Abstract
The sedimentology and geochemistry (major and trace element compositions) of lignite and argillite (carbonaceous shale and claystone) sequences in a Basin in Bali Nyonga, west of the Bamenda Mountain have been investigated to determine their sequences and the prevailing environmental conditions which control their formation. Ten representative samples were obtained fromtrenches, pits, and river and stream valleys in the study area. These samples and their ashes were subsequently examined using X-ray fluorescence spectrometry (XRF), inductively coupled plasma spectrometry (ICP), and X-ray diffraction analysis (XRD). The geochemical results revealed that thelithophile, chalcophile and siderophile elements were dominantly epigenetic in origin, mainly from detrital sources supported by high silica and alumina concentrations in all the samples. The mineral phases identified were quartz, kaolinite, illite, pyrite, hematite, and minor phases of feldspars,pseudorutile. The relatively high silica (54.10 wt%) and alumina contents (27.77 wt%) in these samples can be explained by high detrital input during peat formation. The low contents of MgO and CaO in the analysed samples agree very much with the continental setting of the peat formating basin. A clayey microband derived from alkaline volcanic ashes was identified in the lignite and the dominant composition of these clayey microbands was mixed-layer clay minerals of illite and kaolinte, which were interlayered with organic bands. The modes of occurrence of ash bands indicated that the volcanic activities were characterized by multiple eruptions, short time interval and small scale for each eruption during peat accumulation. The ratios of redox-sensitive traceelement (V/Cr versus Ni/Co and V/V + Ni versus Ni/Co), Sr/Ba, and major oxides ratio (CaO + MgO + Fe2O3)/(SiO2 + Al2O3) from the analysed samples from Bali Nyonga indicate a terrestrial, reductive (oxic), littoral to brackish water environmental conditions which are characteristics of paludal-lacustrine basin that is filled by Tertiary volcanic materials.
Highlights
This basin consists of a sequence of conglomerate, fine to meduim-grained sandstone, argillite, beds within which are continental lignite seam intercalations
Most elements contained in the lignite and carbonaceous shales showed a close association with organic matter or with the inorganic fraction and the changes in the element affinity with either the organic or inorganic fraction may occur during coalification [5]
Clayey microbands derived from alkaline volcanic ashes were identified in lignite and were examined to be mixed-layered clay minerals interbedded with organic bands, and their modes of occurrence indicated that the volcanic activities that deposited them were characterized by multiple eruptions, short time interval and small scale for each eruption during peat accumulation
Summary
This basin consists of a sequence of conglomerate, fine to meduim-grained sandstone, argillite (claystone and carbonaceous shale), beds within which are continental lignite seam intercalations. The sedimentological and geochemical characteristics of lignite-argillite sequences in the study area are still unclear It is at this background that this research work is to carry out the sedimentology and geochemistry (chemical composition of trace and major elements) of lignite and argillite samples from a basin in Bali-Nyonga. In this way, the prevailing geochemical environmental conditions, depositional and a stratigraphical model for the rocks will be designed and this will aid in understanding their relationship with the volcanic rocks in this basin
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