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Scientific objectives

Primary objective

Date clinoform seismic sequence boundaries and sample-associated facies to estimate eustatic amplitudes

Facies, paleoenvironments, and depositional processes associated with the sequence stratigraphic model on prograding continental margins, where sequences are best resolved seismically, have yet to be adequately constrained by scientific ocean drilling. The prediction of sediment distribution within sequences is highly model dependent (e.g., systems tract models of Posamentier et al., 1988; Vail et al., 1991). These models offer great potential for understanding oil and gas resources and for ground water/pollution remediation issues. However, the fundamental assumptions and predictive capabilities of these models can only be tested by drilling on shallow continental shelves. Drilling contrasts upper Miocene–lower Pliocene sequences with smooth, onlapped paleoshelves and rounded breakpoints (seismic sequence boundary ~U10 and below) to upper Pliocene–Pleistocene sequences with eroded and incised, downlapped paleoshelves and more pronounced breakpoints (above U10) (Fig. F7). This tests the hypothesis that paleoshelves below ~U10 were not subaerially exposed at sequence boundaries, whereas those above ~U10 were exposed.

Secondary objectives

1. Drill the Marshall Paraconformity in the offshore basin

Drilling the Marshall Paraconformity offshore provides information on its regional distribution, age, and origin (Figs. F4, F8, F9). The paraconformity has been dated at its onshore type section using strontium isotopes and represents a ~3.4 m.y. hiatus (32.4–29 Ma; Fulthorpe et al., 1996). It is therefore correlative with the postulated early Oligocene eustatic lowstand (Haq et al., 1987). However, a eustatic lowstand is unlikely to have been the direct cause of the paraconformity because correlative features have been inferred to form in water as deep as 4000–5000 m throughout the southwest Pacific (Carter, 1985; Carter et al., 2004c). The limited paleoenvironmental data available suggest that, even at current onshore locations, the Marshall Paraconformity could not have been widely subaerially exposed, although such exposure may have occurred at localized highs (Lewis, 1992).

Instead, the paraconformity probably records intensified current erosion or nondeposition at all water depths, which was accompanied by the development of a current system following the opening of the seaway south of Tasmania (Carter, 1985; Fulthorpe et al., 1996; Carter et al., 2004c). Seismic interpretation supports a current-related origin by indicating that the paraconformity forms the base of the interval of sediment drift deposition. Indeed, immediately post–Marshall Paraconformity, sedimentation involved sediment drift deposition in shallow (Ward and Lewis, 1975), intermediate (Fulthorpe and Carter, 1991; Lu et al., 2003), and deep (Shipboard Scientific Party, 1999a; Carter et al., 2004c) water settings. Leg 181 drilling indicates that the paraconformity developed in deep (bathyal) water ~1–2 m.y. earlier than in shallow water (McGonigal and Di Stefano, 2002). Dating this paraconformity in the offshore Canterbury Basin at Site U1352 tested this hypothesis.

2. Constrain the erosion history of the Southern Alps

The late Oligocene to early Miocene increase in sediment supply to the offshore basin apparently predates the modern transpressional uplift of the Southern Alps, whose main pulse began at ~8–5 Ma (Tippett and Kamp, 1993a; Batt et al., 2000) or ~10–8 Ma (Carter and Norris, 1976; Norris et al., 1978; Adams, 1979; Tippett and Kamp, 1993b). Such onshore results correlate with increases in subsidence rate and sediment supply in the offshore basin (Figs. F5, F6, F11) but may underestimate earlier, but significant, convergence and uplift (Walcott, 1998). The offshore sedimentary prism is the only record of erosion that precedes the current uplift phase, constituting a record of age, volume, and facies of erosion products from the Oligocene to recent.

Sediment volumes within mapped seismic sequences provide a measure of onshore paleoerosion rates. Calculating such volumes involves integration of both industry and EW00-01 MCS data because the commercial data have broader aerial coverage (Fig. F3). Sequence volumes were used to calculate sedimentation rates (Fig. F11) (Lu et al., 2005). Limited available age control precluded estimation of meaningful sedimentation rates for individual sequences because such rates are strongly influenced by sequence duration. Therefore, sedimentation rates averaged over groups of sequences are presented instead. The resulting rates correlate well with estimates of the perpendicular component (convergence) of relative plate motion at the Alpine Fault: both increase during the last 5–8 m.y. (Fig. F11). In contrast, high sedimentation rates from 14.5 to 11.5 Ma occurred during a period of low convergence rate at the fault (Lu et al., 2005). However, these high rates correlate with similar peaks in sedimentation rates elsewhere, such as off New Jersey and West Africa (Steckler et al., 1999; Lavier et al., 2000, 2001), and occur during a period of falling global sea level (Fig. F11). They are probably, therefore, a response to global climatic trends. Drilling results will be used to verify the ages of the progradational units, which will be integrated with sediment volume results to provide an enhanced, sequence-by-sequence record of sedimentation rates for correlation with tectonic and climatic events. Provenance studies will further add to our understanding of the early history of the plate boundary. Mineralogical and isotopic analyses of sand grains, along with Ar-Ar dating of those grains, will allow the matching of outcrop ages and source areas to sequences offshore. Ideally, we would have liked to calibrate sequences through the entire Neogene section, but penetration of the entire section at shelf water depths is technically difficult and time consuming. As a compromise, Expedition 317 sites targeted seismic sequence boundaries down to U4 (~12.4 Ma), although U5 was the oldest sequence boundary actually penetrated.

3. Determine sediment drift depositional histories and paleoceanographic record

Elongate drifts parallel to the shelf edge are unusual features in such an inboard continental margin setting, and their facies are largely unknown. They were probably deposited adjacent to a current flowing northward along the prograding clinoform slope; progradation in parts of the basin occurred by the accretion of successive sediment drifts (Fulthorpe and Carter, 1991; Lu et al., 2003). The drifts can encompass several seismic sequences, and the erosional unconformities in moats are diachronous (Figs. F7, F8, F12) (Lu et al., 2003). Drift formation is not, therefore, linked directly to individual sea level cycles. Data from Site 1119 (Leg 181) confirmed the drift origin of these features; interglacial silty sand and glacial silty clay cycles were recognized in current-deposited sediments (Shipboard Scientific Party, 1999b). However, Site 1119 penetrated only to 494.8 mbsf in 3.9 Ma sediments, which is insufficient to sample more than the upper few meters of the underlying large, elongate drift (D10 of Lu et al., 2003; Carter et al., 2004a). The earlier history of the frontal systems is therefore poorly constrained. In addition, the climatic record of Site 1119 correlates well with that from the Vostok ice core (Carter et al., 2004b), and drilling deeper could provide an extended proxy for Antarctic continental air temperatures. Furthermore, D11 can be considered the type example of a late, simple, elongate drift, and it displays all of the component seismic facies (base, core, and crest) (Lu et al., 2003). It is estimated to be of late Miocene (~11 Ma) to Pliocene (~3.6–3.25 Ma) age (Fig. F12) (Lu and Fulthorpe, 2004; Carter et al., 2004a) and may be the longest lived sedimentary bedform on Earth. Drilling D11 would therefore be essential to determine the lithologies that make up the large volumes of sediment within the elongate drifts and recover the paleoceanographic record contained within the drift deposit.

Because of time constraints imposed in part by the 10 day transit from Townsville, Australia (which included a fuel stop in Wellington, New Zealand), drilling D11 became a secondary objective of Expedition 317. Therefore, all sites originally proposed for drilling D11 (proposed Sites CB-05A through CB-05E) became contingency sites to be drilled only if drilling at shallower slope and shelf sites was not possible. However, insights into sediment drift deposition and paleoceanography were expected from drilling at shelf Site U1351 (which penetrated the paleoslopes of U6 and U7) and slope Site U1352. Drift geometries become gradually less pronounced along strike toward the southwest (from proposed CB-05 sites to the drilling transect sites), and pronounced mounded drift geometries are absent beneath the slope at Site U1352 (Lu and Fulthorpe, 2004). However, the generation of mounded drifts requires specific conditions that are not well understood; a slope contour current alone is insufficient, as indicated by the fact that such drifts are not being formed under the present current regime. It is therefore possible that currents have reworked the sediments at Sites U1351 and U1352 and left a paleoceanographic record of glacial–interglacial cycles, as at Site 1119 (Carter et al., 2004c), without producing distinctive drift geometries.