Lithofacies and Pebble Morphogenesis: Keys to Paleoenvironmental Interpretation of the Nkporo Formation, Afikpo Sub-Basin, Nigeria

Abstract

The Nkporo Formation in the Afikpo Subbasin, Nigeria, is dominantly shaly but contains two sandstone members within it. These sandstones host vein quartz pebble beds and dispersed clasts which have been used for pebble morphometric analysis. As independent functions and dependent variables, pebble morphometric indices of Coefficient of Flatness, Elongation Ratio, Maximum Projection Sphericity, Oblate-Prolate Index, Roundness and Form suggest deposition of the sandstones in fluvio-deltaic to littoral settings. Lithofacies analysis identified six lithofacies, grouped into estuarine delta and marine offshore lithofacies associations. These include trough cross bedded sandstone, ophiomorpha burrowed, wave/rippled and planar cross-bedded lithofacies, Inclined heterolithic lithofacies, fine grained, silty sandstone, lignite/coals, shales/mudstone, interbedded with limestones and oolitic ironstones lithofacies. The estuarine lithofacies association shows a tripartite subdivision into meandering estuarine and tidal point bars overlain by estuary funnel tidal creek sandstones, carbonaceous beds; and estuary mouth tidal and barriers bars and shallow marine shales in a drowned incised valley. The fining upwards lithofacies pattern suggests deepening and transgression of the sea in a tidal estuarine setting.