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INTRODUCTION (continued)

Drilling Objectives

To meet these scientific objectives, the primary drilling focus was to recover sediments representing each of the target intervals over a depth range of 2 km. Moreover, it was essential that these intervals be stratigraphically continuous and have sufficiently high sedimentation rates to resolve orbital cycles to at least the 41-k.y. periodicity. Reconstruction of the nature of the oceanic response to each of the target events further required that relatively unlithified sequences be recovered, from which well-preserved microfossils could be extracted for geochemical analysis. The preservation of these calcareous microfossils needed not to be perfect but sufficient to resolve relative differences in isotopic and elemental ratios of benthic and planktonic fauna reflective of changes in the oceanic thermal and chemical structure.

Overall, these objectives were achieved in material recovered at six sites at Walvis Ridge during ODP Leg 208. The K/P boundary interval was recovered at two sites, Sites 1262 and 1266. Three copies of the PETM were recovered at each of five sites over a depth range of 2300 m (Site 1263) to 4700 m (Site 1262). Two holes were drilled at Site 1264 to recover a nearly complete sequence for the Oligocene through Pleistocene. In addition, successful APC coring at all sites produced excellent and continuous recovery of most of the targeted intervals. Despite some condensed and unconformities, temporal coverage of the Paleogene through Neogene sedimentary sequences was excellent. The upper Maastrichtian interval was largely complete despite the necessary shift to XCB coring to recover chalky intervals at Site 1267.

Site Survey and Coring Strategy

During Leg 74 drilling in the Walvis Ridge area, the PETM and other critical intervals were not recovered at all sites, in part because of poor recovery and in part because of local unconformities. These unconformities, which appear erosional in nature, were most common in the Eocene and Oligocene sediments, particularly at shallow to middepth sites. These sites (DSDP Sites 525, 528, and 529) were located near gaps (channels) in the ridge that might explain the discontinuous nature of sedimentation. Based on seismic surveys, areas to the east of the existing sites appeared to provide more continuous sequences, which have a higher potential of recovering critical intervals. Thus, Meteor Cruise M49/1 (see report by Spieß et al., 2003) surveyed an area along the northeastern flank of Walvis Ridge that included the Leg 72 sites (Fig. F1). The main survey grid focused on a region to the north and east of the Leg 74 sites, away from a large channel that dissects the ridge to the southwest (Fig. F2). A seismic grid was established with several lines crossing existing sites in order to establish ages of key regional reflectors and sedimentary packages. Because of extensive upper Cenozoic downslope sediment transport throughout the region, the survey also targeted several isolated bathymetric highs where sediment transport might be minimal. The resultant M49/1 multichannel seismic profiles were then used, and existing DSDP site data were then used to locate stratigraphically continuous sequences of lower Cenozoic sediment at relatively shallow burial depths.

We designed a depth transect with 5 primary and 11 alternate sites (Fig. F2; Table T1). Based on the data, the unconformities encountered during Leg 74 appeared to be highly localized and more expanded, possibly continuous, sequences were identified. We maximized our potential to recover the P/E boundary and other key target intervals by selecting sites that had relatively thick Paleogene sequences and thin Neogene cover. In some instances, this approach required placing sites in channels where the Neogene was thinner but lower Paleogene horizons and bedding appeared uniform. A large number of alternate sites were identified for flexibility to move to a new site should a local unconformity be encountered at the P/E boundary.

The primary proposed sites for Leg 208, WALV-8A (Site 1264), -8E (Site 1263), -9B (Site 1265), -10F (Site 1266), -11B (Site 1267), and -12A (Site 1262), spanned a water depth range from 2507 to 4760 m. The stratigraphic targets for the two shallowest proposed sites, WALV-8E and WALV-9B (water depths of 2717 and 3059 m), included the PETM and EOGM in upper Paleocene to lower Oligocene chalks and oozes. We avoided areas near DSDP Site 525, where the upper Eocene and lower Oligocene are absent, and areas near Site 529, where several slumps were identified. Proposed Site WALV-10F (Site 1266), located to the south of Site 528 at an intermediate depth of 3811 m, appeared to contain a similar Paleogene sequence, but with much thinner Neogene cover. The deepest proposed site, WALV-12A (Site 1262), was located well north of DSDP Site 527 at 4770 m. Proposed Site WALV-11B was located near Site 527 but at a slightly shallower water depth of 4365 m.

A complete Paleogene sequence, including the PETM and EOGM excursions and the underlying K/P boundary, was the primary objective. Upper Paleocene sediments were drilled at five sites (Sites 1262 and 1264–1267), the upper Maastrichtian and lower Paleocene at two (Sites 1262 and 1267), and the Neogene at all six sites, including Site 1264 from which the lower Oligocene through Pleistocene was recovered. At least two APC and/or XCB cores were taken at each site to ensure recovery of a complete sequence, and cores were overlapped between holes to facilitate compilation of a composite section. During drilling composite sections were assembled using core log data (magnetic susceptibility, natural gamma radiation, and color reflectance) for correlation. In several cases, a third hole was cored to ensure recovery of at least two complete copies of the target intervals. This occurred when the target interval fell within core gaps of either of the first two holes or when the section was only partially cored.

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