Site 1170 is located on the western part of the STR 400 km south of Tasmania and 40 km east of Site 1169, in relatively shallow water (~2705 m) (Fig. F3 in the "Leg 189 Summary" chapter). The site lies within northern subantarctic surface waters, ~150 km south of the Subtropical Front and well north of the Subantarctic Front. Primary objectives of Site 1170 were to (1) core and log an Eocene detrital section deposited during rifting of the STR from East Antarctica for its paleoceanographic and paleoclimatic history; (2) core and log an Oligocene to Holocene pelagic carbonate section to determine, at moderately high resolution, history of the development of the Tasmanian gateway, subsequent expansion of the Southern Ocean, and paleoceanographic and paleoclimatic effects; and (3) core a Cenozoic sequence to construct subantarctic biostratigraphies in the Australian region and to obtain high-resolution records of paleoclimatic change.
Site 1170 is on thinned continental crust on the eastern side of the western block of the STR, which moved with Antarctica until the middle Eocene (43 Ma) and then became welded to the eastern block of the STR (Royer and Rollet, 1997). The block is elongated north-south and is 500 km long by 150 km wide (Exon et al., 1997). Cretaceous and Cenozoic sediments of the Ninene Basin cover most of the block. The block was cut into a number of complex transtensional sub-basins during the Late Cretaceous to Eocene by strike-slip faults that developed as Australia slid northward past Antarctica. The faults, some of which have throws of several thousand meters, trend northwest-southeast or north-south, and the resultant sub-basins filled with an average of 1000-2000 m of Cretaceous through Eocene prograded rift sediments. The stretched and thinned plateau subsided steadily during the Paleogene, and an early late Eocene marine transgression over basement is documented in Deep Sea Drilling Project (DSDP) Site 281 on the STR, 150 km to the southeast (Kennett, Houtz, et al., 1975). By the late Oligocene the sub-basins had filled, most of the high blocks were covered by sediment, detrital sedimentation had ceased, and pelagic sedimentation could no longer keep up with the subsidence, so much of the STR subsided to bathyal depths. Plate reconstructions show that the Tasmanian Gateway began to open during the Eocene-Oligocene transition, initially at shallow depths as a result of subsidence of the rise, and in deeper waters later in the Oligocene as the southern tip of the STR finally cleared Antarctica (Cande et al., 2000; S.C. Cande et al., unpubl. data; Royer and Rollet, 1997) (see Figs. F24, F25, F26 in the "Leg 189 Summary" chapter).
DSDP Site 281 was drilled on a basement high of quartz-mica schist of latest Carboniferous age southwest of the tip of the STR in water 1591 m deep. The thin (169 m) sequence includes upper Eocene basement, conglomerate and glauconitic sandy mudstone, upper Oligocene glauconite-rich detrital sand, Miocene foraminifer-nannofossil ooze, and Pliocene-Pleistocene foraminifer-nannofossil ooze. Evidence from the recovered intervals suggests that the site had subsided into deep (bathyal) waters by the Miocene. The upper Neogene carbonate sequence and associated microfossil preservation were of sufficiently high quality to be incorporated into a widely used Cenozoic oxygen isotopic curve (Shackleton and Kennett, 1975).
Site 1170 is located at the intersection of multichannel seismic profiles Sonne SO36B-58 and Tasmante 125-14 (Fig. F2), in an area of flat-lying Cenozoic sediments, 10 km west of a structural low in the Cretaceous sequence caused by the subsidence of the structurally higher central block of the STR along the bounding north-south fault (Fig. F3). This movement apparently caused rollover into the fault in Oligocene times. The fault scarp to the east rises as much as 500 m above the present seafloor of the western block and was a source of detritus in the early Cenozoic when it was subject to subaerial erosion. A wedge of sediment derived from the eastern block overlies a strong reflector 0.51 s below the seafloor (sbsf) at the site. The top of the Lower Cretaceous is possibly at ~2.4 sbsf two-way traveltime (TWT), the top of the Cretaceous is possibly at ~1.4 sbsf, and the presumed Eocene/Oligocene boundary is at ~0.51 sbsf. A 3.5-KHz echo-sounder profile shows that the site is in an area of slightly irregular seabed (with a relief of 10 m), perhaps caused by scouring.
The site was designed to penetrate 0.51 s (460 m) of Oligocene through upper Neogene ooze and chalk above a strong reflector believed to represent the Eocene/Oligocene boundary. Below this reflector, we planned to drill 0.3 s (310 m) of Eocene sediments. In seismic profiles the wedge of young transparent ooze at the deeper water western Site 1169 apparently pinches out eastward toward Site 1170, where a weakly well-bedded sequence apparently reaches to the surface. Drilling proved this to be a facies change—from soupy nannofossil ooze to firmer foraminifer-nannofossil ooze—rather than an age change. The bedding in the upper sequence at Site 1170 is a little hummocky and appears to downlap slightly onto the Eocene/Oligocene reflector in places. The inferred Eocene through Paleocene sediments below the Eocene/Oligocene unconformity are weakly well bedded, and the underlying Cretaceous sequence is prograded.
Site 1170 is one of two southern sites drilled deeply during Leg 189, especially to advance understanding of circumpolar oceanographic and climatic evolution as a result of the opening of the Tasmanian Seaway and the development of the Southern Ocean. This site was selected, because of its location in middle bathyal depths near the southwestern margin of the STR, to investigate paleoenvironmental conditions in the extreme southeastern part of the Australo-Antarctic Gulf before and during this early opening of the Tasmanian Gateway in the Paleogene, and the history and consequences of subsequent Southern Ocean expansion. We expected to recover sedimentological evidence of a highly restricted, poorly ventilated gulf at the location of Site 1170 during the late Eocene, followed by the development through the Oligocene of more open ocean circulation.
Plate reconstructions show that Site 1170 was at a high latitude (~65°S) in the late Eocene (Cande, 2000; S.C. Cande et al., unpubl. data) and was immediately adjacent to the East Antarctic margin (see Figs. F24, F25, F26 in the "Leg 189 Summary" chapter). The site, therefore, appeared to be well located to record early development of the antarctic cryosphere in the Paleogene. The early evolution of the antarctic cryosphere during the Eocene-Oligocene transition is not well known and it is possible that the antarctic cryosphere then was restricted to certain sectors of the continent. A major, but temporary, expansion of the East Antarctic Ice Sheet is well documented in the lowermost Oligocene (Breza and Wise, 1992; Zachos et al., 1992) for the antarctic sector in the Indian Ocean adjacent to the Kerguelen Plateau. Site 1170 is expected to allow comparison with these records in the region south of Tasmania, assuming that the opening of the gateway did not create a hiatus during this critical interval.