Sedimentological, Facies Architecture and Palaeoenvironmental Reconstruction of Ajali and Mamu Formations exposures in Iyale and Agbenema, Northern Anambra Basin, Nigeria

Abstract

This study provides an integrated investigation of the sedimentology, facies architecture, 
and palaeoenvironmental evolution of the Ajali Sandstone and Mamu Formation in the northern 
Anambra Basin, southeastern Nigeria. Detailed fieldwork along the Iyale–Agbenema corridor 
combined stratigraphic logging, granulometric analysis, petrographic studies, and heavy 
mineral characterization with facies classification and palaeoenvironmental reconstruction. 
The Ajali Sandstone is composed predominantly of medium– to coarse–grained, poorly sorted 
quartz arenites, characterized by prominent cross-bedding, flaser structures, herringbone 
cross-stratification, and Skolithos burrows. Grain-size parameters, C–M patterns, and linear 
discriminant functions collectively indicate deposition in tidally influenced fluvial–deltaic 
systems, shaped by alternating phases of high-energy braided river discharge and nearshore 
marine reworking. Petrographic evidence, complemented by heavy mineral assemblages 
enriched in zircon, tourmaline, and rutile, suggests high textural maturity and points to 
multicycle sediment recycling sourced from uplifted basement complexes and recycled 
sedimentary terranes. In contrast, the Mamu Formation is dominated by laminated clays, 
siltstones, shales, and ferruginised horizons bearing root traces and plant debris. These 
facies reflect deposition within lower delta plain to prodelta environments, marked by 
fluctuating redox conditions, quiet-water sedimentation, and episodic subaerial exposure. 
Together, the Ajali–Mamu successions capture the complex interplay of Late Cretaceous 
transgressive–regressive cycles in southeastern Nigeria, where proximal braided fluvial 
systems repeatedly prograded into coastal plain and shallow marine environments. The 
integration of sedimentological, petrographic, and provenance data highlights this succession 
as a key stratigraphic archive for reconstructing basin evolution, sediment dispersal 
pathways, and palaeoenvironmental dynamics within the Benue Trough system. Overall, these 
findings refine existing depositional models and underscore the controlling role of tidal–fluvial 
interactions in shaping the sedimentary architecture of paralic basins during the Late 
Cretaceous.