3. Site 1119: Drift Accretion on Canterbury Slope ¹

Shipboard Scientific Party ²

BACKGROUND AND OBJECTIVES

General Description

Site 1119 is located 96 km east of the eastern South Island shoreline, offshore from Timaru, within the Canterbury Basin (Fig. F1). The site was drilled in a water depth of 393 m on the upper continental slope and 5 km seaward of the edge of the continental shelf, which here lies at a water depth of 150 m. Farther seaward, the slope levels out onto an offshore platform at ~800-1000 m, which continues at a regional level across the tops of the Campbell Plateau and Chatham Rise. The site was located along Maurice Ewing multichannel seismic line 1E (Fig. F2), in a position ~5 km upslope from the head of a seafloor mass failure zone. Minor irregularities in the seafloor on either side of Site 1119 indicate that small-scale mass movement is also present in the vicinity of the drill hole, as confirmed by the 3.5-kHz profile through the site (Fig. F3A). These facts notwithstanding, drilling at Site 1119 penetrated through an area of seaward-dipping, continuous sub-bottom reflectors, within which any substrate failure is minor and probably shallowly based.

Multichannel profile Ewing-1E penetrates the complete thickness of the Cretaceous-Holocene Canterbury Basin succession and, below the bottom of Fig. F2, images the top of the inferred graywacke basement clearly. The seismic succession is threefold and comprises an onlapping Cretaceous to late Eocene sequence of nonmarine through shallow marine to offshore marine sediment (Onekakara Group), an Oligocene-earliest Miocene interval of flat-lying greensand and limestones (Kekenodon Group), and a Miocene-Holocene interval of eastward-prograding clinoform strata (Otakou Group). The equivalent stratigraphic intervals have been well studied in the nearby onland Canterbury Basin (e.g., Carter, 1988; Field and Browne, 1989) and are also known from offshore exploration wells such as Clipper, which is located 40 km west-southwest of Site 1119 (Hawkes and Mound, 1984).

The clinoform reflectors within the Miocene-Holocene Otakou Group show considerable variability on line Ewing-1E, delineating features that superficially resemble nested submarine channels. Fulthorpe and Carter (1991) have shown that this seismic geometry occurs widely within a shore-parallel zone under the mid-Canterbury shelf-edge and that it represents a major field of sediment waves or drifts whose deposition commenced in the early Miocene. Individual drifts initiate at the toe of the slope and prograde landward, resulting in the creation of a shore-parallel gutter between a growing drift and the shelf-edge. Later, the gutter becomes filled, which causes the drift to accrete to the edge of the shelf (Fig. F4). Drilling was designed to penetrate the upper part of these drifts.

Site Objectives

Site 1119 was drilled to sample the Canterbury sediment drifts. Deposited in estimated paleo-water depths of 400-1000 m, the drifts were probably deposited from northward-flowing Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (McCartney, 1977), which originate by sinking at the Antarctic Polar Front and proximal to the Subantarctic Front, respectively. Flow may also have been driven by the predecessor current to the Antarctic Circumpolar Current. In either case, a copious source of sediment is required to build the drifts. Individual drift examples attain a relief of >1 km and have a shore-normal width of 15 km. Some drift activity continues today, as evidenced by the presence of 5-km wavelength, low-angle drifts at 1200-m depth on the modern seafloor (Fig. F3B). Site 1119 samples will allow the provenance of the Canterbury drift sediments to be established, paleocurrent velocities to be inferred, and the late Miocene-Pliocene history of the important AAIW water mass to be reconstructed. If sand-rich intervals are found in the hole, their presence may imply relative sea-level change during the late Miocene-Pliocene (cf. Fulthorpe and Carter, 1989).

The upper 40 m of sediment at Site 1119, above the conspicuous drift sequence shown on the multichannel seismic profile, appears on 3.5-kHz profiles as a series of transparent layers (muds) separated by thin, parallel reflectors (sands) (Fig. F3A). Although minor surficial slumping obscures the detail, the sands apparently thicken upslope, until at the shelf edge they comprise a hard reflecting zone of probable lowstand shoreface origin. The geometric evidence from the seismic profiles indicates a likely depth of deposition for the upper sediments from Site 1119 of ~240 m at lowstand (and therefore ~370 m for any intercalated offshore highstand strata). The glacial intervals in these sediments were thus deposited nearshore, under the influence of lowstand shorelines, and were provided with terrigenous sediment from the Alpine Fault plate boundary in the west. In apparent contrast, the older large sediment drifts were presumably deposited mainly under the control of regional current activity, from sources yet to be established, but which might include a southern, oceanic input. Confirming the provenance of the drifts, and whether or not a regional paleoceanographic change occurred between their deposition and that of the cyclic upper Pleistocene beds, were high-priority drilling objectives at Site 1119. In addition, continuous hydraulic piston-cores through the probable lowstand sediment wedges will provide an important high-resolution section of the mid-late Pleistocene on the upper slope, allowing its cyclicity and sequence stratigraphy to be established and compared with nearby sites, including DSDP 594 (Kennett et al., 1986), and with similar cyclothemic successions on land (Saul et al., 1999).