Lithofacies Analysis and Depositional Environments of the Eocene Nanka Sands, as Exposed at Alor and Environs, Southeastern Nigeria: Evidence from Field Study and Granulometric Analysis

The genetic relationship between depositional processes and rock properties provides a potentially powerful tool for interpreting ancient depositional environments. Field study and granulometric analysis have been used to detect the lithofacies and depositional environment of the Eocene Nanka Sand. The field data show Nanka Sand as medium- to coarse-grained, loose and unconsolidated, with cross-bedded white to yellow sand intercalated with silty sand and clay, together with bands of fine-grained sandstone and sandy clay. The granulometric study shows the Nanka Sand to be coarse- to medium-grained and negatively skewed. Sorting ranges from moderately well sorted to poorly sorted, and kurtosis is leptokurtic. Frequency histograms display both unimodal and bimodal characteristics. Bivariate results together with linear discrimination functions of the sand samples reveal both shallow agitated marine and fluvial deposits. The paleocurrent shows bimodal-bipolar current directions, while observed sedimentary structures include reactivation surfaces, clay drapes, flaser and wavy bedding, burrows of Skolithos and Ophiomorpha, and herringbone and planar cross-beds, indicating a tidally influenced environment. Three distinct lithofacies were identified: heterolithic cross-bedded sand facies, channel sand facies, and claystone facies. These characteristics indicate that the Eocene Nanka Sand was deposited in tide-dominated estuaries characterized by tidally influenced fluvial to mixed-energy conditions.