Site 1168 is located in middle bathyal (2463 m) water depths on the continental slope of western Tasmania (Fig. F3 in the "Leg 189 Summary" chapter). The site is 70 km to the west of the central Tasmanian coast and 70 km to the east of the boundary of the abyssal plain, which lies at a depth of 4500 m. The site is thus at mid depths between continent and abyssal plain but lies above continental crust. The site was selected at the intersection of reflection seismic lines Tasmante 125-52 and Sonne SO36B-47 (Fig. F2). Site 1168 represents a near-continuous sediment record (883.5 m) from the early late Eocene to the present. This constitutes a moderately transparent sequence of Quaternary to Oligocene calcareous pelagic sediments, seismically 0.8 s thick, and an underlying hummocky sequence of siliciclastic sediments ~0.2 s thick, of which the more uniform upper ~0.12 s was drilled.
The present western Tasmania margin was part of the east Gondwana supercontinent until Australia separated from Antarctica during the Late Cretaceous through early Cenozoic. Early separation from the Cretaceous through the middle Eocene, forming the Australo-Antarctic Gulf, was associated with the movement of the western Tasmania margin in a northwest direction. This was followed by north-south movement that had little effect on the margin on which this site is located. The present structures of the margin were formed by northwest-southeast strike-slip faulting during the Maastrichtian to Paleocene (Hill et al., 1997). A broad epicontinental basin lay between Tasmania and Antarctica from the Cretaceous to the early Paleocene (see Figs. F24, F25, F26 in the "Leg 189 Summary" chapter). The strike-slip motion dissected this basin into a series of narrow northwest-southeast-trending strike-slip basins during the Paleocene. This formed a margin in the Paleogene with characteristics similar to those of the Neogene borderland province of southern California (see Figs. F6, F7 in the "Leg 189 Summary" chapter). The Californian borderland is made up of a number of semi-isolated basins interconnected by shallow sills representing the structural highs. There, the inner basins trap much of the coarser sediments derived from the continent so that the outer basins are increasingly less influenced by sediment gravity flows and are starved of coarse siliciclastic sediments. In contrast, the inner basins are increasingly isolated from the open ocean, which reduces ventilation and decreases oxygen concentrations in deep basinal waters.
The seismic stratigraphy of the western Tasmania margin (Fig. F3) shows that sedimentation continued within a succession of narrow, fault-bounded basins for much of the remaining Cenozoic (see Fig. F18). Site 1168 is located in one such basin, 20 km wide, midway across the continental borderland. Thus, the basin was partially isolated from major terrigenous sediment input and downslope sediment transport during much of the Cenozoic, and it was sufficiently isolated from the open ocean to be poorly ventilated. Seismic evidence indicates that Paleogene sediments sloped gently down toward the northwest as indicated by progradation in the Paleocene-Eocene sequence. The sea would have transgressed southeastward upslope within this basin during the Paleogene. Deposition of the post-Eocene sequence occurred during major subsidence of the western Tasmania margin. This subsidence commenced in the early Oligocene once the southern part of the South Tasman Rise moved sufficiently far north to clear the Antarctic continent. Subsidence has continued during the flight of Australia northward away from Antarctica. Margin subsidence was greater with increasing distance from Tasmania. Basinal filling at Site 1168 occurred during subsidence so that by the latest Neogene sediments moving downslope from shallower waters were no longer blocked by the fault-bounded highs. Site 1168 clearly records Cenozoic subsidence, with sediments deposited in upper bathyal to shelfal depths (>~100 m) in the late Eocene and in middle bathyal depths (~2500 m) in the present day.
The primary objectives of Site 1168 include
Site 1168 is one of a suite of four sites drilled during Leg 189, designed to advance understanding of circumpolar oceanographic and climatic evolution as a result of the northward expansion of the Southern Ocean and the development of the Tasmanian Seaway. This site was selected to provide vitally required information on the changing paleoenvironmental conditions on the northwest side of the Tasmanian gateway as the seaway opened during the Paleogene and then expanded during the rest of the Cenozoic. Its location away from the gateway suggested that, importantly, this site would yield a more complete stratigraphic record than the others. Paleoenvironmental conditions were expected to have undergone dramatic evolution during the Cenozoic in response to plate tectonism. Earlier drilling at DSDP Site 282, just 80 km northwest of Site 1168, revealed an upward progression from a poorly ventilated margin with subdued current activity and dominant terrigenous sedimentation in the upper Eocene section. This changed to increasingly biogenic sedimentation in ventilated conditions during the late Paleogene and early Oligocene, as the margin was influenced by more open ocean conditions. Finally, the sedimentary sequence was exposed to strong current activity with the intensification of the circumpolar circulation. However, much of the Site 282 sequence was poorly cored or absent in unconformities.
It is clear that the western Tasmania margin during the Eocene and early Oligocene was located at the far eastern end of a relatively narrow gulf extending ~2500 km across the full extent of Australia, which we here name the Australo-Antarctic Gulf. The steadily opening gulf was ~800 km wide at the end of the Eocene. Circulation in the eastern Gulf would have been highly restricted, leading to the poorly ventilated margin in which Site 1168 is located. Plate reconstructions show that Site 1168 was at ~60°S in the late Eocene (Cande et al., 2000). This situation contrasts with that of the present-day, in which the margin, at 42°S, is in a fully open ocean, is extremely well ventilated, and faces westward toward the vastness of the northern part of the Southern Ocean. In the last 45 m.y., almost everything has changed drastically on the margin at Site 1168, including tectonism, structure, water depth, ventilation, sediments, biogenic components, water temperature, and geochemistry. Site 1168 was cored to document these changes and provide a basis for understanding their causes.