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Sedimentology and Stratigraphic Architecture of Outcropping Tertiary Facies of The Anambra Basin

Abstract

Sedimentology and stratigraphic architecture of the outcropping Tertiary Facies of the Anambra basin were carried out using twenty lithologic outcrop units. The lithostratigraphic units in the study area include: Ebenebe Sand Member (Palaeocene); Nanka Sandstone (Eocene); Nsugbe Sands-Oligocene). These outcrops were exposed along the Onitsha-Otocha-Omor-Nsukka road and the Onitsha-Enugu Expressway. The univariate, bivariate, multivariate, pebble morphometry and sedimentary structures indicate that the sandstones were deposited in a variety of depositional settings such as fluvial, lagoonal, tidal and shallow marine environments. Petrographic and palaeocurrent results show that the sediments are derived from two sources namely the basement area and from pre-existing sedimentary terrain which exist east and northeast of the study area. The stratigraphic architecture shows various facies associations such as the tidally influenced channels, braided fluvial channels, flood plains and fluvial channels. The sequence stratigraphic analysis reveals that the sand bodies were deposited in varying systems tracts notably the Transgressive System Tracts (TST) and the Highstand System Tract (HST) which are separated by Transgressive Surface (TS) and Maximum Flooding Surface (MFS) with an inferred Sequence Boundary (SB). The sands are medium-coarse grained mainly moderately sorted, subrounded, negatively to positively skewed and leptokurtic in distribution. The sands are mainly quartz arenites with a good to excellent reservoir quality hence have the potential to accumulate hydrocarbons. A new model of a tide dominated depositional system is thus proposed in this study based on integrated ichnological and sedimentological data.