Campanian alluvial processes in SW Anambra Basin, Nigeria: insights from sedimentology and lithofacies analysis of the Lokoja Formation

Abstract

Strata of the Campanian to mid-Maastrichtian Lokoja Formation exposed on road cuts in the Benin Flank, Anambra Basin, Nigeria, were subjected to facies analysis, visual textural analysis, sieve analysis, and pebble morphometry. The aim was to decipher the nature, transport mechanisms, and paleodepositional environment. Four distinct lithofacies—L1, L2, L3, and L4—were identified. Observed data show L1 and L2 as normally graded, poorly sorted trough cross-bedded, and planar cross-bedded matrix-supported gravels, respectively, which were categorized as gravel bars and bedforms lithofacies association representing braided streamflow paleodepositional processes. Similarly, L3 is described as poorly-moderately sorted normally graded planar cross-bedded sand, while L4 shows intercalations of thin beds of mud and sand. L3 and L4 were thereafter characterized as laminated sheet sand and mud lithofacies association depicting sheetflood paleodepositional processes. The braided streamflow and sheetflood paleodepositional processes are hypothesized to depict a middle-outer alluvial fan gross depositional environment. This inference is supported by binary discriminant plots of parameters from statistical analysis of granulometry data from 12 representative samples and pebble morphometry data obtained from 111 gravels from L1 and L2. Furthermore, paleocurrent data, textural analysis, and mineralogy of framework depict textural immaturity and low compositional maturity, which indicate first-cycle sedimentation, sourced from the Precambrian rocks of the Igarra Schist Belt, north of the study area. This contribution reinforces the multiple sediment provenance hypothesis of the Anambra Basin, comprising Precambrian basement and pre-Santonian rocks.