地球科学と気候変動ジャーナル

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Paleo-Environmental Deductions from Grain size Analysis: A Case Study of the Eocene - Oligocene Sand Facies in Osumenyi and Ukpor areas of South-Eastern, Nigeria

Chidera Ikechukwu V, Egbunike Michael E, Okpoko Ephraim I, Okpala Emmanuel C and Onwuka Chisom J

The Eocene–Oligocene sand facies is well exposed around Osumenyi and Ukpor areas in the Anambra Basin of southeastern Nigeria. Outcrop sections were located, measured, described in detail and sampled so as to decipher the paleoenvironment of deposition, using an integration of sedimentary facies and grain size analyses. The sedimentary facies study suggests that the Eocene– Oligocene age in this area consist mainly of three facies association; sandstone, lignite and clay facies. The fine-grained sandstone sub-facies is indicative of shoreface, the medium grained sandstone sub-facies is indicative of estuarine, while the coarse-grained sandstone sub-facies suggests a fluvial environment. The lignite facies indicate a continental origin (swamp environment) while the clay facies are interpreted as fluvially intercepted shallow marine environment. Results of grain-size analysis for the Eocene– Oligocene age show that the sands are medium grained, moderately to poorly sorted, very negatively to positively skewed and very platykurtic to very leptokurtic. Bivariate plots of mean size against standard deviation and skewness against standard deviation support a fluvially dominated shallow marine environment with tidal crosscuts. Result of multivariate functions showed a fluvially intercepted shallow marine environment. This study is significant in providing evidence for the reservoir potentials and aquifer characteristics of the Eocene – Oligocene sand facies for petroleum accumulation and prolific water bearing units within the basin.