Composition and Textural Characteristics of Turonian Ezeaku Formation: Implication for Depositional Environment

Abstract

The depositional environment of the Ezeaku Sandstone outcropping as Amasiri Sandstone in Afikpo Basin in the southeastern part of the Benue Trough was studied using the composition and textural characteristics of the Sandstone. Samples were collected from the Ibii, Ozara-Ukwu, Akpoha, Amasiri-Akpoeze ridges for petrographic and chemical analyses. The results show that the sandstones are feldspathic arenites with matric content less than 15%. Quartz is the most ubiquitous mineral with micro fractures; also, grain margins and surfaces are corroded and altered by calcite. Euhedral quartz overgrowth is seen with dust rim.  The grains are angular to sub rounded, elongate grains were also common. Sorting is poor. Plagioclase and alkaline feldspars were observed, they have a modal composition of about 10 – 12%. Rock fragments were not common in the Ezeaku Formation. The grains were seen to be cemented mainly by calcite cement, silica cement also occur as quartz overgrowth and microcrystalline aggregates. Clay minerals and iron oxides are seen coating grains. The sandstone is compositionally and texturally sub mature. The chemical analysis shows the depletion of silica as a result of calcite replacement in the calcareous sandstone and concretions. Al₂O₃ value is high and it ranges from 9.766% to 23.55%. The sandstone is richer in K₂O than in N₂O, which is typical of arkoses. The results show that the Amasiri Sandstone was deposited in a shallow marine environment under humid conditions that allowed the segregation of calcite minerals into concretions within the Sandstone.

Keywords: Petrography, Calcareous sandstone, Calcareous concretions, Diagenesis, Paragenetic sequence, Sandstone composition, Plate Tectonics