Next Section | Table of Contents

SITE SUMMARIES (continued)

Site 1263

Latitude 28°31.98'S

Longitude: 02°46.77'E

Water depth: 2717 m

Maximum depth of penetration: 345.6 mbsf

Oldest formation: late Paleocene

Time on site: 6.79 days (1515 hr on 29 March–015 hr on 5 April)

Site 1263 (proposed Site WALV-8E) is located along the northwestern flank of Walvis Ridge just a few hundred meters beneath the summit (Figs. F16, F17). At 2.72 km water depth, this site represents the shallow end-member of the Leg 208 depth transect. Situated well above the level of the CCD throughout the Cenozoic, Site 1263 was expected to yield a stratigraphically continuous and expanded sequence of Paleogene pelagic nannofossil ooze. The primary objective was to recover sedimentary sections with sufficient resolution to detail orbital scale changes in bottom water chemistry and circulation at shallow bathyal depths during several of the key paleoceanographic events of the Paleogene including the Eocene–Oligocene transition, EECO, and the PETM. Ideally, this requires 100% recovery of the sedimentary section with minimal coring disturbance, a requirement that is best met by using APC coring. To this end, the site was positioned along a slope just beneath the ridge crest where the Neogene overburden is thin and the Paleogene target intervals are at subbottom depths of <300 m.

Four holes, offset ~20 m from each other, were cored at Site 1263 using the APC and XCB coring systems to recover a 346-m-thick section of upper Paleocene to Pleistocene nannofossil ooze and chalk. Hole 1263A was cored with the APC from the mudline to 284 mbsf and with the XCB to 346 mbsf. Hole 1263B was cored with the APC from 46 to 262 mbsf and with the XCB to 339 mbsf. Hole 1263C was cored with the APC from 90 to 119 mbsf and from 203 to 286 mbsf and with the XCB to 291 mbsf. Hole 1263D was cored with the APC from 272 to 287 mbsf. Nominal recovery averaged ~100% with the APC and ~60% with the XCB. Total nominal recovery for the site was 91%. Using MS data, cores from the four holes were aligned by depth shifting and representative intervals were spliced to create a continuous stratigraphic section for the Eocene and uppermost Paleocene extending from 48 to 340 mcd. The sections above and below were not completely recovered. Cores in those intervals were depth shifted, assuming a constant growth rate of 18% in the composite section. The total length of the composite is 401 mcd.

The sediments recovered at Site 1263 are comprised predominantly of calcareous nannofossil ooze and chalk with relatively little downhole variability (Fig. F18). Calcium carbonate contents are typically 90–95 wt% (except for the P/E boundary interval). As a result, only one lithologic unit and three subunits were recognized. Subunit IA (0–99.1 mcd) is upper Eocene to Pleistocene foraminifer-bearing nannofossil ooze and nannofossil ooze. The Pliocene–Pleistocene interval (0–26 mcd) has sedimentation rates of 1–6 m/m.y. The upper Oligocene to upper Miocene section (26–48 mcd) is condensed, shows evidence of extensive winnowing as well as reworking and downslope transport (slumps and turbidites), and possibly contains one or more hiatuses. The upper Eocene to lower Oligocene section (48–99 mcd) appears to be complete, although reworking is indicated by calcareous microfossils. Subunit IB (99.1–318.0 mcd) is a nannofossil ooze, clay-bearing nannofossil ooze, and chalky nannofossil ooze with occasional layers of volcanic ash and disseminated chert or chert stringers, most of which were easily penetrated by the APC. Eocene Subunit IB has sedimentation rates of 5–15 m/m.y. (Fig. F19). The Paleocene to lowermost Eocene Subunit IC (318.0–400.7 mcd) is composed of nannofossil ooze and chalky nannofossil ooze. Several chert horizons are present in the upper Paleocene section. The P/E boundary interval is represented by a 100-cm-thick "clay" layer at ~335 mcd. The basal contact is sharp with MS rising rapidly and carbonate content decreasing from ~90 wt% to <1 wt% over the first 5 cm of the transition. The upper contact is gradational with carbonate content increasing to >90 wt% over 100 cm. Sedimentation rates in Subunit IC are generally 10–20 m/m.y. and reach 30 m/m.y. near the top of the subunit in the lowermost Eocene.

The P/E boundary clay layer proved to be an exceptionally challenging target to recover at this site. At the level of the clay layer in Hole 1263A (285 mbsf), the APC took only a partial stroke but still penetrated into the top of the layer, 40 cm of which was captured in the core catcher. Switching to the XCB in the next core, we were able to recover another 50 cm of the clay layer. However, the contact with underlying Paleocene ooze, which was softer, appeared unconformable. In Hole 1263B, we elected to use the XCB to penetrate the boundary layer. This time, ~30 cm of the upper portion of the partially silicified clay unit lodged in the core catcher, apparently as the core was being cut. As a result, the remaining portion of the clay layer and uppermost Paleocene were washed away. In Hole 1263C, we again elected to use the APC, this time backing off a few meters to allow the core barrel to attain maximum velocity before striking the critical sediment interval. Although the stroke was only partial, the core barrel penetrated the entire clay layer plus 50 cm of the underlying Paleocene ooze. Because this represented the only copy of the entire P/E clay layer at this site, we decided to drill a fourth hole. In Hole 1263D we applied the same strategy as in the previous hole, firing the APC from a point 2 m above the bottom of the hole. This time the core barrel penetrated just the upper 30 cm of the clay layer. The second APC attempt, again fired from 2 m above the bottom of the hole, penetrated the remaining 50 cm of the clay layer as well as 150 cm of the upper Paleocene. As a result, the basal contact was recovered intact, providing the second complete copy of the critical interval for this site.

The moderate sedimentation rates and generally high carbonate content of the Site 1263 sediments are consistent with the relatively shallow depth of the site. During the Paleogene, the site remained well above the lysocline depth (<2 km). This is most evident with the presence of carbonate-rich upper Eocene and Oligocene sections, which are largely represented by condensed clay units in deeper segments of the ridge and surrounding seafloor. The lone exception is the 1-m-thick P/E boundary interval at 335 mcd, which is present within a thick and uniform sequence of upper Paleocene and lower Eocene foraminifer nannofossil ooze. The benthic foraminiferal extinction event occurs just below the base of this layer at 335.4 to 335.7 mcd. This coincides with a major shift in nannofossil abundances from Fasciculithus to Zygrhablithus. The overall pattern of the P/E boundary interval is consistent with other pelagic records and is inferred to result from seafloor carbonate dissolution resulting from the input of methane-derived CO2 during the event. The fact that this site was at a paleodepth of 1400 m, ~2000 m shallower than Site 1262, attests to the scale of seafloor carbonate dissolution during this event.

The Site 1263 sediment record displays pervasive bedding cycles as expressed in the MS, color reflectance, and other high-resolution core logging data. The middle and lower Eocene, in particular, are characterized by pronounced decimeter- to meter-scale bedding cycles. The variance is concentrated in three frequency bands. The shorter cycles have a frequency close to that of the orbital precession, whereas the longer oscillations have a frequency similar to the 100- and 400-k.y. eccentricity cycles. Both the frequency and amplitude of the bedding cycles are similar to those observed at Site 1262 and should permit high-resolution correlation of units.

Next Section | Table of Contents