| Abstrakt: |
Sedimentological and mineralogical studies from two sandstone formations of pre-Santonian (Awi Formation) and post-Santonian (Lokoja Formation) ages were carried out aiming to characterize their provenance and depositional environments. Detailed field studies, textural and mineralogical analyses involving lithologic descriptions, grain size analysis, thin section, heavy mineral, and X-ray diffraction analyses were carried out. Both sandstones exhibit fining upwards succession and unconformably overlie the basement complex. Other sedimentary structures include grain imbrications, mud drapes, cross stratifications, flaser beds, and ripple marks. Textural studies show that the sandstones have medium to coarse grains and are moderate to poorly sorted, positively skewed, and platykurtic. These characteristics suggest that high-energy conditions prevailed during the deposition of that sandstones. Bivariate analysis of skewness versus sorting showed that the sandstones were deposited in a fluvial environment. Petrographic analysis revealed angular to sub-angular grains with the dominance of quartz and feldspars in the arkosic and sub-arkosic arenites with variable, but low lithic and heavy mineral constituents. The result of the heavy mineral analysis shows dominance of non-opaque minerals (64%) compared with opaque minerals (35.08%). While the heavy mineral assemblage suggests derivation from igneous and metamorphic rocks, the presence of garnet and sillimanite point to nearness to the source. Analysis of clay-sized fraction revealed a dominance of kaolinite with illite and montmorillonite making the subordinate group within the clay minerals present. Clay species ration (high kaolinite: illite ratio) together with results from petrography suggests the possibility of warm and humid paleoclimatic conditions prevalent during deposition. From the integration of textural analysis (multivariate analysis) and observed sedimentary structures (cross-stratification, floodplain deposits, clay drapes), an agitated, fluvio-deltaic to nearshore environment of deposition is indicated by the overall sandstone geometry. This confirms the continental source for the sediments. The modal analysis further suggests a continental block provenance within a tectonic setting dominated by cratonic to recycled orogen. Such a setting is analogous to the Oban Massif for the Awi Formation and the North and Central basement complex and sediments in their second cycle of sedimentation sourced from the post-Santonian deformational episode for the Lokoja Formation. |
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