PALEOEVIRONMENT OF NIGERIA'S AJALI SANDSTONES: A PEBBLE MORPHOMETIC APPROACH.

The Ajali Sandstone is a major clastic formation of Campanian-Maastrichtian age occuring within the Anambra Basin, Southeastern Nigeria. The Sandstones have a high incidence of quartz and feldspar pebbles. Clasts of vein quartz pebbles were selected for morphometric study of decipher the depositional environment of the sandstone. The three mutually perpendicular axes of each pebble were measured and the roundness estimate with the aid of a roundness image set. Morphometric parameters such as size, flatness ratio, elongation ratio, elongation ratio, maximum projection sphericity, form geometry and oblate index were computed. Bivarite scattergrams of roundness versus oblate-prolate index were plotted. Except for the flatness ratio which is non-diagnostic, all the other results of the morphometric studies indicated a fluvial – dominated depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment based on analysis of faceis, sedimentary rock which are suitable for morphometrical research: those with isotropic constitution and high resistivity to wear such as cherts, quartz sandstones and some related types. KEY WORDS: Clastic formation, Fluviatile, Fluviodeltaic, Elongation ratio, Oblate index. Global Jnl Geological Sciences. Vol.2(1) 2004: 37-4