NEOGENE GLACIAL RECORD OF THE LAMBERT GRABEN, EAST ANTARCTICA: IMPLICATIONS FOR ICE SHEET FLUCTUATIONS

This contribution examines the Neogene glacial sedimentary record in the Prince Charles Mountains, bordering the Lambert Graben, and compares it with the offshore record in Prydz Bay drilled by the Ocean Drilling Program. The graben is important glaciologically because it is the largest ice-drainage system in Antarctica, evacuating 16% of the East Antarctic Ice Sheet. The sedimentology of the onshore sequences, combined with core-constrained seismic interpretation of the continental shelf and ice-sheet modelling, allows inferences to be made concerning changes in the dynamics of the ice sheet through the Neogene Period.

The onshore glacial record is represented by the Pagodroma Group in the northern Prince Charles Mountains which comprises four formations separated geographically and topographically. Over 800 m of Pagodroma Group strata have been logged; they record ice-proximal fjordal deposition in a glaciological regime that was much warmer and wetter than the present day - probably resembling modern conditions in Greenland or Svalbard where the glaciers are polythermal. The oldest Pagodroma Group sediments are late Oligocene/early Miocene and are topographically the highest (up to 1400 m.a.s.l.), whereas the youngest are Pliocene and are the lowest (up to 100 m.a.s.l.). Thus step-wise tectonic uplift of the flanks of the Lambert Graben, as well as large-scale fluctuations of the ice margin is a feature of Cenozoic evolution in this region.

Drilling and seismic mapping in Prydz Bay by the Ocean Drilling Program indicates that pre-late Miocene sedimentation of the continental shelf under the influence of glaciation was characterised by linear progradation of the shelf break, but around the Miocene/Pliocene boundary sedimentation became focused to produce the Prydz Trough-Mouth Fan, indicating a switch to ice-stream sedimentation.

Numerical ice-sheet modelling shows that tectonically induced changes in the bathymetry of the Lambert Graben and Prydz Bay are sufficient to alter the ice sheet dynamics dramatically, although uplift of individual blocks within the Prince Charles Mountains probably had little effect in controlling ice flow. Glacial erosion may have played a role in excavating the Lambert Graben by promoting fast-flowing ice through positive feedback.