Ductile Deformation in Deepwater Nigeria

Rion H. Camerlo
Nigeria/Mid-Africa SBU, Chevron, Houston, TX

The dominant structural style of the deepwater Niger delta lying between the extensional zone on the shelf and the outer thrust belt is uncertain. This structural domain is usually referred to as the “Inner Thrust Belt”. Contrasting structural interpretations such as shale diapirs and fault-related folds have been applied to the Inner Thrust Belt.

Several observations indicate that ductile deformation processes represent an important mechanism in deepwater Nigeria and particularly within the Inner Thrust Belt. Small ponded basins with local displacement of the underlying sediment can be found throughout the shoreward section of the “Inner Thrust Belt”. These small basins often coalesce into large smoothly convex downward basins and troughs. These large basins are commonly bounded by large extensional faults. Structural highs surrounding these larger basins commonly have thinning sediment wedges on their flanks, thin sedimentary sections on their crests, and are pierced by mud volcanoes. The crests of many of these structures have domino-style rotated blocks, highly rotated normal faults, and normal faults with large displacements and strong displacement gradients. The basinward edge of the Inner Thrust Belt is composed of features that have geometries very similar to those of salt tectonics, such as shortened diapirs, mini-basins, and allochthobous nappes. Shoreward, such features as rollers and reactive diapirs can be inferred. Basinward of the Inner Thrust Belt open detachment folds, shear fault-bend folds with elevated regional levels toward the inner fold belt, or large monoclinal folds are developed which indicate ductile thickening of the Akata decollement. The basinward limit of the “Inner Thrust Belt” may represent the general boundary between strongly loading influenced deformation (shoreward) from lateral contraction dominated deformation (basinward).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas