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SCAR Report No 16,

Appendix 7

PRYDZ BAY AND MAC.ROBERTSON SHELF
P.E. O’Brien, Antarctic CRC and Australian Geological Survey Organisation
GPOBox 378, Canberra, ACT, 2601
and
G. Leitchenkov, VNIIOkeangeologia, Antarctic Branch
1 Angliysky Av. 190 121, St Petersburg, Russia

Although Prydz Bay is one of the more intensely studied parts of the Antarctic margin there are still many important questions that can be addressed by studying it further. The Mac.Robertson Shelf is relatively poorly known but recent work along its eastern end suggests that it also provides opportunities to address major scientific problems.

Theme 1. Structure, Tectonics And Continental Margin Development

Prydz Bay is a key region in understanding the early history of break-up between Antarctica and Greater India and the development of the Indian Ocean.The major crustal and tectonic feature of this region is the prominent north-east south-west trending rift system, the Lambert Graben, which crosses the continental margin obliquely and extends into the continent toward the south.(Fig.1). Two parallel riftgrabens, divided by a crystalline basement high occur within the shelf, continental slope and rise of Prydz Bay, representing a typical 'Double Rift' system, with intracontinental and pericontinental branches. The pericontinental rift branch shows transfer faults with offsets of up to 100 km (Fig.1).

The inner basement high features half-grabens oriented parallel to the mainrift with rotated crustal blocks that switch polarity across transfer faults. Similar features are recognised in the basement of the pericontinental rift branch (Fig.1) and all reflect the first (early) phase of large-scale extension, which has resulted indevelopment of the broad double rift structure and dog-leg pattern of the intracontinental branch (Fig.1). Another group of tilt blocks occurs beneath the outer shelf and continental rise and shows the latitudinal strikes. This variation in strike of extensional structures was possibly caused by a change of stretching direction from NW-SE to N-S. Continental extension has produced more than 350 km of continental margin underlain by stretched continental crust, implying extension of as much as 300 %. This extension was followed by the opening of the Indian Ocean. The change of stretching regime in the Prydz Bay region probably corresponds to the onset of sea-floor spreading on the Western Australia margin at M11 time (about 132 Ma). Sea-floor spreading in Cooperation Sea started at M10 Time (about 118 Ma).

The Mac.Robertson Shelf west of Prydz Bay also contains a record of the rifting history of this margin. It is a scalped shelf with Precambrian basement,Mesozoic and pre-glacial Cenozoic sediments cropping out on the sea floor. Recent seismic and sampling have identified an inner shelf half graben filled with Cretaceous sediments. Gently dipping Jurassic, Palaeocene and Eocene sediments underlie the outer shelf. The Palaeocene and Eocene units are clearly post-rifting shelf sediments but it is not clear how the Jurassic sediments fit into the to the tectonic history of the margin. The Cretaceous age of the synrift sediments is at odds with interpretations of the rifting of India from Gondwana based on evidence from the Jurassic age of oceanic crust off Western Australia.

Research Possibilities

The geometry of rifting and hence the mechanism of extension during rifting are still poorly constrained in Prydz Bay and the adjoining continental margin. Investigations of these aspects of structure and tectonics would be greatly enhances by additional deep-penetrating seismic surveys with recording parameters designed to see below the strong multiple. Seismic refraction using Ocean Bottom Seismometers would also be useful. Parts of the Mac.Robertson Shelf has never been visited by shipsequipped with seismic reflection systems. Dating of the half grabens associated with extension in Prydz Bay would require ODP-style drilling. The Mac.Robertson Shelf, however, has abundant sea floor outcrop of Mesozoic synrift and Palaeogene post rift sediments which could be accesses using small drilling rigs which need only penetrate about 10 meters to obtain good samples for dating and interpreting thesediments.

The position of the continent-ocean boundary is poorly known and would require additional seismic reflection, refraction, magnetics and gravity studies to constrain. Also, the timing of sea floor spreading in the Cooperation Sea and its relationship to the formation of the Kerguelen Plateau is poorly understood and requires additional magneticdata and more studies of satellite gravity data.

Theme 2. Mesozoic And Palaeogene Environments

Mesozoic non-marine sediments were recovered in ODP sites 740 and 741 and examination of seismic sections from the inshore part of the Bay suggests that they may crop out in places. Two units are present. The younger unit yielded early Cretaceous (middle Albian) palynomorphs and consists of sandstone, siltstone, mudstone and coal beds with abundant plant matter in fining-up cycles indicative of fluvial deposition (Turner and Padley, 1991). The underlying, undated unit also featuresfining-up sandstone-mudstone cycles but is reddish to greenish-grey and lacks coal beds (Turner, 1991). This unit could be Jurassic or be part of the Amery Group which crops out in the Prince Charles Mountains and is of Permian to Triassic age.

The oldest Palaeogene sediments recovered in Leg 119 holes were of late Eocene to Oligocene age in site 742A (Barron, Larsen and Baldauf, 1991). It is possible that the hole reached pre-glacial sediments but the sandstone bed at the base of the hole could also be interbedded with glacial facies. Seismic data indicates a significant thickness of sediment between the horizon reached by site 742A and the top of the early Cretaceous sequence. This sequence was deposited during the preglacial and early glacial part of the Palaeogene and thus may be a unique record of the initial transition into the Cenozoic glaciation.

Although there has been no drilling on the Mac.Robertson Shelf, gravity cores have yielded Jurassic, Cretaceous, Palaeocene and Eocene palynomorphs and foraminifera (O’Brien et al., 1995). These fossils occur in discrete, unmixed assemblages and are well preserved, indicating minimal reworking and transport. Therefore, they probably represent the fossil content of the sediments cropping out on the shelf. Plant matter, glauconite and fish teeth indicate non-marine to shallow marine conditions of deposition.

Research Possibilities

Jurassic to Eocene sediments are poorly known for the Antarctic, largely because of the scarcity of outcrop and drill material. The Palaeocene and Eocene in particular span the period of transition from cool temperate to glacial environments with the accompanying change in flora and fauna. The Eocene to Palaeocene section in Prydz Bay can be most effectively reached using ODP drilling on the western side of the bay in Prydz Channel where erosion by a fast-flowing ice stream has removed much of the thick glacial sediments which hindered drilling on ODP Leg 119. Proposal 790 (O’Brien et al., 1996) includes a proposed site to intersect the preglacial Palaeogene section. The Mac.Robertson Shelf provides and excellent opportunity for coring of these sediments using small sea floor or ship board rigs because of the extensive sea floor outcrop. Additional seismic and sidescan data from the area would facilitate selection of drill sites.

Theme 3. Cenozoic Glacial History

Prydz Bay is a key location for the study of Antarctic Cenozoic glaciation. About 20% of the East Antarctic Ice Sheet drains through the Lambert Glacier into Prydz Bay. Included in this drainage basin are the Gamburtsev Subglacial Highlands which could have been the initial site of ice sheet development on the continent. Sediments delivered by the Lambert Glacier are preserved in the bay, on the continental slope and on the rise and a fortunate juxtaposition of rock types means that ice fed by local, coastal precipitation on the western side of the bay carry distinctly different sediments from ice originating in the interior.

The oldest glacial sediments known in ODP holes in Prydz Bay are late Eocene to early Oligocene (Barron et al., 1991). These sediments accumulated as vertically accreting sheets until a distinct shelf break developed at the limit of glacial deposition. From the mid Oligocene, significant amounts of debris reached the shelf break and prograded seawards. Thin topsets of glacial till accumulated on the shelf during glacial maxima or the shelf was eroded. Glacial maxima probably occurredin the late Oligocene and late Miocene.

Glacial debris was distributed evenly across the bay until the early Pliocene when a change in ice behaviour saw the development of a fast flowing ice stream on the western side. This ice stream cut a channel 100 km across to the shelf edge. Debris entrained in the base of the ice and in its deforming bed was then transported to the shelf edge and deposited in a large trough mouth fan. Seismic and core data from the fan indicate that thin turbidites were deposited during glacial maxima with proximal glacial meltwater plume deposits near the shelf edge. Preliminary thermo- luminescence dating suggests that the last advance to the shelf edge took place during isotope stage 6 (c.a. 140 ka.). Hemipelagic muds and ooze were deposited during episodes of reduced ice extent or an erosion surface developed on the fan. Some 16 sequences representing major glacial-interglacial cycles can be mapped in the post early Pliocene deposits of the continental slope of Prydz Bay. Large slump deposits are not apparent on existing seismic and echo sounder profiles, unlike adjacent parts of the slope, indicating a relatively undisturbed record of glacier advances to the shelf edge may exist in the trough mouth fan. Fine sediments which have bypassed the fan now reside in contourite and distal turbidite drifts on the continental rise.

Research Possibilities

Prydz Bay has great potential for ODP drilling to identify the oldest Cenozoic glacial sediments on the Antarctic margin in the bay itself and to provide relatively complete records of major ice advances in the trough mouth fan. Continental rise drifts have potential to preserve sediments from both glacial and interglacial episodes, particularly for the Plio-Pleistocene. An ODP proposal(Proposal 790) is active at present but still requires some site survey data to be completed. Both the trough mouth fan and continental rise drift deposits would be attractive targets for long piston coring cruises.

References

Barron, J., Larsen, B. and Baldauf, J.G., 1991 - Evidence for late Eocene to early Oligocene Antarctic glaciation and observations on the late Neogene history of Antarctica: results from leg 119. In Barron, J., Larsen, B. et al., 1991 - Proceedings of the Ocean Drilling Program Scientific Results, 119, 869-894.
Cooper, A., Stagg, H., & Geist, E., 1991 - Seismicstratigraphy and structure of Prydz Bay, Antarctica: implications from Leg 119 drilling. In Barron, J., Larsen, B. et al., 1991 - Proceedings of the Ocean Drilling Program Scientific Results, 119, 5-26.
O’Brien, P.E., Harris, P.T., Quilty, P.G., Taylor, F. and Wells, P., 1995 - Post-cruise report, Antarctic CRC marine geoscience, Prydz Bay, Mac.Robertson Shelf and Kerguelen Plateau. Australian Geological Survey Record, 1995/29.
O'Brien, P.E., Leitchenkov, G., Kuvaas, B., Ishihara, T., Harris, P.T., and Cooper, A. K., 1996 - Glacial history and palaeoceanography: Prydz Bay-Cooperation sea, Antarctica. Ocean Drilling Program Proposal 790.
Turner, B. R., 1991 - Depositional environment andpetrography of preglacial sediments from Hole 740A, Prydz Bay, Antarctica. In: Barron, J., Larsen, B. et al., 1991 - Proceedings of the Ocean Drilling Program Scientific Results, 119, 45-56.
Turner, B. R. and Padley, D., 1991 - Lower Cretaceous coal-bearing sediments from Prydz Bay, Antarctica. In: Barron, J., Larsen, B. et al., 1991 - Proceedings of the Ocean Drilling Program Scientific Results, 119, 57-60.