Sedimentology and Stratigraphy of the Nanka Sand

ABSTRACT The Nanka Sand (~ocene, Ypresian Stage) is one of the several lithostratigraphic units comprising the lithic fill of the Anambra Basin, southeastern Nigeria. Its unitstratotype has been selected from a system of gullies in the Nanka town. Here the unit is 305 metres thick and is underlain by the Imo Shale and, farther southwest, overlain by the Ogwashi-Asaba Formation. The stratigraphic column may be divided into four cross-bedded and burrowed tabular sand subunits ranging from 50 to 90 metres in thickness, separated by three gypsiferous clay- and stilt-shale beds averaging 2.5 metres in thickness. The shale beds wedge out southwestwards. A formational status is here proposed for the Nanka Sand on the basis of its mappability, characteristic textural and structural attributes, and overall lithologic homogeneity. #' The unit is composed of loose to very weakly c~nsoli&~ted quartz arenite completely devoid of cement. Other lithotypes, I usually occurring in minor amounts, are shale, mudstone and fefnuginous sandstone, The sand is medium Lo coarse grained, occasionally pebbly, subrounded, poorly to moderately sorted, positively skewed and mesokurtic to leptokwtic in distribuj tior!. The heavy mineral population includes a stable suite cf zircon, tourmaline rutile, and a metamorphic suite I consisting of kyanite, ,'sillimanite and staurolite.