Site 1171 is located near the southernmost tip of the STR ~550 km south of Tasmania and 270 km southeast of Site 1170 in a water depth of 2150 m on a gentle slope to the southwest (Fig. F3 in the "Leg 189 Summary" chapter). At 48º30´S, Site 1171 lies in subantarctic waters south of the present-day position of the Subtropical Front and north of the Subantarctic Front. The major objectives were to (1) core and log an Oligocene to present-day pelagic carbonate section to evaluate expected major paleoceanographic and paleoclimatic effects resulting from the initial opening of the Tasmanian Gateway in the early Oligocene, followed by later development of deep-water Antarctic Circumpolar Current flow; (2) core and log an expected underlying detrital sedimentary sequence of Eocene age, deposited during rifting of the STR from East Antarctica, for paleoenvironmental information; and (3) obtain high-resolution sedimentary records from critical subantarctic latitudes to better understand the role of the Southern Ocean in processes of climatic change during the Neogene on Milankovitch time scales.
Site 1171 is located on thinned continental crust of the STR in the central part of the eastern terrain that was believed to be attached to both Antarctica and Australia until 66 Ma (Royer and Rollet, 1997) (Figs. F4, F5 in the "Leg 189 Summary" chapter). The block is rhombohedral shaped,~100 km across, and bounded by north-south sheared margins (Exon et al., 1997). During the Late Cretaceous to Eocene, the block was broken up into sub-blocks and several complex transtensional sub-basins by strike-slip faults that developed as Australia slid northward past Antarctica (Fig. F2). The faults, some of which have throws of several thousand meters, probably trend northwest-southeast or north-south, and the resultant sub-basins are filled with ~1000 to 2000 m of Cretaceous through Eocene rift sediments.
During the Paleogene, stretching and thinning of the STR led to steady subsidence. By the early late Eocene, initial marine transgression migrated onto the shallow part of the rise as documented in Deep Sea Drilling Project (DSDP) Site 281, 150 km to the northwest of Site 1171 (Kennett, Houtz, et al., 1975). We assumed initially that Site 1171 was probably at the far eastern end of the restricted Australo-Antarctic Gulf and separated from the Pacific Ocean by the Tasmanian land bridge. Drilling showed that it was, in fact, east of the land bridge and in the Pacific Ocean. By late Oligocene times, the sub-basins had filled, most of the high blocks were covered, detrital sedimentation had ceased, pelagic sedimentation could not keep up, and much of the STR had sunk to bathyal depths. The southern tip of the STR cleared Antarctica during the Oligocene (Cande et al., 2000; S.C. Cande et al., unpubl. data), at which time deep-water antarctic circumpolar circulation became established. Site 1171 was selected because of its extreme southern location on the STR, which is nevertheless in sufficiently shallow-water depths to provide a carbonate sequence suitable for studies of Cenozoic biostratigraphy and chemostratigraphy. Thus, the site should provide critical data on the subsidence of the STR during the Oligocene, on the timing of the initial flow of surface water, and the later flow of deep water through the opening Tasmanian Gateway. The site might also provide a record of ice-rafted detritus, valuable in further understanding of the development of the antarctic cryosphere (Miller et al., 1991).
Earlier drilling of two sites (DSDP 281 and 280) has provided critical information about the sedimentary succession in the STR region. DSDP Site 281 (Kennett, Houtz, et al., 1975) was drilled on a basement high of quartz-mica schist of latest Carboniferous age near the southwestern tip of the STR in 1591 m water depth. The 169-m-thick cored sequence includes upper Eocene conglomerate and glauconitic sandy mudstone immediately overlying basement, upper Oligocene glauconite-rich detrital sand, Miocene foraminifer-nannofossil ooze, and Pliocene-Pleistocene foraminifer-nannofossil ooze. The site was sufficiently shallow throughout for preservation of the calcareous microfossils so valuable for paleoenvironmental interpretations. Evidence from the recovered intervals suggests that the site had subsided to middle bathyal depths by the Miocene.
The other nearby, relevant DSDP Site is Site 280 (Kennett, Houtz, et al., 1975), drilled in 4176 m of water on oceanic crust, southwest of the STR and 150 km southwest of Site 1171. The site penetrated a veneer of upper Miocene to upper Pleistocene clay and ooze and was underlain, beneath a sampling gap, by 55 m of siliceous lower Oligocene sandy silt. The lower part of the sequence consists of 428 m of middle Eocene to lower Oligocene sandy silt, containing chert in the upper 100 m, and glauconite and manganese micronodules in the lower succession. The lower 200 m is rich in organic carbon (0.6-2.2 wt%). The middle to late Eocene carbonaceous sequence contains only dinoflagellates and agglutinating foraminifers in the lowest 360 m (Crouch and Hollis, 1996). The environment of deposition during this interval may have been similar to that of the late Eocene at Site 1168. The base of the site is a basaltic sill believed to be associated with oceanic crust, but it is possible that water depths were shallower than abyssal at the onset of rapid seafloor spreading in the middle Eocene.
Site 1171 is located at the intersection of multichannel seismic profiles AGSO 202-05 (Fig. F3) and -06 in an area of flat-lying Cenozoic sediments near a structural low in the Cretaceous sequence. The basement unconformity is possibly at 2.5 s two-way traveltime (TWT) below the seafloor, the top of the Cretaceous may be at ~1.7 s, the top of the Paleocene is at ~0.9 s, and two younger seismic boundaries are at ~0.58 and 0.31 s.
The site was designed to penetrate 0.31 s (~270 m) of ooze and chalk above a very strong reflector, possibly an unconformity, which drilling proved to be an unexpectedly shallow Oligocene/Eocene boundary. This sequence is seismically semitransparent, but it does contain evidence of large sediment waves and downlaps the unconformity. Below the reflector, we planned to drill 0.1 s (~100 m) of highly reflective sediments, which proved to be interbedded hard and soft siliciclastics of Eocene age. Low in this reflective sequence, at ~0.38 s, a possible bottom-simulating reflector might have represented the base of a zone of gas hydrates, but drilling gave no evidence of hydrates. Below the reflective section is a seismically semitransparent 0.2-s-thick section (~175 m) that proved to be Eocene mudstone. Below this sequence is an unconformity at 0.58 s, within the Eocene section.
Below the unconformity within the Eocene (0.58 s), a sequence of older Eocene siliciclastic sediments (0.6 s thick) is poorly and irregularly bedded. Below an unconformable top at 0.92 s (~900 mbsf) are prograded sediments proven to be of Paleocene age. Planned penetration was to a total depth of 940 m, but we had permission to drill as deeply as 1100 m, if appropriate.
Site 1171 was also designed to recover upper Neogene sedimentary sequences of sufficiently high resolution to conduct critically needed paleoclimatic investigations for the subantarctic region. The nutrient dynamics of the subantarctic region are important for better understanding of global carbon cycling, because of effects on atmospheric CO2 and because a majority of the world's intermediate and deep waters are ventilated in the Southern Ocean (Ninnemann and Charles, 1997). The increased biological productivity of the surface ocean needed to decrease atmospheric CO2 may have occurred in the subantarctic region (Mortlock et al., 1991). Stable isotopic and other geochemical records at Site 1171 are expected to assist with such evaluations.