Source area-weathering, provenance and tectonic setting of the campanian - Maastrichtian clay sequences in the Lower Benue Trough of Nigeria

Abstract

Clay deposits are abundant within the Lower Benue Trough (LBT) of Nigeria. The chemical compositions and other attributes of some of these deposits, aside their uses, have not been extensively studied and documented. These natural resources are important in the understanding of the geological processes with reference to the source area-weathering, provenance and tectonic setting of the precursors, using the major, trace and Light Rare Earth Element (LREE) compositions of the clay samples. The aim of this investigation was to understand the geochemistry of the clay deposit, beyond the already known functional applications, for which 18 raw clay and 14 separated <2 μm clay fraction from LBT (Nigeria) were collected and analysed.The SiO2 (64.61–90.28 wt%), Al2O3 (5.10–19.22 wt%), Fe2O3 (0.28–3.99 wt%) and LOI (2.70–11.10 wt%) contents of the clay are characteristic of quartz-rich weathered sediments with kaolinitic clay mineral assemblage. The calculated Chemical Index of Alteration (CIA) (91.17–99.79), Plagioclase Index Alteration (PIA) (99.31–99.87) and Chemical Index of Weathering (CIW) (99.34–99.87), as well as, the geochemical parameter (R~0.2) and alpha indices of Ca (28.87 α AlCa < 297) is by far mobile than Ba (1.14 α AlBa < 7.12), Sr (1.24 α AlSr < 11.33), Rb (1.61 α AlRb < 19.03), Mg (11.19 α AlMg < 95.79) and Na (35.24 α AlNa < 150.52) suggested chemical weathering associated with feldspar decomposition into kaolinite and subsequent deposition in the rifted LBT. The LREE and Y concentrations in the clay samples suggested enrichment during the weathering and kaolinization processes. The La–Th-Sc and Eu/Eu* versus [Gd/Yb]N diagram indicated a granitic gneiss precursor. Major elements discriminant-function multi-dimensional diagram, DF1 (arc-rift- col) versus DF2 (arc-rift- col), for high-silica sediments revealed a continental rift tectonic setting. The campanianMaastrichtian clay sequences in the Lower Benue Trough of Nigeria is a quartz-rich kaolinitic clay derived from chemical weathering of Proterozoic granitic gneiss. The clay was deposited in a continental rift tectonic setting, under a tropical climatic condition.