5. Site 11701

Shipboard Scientific Party2


Site 1170 is located in deep water (2704 m) on the flat western part of the South Tasman Rise (STR), 400 km south of Tasmania and 40 km east of Site 1169. It is 10 km west of a fault scarp, ~500 m high and trending north-south, that separates the lower western and higher central blocks of the STR. The site lies within present-day northern subantarctic surface waters, ~150 km south of the Subtropical Front and well north of the Subantarctic Front. The primary objectives of Site 1170 were to core and log (1) an Eocene detrital section deposited during early rifting between the STR and Antarctica to ascertain marine paleoenvironmental conditions before and leading into the initial marine connection that developed between the southern Indian and Pacific Oceans as the Tasmanian gateway opened during the mid-Paleogene, (2) an Oligocene to Holocene pelagic carbonate sequence to document the paleoceanographic and paleoclimatic responses to the opening of the Tasmanian gateway and subsequent expansion of the Southern Ocean, and (3) an upper Neogene sequence to construct a high-resolution subantarctic biostratigraphy and a high-resolution record of paleoclimatic change.

Plate tectonic reconstructions show the site as being in the broad northwest-southeast Tasmanian-Antarctic Shear Zone during the Cretaceous and moving south with Antarctica until the latest Cretaceous, when it became welded to the remainder of the STR as part of the Australian plate. From the earliest Paleogene, the site was close to the active rift. A shallow sea associated with Paleogene rifting and east-west spreading between Australia and Antarctica placed the site in the far southeastern corner of the restricted Australo-Antarctic Gulf, on the Indian Ocean side of the Tasmanian land bridge. The ridge of the Tasman Fracture Zone (TFZ), 80 km west of the site, formed soon after fast spreading began in the middle Eocene and must have provided east-flowing debris. Marine magnetic lineations show that in the late Oligocene (26-27 Ma) the east-west spreading axis was just west of the TFZ at Chron 8. The passing of the axis probably caused nearby uplift followed by subsidence.

Seismic profiles and regional correlations suggest that the site was subject to steady deposition of prograded siliciclastic deltaic sediments from the Cretaceous into the Eocene, and hemipelagic sedimentation grading to pelagic sedimentation thereafter (Fig. F1). Much of the Cenozoic siliciclastic detritus must have come from the higher central block 10 km to the east, believed to consist largely of continental basement and Cretaceous to Eocene sedimentary rocks. Parts of the central block, which was initially the Tasmanian land bridge, may have remained subaerial and, hence, a source of siliciclastic sediments well into the Oligocene. Seismic profiles suggest that there was a period of current erosion against the fault scarp of the central block, probably during the Miocene. A wedge of sediments was deposited in the depression.

At Site 1170 we cored one advanced hydraulic piston corer/extended core barrel (APC/XCB) hole, two more with the APC, and a rotary-cored hole (Table T2 in the "Leg 189 Summary" chapter). Because suboptimal weather conditions affected the APC coring, construction of a composite section of the triple-cored portion of the sedimentary sequence was possible only to 70 m below seafloor (mbsf) (early late Pliocene). Beyond that, there are limited gaps, but overall core recovery averaged 90.4%. Hole 1170A reached 464.3 mbsf with 81.8% recovery. Hole 1170B was APC cored to 175.8 mbsf with 102.2% recovery, and Hole 1170C reached 180.1 mbsf with 99.7% recovery. Hole 1170D was rotary cored from 425 to 779.8 mbsf with 81.1% recovery. Wireline logging was conducted over ~540-770 mbsf in Hole 1170D with the triple-combination (triple combo) tool string, the geological high-sensitivity magnetic (GHMT)-sonic tool string, and the Formation MicroScanner (FMS)-sonic tool. Logging was terminated when the drill pipe became stuck in the hole, and the bottom-hole assembly (BHA) had to be severed with explosives.

Site 1170, with a total sediment thickness of 780 m, ranges in age from the middle Eocene (43