Site 1120 is located ~650 km southeast of Stewart Island, near the middle of the Campbell Plateau. The site was drilled in a water depth of 543 m and was located at the crossing of National Institute of Water and Atmospheric Research (NIWA) multichannel seismic line 3034 (Fig. F1). A 3.5-kHz profile (NIWA, Tangaroa CR-3034, 23/2/97) reveals an acoustically reflective seafloor that is underlain by three shallow reflectors, perhaps unconformities, at depths of ~5, 13, and 18 m subseafloor. Multichannel profile NIWA 3034-20 penetrated the complete thickness of the Cretaceous-Holocene Campbell Plateau succession (e.g., Beggs, 1978), including probable Cretaceous nonmarine rift-fill sediments at the base.
The seismic succession is punctuated by seven conspicuous reflectors (R1-R7), which probably represent unconformities. Three of these reflectors (R2, R4, and R5) are used to divide the succession into four major parts, corresponding to subdivisions of the New Zealand Plateau-wide Cretaceous-Holocene, Kaikoura Synthem (Carter, 1988; Fig. F2). Between reflectors R6 (basement) and R5, a thin unit (0-0.18 two-way traveltime [TWT]) of probable upper Cretaceous-Eocene terrigenous sands and silts has its thickest occurrence within rift-axis depocenters (sediments up to 0.55 TWT thick) and thins to almost nothing where it onlaps block-faulted basement highs. Reflector R5, which caps the basal part of the succession, is the first flat-lying reflector that is traceable throughout the region, and it probably marks the inception of widespread biopelagic carbonate sedimentation. If this correlation is correct, then reflector R5 corresponds to the contact between the Garden Point and Amuri Limestones on nearby Campbell Island (Beggs, 1978). Few basement highs remained on the Campbell Plateau to shed terrigenous sediment after the late Cretaceous, and the entire succession above reflector R5 is therefore probably biopelagic carbonate (i.e., Amuri Limestone facies). Times of water-mass change, perhaps accompanied by current activity and sublevation, are indicated by the subtle but marked changes in seismic character that take place across reflectors R4 to R1. Two obvious episodes of current drift deposition occur above reflectors R5 and R4 respectively, starting at ~0.7 s and ~0.5 s TWT subseafloor, and minor channeling is apparent along reflector R2 at ~0.15 s TWT subseafloor. The railroad-track, flat-lying nature of the reflections of the biopelagic carbonates across profile NIWA 3034 is disturbed only by minor faulting, seemingly concentrated along reflector R3 but which sometimes extends to the seabed above and into older sediments below.
Site 1120 was drilled to establish the age of the major unconformities in the Campbell Plateau sequence, and to determine the history of the shallow parts of the Antarctic Circumpolar Current (ACC) where it sweeps past and over the southeast corner of the Plateau. The analysis of seafloor magnetic anomalies indicates that the ACC had its inception in the Oligocene, at ~32 Ma, when Australia separated from Antarctica and allowed circumpolar flow to commence (Molnar et al., 1975; Lawver et al., 1992). At this time, the Campbell Plateau was situated immediately down-current from the opening Southern Ocean, and, like eastern South Island, New Zealand (Carter et al., 1996b), was therefore probably exposed to vigorous current activity. It was anticipated that reflector R3 would be of middle-late Miocene in age, thus correlating with the phase of known volcanism and minor faulting in eastern South Island between ~13-10 Ma (Benson, 1969; Field and Browne, 1989). The expanded section of Eocene and Oligocene carbonates below this unconformity is one of the thickest known in the entire New Zealand region and should yield an unparalleled record of the paleoceanographic events associated with the opening of the Tasmania-Antarctica gateway, and the early evolution of the ACC system.
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