HISTORY  OF  THE  EAST  ANTARCTIC  ICE  SHIELD:
CENOZOIC  SEDIMENTS  OF  THE  PAGODROMA  GROUP

Prof. Dr. Werner  Ehrmann

Prof. Dr. Michael J. Hambrey,
Centre for Glaciology, Institute of Geography and Earth Sciences,
University of Wales, Aberystwyth, Ceredigion SY23 3DB, UK
Dr. Barrie McKelvey,
Division of Earth Sciences, School of Physical Sciences, University of New England, Armidale, New South Wales 2351, Australia
Dr. Jason Whitehead,
Institute of Antarctic and Southern Ocean Studies,
University of Tasmania, Private Bag 77,
Hobart, Tasmania 7001, Australia
Dr. Jan Bloemendal,
Department of Geography, University of Liverpool,
Roxby Building, Liverpool L69 3BX, UK

Scientific Background:

The East Antarctic ice sheet has existed, according to many researchers, for more than 33 Ma, but it has fluctuated considerably and has been one of the major driving forces of global sea level and climate throughout the Cenozoic Era. However, the scale and temporal pattern of these fluctuations and associated climatic changes, have been the subject of considerable debate. Two main hypotheses have emerged concerning the stability of the ice sheet in pre-Quaternary time, one that it has remained stable for at least 15 Ma (the ‚stabilist‘ view), and the other that it only achieved stability around 3 Ma (the ‚dynamicist‘ view). These contrasting views have emerged from work over two decades in the Transantarctic Mountains, but a new region, the Lambert Glacier - Amery Ice Shelf drainage basin, which is arguably the largest ice drainage basin on the continent, is emerging as an ideal site to test these hypotheses, and to develop a record for understanding the evolution of the entire East Antarctic ice sheet and the associated climates. 

Field Work:

The four formations of the Pagodroma Group have been described and sampled by Hambrey and McKelvey in 1994/95. The outcrops are situated in Prince Charles Mountains on the western rift margin of the Lambert Graben, and they comprise the Fisher Massif (Mount Johnston Formation, Fisher Bench Formation) and the Amery Oasis (Battye Glacier Formation, Bardin Bluffs Formation).Four sections from the Battye Glacier Formation were logged and sampled in detail by Whitehead in 2000/2001.

Investigations:

Hambrey and McKelvey found that the four formations of the Pagodroma Group were deposited largely in an ice-proximal fjordal setting of the ancestral Lambert Glacier - Amery Ice Shelf drainage system, although occasionally there are signs of grounding in some sections. A climatic regime much warmer than that of today is suggested; suitable modern analogues include areas such as East Greenland and Svalbard.At the University of Leipzig, the clay mineral composition of the sediments has been investigated in order to facilitate correlation of the sedimentary sequences. Another aim was to reconstruct the provenance area of the sediments and the Cenozoic continental weathering conditions.At the University of Liverpool, the geochemical and magnetic signatures of the sediments have been investigated in order to discriminate between different stratigraphic successions, to identify climatic trends and to reconstruct source areas. J. Whitehead coordinated the palaeontological investigations.

Some Results:

(1) The four geographically separated formations of the Pagodroma Group carry indvidual clay mineral signals. The clay assemblages of the Mount Johnston Formation and Fisher Bench Formation are similar, and are dominated by illite and chlorite. In contrast, the sediments of the Battye Glacier Formation are characterized by additional components of kaolinite and smectite. The Bardin Bluffs Formation has the greatest kaolinite concentrations and relatively low illite and chlorite concentrations, but lacks smectite.

(2) The major part of the clay mineral assemblages of the Mount Johnston Formation, the Fisher Bench Formation and the Bardin Bluffs Formation of the Pagodroma Group can be attributed to a nearby source. The metavolcanic rocks and gneisses of Fisher Massif contributed illite and chlorite to the sediments of the Mount Johnston Formation and the Fisher Bench Formation. The Permo-Triassic sedimentary rocks of the Amery Group provided the kaolinite found in the Bardin Bluffs Formation.

(3) The bulk of the clay minerals present in the Pagodroma Group indicates an origin by physical weathering under glacial conditions. Only the smectite and kaolinite of the Battye Glacier Formation suggest an interval of warmer and wetter conditions that resulted in chemical weathering. However, the more likely alternative is that the smectites and kaolinites are of detrital origin, derived from a far-distant source, probably situated beneath the ice. 

(4) Principal components analysis of the XRF data indicate the occurrence of two main groupings, which we infer are determined by the presence in the Battye Glacier and Bardin Bluffs formations of a significant component derived from the Permo-Triassic sedimentary rocks of the Amery Group.

(5) The Battye Glacier Formation (10.7 - 9.0 Ma) consists of two units. The Lower Member reflects erosion of Precambrian basement. Although the Member at present is situated some 250 km inland of the ice edge, it was deposited in a marine, ice-distal environment. It included the accumulation of the fossil bearing McLeod Beds, the monospecific Hiatella sp. mollusc assamblage of which indicates a large input of melt water. The Upper Member reflects increased erosion of Permian-Triassic Amery Group and was deposited in a marine, ice-proximal environment.

Ehrmann, W., Bloemendal, J., Hambrey, M.J., McKelvey, B. & Whitehead, J. (2003): Variations in the composition of the clay fraction of the Cenozoic Pagodroma Group: implications for determining provenance.- Sedimentary Geology, 161: 131-152.

Whitehead, J., Ehrmann, W., Harwood, D.M., Hillenbrand, C.-D., Quilty, P.G., Hart, C., Taviani, M., Thorn, V., McMinn (2006): Late Miocene paleoenvironment of the Lambert Graben embayment, East Antarctica, evident from: Mollusc paleontology, sedimentology and geochemistry.- Global and Planetary Change, 50: 127-147.