The eastern New Zealand continental shelf is underlain by a seaward-dipping, clinoform sequence of early Miocene to Holocene age. Under wide areas of the middle and outer shelf, the simple clinoforms are replaced by major sediment drifts, which were deposited in water depths between ~200 and 1200 m. Individual drifts are 200-800 m thick, 10-15 km wide, and up to several tens of km long (Fulthorpe and Carter, 1991) and contain a record of upper AAIW and thermocline water masses through the Neogene.

SWPAC-1C is targeted to pass through a Pliocene-Pleistocene drape of upper slope mud, and penetrate the main Miocene-Pliocene drift sequence below. The sedimentary record will date the cessation of drift deposition and provide estimates of the paleohydrography of the later parts of the drift sequence. Information on fluctuations in current strength will be compared with the deeper water record available from DSDP Site 594 nearby.

Alternative Site SWPAC-1D (pending approval) is located at 650 m depth, immediately downslope from Site SWPAC-1C. The site is provided as an alternative to Site SWPAC-1C in case adverse weather conditions preclude drilling at Site SWPAC-1C.

The Chatham Rise is eroded and current swept to the degree that phosphatized Miocene chalks are exposed along its crest. Gently northward-dipping sediments of Paleogene and early Neogene age occur north and south of the crest of the rise in about 600 m of water. A thin (<0.05-s thick, Unit A) sequence of Pliocene-Pleistocene sediment is lost in the airgun bubble pulse. 3.5 kHz records show these surficial sediments to comprise a pelagic drape with continuous reflectors that probably represent glacial and interglacial cycles. Below this drape, a 0.04-s-thick transparent interval (Unit B) of probable late Miocene age is separated across Unconformity Y from a 0.1-s-thick sequence with strong parallel reflectors (Unit C; early Miocene). Unit C is separated from the probable Paleogene sediment of Unit D across Unconformity X and tentatively correlated with the mid-Oligocene Marshall Paraconformity. Overall the section is expected to comprise a sequence of largely carbonate biopelagic sediments that have never been buried much below the seafloor.

The major targets of Site SWPAC-2B are to retrieve an unaltered sequence of lower Neogene and perhaps Paleogene sediments that encompass the commencement of ACC-AAIW activity on the margin, i.e., penetrate back to the Oligocene. This sequence will provide a record of AAIW paleohydrography, changing paleoproductivity, and variability or stasis in the position of the Subtropical Convergence. A particular attraction of Site SWPAC-2B is the possibility of achieving a high-quality oxygen isotopic record that spans the Oligocene and early Miocene (i.e., the period of probable inception of both Antarctic glaciation and the consequent delivery of cold water into the ACC-DWBC system).

A major sediment drift occurs between 169W and 175W and at depths of 2200-4500 m on the northeastern slopes of the Chatham Rise. A drift thicker than 0.6 s lies above 3500 m depth, and has been deposited where the DWBC decelerates after passing through Valerie Passage (Carter and McCave, 1994). The drift (Unit B) is well delimited between Unconformities Y and X. A probable paleodrift is delimited by Reflector X' (early Miocene, ~20 Ma) within the base of the unit, which is interpreted to be of mid-Oligocene age. The upper sediments at this site (Unit A) comprise a 0.2-s thick sequence of Pliocene-Pleistocene pelagic drape, which has been modified by the DWBC as attested by the widespread occurrence of sediment waves. Closely spaced parallel reflectors within this drape probably represent muddy calcareous pelagites and purer calcareous pelagites.

The anticipated presence of a substantial carbonate record back to the middle Oligocene (Unconformity X) makes Site SWPAC-5B a prime site at which to evaluate the evolution of the AAC-DWBC system, including information on the NADW component of flow. It is anticipated that the upper part of the sequence will contain a record of volcanic ashes derived from North Island, New Zealand.

This site is located at a 0.7-s-thick sediment sequence near the eastern edge of the Campbell Plateau. Earlier DSDP drilling at nearby Site 275 revealed a lack of Cenozoic sediment and yielded Late Cretaceous ages at shallow subseafloor depths, beneath a veneer of modern ooze and a manganese pavement. The substantial accumulation of Cenozoic sediment targeted by Site SWPAC-6B has been deposited in an ideal position to record changes in the position of the nearby Subantarctic Front and possibly the Antarctic Convergence. These fronts form, respectively, the northern and southern limits of the ACC. The site is also located at an appropriate depth to monitor changes in AAIW activity and should yield a carbonate record throughout.

The Campbell Drift is an extensive sediment accumulation up to 170 km wide, 850 km long, and a maximum of 1.1-s thick, which occurs along the margin of the Campbell Plateau under the path of the DWBC, at depths of 4000-4500 m. DSDP Site 276 was located in the erosional moat between the western edge of the drift and the Campbell Plateau escarpment, and penetrated directly into Oligocene and Eocene siliceous sediment. Thus, DWBC erosion has apparently cut down to the Paleogene in the moat, which means there is an excellent chance of obtaining a continuous Neogene-Paleogene section by drilling through the crest of the drift at Site SWPAC-7B.

Site SWPAC-7B should record the history of the ACC-DWBC and associated water masses as they approach the Pacific gateway. It will be of special interest to compare paleocurrent speeds at this site with sites farther north, as Site SWPAC-7B is located where the DWBC is presently forced by the ACC, which may affect boundary flow to 4000 m or deeper depth.

The outer Bounty Trough is a major injection point of New Zealand derived sediment into the path of the DWBC. During the late Neogene, an abyssal fan developed on an unconformity that is correlated with regional Reflector Y (late Miocene, ~8 Ma; Carter et al., 1994). The fan comprises well stratified Pliocene-Pleistocene sediments which, on the north bank, were deposited in the form of deep-sea sediment waves. These waves probably correspond largely to the hydraulics of overspilling turbidity currents, but with influence also from the DWBC. Nearby surface cores demonstrate that the alternating reflecting and nonreflecting units correspond respectively to glacial periods of turbidity current activity and interglacial biopelagic deposition, that is, that a close relationship exists between seismic stratigraphy and oxygen isotopic stratigraphy (Carter and Carter, 1992). A core near the base of the north channel wall contains a well-preserved late Pliocene calcareous planktonic fauna, indicating excellent preservation of carbonate for the interglacial intervals. An additional attraction to this site is the high average sedimentation rate (glacials and interglacials) of ~15 cm/k.y., implied by the deposition of 350 m of levee sediment over ~2.4 m.y.

Site SWPAC-8A will provide high-resolution information on (1) estimates of the turbidity current speeds of deposition and rates of bedform accumulation and migration of deep-sea mud waves; (2) the record of terrigenous vs. biopelagic input into the DWBC and ENZOSS system over the last 3 m.y.; and (3) the correlation between seismic reflectors and oxygen isotopic stratigraphy.

The 250-km-long, ridge-like Rekohu Drift consists mainly of inferred Miocene drift sediments. The main Rekohu sequence overlies older sediments across Unconformity X and is onlapped by overbank turbidites of the Hikurangi Channel across Unconformity Y. By correlation with other sections, the Unit B sediments at this site are probably calcareous pelagites. Unravelling the evolution of the Rekohu Drift is critical to understanding the development of Hikurangi Channel, and the injection of sediment into the DWBC north of the drift. The Rekohu Drift has clearly acted as an effective barrier to eastward dispersal of terrigenous sediment from the Hikurangi Channel during the Pliocene-Pleistocene.

Site SWPAC-9B should yield a mainly carbonate record of the Miocene paleohydrography of the DWBC, and (if it penetrates unconformity X) important information on the mid-Cenozoic initiation of the system.

Alternate Site SWPAC-10B
The northern platform flank of the Bounty Trough encompasses a full seismic sequence of Units A through D together with the prominent regional unconformities represented by Reflectors Y (late Miocene, ~6.5 Ma) and X (late Oligocene, ~30 Ma). Thus, in contrast to the late Neogene nature of the Bounty Fan site (Site SWPAC-8A), Site SWPAC-10B will only drill through a thin upper Cenozoic record before penetrating the inferred Miocene sediment that underlies regional unconformity Reflector X. Of key interest is the nature of basal Unit B sediments, their record (if any) of DWBC activity and the character and age of the bounding unconformity below.

The gentle folding that affects all units up to Unconformity Y marks a regional event of probable late middle Miocene age that may correlate the known change in motion of the New Zealand plate boundary at about 10 Ma, from a strike slip to strongly transpressional phase. Site SWPAC-10B would provide direct dating for this important regional event.

Alternate Site SWPAC-16A
The East Chatham deep drift is located at the northeastern end of the rise, at the narrowest point of Valerie Passage. The drift, therefore, lies adjacent to the gateway into the Pacific for the DWBC and was deposited from LCDW, which originated in the Weddell Sea. Core from Site SWPAC-16A is therefore anticipated to contain a record of variations in the source material and strength of depositional current of the DWBC. Kasten Core Chat-3K from this locality contains an excellent benthic and planktonic oxygen and carbon isotope record.

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