SOFT-SEDIMENT DEFORMATION STRUCTURES AND GRANULOMETRIC PROPERTIES DISTRIBUTION IN AJALI SANDSTONE RIDGES, WESTERN AFIKPO BASIN, UTURU, NIGERIA

Three Ajali sandstone ridges (L1, L2 and L3) at Uturu being quarried for construction sands were studied for soft-sediment deformation structures (SSDS) and granulometric properties distributions. SSDS that includes recumbents foresets, sands dykes, flame structures and fluid escape tubes were identified only in ridge L3. The geometry of the SSDS indicates sediment loading/density contrast, fluidization and liquefaction as the mechanisms for their formation but with liquefaction as the most dominant mechanism. Gran size analysis and granulometric curves properties calculations show that: Mean grain size ranged from fine (1.18 ɸ) to medium (2.57 ɸ); Sorting ranged from rarely poorly sorted (1.13ɸ) to well sorted (0.37ɸ) but with mean values in each ridge as moderate sorted; Skewness ranged from strongly fine skewed (1.0) to strongly coarse skewed (-1.57); and Kurtosis ranged from very platykurtic (0.27) to very leptokurtic (2.0) but with sands of ridge L3 mainly very platykurtic. Granulometric curves and bivariate plot of properties indicate fluvial deposition with rare marine influence. Results show that there is no significant variation in sediment properties and depositional environments across the three ridges. The localization of SSDS and non-proximity to any fault suggest that liquefaction, as the dominant mechanism for soft-sediment deformation, was not triggered by an earthquake. Possible mechanisms include rapid sediment loading, localised sudden subsidence induced by loading of localised oxidized compressible peats and coal; and increased in sediments’ water saturation via localised groundwater seepage. Fine grains, well sorting, fine to strongly skewed very platykurtic characteristics of sediments made it more susceptible to liquefaction.