Morphological, mineralogical and geochemical features of alluvial sediments in Bol area (Lake Chad Basin): implications for source area-weathering and tectonic settings

Abstract

Abstract This paper discusses the source area-weathering and tectonic settings of alluvial sediments from Lake Chad Basin (LCB). The grain size distribution has been performed by the Robinson-Kӧln's pipetting method. An HACH-HQ11d brand electric pH meter measures the pH and Eh values. Some sand grains and aggregates were examined by scanning electron microscopy with an Energy Dispersive Spectroscopy. The bulk mineralogy was performed with X-TRA Thermo-ARL Diffractometer. The geochemistry of whole samples was assessed by X-ray Fluorescence Spectrometry. Four profiles constituted of different levels characterized by variation in colors and textures have been examined. The textural variation is linked to the alternation of wet and dry periods in the LCB. Microtextural observations reveal sub-rounded to angular particles with secondary silica precipitation on their surfaces. This suggests fluvial transport and short periods of aeolian movements. The clayey and clayey sandy facies are dominated by kaolinite, quartz and illite while the sandy samples are dominated by quartz, kaolinite, illite, goethite and rutile. Amount major elements, SiO2, Al2O3 and Fe2O3 have the highest contents and this is consistent with the mineralogical composition. The sediments are mature and classified as Fe-shale, Fe-sand and (sub)litharenite. The sediments are formed under oxic paleoenvironmental conditions. The rock sources underwent variable degree of weathering which reveals the impact of climate change prevailing in LCB. The trace element distribution shows no chemical differentiation as a result of low sediment recycling. Most of the discriminant plots indicate a felsic nature of the original sources rocks. The samples plot in the fields of passive margin and collision in a lesser extent. The collision setting may be related to the complex history of felsic source rocks which derived from the Pan African orogeny and the Cameroon Volcanic Line activities.