6. Site 1128¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1128 is located on the southern Australian upper continental rise in 3874.4 m of water (Fig. F1). This was primarily a paleoceanographic site designed to intersect pelagic sections that collectively span the entire Cenozoic succession and to constitute the deeper water component of the overall Leg 182 shelf-to-basin transect sampling strategy. Site 1128 is located on gently dipping seafloor immediately seaward of a relatively steep, eroding lower slope. The site was positioned to sample a thick stratigraphic section of presumed Cenozoic age, together with the uppermost part of the underlying presumed Mesozoic succession (Fig. F2; see "Seismic Stratigraphy"). The absence of any suitable middle to lower slope site (between 1500 and 2500 m water depth) meant that this site was a particularly important component of the depth transect. Although the sedimentary section underlying the modern continental rise appears to be laterally extensive (extending for some 350-400 km along the Great Australian Bight foot of slope) and >10 km thick, there are no stratigraphic ties to enable reliable predrill stratigraphic predictions. The estimation of depth to the Cenozoic/Mesozoic boundary was simply based on identification of a significant unconformity surface (Fig. F2) at approximately the expected depth. Anticipated lithologies at Site 1128 were pelagic nannofossil foraminifer oozes, possibly with intercalated clay-rich layers representing deposition below the carbonate compensation depth (CCD).

The principal objective at this site was to sample the Cenozoic succession in a deep oceanic setting to obtain a complete record of Circum-Antarctic Current evolution within the developing seaway between Australia and Antarctica that resulted from the Tasman Gateway opening during the Eocene. Because the condensed section at the base of the Cenozoic in the Jerboa-1 well contains lower Oligocene faunas, there was a high probability that the intermediate and deep pelagic successions would together contain a more expanded record of this critically important time of Antarctic ice-cap evolution and Southern Ocean paleoceanographic development.

Additional objectives were to

1. Determine depositional and diagenetic facies in an upper continental rise setting;
2. Recover a condensed Cenozoic section recording the history of CCD fluctuations and at least a partial history of deep-water mass variations during the evolution of the Southern Ocean;
3. Categorize the heatflow regime on the upper continental rise; and
4. Seek evidence for fluid flow within the upper part of this deep sedimentary basin.