Provenance of the Campanian-Maastrichtian Shales, Anambra Basin: Evidence from Clay Mineralogy

Abstract

The origin, formation and history of the Campanian-Maastrichtian Anambra basin-fill are issues of considerable interest and importance especially in the reconstruction of the basin evolution and assessment of the hydrocarbon potential. Previous sandstone petrographic studies point to the easterly Cameroon Precambrian basement complex as the primary source of the Anambra Basin detritus and ascribe the high compositional maturity index to intense chemical weathering both at source and during transport.  The provenance interpretation presented in this work is based on results of mineralogical analysis of the mud rocks sampled at regular intervals of the Campanian-Maastrichtian succession and analyzed for clay minerals, using X-ray diffractometer technique. The results show that quartz and kaolinite are the most abundant minerals in the mud rocks. Illite is resident in lesser quantities in all samples, and some samples also contain appreciable amounts of smectites and mixed layer (illite/smectites) clays. The relative abundance and distribution of clay minerals in the shales analyzed in this work suggests primary derivation from an igneous complex dominated by intermediate to basic rocks.Trends in the quartz, clay mineral ratios and the kaolinite abundance strongly suggest derivation from the Precambrian basement complex of the Cameroon and the arkosic pre-Campanian strata of the Abakaliki Anticlinorium, and supply into the Anambra Basin by rivers.  This interpretation is consistent with views held by earlier workers that erosion of the pre-Campanian sediments and weathering of the adjoining basement blocks sourced the kaolinite-rich mud rocks of the Anambra Basin. The result of this research represents a useful contribution to the on-going review of the sedimentology and hydrocarbon potentials of the Anambra Basin.

Keywords: Provenance; Campanian-Maastrichtian; Anambra Basin; Clay Mineralogy