Savanna Journal of Basic and Applied Sciences 4(2):1-16
Geochemistry, Textural and Mineralogical Maturity indices of Dukku Sandstone Member, Type Section of Gwandu Formation, Sokoto Basin, Northwestern Nigeria
Ola-Buraimo A.O. and Usman S.K. Hassana
ABSTRACT
Outcrop samples were collected from Dukku Sandstone here referred to as member of Gwandu Formation with the view towards determining the maturity and sandstone classification. Field lithological log description was noted by describing their textural characteristics, structures and post depositional effect such as diagenesis. The collected sandstone samples were subjected to geochemical analysis using atomic absorption spectrometry (AAS), petrographic and heavy mineral analyses. Field result shows that Gwandu sequence varies at the base from fine laminated herringbone structure sandstone, upward finning to claystone, intercalation of silty claystone and massive variegated heterolith claystone; capped by oolitic ironstone at the top. Geochemical analysis result of the sandstones revealed quantitative concentration of SiO2 and Al2O3 varying from 54.8257.35 and 29.02-31.23 respectively, while other heavy metals, trace and rare earth elements (REE) are very low in concentration. Relative high concentrations of Fe2O3 (4.06-4.94) indicated high ferromagnesian and iron minerals. CaO ranges from 1.44-1.96; reflected deposition in marine environment and due to dissolved diagenetic calcite cement. All other major oxides decrease with increase in SiO2 due to mineral dissolution with distance of deposition. Maturity of the sandstones was measured by SiO2/Al2O3 index, values range from 1.751.94 indicative of amount of clay and mineralogical immaturity. Na2O+K2O (alkali content) values range from 1.68 and 1.95 with an average value of 1.79, depictive of presence of feldspar and low chemical maturity. The Fe2O3/K2O ratio ranges from 4 to 5 is high, thus, low SiO2/Al2O3 ratio and a high Fe2O3/K2O ratio are described to be mineralogically less stable. The MMI values range between 1.80 and 2.91 classified in the Q=75-50% (F+RF) =25-50%; suggested that the sediments are immature. The cross plot of Log Fe2O3/K2O and Log SiO2/Al2O3 classified the sandstones to be mainly litharenite, relatively rich in ferrous content and rarely arkosic in nature, thus, mineralogically immature. The ZTR Index for the sandstone is 67.41%., suggestive of mineralogically immature sediments but the sub-rounded to rounded petrographic view suggested a texturally mature sediment deposited in a tidal flat of a marginal marine environment.
