Basin Details and Geological Overview
Kerguelen Plateau is one of the largest volcanic plateaus in the world and the largest in the Southern Ocean. It lies about 3 000km to the southwest of Fremantle (Western Australia) and is nearly three times the size of Japan or four times the size of the British Isles. It extends for more than 2 200km in a northwest-southeast direction and lies in deep water (1 000 to 4 000 metres). Geological sampling and scientific drilling shows that it was emergent or under shallow water for up to 40 million years of its history. Wood fragments and coal found in Late Cretaceous sediments indicate that the plateau may have been covered with forests.
A major study of the Kerguelen Plateau and particularly, the Raggatt Basin, was undertaken by Geoscience Australia in the 1980s in collaboration with French scientists from Le Laboratoire de Geophysique Marine de l'EOPG, Institut de Physique du Globe, Strasbourg. Since then, Geoscience Australia has conducted two major seismic reflection surveys over the southern Kerguelen Plateau and the Labuan Basin (1997 AGSO surveys 179 and 180).
Interpretation of the seismic data together with available geological and geophysical information was the basis of the major framework study conducted by Geoscience Australia in 1999-2000 (GA Record 2002/05). Two areas of interest are the Elan Bank and the Labuan Basin. For the first time deep multi-channel seismic reflection data has been collected and interpreted in these regions contributing to better understand their origin and structural characteristics.
Due to its large size and remote location, the crustal structure of the plateau remains poorly understood. ODP drilling revealed that the plateau is underlain mostly by magmatic crust generated in the Barremian-Cenomanian (119 - 95 Ma) by excessive volcanism attributed to a large hotspot. The plateau is often described in the literature as a Large Igneous Province or LIP (Coffin and Edholm, 1994). Examples of LIPs include Iceland, Hawaiian Ridge and the Ontong-Java Plateau. The oceanic origin of the plateau has recently been challenged by ODP drilling results on the Elan Bank (Site 1137, Leg 183, Coffin et al. 2000). Gneissic metamorphic and felsic igneous clasts recovered in a fluvial volcanoclastic conglomerate provided unambiguous evidence of its continental origin. Together with geochemical evidence pointing to the presence of continental lithosphere under the southern part of the plateau (Mahoney et al. 1995, Storey et al. 1992), this recent discovery may indicate a more significant involvement of continental crust in the foundations of the plateau than was previously assumed.
High-resolution seismic data collected by Geoscience Australia show considerable stucturing beneath the lava flows ("volcanic basement") on some lines. It seems likely that the plateau is floored by a combination of different crustal blocks, including continental fragments and magmatic crust.
Sediment thickness and basement relief
Sediment thickness has been estimated across the whole southern part of the plateau using both Australian and French seismic reflection data. Depth to basement maps and 3D images allow the visualization of major structural elements (below) such as a rift system in the central part of the plateau and the shape of major sedimentary basins (Kerguelen-Heard and Raggatt basins).
Several distinct provinces are defined within the Kerguelen Plateau and the adjoining Labuan Basin:
Northern Kerguelen Province
Most of the Northern Kerguelen Province (45°-50°S) was formed during the Oligocene-Miocene (less than 40 Ma) and includes the volcanically active Kerguelen Archipelago. Recent drilling on the Skiff Bank (Site 1139) recovered Late Cretaceous trachytes, which may indicate a wider presence of Cretaceous basement underneath the Northern Kerguelen.
Central Kerguelen Province
The Central Kerguelen Province(50°-55°S) includes the volcanically active Heard and McDonald islands and contains a major sedimentary basin (Kerguelen-Heard Basin). Recent Ar/Ar dating of the ODP samples showed that basalts from the top of volcanic basement are Albian age (102 Ma). The Albian constructional phase of volcanism, was followed by an Oligocene-Miocene episode that produced mostly intrusive and extrusive volcanics ranging from alkaline basalts to trachytes and rhyolites. Volcanism was confined to the eastern part of the province and around Heard Island.
The Kerguelen-Heard Basin is a 'sag' basin containing more than 2 000m of Cainozoic sediments. The basin covers an area greater than 40 000 km2. A prominent seismic reflector (the 'acoustic discordance') within the basin appears to be caused partly by a diagenetic 'front' associated with a Late Paleogene - Early Neogene unconformity. Deformation of the section above this unconformity is caused by deposition dominated by vigorous bottom current activity during the Neogene.
Southern Kerguelen Province
Volcanic basement beneath the Southern Kerguelen Province (south of 55°S) is Aptian-Albian age (119-110 Ma). Three spectacular rift systems have been mapped in this province. The 77° graben extends N-S for more than 400km; the 59° graben trends approximately EW, to the south of the 77° graben; and Southern Kerguelen Plateau rift zone trends NW-SE, to the south of the 59° graben. The age and origin of these rift systems remains unclear. It is interpreted that 77° graben formed in the Latest Cretaceous (~75 Ma), although, faults adjacent to the rift have been reactivated at different times with the latest movements taking place in the Miocene.
The Raggatt Basin is a major 'sag' basin in the southern province. The basin occupies an area of approximately 58 000km2 and contains at least 2 000 metres of Cainozoic sediments. Seismic data show that the eastern flank of the basin is underlain by a buried igneous ridge. This ridge is characterised by sequences of dipping intra-basement reflections which can be traced for up to 120km westwards beneath the Raggatt Basin. The sequence is at least 2s TWT (ca 4-5km) thick and dipping reflection are likely to represent sub-aerial lava flows.
The western and northern boundaries of the basin are less clearly defined. It appears that the basin extends generally in the NW-SE direction to at least 56°S being intersected almost in the middle by 77° graben. In the southern Raggatt Basin, prominent mound features can be seen in the Late Cretaceous-Palaeocene section. ODP 748 intersected upper Campanian and Maastrichtian biogenic carbonates within the same stratigraphic level, which suggests that the mounds are likely to have a biogenic rather than a volcanic origin. Seismic interpretation indicates that these mounds were growing until the Palaeocene.
The occurrence of petroleum systems in the Raggatt Basin is possible but unproven. There are potential source, reservoir and seal facies and trapping mechanisms. Prospective areas occur in water depths of around 2km. The most viable play may be Cretaceous limestone reservoirs in biogenic mounds sealed by the drape of Palaeogene fine-grained sediments, with possible hydrocarbons and sourced from mid Cretaceous marine shales or Early Cretaceous coaly sediments.
Elan Bank extends westward from the boundary between the central and southern Kerguelen Plateau. Gneissic metamorphic and felsic igneous clasts recovered from the primarily basaltic basement complex at ODP Site 1137 on the bank reveal its continental origins. The margins of the bank are characterised by massive lava flows and highly reflective layered crust at its base. These volcanics have obvious similarities with volcanic sequences identified on volcanic passive margins (e.g., the Wallaby and Exmouth plateaus off Western Australia). The volcanic sequences on Elan Bank may have formed either during the (?) Valanginian breakup of India/Elan Bank and Antarctica, or during later Albian breakup of India and Elan Bank, when the bank was probably transferred from the Indian plate to the Antarctic plate via a ridge jump. In either case, massive Albian volcanism overprinted and radically altered the continental sliver forming the core of the Elan Bank.
William's Ridge is underlain by a 12-15km thick crust, which is significantly thicker than the adjacent Labuan Basin crust, but similar to that of the Kerguelen Plateau. It rises to only 500 metres below the sea level and appears to consist of two blocks separated by a narrow fault-bounded valley. Williams Ridge has not been drilled and the stratigraphy and nature of the basement are unknown.
A spectacular dipping reflector sequence has been intersected on Line 179-07 on the western flank of the ridge. It is 50km long and about 1.2 sec thick, with the top part of the sequence faulted in the west. Massive lava flows normally have a strong magnetic signature, however dipping sequence on the William's Ridge has no magnetic anomaly. This may indicate the presence of significant amounts of sediment or sedimentary origin of the sequence.
The Labuan Basin flanking the eastern margin of the Kerguelen Plateau is the largest sedimentary basin in the region. It is about 1 000km long, 250km wide and contains 3-5km of sediment. Structural style and sedimentary fill of the basin are highly variable. The nature of the crust underlying the Labuan Basin and the age of its sedimentary fill are very poorly constrained. Its basement has never been sampled and most of the evidence comes from comparisons to the adjacent Kerguelen Plateau.
Plate tectonic reconstructions of the Southern Ocean show that prior to the onset of the fast spreading between Australia and Antarctica in the Eocene, the Labuan Basin reconstructs against the Diamantina Zone. Pre-Eocene reconstructions using satellite gravity imagery suggest a possible structural similarity between these regions. Large basement ridges in the outer part of the Diamantina Zone and in the eastern Labuan Basin appear to form one continuous province.
The Labuan Basin is floored by a highly faulted and heterogeneous crust, 7-8km thick. Interpretation of the seismic and magnetic data has allowed a subdivision the basin into 3 structurally different domains:
- Western Labuan dominated by SW facing extensional style faults
- Eastern Labuan characterised by large dome-shaped basement highs lacking magnetic signature and a very deep linear trough (9s TWT) in the east
- Southern Labuan with the smooth basement largely unaffected by faulting and uninterrupted sedimentary sequences.
The origin of the crust in the Labuan Basin is likely to be heterogenic. The Western domain is likely to represent extended and downfaulted magmatic crust of the Kerguelen Plateau. Large amagnetic bodies in the eastern domain may have formed as peridotite intrusions, whereas smooth basement of the Southern Labuan may represent oceanic crust of Valanginian age trapped on the Antarctic plate. Metamorphic and granitic rocks dredged from a basement high in the in the northern part of the Labuan Basin and on one of the Southern Kerguelen outcrops may indicate the possible presence of continental fragments within the Labuan Basin crust.
There are no direct or indirect indicators of hydrocarbons known from the deep water (> 4km) Labuan Basin and its petroleum prospectivity is speculative. However, one positive indicator has been noted on seismic data: a bottom-simulating reflector that may indicate the presence of gas hydrates. If present, the hydrates could be of biogenic, thermogenic or mixed origin. A likely possible source rock could be Late Cretaceous marine shales. Basal and near-basal Late Cretaceous sandstones and younger turbidites are possible reservoir facies. These potential reservoirs would be predominantly in the west of the basin close to the Kerguelen Plateau.
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