Site 1263 (proposed Site WALV-8E) is located at a water depth of 2717 m just below the crest of a north-south–trending segment of Walvis Ridge (Fig. F1). The site is ~250 m deeper than Deep Sea Drilling Project (DSDP) Site 525 to the southwest, where a 574-m-thick sedimentary sequence was recovered (Moore, Rabinowitz, et al., 1984). All sediments in this sequence are rich in biogenic carbonate. The Neogene section is truncated by a hiatus at 270 meters below seafloor (mbsf) so that upper Oligocene sediments overlie middle Eocene sediments. The upper Oligocene to Pleistocene sequence consists of nannofossil ooze and foraminifer-bearing nannofossil ooze. The Paleogene section includes nannofossil ooze and foraminifer-bearing nannofossil ooze and chalk and extends to 452 mbsf. DSDP Leg 74 shipboard paleontologists identified the Cretaceous/Paleogene (K/P) boundary at 452 mbsf. The lowermost part of the Site 525 sequence comprises Campanian–Maastrichtian sediments on top of basement. The sediments are cyclic in nature and are composed of nannofossil marly chalks and limestones and siltstones/sandstones of turbidite and/or slump origin down to 452 mbsf. The lowermost part of the sequence consists of sedimentary units in a basement complex. Site 525 was cored using a combination of the hydraulic piston corer and rotary drilling; as a result, recovery was poor.
Our main objective for this site was the recovery of undisturbed sediments recording critical intervals in the early Cenozoic, specifically the Paleocene/Eocene Thermal Maximum (PETM) and the period of global cooling and growth of polar ice caps across the Eocene/Oligocene (E/O) boundary into the earliest Oligocene (early Oligocene Glacial Maximum). We planned to recover 100% of the sedimentary section in multiple holes to make it possible to establish a cyclostratigraphy and to develop an astronomically tuned timescale. We aimed to fully document events across the critical intervals as well as short-lived episodes of climate fluctuation during the early Eocene Climatic Optimum. Site 1263 forms the shallow anchor of the Leg 208 Paleocene/Eocene (P/E) boundary depth transect.
We chose a location just below the ridge crest where the Neogene section is thin (estimated at ~50 m) to facilitate recovery of much of the Paleogene section using the advanced piston corer (APC) rather than the extended core barrel (XCB). Total sediment thickness was estimated to be 380 m (400 ms two-way traveltime [TWT] below seafloor), and the basal sediments are Campanian in age. At Site 525, a major unconformity was encountered across the E/O boundary. The Paleogene sections to the northeast of Site 525 along seismic reflection line GeoB 01-031 appear to be more complete. The seismic profiles show a series of reflectors (R1, RP/E, and RK/P) that can be traced from Site 525 to Site 1264 (proposed Site WALV-8A) (Fig. F2) to Site 1263 (common depth point 400, line GeoB 01-046) (Fig. F3) and indicate that the E/O unconformity fades laterally away from Site 525. This unconformity at Site 525 is represented in seismic line GeoB 01-031 as a zone of high-amplitude and truncated reflectors. This zone is also observed in the vicinity of Site 1264 but appears to be bounded by an additional section not present at Site 525. The next deeper prominent reflectors are the P/E boundary reflector (RP/E) (290 ms TWT below seafloor) and the K/P boundary reflector (RK/P) (370 ms TWT below seafloor). The correlation of reflectors from Site 525 to the proposed Sites WALV-8A, -8B, and -8C suggests that the Paleogene sequence is most complete at Sites 1263 and 1264. For the E/O and P/E boundary reflectors at Site 1263, we estimated relatively shallow burial depths of ~171 and ~276 mbsf, respectively, whereas at Site 1264, the depths were estimated to be ~238 and ~380 mbsf (velocity = 1.8 m/ms), respectively. Because the target reflectors at Site 1264 are considerably deeper than desirable for our planned Paleogene work, Site 1263 was selected as the primary site for recovery of a complete upper Maastrichtian–Paleogene sequence.
As expected, coring in four holes at Site 1263 yielded an expanded sequence of Paleogene sediment including intervals spanning climatic extremes at the E/O and P/E boundaries (see "Site 1263" in "Site Summaries" in the "Leg 208 Summary" chapter). Much of the upper Oligocene and Miocene was missing or condensed. Most of the section was cored using the APC, although the XCB had to be deployed for the deepest intervals (greater than ~250 mbsf). The P/E boundary interval, which was slightly deeper than estimated, was recovered at 284 mbsf using both the XCB and APC systems. Although recovery was less than optimal, it appears that 100% of the stratigraphic section was recovered in the four holes. The upper Paleocene and lower Eocene sediments exhibit distinct bedding cycles which can be correlated to cycles observed at Site 1262 and thus should be suitable for establishing a highly resolved record of regional deepwater chemistry and circulation.
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