Description of soft-sediment deformation structures in the Cretaceous Bima Sandstone from the Yola Arm, Upper Benue Trough, Northeastern Nigeria

The upper member of the Bima Sandstone (B3), a Cretaceous sedimentary unit in the Yola Arm of the Upper Benue Trough, has abundant soft-sediment deformation structures that include cusps, droplets, convolute bedding, deformed cross-bedding and sand volcanoes. The structures in most outcrops are sandwiched between undeformed cross-bedded strata that have no major textural differences with the sediments hosting them. The cusps are both simple internal cusps and interpenetrative cusps and are formed by post-depositional fluidization triggered by seismic shocks, where the interpenetrative cusps serve as conduits through which sands rose to the surface to form sand volcanoes. The droplets are the discrete type associated mostly with complex deformed cross-beddings, while the convolute bedding forms concentric antiforms and synforms without any evidence of faulting and gradually die out vertically upward. The deformed cross-bedding is represented by both simple and complex recumbent folds of flood and seismically induced origin respectively. The source of the seismic shocks may be episodic syndepositional Mesozoic volcanism of the Jurassic to Albian times within the Upper Benue Trough.