New Insights into a major Early-Middle Triassic Rift Episode in the NW Shelf of Australia

Abstract

The offshore Roebuck Basin is an amazingly complex and unexpected piece of North West Shelf geology. Large scale seismic geometries within the Triassic section is interpreted to define a complex of lava deltas associated within even a bigger scale rift complex. The location of the magmatic outpouring can be associated with a failed triple junction. Gravity and magnetic modelling supports this model and the apparent thickness of the volcanic complex is up to 10km. The impact of this rifting is felt regionally along the three arms of the proposed triple junction. The northern arm is easily identified and extends up to the proto-Barcoo and proto-Caswell sub-Basins. The eastern arm propagates with strike-slip motion through the Fitzroy Trough creating numerous transpressional and transextensional features in the Palaeozoic and Early Triassic stratigraphy - previously known as the Fitzroy Movement. The western arm is only recognised to the north of Wombat Plateau. In this area, the Early Triassic thins dramatically and sets up the uplifted outboard edge of a broad epicentre extending across the present-day Northern Carnarvon Basin. The sequences described above across all three arms are overlain by a major unconformity that is interpreted to be a product of these rifting/uplift events. This paper highlights that a regional approach, incorporating data from multiple sources, geographical areas and formations, assists with the broader understanding of tectonic history of the North West Shelf during the Early Triassic to Middle Triassic.