Source Area Weathering and Tectonic History Inferred from the Geochemistry of the Maastrichtian Sandstone from Patti Formation, Southern Bida Basin, North Central Nigeria

Abstract

Sandstones sampled from Patti Formation, Southern Bida Basin, were studied geochemically using Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES) and an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) technique to evaluate their weathering and tectonic setting as well as to deduce the paleo-climatic conditions that existed during their deposition. Geochemically, SiO2 range from 73.9% to 86.2%, Al2O3 (6.7%~17.1%), Fe2O3 (1.1%~1.9%), K2O (0.1%~0.7%) while MgO, CaO, Na2O, P2O5, MnO and TiO2 were <1%. Enriched in Ba (Av. 622.94), Sr (Av. 153.63), Rb (Av. 55.08) and Zr (Av. 51.86) relatively similar in composition to UCC. High SiO2 but low other major oxides signify high mobility during processes of weathering. This was confirmed by high value (>80%) for indices like chemical index of alteration, chemical index of weathering, plagioclase index of alteration, mineralogical index of alteration and relatively lower values for weathering index of parker, recently used alpha indices (αAl E) of sodium (326.17αΑl Na<344.40), magnesium (100.54αΑl mg<398.55), calcium (12.07αΑl Ca<198.99), potassium (4.43αΑl K<64.33), strontium (0.84αΑl E<21.40), barium (0.45 αΑl Ba<10.52) and rubidium (0.0008αΑl Rb<0.06), supported by AI2O3-(CaO*+Na2O)-K2O and CIA vs. SiO2 plots that imply intense weathering in the source area. The obtained high CIA values (>80) indicates a steady-state of weathering under a warm/humid climate as confirmed by the SiO2 vs. Al2O3+ K2O + Na2O plot. High average SiO2 (75.41wt%) with K2O/Na2O ratio >1 (15.63), low Fe2O3 (1.27wt %), Al2O3 (15.82wt%) and TiO2 (0.46) suggest passive margin tectonic setting. This is supported by enriched ΣREE (209.64 ppm), ΣLREE (195.78), LREE/HREE (27.78) and negative Eu/Eu* (0.68), plots of log (K2O/Na2O) vs. SiO2 and SiO2/Al2O3 vs. K2O/Na2O. Major elements discriminant-function multi-dimensional diagram, DF1 (arcrift-col) vs. DF2 (arc-rift-col), for high-silica sediments revealed a continental rift tectonic setting. Thus, the Patti Formation sandstone underwent a high degree of weathering under a humid climatic condition within a continental rift tectonic setting.