Site 1122 is located 275 km south of the Chatham Islands, midway between the Chatham and Bounty Islands, and 830 km east of Dunedin, eastern South Island (Fig. F1). The site was drilled in a water depth of 4432 m on the left (north) bank levee of the abyssal Bounty Fan. The fan is located in the most seaward axial deep of the Bounty Trough, a Cretaceous rift basin formed during the separation of New Zealand and Antarctica across the newly forming mid-Pacific Rise (Carter and Carter, 1993, 1996; Davey, 1993). The Bounty Channel feeds sediment along the axis of the trough and onto the adjacent 95-Ma oceanic crust of the Southwest Pacific abyssal plain.
Site 1122 is located on New Zealand Oceanographic Institute (now NIWA) single-channel seismic and 3.5-kHz line 88-2023 (Figs. F2A, F2B, F3). The three main units that compose the Bounty Fan at this crossing (seismic Units C-A; Table T1) all show strong angular onlap onto the folded surface of pre-Unit C sediments. In ascending order, and at the drill site (Fig. F2B), these units consist of 150 ms of irregularly and lightly reflective sediment (Unit C, capped by a strong reflector, R-7), 110 ms of thin-bedded and regularly reflective sediment passing up into strongly reflective sediment (Unit B, capped by reflector R-5), and upper Unit A which is 470 ms thick and comprises a field of spectacular sediment waves. The waves initiate as small features just above reflector R5, (the Unit B/A boundary), and grow in both wavelength (up to 6 km apparent) and height (up to 17 m) as they rise through the sediment pile to the seabed, where they were apparently still active in the recent past (Carter et al., 1990). The sediment waves are best developed, and were drilled, beneath the 315-m-high north bank levee of the Bounty Channel. Similar waves, though of smaller amplitude, occur underneath the 90-m lower crest of the south bank of the Bounty Channel. Sediment waves have been described from a number of Northern Hemisphere deep-sea environments, including submarine fans where they are most prominent on channel right banks (Damuth, 1979; Normark et al., 1980; Brew, 1995). Fan levees are inferred to have been built by overspilling turbidity currents, whose top surfaces are deflected right or left, according to the effect of Coriolis force on fluids in motion in the Northern and Southern Hemispheres, respectively (Menard, 1955; Komar, 1969). As a turbidity current overtops its levee, it may develop a series of antidune surges that cause the formation and growth of sediment waves across the levee crest (Normark et al., 1980).
The sediment waves form the upper unit (A) of the fourfold seismic stratigraphy we describe (Table T1). A zone of harder acoustic reflection extends down from the floor of the present-day channel obliquely to the left (south), where it has its initiation at or a little above reflector R9 (Fig. F2A). This zone marks the movement through time of the axis of the paleo-Bounty Channel. Seismic Units C-A compose the core of the Bounty Fan, and all appear to have been deposited as part of the north bank levee of the Bounty Fan's main sediment feeder, the Bounty Channel. The fan sediments rest on a regional unconformity (Y), here equivalent to reflector R9, below which the sediments of Unit D are gently folded and eroded. The lateral equivalents of Unit D are exposed in the inner and outer sills of the Bounty Trough, where dredged samples have provided Miocene microfossils, suggesting that the deformation and unconformity formation were of middle late Miocene age. The lower part of Unit A has been dated previously as late Pliocene (Mangapanian), on the basis of a microfauna cored from the lower north wall of the channel (Carter et al., 1994).
Site 1122 was drilled to establish the history of deposition of the Bounty Fan, and the degree to which fan-growth has been affected by the fact that the current fan is building out into the path of the Southwest Pacific Deep Western Boundary Current (DWBC). For instance, a breach in the left bank levee of the fan, at depths of ~4650 m, may have been caused by DWBC erosion or may be a turbidity current avulsion point maintained by the boundary flow. The current may also be causing sediment unmixing and sand mobility across the middle and lower fan (Carter and Carter, 1996). The sediment waves under the left bank levee are the subject of an earlier study by Carter et al. (1990), who described core and 3.5-kHz profile evidence for the presence of alternating cycles of glacial turbidite deposition and interglacial biopelagic ooze accumulation. Carter and Carter (1992) therefore interpreted the pattern of regular reflectors on deep seismic profile NZ01 2023 as evidence for a similar glacial/interglacial pattern of lithologic change with depth, and, by comparison with the global isotope stage record, predicted an isotope Stage 100 (~2.4 Ma) age for reflector R7 of this study (Table T1). The Site 1122 cores were expected to yield a test of this prediction. Finally, because of the location of the site just south of the Subtropical Convergence (STC), information from Site 1122 should help test for the stability of position of the STC between glacial and interglacial times (Fenner et al., 1992; Nelson et al., 1993; Weaver et al., 1998).
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