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BELANTOSTRAT project

Antarctica phase III (1993-1997)

Belgian contribution to the "Antarctic offshore acoustic stratigraphy project" (BELANTOSTRAT)

Promotor

Professor Marc De Batist
Universiteit Gent
Renard Centre of Marine Geology
Krijgslaan 281, S8
B-9000 GENT
Phone: +32 (0)9 264 45 87
Fax: +32 (0)9 264 49 97
E-mail: marc.debatist@rug.ac.be

Topics

The scientific objective of ANTOSTRAT - the "Antarctic Offshore Acoustic Stratigraphy" Project - is to extract Antarctica's Cenozoic glacial history from the sediments of its continental margins. The Belgian scientific contribution to ANTOSTRAT consisted of seismic-stratigraphic studies of three portions of the Antarctic continental margin.

The studied areas are situated along portions of the Antarctic continental margin that exhibit different characteristics, such as different glacial regimes and glacial evolution (West-Antarctica as opposed to East-Antarctica), different depositional processes and environments (semi-enclosed basins, quasi-starved distal glacial-marine environments, deep-sea fan and drift environments, etc.)or different preservation potential (subsiding shelf edge in Bransfield Basin as opposed to stable shelf edges in Weddell Sea or along the Amundsen-Bellingshausen margin).

1. The Trinity Peninsula and South Shetland Islands margins of central Bransfield Basin:

Swath bathymetry data acquired in central Bransfield Basin during the GEBRA-93 survey reveal new morphological features and trends that shed a light on the ongoing processes of back-arc basin formation and evolution. Basin compartmentalisation and progressive deepening towards the north-east via a series of bathymetrical steps suggests a progressive increase in basin maturity in that direction. The different shapes and sizes of large volcanic edifices dominating the basin-floor morphology can be interpreted in terms of successive evolutionary stages of incipient sea-floor spreading.

New high-resolution reflection seismic data indicate that the upper-slope deposits along the Trinity Peninsula margin contain resolvable records of at least three periods - since the Pliocene, beginning of the opening of Bransfield Basin - during which ice sheets advanced to the shelf edge for a significant amount of time. The record of glacial periods of lesser extent is probably not preserved in the upper-slope and shelf deposits as suggested by the strong erosional unconformities.

Magnitude of slope progradation varies along the Trinity Peninsula margin and appears to be related to local sources of sediment supply associated with separate glacial troughs. Ice-stream activity within these troughs appears to have varied through time. A thick stack of prograding units is preserved at the mouth of Orleans Trough. Off the mouth of the Antarctic Sound, most of the correlative section has been removed by canyon incision at the Gebra Valley.

Development of basin-floor strata appears to be temporally distinct from the development of the prograding upper-slope wedges, the toes of which downlap the basin floor strata. A drill hole (recently submitted ANTOSTRAT-ODP Proposal) through this basin-floor section would provide an excellent opportunity to constrain the age of the glacial cycles associated with the progradational wedges.

2. The West-Antarctic continental margins of the Bellingshausen and Amundsen seas:

A regionally-spaced reflection seismic data set has been acquired from the hitherto largely unexplored Bellingshausen and Amundsen Seas along the West Antarctic margin during the ANT Xl/3 survey in 1994. These data show the large-scale stratigraphic architecture of the continental shelf, slope and rise, and contain a record of the long-term glacial history of the area. On all seismic profiles, the same variation of outer-shelf geometries is observed: (i) a lower unit of mainly aggradational sequences; (ii) a middle unit of strongly prograding sequences; and, (iii) an upper unit exhibiting both progradation and aggradation. The lower aggradational sequences are thought to represent conditions before the onset of the major glacial advance of a grounded ice sheet, whereas the overlying sequences probably record several extended periods of ice-sheet grounding on the shelf since the Middle Miocene.

A prominent erosional surface defines the base of a prograding wedge occupying the continental slope along the margin. It is tentatively correlated with the transition from aggrading to prograding sequences on the outer shelf, and may thus reflect an intensification of the bottom current regime in the lower parts of the palaeo-slope at response to the onset of glacial conditions on the continental shelf.

Sedimentation on the continental rise appears to have resulted in the construction of large sediment drifts that originate from the interaction of channelised turbidity currents traversing the continental rise, and along-slope bottom currents. These drift deposits are believed to contain a good and easily recoverable record of glacial history of the adjacent continental shelf, and have been the main target of a recently submitted ANTOSTRAT-ODP proposal.

3. The East-Antarctic continental margin of the North-eastern Weddell Sea:

Re-examination of high-resolution, analog seismic records in the vicinity of ODP Site 693 has allowed a number of fine-scale unconformities to be identified in addition to those defined previously. Three sub-sequences have been recognised within seismic units W6 and two in seismic unit W7. They all occur within the seemingly homogeneous Pliocene strata at ODP Site 693, consisting of clayey mud, diatom mud and silty and clayey diatom- bearing mud. They coincide with stratigraphic horizons characterised by spikes in smectite percentage in an overall illite-dominant lithology.

Smectite spikes probably indicate a change in sediment source from the Antarctic continent where glacial activity produced illite in response to direct rock erosion, to the continental shelf where previously hydrolysed Cretaceous and Tertiary sediments were exposed. Eroded detritus could have been transported to the shelf edge by ice sheets. Sediment-laden melt water debauching from the ice-sheet grounding line may have created low-angle erosional unconformities in the middle-slope deposits and may have initiated mass flow that moved downslope towards Wegener Canyon across the mid-slope bench. The smectite-horizons found at ODP Site 693 would represent overbank deposition. The unconformities and sequence boundaries identified on seismic sections on the slope off Cape Norvegia - outside the immediate influence of the glacial prograding wedge deposits - are therefore probably directly related to processes of ice-sheet expansion.

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