In the Northern Barbados Ridge area, the décollement zone beneath the accretionary prism lies in a lower Miocene radiolarian claystone. Seismic data allow recognition of this continuous stratigraphic horizon seaward of the accretionary prism. Logging-while-drilling (LWD) data show that the radiolarian claystone is of low density relative to surrounding sediments and indicate that the interval densified heterogeneously with progressive deformation. The low-density interval is relatively weak, and possibly overpressured, which together foster décollement zone propagation. Characterization of densification processes in this interval by image analysis shows that reorientation of clays by shear-enhanced consolidation and tectonic overburden is the dominant densification process. The correlation between densification and clay reorientation suggests that predeformational conditions play a deciding role in décollement propagation.
3D seismic reflection data show that the topographic relief of the décollement zone arcward of the deformation front is greater than that in the protodécollement zone and is apparently controlled by basement structure. Thus, it is unlikely that the processes forming the décollement zone are truncating relief in the protodécollement surface, but rather sliding over it along a weak stratigraphic interval. Although localization of the décollement may be originally due to low density and weak sediments, strain softening in a smectite-rich lithology and, perhaps, development of a low permeability cap may contribute to the continued deformation in this stratigraphic interval.