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 400 km; 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 58,000 km2 and contains at least 2000 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 120 km westwards beneath the Raggatt Basin. The sequence is at least 2s TWT (ca 4-5 km) 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 2 km. 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.