Pattern of Geochemical and Sedimentological Variability of the Albian to Cenomanian Upper Bima Sandstone, Benue Trough, Nigeria: Implications on Tectonic provenance and Source Area Weathering

Abstract

This study presents the pattern of geochemical and sedimentological variability of the Albian to Cenomanian Upper Bima sandstone in Upper Benue Trough, NE Nigeria. The intent is to apply sedimentological investigations and major element concentrations of the upper Bima sediments (fine grained sandstones) in order to reveal source area weathering, provenance and tectonic setting of inferred source areas. The investigation approach involved field studies and collection of samples from different outcrop locations, followed by laboratory studies involving grain-size analysis (GSA) and major elements analyses using the Atomic Absorption Spectroscopy (AAS) method. Field studies show that the sandstones are indurated with evidence of soft-sediment deformation and range in color from white in fresh sample to reddish brown on weathering. In addition, the sandstone units are cross-bedded and show graded bedding exemplified by fining upward sequence. Textural examination indicates that the sandstones range from fine to medium to coarse grained, with graphic mean grain size of 0.16 to 2.21 mm. Standard deviation (sorting) ranges from 0.98 to 1.77Ø and implies moderately sorted to poorly sorted sediments. Inferred from the textural indices, the depositional environmental discrimination of the Bima Sandstone revealed a fluvial/river system-dominated sedimentary process. In addition, major elemental oxides show SiO₂ content greater than 75% for the Upper Bima Sandstone samples with depletion of mobile oxides such as Na₂O, CaO and K₂O through weathering and other sedimentary processes. On the basis of the major elements, the sandstones are classified as sublitharenite, and arkose types. Provenance and tectonic setting discrimination using geochemical data revealed typical felsic igneous-dominated cratonic environment while ternary plots (e.g. SiO₂/20 - K2O+Na₂O - TiO₂+Fe₂O₃+MgO) reflect passive continental margin setting for the Bima Sandstone. Tectonic setting discrimination diagrams based on major elements suggest a granitic provenance in a passive continental margin. Consequently, the source area is constrained to the Precambrian basement rock units of Adamawa areas to the north and southern parts of the Yola Basin. The Chemical Index of Alteration (CIA, average 79.63) reflects intense degree of chemical weathering in a tropical wet climate, and an A-CN-K plot suggests an upper continental crust provenance dominated by felsic to intermediate igneous rocks (tonalite, granodiorite and granite) of average granitic composition.

Keywords: provenance, tectonic setting, sandstone, geochemistry, Upper Bima Sandstone, Benue Trough