GEOLOGICAL SETTING
The evolution of the ACC-DWBC system took place during the Oligocene and earliest Miocene (32–20 Ma), when plate movements created the first deep-water oceanic gaps south of Australia and South America. The stratigraphic record of these events, and of the development of the modern ACC-DWBC, occurs in Cenozoic sediments located on and just east of the New Zealand microcontinental plateau (Fig. 4). South Island New Zealand is today transected by the Alpine Fault boundary between the colliding Australian and Pacific plates, but the greater part of eastern New Zealand has been unaffected by major tectonic events since the phase of Late Cretaceous rifting that created the south Pacific Ocean and first delineated the New Zealand Pacific continental margin. In short, since the Late Cretaceous, eastern New Zealand, which includes the submerged continental crust of Campbell Plateau and Chatham Rise, separated by the rift re-entrant of the Bounty Trough, has been a trailing-edge passive margin, and subject to thermotectonic subsidence and marine transgression (Cotton, 1955; Carter, 1988a).
The geological history of eastern New Zealand therefore consists of Late Cretaceous rift-valley filling (Bishop and Laird, 1976), followed by peneplanation and a marine transgression that reached its climax in the Oliocene, when almost the entire New Zealand plateau was submerged, terrigenous sources were flooded or buried, and regional carbonate sedimentation reigned supreme (Suggate et al., 1978). In the west, tectonic activity associated with the development of the transform plate boundary through New Zealand started in the late Eocene (Turnbull, 1985; Turnbull and Uruski, 1995; Sutherland, 1995), and, by the copious volumes of early Miocene terrigenous sediment from mountains along the Alpine Fault were being shed eastward into the Canterbury Basin, where they built the progradational sedimentary prism that underlies the modern coastal lowland and continental shelf of eastern South Island (Carter and Norris, 1976; Norris et al., 1978). Mountain building accelerated, and, presumably, sediment yields increased, at ~6.5 Ma in the late Miocene, when a shift in the pole of rotation resulted in a stronger element of collision across the Alpine Fault boundary (Walcott, 1998). These various plate boundary events had only minor effects in the eastern (offshore) parts of the New Zealand plateau (Fig. 4B). Apart from localized episodes of volcanism and mild folding-faulting associated with changes in regional stress patterns (e.g., Oliver et al., 1950; Carter, 1988b; Carter et al., 1994; Campbell et al., 1993), stasis or very slow postrift subsidence continued, and major sources of terrigenous sediment were absent. Sediment accumulation on highs was either precluded by strong water motion (e.g., Chatham Rise) or consisted of biopelagic chalk and ooze (e.g., Campbell Plateau), and terrigenous sedimentation was restricted to lows (e.g., Bounty Channel-Fan complex) or to sites adjacent to the prograding eastern New Zealand sediment prism (e.g., DSDP Site 594; Kennett, von der Borch, et al., 1986).