New Data on the Stratigraphy of the Ameki Group, Se Nigeria: Implications for the Eocene Stratigraphic Nomenclature

Abstract

Detailed down-dip, bed-by-bed sedimentary logging of outcrops of the Ameki Group in the Awka-Onitsha area of south-eastern Nigeria, was performed to identify and characterize components of the Group in the area. Thirteen exposures were measured and analyzed for lithological changes, composition, sedimentary structures, trace fossils, and stratigraphic surfaces. Stratigraphic analysis shows that the Ameki Group in the study area consists of two stratigraphic components- a basal sandstone component (> 200m) and an overlying mud rock component (>20m). Facies analysis reveals that the sandstone component consists of two facies associations: (i) a tidally-influenced channel facies association comprising strongly ferruginized, pebbly- to medium grained, fining-upwards sandstone that is inter-bedded with kaolinitic claystone and ironstone of variable thickness and(ii) sand-rich heterolithic facies association that reflects sand flat sedimentation. The mud rock component of the Ameki Group also contains two facies associations: (i) clay-rich heterolithic facies association that reflects deposition in mixed flats and (ii) carbonaceous claystone-lignite facies association that reflects mudflats and swamp sedimentation. Field relations and facies analysis confirm that the contact between the channelized, basal sandy component of the Ameki Group and the underlying Imo Formation is defined by the upward transition from dark-coloured, fine grained marine facies, to coarse grained fluvial channel sandstone. Similarly, the contact between the of the muddy component of the Ameki Group and the overlying Ogwashi Asaba Formation is defined by the transition from dark coloured mud flats/swamp facies to light coloured pebbly-coarse grained coastal plain sandstone and lignite. These new information should serve as a major contribution toward the resolution of the problems of the stratigraphy and contact relations of the Ameki Group in this part of southern Nigeria.