IODP Expedition 317 Scientific Prospectus

doi:10.2204/iodp.sp.317.2009

Scientific objectives

Primary objective

The 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. Prediction of the distribution of sediments 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 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 will contrast upper Miocene–lower Pliocene sequences with smooth, onlapped paleoshelves and rounded breakpoints (below Unconformity U9; principally Sites CB-01A to CB-04B) and upper Pliocene–Pleistocene sequences with eroded and incised downlapped paleoshelves and more pronounced breakpoints (above Unconformity U9; all sites) (Fig. F7). This will test the hypothesis that paleoshelves below Unconformity U9 were not subaerially exposed at sequence boundaries, whereas those above Unconformity U9 were exposed.

Secondary objectives

Drilling the Marshall Paraconformity offshore will provide information on its regional distribution, age, and origin (Fig. F4). Based on ties to exploration wells, the EW00-01 data resolve, for the first time, a seismic surface potentially correlative with the onshore Marshall Paraconformity (Figs. F7, F8, F9). The paraconformity has been dated at its onshore type section using strontium isotopes as representing a hiatus lasting from 32.4 to 29 Ma (Fulthorpe et al., 1996). It is therefore correlative with the postulated mid-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 depths 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 locations now located onshore, the Marshall Paraconformity could not have been widely subaerially exposed, though such exposure may have occurred on localized highs (Lewis, 1992).

Instead, the paraconformity probably records intensified current erosion, or nondeposition, at all water depths, which accompanied the development of a partial Antarctic Circumpolar 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 deepwater settings (Shipboard Scientific Party, 1999a; Carter et al., 2004c). There are indications from Leg 181 drilling that the paraconformity developed in deep (bathyal) water ~1–2 m.y. earlier than in shallow water (McGonigal and Di Stefano, 2002). Dating the paraconformity in the offshore Canterbury Basin at Site CB-04B or at deepwater contingency Site CB-06B will test this hypothesis by sampling it where paleowater depths were intermediate.

The late Oligocene–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 preceded the current uplift phase; it constitutes a record of age, volume, and facies of erosion products from the Oligocene to the Holocene.

Sediment volumes within mapped seismic sequences provide a measure of onshore paleoerosion rates. Calculating such volumes involves integration of both the commercial and EW00-01 MCS data since the commercial data have broader areal coverage (Fig. F3). Sequence volumes have been used to calculate sedimentation rates (Fig. F11) (Lu et al., 2005). The limited available age control precludes estimation of meaningful sedimentation rates for individual sequences since 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 during deposition of S2–S4 occur 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 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 will verify ages of the progradational units for integration 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, with Ar-Ar dating of those grains, will allow matching of outcrop ages and source areas to sequences offshore. Ideally, we would like to calibrate sequences through the entire Neogene section, but penetration through the entire section at shelf water depths will be technically difficult and time consuming. As a compromise, Sites CB-01A and CB-3B penetrate to Unconformity U4 (~12.4 Ma).

The shelf-edge-parallel elongate drifts are unusual features in such an inboard continental margin setting; 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 was by the accretion of successive sediment drifts (Fulthorpe and Carter, 1991; Lu et al., 2003). 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. 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 only penetrated to 494.8 mbsf in sediments of 3.9 Ma, insufficient to sample more than the upper few meters of the underlying large elongate drift (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, Drift D11 can be considered the type example of a late simple elongate drift, and 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 into Drift 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, drilling into Drift D11 is no longer a primary objective of this expedition. Therefore, all sites originally proposed for drilling into Drift D11 (Sites CB-05B, CB-05C, CB-05D, and CB-05E) are henceforth considered as contingency sites and to be drilled only if drilling at shallower slope and shelf sites is not possible (see "Risk and contingency strategy"). However, it is still possible that insights into sediment drift deposition and paleoceanography may be obtained from drilling at primary slope Site CB-04B. Drift geometries become gradually less pronounced along strike toward the southwest (from CB-05 sites to Site CB-04B) and pronounced mounded drift geometries are absent at Site CB-04B (Lu and Fulthorpe, 2004). However, generation of mounded drifts requires specific conditions, which are not well understood; a slope contour current alone is insufficient, as is 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 Site CB-04B and left a paleoceanographic record of glacial–interglacial cycles, as at Site 1119 (Carter et al., 2004c), without producing distinctive geometries.

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