STRUCTURAL EVOLUTION AND MOBILE SHALE DEFORMATION IN THE NIGER DELTA

The Niger Delta is a Tertiary detached linked system driven by sediment loading and gravity, built on Cretaceous oceanic crust. Regional cross-sections and restorations of the eastern delta constructed from 2D and 3D seismic data show the structural evolution of paired extensional-contractional belts and the geometric aspects of detachment fold deformation in mobile shale. The delta consists of an updip extensional belt and downdip zones of transitional and contractional deformation linked by a regional detachment. The extensional belt is characterized by zones of N-dipping (counter-regional) and S-dipping (regional) normal faults. Sediment accommodation in the regional fault trend is largely by lateral movement of mobile substrate. The transitional belt is characterized by low relief, shale-cored detachment folds and normal faults. The contractional belt consists of 2 parts, the high relief shale-cored detachment fold belt (mobile shale) and the fold/thrust belt. In the mobile shale belt, anticlines are generally symmetric and characterized by parallel-folded cover and highly variable thickness in the underlying ductile shale zone. Palinspastic restoration of the mobile shale by area balance shows a high degree of lateral and vertical mobility. Isostatic restoration of the depositional wedge that is the precursor to the mobile shale suggests lateral movement of 10’s of kms from the extensional to the contractional domain. The fold and thrust belt is characterized by a train of asymmetric fault-related folds. The zone of ductile substrate is thin in this area, which may account for the change in structural style from high relief detachment folds in the mobile shale belt to a more “classic” fold/thrust belt style to the south. Basement ramps also play a role in the localization of the fold/thrust belt.