Areas AC07-1, AC07-2, AC07-3, AC07-4 and AC07-5
Ashmore Platform, Bonaparte Basin
Release Area Geology
Regional tectonic setting
The Ashmore Platform is an extensive, elevated and highly structured block (Figure 4). It borders the Vulcan Sub-basin to the east, the northern Browse Basin to the south and deepens into the Timor Trough to the west. On the platform, 1500–2000 m of flat-lying Cretaceous and Cenozoic strata overlie at least 4500 m of heavily faulted and folded Permo-Triassic sediments. To the south, rifting during the Late Jurassic break-up of the Argo margin led to tilted fault-block development on the Ashmore Platform prior to widespread peneplanation, subsidence and burial in the Cretaceous–Cenozoic. It has been subjected to fault reactivation due to the Miocene–Pliocene convergence of the Australian Plate and the Southeast Asian Microplates.
The Ashmore Platform is complexly faulted. A westerly concave fault zone divides the platform into two segments; a western terrain with mainly west dipping faults, and an eastern terrain with both east and west dipping faults. The deeper Permo-Triassic section of the western terrain is dominated by westwards-dipping faulting, developed in response to pre-breakup rifting. In the east, the steeper eastwards-dipping faulting developed in response to the formation of the Vulcan Graben. The overlying Cretaceous–Cenozoic section is characterised largely by the upward propagation of this deep-seated faulting; related synthetic and secondary faulting resulted in the widespread presence of hour-glass structures.
The regional tectonic and stratigraphic history of the Vulcan Sub-basin has been summarised by Kennard et al (1999) and Edwards et al (2004). The Vulcan Sub-basin is a northeast-trending, Mesozoic, extensional depocentre in the western Bonaparte Basin. The sub-basin comprises a complex series of horsts, graben and marginal terraces, and abuts the Londonderry High to the east-southeast and the Ashmore Platform to the west-northwest (Figures 2 and 3). The structurally significant and proven hydrocarbon source provinces of the Swan Graben and Paqualin Graben die out to the northeast beneath the younger (Neogene) Cartier Trough. The Montara Terrace flanks the Swan Graben to the east, and the Jabiru Terrace flanks the eastern margin of the Cartier Trough. The southern boundary of the Vulcan Sub-basin with the northern Browse Basin is somewhat arbitrary. O’Brien et al (1999) consider that the boundary is marked by a fault relay zone that overlies a major northwest-trending Proterozoic fracture system.
Stratigraphy
The stratigraphy of the Vulcan Sub-basin is shown in Figure 5 after Edwards et al (2004). The stratigraphy of the Ashmore Platform is similar to that for the Vulcan Sub-basin, except that the Jurassic sediments are either thin or absent. The Cretaceous to Cenozoic sediments have a maximum thickness of 2000 m, forming a thin veneer over Permo-Triassic tilted fault blocks (Figure 6).
Sedimentation in the area was probably initiated as a result of Late Carboniferous
to Early Permian rifting with deposition of a thick succession of shallow
marine to fluvio-deltaic sediments during the Permian to Triassic (Hyland
Bay Formation, Mount Goodwin Formation and Sahul Group). North–south
transpression in the Late Triassic resulted in uplift and erosion. The Plover
Formation was deposited in a fluvio-deltaic environment throughout the Early–Middle
Jurassic. Widespread faulting and uplift commenced in the late Middle Jurassic,
concurrent with intracratonic rifting in the adjacent Vulcan Sub-basin,
and led to extensive erosion of Triassic–Jurassic strata. Post-rift
thermal subsidence in the Valanginian resulted in transgression and the
re-commencement of shallow marine deposition on the platform (Echuca Shoals
Formation). Subsidence continued through the Cretaceous with the deposition
of marine siliciclastics (Bathurst Island Group) followed by subtropical
and tropical carbonates in the Paleogene and Neogene (Woodbine Group). Sedimentation
was interrupted by a major Oligocene hiatus that is recognised throughout
the North West Shelf.