Problems of classification and maturity evaluation of a diagenetically altered fluvial sandstone

Abstract

The fluvial sandstones of the late Cretaceous Makurdi Formation of the middle Benue Trough of Nigeria are composed of feldspathic wackes (average matrix content 25.4%) and subarkosic arenites (average matrix content 9.3%). Except for the differences in the matrix content, the framework mineralogy of the two groups of sandstones is very similar. The interstitial clay matrix is both primary and diagenetic in origin. Regression analysis shows an inverse correlation between the proportion of matrix and the incidence of detrital feldspars in the sandstones. It implies that postdepositional alterations of feldspars have given rise to a mineral assemblage which is more mineralogically and less texturally mature than the original deposit, thus invalidating genetic significance in sandstone nomenclature. Neither the clay content nor the zircon-tourmaline-rutile (ZTR) index is a reliable measure for textural or mineralogical maturity of the Makurdi sandstones. It is proposed that several other attributes, such as roundness, ratio of undulose-to-nonundulose quartz, and MQ/PQ ratio which are less susceptible to diagenetic alteration, can be used as reliable indices of textural maturity. A mineralogical maturity scale, based on inert versus labile components (Q/F+L) of framework composition of a sandstone, is proposed to facilitate quantitative analysis of maturity data.