Typology of Fe-Coated Grains and Diagenetic History of Agbaja Ironstone Formation Exposed at Mount Patti and Koto – Karfi, Southern Bida Basin, Nigeria

Abstract

Ooidal ironstones from theMaastrichtian Agbaja Formation, Bida Basin in the north central Nigeria were investigated and sampled around Mount Patti and Koton – Karfi where they formed consistent cap over the older sedimentary sequence. The ironstone samples collected from the outcropping sections were subjected to microscopic and mineralogical studies in order to examine their texture diagenetic history and origin. The ironstone iswell indurated, dark to reddish brown and occurs as oolitic – pisolitic, laminated – massive and concretional lithotypes. The ironstones as well as the associated lithologies are characterized by cross and parallel laminations, ripple marks with burrows of Thalassinoides and Skolithos ichnogenera. Thin section and Scanning Electron Microscopy images reveal scars on the surface of some Fe – coated grains which is interpreted to have been due to attrition, abrasion and impart during transportation. The main textural components include: true and superficial ooids, oncoids, matrix, pores, lithorelicts and bioturbations with varying level of iron replacement. X – Ray Diffraction result reveal that kaolinite and quartz are the main detrital precursors in the ironstone while goethite, hematite and magnetite form the diagenetic Fe-bearing minerals. Finning upward depositional characteristics, bioturbations, well as broken and scoured surface coated grains suggest a high energy shelf environment with agitated hydrodynamic conditions. Diagenetic processes responsible for the formation of the ironstones include precipitation, replacement, recrystallization and dehydration of Fe-oxide. The study shows that the ironstones are mainly post-depositional in origin with evidences of progressive replacement of the precursor kaolinitic coated grains and groundmass by iron oxide.