ABSTRACT
An investigation of Ajali Sandstone, Anambra Basin was undertaken to describe its sedimentology and geochemical characteristics in view of the deposition environment, paleo-climate, provenance of the sediments and the economic potentials. A total of fifty one (51) outcrop samples were collected at an interval of 0.6m. Following a lithostratigraphic logging, samples were analysed for Grain size analysis, Pebble morphometric analysis, Petrographic analysis, Heavy mineral analysis, and X-ray fluorescence (XRF) fusion analysis from which the results, discussion and conclusion were drawn.
The results obtained from the standard deviation (sorting) of the five(5) sampled locations averages 0.78ф, 0.63ф, 0.96ф, 0.88ф and 3.3ф respectively. This indicates deposition in a fluviatile environment. The bivariant plots for environmental discrimination, the environment indicator plots from the Pebble analysis (roundness vs elongation; sphericity vs oblate-probate (OP) index) also indicates a fluvial paleo-environment of deposition. The sediments were transported by bottom suspension as indicated by the CM diagram. The mineralogical maturity index (MMI) of 17.4, and Zircon – Tourmaline –Rutile (ZTR) index of 67.96%, indicate a mineralogically matured sediments and chemically immature to sub-mature sandstone respectively. The mineralogical and elemental composition with SiO2> 90%, the Quartz-Feldsper-Lithic (QFL) Ternary plot and the log (SiO2/Al2O3) ≥ 1.5 confirm the sediments as siliciclastic quartz arenites, related to those from granitic and metamorphic provenance. This is collaborated by the (Al2O3/TiO2) ratio which averages 3.5, typifying a basement source rock, further supported by the heavy mineral assemblages which are from granitic and metamorphic provenance (Basement rock). The SiO2/Al2O3 ratio of 180.24(ave.) authenticates the mineral maturity of the Sandstone as matured. The ternary plot shows that the Ajali Sandstone, are sediments obviously from igneous and metamorphic source rock, during a prevailing humid climate. The weathering indices of Oxides: SiO2 (98.2%); Al2O3 (0.79%); Fe2O3 (0.49%) and TiO2 (0.35%), conforms to the British International System (BIS) specification of Silica for industries. As a result the Ajali Sandstone finds application in glass making, iron and ceramic industries.
An isopach map of the Ajali Formation was generated from which the gross rock volume of the basin was calculated and found to be 1.25 billion m3 (1.25 million km3), the isopach map reveals a 420m thick well within the Ajali Formation. This 420m thickness is a focal zone for hydrogeological, mineral exploration and or hydrocarbon exploitation processes within the Anambra Basin. The volumetric together with the textural and geochemical characteristics of the Ajali Sandstone is an indication that the formation maybe considered for economic exploration and exploitation activities of the resources within the Basin.
