Grain Size Distribution Patterns within the Calabar River: Implications for Paleoenvironmental Reconstruction

Abstract

This study investigated sediment distribution patterns within the Calabar River, southeastern Nigeria, with the aim of improving the recognition of tidal facies and enhancing paleoenvironmental reconstruction of ancient estuarine deposits. A total of 50 geo-referenced bottom sediment samples were collected in September 2011 using a Van Veen grab sampler along a 24 km tidal channel covering approximately 17.45 km². Tidal current measurements and bathymetric surveys were conducted during spring, mean, and neap tidal cycles. Maximum surface current velocities ranged from 0.41 to 0.61 m s⁻¹ at Adiabo Bridge and from 0.65 to 0.95 m s⁻¹ at Marina Beach, while near-bottom velocities varied from 0.27 to 0.43 m s⁻¹ and 0.46 to 0.67 m s⁻¹, respectively. Velocity profiles revealed an ebb-dominant hydrodynamic regime indicative of tidal asymmetry and net seaward sediment transport. Bathymetric analysis showed channel depths ranging from 2 to 20 m, with the deepest sections occurring within the central channel. Grain-size analysis revealed predominantly bimodal distributions comprising mud, sand, gravel, and mixed sediment fractions. Textural classification showed that sand and muddy sand constitute the dominant sediment types throughout the river system. Statistical analyses of mean grain size, sorting, skewness, and kurtosis indicated strong spatial variability linked to hydrodynamic energy conditions and sediment transport processes. Three sedimentary facies were identified: Facies A (upstream), extending approximately 6 km and characterized by fine-grained sands and muddy sands deposited under flood-dominated conditions; Facies B (central), extending approximately 8 km and dominated by medium sands with gravelly patches associated with maximum tidal energy and depths reaching 20 m; and Facies C (downstream), extending approximately 10 km and consisting mainly of medium sands and muddy sands deposited under ebb-dominated conditions. The facies succession reflects the combined influence of tidal asymmetry, channel morphology, and sediment supply on sediment distribution. These findings demonstrate that grain-size statistical parameters provide reliable indicators of depositional processes and facies architecture in meso-tidal rivers. The Calabar River therefore serves as an important modern analogue for interpreting ancient tidal-channel and estuarine deposits preserved in the geological record.