SEISMITES IN THE FLUVIATILE BIMA SANDSTONES - IDENTIFICATION OF PALEOSEISMS AND DISCUSSION OF THEIR MAGNITUDES IN A CRETACEOUS SYNSEDIMENTARY STRIKE-SLIP BASIN (UPPER-BENUE, NIGERIA)

The Upper Benue Basin (northeastern Nigeria) is interpreted as a complex strike-slip basin which consists of a set of sub-basins controlled by synsedimentary N60 degrees E-trending, strike-slip faults which were active during Early Cretaceous times. The oblique planar cross-beds made of the medium- to fine-grained sandstones of the upper member of the Bima Sandstone Formation are affected by various soft-sediment i.e., early diagenetic but soft-sediment deformational structures. These are present not only close to the synsedimentary faults but also on a larger regional scale. Three main types of structures (undulated and recumbent-folded cross-bedded lamination, dislocated convolutions and concordant microbreccias, discordant fluidised intrusions) are described. These are related to processes of sediment deformation favored by sand hydroplasticity, liquefaction and fluidization. The local organization of these soft-sediment intrastratal deformational structures, their spatial distribution with respect to synsedimentaly faults, and their independance with respect to marginal paleoslopes, suggest that they cannot be explained by slumping or current drag. All the deformations are related to pore water expulsion and it is proposed that the required sand reorganization was triggered by seismic waves. The spatial and chronologic distribution and intensity of the various types of deformation structures suggest that they were produced around different epicenters repeated by earthquakes of varying magnitudes associated with the synsedimentary strike-slip tectonics.