Sedimentologic and Petrographic Study of Outcrops of Ajali Sandstone in Okigwe, Uturu and Isiukwuato, Anambra Basin Southeastern Nigeria

Abstract

The Ajali Formation outcrops at Okigwe, Ihube, Onyekaba mine, Uturu and Isiukwuato in the Anambra Basin southeastern Nigeria were evaluated for an integrated field, sedimentologic and petrograpic methods. The study was aimed at presenting a better understanding of the depositional processes and provenance. Five lithofacies were altogether described namely: planar Cross bedded sandstone facies A, Massive sandstone facies B, Ferruginized sandstone facies C, herringbone cross bedded sandstone facies D, and Bioturbated sandstone facies E. The facies were deposited in a fluvially influenced shallow marine environment. The overall grain size interpretation shows that sediments from the study area are fine to coarse grained with values ranging from 0.2 to 2.0Φ, poorly sorted to very well sorted with range value of 0.28 to 1.2Φ, nearly symmetrical to strongly fine skewed with values of -0.04 to 0.59Φ and platykurtic to very leptokurtic with values of 0.79 to 2.46Φ. An inspection of the cumulative frequency curves of the studied samples shows that the sediment population is dominated by saltation population. Texturally, the sandstone sediments are sub-mature containing grains that are moderately to poorly sorted and not well rounded. The Mineral Maturity Index of the sandstone in the study area ranges from 24-49 which falls within the mature to super mature class. The high quartz content and insignificant amount of feldspar and rock fragments further support this assertion. The study suggest the sandstones must have been subjected to substantial high degree of reworking that resulted in the removal of the ferromagnesian minerals and feldspar typical of recycled sediments.