Synthesis of results: logging while drilling, northern Barbados accretionary prism

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 resistivitybased 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 lowdensity radiolarian claystone characterizes the proto-decollement zone, facilitates normal faulting in the incoming section, and localizes the decollement zone beneath the accretionary prism. Site 1045 data indicate that a low-density interval also characterizes the strong negative-polarity seismic reflections from the decollement zone. Both the individual LWD sites and an inversion of the 3-D seismic data for density indicate that the low-density decollement zone progressively consolidates with underthrusting. A northeasterly trending band of negative-polarity reflections in the decollement 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 decollement zone is caused primarily by 1Moore, J.C., 2000. Synthesis of results: logging while drilling, northern Barbados accretionary prism. In Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 171A, 1–25 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Earth Sciences, University of California at Santa Cruz, Santa Cruz CA 95064, USA. cmoore@es.ucsc.edu Initial receipt: 14 December 1999 Acceptance: 7 July 2000 Web publication: 2 October 2000