PROVENANCE OF THE UPPER ORDOVICIAN AJUA SHALE AND ELMINA SANDSTONE FORMATIONS, SEKONDIAN GROUP – GHANA

The mineralogical and geochemical characteristics of siliciclastic sedimentary rocks are key indicators of their provenance and depositional settings. Despite previous studies on the Sekondian Group in Ghana, a detailed petrographic investigation into the Ajua Shale and Elmina Sandstone Formations has remained limited. This study addresses this gap by integrating petrographic, mineralogical, and geochemical analyses to infer their provenance, weathering history, and tectonic setting. A total of 47 thin sections (20 sandstones from each Formation and 7 sandy shale samples from the Ajua Shale Formation) were analyzed petrographically using point-counting methods under a polarizing microscope. X-ray diffraction (XRD) was conducted on ten samples for mineralogical characterization. Additionally, 40 powdered samples underwent major and trace element geochemical analysis using X-ray fluorescence (XRF). Data were interpreted using standard ternary diagrams, weathering indices (Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), Index of Compositional Variability (ICV), and tectonic discrimination plots. Petrographic analysis classifies the sandstones predominantly as arkosic and sub-arkosic arenites, characterized by sub-mature textures and low chemical maturity, with high feldspar and quartz content. Geochemically, both formations exhibit weak to moderate chemical weathering, reflected in the CIA and PIA values. The Al₂O₃/TiO₂ and TiO₂/Zr ratios, along with enrichments in Zr, Hf, and Y, suggest felsic to intermediate igneous sources, likely the Birimian granitoids and Pan-African felsic gneisses. Tectonic discrimination diagrams position the sandstones mainly in dissected arc and active continental margin fields, although field evidences also hint at passive margin conditions due to low sedimentation rates and the absence of volcanism. The integrated petrographic and geochemical data indicate that the Ajua Shale and Elmina Sandstone Formations were derived from felsic-intermediate continental sources, deposited under semi-arid to arid climates, and influenced by active margin tectonics, with possible overprinting from glacial and passive margin settings during the Late Ordovician.