SEDIMENTATION IN PRYDZ BAYCenozoic
Hambrey et al. (1991) interpret the massive diamictites as mostly water-lain till formed by rainout from the base of glacial ice close to the grounding zone. They interpret some intervals with deformed bedding, particularly in the dipping, foreset units as debris flows formed from glacial debris with minor amounts of remobilized turbidites and ice-rafted sediment. Hambrey et al. (1991) describe some massive units with preferred orientation of clasts as possible subglacial till units, an interpretation supported by evidence of overcompaction of some intervals in the upper, flat-lying sequence (Solheim et al., 1991).
Hambrey et al. (1991) interpret the succession along the Leg 119 transect as indicating a history of earliest glaciation of the shelf in the Eocene to early Oligocene reworking preglacial Eocene sediments and depositing the lower, gently dipping sediments on the shelf. The ice then moved onto the shelf as a floating tongue and prograded the shelf edge in the early Oligocene. A major expansion of ice that Hambrey et al. (1991) regard as the largest pre-Quaternary expansion occurred during the late Oligocene to early Miocene.
Seismic data through the Leg 119 drill sites indicate that the flat-lying upper sequences pass seaward into foreset beds that prograde the continental shelf edge (Fig. 8; Cooper et al., 1991a, 1991b). Farther west where Prydz Channel crosses the shelf, the topsets are thin to absent and deposition has been concentrated on the upper slope in a trough mouth fan called the Prydz Channel Fan (Fig. 9). Landward of Four Ladies Bank, the topsets pinch out and the Amery Depression is covered with a thin layer of till and glaciomarine clayey silts and sands and siliceous muddy ooze overlying Cretaceous and older sediments. Leitchenkov et al. (1994) suggested that the shelf prograded more or less evenly across the bay until some time from the late Miocene to Pliocene. A readily mappable erosion surface (pp12, Surface A of Mizukoshi et al., 1986) marks the development of Prydz Channel and the start of Prydz Channel Fan sedimentation at this time. The fan has been the major depocenter since then although the Four Ladies Bank has continued to receive topsets of till (Fig. 9).
The Prydz Bay continental slope and rise are underlain by thick (more than 6000 m) drift sediments, some in elongated ridges aligned along the margins of deep channels, others having no clear correlation with channels, but all of them elongate approximately orthogonal to the continental margin (Kuvass and Leitchenkov, 1992). The seismic geometry of these drifts suggests that they have been deposited as a result of the interaction of downslope mass flow and strong bottom (contour) currents. By analogy with other drift deposits on the Antarctic margin (Rebesco et al., 1997; Shipboard Scientific Party, 1999), the drifts are composed of alternating clastic- and biogenic-rich intervals that reflect alternations of glacial and interglacial conditions. Such records can be compared to the proximal records of the continental shelf and upper slope to understand the relationship between oceanographic conditions and the advance and retreat of the ice sheet.
The most conspicuous sediment drifts are developed in the western part of the Cooperation Sea between Wilkins and Wild Canyons and are referred to as the Wilkins and Wild Drifts (Fig. 3). Kuvaas and Leitchenkov (1992) recognized two major seismic unconformities (P1 and P2). Additional data and reinterpretation have allowed the mapping of a third surface younger than P1 and P2. Surface P1 within these sediments marks the transition from a lower homogeneous part of section, with mostly irregular reflectors, to an upper, heterogenous one in which a variety of well-stratified seismic facies are present. More distal data suggests that P1 may be as old as Cretaceous. Surface P2 marks a change to submarine canyons and to related channel and levee deposits and chaotic seismic facies. Kuvaas and Leitchenkov (1992) interpret this transition resulting from the onset of continental glaciation in the Eocene or the arrival of grounded ice sheets at the shelf edge in the early Oligocene, as indicated by ODP Sites 739 and 742 (Barron et al., 1991). This sedimentation change produced thick, prograding foresets above the P2 unconformity beneath the Prydz Bay outer shelf (Kuvaas and Leitchenkov, 1992).
Surface P3, above P2, represents the base of deposits containing abundant, well-stratified sediment drift facies, including sediment waves. Sediment wave geometry implies that strong, westerly flowing bottom currents played a significant role in drift formation. The changes at this level could have been related to initiation of the ACC after the opening of Drake Passage around the Oligocene/Miocene boundary or may relate to a major ice expansion during the Oligocene or Miocene.
Quaternary to Modern Sediments
The parts of Prydz Bay shallower than 690 m are extensively ploughed by iceberg keels (O'Brien and Leitchenkov, 1997) and so are covered with a layer of disturbed sediment. Surface sediments and their diatom floras are described in Harris et al. (1998). Areas deeper than this have undisturbed Quaternary sections. The Amery Depression inshore from grounding zone wedges in Prydz Channel is floored by fluted subglacial till and draped in places by clayey diatom ooze deposited since ice retreat in the last 12,000 years (Domack et al., 1998; O'Brien et al., 1999). The tills are dark gray pebbly sandy clays with high magnetic susceptibility in most of the Amery Depression but in the southwestern corner of the Bay, in the Lambert Deep, tills are brown-red with low susceptibility derived from pre-Cretaceous red bed sediments in the Lambert Graben (Domack et al., 1998). Prydz Channel seaward of the grounding zone wedges, is floored by smooth seafloor and iceberg scours draped by gray clays and diatom ooze. These draped iceberg scours are probably relict features from LGM low sea levels.
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