La/Th-Hf, Th-Hf-Co and rare earth element contents of sediment samples suggest felsic protoliths of upper continental crust in a passive margin tectonic setting. An insignificant contribution of mafic components from the source is, however, inferred based on the Ni and Cr contents of the sediment samples. Combined Eu anomalies <0.85 and (Gd/Yb)_n ratios <2.0 (1.53- 1.82, average 1.65) suggest post-Archean protoliths.