3. Synthesis of Results: Logging While Drilling, Northern Barbados Accretionary Prism¹

J. Casey Moore²

ABSTRACT

Leg 171A collected logging while drilling (LWD) data at three sites in the northern Barbados accretionary prism and two in the section just seaward of the prism. These borehole logs, plus extensive information from previous Deep Sea Drilling Project and Ocean Drilling Program legs and a three-dimensional (3-D) seismic survey, provide new insights on the evolution of this accretionary prism.

Application of multivariate statistical methods to the LWD data quickly, reliably, and objectively define logging units that correlate well with the lithologic units from cored holes. Calculation of resistivity-based porosities allowed estimation of borehole velocities that were not directly measured by the LWD tools.

LWD data combined with previous coring results indicate that a low-density radiolarian claystone characterizes the proto-décollement zone, facilitates normal faulting in the incoming section, and localizes the décollement zone beneath the accretionary prism. Site 1045 data indicate that a low-density interval also characterizes the strong negative-polarity seismic reflections from the décollement zone. Both the individual LWD sites and an inversion of the 3-D seismic data for density indicate that the low-density décollement zone progressively consolidates with underthrusting. A northeasterly trending band of negative-polarity reflections in the décollement zone is an exception to this progressive densification process and shows anomalously low density caused by an arrested consolidation. Fluid flow from depth may account for the arrested consolidation of the northeasterly band of anomalously low density. Densification of the décollement zone is caused primarily by collapse of pore spaces between clay minerals and secondarily by collapse of radiolarian porosity, mineral phase changes, and zeolite precipitation. Previous coring results and, arguably, the seismic reflection data indicate that the décollement remains localized in an interval of low-density radiolarian claystone. Weakening of this smectite-rich lithology during shear explains the tendency of the décollement to remain localized in its initial position. An inadvertent two-hole hydrologic test conducted during LWD operations indicates significantly higher permeability over the scale of 50 m than that inferred previously from single borehole tests.