Lambert Glacier Region

The Pagodroma Group is a succession of massive diamicts and boulder gravels with minor stratified diamicts, laminites, sand and gravel, cropping out in the Prince Charles Mountains along the western margin of the Lambert Graben (Figs. 10.2 and 10.4). The best known outcrops are at Amery Oasis and Fisher Massif (Bardin, 1982; Hambrey and McKelvey, 2000a; McKelvey et al., 2001; Whitehead et al., 2003, 2006a), but equivalent exposures are known from Mount Menzies, the last exposed rock in this region heading polewards. The total outcrop distance of known glacigenic strata along the flank of the graben is about 800 km. The graben is occupied by the Lambert Glacier, a south-north-flowing outlet glacier, whose total ice-drainage area represents 13 per cent or 1 million km, of the EAIS. It has acted as a conduit for ice flowing into Prydz Bay since its initial formation at the Eocene-Oligocene transition (Barron et al., 1991; Strand et al., 2003). Although the Pagodroma Group is regarded as the East Antarctic equivalent of the Sirius Group, there are major differences in lithofacies, facies thickness and geometry, fossil content and palaeoenvironment. For example, these

Figure 10.4: Glacial erosional features and sedimentary characteristics of the Pagodroma Group, Prince Charles Mountains. (A) Uplifted segment of a palaeofjord bottom and sidewall (arrowed), filled with nearly 300 m of ice-proximal fjordal sediment of the Miocene Fisher Bench formation, Fisher Massif. (B) Striated pavement on Amery Group Sandstone, Radok Lake, Amery Oasis; this surface is coeval with the lower part of the Pliocene Bardin Bluffs formation. (C) Typical massive to crudely stratified boulder gravel of the Miocene Fisher Bench formation, Fisher Massif, interpreted as ice-contact deposits associated with a grounding-line fan. (D) Weakly stratified diamict and boulder gravel of the Pliocene Bardin Bluffs formation at the type locality, interpreted as ice-proximal deposits; the cliff face here is about 60 m high. (E) Laminated silt and thin diamictites representing deposition adjacent to a grounding-line fan as quiet water cyclopels and cyclopsams, interrupted by gravity-flow deposition, Miocene Battye Glacier formation, Dragons Teeth, Amery Oasis (redrawn from Hambrey and McKelvey, 2000a).

Figure 10.4: Glacial erosional features and sedimentary characteristics of the Pagodroma Group, Prince Charles Mountains. (A) Uplifted segment of a palaeofjord bottom and sidewall (arrowed), filled with nearly 300 m of ice-proximal fjordal sediment of the Miocene Fisher Bench formation, Fisher Massif. (B) Striated pavement on Amery Group Sandstone, Radok Lake, Amery Oasis; this surface is coeval with the lower part of the Pliocene Bardin Bluffs formation. (C) Typical massive to crudely stratified boulder gravel of the Miocene Fisher Bench formation, Fisher Massif, interpreted as ice-contact deposits associated with a grounding-line fan. (D) Weakly stratified diamict and boulder gravel of the Pliocene Bardin Bluffs formation at the type locality, interpreted as ice-proximal deposits; the cliff face here is about 60 m high. (E) Laminated silt and thin diamictites representing deposition adjacent to a grounding-line fan as quiet water cyclopels and cyclopsams, interrupted by gravity-flow deposition, Miocene Battye Glacier formation, Dragons Teeth, Amery Oasis (redrawn from Hambrey and McKelvey, 2000a).

deposits contain in situ diatom assemblages that can be used to date the formations directly (Whitehead et al., 2003, 2004).

The four formally defined formations within the Pagodroma Group (Table 10.1) have age ranges from early Miocene (or possibly Oligocene) to Pliocene-Pleistocene (Hambrey and McKelvey, 2000b; Whitehead et al., 2003, 2006a). The strata at Mount Menzies (Whitehead and McKelvey, 2002) have not yet yielded any datable fossils. The massive diamicts and boulder gravels indicate deposition in an ice-proximal environment near a grounding line, whereas the stratified facies represent more distal iceberg deposition. The diamicts contain striated and facetted clasts within a matrix that is finer grained than modern Antarctic tills. Dropstone structures occur in thin laminated beds and resemble glacigenic marine rhythmites (cyclopels and cyclopsams) that commonly occur to the sides and front of a grounding-line fan (Fig. 10.5). The depositional environments are considered to be analogous to the modern fjords of East Greenland with fast-flowing polythermal tidewater glaciers (Hambrey and McKelvey, 2000b). The Battye Glacier Formation contains diatom-bearing beds (opal contents up to 15wt.%) with in situ mollusc assemblages, which are assumed to have been deposited in ice-distal settings (Whitehead et al., 2006a). The Pagodroma Group has a cumulative thickness of > 800 m and provides evidence for major shifts in the position of the grounding line of the Lambert Glacier throughout the Neogene (Hambrey and McKelvey, 2000a).

The Pagodroma Group represents remnants of a much more extensive fjord-fill sequence that has been uplifted above sea level (a.s.l.) (Fig. 10.4). Palaeofjord floors have been uplifted to different levels at Amery Oasis and Fisher Massif, the highest being nearly 1,500 m on the latter. In contrast, the main Lambert trough has been excavated to 2,000 m below sea level, the sediment from which has been delivered into Prydz Bay since the Eocene/ Oligocene transition, where it forms the prograded continental shelf, including the huge late Neogene Prydz Trough-Mouth Fan that was drilled during Ocean Drilling Program (ODP) Leg 188 (Passchier et al., 2003; O'Brien et al., 2004). The Prydz Bay continental margin has been drilled by ODP Legs 119 and 188 (see Barron et al., 1991; Cooper et al., 2004; Whitehead et al., 2006b). Numerical modelling of ice-sheet expansion and recession in the Lambert Glacier-Prydz Bay region has demonstrated that when major phases of erosion and sediment delivery took place, the ice dynamics were strongly controlled by the changing bathymetry of the ice-eroded graben (Taylor et al., 2004).

The clay mineralogy of the Pagodroma Group indicates a dominance of illite and chlorite in the Middle Miocene or older Mount Johnston and Fisher Bench Formations, indicative of physical weathering and erosion of

Figure 10.5: Distribution of Neogene (15-2 Ma) terrestrial volcanic outcrops in West Antarctica.

Figure 10.5: Distribution of Neogene (15-2 Ma) terrestrial volcanic outcrops in West Antarctica.

local metavolcanic and gneissic source rock terrains under glacial conditions (Ehrmann et al., 2003). In contrast, the Middle-Late Miocene Battye Glacier Formation contains significant contributions of smectite and kaolinite (Whitehead et al., 2006a), and the Pliocene-Pleistocene Bardin Bluffs Formation has the highest kaolinite concentrations in the absence of smectite. Formation of kaolinite and smectite in source rock terrains generally requires prolonged exposure of source rocks to chemical weathering. The chemically altered nature of the strata is confirmed by bulk chemical and rock magnetic studies (Bloemendal et al., 2003; Passchier and Whitehead, 2006). Recycling of weathered materials from local sedimentary source rocks cannot be ruled out (Ehrmann et al., 2003). The origin and significance of the chemically weathered materials in the Middle Miocene and younger Pagodroma Group formations remains inconclusive at this time and further field and laboratory studies are necessary to resolve this issue.

The two regions described above represent the best studied examples of pre-Quaternary onshore glacigenic successions, but there are others elsewhere, e.g. the Antarctic Peninsula (Hambrey et al., 2008) and the Grove Mountains (Fang et al., 2004). When a better-dated Sirius Group can be linked to the offshore record in McMurdo Sound, and the data from the Prince Charles Mountains fully integrated with the emerging Prydz Bay record, we will have a good understanding of the behaviour of the EAIS since its inception in these segments of the Antarctic margin.

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