The Sedimentology of Gambaga Massifs of the Northeastern Voltaian, Ghana.

Abstract

The sedimentary facies of the investigated area of the northeastern Voltaian, with respect to the sedimentary structures, composition as well as the depositional environments, can be classified into six (6) facies. The sediments are deposited dominantly in a shallow marine environments with records of shoreface/foreshore, Aeolian and tidal environments. The sediments were deposited in the various environments by both low and high energy media. The area shows a shallowing and thickening upward facies sequence. The paleocurrents of the sediments show a dominant southwestern source with minor contribution of sediments from the south and south east eastern parts. The sediments are composed of quartz, dominantly monocrystalline with undulatory extinction (strain) with minor polycrystalline, potassium feldspars, plagioclase feldspars, micas, sericites as the matrix, zircons and some accessory minerals from the petrographic study of the collected samples. The sediments studied are moderately – highly weathered in a relatively gentle – flat terrain. The composition of the clastic sediments shows high temperature regimes, dry/arid – moderate humid climatic conditions in a most probable poor rainfall environments. The sediments of the studied area from petrography, are the weathering products of granitic rocks, granitic gneisses and sedimentary rocks. The source area of the investigated area can be most likely traced to the adjacent Birimian Supergroup. Geochemically, the studied samples of the Gambaga – Nakpanduri area can be grouped into sublitharenite, subarkose and quartz arenites using log (Fe2O3/K2O) versus log (SiO2/Al2O3) bivariate plot. The weathering indices, thus, Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Thorium – Uranium ratio (Th/U) and Index of Compositional Variability (ICV) all supports a moderate – highly weathered and matured (ICV <1) sediments. Both the petrography and the geochemistry supports recycled sediments of the study area with evidence of quartz overgrowth and Th/Sc versus Zr/Sc plot respectively. The source area of the sediments has experienced semi-arid climatic conditions in flat – gentle topographic terrain with accompanying high temperature regimes. The sediments are largely the weathered products of felsic rocks with subordinate contribution of mafic and an older sedimentary source. The major elements as well as the trace elements supports an active continents margin setting and a passive margin tectonic setting. The studied area based on lithological features has been classified into three formations of lower, middle and upper parts which correlates with Tossiegou, Poubogou and Panabako formations of the Bombouaka Supergroup rock suite of earlier workers. The lower formation is made up of sandstones with asymmetric ripple marks. The middle formation is made up of sandy shales, siltstones with minor sandstones. The sandy shales are cross stratified with papery fissility, bifurcated ripple marks, flute casts, climbing ripple marks. The upper formation is made up of feldspar and quartz rich sandstones. They are medium to coarse grained with characteristic cross beds, penetrative burrows (Skolithos), over turned cross beds, ripple marks and wavy laminations.