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Site survey data and interpretation

Data acquisition

A large amount of multichannel seismic (MCS) data, collected in 1982 for a petroleum exploration consortium (BP Shell Todd), is available for the offshore Canterbury Basin (CB-82 profiles; e.g., Fulthorpe and Carter, 1989). Although the data cover an extensive area, they exhibit relatively low vertical resolution (20 m). Therefore, this survey was augmented in 2000 by two-dimensional high-resolution MCS profiles (R/V Maurice Ewing Cruise EW00-01; Figs. F3, F7, F8, F9). The survey grid lies approximately midway between the Banks and Otago peninsulas on the present-day middle to outer shelf and slope, above the late Miocene to recent depocenter and over the area where the largest sediment drifts developed (Fig. F3). The seismic source consisted of two generator-injector (GI) air guns (45/45 in3), and the streamer was deployed with 12.5 m groups in 96- and 120-channel configurations. A total of 57 profiles (~3750 line-km) were collected, covering ~4840 km2. Line spacings are 0.7–3.0 km in the dip direction and 2.0–5.5 km in the strike direction. The penetration of 1.7–2.0 s below seafloor is sufficient to image the entire Oligocene to recent section. Data processing used Focus software, and the data were loaded into the GeoQuest interpretation system. High resolution was achieved by using reliable high-frequency sources (maximum frequency = 500 Hz), small sample intervals (1 ms), and high fold (48–60). Vertical resolution (~5 m for two-way traveltime <1 s) is as much as 4–5 times better than that of existing commercial MCS data (Fig. F10). However, seafloor and peg leg multiples are pronounced beneath the shelf. In order to deal with this problem, prestack deconvolution and FK-filtering were applied to critical sections, yielding some improvements in quality.

Commercial low-resolution MCS grid data were used to extend interpretations beyond the EW00-01 grid. CB-82 data were particularly useful for tying to exploration wells, determining sediment drift distribution, and locating clinoform rollovers, onlap, and canyons associated with the oldest sequences. The grid consists of 81 profiles, representing ~6000 line-km (Fig. F3). The record length is 5 s, and the sample rate is 4 ms. Digital copies of all stacked profiles were loaded into the GeoQuest system. Paper copies of migrated profiles were also available, and 20 of the digital profiles were migrated as part of this project.

Seismic stratigraphy

The EW00-01 MCS data were interpreted to provide a high-frequency sequence stratigraphic framework for the offshore Canterbury Basin (e.g., Fig. F7). Nineteen regional seismic sequence boundaries (U1–U19) were identified in the middle Miocene to recent shelf-slope sediment prism of the offshore Canterbury Basin (Lu and Fulthorpe, 2004). Three larger seismic units were defined based on seismic architecture and facies (Fig. F7):

  1. U1–U4 mostly lack distinct clinoform breaks within the seismic coverage.

  2. U5–U8 feature rounded breaks; internal reflection geometries are predominantly sigmoid, and paleoshelves are smooth and defined by onlap and truncation.

  3. U9–U19 are downlapped on paleoshelves and truncate underlying reflections near paleoshelf edges; internal reflection geometries are oblique, and U- and V-shaped channels incising paleoshelves indicate exposure during sea level lowstands.

The predrilling ages of U3–U10 were initially based on ties to the Clipper exploration well (Hawkes and Mound, 1984), and the ages of U11–U19 were based on ties to ODP Site 1119 (Fig. F11) (Shipboard Scientific Party, 1999b; R.M. Carter, pers. comm., 2002). The Endeavour (Wilding and Sweetman, 1971) and Resolution (Milne et al., 1975) exploration wells were tied to the EW00-01 survey using the commercial CB-82 MCS profiles to calibrate the lowermost seismic sequence boundaries (U1–U3) (Fig. F7). Late Pliocene to recent age seismic sequence boundaries U11–U19 were considered the most reliable because they were derived from the continuously cored Site 1119. The Site 1119 section is virtually complete; only one downlap unconformity (U18 at ~87 meters below seafloor [mbsf], representing the Stage 7/8 boundary at ~252–277 k.y.) has a significant hiatus (~25 k.y.). U19 (~48 mbsf) corresponds to the Stage 5/6 boundary (~113 ka) at Site 1119, and the age of the deepest sediment recovered at Site 1119 is ~3.9 Ma (Carter et al., 2004a). The predrilling ages of lower Pliocene and Miocene unconformities were less well constrained.

Expedition 317 drilling indicates that predrilling age estimates for most seismic sequence boundaries agree reasonably well with drilling results (see "Synopsis"). Only U7 was off by a significant margin (4.3–5.3 Ma versus the predicted 9 Ma; see "Synopsis"). A tentative correlation with oxygen isotopic records based on predrilling age estimates suggests a eustatic origin for the seismic sequence boundaries. When cycles of comparable frequency are compared, the number of seismic sequences is similar to that of coeval cycles on the Miocene to recent δ18O values of seawater (δ18Osw) record of Billups and Schrag (2002) (Fig. F11). However, local processes also exerted fundamental control on sequence architecture. Along-strike currents strongly influenced sequence development, and large sediment drifts dominate parts of the Neogene section.

Sediment drifts and paleoflow

The presence of long-lived drifts beneath the modern shelf confirms that currents swept the New Zealand plateau as early as 15 Ma (Fig. F12) (Lu and Fulthorpe, 2004), and onshore outcrops of Kekenodon Group Greensand and limestone indicate that the current influence extends back to the late Oligocene (Ward and Lewis, 1975). The STF, Subantarctic Front (SAF), and associated currents may have existed close to their present positions relative to New Zealand by the middle or latest Miocene (Carter et al., 2004c; Nelson and Cooke, 2001). The STF is represented by the Southland Front along the eastern South Island (Fig. F2). The δ13C record at Site 1119 is interpreted as reflecting glacial–interglacial alternations of subtropical and subantarctic water caused by movement of the STF across the site over at least the last two glacial–interglacial cycles (Carter et al., 2004a, 2004b). Falling sea level deflected the front basinward during glacial periods, with the counterintuitive result that the site experienced a warmer watermass during glacials; intervals of interbedded sand and mud mark the passage of the front basinward and landward across the site (Carter et al., 2004b). Sand beds occurring near the peak of glaciation are interpreted as representing the proximity of the STF and SAF, which may have coalesced near the site, intensifying current strength (Carter et al., 2004b).

Currents formed at least 11 large, elongate drifts within the lower Miocene to recent section (e.g., Fig. F12) (Lu et al., 2003; note that timing is based on predrilling ages). These drifts were initiated near the slope toe and aggraded to near-shelf water depths. Drift deposits are as thick as 1000 m and have mounded morphologies with channel-like moats along their landward flanks. Internal geometries define two elongate drift end-members: simple and complex. Early (middle Miocene) simple drifts are small and are concentrated in the southern part of the survey area (Figs. F3, F7). Drift thickness and longevity increased as the shelf aggraded, increasing accommodation space, while the locus of drift development migrated northeastward. Late (late Miocene to recent) simple drifts are therefore larger and occur in the northeastern part of the survey area (Figs. F3, F8). Late simple drifts are divided into three parts (base, core, and crest) based on seismic facies. These facies form in response to progressive confinement of current flow within the moat. Complex drifts (multicrested and multistage; Fig. F3) may form as current pathways migrate in response to sea level change, modulated by paleoslope inclination, and as a result of fluctuations in the rate of sediment supply.

Current erosion in drift moats forms diachronous unconformities that cut across sequence boundaries. Several sequence boundaries pass through some of the larger drifts, indicating that they existed throughout several cycles of relative sea level change (Figs. F7, F8). Drift deposition controls sequence thickness distributions: sequences are thickest at drift mounds and thinnest within moats. In addition, currents focus deposition on the slope, reducing the rate of basinward movement of the shelf edge but increasing that of the slope toe. As a result, slope inclination is minimized (cf. Figs. F8, F9). The cessation of drift development and the replacement of along-strike processes with downslope processes result in increased rates of shelf-edge progradation and slope steepening as the accommodation space over the expanded slope is filled. Slope platforms can form above extinct drifts, reducing accommodation and locally accelerating shelf-edge progradation and slope steepening. The termination of large, elongate drift development (~3.25 Ma at Site 1119; Carter et al., 2004a) may have resulted from the initiation of late Pliocene to Pleistocene high-amplitude sea level change, which enhanced downslope processes by exposing the shelf edge. In spite of the demonstrable presence of a current, seismic evidence for current activity is lacking and coeval strata are clinoformal along strike from some large, elongate drifts (Fig. F9). Therefore, elongate drift formation is the product of multiple controls, including current intensity, seafloor morphology, and sediment input.