Sedimentology and Diagenetic Evolution of Maastrichtian Fugar Sandstone, Western Flanks of Anambra Basin, Nigeria

Abstract

Diagenetic studies unravel the different diagenetic processes a rock has been subjected to and this plays an important role in the formation of sedimentary rocks, therefore the detailed study of the sedimentology and diagenetic evolution of the Fugar Sandstone depicts the diagenetic history and the paragenetic sequence of the cements present. The Fugar sandstone is an extension of the Upper Cretaceous Ajali Sandstone unit of the Anambra Basin in South western Nigeria and this study was aimed at understanding the sedimentological and petrographical characteristics of the sandstone with a view to model the sandstone diagenesis. The field study and granulometric analysis results reveals that the sandstones are unconsolidated, friable, fine to medium-coarse grained, poorly sorted, planar and herringbone cross-bedded and exhibits a repeated succession of fining upward cycles with the inferred depositional environment of fluvial to marginal-marine. The samples were analyzed using a range of techniques: X-ray diffraction, X-ray fluorescence and Optical microscope which shows that the sandstone is quartz arenite with an average of 95% quartz consisting of subangular to subrounded grains and this implies that the sandstone is mineralogically matured but texturally immature. Detailed diagenetic study using thin section petrography reveals that the rocks have undergone different diagenetic processes; compaction, cementation, dissolution, alteration and replacement, and the paragenetic order of the cementing minerals were from authigenic quartz which occurs as quartz overgrowth, clay and then Iron oxides which serves as the predominant mineral cements in the rocks.