Eocene tidal sedimentation in the Anambra basin,Southern Nigeria

The Eocene Nanka Formation of the Anambra basin in southern Nigeria consists, in its type area, of four sand subunits each 50–90 m thick, separated by three gypsiferous glauconitic shales each about 2.5 m thick. The sand subunits are unconsolidated, uncemented quartzarenites, planar and trough cross-stratified, flaser and lenticular bedded, and burrowed (Ophiomorpha and Skolithos). Texturally the sands are medium to coarse, moderately sorted, positively skewed and leptokurtic. Two parallel belts, each with a distinctive paleocurrent pattern, are identified: one lying along the present eastern and northern margin of the formation, characterised by a shore-normal paleocurrent pattern; and the other lying on the west, distinguished by a shore-parallel orientation.An integrated analysis of stratigraphic, petrographic, textural, and structural data of the formation enabled reconstruction of an environmental model for the sand body. The model depicts sedimentation in a tidally influenced marine shoreline environment in which an intertidal and a subtidal zone can be delineated. Each zone is characterised by an assemblage of several sedimentary features. Interlaminations of shale and thin sand lenses, gypsiferous and glauconitic shale beds, flaser and lenticular bedding, herringbone structures, and shore-normal paleocurrent pattern are among the features that delineate the intertidal facies. The abundantly cross-stratified, coarse elastic sand-bars with rapid fining-upward sequences, burrowed pebbly horizons overlain by fine sand, and shoreparallel paleocurrent are the distinguishing features of the subtidal facies. The sandflat facies of the intertidal zone and the sandbar facies of the subtidal zone are, however, inseparable in most cases.