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SCAR Report No 16

Appendix 7

CONTINENTAL SHELF SEDIMENTARY BASINS
P J Barrett, School of Earth Sciences, Victoria University of Wellington,
P O Box 600 Wellington, New Zealand.

The most direct stratigraphic record of Antarctic climatic history over the last 100 million years is to be found in the blanket of sediment that covers the Antarctic continental shelf (Fig. 6). This record has been the subject of spasmodic study from the turn of the century to the end of the 1960s, followed by substantial activity in the 1970s and 80s with extensive seismic surveys and around a dozen drill holes, followed in the 90s by a period of review and assessment (Webb, 1990, Cooper et al., 1993, this meeting). With continuing concern over future climate and sea level change, a better knowledge and understanding of the past history of the Antarctic continent is needed to raise awareness of the sensitivities and thresholds in the earth-ocean-atmosphere-ice system, and data from shelf sediments should contribute to this.

Sediment on the continental shelf varies in thickness from nothing to around 14 km. The areas of greater thickness, the sedimentary basins, are typically tens to hundred of km across and vary in shape from subcircular depressions to half grabens to linear troughs. The troughs tend to be either parallel or perpendicular to the shelf margin. From the limited knowledge obtained thus far on the age of sediments in the basins, there appear to have been two main period of basin development, one a consequence of the stretching as Gondwanaland fragmented and firstly India and southern Africa and then Australia moved northwards in early and late Cretaceous times respectively. This rifting and fracturing set the pattern of basin geometry around the Antarctic margin and resulted in the first (syn-rift) episode of basin filling. A period of quiescence was followed by renewed differential subsidence through the Cenozoic with some basins accumulating sediment largely in the Paleogene (eg Victoria Land Basin, Ross Sea) and others in the Neogene (eg. Eastern Basin, Ross Sea).

Younger (post-rift) sediments in both basins and troughs commonly show an internal geometry of seaward dipping reflectors that has led to their description first as deltas (Hinz & Block, 1984) and then more neutrally as prograding sequences (Cooper et al., 1993) (Fig 1b). These features have also been interpreted as till-deltas (Alley et al. 1989), in which the join between topset and foreset bed records the contemporaneous grounding line of a marine ice sheet. It is plainly important to test such a model on both modern sediments and older sedimentary sequences when the feature is well-developed; however the practical difficulties of coring through a marine ice sheet and tens of m into the sediment beneath suggest that any testing may have to be carried out on older sequences, at least initially.

The differential subsidence history of shelf basins is important to recognise and exploit for two reasons. One is that the different basins active at different times in the same region will record events from different time period and when brought together allow a more complete history to be assembled for the region. The other is that in periods of rapid subsidence a basin has the potential for providing the high resolution record needed for icesheet-driven depositional systems, where cycles are only a few tens of thousands of years. However success in piecing together records from different basins requires unambiguous correlation and an accurate chronology, for which data from both seismic surveys and drill holes must be combined.

ANTOSTRAT (1988) identified 5 main regions on which to focus, with each considered to record the history of different parts of the Antarctic ice sheet, e.g. Antarctic Peninsula ice, West Antarctic Ice Sheet, East Antarctic Ice Sheet. Each of these parts of the ice sheet has a different sensitivity and response to climatic forcing (both direct and indirect - eg sea level change) because of their different size and geographic/geologic setting. These histories need to be documented and understood separately in order to understand the global influence of the Antarctic ice sheet as a whole on past climate and sea level change. The regions are (Fig. 1a).

Antarctic Peninsula -Pacific side - Neogene AP ice sheet
Weddell Sea- East Antarctic Paleogene ice sheet history with record of earliest ice
Prydz Bay - East Antarctic Paleogene ice sheet history with record of earliest ice
Wilkes Land - East Antarctic Paleogene/Neogene ice sheet history with record of
later ice growth
Ross Sea - West Antarctic Paleogene/Neogene ice sheet history with latest ice growth

Other useful regions need to be identified for study. For example, the sequence off James Ross Island on the Atlantic side of the Antarctic Peninsula overlies the Paleocene strata of Seymour Island and is likely to contain a detailed record of sedimentation and climate in this region through the Eocene and Oligocene when the first ice sheets were forming, according to current knowledge. The 5 ANTOSTRAT regions were selected for seeking the best glacial record and thus are places where ice has been focussed. Almost as important should be the basins around the east Antarctic margin away from the main outlet glaciers and recording contemporaneous complementary data on the local marine environment - temperature, currents , etc.

Although it is dangerous to be too much influenced by models, it is useful in planning to take into account ice sheet behaviour patterns as described by current ice sheet models. For example, Huybrechts (1993) suggests that the earliest large ice sheet developed over the Gamburtsev Mountains (with temperature 20 deg above present) and engulfed Queen Maud Land before extending Australia-wards to cover all of East Antarctica (15 deg above present, and then enlarge further over West Antarctica (9 deg above present) and the Antarctic Peninsula before reaching its present state and extent. The sedimentary record in basins around the periphery should reflect the differences in extent and timing of the limits of the icesheet at the different stages of its history. Furthermore with the acknowledged cyclic character of glaciations in the Quaternary and very likely throughout the Neogene, we should expect such cycles in the Paleogene and seek to document their extent (linked to temperature range) and frequency. Paleogene glaciations may not have involved the full development of this cycle.

References.

Alley R.B., Blankenship D.D., Rooney M.T. & Bentley C.R. 1989 Sedimentation beneath ice shelves - the view from Icestream B. Marine Geology 85, 101-120.
Cooper A.K., Eittriem S., ten Brink U., Zayatz I. 1993. Marine glacial sequences of the Antarctic continental margin as recorders of Antarctic ice sheet fluctuations.
Hinz K., & Block M. 1984. Results of geophysical investigations in the Weddell Sea and in the Ross Sea, Antarctica, Proceedings 11th World Petroleum Congress, London, 1983.
Huybrechts, P., 1992. The Antarctic ice sheet and environmental change: a three-dimensional modelling study. Rep. Polar Research, 99, 241 p.
Webb P.N. The Cenozoic history of Antarctica and its global impact. Antarctic Science 2: 3-21.