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

摘要

The genetic relationship between depositional processes and rock properties provide 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 shows Nanka Sand as medium to coarse grain sand that is loose and unconsolidated, with cross bedded white to yellow sand having intercalation of silty sand and clay with bands of fine grained sandstone and sandy clay on top. The granulometric study shows Nanka Sand to be coarse to medium grain and negatively skewed. Sorting is moderately well sorted to poorly sorted, and kurtosis is leptokurtic. The frequency histogram showed both unimodal and bimodal characters. 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 the observed sedimentary structures include reactivation surfaces, clay drapes, flaser and wavy beddings, burrows of Skolithos and Ophiomorpha , with herringbone and planar cross beds, indicative of tidally influenced environment. Three distinct lithofacies observed were heterolithic, cross bedded sand facies, channel sand facies, and clay stone facies, and these has allowed the depositional environment of the Eocene Nanka Sand to be interpreted as a tide dominated estuaries characterized by tidally influenced fluvial to mixed energy environment.