Site 1165 is situated on the continental rise offshore from Prydz Bay (Fig. F1). It targets mixed sediment-drift channel-levee sediments of the central Wild Drift, an elongate sediment body formed by the interaction of sediment supplied from the shelf and westward-flowing currents on the continental rise. The site is in 3525 m of water and was selected to provide a record of sedimentation that extends back to the onset of contour current-influenced deposition on the rise.
The Prydz Bay continental slope and rise are underlain by >6000-m-thick post-Lower Cretaceous sediments. Kuvaas and Leitchenkov (1992) recognized two prominent seismic unconformities (P1 and P2). Reinterpretation of prior and recent seismic reflection data by the Prydz Bay Antarctic Offshore Stratigraphy Project (ANTOSTRAT) regional working group has resulted in the mapping of several unconformities (e.g., P3 and PP15) younger than P1 and P2 (Fig. F2). Surface P1 marks the transition from a lower, homogeneous acoustic unit with intermittent and irregular reflectors to an upper, heterogeneous unit denoted by well-stratified seismic facies. Kuvaas and Leitchenkov (1992) interpreted P1 to be a result of changes brought about by the start of glaciation in late Eocene to early Oligocene times; however, more-distal, recent seismic reflection data and drilling results near Kerguelen Plateau suggest that P1 may be as old as Cretaceous age (G. Leitchenkov, pers. comm., 1998). Surface P2, which denotes a distinct upward transition to seismic facies characteristic of submarine canyons with channel and levee deposits, reflects an increase in the bottom-current activity along the continental margin. The change in depositional environments may be due to initiation of the Antarctic Circumpolar Current in late Oligocene time (Kuvaas and Leitchenkov, 1992) or an episode of glaciation in early Oligocene time, when grounded ice sheets arrived at the continental shelf edge (ANTOSTRAT working group). P2 marks a time when large amounts of glacially derived sediment were initially deposited in thick prograding foreset beds (above P2) on the continental slope across the front of Prydz Bay and were carried oceanward by downslope turbidity flows and contour currents.
Unconformity P3 lies above P2 and forms the base of deposits that contain abundant well-stratified sediment-drift facies, including sediment waves. Such features imply that strong, presumably westerly flowing, bottom currents played a significant role in drift formation (Fig. F2). The changes at this level could be related to enhanced current activity resulting from the opening of deep-ocean gateways (e.g., South Tasman Rise from Antarctica and/or Drake Passage). A shallower unconformity, PP12 (i.e., Reflector "A" of Mizukoshi et al., 1986), marks the start of deposition of the Prydz Channel Trough Mouth Fan and the distal well-stratified hemipelagic and turbidite deposits on the continental rise. Sediments above PP12 are believed to be of late Miocene or younger age from correlations with Ocean Drilling Program (ODP) Site 739 (ANTOSTRAT working group).
Site 1165 was placed in the central part of the Wild Drift where sediment is relatively thin to acquire a nearly complete record of the section down to unconformity P3 (Fig. F2). This site location would provide a record of ocean and ice sheet interactions (i.e., glacial-interglacial fluctuations of grounded ice within Prydz Bay) throughout the Neogene. Drilling at this site would also provide information on the timing of the major change in continental rise sedimentation denoted by regional unconformity P3 in addition to evidence of reasons for the change. The depth to P3 at Site 1165 is ~1020 mbsf, and if the position of the unconformity is—as anticipated—approximately lower Miocene, then average sedimentation rates would be ~40 m/m.y. If older, then rates will be lower. Glacial events recorded in sediments recovered at Site 1165 will be compared with those observed in slope and shelf drill sites from Leg 188 and prior ODP Leg 119 sites in Prydz Bay. The ultimate objective is to obtain a proximal continental-rise record of Antarctic glacial and interglacial periods for comparison with sites around Antarctica and with those near Northern Hemisphere ice sheets.