171A Scientific Prospectus


Operations will commence by dropping beacons for all sites, and the sites will then be drilled in the following order: NBR-11A, 5A, 9A, 10A (Figs. 1, 2).

Primary Sites
Proposed Site NBR-11A

Proposed Site NBR-11A is located at the oceanic "reference" Site 672, 6 km east of the deformation front. This site showed incipient deformation and a geochemical anomaly at the stratigraphic level of the projected décollement zone. LWD here will provide information on the inception of deformation and fluid flow in the incoming sedimentary section, as well as a general overview of physical properties of the oceanic sedimentary section.

Proposed Site NBR-5A

Proposed Site NBR-5A will establish the physical properties of the negative polarity reflections in the Barbados prism. It is located in an area of negative polarity about 2500 m west of the deformation front. Shipley et al. (1994) predict that the negative polarities are dilatant zones. Accordingly, they may be characterized by "hydrofractures" or zones of fluidized sediment more numerous than those encountered at Site 948. Because the depth of the décollement is 400 m as opposed to the more than 600 m at Site 947, and the negative amplitude is less than at Site 947, proposed Site NBR-5A can be successfully completed. The site has never been cored; however, safety problems are not anticipated because nearby penetrations show negligible hydrocarbons. Correlations from nearby holes and the three-dimensional seismic data should provide basic lithologic information.

Proposed Site NBR-9A

Proposed Site NBR-9A, located at CORK Site 949, 1800 m west of the deformation front, will establish the physical properties of a décollement zone with intermediate reflection polarity characteristics, and determine the physical properties profile at this borehole seal site. This site is also cut by an imbricate thrust fault that is actively deforming the accretionary prism, and will provide information on the physical properties of thrusts.

Proposed Site NBR-10A

Proposed Site NBR-10A, located at Site 676, will determine the character of the initial deformation of the accretionary prism. This site is located about 800 m inboard of the deformation front, and penetrates the incipiently developed décollement zone and several thrusts in the offscraped section.

Alternate Sites
Proposed Site NBR-1A

Proposed Site NBR-1A is an alternate for proposed Site NBR-11A. This site, approved for Leg 156, is located about 4 km closer to the deformation front than NBR-11A and provides similar reference information.

Proposed Site NBR-8A

Proposed Site NBR-8A is an alternate to NBR-5A. All primary features and objectives are the same.

Proposed Site NBR-12

Proposed Site NBR-12 is a reoccupation of Site 541. Permission is currently being sought for this site. The site is located where the décollement is of positive polarity. Site 541 was continuously cored and contains several thrust faults from which we would like to obtain LWD signatures.

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