DEPOSITIONAL ENVIRONMENT OF EOCENE NANKA SANDSTONE, ANAMBRA BASIN, SOUTH EASTERN NIGERIA

Lithofacies, Chemical and Petrographic analysis of the Eocene Nanka Sandstone was undertaken in Anambra Basin, Southeastern Nigeria. The aim of the study was to deduce the depositional environment and provenance, propose a depositional model, evaluate the chemistry of the sandstone with the view to understanding its relationship to the devastating erosion menace in southeastern Nigeria, and to propose remedies. The Sedimentological study reveals four lithofacies: (i) Planar bedded sandstone facies; (ii) Hetrolithic cross-bedded sandstone facies; (iii) Cross-bedded sandstone facies, and (iv) Wavy-bedded sandstone facies. The sedimentological analysis also reveals five depositional facie namely tidal channels, fluvial channel, tidal sand bars, subtidal mudflats and muddy shelves. Based on the presence of drapes on bed forms, hetrolithic facies, mud layers, cross-bedding, wavy bedding, thick-thin alternation of sand and clay layers showing cyclicity (muddy rhythmites), I interpret the measured outcrops to represent tide-dominated estuarine facies.

The sandstone are medium to fine grained quartzarenites, moderately sorted to moderately well sorted and texturally mature. Heavy mineral analysis reveals a provenance of metamorphic and intrusive rocks probably from northern and northeastern part of Nigeria. The chemical analysis through spectrophotometric method showed that the percentage of the mobile cations Na, K, Mg and Ca ranges from 96 - 98% in most samples, while the immobile cations Si, Al and Fe make up less than 3% thus, the cause of the chemical instability of the sand stone. The palliative measures proposed include construction and maintenance of catch-pits for runoffs, as well as mechanical leveling by bulldozing to level out small and incipient sheet and gully erosion sites as soon as they are noticed.