The Prydz Bay Region

Drilling in Prydz Bay was undertaken by two ship-borne legs of the ODP (Fig. 8.5). In contrast to the western Ross Sea cores, core-recovery rates here were much less satisfactory; hence interpreting depositional processes is more questionable. Nevertheless, plausible scenarios have been derived, albeit lacking precise constraints owing to core loss. Prydz Bay represents the continuation of the Lambert Graben, which contains the Lambert Glacier-Amery Ice Shelf System, an ice drainage basin covering approximately 1Mkm2, draining 13% of the East Antarctic Ice Sheet by surface

0 200 400

Figure 8.5: Location of drill sites in Prydz Bay, East Antarctica, from ODP legs 119 and 188. Continental shelf sites 742 and 1166 include strata the cross the E/O transition. From shipboard Scientific Party (2001a). Reproduced with permission of the Ocean Drilling Program, College Station, Texas.

0 200 400

Figure 8.5: Location of drill sites in Prydz Bay, East Antarctica, from ODP legs 119 and 188. Continental shelf sites 742 and 1166 include strata the cross the E/O transition. From shipboard Scientific Party (2001a). Reproduced with permission of the Ocean Drilling Program, College Station, Texas.

area. Thus, the record in Prydz Bay provides a signal of the ice sheet as a whole since its inception, and complements the Oligocene (?) to Pliocene uplifted glaciomarine record in the Prince Charles Mountains (see Haywood et al., 2008; Chapter 10). Prydz Bay itself is dominated by a trough-mouth fan that prograded during phases of glacier advance to the shelf break. Like the western Ross Sea, large data-sets are available covering all aspects of core analysis from ODP Legs 119 and 188 (Barron, Larsen, & Shipboard Scientific Party, 1991; Cooper and O'Brien, 2004; Cooper et al., 2004) and a convenient summary has been provided by Whitehead et al. (2006).

ODP Leg 119 obtained two cores, 739 (480 m) and 742 (316 m), the lower parts of which were loosely dated as Middle Eocene to Early Oligocene. The dominant facies recovered was massive diamictite, with minor stratified diamictite and sand (Hambrey et al., 1991, 1992). Poorly consolidated finegrained facies may well have been washed away during the drilling, however, since core-recovery rates were less than 50%. A few broken shell fragments are present, but there is a dearth of material suitable for precise dating. The base of

Site 742 is represented by a zone of soft-sediment deformation. The Oligocene succession forms part of a prograding unit as defined in seismic profiles, but is truncated by a regional unconformity. Above lies a flat-lying sequence of more diamictite, some with preferred clast orientation and overcompaction, of late Miocene to Pliocene age (Cooper et al., 1991).

Leg 188 drilled Site 1166 on the continental shelf near Sites 739 and 742 in order to obtain a more complete record of the E/O transition, but again core recovery was poor (19%). From the base upwards, Late Eocene matrix-supported sand was followed by a transgressive surface and the Late Eocene to Early Oligocene graded sand and diatom-bearing claystone with dispersed granules. These facies were capped, above an unconformity, by ''clast-rich clayey sandy silt'' (diamicton/ite) of Neogene age (Shipboard Scientific Party, 2001a,b,c,d).

The interpretation of the Leg 119 facies is as follows: the deformed bed at the base of Site 742 may represent the first stages of glaciation, with the ancestral Lambert Glacier extending across the continental shelf for the first time. Then the bulk of the recovered facies in Sites 739 and 742 (diamictite) records deposition from the grounding-line of a tidewater glacier margin, by debris rain-out and submarine sediment gravity flow beyond the shelf break, conditions which characterize much of Early Oligocene time. The overlying Miocene succession is quite different, and represents successive advances across the already prograded shelf to produce sub-glacial, ice-proximal and ice-distal facies in alternation, forming the flat-lying sequence (Hambrey et al., 1991). Leg 188's Site 1166 begins at the base with Late Eocene sand of fluvio-deltaic character, and is inferred to be pre-glacial. The overlying Late Eocene to Early Oligocene sand and claystone represent shallow marine and open marine conditions, respectively, but in a proglacial setting as indicated by ice-rafted granule-sized material. The Neogene strata that lie unconform-ably above represent full glacial conditions.

Combining the sedimentary and seismic records, along with bathymetric data from the over-deepened Lambert Graben, a conceptual model of erosion and deposition has been developed (Fig. 8.6). Considering the whole Prydz Bay region in 3D, it is apparent that Early Oligocene time was characterized by sedimentation at the shelf break. A major change then took place in the Late Miocene to mid-Pliocene following the development of a fast-flowing ice stream that now occupied a more constrained channel, leading to the growth of the trough-mouth fan (Shipboard Scientific Party, 2001a,b,c,d; Taylor et al., 2004).

Comparing data-sets from the western Ross Sea and Prydz Bay, it is apparent that ice first reached the continental shelf edge in earliest Oligocene time in Prydz Bay, while the Victoria Land coast was influenced by iceberg

Figure 8.6: Conceptual model of styles of glacial sedimentation during phases of advance and recession of the ancestral Lambert Glacier (from Hambrey and McKelvey, 2000b). (A) ''Cold polar'' glaciation as occurs today. (B) Expanded ice in the Neogene Period. (C) Recessed ice in the Neogene period. Both B and C are characterized by polythermal glaciation (comparable to the High Arctic today), and may also have been a feature of the Palaeogene period. Data are too limited, however, to define the palaeoclimate for this earlier period. Reproduced with permission of The Geological Society of America, Boulder, Colorado.

Figure 8.6: Conceptual model of styles of glacial sedimentation during phases of advance and recession of the ancestral Lambert Glacier (from Hambrey and McKelvey, 2000b). (A) ''Cold polar'' glaciation as occurs today. (B) Expanded ice in the Neogene Period. (C) Recessed ice in the Neogene period. Both B and C are characterized by polythermal glaciation (comparable to the High Arctic today), and may also have been a feature of the Palaeogene period. Data are too limited, however, to define the palaeoclimate for this earlier period. Reproduced with permission of The Geological Society of America, Boulder, Colorado.

rafting. Late Oligocene time saw repeated ice expansions recorded in the CIROS-1 core by a succession of basal tills, and probably by the major unconformity in Prydz Bay. Subsequently, frequent expansions took place to the shelf edges in both areas, resulting in a fragmented stratigraphic record.

0 0

Post a comment