OF THE EAST ANTARCTIC ICE SHIELD:
SEDIMENTS OF THE PAGODROMA GROUP
Prof. Dr. Werner
Michael J. Hambrey,
for Glaciology, Institute of Geography and Earth Sciences,
University of Wales, Aberystwyth,
Ceredigion SY23 3DB, UK
Dr. Barrie McKelvey,
of Earth Sciences, School of Physical Sciences, University of
New England, Armidale, New South Wales
Dr. Jason Whitehead,
Institute of Antarctic and Southern Ocean Studies,
University of Tasmania, Private Bag 77,
Hobart, Tasmania 7001, Australia
Dr. Jan Bloemendal,
of Geography, University of Liverpool,
Roxby Building, Liverpool
L69 3BX, UK
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.
|Fig. 1: Location
map of the Pagodroma Group formations sampled, with their locations in
the Antarctic context (A) and the context of the Lambert Glacier basin
(B). Mount Johnston Formation and Fisher Bench Formation were sampled at
Fisher Massif, Battye Glacier Formation and Bardin Bluffs Formation were
sampled in Amery Oasis.
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.
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.
(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
(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.
|Fig. 2: The different
clay mineral assemblages in the four formations of the Pagodroma Group
indicate different source areas.
Bloemendal, J., Ehrmann, W., Hambrey,
M.J., McKelvey, B.C. Matthews, R. & Whitehead, J.M. (2003):
Geochemical and rock magnetic records from sediments of the
Cenozoic Pagodroma Group, Prince Charles Mountains, East Antarctica:
implications for provenance and weathering.- Antarctic Science,
15 (3): 365-378.
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,
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.