Lithofacies Architecture and Depositional Environment of the Lamja Formation, Yola Arm, Upper Benue Trough, Nigeria

Abstract

The Lamja Formation records the terminal marine-influenced phase of the Cretaceous succession in the Yola Arm of the Upper Benue Trough, but its outcrop-scale facies architecture remains incompletely constrained. Four sections at Gwalura Village (LS1), Lamza Sama 1 (LS2), Lamza Sama 2 (LS3) and Lokoro (LS4) were measured and described using lithology, bed thickness, sedimentary structures, fossils and vertical contacts. The exposed measured intervals range from approximately 5.2 to 20.26 m. Gwalura is the thickest and most heterogeneous section, Lamza Sama 1 contains mudrock and sandstone beneath recent alluvium, Lamza Sama 2 records repeated mudrock-sandstone and marine-influenced intervals, and Lokoro is comparatively sand-rich. Five lithofacies groups were recognised: sandstone, shale, shale-mudstone intercalation, limestone and coal. Planar-bedded and planar-laminated fine sandstone records tractional deposition in relatively higher-energy settings, whereas fissile shale and intercalated mudrock indicate suspension settling in low-energy prodelta to offshore-transition environments. Thin fossiliferous limestone marks episodes of reduced siliciclastic supply and marine influence, while coal represents peat accumulation in waterlogged delta-plain swamps. Repeated mudrock-to-sandstone stacking records progradation, whereas renewed mudrock and limestone deposition records landward shoreline movement or marine flooding. The four investigated sections therefore document a shallow-marine to paralic deltaic system with recurring shifts among prodelta, delta-front, marine-influenced and delta-plain conditions. These observations are compatible with transgressive-regressive change, but formal sequence boundaries and maximum flooding surfaces require lateral correlation, independent age control and supporting geochemical or palynological evidence.