FIGURE CAPTIONSFigure 1. Antarctica and surrounding continents at the Cretaceous/Tertiary boundary, during early Oligocene and during earliest Miocene times showing the change from meridional to Antarctic Circumpolar Current circulation that brought about the thermal isolation of Antarctica (modified from Kennett, 1978).
Figure 2. Global sea level, tectonism, and ice volume (after Barrett, 1994). Left curve is from Atlantic benthic foraminifers (Miller et al., 1987); right curve is from seismic sequence analysis (Haq et al., 1987).
Figure 3. Bathymetry of the offshore Tasmanian region, making use of Tasmante swath bathymetry. Solid circles = Leg 189 sites. Crosses = DSDP sites. Squares = Marion Dufresne 1997 piston core sites. Seismic profiles shown in Figure 8 are solid lines. Contours are in meters.
Figure 4. Trajectory of the South Tasman Rise with respect to Antarctica (J.-Y. Royer, unpubl. data). Because Antarctica has remained stable relative to the South Pole since the Cretaceous (e.g. Müller et al., 1993; DiVenere et al., 1994), it also shows the paleolatitudes of the South Tasman Rise. The close correspondence between the hot spot trace (dashed line) and the trace with Antarctica fixed (solid line) adds confidence to the interpretation (ages after Cande and Kent, 1995, magnetic reversal time scale; magnetic chrons are in parentheses).
Figure 5. Plate tectonic setting of the Tasmanian region within Gondwana during the earliest Late Cretaceous (95 Ma) modified after Royer and Robert (1997), showing a zone of strike-slip faulting (Tasmanian-Antarctic Shear Zone) and areas of rift sedimentation. W-STR = west South Tasman Rise; E-STR = east South Tasman Rise; ETP = East Tasman Plateau.
Figure 6. Paleogeographic map of Tasmanian region during the middle Eocene (40 Ma) based partly on Royer and Rollet (1997, fig. 10C) and on Leg 189 results. Solid circles = DSDP Leg 29 sites. Open circles with dots = Leg 189 sites. Late Cretaceous oceanic crust was covered with deep Pacific Ocean, and Eocene oceanic crust with bathyal ocean. Most of the Tasmania-South Tasman Rise region was covered in shallow seas. Shallow-water connections between the restricted Australo-Antarctic Gulf to the west and Pacific Ocean may have been established both north and south of the South Tasman Rise.
Figure 7. Paleogeographic map of the Tasmanian region at the Eocene/Oligocene boundary (33.5 Ma) based partly on Royer and Rollet (1997, fig. 8) and Leg 189 results. Solid circles = DSDP Leg 29 sites. Open circles with dots = Leg 189 sites. Most oceanic crust was covered with deep water, and most of the Tasmania/South Tasman Rise region was covered in bathyal seas. A deep water connection between the restricted Australo-Antarctic Gulf to the west and the Pacific Ocean had just been established south of the STR, allowing the onset of circum-Antarctic circulation.
Figure 8. Cross-sections from seismic profiles across the region showing DSDP and proposed high-priority ODP sites. P1 = West Tasmania; P2, P3 = South Tasman Rise; and P4 = East Tasman Plateau. Site locations are shown in Figure 3.
Figure 9. Map of structures and minimum sediment thickness of the area west and south of Tasmania after Hill et al. (1997a), based on multichannel seismic profiles. ODP and DSDP sites are shown. TWT = two-way traveltime.
Figure 10. Sketch map of Cretaceous and Paleogene movements, relative to a fixed Tasmania, of the western South Tasman Rise block, eastern South Tasman Rise block, and East Tasman Plateau (modified from Exon et al., 1997b).
Figure 11. The oceanographic fronts in the Southern Ocean south of Tasmania after Rintoul and Bullister (1999). STF = Subtropical Front. SAF = Subantarctic Front. PF = Polar Front.
Figure 12. Postdrilling interpretation for local seismic profile SO 36-47, across Site 1168, showing broad ages and lithostratigraphic units. The site is on a gentle slope to the west. Note the thick Oligocene sequence. The long-wavelength hummocks in the late Eocene may represent delta distributaries. There is almost no seismic character in the younger sequences, but three age/lithostratigraphic boundaries are marked by reflectors: Me, Oe, and El. Siliciclastic material has been shed from a ridge upslope into the Eocene and Oligocene sequences. TD = total depth.
Figure 13. Postdrilling interpretation for local seismic profile AGSO 125-14, across Site 1170, showing broad ages and lithostratigraphic units. This shows the site's position just west of the high central block and the evidence of Miocene scouring that removed the Oligocene and late Eocene sequences. Note the thick Pliocene-Pleistocene and the reduced Oligocene sections at this site. TD = total depth.
Figure 14. Postdrilling interpretation for local seismic profile AGSO 202-5, across Site 1171, showing broad ages and lithostratigraphic units. The site is in a north-south strike-slip basin on the central block of the South Tasman Rise, just west of the trace of the Balleny Fracture Zone block (and the eastern block of the Rise). It is the only site that penetrated the Paleocene, and the relationships at the Paleocene/Eocene unconformity indicate that the fracture zone's last movement was in the late Paleocene. This probably represents the time of initial spreading to the south. Note the thick Eocene, the very thin Oligocene, and the thin Pliocene-Pleistocene sections at this site. TD = total depth.
Figure 15. Postdrilling interpretation for local seismic profile AGSO 125-4, across Site 1172, showing broad ages and lithostratigraphic units. Drilling directly dated the three upper unconformities (A-C), and helped control the possible ages of the lower unconformities (D and E). Note the thick middle Eocene and Miocene to Holocene, and the virtually absent Oligocene and late Eocene sections at this site. TD = total depth.
Figure. 16. Summary of lithologies, Leg 189 sites 1168 through 1172. TD = total depth.
Figure 17. Comparison of major age stratigraphic units by depth for Leg 189 Sites 1168 through 1172. TD = total depth. L* = straddles the Eocene/Oligocene boundary.
Figure 18. Summary stratigraphic and sediment facies for Leg 189 sites 1168 through 1172, arranged west to east. Major lithologies against time show major hiatuses.
Figure 19. Sedimentation rate summary curves for Leg 189 sites 1168 through 1172.
Figure 20. Downhole natural gamma and medium resistivity results from Holes 1168A, 1170D, 1171D, and 1172D. Higher gamma values are often indicative of an increased terrigenous component and/or organic material in the sediment, whereas higher resitivities are often associated with increased lithification and/or decreased porosity. The distinct lithologic change close to the Eocene/Oligocene boundary, from siliciclastic glauconitic claystone to nannofossil chalk and ooze, can be clearly seen in Holes 1168A (~260 mbsf), 1171D (~270 mbsf) and 1172D (~360 mbsf).
Figure 21. Summary of organic geochemistry results. The upper panel shows carbonate content (in weight percent CaCO3), whereas the lower panel presents total organic matter content and hydrogen indices. Also shown are regional correlations and approximate location of lithostratigraphic units and age.
Figure 22. Summary of interstitial water geochemistry results for Sites 1168, 1170, 1171, and 1172. The upper panel shows Cl- and methane profiles. Intervals with Cl- values less than mean seawater (i.e., fresher waters) are shaded. The lower panel shows concentration-depth profiles for selected interstitial water constituents. The approximate location of the Eocene-Oligocene (E/O) transition is also shown.
Figure 23. Schematic diagram showing inferred surface-water circulation of the Australian Antarctic region during the middle Eocene (43.7 Ma) with the Tasmanian Gateway closed. Plate tectonic reconstruction is modified from Cande et al. (2000). Note the influence of the East Australian Current on the Antarctic margin in the absence of circum-Antarctic circulation. Weak gyral circulation is inferred in the highly restricted Australo-Antarctic Gulf. ODP Leg 189 and DSDP Leg 29 site locations are indicated.
Figure 24. Schematic diagram showing inferred surface water-circulation of the Australian Antarctic region during the earliest Oligocene (33 Ma). Plate tectonic reconstruction is modified from Cande et al. (2000). At this time, the Tasmanian Gateway was initially open to deep-water circum-Antarctic circulation between the South Tasman Rise and Antarctica. Note that early development of circum-Antarctic circulation began to decrease the influence of the East Australian Current on the Antarctic continental margin.
Figure 25. Schematic diagram showing inferred surface water circulation of the Australian Antarctic region during the late Oligocene (26 Ma). Plate tectonic reconstruction is modified from Cande et al. (2000). At this time, the Antarctic cryosphere had increased significantly. The expansion of the Antarctic Circumpolar Current (ACC) continued as the South Tasman Rise and Tasmania moved northward from Antarctica. Note increasing decoupling of the East Australian current from the Antarctic margin associated with the increasing strength of the ACC.
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