Sediments recovered in the Great Australian Bight during Leg 182 (Fig. F1) record siliceous and carbonate deposition in a middle- to high-latitude environment. The lower Eocene sediments at Site 1128 consist of shallow-water terrigenous sands and carbonates that deepen upward into Oligocene pelagic ooze and chalk. The middle Eocene-lower Oligocene sediment sequence was well recovered on the continental rise at Site 1128 in a water depth of 3875 m and contains an expanded (>350 m) siliceous biogenic record of Circum-Antarctic Current evolution.
Results indicate that the upper part of this basin contains a thick almost-continuous biosiliceous record of Southern Ocean development through the Paleogene. Calcareous microfossils indicate the presence of a hiatus (~18 m.y.) at the Miocene/Oligocene boundary (70 meters below seafloor [mbsf]), where the entire upper Oligocene is missing. Sedimentation rates were 50-60 m/m.y. during most of the early Oligocene and 4 m/m.y. in the late Eocene. In spite of poor core recovery and barren intervals, the sedimentation rates in the lower Eocene have been estimated at 40-45 m/m.y. (Shipboard Scientific Party, 2000a).
Biostratigraphic results from nannofossils and planktonic foraminifers suggest a largely continuous conformable succession across the Oligocene/Eocene boundary (Shipboard Scientific Party, 2000a). The expanded lower Oligocene section is indicated by nannofossils at Site 1128, where biostratigraphic resolution below 70 mbsf was largely achieved using calcareous nannofossils, because foraminifers were rare or absent in sediments dominated by siliceous oozes and packstones. The middle Eocene is represented by poorly preserved impoverished assemblages in various poorly recovered lithologies.