The Northwest Australian Continental Margin

The Northwest Shelf of Australia offers a typical example of a `passive' continental margin. Major intra-cratonic basins of Permian to Middle Jurassic age developed along the present coastline, superimposed to either orthogonally trending Palaeozoic basins or Precambrian basement rocks. In each of these depocentres distinct lithotectonic units can be recognized that are related to phases of rifting and subsequent continental break-up. The pre-break-up rift valley and intra-cratonic basin stages are represented by a very thick Permian to Middle Jurassic series of mainly fluvio-deltaic sediments but with occasional marine incursions. Break-up took place near the end of the Middle Jurassic and was accompanied by large-scale block faultings with associated uplift and sub-areal erosion. Gradually late Jurassic to early Cretaceous marine sediments transgressed over the eroded surface: within the general transgressive episode, late Callovian, late Oxfordian to Kimmeridgian, late Tithonian to early Cretaceous marine incursions may be singled out. Open marine conditions, related to the breakup of Gondwanaland and opening of the Indian Ocean, became widespread during the Albian in the southern part of the Australian Northwest Shelf and during the Cenomanian in the northern part. The deposition of a thick prograding wedge of mainly carbonate sedimentation since the mid-Eocene resulted in a northwesterly regional tilt of the Shelf. Hydrocarbon occurrences are related to the tectonic evolution. Early Triassic, early Middle Jurassic, late Oxfordian--Kimmeridgian and early Cretaceous marine incursions are directly related to the deposition of potential source rocks in restricted basins. A regressive phase led to the deposition of Triassic fluviatile sediments with excellent reservoir potential. Break-up tectonism and subsequent marine transgression provided the relevant trapping mechanism and probably the migration paths for the major gas--condensate discoveries of the Rankin Platform. The prolonged high rate of subsidence and accompanying thick sedimentation have ensured that hydrocarbon generation occurred, despite the low geothermal gradient.