Grain-Size Distribution and Paleohydrodynamic Analysis of the Maastrichtian Ajali Sandstone, Anambra Basin, Nigeria

The Maastrichtian Ajali Sandstone of the Anambra Basin, southeastern Nigeria, was investigated to assess its grain-size distribution and paleohydrodynamic characteristics, to interpret its depositional environment and flow regime. Twenty representative samples were analyzed for statistical grain-size parameters, including mean grain size (۱.۲۹–۲.۱۰ϕ), standard deviation (۰.۴۱–۱.۱۳ϕ), skewness (−۰.۳۹ to ۰.۴۰ϕ), and kurtosis (۰.۶۴–۱.۵۰ϕ). These values indicate that the sandstones are coarse to medium-grained, moderately to moderately well sorted, and exhibit both positive and negative skewness, ranging from platykurtic to leptokurtic, indicating deposition under fluctuating high-energy conditions, including channel and possible beach environments. Bivariate and multivariate plots confirm a unidirectional, high-energy fluvial system, while the log-probability curves show mixed transport mechanisms (traction, saltation, and suspension). Paleohydraulic reconstructions suggest flow velocities of ۰.۵–۱.۲ m/s, consistent with braided river systems. Provenance analysis further indicates sediment derivation from the nearby Oban Massif and Basement Complex. Collectively, the data indicate that the Ajali Sandstone was deposited in a braided fluvial system influenced by episodic high-energy events in a semi-arid climatic setting, with minor tidal modifications in marginal zones.

کلیدواژه ها:

Fluvial sedimentation ، Paleoenvironment ، Sediment transport