Frank Bilotti¹, John H. Shaw²

(1) Texaco, Bellaire, TX
(2) Harvard University, Cambridge, MA

ABSTRACT: Modeling the compressive toe of the Niger Delta as a critical taper wedge

The compressive toe of the Niger Delta is driven by gravitational collapse of shelf sediments. The compressive deformation occurs above a regional detachment in the marine Akata Fm. and the accumulated compressive deformation creates a regional seafloor slope away from the delta. This system of sloping seafloor, basal detachment and internally deforming wedge of material can be modeled as a critical-taper accretionary wedge like those at active margins. The theory of critical taper wedge mechanics is that the wedge grows with a constant surface slope or taper as material is added to the toe. The constant taper is maintained by internal deformation of the wedge, which is always on the verge of failure.

There are important differences between gravitationally driven wedges and accretionary wedges at convergent margins. At convergent margins the primary source of material input to the wedge is from sediments scraped from the subducting seafloor; otherwise, there is little modification of the overall shape of the wedge. In the case of the Niger Delta material is added to this system both at the deformation front and through sedimentation from the delta. This additional sedimentary input both modifies the taper of the wedge and provides more drive to the overall gravitational system. This modification of the wedge's taper is more similar to sub-aerial wedges, which are subject to erosion that constantly modifies the taper of the wedge.

The complex nature of the Niger Delta wedge system results in deformation that does not simply propagate toward the foreland. The syntectonic sediments over emergent deepwater fault-related folds record this complex history and are used here to unravel the sequence of deformation throughout the deepwater Niger Delta. Combined with a regional stratigraphic mapping effort, we strive to understand the relationships between temporal depocenters and periods of outboard contraction.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado